<%BANNER%>

Advanced power electronic for wind-power generation buffering

University of Florida Institutional Repository
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ADVANCED POWER ELEC TRONIC FOR WIND-POWER GENERATION BUFFERING By ALEJANDRO MONTENEGRO LEN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Alejandro Montenegro Len

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To my brother

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iv ACKNOWLEDGMENTS I would like to first expre ss my gratitude to Charles Edwards, the principle engineer at S&C Electric Co. (Chicago, IL) for his patience and the knowledge he shared throughout the project. I would also like to acknowledge Kenneth Mattern (manager at S&C Electric Co., Power Quality Division) for his constant encouragement and confidence in my ability. I am grateful to S&C Electric Company in general for all of their contribution and concern. Additionally, I would like to thank Alexander Domijan (my supervisory committee chair) for his funding during my graduate studies. My gratitude also goes to my supervisory committee (Dr. Ngo, Dr. Arroyo, and Dr. Goswami) for all of their time and effort. I would furthermore like to acknowledge my family in Spain, for supporting me and believing in me throughout my stay in the United States. I would finally like to express my love and gratitude to my girlfriend, Andrea Vict oriano, for her help with the proofreading and for always being the shoul der I could lean on th roughout the project

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES...........................................................................................................viii LIST OF FIGURES...........................................................................................................ix ABSTRACT....................................................................................................................xvii CHAPTER 1 INTRODUCTION........................................................................................................1 Wind-Energy Outlook..................................................................................................1 Electrical Issues............................................................................................................3 Solutions to Wind-Power Fluctuations.........................................................................9 State of the Art..............................................................................................................9 Objective.....................................................................................................................11 2 SYSTEM DESIGN.....................................................................................................14 Introduction.................................................................................................................14 Control Scheme..........................................................................................................14 Positive Sequence Calculation............................................................................14 Real Power Calculation Using dq Components..................................................20 Phase Locked Loop.............................................................................................21 Control Algorithm Design...................................................................................26 Inner regulators............................................................................................27 Outer regulators............................................................................................35 Per-Unit System Model..............................................................................................56 Inverter Output-Filter Design..............................................................................56 Harmonic content.........................................................................................57 Switching frequency.....................................................................................60 Passive filter design......................................................................................61 Passive filter damping..................................................................................65 Direct-Current Link Capacitor Design................................................................68 Energy Storage Design........................................................................................69 Chopper Inductor Design....................................................................................71 Per-Unit System Model Summary.......................................................................72 Simulated Model.........................................................................................................73

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vi 3 SYSTEM DESCRIPTION..........................................................................................78 System Overview........................................................................................................78 Electrical Network Model...........................................................................................80 Synchronous Machine.........................................................................................80 Voltage regulation...............................................................................................81 Prime Mover........................................................................................................81 Synchronous Machine Control Algorithm..........................................................83 Wind-Farm Model......................................................................................................87 Wind-Farm Control Algorithm............................................................................90 Wind-Farm Power-Factor Correction..................................................................90 Wind-Farm Soft-Start System.............................................................................94 Power Stabilizer..........................................................................................................97 Power-Stabilizer Hardware Description..............................................................97 Interface board..............................................................................................99 Digital signal processor..............................................................................105 Field-programmable gate array..................................................................106 Intelligent power module...........................................................................107 Isolation interface circuit............................................................................108 Power Stabilizer Software Description.............................................................108 Description of DSP program......................................................................109 FPGA program description........................................................................114 4 SYSTEM PERFORMANCE....................................................................................121 System Data..............................................................................................................121 Power Stabilizer Transient Response.......................................................................121 Direct-Current Link Voltage Control................................................................121 Reactive Current Control...................................................................................123 Passive Filter Performance.......................................................................................126 Voltage Regulation...................................................................................................127 System Losses...........................................................................................................128 Power Limiter Results..............................................................................................130 Power Limiter 1 (High Pass Filter)...................................................................131 Power Limiter 1 (Adaptive High Pass Filter)....................................................136 Power Limiter 2.................................................................................................138 Power Limiters Comparison Study...........................................................................145 5 SUMMARY..............................................................................................................148 Conclusions...............................................................................................................148 Further Work............................................................................................................150 APPENDIX A MATHEMATICAL TRANSFORMATIONS..........................................................151 B MATLAB CODES...................................................................................................158

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vii C POWER STABILIZER CONTROL MODULES....................................................168 LIST OF REFERENCES.................................................................................................172 BIOGRAPHICAL SKETCH...........................................................................................176

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viii LIST OF TABLES Table page 1-1 Technical specifica tions of IEC and IEEE................................................................4 1-2 Wind-farm output-power requirements.....................................................................8 1-3 Large-scale wind-power output-leveling projects...................................................10 1-4 Conceptual wind-pow er filtering projects...............................................................12 1-5 Basic system configurations....................................................................................13 2-1 Outer regulator assignation.....................................................................................35 2-2 Rate-of-change limits or PPA for a 10 MW wind farm..........................................47 2-3 Generalized Harmonics of line-to-line voltage.......................................................59 2-4 L filter vs. LCL filter...............................................................................................61 2-5 LCL filter design.....................................................................................................64 2-6 LCL equivalent impedance with damping resistance.............................................65 2-7 Per-unit system........................................................................................................65 2-8 Per-unit system parameters.....................................................................................73 2-9 Designed system results and simulated system results comparison........................76 3-1 Synchronous machine output voltage profile at rated speed...................................82 3-2 Alternatives for the power stabilizer controller.....................................................106 3-3 FPGA code words.................................................................................................120 4-1 System parameters.................................................................................................122 A-1 Mathematical transformations summary...............................................................157 C-1 Control Modules....................................................................................................168

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ix LIST OF FIGURES Figure page 1-1 Wind-power output for two wind farms during one month......................................5 1-2 Power fluctuation comparison...................................................................................6 1-3 Typical power curve of a wind turbine.....................................................................6 1-4 Wind-farm output power vs system frequency.........................................................7 1-5 Control strategies along the power curve..................................................................8 1-6 Wind-farm generation buffering concept................................................................13 2-1 Unbalanced system..................................................................................................15 2-2 Space vector trajectory of an unba lanced system in the d-q-o plane......................16 2-3 Space vector trajectory projection over the d-q plane.............................................16 2-4 Direct and quadr ature components of an unbalanced system.................................17 2-5 Representation of an unbalanced system in the frequency domain.........................17 2-6 Positive-sequence extraction algorithm..................................................................19 2-7 Voltage waveforms for an unbalanced fault event..................................................19 2-8 Response of the positivesequence extraction algorithm........................................20 2-9 Distortion of phase angle due to a negative sequence component..........................22 2-10 PLL diagram............................................................................................................23 2-11 PLL simplified model..............................................................................................24 2-12 PLL system step response.......................................................................................25 2-13 Root locus for two di fferent regulator gains...........................................................25 2-14 PLL system response to an unbalanced system condition......................................26

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x 2-15 PLL system response to a frequency excursion......................................................26 2-16 System description..................................................................................................27 2-17 Simplified system model.........................................................................................28 2-18 Electrical representati on of the dq components......................................................30 2-19 System model block diagram..................................................................................30 2-20 Inverter current regulato r-system model block diagram.........................................31 2-21 Inverter current regulator-sys tem model simplified block diagram........................32 2-22 Simplified current control diagram.........................................................................32 2-23 Current regulator step response...............................................................................33 2-24 Chopper equivalent system.....................................................................................34 2-25 Chopper current controller......................................................................................35 2-26 Powers' definition....................................................................................................36 2-27 System model..........................................................................................................37 2-28 DC link equivalent system block diagram..............................................................37 2-29 DC link simplified system block diagram...............................................................38 2-30 DC link voltage regulator step response.................................................................38 2-31 Simplified system model.........................................................................................40 2-32 Source impedance voltage drop..............................................................................41 2-33 Transfer functions of inverte rs quadrature current component..............................42 2-34 Transfer functions of inve rters direct current component......................................42 2-35 Voltage regulator system block diagram.................................................................44 2-36 Positive sequence extraction algorithm equivalent system.....................................44 237 Voltage regulator detailed block diagram...............................................................45 238 Voltage regulator simplified control diagram.........................................................45 239 System response to a 5% change in voltage reference............................................45

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xi 2-40 Adaptive control scheme.........................................................................................46 2-41 Power Regulator general control scheme................................................................47 2-42 Power limiter 1. Control block diagram..................................................................48 2-43 Power limiter 1. Performance using di fferent cut-off frequencies (unlimited power and energy)....................................................................................................49 2-44 Power limiter 1. Performance using different cut-off frequencies (Pinverter=1.0 MW and Einverter=.5 MJ).......................................................................................49 2-45 Power limiter 2. Limiters details.............................................................................50 2-46 Power limiter 2. Control block diagram..................................................................51 2-47 Power limiter 2. Compensation performance.........................................................51 2-48 Power limiter 2. Inverter response for a sampling time of 2 seconds.....................52 2-49 Power limiter 2. Inverter response for different power ratings. Sampling time 2 seconds.....................................................................................................................53 2-50 Power limiter 2. Inverter response for different ESS sizes. Sampling time 2 seconds.....................................................................................................................53 2-51 Power limiter 3. Control block diagram..................................................................54 2-52 Power limiter 3. Compensation performance.........................................................55 2-53 Power limiter 3. Inverter response for a sampling time of 2 seconds.....................55 2-54 Inverter topology......................................................................................................57 2-55 Line-to-line and line-to-neutral voltage of a three phase inverter...........................57 2-56 RMS Line-to-line voltage harmonic spectrum........................................................58 2-57 Static Synchronous Generator diagram...................................................................59 2-58 LCL filter topology.................................................................................................61 2-59 LCL equivalent block diagram................................................................................62 2-60 Single phase equivalent filter m odel at the fundamental frequency.......................62 2-61 Single phase equivalent filter model at the hth harmonic........................................63 2-62 LCL equivalent impedance with damping resistance.............................................66

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xii 2-63 Single phase harmonic generator equivalent circuits..............................................66 2-64 LCL gain frequency response.................................................................................67 2-65 Inverter frequency analysis.....................................................................................67 2-66 Capacitor Voltage vs. Energy Storage....................................................................70 2-67 ESS-Chopper topology............................................................................................71 2-68 Equivalent circuit for maxi mum current ripple calculation....................................72 2-69 System overview.....................................................................................................74 2-70 Per-unit electric system model................................................................................74 2-71 Power Stabilizer Control Scheme...........................................................................75 3-1 Equivalent system model........................................................................................79 3-2 DC gen-set...............................................................................................................83 3-3 Two single quadrant chopper circuit.......................................................................83 3-4 Synchronous generator control system...................................................................84 3-5 Frequency deviation................................................................................................85 3-6 DC-GEN set control scheme...................................................................................85 3-7 System frequency response for f=-1Hz................................................................86 3-8 Frequency contro l equivalent system......................................................................87 3-9 Equivalent model frequency response for f= 0.01666 pu..................................88 3-10 Dynamic model used for transient studies..............................................................88 3-11 Static model used for steady-state studies...............................................................88 3-12 Wind-farm model....................................................................................................89 3-13 Wind-farm controller...............................................................................................90 3-14 Wind-farm power regulator & current regulator step response ( P=100%)..........91 3-15 Induction generator PQ curve.................................................................................92 3-16 Wind-farm PF correction capacitor bank................................................................93

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xiii 3-17 PF correction capacitor bank current waveforms....................................................93 3-18 Capacitor bank impedance frequency scan.............................................................94 3-19 Machine control scheme operating states................................................................95 3-20 Electric power system start-up................................................................................96 3-21 Detail of the transition from start-up mode to run mode.........................................96 3-22 Power Stabilizer system overview..........................................................................97 3-23 Interface board overview.......................................................................................100 3-24 AC voltage scaling circuit (i nput [-1000+1000V], output [0 +3V]).....................101 3-25 DC voltage scaling circuit (i nput [0 +1000V], output [0 +3V])...........................101 3-26 CT current scaling circuit (input [-5 +5A], output [0 +3V]).................................101 3-27 LEM current scaling circuit (i nput [-0.36 +0.36A], output [0 +3V])....................101 3-28 Power supplies voltage monitoring......................................................................102 3-29 Systems critical signals during turn on................................................................103 3-30 Systems critical signals during turn off...............................................................103 3-31 Darlington drivers.................................................................................................104 3-32 IPM status signa ls interface circuitry...................................................................104 3-33 DAC circuit...........................................................................................................105 3-34 DSP built-in PWM output performance vs. FPGA...............................................107 3-35 IMP power circuit configuration...........................................................................108 3-36 Isolated interface board.........................................................................................109 3-38 Power stabilizer control algorithm sampling rates................................................110 3-37 Interconnections between the different sub-systems of the power stabilizer........111 3-39 Power stabilizer control stages..............................................................................113 340 Power stabilizer start-up sequence........................................................................113 3-41 FPGA system overview.........................................................................................116

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xiv 3-42 Up/Down counter..................................................................................................117 3-43 PWM ge nerator.....................................................................................................117 3-44 One phase dead-time generator detailed diagram.................................................119 3-45 Dead-time generators waveforms........................................................................119 3-46 Watchdog logic.....................................................................................................120 4-1 DC link voltage response for different Kp gains...................................................121 4-2 DC link voltage response for different Ki gains...................................................122 4-3 Iqref command step change from -0.5 to 0.5 A per unit. Integral gain effect........123 4-4 Iqref command step change from -0.5 to 0.5 A per unit. Proportional gain effect.......................................................................................................................124 4-5 Iq current regulator output for different Kp..........................................................124 4-6 Iqref command step change from 0.5 to 0.5 and back to -0.5 A per unit............124 4-7 Power stabilizer harmonic inj ection response for Ki=18 and Kp=1.....................125 4-8 Current regulator frequency response...................................................................126 4-9 Front-end inverter current waveform....................................................................126 4-10 Frequency spectrum of the LCL currents..............................................................127 4-11 Simplified system description...............................................................................127 4-12 Power stabilizer voltage regulation performance..................................................128 4-13 Energy storage charge/discharge cycle.................................................................129 4-14 Control scheme with a losses compensation term.................................................129 4-15 Power stabilizer equivalent system.......................................................................130 4-16 Wind-power conditions under study.....................................................................130 4-17 Measured and modeled hi gh pass filter results for Kc=0.0064 W/J, fcut_off=0.005 Hz......................................................................................................132 4-19 Measured high pass filter performance for different cut-off frequencies. System parameters Kc=0.0064 W/J.....................................................................................134

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xv 4-20 Modeled high pass filter performance fo r different cut-off frequencies. System parameters Kc=0.0064 W/J.....................................................................................135 4-21 Measured high pass filter performa nce for different energy storage sizes. System parameters, Kc=0.0064 W/J, fcut-off=0.005 Hz...........................................135 4-22 Cut-off frequency trajectory of the adaptive high pass filter for a given energy deviation.................................................................................................................136 4-23 Measured adaptive high pass f ilter performance for different Kfs. System parameters, Kc=0.0064 W/J, fcut-off-origin=0.005 Hz.................................................137 4-24 Measured adaptive high pass filter performance for different energy storage sizes........................................................................................................................137 4-25 Multiple sampling concept....................................................................................139 4-26. Measured and modeled power limiter 2 results for Kc=0.0064, RR=2 MW/minute, A=0.3 MW/minute, I=1MW/2 seconds fcut-off=0.005 Hz.................140 4-27 Measured power indexes activity. System parameters: Kc=0.0064 W/J, RR=2 MW/minute, A=0.3 MW/minute, I=1 MW/2 seconds, and fs=10Hz.....................141 4-28 Measured power limiter 2 response to different Kc System parameters: RR=2 MW/minute, A=0.3 MW/minute, I=1MW/2 seconds, and fs=10Hz......................142 4-29 Measured power limiter 2 response to different ramp rate limits.........................142 4-30 Measured power limiter 2 response to different average power fluctuation limits.......................................................................................................................143 4-31 Effect of linear interpolation on th e average power fluctuation index activity. The sampling time of the original wind-power data is 2 seconds..........................144 4-32 Measured power limiter 2 response to different instantaneous power fluctuation limits.......................................................................................................................144 4-33 Measured power limiter 2 response to different sampling frequencies.................145 4-34 Measured synchronous machine output power for the different power limiter control schemes......................................................................................................146 4-35 Measured synchronous machine output power for the different power limiter control schemes......................................................................................................147 4-36 Frequency regulator output fo r the different power limiters.................................147 A-1 Relationships among ds-qs, and abc axes.............................................................153

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xvi A-2 Stationary ds-qs components in the time domain.....................................................153 A3 Relationship among ds-qs and dr-qr axes...............................................................154 A-4 Direct and qua drature components........................................................................155 A-5 Time domain representation of abc and d-q components.....................................156

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xvii Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ADVANCED POWER ELEC TRONIC FOR WIND-POWER GENERATION BUFFERING By Alejandro Montenegro Len May 2005 Chair: Alexander Domijan, Jr Major Department: Electrical and Computer Engineering As the cost of installing and operating wind generators has dropped, and the cost of conventional fossil-fuel-based generation has risen, the economics and political desirability of more wind-based ener gy production has increased. High wind-power penetration levels are thus expected to augm ent in the near future raising the need for additional spinning reserve to counteract the e ffects of wind variations. This solution is technologically viable, but it has high associ ated costs. Our study presents a different solution to short-term wind-pow er variability, using advan ced power electronic devices combined with energy-storage systems. New control schemes (designed to filter power swings with a minimum of energy) were designed, modeled and verified through experimental tests. We also determined the procedure to extr act the corresponding perunit model parameters for simulations and test purposes.

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xviii We first reviewed D-Q transformations with emphasis on modeling of the system and control algorithm. System components were then designed using criteria similar to those used to design medium-voltage power products. We tested a proof-of-concept for performa nce of the power converter in a scaleddown isolated system using real wind-power data. Tests were conducted under realistic system conditions of wind-penetration le vel and energy-storage levels, to better characterized the impacts and benefits of th e Power Stabilizer. We described the scaleddown isolated electric power system used in the testing. We also analyzed the performance of the wind-farm model and the synchronous machines governor to gain an insight into the model systems limitations. Simulation results carried out in Mathem atical Laboratory (MATLAB) and Power Systems Computer Aided Design (PSCAD) were compared to e xperimental data to verify the performance of the power converter under different system conditions and algorithms. Power limiters were also contrasted and evaluated for frequency deviations and attenuated power fluctuations. In summary we can say that, among all th e power limiters considered in our study, the adaptive high pass filter pres ented the best performance in terms of system robustness and effectiveness.

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1 CHAPTER 1 INTRODUCTION Wind-Energy Outlook Wind power has been used for at least 3000 years, mainly for milling grain, pumping water, or driving vari ous types of machines. However, the first attempt to use wind turbines for producing electricity date back to the 19th century. In 1891, Poul La Cour in Demark built an experimental wind turbine driving a dynamo. The oil crisis of the 1970s revived interest in wind turbines. Nowadays, the po wer is the fastest growing source of energy in the world and its growth rates have exceeded 30% annually over the past decade [1]. Cumulative global wind-en ergy generating capac ity approached 40,000 MW by the end of 2003 [2]-[3]. The main driv ers for developing of the wind industry in the United States are Federal Renewable Energy Policies, part icularly the Production Tax Credit (PTC) that provides a 1.5 cent per kilowatt-hour credit for electricity produced from a wind farm during the first 10 years of operation. This wind energy PTC expired December 31, 2003 but will be reinstated through 2005 as part of a major tax package (H.R. 1308). State-level renewable energy initiatives, su ch as the Renewable Portfolio Standard, or green pricing. The Database of State Incentive for Rene wable Energy [4] gives more information on incentives. These government initiatives, together with technological advances, plus the need for a new source of energy capab le of meeting the worlds growing power demand and the rising prices of conventional fossil fuel-based generation, make the wind power one of the most promising industries in the future.

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2 According to the European Wind Energy Association and Greenpeace, no barriers exist for wind to provide 12% of the worl ds electricity by 2020. The American Wind Energy Association forecasts that wind power will provide 6% of the USs electricity by 2020 if the wind industry maintains an annual growth rate of 18%. The positive effects of using such type s of renewable resources are well known. However, wind-power plants, like all other en ergy technology, have some drawbacks that should be mentioned. These problems can be divided into major groups: environmental issues and interc onnection issues. Environmental issues Most significant among these are the following: Sound from turbines: Some wind turbines built in the early 1980s were very noisy. However, manufactures have been working on making the turbines quieter. Today, an operating wind farm at a distan ce of 750 to 1,000 feet is no noisier than a moderately quiet room. Research in aeroacoustics is still being carried out to further reduce noise from wind on the blades. Bird death: Wind turbines are often mentioned as a risk to birds, and several international tests have been performed. The general conclusion is that birds are seldom bothered by wind turbines. Studies show that for example, overhead power pole lines are far more hazardous fo r birds than wind turbines [2]. Wind-tower shadow effect: Wind turbines, like other tall structures cast a shadow on the neighboring area when the sun is visi ble. It may be irritating if the rotor blades chop the sunlight, causing a flickeri ng effect while the rotor is in motion, especially when the sun is low in the sky. Interconnection issues. Connecting wind turbine to ope rate in parallel with the electric power system influences the syst em operating point (load flow, nodal voltages, power losses, etc). These changes in the elect ric power system state bring up new systemintegration issues that system operators a nd power quality engin eers must take into account. These interconnection issu es can be divided into operat ional issues and electrical issues. Operational Issues: These include unit commitment and spinning reserve.

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3 The unit commitment problem is to schedule specific or available generators (on or off) on the utility sy stem to meet the required loads at a minimum cost, subject to system c onstraints. The most conservative approach to unit commitment and ec onomic dispatch is to discount any contribution from interconnected wind resources because of wind variability. Operating reserve is further define d to be a spinning or nonspinning reserve. Any probable load or gene ration variations that cannot be forecasted, such as wind power, have to be considered when determining the amount of operating reserve to carry out. Electrical issues: These factors are consider ed in the next section. Electrical Issues Wind-turbine generator-system operation ha s some negative influence on power systems. This influence on the electric power system depends on wind variations and on wind-turbine technolo gy. Impacts on the electric power system can be grouped as follows: Power quality: Voltage variations, flicke r, harmonics, power-flow variations Voltage and angle stability Protection and control The IEEE 1547 [5] and the IEC 61400-21 [6] st andards are the bases to evaluating the impact of such wind-turbine genera tion systems on the electric power system. According to the IEEE 1547 [5, page 2] abstract, This standard focuses on the technical sp ecifications for, and testing of, the interconnection itself. It provides requir ements relevant to the performance, operation, testing, safety cons iderations, and maintenance of the interconnection. It includes general requirements, response to abnormal conditions, power quality, islanding, and test specifications and requirements for design, production, installation evaluation, commi ssioning, and periodic tests. The stated requirements are universally needed for interconnection of distributed resour ces (DR), including synchronous machines, induction machines, or power inverters/converters and will be sufficient for most installations. The cr iteria and requirements are applicable to all DR technologies, with aggregate capaci ty of 10 MVA or less at the point of common coupling, interconnected to electric power syst ems at typical primary and/or secondary distribution voltages.

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4 According to the IEC 61400-21 [6, page 9] abstract, The purpose of this part of IEC 61400 is to provide a uniform methodology that will ensure consistency and accuracy in the measurement and assessment of power quality characteristics of grid connected wi nd turbines (WTs). In this respect the term power quality includes those electric characteristics of the WT that influence the voltage quality of the grid to which the WT is connected. This standard provides recommendations for preparing the measurements and assessment of power quality charact eristics of grid connected WTs. Table 1-1 shows technical specifications for interconnection and power assessment covered in both standards. Table 1-1. Technical specifications of IEC and IEEE Interconnection system response to excursions Power quality assessment IEEE Voltage Frequency IEC Voltage Frequency Voltage fluctuations: Continuous operation Switching operation Harmonics As shown in Table 1-1, both standards overlooke d one of the most significant characteristics of wind farms: its variability (i.e., power fluctuations) [7], the most important ones being Gusty wind variations having a spectru m of frequencies from 1-10 Hz. Shadow effect having a spectrum of freque ncies from 1-2 Hz and producing torque variations up to 30%. Complex oscillations of the turbine tower, rotor shaft, gear box, and blades with spectrum frequencies from 2-100 Hz, and creating torque vari ations up to 10%. Figure 1-1 shows actual output power da ta collected by NREL from two large wind-power plants in the Unite d States. The small wind farm has a capacity of about 35 MW, and the large one has a capacity of 150 MW.

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5 0 0.5 1 1.5 2 2.5 3 x 106 0 10 20 30 40 Power (MW)Time (s) 0 0.5 1 1.5 2 2.5 3 x 106 0 50 100 150 Trent Mesa Project. Wind power output May 2003Power (MW)Time (s) Figure 1-1. Wind-power output for two wind farms during one month (May 2003). A) Nominal capacity 35 MW. B) Nominal capacity 150 MW. Even though the technology used in construc ting the small wind farm is more than a decade older than the large one, power fluctuations keep being an issue. Figure 1-2 is a close-up of Figure 1-1 and shows the magnitude of these power fluctuations. Wind turbine manufactures usua lly provide power curves (Figure 1-3) to developers to determine the amount of power th at will be transferred into the grid for a single turbine, given the wind speed. However, those fi gures represent only the mean values, since a series of stochastic values cannot be controlled, and create additional power fluctuations. Wind-output power fluctuations can have different eff ects on the electric power system, but the most significant ones are volta ge variation and fre quency variation in small or isolated systems. A B

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6 0 1 2 3 4 5 6 7 8 x 104 0 10 20 30 40 Texas Wind Power Project Wind power output May 2003 Power (MW)Time (s) 0 1 2 3 4 5 6 7 8 0 50 100 150 Trent Mesa Project. Wind power output May 2003Power (MW) 0 1 2 3 4 5 6 7 8 x 104 0 10 20 30 40 Power (MW)Time (s) 0 1 2 3 4 5 6 7 8 x 104 40 50 60 70 80 Trent Mesa Project. Wind power output May 2003Power (MW)Time (s) Figure 1-2. Power fluctuation comparison. A) Nominal capacity 35 MW. B) Nominal capacity 150 MW. Figure 1-3. Typical power cu rve of a wind turbine. As the power fluctuates, the reactive pow er required by the turbines changes as well, and therefore voltage variations are ex pected, especially when the wind farm is A B

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7 located at weak points in the system. To comp ensate for such voltage variations and keep the voltage close to its rate d value, several solutions are available: simple capacitor banks, static voltage compen sator (SVC), or static compensators (STATCOM). A different approach must be taken fo r frequency variations due to power fluctuations. Normally, wind farms connected to big systems do not present a major problem in terms of frequency variations, because of the stiffness of the system. However, with small or isolated systems th at contain slow or no automatic generation controls, a mismatch between generated and absorbed power can significantly affect system frequency unless spinni ng reserves are significant. Figure 1-4 shows the effect of wind-power fluctuation on an isolated syst em with a wind penetration level of 1%. To counter these negative e ffects, countries and small isolated systems with high wind-penetration factors developed special purchase power agreement (PPA) requirements or indexes fo r wind-farm developers (Table 1-2). Figure 1-4. Wind-farm output power vs system frequency.

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8 Table 1-2. Wind-farm output-power requirements Ramp Rate dP/dt Instantaneous Average (max variation) Netherlands <12 MW per min Denmark <0.1Pnom per min <0.05 Pnom per 60 sec period Hawaii <2 MW per min 1 MW change per 2 sec scan <0.3MW per 60 sec period Germany <0.1Pnom per min Scotland No limit for Pnom<15 MW/min Pnom/15 for Pnom=[15-150] MW/min 10 MW for Pnom>150 MW per min These power requirements guarantee minimum impact on system voltage and frequency control. However, todays wind farms have limited capacity to reduce the rate of change of power, especi ally the down ramp rate. At high wind speeds (above the rated wind sp eed), active and stal led pitch controls, among other strategies, can help keep the out put power under control. However, modern wind turbines are designed to obtain as much power as possible at low wind speeds (Figure 1-5), making them very vul nerable to wind variations. Figure 1-5. Control strategi es along the power curve

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9 Solutions to Wind-Power Fluctuations To reduce the effects of wind-power varia tions and meet the PPA requirements for electric utilities, two solu tions can be considered: Higher spinning reserves Wind farm buffer Increasing spinning reserves is a costly so lution. A better approach would be to use an energy-storage system that could de liver the required power when needed. Work has been done in developing largescale energy storage systems that have overcome these issues by absorbing undesira ble power fluctuations and providing firm, dependable peaking capacity [8]. However, a less costly solution should be explored based exclusively on power-fluctuation inde xes (such as ramp rate indexes or instantaneous fluctuation indexes). State of the Art Storing wind power is not a new concept; in fact, back in 1900, the father of the modern wind turbine, Poul La Cour, tackle d for the first time the problem of energy storage. He used the electricit y from the wind turbines for el ectrolysis and to store energy in the form of hydrogen. However, with time, system requirements, energy storage systems, and wind turbine ratings have changed. Nowadays, the average wind turbine inst alled is around 1 MW, according to the European Wind Energy Association, and windpower farms usually c onsists of ten to several tens of wind-turbine generators of rated power up to 2 MW. Thus, the amount of energy storage needed to stabilize the pow er output change in the short term has increased. Table 1-3 shows some recent projects dealing with output leveling of windenergy conversion.

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10 Table 1-3. Large-scale wind-power output-leveling projects Project name Wind farm size Energy storage system Active power reference control scheme Subaru Project [9]. Tomamae wind-power station. 1.65 MW *16 (Vestas). 1.5 MW 5 (Enercon). Total Capacity 30.6 MW VanadiumRedox Flow Battery PVRB nominal =4.000kW EVRB=6.000kWh S inverter=6.000kVA Moving Average of wind farm output determined as Pbattery=Pwind average (tt)-Pwind(t) (for t=8 seconds to 8 hours) King Island [10]. Energystorage system provided by Pinnacle VRB 250 kW*3 850 kW*2 Total Capacity 2.45 MW VanadiumRedox Flow Battery PVRB nominal=200kW PVRB short-term ( 5 minutes)=300kW PVRB short-term (10 seconds)=400kW EVRB =1100kWh Isochronous frequency mode over the VRB power range. Speed droop characteristic during instantaneous and short-term load (> 200 kW). Oki project by Fuji Electric 600 kW *3 Total Capacity 1.8 MW Flywheel E flywheel = 100 kW 90 sec P inverter flywheel side= 110kVA P inverter power system side= 150kVA Power ramp rate limiting

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11 However, small-scale concepts and tec hnical/economic feasibility studies have been proposed (Table 1-4). Each of these projects has a different objective (frequency control, power smoothing, load leveling, etc.). However, they all end up using one of the topologies and energy-storage systems shown in Table 1-5, where the flywheel or capacitors may be replaced by some other en ergy-storage medium. Tables 1-3 and 1-4 show that the amount of energy needed for wind-power balancing using current technology and current pricing is so significant, that a more flexible and integrated approach is needed. Our study focused on developing new power smoothing control algorithms. The new integrated approach used a shunt-conne cted voltage-source converter with added storage included on the DC link bus. The system can Exchange active power with the system. Regulate voltage at the point of common coupling Increase power quality and system stability Objective Our purpose was to develop, simulate, and implement a proof-of-concept prototype advanced-power electronic device capabl e of controlling and smoothing the power fluctuations of a wind farm using an op timal amount of energy. The wind-power generation buffering concept is shown in Figure 1-6. The Power Stabilizer was designed to store excess power during periods of in creased wind-power generation and release stored energy during periods of decreas ed generation due to wind fluctuations. We tested the performance of th e advanced electronic device on DC-synchronous machine set Passive load DC-asynchronous machine set Wind-farm buffer or also called Power Stabilizer

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12 Table 1-4. Conceptual wind-power filtering projects Wind farm size Energy storage system Active power reference control scheme Comments 20 MW Zinc-bromide battery PZBB nominal (charge) =-750kW PZBB nominal (discharge)=1500kW EZBB =1500 kWh Limiting instantaneous power fluctuations based on a 2 seconds interval Pinstantaneous ( t=2 seconds) = .3 MW Average power levels over a 2 hour window Paverage(tt=2hours)= kW Technical and economical feasibility evaluation [11] Maximum power oscillation 2.5 MW Super-conducting magnetic energy storage (SMES) Active power reference is chosen to control system frequency Simulation study [12] 300kW Electric double layer capacitor P ECS =100 kW E ECS =1.1 kWh ESS active power reference is determined by detection power oscillation components using a high pass filter Simulation study [13] 6GW Redox-flow battery (Regenesys ) E=62004 MWh P=255MW Power balancing Feasibility study [14] 45KW Flywheel E flywheel= 12MJ P drive=45kW The active power demand is extracted via a 2nd order Butterworth high pass filter, with a 5mHz bandwidth Practical results [15]-[16] 55kW Lead Acid Battery E battery=35kWh P converter=50kVA Power smoothing Practical results [17]

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13 A B C A B C 0.69 #2#1 12.47 ? L1 L2 L3 Vestas VRCC V47 VnaS NAS VnbS NBS VncS NCS A B C A B C 0.48 #2 #1 12.47 ? g1 g2 g3 g4 g5 g6 2 1 300.0 2 3 2 5 2 4 2 6 2 2 dcCur gc1 2 5 gc2 2 5 1.0 Chopper Reactor1.0 Energy Storage Capacitor A B C A B C 12.47 #2#1 69.0 ? INVERTER WIND FARM 0 20 40 60 80 100 120 8800 9000 9200 9400 9600 9800 10000 10200 10400 10600 10800 Wind Power OutputPower (W)Time (s) 0 20 40 60 80 100 120 -1000 -800 -600 -400 -200 0 200 400 600 Power Stabilier Power OutputPower (W)Time (s) 0 20 40 60 80 100 120 8800 9000 9200 9400 9600 9800 10000 10200 10400 10600 10800 Wind Power OutputPower (W)Time (s) Wind power + Wind farm buffer Power Figure 1-6. Wind-farm generation buffering concept Table 1-5. Basic system configurations System configuration Voltage source inverter ESS connected at the DC link si de [19]-[21]-[24] Synchronous MachineVoltage Source Inverter ESS (flywheel) Electric System Wind TurbineDC link ESS connected at the AC side [ 18]-[20]-[22]-[23]-[25] Induction MachineVoltage Source Inverter ESS (flywheel) Electric System Wind Turbine DC link Current source inverter (shunt connected) [13] Induction MachineCurrent Source Inverter ESS (capacitors) Electric System Wind Turbine Chopper (DC/DC converter) ECS Energy storage system Available options [26] Compressed air energy storage Battery storage Electro-chemical flow cell systems Fuel cell/electrolyse r/hydrogen systems Kinetic energy (flywheel) storage Pumping water

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14 CHAPTER 2 SYSTEM DESIGN Introduction One of the most difficult tasks when de signing a control algorithm for a power electronic converter is to calcula te the regulators gains. Dete rmination of the controllers parameters is based on the electric power sy stem they are connected to, and also on the power electronic converter t opology. This chapter details the design of the different regulators involved in the cont rol of the Power Stabilizer and also the design of the different components th at define its topology. The system design was carried out per unit, so results can be extrapolated to any system size, to facilitate implementation of the control scheme in a fixed-point digital signal processor. The system design was also co mpared to simulation results to assure the correctness of the design methodology used Control Scheme Positive Sequence Calculation Three-phase systems are not always bala nced, especially during fault conditions, and it is expected to have positive, ne gative and even zero sequence components. However, for voltage regulation purposes, only the positive sequence component is of importance. Before going into detail on the positive extraction algorithm de scription, we will explain first where the transformations given in Appendix A fail in coupling the different symmetrical components. Consider the following set of phasors

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15 240 120 00 1 0 1 5 0c b aV V V (2-1) Figure 2-1 shows the time domain representati on of this three-phase unbalanced system. Figure 2-1. Unbalanced system If we now calculate the symmetrical compone nts of this unbalanced system, we obtain 180 0 180 2 0 1167 0 167 0 833 0 V V V (2-2) The symmetrical components transformation is a good tool to determine the type of distortion or asymmetry the system has. Howe ver, it has the drawback of having to use phasors as input instead of time domain sign als. Therefore a differe nt transformation was needed in order to extract the positive sequence component out of the rotating space vector. Figure 2-2 shows the trajectory followed by the rotating space vector of the unbalanced system in the d-q-o plane using Cl arkes transformation. This trajectory is

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16 clearly distorted from the ideal one, and the sp ace vector no longer follows a circular path (Figure 2-3). Figure 2-2. Space vector trajectory of an unbalanced system in the d-q-o plane Figure 2-3. Space vector trajectory projection over the d-q plane Figure 2-4 shows the Vdr and Vqr components (Parks transformation) of the unbalanced system in the time domain for = 0.

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17 Figure 2-4. Direct and quadrature components of an unbalanced system It is clear that the Vdr component is not constant any more, and it contains a 2nd harmonic due to the negative sequence. This eff ect can also be explained in the frequency domain as shown in Figure 2-5. The rotating reference frame aligns with the fundamental frequency, w=2 f, and therefore a negative sequence (-w) appears as a 2nd harmonic a dc component appears as a 1st harmonic a positive sequence (w) ha s a constant value. abc axisdrqr axis 0.167 0.833-ww dc w dc -w -2w Figure 2-5. Representation of an unbala nced system in the frequency domain

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18 Thus, it can be concluded that Clarkes and Parks transformations do not provide suitable components that can be used in a voltage regulation cont rol algorithm. It is therefore necessary then to redefine the tran sformations in order to extract the desired components. Assuming the three-phase electric system has positive and negative sequence components ) 3 4 cos( ) 3 4 cos( ) 3 2 cos( ) 3 2 cos( ) cos( ) cos( wt V wt V V wt V wt V V wt V wt V Vn p c n p b n p a (2-3) Clarkes transformation can be used to obtain qs qs n p qs ds ds n p dsV V wt V wt V V V V wt V wt V V ) sin( ) sin( ) cos( ) cos( (2-4) where dsV and qsV are the d-q components of the positive sequence, while dsV and qsV are the d-q components of the negative sequence. If we now assume that the symmetrical co mponents remained constant for at least a quarter of cycle, the equa tions can be rewritten as t t t t t t t t t t t tqs ds qs qs ds ds qs ds qs qs ds dsV 2 V 2 1 V 2 V V 2 1 V V 2 V 2 1 V 2 V V 2 1 V (2-5)

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19 These components can now be transformed using the rotating reference frame in order to obtain the pos itive sequence component. Figure 2-6 shows the block diagram of the algorithm used to extract the positive-sequence component. The same concept could be used if the negative sequence magnitude is needed. abd dsqs VdsVqs Delay (1/f/4) + + 0.5 Delay (1/f/4) + 0.5 Vds+Vqs+ dsqsdrqr x + ++Vdr+ Vqr+ xVmagnitude positive sequenc e VdrVqr VaVbVc dsqsdrqr FilterSliding window filter Figure 2-6. Positive-sequence extraction algorithm Figure 2-8 shows the algorithm performance when an unbalanced fault condition takes place at t=0.02 sec (Figure 2-7). The data used for this example is given by Equation 2-2. Figure 2-7. Voltage waveforms for an unbalanced fault event

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20 Figure 2-8. Response of the positive-sequence extraction algorithm. A) Positive sequence using and 1 cycle filters. B) Positive sequence using Vdr with 1 cycle filter The meaning of the different plotted variables is the following: Vpositive-sequence magnitude is the output of the positive-se quence extraction algorithm. As expected, its time response is only one quart er of a cycle. Howe ver, the transient response is very abrupt an uneven. Vpositive-sequence magnitude (1/2 cycle filter) is the filtered signal of Vpositive-sequence magnitude using a half cycle sliding window filter. Vpositive-sequence magnitude (1 cycle filter) is the filtered signal of Vpositive-sequence magnitude using a one-cycle sliding window filter. Its transien t response is the slowest but at the same time the smoothest among the three signals. Vdr filtered is the filtered signal of Vdr The one cycle sliding window filter (also called moving average) rejects all harmonics Therefore there is no need to use the Vds+ and Vqs+ calculator to extract the positive sequence. However its transient response is not as smooth as the Vpositive-sequence magnitude (1 cycle filter) one. Real Power Calculation Using dq Components As shown in Appendix A Parks tran sformation matrix is not unitary (1 dqo t dqoT T ) and therefore is no t power invariant. The total instantaneous power in abc qua ntities can be transformed into q-d-o quantities as shown in Equation 2-6. A B

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21 This relationship between dqo quantities and the instanta neous power is later used in the control system to determine th e amount of direct-current component (drI ) needed to meet the power fluc tuation requirements. o o qr qr dr dr o qr dr o qr dr o qr dr dqo t dqo o qr dr o qr dr dqo t o qr dr dqo c b a t c b a c c b b a a abcI V I V I V I I I V V V I I I T T V V V I I I T V V V T I I I V V V I V I V I V P 3 1 2 3 3 1 0 0 0 2 3 0 0 0 2 31 1 1 1 (2-6) Phase Locked Loop The phase angle of th e utility voltage ( ) is of vital importance for the operation of most of the advanced power el ectronic devices connected to the electric utility, since it has a direct effect on th eir control algorithms. A simple and fast method to obtain the phase angle of the utility voltage is to use Clarkes transformation as shown in Equation 2-7. ds qs c b a qs dsX X X X X X X arctan 2 3 2 3 0 2 1 2 1 1 3 2 (2-7)

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22 However, this approach is not robust since it is very sensitive to system disturbances. The phase angle distorts as the utilitys voltage becomes affected by different power quality events, such as voltage unbalance, voltage sags, frequency variations, etc. Figure 2-9 shows the voltages phase angle under unbalanced conditions using Equation 2-7. The angle distortion is due to the negative sequence component of the unbalanced three-phase system. Figure 2-9. Distortion of phase angle due to a negative sequence component In order to lock the phase angle of the utility voltage in a robust way, a phase locked loop (PLL) was used. Assuming a balanced three phase system the control model of the PLL was obtained using Parks transforma tion as shown in Equation 2-8. ) 240 cos( ) 120 cos( ) cos( 2 1 2 1 2 1 ) 240 sin( ) 120 sin( ) sin( ) 240 cos( ) 120cos( ) cos( 3 2 2 1 2 1 2 1 ) 240 sin( ) 120 sin( ) sin( ) 240 cos( ) 120 cos( ) cos( 3 2* * * * * * *wt V wt V wt V V V V V V Vc b a o qr dr (2-8)

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23 Where is the PLL phase angle output, is the utilitys phase angle, and dt d w Thus, if (t=0)=0, we can substitute wt for (t) and obtain ) 240 cos( ) 120 cos( ) cos( 2 1 2 1 2 1 ) 240 sin( ) 120 sin( ) sin( ) 240 cos( ) 120 cos( ) cos( 3 2* * * V V V V V Vo qr dr (2-9) Using trigonometric identities, Equation 2-9 results in 0 ) sin( ) cos( 0 ) sin( ) cos(* V V V V Vo qr dr (2-10) Where is the error between the utility angle and the PLL output. If the is set to zero, Vdr=V and Vqr=0. Therefore, it is possible to lock the utility angle by regulating Vqr to zero without needing any information rega rding the magnitude of the utility voltage. Figure 2-10 shows the details of the PLL al gorithm used in our study. The limits of the controller integrator and the limiter were rad/sec. Thus, the PLL was able to track the system frequency as long as this was within 2 60 rad/sec or 55 to 65 Hz range. To use linear control techniques for the de sign and tuning of PLL controller, it was assumed that: For small values of the term sin ( ) behaved linearly, i.e., sin( ) Wref was assumed to be a constant perturbation. Limiters behave linearly for small contro l actions, and therefor e can be removed. abd dsqs VdsVqs dsqsdrqr VdrVqr VaVbVc Ki Kp + + 30 -30 s 130 -30 + + Wref=2 f s 1 Figure 2-10. PLL diagram

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24 Figure 2-11 shows the PLL control loop af ter eliminating the non-lineal terms. Ki Kp + + s 1 s 1 + PLL controller Plant transfer FunctionControl action Figure 2-11. PLL simplified model The closed loop transfer function of Figure 2-11 determines the dynamic characteristics and stability of the system, and can be expressed as I p I pK s K s K s K H 2 (2-11) The control system (Kp and Ki) was designed to satisfy two performance objectives < 10% overshoot Settling time inside the 2% band error lower than 2 secs The criterion to select the settling ti me was a tradeoff between high distortion rejection and tracking of normal system frequency variations. The PLL closed loop transfer function was compared to a standard second order transfer function to determine the regulat ors gains. The obt ained values were 4 85 2 7 0 2 2 1 8 2 7 0 4 4 sec 2 t ) overshoot 5% for ( 7 02 2 2 s n p s n IK t K Figure 2-12 shows the systems closed-l oop step response for two different PI regulators.

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25 The originally designed regul ator did not meet the system requirements due to the effect of the zero introduced by the PLL regul ator. This additional zero increased the overshoot, but it had very little influence on th e settling time. Thus, it was necessary to tune the original regulator gains in or der to meet the system requirements. Figure 2-12. PLL system step response Figure 2-13 shows the root locus of the si ngle-input single output PLL system for the two regulators. Figure 2-13. Root locus for tw o different regulator gains

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26 Figures 2-14 and 2-15 show the PLL sy stem response to a negative sequence condition (V2=16.6%) and a system frequency excursion (w=2 60+30 rad/sec). Figure 2-14. PLL system response to an unbalanced system condition Figure 2-15. PLL system respons e to a frequency excursion. A) Angle. B) PLL error. Control Algorithm Design Parks transformation was used to model the systems equations to facilitate the design of the control system. The usage of a rotating reference frame had the following advantages: Improvement of the steady-state perf ormance of the current controllers: Sinusoidal signals were transformed into dc components, and accordingly it is possible to achieve small signal errors. High bandwidth current controllers: Feedback signals and reference signals were not sinusoidal, but dc. A B

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27 Decoupling of active and reactive power: This was very useful when trying to control voltage at the point of coupling while meeting the system requirements in terms of power fluctuations. Figure 2-16 shows the overall system topol ogy as well as the sign notation that was used in the control system desi gn. In general, power flowing out of the inverter will be considered to be positive. The objective was to smooth out wind-power fluctuations using the power stabilizer as a buffer. The energystorage voltage was expe cted to change in order to accommodate for t hose changes in wind power. IwindVpccVfIinvVinvVdcVchopperVstorageIchopperCdcLfLxfrmCf WIND FARM UTILITY SYSTEM Xsource Transformer equivalent impedance Filter Inverter DC link bus Chopper ESS P + Figure 2-16. System description Inner regulators Inverter system model. For the following set of equations, it was assumed that the inverter behaved as an ideal controllable voltage source, neglecting the effects of the current harmonics. Systems non-linearities, such as saturation or dead-time effects were taken into consideration later on in the design. The capacitor filter was neglected in the analysis, since the filter current represented a small portion of the inverters current. The system can then be represented as shown in Figure 2-17.

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28 Vpcc aR L Vinv aVDC LINKC Vpcc bR L Vinv b Vpcc cR L Vinv c Iinv aIinv bIinv c Figure 2-17. Simplified system model The system equations for the simplified model are pcca pcca pcca invc invb inva invc invb inva invc invb invaV V V I I I dt d L I I I R V V V (2-12) Applying Parks transformation we get o pcc qr pcc dr pcc dqo o inv qr inv dr inv dqo o inv qr inv dr inv dqo o inv qr inv dr inv dqoV V V T I I I T dt d L I I I T R V V V T1 1 1 1 (2-13) o pcc qr pcc dr pcc dqo dqo o inv qr inv dr inv dqo dqo o inv qr inv dr inv dqo dqo o inv qr inv dr invV V V T T I I I T dt d L T I I I T R T V V V1 1 1(2-14) o pcc qr pcc dr pcc o inv qr inv dr inv dqo o inv qr inv dr inv dqo dqo o inv qr inv dr inv o inv qr inv dr invV V V I I I dt d T I I I dt T d T L I I I R V V V1 1(2-15) o pcc qr pcc dr pcc o inv qr inv dr inv dqo dqo o inv qr inv dr inv dqo dqo o inv qr inv dr inv o inv qr inv dr invV V V I I I dt d T T L I I I dt T d T L I I I R V V V1 1(2-16)

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29 o pcc qr pcc dr pcc o inv qr inv dr inv o inv qr inv dr inv dqo dqo o inv qr inv dr inv o inv qr inv dr invV V V I I I dt d L I I I dt T d T L I I I R V V V1 (2-17) Where dt d dt d dt d dt T ddqo 0 ) 240 cos( ) 240 sin( 0 ) 120 cos( ) 120 sin( 0 ) cos( ) sin( 1 ) 240 sin( ) 240 cos( 1 ) 120 sin( ) 120 cos( 1 ) sin( ) cos(1(2-18) It can be shown that 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 ) 240 cos( ) 240 sin( 0 ) 120 cos( ) 120 sin( 0 ) cos( ) sin( 2 1 2 1 2 1 ) 240 sin( ) 120 sin( ) sin( ) 240 cos( ) 120 cos( ) cos( 3 2* * * 1 dt T d Tdqo dqo(2-19) Thus, the equations for the simplif ied model in the d-q plane are o pcc qr pcc dr pcc o inv qr inv dr inv o inv qr inv dr inv o inv qr inv dr inv o inv qr inv dr invV V V I I I dt d L I I I L I I I R V V V 0 0 0 0 0 0 0 (2-20) The zero-sequence component can be remove d, since the system is a three-phase three-wire inverter with the DC link bus isolated from th e AC side (the DC link midpoint will not be tapped to neutra l). Removing the zero sequence we obtain dr inv qr pcc qr inv qr inv qr inv qr inv dr pcc dr inv dr inv dr invI L V dt dI L I R V I L V dt dI L I R V (2-21)

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30 Equation 2-21 can be represented as a c oupled electrical system as shown in Figure 2-18. R L Vpcc drVinv dr Iinv drL Iinv qr R L Vpcc qrVinv qr Iinv qrL Iinv dr Figure 2-18. Electrical represen tation of the dq components. A) Direct circuit. B) Quadrature circuit. Using Laplaces transformation we can re -write the equations as Equation 2-22. ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( s I L s V s I Ls R s V s I L s V s I Ls R s Vdr inv qr pcc qr inv qr inv qr inv dr pcc dr inv dr inv (2-22) Thus, the block diagram of the system is represented in Figure 2-19. Vpcc drVinv drIinv dr Ls R 1 + L L Vinv qrIinv qr Ls R 1 + Vpcc qr + Figure 2-19. System model block diagram A B

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31 The inverters critical control variable wa s the inverters current. This was due to the fact the outer control loops such voltage regulators, power regulators, etc, were based on the inner current regulators. That was w hy the current controllers were designed to meet two basic requirements, which we re high accuracy and high bandwidth. The inverters terminal-voltage needed to generate the desired inverter current can be determined as dr inv qr pcc drop dr inv qr pcc qr inv qr inv qr inv qr inv dr pcc drop qr inv dr pcc dr inv dr inv dr invI L V V I L V dt dI L I R V I L V V I L V dt dI L I R Vqr dr (2-23) The voltage drop due to the filter inductance was compensated using a PI controller. Figure 2-20 shows the inveters current controller implementation for the system given in Equation 2-23. Vpcc drVinv drIinv dr Ls R 1 + L L Vinv qrIinv qr Ls R 1 + Vpcc qr + SYSTEM MODEL + + Vdrop dr + + + Vdrop qr Ki Kp + + s 1 + Ki Kp + + s 1 + Iinv drIinv qr Iinv dr refIinv qr ref L L CURRENT REGULATORSdr pccV qr pccV Figure 2-20. Inverter current regul ator-system model block diagram The character ^ over a constant or variable indicates that the quantity is estimated, and therefore subject to measurement errors.

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32 To design the current regula tor gains, cross-coupling factors were assumed to cancel each other out. Under these conditions the simplified current regulator block diagram is shown in Figure 2-21. Iinv dr Ls R 1 Iinv qr Ls R 1 SYSTEM MODEL Ki Kp + + s 1 + Ki Kp + + s 1 + Iinv drIinv qr Iinv dr refIinv qr ref CURRENT REGULATORS Figure 2-21. Inverter current regulato r-system model simplif ied block diagram Figure 2-21 shows that: The system behaves linearly, and therefore linear control techniques can be used to determine the regulators gains. Both regulators are identical. Only an estimation of L and R (filter inductance + transformer equivalent impedance) are needed to design the current regulator. Given the filter/transformer characteristics in p.u., the closed-loop transfer function of Figure 2-22 is shown in Equation 2-24. Ki Kp + + s 1 + Iref Ls R 1 I Control Action Figure 2-22. Simplified current control diagram

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33 I p I p refK s K R Ls K s K I I s H 2) ( (2-24) Using the following system data, the transf er function is given in Equation 2-25. X=Xtransfomer+Xfilter=5%+10% = 0.15 L= 400 H X/R=10 R=0.015 Note: More on the system parameters can be found in the per-unit mode section. I p I pK s K s K s K s H 015 0 0004 0 ) (2 (2-25) The Figure 2-23 shows the system step respons e for two different current regulator gains. Figure 2-23. Current regulator step response Even though the current regulator with the highest gains had a faster settling time, the control action required to obtain such a response doubled the regulator with the lowest gains. To avoid possible system satu rations the control ac tion was kept below 1 pu. The best PI controller pe rformance was achieved when the plants dominant pole was cancelled by the controller (Equa tion 2-26). Thus, the zero at p IK K was assigned to the time constant of the plant, which was, L R K Kp I.

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34 s K K s K s K K PIp I p I p (2-26) The synthesis was done by selecting the integr al time constant of the PI equal to that of the load. For our st udy the selected values were 1 5 37 p IK L R K Chopper system model. The analysis of the chopper sy stem was less complex than the inverter one, since no transformations were involved. Again, it was assumed that the chopper behaved as an ideal controllable volta ge source and therefor e the effects of the current harmonics were neglected. VchopperVstorageIchopper Figure 2-24. Chopper equivalent system The system equations for the chopper equivalent circuit (Figure 2-24) are given in Equaion 2-27. dt dI L V V V dt dI L Vchopper storage chopper chopper chopper storage (2-27) The choppers terminals voltage needed to generate the desired chopper current can be determined as drop storage chopperV V V (2-28)

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35 The voltage drop due to the chopper inducta nce was compensated using a simple P controller. The gain of the c ontroller was found by converting the continuous system into discrete time system as shown in Equation 2-29. t L K I K V V dt I I L V dt I L V VL chopper L storage chopper chopper chopper storage chopper storage chopperref where ) ( (2-29) Where KL is the regulators gain and t is half of the sampling time period. Figure 2-25 shows the implementation of the choppers curr ent regulator. Ichopper+ KL Vstorage+ Ichopper refVchopper Figure 2-25. Chopper current controller Outer regulators There were a total of three controllable currents, which consisted of Ichopper_ref, Iinvdr_ref, and Iinvqr_ref.. However, there were four variables that needed to be controlled, which were voltage at the dc link bus, volta ge at the point of co mmon coupling, voltage at the energy storage system, a nd wind farm power fluctuation. Table 2-1 shows how these variables were assigned to the respective current regulators. Table 2-1. Outer regulator assignation Inner current regulator Variable to be controlled Comments Iinv dr ref Vstorage, P wind The direct current compone nt will be responsible for controlling the state of charge of the ESS and for smoothing the wind farm output power Iinv qr ref Vpcc The quadrature current component will be deployed for voltage regulation purposes Ichopper ref Vdc link The chopper current will regulate the DC link bus voltage.

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36 DC link Voltage regulator. The DC link bus was the bridge between the energy storage system (chopper) and the inverter. Theref ore, it was a critical variable in the overall system. Poor DC vo ltage regulation could bring the system down, since the inverter and chopper would not be able to m eet their respective voltage requirements. The DC link system can be modeled as shown in Figure 2-26. Vpcc aR L Vinv aVDC LINKCdclink Vpcc bR L Vinv b Vpcc cR L Vinv c Iinv aIinv bIinv c Lchopper Cstorage PchopperPdc linkPlossesPlossesPout Ichopper Vstorage P + Figure 2-26. Powers' definition Using the energy balance theorem we can write ...) (inverter filter P P V I P P P P Plosses inv storage chopper link dc inv losses link dc chopperout out (2-30) Assuming 0lossesP we have outinv storage chopper link dc link dcP V I dt V d C ) ( 2 12 (2-31) Linearizing Equation 2-31 around the nominal point of the energy-storage voltage (Vstorage), we can re-write the equations as storage out chopper link dc storage link dc inv storage chopper link dc link dcV s P s I C V s V s P V s I s s V Cout) ( ) ( 2 ) ( ) ( ) ( ) )( ( 2 12 2 (2-32) Figure 2-27 shows the block diagram of the system model.

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37 s C Vlink dc storage 2 + storage outV P +2 link dcVchopperI Figure 2-27. System model Considering outinvP as a disturbance, the transf er function of the system is s C V s I s V V s I s s V C V I dt V d Cdc storage chopper dc storage chopper dc dc storage chopper dc dc 2 ) ( ) ( ) ( ) )( ( 2 1 ) ( 2 12 2 2 (2-33) The system model and the DC link voltage regu lator can be represented in the form of a block diagram as shown in Figure 2-28. s C Vlink dc storage 2 storage outV P +2 link dcVchopperI + storage outV P + Ki Kp + + s 1 ref link dcV2+ SYSTEM MODEL DC LINK VOLTAGE REGULATOR Figure 2-28. DC link equivale nt system block diagram Under ideal conditions the terms storage outV P cancel each other out, resulting in a simplified block diagram (Figure 2-29).

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38 s C Vlink dc storage 2 2link dcV Ki Kp + + s 1 ref link dcV2+ SYSTEM MODEL DC LINK VOLTAGE REGULATOR Figure 2-29. DC link simplified system block diagram The closed-loop transfer function of the simplified DC link system is: storage I p storage link dc I p storageV K s K V s C K s K V s H 2 2 22 (2-34) Figure 2-30 shows the system step response for two different regulator gains, using the following system data: Cdc link =15700F Vstorage nominal =1.533 p.u. Figure 2-30. DC link voltage regulator step response To avoid a possible saturation of the DC li nk voltage regulator, the controller with lower gains was chosen. In this case, the control action was the chopper current, and it was designed to always be below 1.pu.

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39 Point of common coupling voltage regulator. The voltage support capability of the inverter depended on the available line impedance back to the utility source voltage, and its dynamics response was directly affected by the line parameters. The regulation of the voltage at the point of common coupling was accomplished by changing the amount of reactive current generated / absorbed (Iinv qr) by the inverter. It was also possible to improve the voltage regulation controlli ng the real current component. However, as it will be shown, th e voltage regulation range was significantly reduced. The system model used in our study (Figure 2-31) was a simplified version of the actual system. It consisted of the source (m odeled as an infinite bus with a series impedance), and the inverter (modeled as a controllable current source). System nonliberalities, such as switching of the semiconduc tor devices, transformer saturation, etc, were neglected. The system of the equations for Figure 2-31 is c source b source a source c inv b inv a inv source c inv b inv a inv source c pcc b pcc a pccV V V I I I dt d L I I I R V V V (2-35) Applying Parks transformation and removi ng the zero-sequence component, it can be shown that dr inv source qr source qr inv source qr inv source qr pcc qr inv source dr source dr inv source dr inv source dr pccI L V dt dI L I R V I L V dt dI L I R V (2-36)

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40 V source aRsourceLsourceVpcc a V source bRsourceLsourceVpcc b Vsource cRsourceLsourceVpcc c Iinv aIinv bIinv c R L Vinv aC R L Vinv b R L Vinv c Iinv aIinv bIinv c Vpcc aVpcc bVpcc c EQUIVALENT SYSTEM MODEL Figure 2-31. Simplified system model Using Laplaces transformation and re-organ izing the terms, we obtain the transfer functions shown in Equation 2-37 ) ( ) ( 1 ) ( ) ( ) ( ) ( ) ( 1 ) ( ) ( ) ( s V s V L s I w s I L R s sI s V s V L s I w s I L R s sIqr source qr pcc source dr inv qr inv source source qr inv dr source dr pcc source qr inv dr inv source source dr inv (2-37) source source qr source qr pcc source dr inv qr inv source source dr source dr pcc source qr inv dr invL R s s V s V L s I w s I L R s s V s V L s I w s I ) ( ) ( 1 ) ( ) ( ) ( ) ( 1 ) ( ) ( (2-38)

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41 Defining the voltage drop, V, as the voltage across the source impedance (Figure 2-32), it was possible to find th e amount of current needed to obtain the desired voltage drop (Equation 2-38). V source aRsourceLsourceVpcc a V source bRsourceLsourceVpcc b Vsource cRsourceLsourceVpcc c Iinv aIinv bIinv c V Figure 2-32. Source impedance voltage drop ) ( 1 ) ( ) ( ) ( 1 ) ( ) (2 2 2 2 2 2s V L R s w L R s L s V w L R s L w s I s V L R s w L R s L s V w L R s L w s Iqr source source source source source dr source source source qr inv dr source source source source source qr source source source dr inv (2-39) The Bode plots of the Equation 2-39 for a system with a source impedance of 10%, and X/R=10 are shown in Figure 2-33 and Figure 2-34. Even thought the Bode plots of I inv dr and I inv qr look very similar, the effect on the amount of voltage drop for a given source impedance were significantly different. There are two ways of controlling the amount of voltage drop at the source impedance; regulating ) (s Vqr and/or) (s Vdr However, in order to increase system stability and gain robustness, the phase shift between the utility voltage and the voltage at

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42 the point of common coupling must be as sma ll as possible. Therefore, it was preferable to regulate Vpcc by controlling ) (s Vdr exclusively. ) ( ) ( s V s Idr qr inv ) ( ) ( s V s Iqr qr inv Figure 2-33. Transfer functions of inve rters quadrature current component ) ( ) ( s V s Iqr dr inv ) ( ) ( s V s Idr dr inv Figure 2-34. Transfer functions of in verters direct current component Comparing Figure 2-33 to Figure 2-34, it is clear that, in the low frequency range, the cross coupling between qr invI and drV is much greater than the direct gain between

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43 dr invI and drV This means that the voltage can be regulated by controlling only the quadrature current. Thus, for instance, the steady-state bu s voltage of a system with a source impedance of 10%, and X/R=10 in terms of dr invI and qr invI is ) ( ) ( 1 ) ( ) ( 0 ) ( ) ( 1 ) ( ) ( 0s V s V L s I w s I L R s V s V L s I w s I L Rqr source qr pcc source dr inv qr inv source source dr source dr pcc source qr inv dr inv source source (2-40) For0dr invI, 2 2 2 2 qr inv source qr source qr inv source dr source dr pcc dr pcc pccI R V I wL V V V V (2-41) For 0 qr invI 2 2 2 2 dr inv source qr source dr inv source dr source dr pcc dr pcc pccI wL V I R V V V V (2-42) If pu Iqr inv1 (capacitive), pu Vdr source1 pu Vqr source0 , 1 0 sourceX and 10 source sourceR X then pu Vpcc1 1 The amount of dr invI needed to obtained the same voltage would be pu I I wL V I R V Vdr inv dr inv source qr source dr inv source dr source pcc67 3 1 12 2 This proves that for a system where the ratio X/R>1, the PCC bus voltage can be regulated in an efficient way by in jecting only quadrature current. The design of the voltage regulator re quires the knowledge of the source impedance. However, this impedance varies with time and on online estimation can be very complex if transient situa tions are present in the system.

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44 For steady-state conditions th e transfer function between V and I inv qr can be reduced to just the source impedance of value Xsource=wLsource. Therefore, the control block diagram of the voltage regulator can be interpreted as shown in Figure 2-35. ref dr invI Ki Kp + + s 1 sequence positive ref pccV + CURRENT CONTROLLERdr invI SOURCE IMPEDANCE pccV + +drV dr sourceV POSITIVE SEQUENCE EXTRACTION SYSTEM MODEL VOLTAGE REGULATOR Figure 2-35. Voltage regulator system block diagram The current controller is re presented as a second order tr ansfer function in Equation 2-43. regulator current I regulator current p regulator current I regulator current p regulator currentK s K s K s K s H 015 0 0004 0 ) (2 (2-43) For Kp=1 and Ki=36 the current co ntroller transfer function is 36 015 1 0004 0 36 ) (2 s s s I I s Href inv inv regulator current The positive sequence extrac tion transfer function can be modeled in continuous time as shown in Figure 2-36. pccV+ sequence positive pccV s Td1Td Transport delay Integrator Figure 2-36. Positive sequence extrac tion algorithm equivalent system

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45 For the simplified voltage regulator system, there was not need for any transformation. Only the modeling of the pos itive sequence 1 cycle sliding window filter (Td=16.666msec) was required (Figure 237). ref dr invI Ki Kp + + s 1 sequence positive ref pccV+ dr invI wLsource pccV + +drV dr sourceV SYSTEM MODEL VOLTAGE REGULATOR 36 015 1 0004 0 362 s s s+ Td s Td1 Figure 237. Voltage regulat or detailed block diagram The system was further simplified assu ming that the current regulator time response was much faster than vo ltage regulator time response (Figure 238). ref dr invI Ki Kp + + s 1 sequence positive ref pccV+ 1dr invI wLsource pccV + +drV dr sourceV SYSTEM MODEL VOLTAGE REGULATOR+ Td s Td1 Figure 238. Voltage regulator simplified control diagram Figure 239 shows the system step re sponse for a given voltage regulator under different system conditions. A B Figure 239. System response to a 5% change in voltage reference for Kp=2 Ki=250. A) With saturation. B) Without saturation

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46 The settling time was a function of the syst em impedance, and therefore it was not possible to predict the syst em response without knowing the source impedance. One solution was to use an adaptive parameter tune r capable of adjusting the regulator gains according to the identified plant dynamics. Th e block diagram of the adaptive control scheme is shown in Figure 2-40. ref dr invI Ki Kp + + s 1 sequence positive ref pccV + + Td s Td1 PLANT pccV Adaptive Parameter Tuner Figure 2-40. Adaptive control scheme However, due to the difficulty in disti nguishing between cha nges in the system impedance, load variations, and utility volta ge, a simpler but robus t solution was adopted. It consisted of a classic PI regulator, with gains that were tuned in the field. The drawback was a slower response that could occur for any given condition. Power regulator. The power regulator required to c ontrol the power fluctuations of a wind farm was the most complicated control scheme among all the described so far. It involved non-linear algorithms wh ich made the system very sensitive to instabilities due to non forecasted conditions. The basic idea behind th e power regulator was to determine the amount of the real power required by the inverter in order to meet the utilitys power fluctuation limits. A generic power regulator control scheme is shown in Figure 2-41.

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47 X IwindaVpcca X IwindbVpccb X IwindcVpccc + + + Limiters Ramp Rate Average Inst Wind Power Centering algorithm Vstorage + ESA required power+ +Allowed centering power Power Inverter Reference Figure 2-41. Power Regulator general control scheme First the wind farm power was calculated a nd the compared to rate-of change limits (Table 2-2). If limits were exceeded, the difference would be compensated by the inverter. The centering algorithm was a control scheme used to hold the energy storage near its nominal value, to be ready for the next supply or absorption cycle. If the wind farm power was causing the limiters to activate, this centering action would not take place. That way a higher priority was given to the power limiters. Table 2-2. Rate-of-change limits or PPA for a 10 MW wind farm Parameter Value Instantaneous 1 MW change per 2 second scan Sub minute average average of 0.3MW change per 2 second scan for any 60 second period Ramp rate 2 MW per minute up, and down when operationally possible The first proposed control scheme of th e power limiter consisted of a high pass (HP) filter which canceled the high frequency power fluctuations independently of the rate-of-change limits. Figure 2-42 shows the HP filter control block diagram. A small bias power was added to assure the charging of the energy storage system. This approach had three major drawbacks:

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48 Rate-of-change limits might not meet unless inverters power and energy requirements were increased. Optimal cut-off frequency design was unknown. Inverter duty cycle was higher than the next approaches. High Pass Filter + Storage (integrator) Centering Charge/ Discharge Constant Wind Farm Output Inverter Power + + + Storage Nominal + Centering State of Charge Figure 2-42. Power limiter 1. Control block diagram Wind farm power data records were used to test the power limiter control scheme under different system conditions. Figure 2-43 and Figure 2-44 show the inverter requirements as well as the system perfor mance for different cut-off frequencies. Note: Inverter size requirements cannot be extrapolated from the 15 minutes simulation. A more detailed study must be performed using long wind-power data records (perhaps years). It was also not cost effective to correct every possible scenario. Therefore, the number of times and/or amount that the wind farm may exceed the power index limits, with a Power Stabilizer installed, needs to be determined, when traded off against inverter and storage ratings. The main advantages of the HP power limiter were its simplicity and its stability under unexpected power fluctuations. The control scheme was implemented in MATLAB in order to test the power limiter performance under different system conditions. Appendix B gives more information on the MATLAB code.

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49 Figure 2-43. Power limiter 1. Performance usi ng different cut-off frequencies (unlimited power and energy). A) Power to utility. B) Power-stabilizer output power. C) Power stabilizers energy storage. Figure 2-44. Power limiter 1. Performance using different cut-off frequencies (Pinverter=1.0 MW and Einverter=.5 MJ). A) Power to utilit y. B) Power-stabilizer output power. C) Power stabilizers energy storage. A B C A B C

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50 The second proposed control scheme of th e power limiter consisted of a power limiter with the three rate-ofchange limiters in cascade (Figure 2-45). The ramp limit was first applied, followed by the sub minute limit, and finally the scan-to-scan limit. Scan-toScan Limiter Subminute Limit Calculator Ramp Limit Calculator Input Output S R+R -R +S -S Figure 2-45. Power limiter 2. Limiters details As mentioned earlier, the cen tering of the energy stor age energy was needed so that it could supply or absorb power from its nominal state. Therefor e this energy must be taken into account when calculating the rate -of-change limits, sin ce it was real power being interchanged with the system. Thus, the power limite r control scheme had two limiters in parallel (Figure 2-46); one limiter acted upon the wind farm output only, another limiter acted on the wind farm power plus the desired centering power. If the inverter were big enough to suppl y or absorb the ex cess power and energy from the wind farm, the power limiter would keep the power within that allowed by the rate-of change limits. The problem occurred when the power or energy storage is beyond the rating of the inverter, since the history of wh at is actually delivered to the utility could be wrong. Thus, a saturation limiter was needed in order to adjust the buffer input data. The control scheme was implemented in MATLAB in order to test the power limiter performance under different system conditions. Appendix B gives mode information on the MATLAB code.

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51 Limiter Limiter + + + Centering Charge/ Discharge Constant + Wind Farm Output Inverter Power -+ + + + + + Desired Power (Wind+Storage Centering) Centering Power Allowed ESA Required Supply/Absorb + Storage Nominal + Desired Centering State of Charge Previous scans (BUFFER) + +1 PU -1 PU + + Last thing to be updated/evaluated Power Limiter Figure 2-46. Power limiter 2. Control block diagram Wind farm power data stored on a 2 sec ond basis was used to test and size the power limiter control scheme under different scenarios. The following figure shows the system performance for a period of 15 minutes. Zoom in Figure 2-47. Power limiter 2. Compensation performance

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52 The inverter power and energy required to meet the rate-of-change limits for the 15minute simulation is shown in Figure 2-48. Figure 2-48. Power limiter 2. Inverter response for a sampling time of 2 seconds. A) Power-stabilizer output power. B) Power stabilizers energy storage. To test the stability of the control algo rithm, saturation effects were taken into account. Figure 2-49 and 2-50 show the syst em performance for different under-rated inverters. Rate-of-change limits were not met, but the system was stable. It is very difficult anticipate all of the types of misbehavior that might occur in the system, and that there could be unusual power fluctuations from the wind farm could get the inverter into a mode where it would c ontinue to swing the power around in an undesirable manner. Therefore it was r ecommended to include some type of misbehavior detector in the power limiter cont rol scheme to protect the inverter and the system. A B

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53 Figure 2-49. Power limiter 2. Inverter res ponse for different power ratings. Sampling time 2 seconds .A) Power-stabilizer ou tput power. B) Power stabilizers energy storage. Zoomin Figure 2-50. Power limiter 2. Inverter res ponse for different ESS sizes. Sampling time 2 seconds. A) Power-stabilizer output power. B) Power stabilizers energy storage. A B A B

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54 The third control scheme considered for the power limiter co nsisted of a power limiter with the three rate-of-change limiters in parallel (Figure 2-51). The limiters input was the power out to the utility instead of the wind power, plus the centering power, for a more accurate control of the power fluctuations seen by the utility. Each limiter determined the maximum and minimum amount of power allowed changing per scan. Then the absolute maximu m and minimum were calculated in order to establish the centering power limits and the required power from the inverter. + Inverter Wind Farm Output Total Inverter Power + + Power to Utility Ramp Limiter Subminute Limiter Scan-toScan Limiter Min Max + + + Upper Lower Lower Lower Upper Upper Upper Lower + + + + -1 -1 upper lower upper lower Kcenter or other controller + + + -1 Inverter Power Before Centering Centering Power These should not have a simulataneous non-zero output Upper Centering Limit Lower Centering Limit + Storage Setpoint Storage (Joules or Volts) + Figure 2-51. Power limiter 3. Control block diagram Figure 2-52 shows the response of the power limiter 2 using the same wind-power data records used previously. It can be concluded from Figure 2-47 and Figure 2-52 that both power limiters have the same response under normal conditions.

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55 Figure 2-52. Power limiter 3. Compensation performance Figure 2-53. Power limiter 3. Inverter response for a sampling time of 2 seconds. A) Power-stabilizer output power. B) Power stabilizers energy storage. A B

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56 Per-Unit System Model The reasons why to convert system variables into per unit are: System easily scalable Facilitate fixed point operations Power system components can be treated uniformly no matter what voltage level The two variables selected as based values are ) ( 1 ) ( 1max maxA pu I I V pu V Vline base neutral line base Thus, the rest of variables can be calculated as Base Impedance : 1base base line RMS neutral line RMS baseI V I V Z Base Power (3 phase): W I V I V P Pbase base line RMS neutral line RMS power rate inverter base5 1 2 2 3 3 Inverter Output-Filter Design The purpose of inverter filter was to attenuate the high frequency switching harmonics produced by the inverter in order to avoid disturbing other EMI sensitive equipment on the grid. Its optimal design is very complex and it involves coupled design constraints and non-linear equations. The inverter topology for which the filter would be designed wa s a 6-pulse 3-wire inverter, without DC bus mi d-point tapped to neutral (Figure 2-54), where power semiconductors were considered as ideal switches.

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57 Vinv aCVinv bVinv c Iinv aIinv bIinv c Figure 2-54.Inverter topology Harmonic content The typical line-to-neutral a nd line-to-line voltage of a th ree phase inverter using a PWM strategy is shown in Figure 2-55. Figure 2-55. Line-to-line and line-to-neutr al voltage of a three phase inverter Figure 2-56 shows the harmonic spectrum of the line-to-line voltage under the following conditions: fsw = 4860 Hz f1=60 Hz Frequency Modulation, 811 f f frequency l fundamenta frequency switching msw f Vdc = 2.04 pu Vsource max line-to-neutral = 1 pu Peak amplitude of the control signal 2_ max link dc desiredV V Amplitude of the triangular signal = 1 A B

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58 Amplitude modulation, 1 signal triangular the of amplitude signal control the of amplitude peak ma Figure 2-56. RMS Line-to-line voltage harmonic spectrum The main harmonic components of the lineto-line output voltage were calculated using the tabulated table in [28] as K V h Vdc l l rms inverter (2-44) K values for the different harmonics can be found in Table 2-3. The front-end inverter was designed to be have as a static synchronous generator (SSG) capable of producing a se t of adjustable voltages, wh ich may be coupled to an ac power system to exchange independently cont rollable real and reactive power. This was accomplished by the usage of a synchronous indu ctor which linked the inverter output to the ac supply side (Figure 2-57).

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59 Table 2-3. Generalized Harmonics of line-to-lin e voltage for a large a nd odd mf that is a multiple of 3 K (Generalized Harmonics of Vrms l-l) ma h 0.2 0.4 0.6 0.8 1 1 mf 2 mf 4 0.122 0.010 0.245 0.037 0.367 0.080 0.490 0.135 0.005 0.612 0.195 0.011 2mf 1 2mf 5 0.116 0.200 0.227 0.192 0.008 0.111 0.020 3mf 2 3mf 4 0.027 0.085 0.007 0.124 0.029 0.108 0.064 0.038 0.096 4mf 1 4mf 5 4mf 7 0.100 0.096 0.005 0.021 0.064 0.051 0.010 0.042 0.073 0.030 VinvL Vsource I Inverter synchronous InductorSSG Figure 2-57. Static Sync hronous Generator diagram Thus, the inverter voltage harmonics w ould generate current harmonics, which amplitude would not only be a function of the inverters mf and ma, but the synchronous inductor as well. The inverter curren t harmonics can be calculated as: For h=1 (fundamental frequency): L h f V V h Il l rms source l l rms inverter rms 12 1 3 (2-45) For h>1 (assuming no harmonics are pr esent in the utility bus voltage): L h f K V L h f h V h Idc l l rms inverter rms 1 12 1 3 2 1 3 ) ( (2-46) Where f1 is the fundamental frequency.

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60 The inverter current harmonics must be attenuated in order to avoid interference with communication circuits and other types of equipment, the increase of system losses, resonance conditions, and malfuncti on of power electronic devices. The IEEE 519 Standard [29] is a recommende d practice to be used for guidance in the design of power systems with nonlinear loads and ther efore should be taken into account on the design of the swit ching ripple filter. The worst case scenario is for general distribution systems (120V through 69000V) w ith a TDD < 5% for current harmonics below the 50th. TDD is the total demand distortion and is defined as 100 (%)50 2 2 L h hI I TDD (2-47) The maximum demand load, which is IL, can be estimated from data used to size the inverter isolation transformer. Switching frequency The selection of the swit ching frequency was based on the recommendations given by [28], which stated that: Because of the relative ease in filtering harmonic voltages at high frequencies, it is always desirable to use as high a switching frequency as possible. In most applications, the switching frequency is selected to be either less than 6 kHz or greater than 20 kHz to be above the audible range. In order to avoid sub-harmonics, sync hronous PWM must be used. Synchronous PWM requires that mf be an integer. In the 3-wire three-phase inverters, only the harmonics in the line-to-line voltage are of concern, and only the odd harmonics exit as sidebands, centered around mf and its multiples, provided mf is odd. If mf is chosen to be an odd multiple of 3 the most dominant harmonics in the lineto-line voltage (eve n harmonics of mf) will be cancelled out.

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61 For high power applications (kVA) where sw itching losses play a major role in the overall system design, the switching frequency is usually selected between 3 kHz and 5 kHz. Taking all these elements into consideration, the optimal switching frequency selected for the inverter and for the chopper was Hz m f ff sw4860 81 601 Passive filter design The switching ripple filter t opology selected for the inve rter filter was based on a LCL network as shown in Figure 2-58. VinvLfVsource Iinverter Inverter Filter or synchronous InductorLCL switching ripple filter CfFilter capacitorIgrid LtIsolation transformer equivalent impedance Figure 2-58. LCL filter topology The main advantages of the LCL filter compare to the L filter are summarized in Table 2-4. Table 2-4. L filter vs. LCL filter Characteristics L filter LCL filter Control method Hysteresis contro llers Fixed switching frequency control methods Attenuation above resonance frequency ) ( ) (s V s Iinverter grid 20dB ( first order system) 60 dB (third order system) Total line filter inductance for a given grid current ripple magnitude High line inductance, and therefore poor transient performance Low line inductance, and fast transient performance

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62 The LCL inverter filter equations are the following: dt dV C I I dt dI L V V dt dI L V Vf f fc f grid inverter grid t source c inverter f c inverter (2-48) Applying Laplaces transform, the LCL inve rter filter can be modeled as shown in Figure 2-59. Figure 2-59. LCL equivalent block diagram The inverter filter was divided into two different equivalent filter models based on the frequency under study. Thus, we have: Equivalent filter circuit configur ation at fundamental frequency. Under these conditions the inverter was cons idered as an ideal sinusoidal voltage source. This was the lineal inverter model valid for the design of the system controllers. Figure 2-60 shows the filter equivalent system at fundamental frequency. Vinv (f1)LfVsource(f1) Iinverter(f1) CfIgrid(f1) Lt Figure 2-60. Single phase equivalent filte r model at the fundamental frequency

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63 Equivalent filter circuit configuratio n for the h harmonic (for h>1). At high frequencies the converter was considered to be a harmonic generator, while the grid can be considered short-circuited. Vinv (hf1)LfVsource(hf1)= 0 Iinverter(hf1) CfIgrid(hf1) Lt Figure 2-61. Single phase equivalent filter model at the hth harmonic Thus, the current ripple attenu ation, passing from the invert er side to the grid side can be calculated as 1 1 ) ( ) ( ) ( 1 ) ( ) ( ) ( 1 ) ( ) (2 3 2 3 t f rms inverter rms grid f t f f t t f n l rms inverter rms inveter f t f f t n l rms inverter rms gridL C s s I s I L L s L C L s L C s s V s I L L s L C L s s V s I (2-49) There are different ways of designing the LCL inverter filter, as well as different specifications or constrains. Table 2-5 is a summary of the most common parameter used to design the inverter filter. It can be inferred from Table 2-5 that there is no a uni que approach or limit when designing the LCL filter. The LCL parameters selected fo r the inverter filter design are impedance) equivalent er transform typical ( 62 132 % 5 577 79 % 33 3333 25 265 % 10H L X F C X H L Xt Lt f C f Lf f

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64Table 2-5. LCL filter design Parameter Description Equations Limits Maximum Peak to Peak value Note: Maximum current ripple at Vsource(t)=0 differs from Vsource(t)=Vmax For ma 1 sw f a dc sw f a n l source ripplef L m V f L m V Max I 4 3 3 4 2 2 1max max For ma 1 sw f dc ripple inverterf L V I 7max Peak to Peak value: 15%-25% of rated current [35] 31% of rated current [32] Current ripple Most significant harmonic components For ma 1 f dc f rms inverterL h f V m h I 12 1 2 0 3 2 Most significant harmonic component (mf) 10% of rated current [30] 1.6% of rated current [31] Laplace domain 1 1 ) ( ) (2 t f rms inverter rms gridL C s s I s I Attenuation of harmonic content Frequency domain h f Z h f Z h f Z m h I m h It f fL C C f rms inverter f rms grid 1 1 12 2 0.2 attenuation [30] 0.5 attenuation [32] Voltage drop across the filter during normal operation f inverter LL f I Vf 1 max max2 Total value of inductance should be lower than 10% to limit the voltage drop and the dc link voltage[30],[33] 1.7% on the inverter kVA base [34] Filter resonant frequency f t f resonantL L C f// 2 1 Resonance frequency between 10 times the line frequency and half of the switching frequency[30][33] Filter capacitor reactive power 2 12 3 (%) 100 1n l rms source power rated inveter C fV f P Q Cf QCf: <5%[30]-[33]-[34] 15% [35]

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65 The electrical characteristics of the LCL filter for the system parameters given Table 2-7 in are summarized in Table 2-6. Table 2-6. LCL equivalent impedance with damping resistance Parameter Equation Stiff system Current ripple (peak to peak) sw f dc ripple current inverterf L V I 7max 0.2264 pu (App) Current ripple (most significant harmonic) f sw dc sw rms inverterL f f V f f I ) 2 ( 2 1 2 0 3 21 1 0.00003 pu (Arms) Harmonic attenuation 1 1 1 1 12 2 2 2 2 f f Z f f Z f f Z f f I f f Isw L sw C sw C sw rms inverter sw rms gridt f f -18.5 dB Max Voltage drop f inverter LL f I Vf 1 max max2 0.1 pu (Volts) Filter resonant frequency ) // ( 2 1f t f resonantL L C f 1897 Hz Filter capacitor reactive power 100 2 3 (%)2 1 power rated inveter n l rms source f CP V f C Qf 3.0% VAr (pu IMAX fC03 0 ) Table 2-7. Per-unit system Variable Per unit neutral line MAXV 1.0 line line RMSV 1.22474 neutral line RMSV 0.70677 nominal inverter MAXI 1.0 nominal inverter RMSI 0.70711 baseZ 1.0 inverterP 1.5 dcV 2.0412 Passive filter damping To determine the system stability, the LCL inverter filter damping resistances must be taken into account when calculating the sy stem attenuation at resonance frequency. The system resistors are given in Figure 2-62.

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66 Lt Lt Lt Lf Lf Lf CfCfCf RtRtRtRfRfRf VinveterVsource Figure 2-62. LCL equivalent impe dance with damping resistance Using a X/R=10 for all inductors, the damp ing resistances of the LCL filter are 005 0 10 % 5 01 0 10 % 10t t L Lt f f L LR R X X R R X Xt f f The LCL inverter filter could resonate due to harmonics generated either from the source or from the inverter. Th e two equivalent circuits are Lt Lf Cf RtRf Vinveter VcapacitorA Lt Lf Cf RtRf Vsource VcapacitorB Figure 2-63. Single phase harmonic generator equivalent circuits. A) Inverter as a harmonic generator. B) Sour ce as a harmonic generator Thus, the V Vcapacitor transfer functions are give n in Equations 2-50 and 2-51: t f fL C L inverter capacitorZ Z Z V V 1 1 1 1 (2-50) f f tL C L source capacitorZ Z Z V V 1 1 1 1 (2-51)

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67 The Bode frequency response of both models is given in Figure 2-64. Figure 2-64. LCL gain frequency response It can be deduced from Figure 2-64 that there is a signi ficant gain at the resonance frequency (small system damping resistance) and therefore harmonics close to this frequency could be amplified by the LCL filter. From the inverter point of view ther e are two sources of disturbances: 1. Voltage harmonics due to the PWM 2. Disturbances amplified by the current regulator Figure 2-65. Inverter frequency analysis

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68 Figure 2-65 shows the current regulator fr equency response, the filter frequency response, and the inverter lin e-to-neutral harmonic spectrum. It can be inferred from Figure 2-65 that the current regulator atte nuates any signal with a frequency > 400Hz (cut-off frequency), and the inverter volta ge harmonics do not make the LCL filter resonate. From the point of view of the voltage s ource there are two sources of disturbances: Large infrequent transient, such as capacitor bank switching. This type of disturbance may ring the filter, but it will damp out in a few cycles. System harmonics. A detail study of the syst em it is required to determine if it is likely. Direct-Current Link Capacitor Design The DC link bus voltage had the following constrains: IPM Max voltage 1200V. Line to line voltage 480 V. This w ould allow the use st andard isolation transformers Minimum DC link voltage = V Vline to line750 2 480 1 1 1 1max Minimum voltage to guarantee system controllability. IPM trip level = 900V. Capacitor switching vo ltage transients tend to raise the DC link voltage and could damage the IGBTs A trip level of 900 V allows ridingthrough the majority of the capacitor switching transients. Low DC-link voltage was desirable in order to reduce the switching losses. Given these system restrictions the sel ected DC-link voltage was 800V. In per unit pu Vlink dc0412 2 3 2 480 800 The dimensioning of the DC link capac itor was determined by the following constrains:

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69 Maximum permissible current stress for a required working life (current ripple) Existence of any zero sequence component System controllability ( avoid large gains) Max ripple voltage 10% For a traditional STATCOM configuration, the DC-link capacitor is necessary for an unbalanced system operation and harmonic absorption. For a configuration with energy storage, the DC link capacitor main f unction is to reduce th e DC current ripple from/into the ESS and therefore a sma ller DC-link capacitor could be used. The time constant selected for our study was msec 5 21 Power Inverter Energy link DC Thus, the DC-link capacitor in per-unit model is F C V C J W Plink dc link dc link dc pu pu pu pu inverter pu15700 2 1 Energy link DC 033 0 Energy link DC 5 1 Energy link DC Energy link DC sec m 22 Power Inverter Energy link DC2 Energy Storage Design The Energy Storage System (ESS) design parameters were The voltage at the energy storage system (ESS) was designed to vary from 95% to 0.95*50% of the DC link voltage. sec 45 20 Powe r Inverter ESS the in Energy Total Note: More on the design and size of the Power Stabilizer energy storage system can be found in [36]. The center voltage of the ESS can be calculated as

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70 2 min storage 2 max storage 2 min storage 2 storage 2 storage 2 max2 1 2 1 2 1 V V V V V C V V Ccenter storage center storage center storage storage (2-52) Thus, for a system with a pu Vdc0412 2nominal the ESS nominal voltage was pu V V V pu V pu Vcenter storage storage storage533 1 8 5 9391 1 2 1 9695 0 0412 2 50 0 95 0 9391 1 0412 2 95 02 min storage 2 max storage min max Figure 2-66 shows the relationship betw een the capacitor voltage and the energy storage. The capacitance of the ESS in per-unit can be calculated from the time constant Power Inverter ESS the in Energy Total as F C V C Energy Max J Energy Max P Energy Max Power Energy Maxstorage storage storage pu pu pu inverter pu31 16 2 1 67 30 sec 45 202 max Figure 2-66. Capacitor Voltage vs. Energy Storage

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71 Chopper Inductor Design The purpose of chopper inductor was to re duce the current ripple produced by the chopper in order to guarantee the ESS working life (Figure 2-67). The current ripple current selected for this application was 30%. Under this condition the chopper inductance in per unit is calculated as shown in Equation 2-53. storage link dc chopper chopper chopper chopperV V V dt dI L V where (2-53) Cdc-link IchopperVdc-link Cstorage VstorageLchopper Figure 2-67. ESS-Chopper topology In the worst case scenario the DC-link volta ge is at its nominal value, while the voltage at the ESS is at its minimum. Thus, the maximum voltage drop across the chopper inductor is pu 0717 1 9695 0 0412 2min nominal max storage link dc chopperV V V Discretization of the chopper inductor voltage drop differe ntial equation yields the Equation 2-54. t I L Vchopper chopper chopper (2-54) Where chopperI is the ripple current, and swf t 1 2 1 as shown in Figure 2-68.

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72 Cdc-link IchopperVdc-link=2.0412 pu Cstorage Vstorage=0.9695 puLchopper Ichopper Time Ichopper Tsw Lchopper Time Triangular waveform t Figure 2-68. Equivalent circuit for maximum current ripple calculation Thus, the chopper inductor in per unit is: H L A pu W P I I I I I t V Lchopper rated chopper rated chopper chopper chopper chopper chopper chopper500 ) ( 7352 0 0412 2 5 1 V ripple) (30% 03 0 4860 2 1 0717 1link dc nominal inverter max Per-Unit System Model Summary Table 2-8 is a summary of the per-unit syst em parameters used in the control and modeling of the system.

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73 Table 2-8. Per-unit system parameters Variable Per-unit Model Specs neutral line MAXV 1.0V line line RMSV 1.22474V neutral line RMSV 0.70677V nominal inverter MAXI 1.0A nominalinverter RMSI 0.70711A baseZ 1.0 inverterP 1.5W al no dcVmin 2.0412V center storageV 1.533V link dcC 15700 F 21.5 msec time constant storageC 16.31F 20.45 sec time constant (at maximum ESS voltage) chopperL 500.0H 30% current ripple chopperR 0.018849 X/R=10 fL 265.25H 10% Impedance fR 0.01 X/R=10 fC 79.57 F 3% VArs (3333.3% Impedance) tL 132.63H 5% Impedance tR 0.05 X/R=10 Simulated Model Power Systems Computer Aided Design ( PSCAD) was used for the modeling and simulation of the power stabilizer. The PSC AD model was based on the per-unit system, so that the system performance could be compared to any given unit size. Figure 2-69 shows the main components of the system. The PSCAD model can be divided into two main subsystems; the electric system (Figure 2-70) and the control algorithm (Figure 2-71).

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74 The majority of components used in the modeling were part of the PSCAD library. Only the power limiter 2 had to be impl emented in FORTRAN and linked to PSCAD given the complexity of its design. IwindVpccVfIinvVinvVdcVchopperVstorageIchopperCdcLfLxfrmCf WIND FARM UTILITY SYSTEM XsourceLCLINVERTER Chopper ESS Figure 2-69. System overview DC link bus g1 g2 g3 g4 g5 g6 2 1 2 3 2 5 2 4 2 6 2 2 0.00026525 0.00026525 0.00026525 0.01 0.01 0.01 0.00013262911 0.00013262911 0.00013262911 Vpcca Vpccb Vpccc Iinvb Iinva gch2 gch1 2 5 2 2 0.0005 Iinvc Vfa Vfb Vfc 0.005 0.005 0.005 0.018849 15700.0 + 16310000.0 + V_dc Vstorage 79.57 79.57 79.57 Vpwma Vpwmb Vpwmc A B C V R=0 Vutility LCL filter Inverter Chopper Figure 2-70. Per-unit el ectric system model Table 2-9 shows the model performance as well as the designed specifications for comparison. It can be observed that desi gned and simulated system closely agree.

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75 X IwindaVpcca X IwindbVpccb X IwindcVpccc + + + Limiters Ramp Rate Average Inst Wind Power Centering algorithm Vstorage + ESA required power+ +Allowed centering power X IinvbVfb X IinvcVfc Power Inverter ReferenceVdr Iinvb IinvaIinvc Idr invIqr inv Idref Vpccb VpccaVpccc Vpcc ref + IdrefIqref + + Idr invIqr invPI regulator PI regulator + Vdf+ wLIqinv + Vqf+ wLIdinv+ VdinvVqinv + + VstorageInverter Power V2dcref V2d c+ PI regulator + + Ichopper+ KL Vstorage+ Vd c (for modulation index calculation) Vchopper pu Pinverter reference updated every 0.1 seconds 40 kHz X VfaIinva Ichopper ref Vchopper + PWMchopper PLL (2 sec time constant) Clark V1 Vd sVqs Vo s abc Id sIqs Io sClark I1 sin(theta) cos(theta) Theta ds-qsdr-qrPark I1ds-qsabc ds-qsPLL VdrVqr ds-qsdr-qrPark V1 sin(theta)cos(theta) sin(theta)cos(theta) Delay (t-1/f/4) Delay (t-1/f/4)Vd sVqsVd sVqs + + + 0.5 0.5Vd sVqs Vdr positive sequenceVqr positive sequence ds-qsdr-qrPark V1 positive sequence cos(theta)Vds positive sequenceVqs positive sequencesin(theta) X X + + Filter Sliding window filter (1 cycle) Vpcc positive sequenceTo PCC voltage regulatorVpcc positive sequence Vds invVqs inv ds-qsdr-qriPark V1 sin(theta) cos(theta) iClark V1abc ds-qsTo current regulator To current regulator + + + + + + + + + PWM Va PWM VbPWM VcVa pu V b puV c puVflat a Vflat bVflat cVflatVd c (for modulation index calculation) 2 0 VdqinvKdqinv dinv qinv dqinv qinv dinv dqinvV V K V V V 2 2Constant angle Max MagnitudeVdqinv_limited ed dinv_limit dqinv ed qinv_limit 2 dqinv ted dqinv_limi ed dinv_limit1 V K V K V V Vdinv_limitedVqinv_limited + Klimit 1.5 pu -1.5 pu Current Limiter MVdc_link -Vdc_link Vchopper_limited Ki s 1 Kp + + M -M M -M Ki s 1 Kp + + 1pu -1 pu 1pu -1 pu M PI regulatorVpcc_offsetVpcc_offset Power Limiter Voltage regulator Current regulator Limiters & Transformations Flat-top Chopper Control Scheme Rotating Reference Frame transformation & positive sequence calculation Figure 2-71. Power Stabilizer Control Scheme

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76 Table 2-9. Designed system results and simulated system results comparison. System conditions: V dc link =2.04 pu, V source max =1 pu, stiff system Parameter Model response Design system/Comments Inverter current ripple ) ( 2241 0 2241 0 198 0 4 3 3 4 2 2 1max max maxApp pu Max I f L m V f L m V Max Iripple inverter sw f a dc sw f a n l source ripple inverter ) ( 02986 0 2 2 1 2 0 3 21rms A pu m h I L h f V m h If rms inverter f dc f rms inverter Harmonic attenuati on ) ( 0035 0 02986 0 1188 0 2 5 18 2 2 2 2 2 2 21 1 1 1 1 1 1 1rms A pu f f I bB f f I f f I f f Z f f Z f f Z f f I f f Isw rms grid sw rms inverter sw rms grid sw L sw C sw C sw rms inverter sw rms gridt f f Current regulator step response Iqref step change [-1 1] (from capacitive to inductive) No PCC voltage regulator Stiff system Limiters: Vmax=1.15, Vflat=1 Dc link step response Vdc ref step change [2.0412 2.3] Stiff system

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77 Table 2-9. Continued Parameter Model response Design system/Comments Voltage regulation Vpcc ref step change [1.0 1.05] Variable line impedance: [1% 2% 4% 5% 10% 20% ] Current regulator bandwidth Cut-off frequency 400 Hz Iqref =0.2sin (wt) Idref =Iess+0.2cos (wt) f=[120 300 420 540 660 1020]Hz Vdc-link=2.0412pu (V) Vsouce max l-n=1 pu (V) Stiff system Power filtering Power limiter 2 simulation results for a sampling time of 2 seconds

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78 CHAPTER 3 SYSTEM DESCRIPTION System Overview The performance of the power advanced el ectronic device was tested in a test bench based on: DC motor synchronous machine set Passive load DC motor asynchronous machine set Wind farm buffer The basic idea was to reprodu ce the basic electrical com ponents of a small isolated system in order to asses the benefits of smoothing wind-power fluctuations. Figure 3-1 shows this main idea. Bulk generation. The system models bulk gene ration was represented with a single synchronous machine, which the main function was to control the system frequency and voltage. Load. The power systems load composition was strongly dependent on the time of day, month, and season, but also on weather. A typical load profile was studied in [37], and can have the following approximate composition: Induction motors, 60 per cent Synchronous motors, 20 per cent Other ingredients (passive loa d, electronics...), 20 per cent

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79 Asynchronous Generator LOAD DC MotorPF correctionLine Impedance Z=5%, X/R=100 Synchronous MachineWind Farm model Electric Network Model Tachometer (w>1800 rpm) 3 phase four wire system Vphase-phase=480 V + 180 Vdc 1st QUADRANT CHOPPER Gate Driver Control signal Vrated field= ?? DC volts Fixed Magnetic field ?? Power=?? 3 phase input Vl-l=240 VPF caps 3 phase four wire system Vphase-phase=480 VP=4.5kW Power Stabilizer DC Motor Tachometer (w>1800 rpm) + 180 Vdc 1st QUADRANT CHOPPER Gate Driver Control signal Vrated field= ?? DC volts Fixed Magnetic field ?? Power=?? 3 phase input Vl-l=240 V3 phase four wire system Vphase-phase=480 V SE350 Voltage Regulator (1% voltage regulation) VnaS NAS VnbS NBS VncS NCS g1 g2 g3 g4 g5 g6 2 1 300.0 2 3 2 5 2 4 2 6 2 2 gc1 2 5 gc2 2 5 1.0 Chopper Reactor1.0 Energy Storage Capacitor Choke InductorFilter5% base on Power Stabilizer rated Power and Voltage 10% base on Power Stabilizer rated Power and Voltage 3% base on Power Stabilizer rated Power and VoltageVdc link Ia, Ib, Ic INV Va, Vb, Vc Filter Va, Vb, Vc PCC GM1 GM2 GM3 GM4 GM5 LOAD 1 LOAD 2 LOAD 3 LOAD 4 LOAD 5 LOAD 6 IM1 Figure 3-1. Equivalent system model Dynamic loads usually consume between 60 to 70 % of the total power system energy. However, their dynamics are of special importance for voltage stability studies due to their reactive power requi rements. Thus, since only r eal power fluctuations were of interest in our study, the systems load wa s reduced to a one three-phase passive load. Renewable Resources. Renewable resources are grow ing faster than traditional energy sources, with the fastest growth being in wind and solar energy. It is expected that in the near future, they will play a significant role in the generation mix. The systems renewable resources were m odeled using a single induction generator that would represent 15% of the system capacity. This number was very conservative compared to other grids such as Western De nmark with a penetra tion level of 63% of peak load and the Island of Crete, where wind power has a penetration level close to 40%.

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80 Power Quality Devices. Because of wind powers high penetration factor in the near future, new advanced power electronic de vices as well as grid operation procedures have to emerge to minimize the imp act of non-dispatchable wind power. In modeling the system, only a proof of concept wind farm buffer was considered to study different control schemes that could reduce wind-power fluctuations. Electrical Network Model Synchronous Machine The first requirement of a reliable service is to have the synchronous generators with adequate capacity to meet the load demand. Any unbalance between the generation and load initiates a transient that cause s the synchronous machine to accelerate or decelerate due to the appearance of net torques on the rotor. It can be shown that the interconnection of j finite machines with inertia constants Mj can be reduced to a single finite machine wi th inertia H, where H can be calculated as shown in Equation 3-1. jH H H H 1 ... 1 1 12 1 (3-1) The synchronous machine selected for modeling the electrical system is a threephase, brushless, self excited, exte rnally regulated, AC generator. The ratings of the synchronous machine were Rated Power 7.0 kW intermittent, 5.4 kW Continuous Rated Voltage 240/480 3ph 60 Hz Rated Speed 1800 rpm

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81 The system voltage selected for the m odel is 480V; therefore the synchronous machines coils were connected in a high series Y configuration. Voltage regulation Load voltage regulation was mainly carried out by the generators exciter using an external voltage regulator. The automatic voltage regulator received both its input power and voltage sensing from the generators outpu t terminals. The DC output voltage of the exciter field required to maintain consta nt the generators terminal voltage was automatically changed by the voltage regulat or, which had a voltage regulation accuracy of 1%. The voltage regulator se t point was 480V, line to line. Due to synchronous machine imperfections and asymmetries, output voltage was not an ideal sinusoidal waveform, as shown in Table 3-1. The most significant distortions were the second, third, fourth, and fifth ha rmonics, with an unbalance of approximately 1%. Such types of distortions were not very common in electric systems and may have an impact on the control system. Simple sliding windows were used to filter/reduce their impact. Prime Mover The prime movers of large generators are principally hydraulic turbines, steam turbines, and combustion turbines. In our model the prime mover that was used to produce the mechanical torque was a DC machine with the following specs: Rated Power: 7.5 HP Armature Voltage 240 V dc Field Voltage 150 V dc Rated Speed 1750 rpm

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82 Both machines were connected in cascade through their shaft, so power could be transferred from one machine to another. Figure 3-2 shows the system configuration as well as the variables used in the control. Table 3-1. Synchronous machine output voltage profile at rated speed Features Synchronous Machine Voltage profile for unloaded condition Synchronous Machine Voltage profile for unloaded condition Waveform FFT Unbalance A single quadrant chopper was used for sp eed control of the DC machine. Chopper circuit specs are shown in Figure 3-3. Note: Figure 3-3 shows that the DC power supply was used by the two choppers required in the model. One was for the prim e mover of the synchronous machine, and the other one a different DC machine that would represent wind speed variations.

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83 Synchronous Generator 7hp DC Motor 7.5hp Encoder (w>1800 rpm) 3 phase four wire system Vphase-phase=480 V single phase input Vl-n=120 V Istator generator Wrotor2 Pout, Qout (V and I) Ia, Ib generator Vab, Vbc generator EncoderElectrical field constant=7.6 Ripple period SE350 Voltage Regulator (1% voltage regulation) Interface Board (signal conditioning and filtering) (Output Range 0v to 3v) (cut-off frequency=100kHz) TO DSP Single quadrant chopper Electric system Figure 3-2. DC gen-set 670 VCfilter Lchoke Rlimit 480 Volts L-L RMS 100 100W 0.83mH Irms=44Ams 3.3 mF 900 VoltsCHOPPER2 CHOPPER1 Vdc source Fuses FWH100 Contactor R 15k W R 15k W C=2200uF 450V C=2200uF 450V C=2200uF 450V C=2200uF 450V C=2200uF 450V C=2200uF 450VBDD6 U 160 N 16 Cfilter R 750 50 ln sec 300 2 DC machine 1 DC machine 2 Prime mover for synchronous machine Prime mover for induction machine Figure 3-3. Two single quadrant chopper circuit Synchronous Machine Control Algorithm The control of a synchronous machine is ba sed on three separate control systems: The excitation system or voltage regulato r that controls the synchronous machines terminal voltage. The governor that controls the mechani cal power monitoring the shafts speed.

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84 The supervisory controller which sends signa ls to each generator in the system to meet the load demand. Boiler Boiler Control Control Center Servomotors and Actuators Steam turbine Set points+ Machine Dynamics Generator Speed Pmechanical+ Speed Speed Electrical Network Pelectrical PelectricalPelectricalPelectricalRectifier VtVt Excitation system VfieldVfield + Vreference Governor Excitation system Figure 3-4. Synchronous ge nerator control system Figure 3-4 shows a generic block diagram of the different control systems required for the control of one generator. In our small-scale model, only the gover nor and the excitation were considered, since the master controller was designed to be quite slow, which is usually not involved in the mechanical dynamics of the shaft. The accurate modeling of the systems fre quency control requires the knowledge of a series of parameters that most of the tim es are impracticable to get. The frequencys response after a disturbance depends on: System inertia, H Network power frequency characteristic, D Speed governors transfer function Load shedding scheme Equation 3-2 represents the frequency devi ation response of a system with inertia H, and load frequency dependency D. Figure 3-5 shows the system frequency response for different H and D, for a -50% load step change ( P).

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85 w D P H w dt dwn 2 (3-2) These two factors, H and D, dominate th e system frequency deviation during the initial seconds following a power perturbation, since large time constants are involved in the control of the system frequency. Such parameters are usually very difficult to determine or estimate due to the systems non-linearity. If all these factors were know n, it could be possible to implement a control scheme that could represent the systems frequency response. Figure 3-6 shows a control scheme for a synchronous machine when system frequency response parameters are known. Figure 3-5. Frequency deviation Vab, Vbc generator 5 channels Ia, Ib generator A/D inputs Frequency and Power calculation f ref+ Stator Current Calculation 1/k Tref2 + I stator ref2 PI controller Servo-Valve System model Turbine system model PI controller PWM Vab, Vbc generator Ia, Ib generator f grid c gate position Transient Droop Droop (4%) + + Torque calculation + P Pout2Pref2 Wrotor2Ichopper generatorI chopper generator Vdc source4860Hz Figure 3-6. DC-GEN set control scheme

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86 This control algorithm was further simp lified, and a single frequency control regulator was implemented to tightly control the system frequency. Figure 3-7 shows the synchronous machine fr equency response to a step change of -1 Hz in the reference for two different ga ins with a 90% system load (4.5kW out of 5kW). The frequency regulators transfer function and gains were adjusted to simulate any other system frequency deviation respons e. The only limitations were the DC motor maximum electrical specs, which should not be exceeded in order to avoid possible control-action saturations and system damage. Figure 3-7. System frequency response for f=-1Hz From Figure 3-7, it is also possible to identi fy the DC-gen set transfer function, adjusting the system response to a second orde r equation. Such parameters were valid for a particular point of operation, and should be recalculated every time the system changes. Nevertheless, their knowledge he lped in the tuning of the frequency regulator, even with approximated coefficients. Thus, with 90% load the system DC-Gen set transfer function was estimated to be:

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87 225 33 225 0 3 2 ) ( ) ( ) ( ) ( ) (2 2 2 2 s s s s K s d s w s V s w s Hn n n chopper cycle duty pu in armature Thus, it possible to represent the DC-Gen se t frequency control scheme as shown in Figure 3-8. + w 225 33 225 0 32 s s s Ki Kp w ref duty cycle Figure 3-8. Frequency cont rol equivalent system Figure 3-9 shows the equivalent syst em response to a step change of 1Hz/60Hz=0.01666pu for two different gains. As expected, the systems behavior was similar to the model, and it was used in the design of the fr equency regulator. Wind Farm Model There are countless studies, books and reports on the modeling of wind turbines and wind farms. Among them, transient studies, such as flicker assessment, harmonics impact, fault ride-through, etc, have become one of the most popular ones. Nevertheless, there are long term studies, such as voltage stability, protection a nd control, and powerflow variations that, even though they do not re quire a detailed system representation, are of vital importance for the overall electric system stability. Transient and long term studi es require different appr oaches when modeling the wind turbines, mainly due to the different time constants involved in the different analysis. Figure 3-10 and 3-11 illustrate the different basic struct ures of a wind turbine for the different types of analysis.

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88 Figure 3-9. Equivalent mode l frequency response for f= 0.01666 pu 32 1 v A Pwind v Wind Power wind p wheel windP c P ) ( Pitch control 1 5 4 3 2 161 ) (c x pe c c c c c c v R w ; 1 035 0 08 0 1 13 Wind Wheel Power angle pitch Blade transients Mechanical transients Drive train Model w D Tg Tw dt dw J Tw Generator Tg wwindwwindwgenerator wwind Converter (optional) Converter (optional) Pg Generator controller Grid Figure 3-10. Dynamic model used for transient studies [38] cjX mjXr sR R ') (' r sX X j ) ( wind f P Figure 3-11. Static model used for steady-state studies [39] According to an extensive research study undertaken by the National Renewable Energy Laboratory (NREL) on the short-term power fluctuation of large wind-power plants, the persistence of th e wind and the output of the wi nd-power plant are very strong

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89 within one second time step. In other words, the likelihood of wind power changing from one level Pi to a nother level Pj=Pi*( 1.1) at the next second is very small (<1.5%) [40][41]. Therefore, since only power fluctuations were of intere st for our study, no fast wind turbine dynamics were incorporated into th e system model or system control. The proposed wind-farm equivalent model is shown in Figure 3-12. A 7.5 HP DC motor acted as a prime mover and its control was designed to assure that a 5 HP induction generator followed a given power fluctuation profile. Data logging Supervisory system Asynchronous Generator 5hp DC Motor 7.5hpPF correction Encoder (w>1800 rpm) 3 phase four wire system Vphase-phase=480 V Vrated field= 150 DC volts Irated=1.15 Amps Lfield=6.76 H Rfield=107 =0.063 secs single phase input Vl-n=120 V Ichopper wind farm Wrotor1 Pout, Qout (V and I) Ia, Ib wind farmPF capsVab, Vbc wind farm EncoderElectrical field constant=7.6 Ripple period Interface Board (signal conditioning and filtering) (Output Range 0v to 3v) (cut-off frequency=100kHz) GRID Single quadrant chopper DSP TI F2812 real time data transfer DC machine controller Power reference wind power data Figure 3-12. Wind-farm model Actual wind-power output data from a wind-power plant in Hawaii was used for the analysis of the system performance. Before the data was incorporated into the control algorithm as reference values, they were sc aled down according to the system needs and penetration factors wanted. For instance, a 10 MW rated power wind farm that represents 15% of the system total capacity would have to be scaled down as 15 0 10 MW) ( data output power wind Actual Power Reference Machine Induction W) 750 or ( 15% level n penetratio farm Wind kW 5 capacity system Model MW

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90 The goal was to be able to reproduce any kind of power variati on (or wind profile) typical of wind farms, and its im pact on the electric power system. Wind-Farm Control Algorithm To generate the required wind-power fluctuations, the following control algorithm was implemented in a DSP. A/D inputsVa, Vb,Vc wind farm Ia, Ib wind farm 6 channels + I chopper ref PI controller PWM TO CHOPPERIchopperI chopper PI controller Pwind farm Pwind ref + Vdc power supply4860Hz + + sT s Kderivative 1 Power regulator Current regulator Figure 3-13. Wind-farm controller A derivative control action was added to the PI controller to allow for a fast time response. Figure 3-14 shows the wind-farm model ti me response to a 750W step change (from 0% to full load). It can be deduced fr om the current chopper reference curve that the power regulators derivative control action was of vital importance in the damping of the output power ( P wind farm). The system time constant was around 0.25 seconds, allowing for changes in power references every second. Wind-Farm Power-Factor Correction When an asynchronous machine is acting as a generator, excitation currents needed to create the field excitation are drawn from the electric power system it is connected to.

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91 Asynchronous generators are usually equi pped with a power factor correction system for phase compensation of individual ma chines and/or to regulate voltage at the point of interconnection. Figure 3-14. Wind-farm power regulator & current re gulator step response ( P=100%). A) Current regulator output. B) Power regulator output. Figure 3-15 shows the PQ curve of the i nduction generator used for the modeling of the wind farm. It can be inferred from Figure 3-15 that As the power generated increases so does the reactive power drawn from the system. The PQ curve is strongly biased. Usually, only the non-load reactive power consumed by the asynchronous generator is compensated by means of cap acitor banks. However, the windfarm manufacturers also offer the possibility of 100% compensati on using different capacitor A B

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92 bank steps. The largest capacitor bank is always switched on first, once the generator has been cut-in. Figure 3-15. Induction generator PQ curve In order to reduce the amount of reactiv e current drawn from the electric power system, a power factor correction capacitor bank was connected in shunt with the induction machine. The wind-farm power factor was not co rrected 100% to av oid possible selfexcitation conditions during unwan ted island mode operations. The amount of required capacitance require d to compensate for a given reactive power is given by Equation 3-3. 2 32 3n l phaseV f Q C (3-3) Thus, a C 15 uF capacitor (per phase) was required to compensate for Q=1300Var, which meant a reduction of approxi mately 70% in reactive current at the non-load operation point. Figure 3-16 shows the location and configuration of the PF capacitor bank.

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93 Figure 3-17 shows the power factor correcti on capacitor bank effect on the wind-farm output current for the non-load cond ition. It can be concluded that The required reactive current drawn fr om the electric power system was significantly reduced. Capacitor current created undesired harm onic components that had an impact on the total output wind-farm current. Asynchronous Generator 5hp DC Motor 7.5hpPF correction Encoder Single quadrant chopper 15 uF 280 V 15 uF 280 V 15 uF 280 V GRID Figure 3-16. Wind-farm PF correction capacitor bank A closer look at the capacitor harmonic cu rrent content revealed that the capacitor impedance did not decrease li nearly with the frequency. Figure 3-18 shows the capacitor impedance at different frequencies. Figure 3-17. PF correction capacitor bank curr ent waveforms. A) Synchronous machine voltage. B) Capacitor bank current. C) Induction machine current. D) Wind farm current. A B C D

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94 Figure 3-18. Capacitor bank impedance frequency scan Capacitor current distortion was mainly due to its non-linear beha vior at different frequencies. Thus, synchronous machine voltage harmonics content, in particular 2nd, 3rd, 4th, and 5th harmonics, would exaggerate this nonlinearity, making it more visible. Wind-Farm Soft-Start System Wind turbines use soft-start systems to smooth the connection and disconnection of the generator to the electric power system. It helps minimize high changes of voltage and current in the grid, while protecting the mech anical parts of the wi nd turbine against high torque forces present during the start-up and shut-down. In the majority of cases, thyristors are us ed as soft-starters, and these are bypasses electro-mechanically, once the wind turbin e has been started to avoid semiconductor losses. Induction machines inrush current can so metimes reach values up to eight times their nominal current, causing protection sy stems to disconnect the machine from the power grid. The same type of inrush current s was present in our model during the start-up of the induction generator. In order to avoid this inrush current, the induction generator was brought online at the same time the s ynchronous machine was brought up to the rated speed (1800 rpm). Figure 3-19 shows a flow char t describing the procedure

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95 followed to bring the entire system to no minal values, avoiding any unwanted inrush current or transient. The system operated in various modes, some of them stationary, and some of them temporary. The control sequence was the following: 1. The duty cycle of the synchronous machine s prime mover was increased linearly, so that synchronous machines V/F ratio wa s kept as constant as possible (except when the voltage regulator took co ntrol over the excitation field). 2. A speed regulator controlled the duty cy cle of the induction machines prime mover in order to track the synchronous machines speed during the ramp-up stage. Thus, the induction machines inrush current was minimized. 3. Once both machines were closer to thei r nominal speed (1800 rpm), the control system smoothly changed the mode of operation, where the synchronous machines speed and induction machines output power were tightly controlled. Reset Mode Start-up Mode Run Mode Stop Mode Chopper DC power supply OK? Yes No Actions Disable Chopper PWM1 Disable Chopper PWM2 Initialize regulators Actions Clear errors Enable Chopper PWM1 Enable Chopper PWM2 Actions Ramp-up duty cycle of synchronous machine's primer mover Enable speed regulator of induction machine's primer move ( speed reference induction machine equals synchronous machine actual speed) IM speed>90% rated speed ? & SM speed>99% rated speed? Yes No IM speed<90% rated speed ? & SM speed>99% rated speed? Chopper DC power supply OK? Actions Disable all control actions Actions Frequency regulation Power regulation SM speed>120% rated speed? YesNo No Figure 3-19. Machine control scheme operating states

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96 Figure 3-20 shows the systems performa nce during the start-up sequence. As expected, the induction machines inrush cu rrent during the star ting stage was also insignificant. Figure 3-21 shows a detail of the transient of the i nduction machine speed during the transition between start -up mode and run mode. Figure 3-20. Electric power system start-up. A) Machines duty-cycle. B) Machines speeds. C) Automatic voltage regulat or output. D) Synchronous machine output voltage. E) Wind-farm current. Figure 3-21. Detail of the transition from start-up mode to run mode A B C D E

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97 Power Stabilizer Power-Stabilizer Hardware Description The main components that form part of the wind-farm buffer were: Interface board Digital Signal Processor (DSP) Field Programmable Gate Array (FPGA) Intelligent Power Module (IPM) Gate Drive board (isola ted interface circuit) All these components were linked to each other one way or another, and their designs were very interrelated. The ma jor difficulty found when designing and implementing the Power Stabilizer system was the electrical noise. Sharp edge digital signals in close proximity to high switchi ng frequency power semiconductors at high voltage levels required special care in order to make them work in harmony. Figure 3-22 shows the overall pow er stabilizer system as well as all the major links between all key components. Power Stabilizer control algorithm Energy Storage Choke InductorFilter5% base on Power Stabilizer rated Power and Voltage 10% base on Power Stabilizer rated Power and Voltage 3% base on Power Stabilizer rated Power and Voltage signal conditioning and filtering Output Range 0v to 3v Digital Outputs A/D 16 inputs (12 bit) Va, Vb, Vc PCC Va, Vb, Vc Filter Ia, Ib, Ic INV Vdc link Ichopper Vstorage Va, Vb, Vc PCC Va, Vb, Vc Filter Ia, Ib, Ic INV Vdc linkIchopperVstorage 3 channels3 channels3 channels1 channel1 channel1 channel PWM generation System diagnosis Error signalsVstorageIa, Ib, Ic INV Va, Vb, Vc Filter Va, Vb, Vc PCC DSP FPGA Ia, Ib, Ic wind farm Ia, Ib, Ic wind farm 3 channels ResetINTERFACE BOARD GATE DRIVE BOARD g1g2g3g4g5g6gc1gc2 g1 g2 g3 g4 g5 g6 2 1 2 3 2 5 2 4 2 6 2 2 gc1 2 5 gc2 2 5 Chopper Reactor Capacitor Vdc link Ichopper GRID Iwind g1g2g3g4g5g6gc1gc2 Data Bus (PWM & Watch Dog) Address Synchronization signal Figure 3-22. Power Stabilizer system overview

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98 The following sequence shows how the pow er stabilizers control scheme functions: 1. Signals from the electric system were scaled down by the interface board to the appropriated levels before these were sent to the DSPs A/D converter. 2. To synchronize the PWM-triangular waveform and the DSPs interrupt service routine frequency that executed the ma in control algorithm, the FPGAs clock signal was used as the main time base. Thus, every 25.72 s (or eight times the PWM switching frequency) the FPGA sent a signal to the DSP to initialize the execution of the main control algorithm. The DSP processed the input signals and determined the control actions required to meet particular systems specifications. 3. Once control actions were calculated by the DSP, these were sent to the FPGA in the form of clock cycles. The FPGA then compared these signals to a digital triangular waveform (an up/down counter) in order to generate the PWM signals. 4. PWM signals were then passed through a digital dead-time ge nerator to avoid possible shoot-through currents th at could damage the IPMs. 5. PWM signals were then sent to the inte rface board, where the appropriate scaling was carried out before these were sent to the gate drive board. 6. Once the PWM signals reached the gate driv e board, these were isolated from the high power side in order to avoid noise problems. 7. IPMs executed the desired control actions. Other signals that were involved in our design and were not shown in the overall diagram are Protection signals:Trip signals due to over-curre nt, and over-voltage conditions were present in the interface board and they were provided for the safe operation of the system. These signals, when active, di sabled the PWM cont rol actions without the intervention of the DSP. The reason fo r such high-speed response was that the DSPs interrupt latency could not protect hardware when responding to over-current through interrupt service routine software. WatchDog: The FPGA also contained a special error signal called WatchDog. It basically provided a safeguard against DSP crashes by automatically disabling PWM control actions if it was not servi ced by the DSP at re gular intervals. Data acquisition signals: To tune and improve system performance a data acquisition system to monitor system va riables was required. Thus, the interface

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99 board was equipped with two D/A converter s that in real time could show the DSPs internal variables. Up to 8 different variables were sampled in real time. Data acquisition system: A secondary data acquisition system was also available through the DSPs JTAG cont roller interface with a refreshing time limited to 100 ms (close to 4000 times slower than the DAC system). Interface board The interface boards main function was signaling conditioning while providing trip signals due to over-current and/or over-voltage conditions. Figure 3-23 shows an overview of the different functions implemen ted in the interface boa rd. These functions were Signals scaling (voltage and current) Trip signals conditioning Power supply voltage monitoring FPGA-Gate drive board interface (input and output signals) D/A circuit Signals scaling. The electric power signals had to be scaled down before their conversion into digital. The DSP ADC data sh eet specified that th e input range voltage was from 0.0 V to 3.0 V. Therefore, a signal conditioning circuit was designed to bring down voltages within V range. Figures 324, 3-25, 3-26, and 3-27 show the circuit topologies used in the scaling of the different voltages and currents pr esent in the system. Voltage at the DC link bus, voltage at the energy storage energy bus, and power stabilizer currents were the only inputs with trip signals Moreover, only the DC link voltages scaling & trip circ uitry was duplicated, with the purpose of avoiding system damage due to the loss of one of the inst rumentation amplifiers. The DC link voltage would tend to rise without control, if the system used to measure its value failed.

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100 J11 SHROUDED LATCH HEADER FOR RIBBON CABLE STRAIGHT PIN 10 1 2 3 4 5 6 7 8 9 10 J16 SHROUDED LATCH HEADER FOR RIBBON CABLE STRAIGHT PIN 10 1 2 3 4 5 6 7 8 9 10 J4 PHOENIX 2 1 2 J1 PHOENIX 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 J13 SHROUDED LATCH HEADER FOR RIBBON CABLE STRAIGHT PIN 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 J7 PHOENIX 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 J6 SHROUDED LATCH HEADER FOR RIBBON CABLE STRAIGHT PIN 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 J15 SHROUDED LATCH HEADER FOR RIBBON CABLE STRAIGHT PIN 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 J8 PHOENIX 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 AREA1 VFILTER, VPCC,VDCLINK, VSTORAGE VFILTERA VFILTERN VFILTERB VFILTERN VFILTERC VFILTERN VPCCA VPCCN VPCCB VPCCN VPCCC VPCCN V_DC_LINK_P1 V_DC_LINK_N1 V_DC_LINK_P2 V_DC_LINK_N2 VSTORAGEP VSTORAGEN VFILTERAOUT VFILTERBOUT VFILTERCOUT VPCCAOUT VPCCBOUT VPCCCOUT V_DC_LINK1 V_DC_LINK2 VSTORAGE GND DC_LINK_ERROR1 DC_LINK_ERROR2 VSTORAGE_ERROR +15V +5V -15V GND +3.3V VEXTRAP VEXTRAN VEXTRA_ERROR VEXTRA AREA2 IWIND,IESA, ICHOPPER IWINDAP IWINDAN IWINDBP IWINDBN IWINDCP IWINDCN IESAAP IESAAN IESABP IESABN IESACP IESACN ICHOPPERP ICHOPPERN IWINDA IWINDB IWINDC IESAA IESAB IESAC ICHOPPER GND IESAA_ERROR_P IESAA_ERROR_N IESAB_ERROR_P IESAB_ERROR_N IESAC_ERROR_P IESAC_ERROR_N ICHOPPER_ERROR_P ICHOPPER_ERROR_N +3.3V +5V +15V GND -15V J10 PHOENIX 10 1 2 3 4 5 6 7 8 9 10 AREA4 PWM BUFFER PWM_UP PWM_UN PWM_VP PWM_VN PWM_WP PWM_WN PWM_CP PWM_CN GND UP1A UP2A VP1A VP2A WP1A WP2A UN1A UN2A VP1A VN2A WN1A WN2A UP1B VP1B UP2B VP2B WP1B WP2B UN1B UN2B VN1B VN2B WN1B WN2B GND +15V OK J9 SHROUDED LATCH HEADER FOR RIBBON CABLE STRAIGHT PIN 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 AREA5 UV +15V +5V +3.3V -15V GND UV+24V UV+15V UV+5V UV+3.3V UV-15V GND +15V OK +24V J12 SHROUDED LATCH HEADER FOR RIBBON CABLE STRAIGHT PIN 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 J2 SHROUDED LATCH HEADER FOR RIBBON CABLE STRAIGHT PIN 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 AREA6 B IPM ERROR B GND U_FOSB V_FOSB W_FOSB FOSB U_FOSERRORB V_FOSERRORB W_FOSERRORB FOSERRORB GND +15V OK +3.3V AREA6 A IPM ERROR A GND U_FOSA V_FOSA W_FOSA FOSA U_FOSERRORA V_FOSERRORA W_FOSERRORA FOSERRORA GND +15V OK +3.3V J14 SHROUDED LATCH HEADER FOR RIBBON CABLE STRAIGHT PIN 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 AREA 3 D/A D0 D1 D2 D3 D4 D5 D6 D7 D9 D8 D10 D11 A0 A1 GND D/A OUTPUT1 D/A OUTPUT2 D/A OUTPUT3 D/A OUTPUT4 D/A OUTPUT6 D/A OUTPUT5 D/A OUTPUT7 D/A OUTPUT8 GND +15V +5V GND -15V LS MS CS1 CS2 J3 PHOENIX 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 J5 PHOENIX 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Figure 3-23. Interf ace board overview

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101 R51M 3/4W +5V + R1 R2 -U1 AD620 3 1 8 2 6 5 7 4 R2 4.99k 0.6W 1% +15V VFILTERA D1 1N914B 0 TO +3V RANGE C5 0.1uF 50V 10% +10V CLAMP VOLTAGE -15V R4 1.2 k 0.4W 1% C3 0.001uF 50V 10% C1 0.1uF 50V 10% C2 0.001uF 50V 10% R6 4.99k 0.6W 1% R11M 3/4W 0 TO +10V RANGE -5V TO +5V RANGE VFILTERAOUT R3 2.8k 1/4W 1% VFILTERN D2 1N5817 Figure 3-24. AC voltage scaling circu it (input [-1000+1000V], output [0 +3V]) U10F 74AHC14 13 12 14 7 R401M 3/4W R47 1.2 k 0.4W 1% + R1 R2 -U7 AD620 3 1 8 2 6 5 7 4 C28 0.001uF 50V 10% +3.3V V_DC_LINK_N1 +U9A LM239 7 6 1 3 12 C34 0.1uF 50V 10% 0 TO +10V RANGE, 9V TRIP (DC LINK TRIP VOLTAGE =900 V) V_DC_LINK1 +3.3V +15V R41 10K 1/4W 1% R421M 3/4W -15V C27 0.001uF 50V 10% R46 2.8k 1/4W 1% C36 0.1uF 50V 10% +15V RP1A 1.0k 16 1 D14 1N5817 +10V CLAMP VOLTAGE D13 1N914B R43 10K 1/4W 1% C26 0.1uF 50V 10% 0 TO +3V RANGE, 2.7V TRIP (DC LINK TRIP VOLTAGE =900 V) C29 0.1uF 50V 10% +TRIP LEVEL VOLTAGE V_DC_LINK_P1 DC_LINK_ERROR1 Figure 3-25. DC voltage scaling circu it (input [0 +1000V], output [0 +3V]) + R1 R2 -U15 AD620 3 1 8 2 6 5 7 4 C51 0.1uF 50V 10% R80 5.49k 1% C49 0.001uF 50V 10% R84 100 1/4W R79 1.0k 1/4W 1% -15V R85 2.8k 1/4W 1% 0 TO +10V RANGE TP2 TP1 IWINDAP 0 TO +3V RANGE +15V D27 1N914B GAIN 10 T1 AC1005 1 2 3 PIN1 PIN2 PIN3 +10V CLAMP CURRENT C48 0.1uF 50V 10% -0.5V TO 0.5V RANGE +5V IWINDAN D28 1N5817 R86 1.2 k 0.4W 1% IWINDA Figure 3-26. CT current scaling circ uit (input [-5 +5A], output [0 +3V]) C59 0.001uF 50V 10% +TRIP LEVEL CURRENT +15V R98 137 0.6W 1% +U20A LM239 7 6 1 3 12 C58 0.1uF 50V 10% -TRIP LEVEL CURRENT +15V D33 1N914B U21B 74AHC14 3 4 14 7 +5V 0 TO +10V RANGE, 1V & 9V TRIP MLEM1 LA 25-NP/SP14 IESAAN +U20B LM239 5 4 2 3 12 C60 0.1uF 50V 10% +15V IESAA_ERROR_P D34 1N5817 C61 0.1uF 50V 10% +3.3V -15V R101 2.8k 1/4W 1% RP1E 1.0k 16 5 C62 0.1uF 50V 10% -15V IESAAP +3.3V R102 1.2 k 0.4W 1% IESAA_ERROR_N +10V CLAMP CURRENT + R1 R2 -U19 AD620 3 1 8 2 6 5 7 4 R97 1.0k 1/4W 1% 0 TO +3V RANGE, 0.3V AND 2.7V TRIP RP1F 1.0k 16 6 U21C 74AHC14 5 6 IESAA Figure 3-27. LEM current scaling circuit (input [-0.36 +0.36A], output [0 +3V]) Power supply monitoring. During the start-up and shut down of the system, it was very important to have complete control in order to avoid possibl e unwanted turn-on of the power semiconductors. Moreover, if one of the power supplies would fail during

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102 normal operation of the power stabilizer, a shut down of the IGBT gating was required in order to protect the rest of the system. Figure 3-28 shows the power supplies voltage monitoring circuitry used in the interface board. R71 1.0k 1/4W 1% R76 1.24k 1/4 W 1% C40 0.1uF 50V 10% +3.3V +3.3V U10B 74AHC14 3 4 D25 1N914B UV+24V R82 10k 1/4W 1% U16 MAX8214 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 VREF IN1 IN2 IN3+ IN3IN4+ IN4DIN VDD MS OUT1 OUT2 OUT3 OUT4 DOUT GND Z4 1N755A A K -15V R70 21.5k 1/4W 1% +5V R74 1.24k 1/4 W 1% D19 1N914B R83 1.24k 1/4 W 1% +5V +5V U10E 74AHC14 11 10 D26 1N914B R77 10k 1/4W 1% U10C 74AHC14 5 6 C46 0.1uF 50V 10% R73 3.48k 1/4W 1% R66 13k 1/4W 1% R78 1.87k 1/4W 1% Q1 2N7000 R69 10k 1/4W 1% +15V OK R81 1.24k 1/4 W 1% +15V D21 1N914B Z3 1N967B A K R72 1.24k 1/4 W 1% D23 1N914B Z2 1N755A A K UV+15V +3.3V U14 MAX8214 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 VREF IN1 IN2 IN3+ IN3IN4+ IN4DIN VDD MS OUT1 OUT2 OUT3 OUT4 DOUT GND D22 1N914B +3.3V R64 1.24k 1/4 W 1% R62 10k 1/4W 1% U10D 74AHC14 9 8 +24V R75 10k 1/4W 1% UV+3.3V C50 0.1uF 50V 10% R65 10k 1/4W 1% Q2 IRF4905/TO UV+5V R63 10k 1/4W 1% U10A 74AHC14 1 2 14 7 UV-15V +15V Figure 3-28. Power supplie s voltage monitoring Only when the power supplies were OK a 15 V source (labeled +15V OK) was switched on, allowing the Dar lington drivers of the FPGA to control the optocouples (fault and control signals) of the IPMs gate drive. Figure 3-29 shows the systems critical signals during pow er-up. It can be inferred that the voltage monitoring circuit ( +15 Vo lts OK signal ) was a critical element to assure that no PWM signals reached the gate drive until all the voltages were stable. Figure 3-30 shows the systems critical signals during s hut-down. In this case no glitches were present. However the voltage monitoring circuitry provided a clean system shut-down, avoiding possibl e system malfunctions.

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103 Figure 3-29. Systems critical signals during tu rn on. A) AC side. B) 15 volts signal. C) FPGAs PWM phase A. D) FPGA voltage supply. Figure 3-30. Systems critical signals during turn off. A) AC side. B) 15 volts signal. C) FPGAs PWM phase A. D) FPGA voltage supply. FPGA-Gate drive board interface. Gate drive board fau lts and on/off control signals were transferred to and from the syst em controller (FPGA) using optocouplers, so high and low side control signals could be referred to a common logic level. In our application board, Darlington transistors (Figure 3-31) were used to drive the diode side of the optocouplers. A B C D A B C D

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104 PWM_VN WN2A UP1A VP2A VN1B VN2B +15V OK WN1A UN1B PWM_UP PWM_VP WP1B WP2B U32 MC1413 9 1 2 3 4 5 6 7 16 15 14 13 12 11 10 COM IN1 IN2 IN3 IN4 IN5 IN6 IN7 OUT1 OUT2 OUT3 OUT4 OUT5 OUT6 OUT7 WN1B UN1A UP2A UN2A Z6 1N967B A K PWM_WP VP1B VP1A UP2B PWM_WN PWM_CP VN2A WP2A PWM_CN PWM_UN UP1B U31 MC1413 9 1 2 3 4 5 6 7 16 15 14 13 12 11 10 COM IN1 IN2 IN3 IN4 IN5 IN6 IN7 OUT1 OUT2 OUT3 OUT4 OUT5 OUT6 OUT7 VP2B WP1A UN2B WN2B VP1A FROM FPGA TO GATE DRIVE BOARD (isolated interface board) Figure 3-31. Darlington drivers Status signals from the IPM, once isolated by the gate drive boa rd, are scaled down in order to be processes by the FPGA (Figure 3-32). R131 30k 1/4W 1% R122 5.5k 1/4W 1% R123 5.5k 1/4W 1% FOSERRORA U_FOSERRORA U33D 74AHC14 9 8 R129 30k 1/4W 1% U33C 74AHC14 5 6 C91 0.001uF 50V 10% W_FOSA +3.3V C90 0.001uF 50V 10% FOSA R138 10k 1/4W 1% R130 30k 1/4W 1% C89 0.001uF 50V 10% C88 0.001uF 50V 10% C41 0.1uF 50V 10% R128 30k 1/4W 1% U_FOSA R136 10k 1/4W 1% R121 5.5k 1/4W 1% V_FOSA R137 10k 1/4W 1% R120 5.5k 1/4W 1% V_FOSERRORA +15V OK W_FOSERRORA R139 10k 1/4W 1% U33B 74AHC14 3 4 U33A 74AHC14 1 2 14 7 TO FPGA FROM GATE DRIVE BOARD (isolated interface board) Figure 3-32. IPM status signals interface circuitry D/A circuit. Real time system diagnostics could be carried out by the usage of D/A converters controlled by the DSP. The goal was to be able to identify potential problems and to evaluate system performance. T hus, every time the DSP executed the control algorithm, the DACs were updated with new values from the DSP. The number of DAC-channels available was 8, and the range of output voltage was programmable from the DSP, having a maximum of V. Figure 3-33 shows the circuit used for the control of the DAC.

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105 +5V C83 0.1uF 50V 10% D4 GND R118 10k 1/4W 1% D5 +5V D0 D/A OUTPUT4 MS\ D6 D/A OUTPUT5 GND C76 0.1uF 50V 10% D9 +5V +15V D8 C81 0.1uF 50V 10% C87 1uF 50V 10% D/A OUTPUT3 R119 10k 1/4W 1% D3 D/A OUTPUT1 D/A OUTPUT7 U27 AD664 (44 pin) 28 29 30 31 32 33 35 36 37 38 39 40 8 7 10 9 12 13 15 14 6 5 4 21 42 19 20 23 24 25 2 27 26 18 1 22 11D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 VOA RFA RFB VOB RFC VOC RFD VODVEE VCC AGND DGND VLLDS0 DS1 QS0 QS1 QS2 LS MS TR CS RD RST VREF C84 1uF 50V 10% U30 AD587JN 4 2 6 8 5GND+VINVOUT NOISE TRIM +5V D10 A1 +5V +15V LS\ +15V -15V U28 AD664 (44 pin) 28 29 30 31 32 33 35 36 37 38 39 40 8 7 10 9 12 13 15 14 6 5 4 21 42 19 20 23 24 25 2 27 26 18 1 22 11D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 VOA RFA RFB VOB RFC VOC RFD VODVEE VCC AGND DGND VLLDS0 DS1 QS0 QS1 QS2 LS MS TR CS RD RST VREF D2 D/A OUTPUT6 C79 0.1uF 50V 10% C85 0.1uF 50V 10% A0 D11 D/A OUTPUT8 -15V D/A OUTPUT2 C86 0.1uF 50V 10% C78 0.1uF 50V 10% U29 AD587JN 4 2 6 8 5GND+VINVOUT NOISE TRIM D7 CS2 C80 0.1uF 50V 10% D1 +15V C77 0.1uF 50V 10% CS1 +5V C82 0.1uF 50V 10% Figure 3-33. DAC circuit Digital signal processor The controller selected for this app lication was the DSP F2812 from Texas Instruments. The main features of the DSP development board are: TMS320F2812 Digital Signal Pro cessor operating at 150 MHz 32-Bit CPU 18 K on chip RAM 12-Bit ADC, 16 Channels 56 Individually Programmable, Multiplexed GPIO Pins

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106 128 K on chip FLASH ROM 64 K words on board RAM Expansion Connectors Onboard IEEE 1149.1 JTAG controller Different alternatives were evaluated fo r the control of the power stabilizer. However, mainly due to cost reasons a nd processing speed, the DSP F2812 was the chosen option for this type of application. The alterna tives under consideration are summarized in Table 3-2. Table 3-2. Alternatives for th e power stabilizer controller Controller Price Comments Matlab Real Time Workshop Matlab Real-Time Windows Target $7,500.00 $2,000.00 Maximum speed: 10 kHz Real-Time Linux Freeware Long learning curve to be proficient at it. National Instruments : NI PXI-8186 Real-Time (42 kHz single PI loop) + LabView RealTime $4495.00 Data acquisition system with onboard processor Microstar Laboratories: DAP5400a $4000.00 Data acquisition system with onboard processor Texas Instruments TMS320C2812 DSP development kit $300.00 Field-programmable gate array A Field Programmable Gate Array or FPGA is an integr ated circuit that can be programmed, and it is specially designed for prototyping integrat ed circuit designs. In this application the FPGA developed two basic tasks: PWM generation ( digital triangular waveform, circuit comparator, and deadtime generator) Error signals processing Despite the fact that the DSP F2812 was capable of generating up to 10 independent PWM outputs, signals were al lowed to change only twice per cycle. Figure 3-34 explains this limitation in the built-in PWM circuit.

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107 As it will be elucidated later, a PWM ge nerator was implemented in the FPGA, so that the systems response was improved by allowing multiple changes within a cycle. Moreover, simple logic was included in th e PWM generator so dead-time and error signals could be taken into account before PWM signals were sent to the gate drive board. The basic features of the development board used with the FPGA are Actel APA075 ProASIC Plus FPGA Maximum System Gates 75000 Embedded RAM bits 27*1024 Maximum user I/O 158 On board clock oscillator ( 40 Mhz) Eight LEDs Four switches Triangular waveform Time TMX320F2812 PWM output Desired PWM output (FPGA output) Signal Step change Missed pulse Timer value (counts) Figure 3-34. DSP built-in PWM output performance vs. FPGA Intelligent power module The Intelligent Power Module or IPM is an isolated base module, composed of several IGBTs and designed to reduce the syst ems development time, thanks to its builtin gate drive circuit. The IPM module select ed for this applicat ion was the PM50RSA120 IPM from Powerex. Its main features were Gate Drive Circuit

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108 Protection Logic: short circuit, over cu rrent, over temperat ure, under voltage Maximum collector-emitter voltage 1200 V Collector Current 50 Amps Maximum PWM input frequency 20 kHz Minimum dead-time 3.0 s Figure 3-35 shows the power circuit conf iguration of the PM50RSA120. The brake section (B) of the IPM was not used in th e implementation of th e power stabilizer. W V U B P N Figure 3-35. IMP power circuit configuration Isolation interface circuit One of the critical components in the design of the power stabilizer was the interface circuit between the IPM and the low side contro l signals. Even though the IPM had a built-in gate drive, there were still so me interface circuit requirements that had to be satisfied in order to avoid noise probl ems. The isolation interface required was provided by optocouplers, which would be driv en by the Darlingtons transistors of the interface board. Figure 3-36 shows the isolated inte rface circuit used for the IPM. Power Stabilizer Software Description The software tools used in the progra mming of the FPGA and the DSP are Actel Libero Gold Integrated Design Environm ent (IDE) and Texas Instruments Code Composer Studio (CCS). Different C code generation tools were considered in order to reduce system development time and to increase flexibility and readability in the design. The different

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109 alternatives evaluated were VisSim/Embedded Controls Developer from Visual Solutions and Matlab embedded target for TI C2000 toolbo x. However, neither one of them offered the flexibility, readability and efficiency needed for this type of application. GND V_N1 V_WP1 C36 10uF V_VPC C35 0.1uF R43??? VN1 V_UP1 SHIELD 20 KOhms U69 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4GND VO VL VCC C20 0.1uF VN2 V_UP1 R48 10k C32 0.1uF V_N1 V_N1 R521.5k C27 10uF V_F0 U61 M57140-01 1 2 3 4 5 6 14 13 12 11 10 9 8 7 COM_IN COM_IN COM_IN +20V_IN +20V_IN +20V_IN +15V_4 COM_4 +15V_3 COM_3 +15V_2 COM_2 +15V_1 COM_1 F0S UN2 TL C21 10uF R511.5 K R44 10k W_N C23 0.1uF TU C29 0.1uF V_VP1 C19 330uF UP2 UN1 R49??? R46 10k P_20V SHIELD 20 KOhms U70 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4GND VO VL VCC SHIELD 20 KOhms U72 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4GND VO VL VCC V_NC R47??? SHIELD 20 KOhms U71 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4 GND VO VL VCC U_P R53 10k WN2 SL C33 10uF C30 10uF C24 10uF RU V_F0S V_UPC WN1 SHIELD 20 KOhms U65 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4 GND VO VL VCC W_P C34 0.1uF F0 SHIELD 20 KOhms U64 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4GND VO VL VCC C25 0.1uF J17 VOLTAGE SUPPLY 20 V F 1 2 RL R45??? J18 CONN 20 F 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 U_F0S BR C22 0.1uF V_VP1 N_20V BRC SU SHIELD 20 KOhms U66 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4GND VO VL VCC SHIELD 20 KOhms U62 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4GND VO VL VCC VP2 C31 0.1uF UP1 V_P R50 10k WP2 J16 CONN 20 F 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 C28 0.1uF SHIELD 20 KOhms U63 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4 GND VO VL VCC W_F0 R54??? WP1 SHIELD 20 KOhms U67 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4 GND VO VL VCC V_NC R55 10k U_F0 V_WP1 SHIELD 20 KOhms U68 HCNW4506 1 2 3 4 5 6 7 8 NC1 ANODE CATHODE NC4GND VO VL VCC V_N U_N +20V W_F0S VP1 V_WPC C26 0.1uF FROM INTERFACE BOARD (control signals) TO INTERFACE BOARD (status signals) Figure 3-36. Isolated interface board Figure 3-38 shows the name of the variable s used in the programming of the DSP and FPGA, and also the different links between the different sub-systems. Description of DSP program The programming of the DSP power stabil izer control and communications was probably the most complex one in our entire project, since it re quired the knowledge not only of the control algorithm, but of the system configuration as well. To increase the readability of the code, a series of object oriented modules were created in C. The majority of the modules were generated using the IQmath, a quasi floating point toolset, from Texas Instruments [42], with the intent ion of increasing the

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110 control algorithms accuracy. A detailed description of the main modules used in the implementation of the control scheme can be found in Appendix C. The power stabilizer control algorithm wa s split in main blocks according to the time constants involved. These two blocks we re the power limiters control loop, and the current controller loops. Figure 3-37 explains how th e control algorithm was divided into two sections and the com ponents that were included in each one. On the one hand, power fluctuations due to changes in wind speed were not expected to have significant changes within time intervals sma ller than 100 milliseconds. Thus, a control loop with a sampling frequenc y equal 10 Hz was used to determine the amount of power required to filter out wind-fa rm power variations. On the other hand, since the inverter performance was based on how accurately the system current was controlled, inner current loops had a faster sampling rate. The splitting up of the control scheme into two main loops released some time out of the DSP, and reduced the amount of data needed to be stored. X IwindaVpcca X IwindbVpccb X IwindcVpccc + + + Limiters Ramp Rate Average Inst Wind Power Centering algorithm Vstorage + ESA required power+ +Allowed centering power X IinvbVfb X IinvcVfc Power Inverter ReferenceVdr Iinvb IinvaIinvc Idr invIqr inv Idref Vpccb VpccaVpccc Vpcc ref + IdrefIqref + + Idr invIqr invPI regulator PI regulator + Vdf+ wLIqinv + Vqf+ wLIdinv+ VdinvVqinv + + VstorageInverter Power V2dcref V2d c+ PI regulator + + Ichopper+ KL Vstorage+ Vd c (for modulation index calculation) Vchopper pu 38880 Hz X VfaIinva Ichopper ref Vchopper + PWMchopper PLL (2 sec time constant) Clark V1 Vd sVqs Vo s abc Id sIqs Io sClark I1 sin(theta) cos(theta) Theta ds-qsdr-qrPark I1ds-qsabc ds-qsPLL VdrVqr ds-qsdr-qrPark V1 sin(theta)cos(theta) sin(theta)cos(theta) Delay (t-1/f/4) Delay (t-1/f/4)Vd sVqsVd sVqs + + + 0.5 0.5Vd sVqs Vdr positive sequenceVqr positive sequence ds-qsdr-qrPark V1 positive sequence cos(theta)Vds positive sequenceVqs positive sequencesin(theta) X X + + Filter Sliding window filter (1 cycle) Vpcc positive sequenceTo PCC voltage regulatorVpcc positive sequence Vds invVqs inv ds-qsdr-qriPark V1 sin(theta) cos(theta) iClark V1abc ds-qsTo current regulator To current regulator + + + + + + + + + PWM Va PWM VbPWM VcVa pu V b puV c puVflat a Vflat bVflat cVflatVd c (for modulation index calculation) 2 0 VdqinvKdqinv dinv qinv dqinv qinv dinv dqinvV V K V V V 2 2Constant angle Max MagnitudeVdqinv_limited ed dinv_limit dqinv ed qinv_limit 2 dqinv ted dqinv_limi ed dinv_limit1 V K V K V V Vdinv_limitedVqinv_limited + Klimit 1.5 pu -1.5 pu Current Limiter MVdc_link -Vdc_link Vchopper_limited Ki s 1 Kp + + M -M M -M MVpcc_offsetVpcc_offset10Hz Figure 3-37. Power stabilizer control algorithm sampling rates

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111 MEASUREMENT Measurement & Interface board VPCCA VPCCN VPCCB VPCCN VPCCC VPCCN IWINDAP IWINDAN IWINDBP IWINDBN IWINDCP IWINDCN VFILTERA VFILTERN VFILTERB VFILTERN VFILTERC VFILTERN IESAAP IESAAN IESABP IESABN IESACP IESACN UP1A UP2A VP1A VP2A WP1A WP2A UN1A UN2A VN1A VN2A WN1A WN2A BRCA U_F0SA V_F0SA W_F0SA F0SA +20V GND V_DC_LINK_P1 V_DC_LINK_N1 V_DC_LINK_P2 V_DC_LINK_N2 ICHOPPERP ICHOPPERN VSTORAGEP VSTORAGEN UP1B UP2B VP1B VP2B WP1B WP2B UN1B UN2B VN1B VN2B WN1B WN2B BRCB U_F0SB V_F0SB W_F0SB FOSB +20V GND D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 WR A0 A1 A2 LDAC GND D/A OUTPUT1 D/A OUTPUT2 D/A OUTPUT3 D/A OUTPUT4 D/A OUTPUT5 D/A OUTPUT6 D/A OUTPUT7 D/A OUTPUT8 IWINDA IWINDB IWINDC VPCCA VPCCB VPCCC VFILTERA VFILTERB VFILTERC IESAA IESAB IESAC V_DC_LINK1 V_DC_LINK2 ICHOPPER VSTORAGE PWM_UP PWM_UN PWM_VP PWM_VN PWM_WP PWM_WN PWM_CP PWM_CN GND GND DC_LINK_ERROR1 VSTORAGE_ERROR DC_LINK_ERROR2 IESAA_ERROR_P IESAA_ERROR_N IESAB_ERROR_P IESAC_ERROR_P ICHOPPER_ERROR_P IESAB_ERROR_N IESAC_ERROR_N GND UV +24V UV +15V UV +5V UV +3.3V UV -15V +24V +15V +5V +3.3V GND -15V GND ICHOPPER_ERROR_N U_FOSERRORA V_FOSERRORA W_FOSERRORA FOSERRORA U_FOSERRORB V_FOSERRORB W_FOSERRORB FOSERRORB GND GND GND GND GND GND GND DSP DSP Development Board D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 WR A0 A1 A2 LDAC GND PWM_D0 PWM_D1 PWM_D2 PWM_D3 PWM_D4 PWM_D5 PWM_D6 PWM_D7 PWM_D8 PWM_D9 PWM_D10 PWM_D11 PWM_A0 PWM_A1 PWM_WR IWINDA IWINDB IWINDC VPCCA VPCCB VPCCC VFILTERA VFILTERB VFILTERC IESAA IESAB IESAC V_DC_LINK1 V_DC_LINK2 ICHOPPER VSTORAGE PWM_LATCH GND GND RESET SF STOP L6TRAFO EQUIVALENT 5% 1 2 L4L FILTER 10% 1 2 C2 C DC LINK S1 HW STOP BUTTON PWR1 POWER SUPPLY +24V +15V +5V +3.3V -15V GND GND L7L FILTER 10% 1 2 IPM1 IPM 1 FPGA FPGA Develpment Board PWM_D0 PWM_D1 PWM_D2 PWM_D3 PWM_D4 PWM_D5 PWM_D6 PWM_D7 PWM_D8 PWM_D9 PWM_D10 PWM_D11 PWM_A0 PWM_A1 PWM_WR PWM_LATCH PWM_UP PWM_UN PWM_VP PWM_VN PWM_WP PWM_WN PWM_CP PWM_CN GND GND DC_LINK_ERROR1 VSTORAGE_ERROR DC_LINK_ERROR2 IESAA_ERROR_P IESAA_ERROR_N ICHOPPER_ERROR_P IESAB_ERROR_P IESAB_ERROR_N IESAC_ERROR_P IESAC_ERROR_N GND UV +15V UV +24V UV +5V UV +3.3V UV -15V ICHOPPER_ERROR_N U_FOSERRORA V_FOSERRORA W_FOSERRORA FOSERRORA U_FOSERRORB V_FOSERRORB W_FOSERRORB FOSERRORB RESET HW STOP SF STOP VDD C3 C FILTER 10% C5 C FILTER 10% L2L FILTER 10% 1 2 C1 C FILTER 10% IPM2 IPM 2 C4 C STORAGE A/D GPIO L5L CHOPPER 1 2 L3TRAFO EQUIVALENT 5% 1 2 WF1 WIND FARM 1 2 3 Phase A Phase B Phase C IPM INTERFACE B IPM interface board B GND +20V U_F0S V_F0S W_F0S F0S BRC VP1 VP2 WP1 WP2 UN1 UN2 VN1 VN2 WN1 WN2 UP1 UP2 V_UPC U_F0 U_P V_UP1 V_VPC V_F0 V_P V_VP1 V_WPC W_F0 W_P V_WP1 V_NC V_N1 BR U_N V_N W_N F_0 IPM INTERFACE A IPM interface board A GND +20V U_F0S V_F0S W_F0S F0S BRC VP1 VP2 WP1 WP2 UN1 UN2 VN1 VN2 WN1 WN2 UP1 UP2 V_UPC U_F0 U_P V_UP1 V_VPC V_F0 V_P V_VP1 V_WPC W_F0 W_P V_WP1 V_NC V_N1 BR U_N V_N W_N F_0 L1TRAFO EQUIVALENT 5% 1 2 Figure 3-38. Interconnections between the diffe rent sub-systems of the power stabilizer

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112 As stated in the previous chapter, th e switching frequency selected for the power stabilizer was 4860 Hz. However, the inner l oop regulators had a samp ling frequency that was 8 times the PWM frequency. The goal wa s to reduce inverter time responses to system transients, increasing the power stabilizers bandwidth. This technique is called re-sampled uniform PWM [43], and is a dig ital approximation to the naturally sampled PWM technique. A flow chart illustrating the different st ages the power stabilizer undergoes is shown in Figure 3-39. The time base used for the power stabilizer control algorithm was based on the FPGA synchronization signal that would force the DSP to run at 38,880 Hz. During the start-up of the machine, the fr ont-end inverter of the power stabilizer acted as a three phase rectifier, charging the DC link capacitor bank. Once the simulated electric network was running at rated frequency and volta ge, the chopper of the power stabilizer would charge the energy storage sy stem to 90% its nominal value. Then the control system would revers e the power through the chopper, so the DC link voltage could be charged to its nominal value. Once both systems, the DC link bus and the ESS, were within a reasonable range and close to their nominal values, the power stabilizer switched to normal mode, where powe r could flow in both directions. Figure 340 shows the power interchange between the electri c power system and the power stabilizer, during the different stages.

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113 Modules Initialization A/D offsets calculation A/D offsets calculation Power Limiter control scheme Data acquisition Watchdog reset Data convertion Overcurrent protection Wait mode Charging Energy Storage Vdc_link>590V? 105%90% ? Vdc_link>590V? Charging DC link DC Energy>95% ? Vdc_link>590V? Run mode Yes No Yes No Yes No Yes No Yes No Operation Mode Background control loop (f=10 Hz) Foreground control loop (f=38880 Hz) FPGA synchronization signal (f=38880 Hz) Figure 3-39. Power stabilizer control stages DC link charge during machines start-up sequence ESS is charge to 90% of its nominal value DC link is charged to 800Volts Normal operation STAGE 1 STAGE 2 STAGE 3 STAGE 4 Figure 340. Power stabilizer start-up sequence

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114 During the Power stabilizers normal opera tion, several tasks, besides the control algorithm, had to be carried out. These actions were Update control actions Update data logging fo r PC-DSP communications Refresh DAC Update FPGA PWM signals FPGA program description All the main system components ended up using part of the FPGA for different purposes. However, the main functions were for PWM and error generation. The FPGA code was split into two independe nt systems, one for the control of the different machines and the other for the control of the power stabilizer. Figure 3-41 shows the main modules implemented in the FPGA (the software tool used for the programming is ViewDraw from Actel ). Among all these modules, only three were of vital importance for the proper performance of the system. The main modules were PWM generator and the DSP synchronization Dead time generator Watchdog logic PWM generator and DSP synchronization. The basic idea consists of using an up/down counter as a triangular waveform generator, so the up/down counts can be compared to voltage demand for phase a, phase b, phase c, and the chopper. The PWM triangle count used a counter th at was one more bit than the up/down triangular count would require. For instance, for an up count of 0-1023, and a down count of 1023-0, instead of using a 10 bits counter, the one used for this approach would be 11 bits (N bits). The higher order Nth bit was used to invert th e lower order N-1 bits when it was high (Figure 3-42). Only the lower order N-1 bits were used in the triangle

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115 comparator, since a 10-bit word was sent from the DSP to the FPGA, containing information on the voltage commands. The format of the word sent from the DSP to the FPGA required a small code manipulation, before it was actually compared to the up/down triangle. Since the DSP voltage commands were represented by signed in tegers, a flip of the highest order bit was all that was need to convert a signal from si gned 1.x to unsigned integer. For instance, a (10bits) #0000000000 would be converted into #1000000000, which is about half of the count range. B#1000000000 would convert to B#0000000000, the minimum, and B#0111111111, would be B#1111111111, the maximum. As far as the maximum value that could be sent to the FPGA, there were a couple of remarks; it was possible to send valu es that ranged between the minimum and maximum triangle count. However, this pr esented the following problem; when the values were B#0000000000 >= B#0000000000, the output was high, and when it was B#1111111111 >= B#1111111111, the output was also high. This gave a high bias to the output for an average around zero. This eff ect was canceled out by the fact the number range was one count larger in the negative dire ction. The other problem was that as very narrow notches were being generated, the underlap circuitry that prevented shootthroughs did a poor job of reproducing them. The device in the pole with the high duty cycle continued to switch until the notch di sappeared. Suddenly, the underlap circuit was not required to do anything, and the voltage t ook a small jump in the direction of the high duty cycle switch. The low duty cycle switch stopped conducting when the notch time got to be shorter than the unde rlap time. The dead-time presented a problem in that the current defined what the voltage ended up bei ng during this time. However, when the low

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116 duty-cycle switch stopped coming on, the prob lem become worse. One way to reduce both of these effects was not to send a va lue that would stop th e low duty cycle IGBT from switching. This value was approximately defined as: frequency modulation time dead 2 1 tage output vol Maximum Thus, if the dead-time is 2 sec, the maxi mum value that can be sent to the FPGA is 0.96 pu. Figure 3-41. FPGA system overview

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117 Figure 3-42. Up/Down counter. A) Digital triangular wavefo rm. B).Detail of the countfold effect. The output of the comparators (A>=B) were cleaned up by the D flip-flops at the outputs since the comparator outputs could transi ently glitch as the l ogic settled, after a rising clock-edge. These PWM state signals were then sent to dead-time generators as shown in Figure 3-43. PWM generator Deadtime generator DSP synchronization signal finput=40 Mhz f output=10 Mhz Figure 3-43. PWM generator A B

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118 The DSP synchronization pulse was a latched (and one cl ock delayed) signal and generated 8 times per PWM cycl e (4860Hz). It was true one clock cycle after any count, when the least significant 8 bits were low. Therefore, the DSP control loop ran at a frequency of 38,880Hz Note: Due to clock limitations, the actual PWM frequency was 10MHz/211 = 4882.8 Hz, and the DSP control loop frequency was 10 MHz/28 =39063 Hz. These values were only 0.47% off compared to the designed values. Dead-time generator. The dead-time generator ensured that no shoot-through conditions were given in the IPM module. The IPM minimum dead-time was 3sec, consequently a 4sec dead-time was used in order to guarantee system performance during switching conditions. Figure 3-44 shows how the dead-time gene rator was implemented in the FPGA. Two counters were turned on/off according to the PWM input signal. Their output signals (counts) were then compared to a constant ( number of counts). In th is case the constant selected was 40. Thus, the delay-time was: sec 4 10 40 frequency clock constant time dead Mhz PWM output signals were then enabled or disabled based on a trip signal that included information about all the po ssible flags/errors in the system. Figure 3-45 shows the FPGA dead-time perf ormance for phase A of the front-end inverter.

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119 PWM PWM upper IGBT PWM lower IGBT ERROR-SIGNAL CLOCK-SIGNAL Figure 3-44. One phase dead-tim e generator detailed diagram Figure 3-45. Dead-time generators waveforms Watchdog logic. The F2812 DSP has a built-in watchdog timer that provides a safeguard against CPU crashes by automatically initiating a reset if not serviced by the CPU at regular intervals. This is a very useful tool for applications where the PWM signals are generated from the DSP, since any CPU reset will put the PWM outputs to a high impedance state, which should turn o ff the power converter. However, for our application, the PWM generation was carried out by the FPGA, and therefore a built-in watchdog timer had to be implemented. Figure 3-46 shows the logi c used to realize the watchdog timer that mimicked the one implemented in the DSP.

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120 29 different error signals Watchdog counter Figure 3-46. Watchdog logic The Watchdog timer was a 9-bit counter that generated an error signal (terminal count) after sec 2 51 10 29 Mhz, if not serviced by the DSP. The way the DSP attended the watchdog timer was by sending a specific word to the FPGA. The following table explains meaning of the different words used for the control of the watchdog timer, and also the reset signals. Table 3-3. FPGA code words Objective Value written into the data bus Description Reset error signals 0x055 This valu e will clear all the flip-flops used to latch error signals Watchdog reset 0x0AA This value will reset the watchdog timer Front-end inverter and Chopper turn off signal 0x0EE This value will disable both, the inverter and the chopper of the power stabilizer Inverter turn-on signal 0x088 This valu e will enable the inverter part of the power stabilizer Chopper turn-on signal 0x033 This valu e will enable the chopper part of the power stabilizer Note: It is assumed that a DSP general purpose I/O pin was assigned to latch the FPGA data input. The latched value was clea red by the DSP synchronization signal; so that no values were held in the latch for more than one D SP control loop cycle. In this application the GPIO had the name of DSP_WD.

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121 CHAPTER 4 SYSTEM PERFORMANCE System Data Table 4-1 summarizes the system paramete rs used in the modeling of the power stabilizer system. A comparison with the ideal values based on the per-unit designed system is also included. These differences caused a small deviation from the ideal system. However, the overall system performance wa s not significantly decreased. Power Stabilizer Transient Response The following sections illust rate the power stabilizer re sponse due to step-changes in the different values. System performance was analyzed under various scenarios. Direct-Current Link Voltage Control One of the key variables in the overall system performance was the voltage at the DC link bus, since PWM control actions were based on it. Thus, its value had to be tightly controlled. Figure 4-1 and Figure 4-2 show the DC link voltage responses to a step-change in the voltage reference for different regulator gains. Figure 4-1. DC link voltage re sponse for different Kp gains

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122 Table 4-1. System parameters Variable Per-unit model Ideal model Real model DC link time constant (sec) 0.022 0.022 0.032 ESS time constant (sec) ( @ Vess max) 20.45 20.45 22.23 Chopper current ripple (pu) 0.3 0.3 0.34 PWM (Hz) 4860 4860 4860 X/R 10 10 18 Xfilter (pu) 0.1 0.1 0.103 Xtrafo (pu) 0.06 0.06 0.074 Cfilter size (Vars pu) 0.03 0.03 0.029 Vmax line-neutral 1 391.92 391.92 I max inverter nominal 1 0.256 0.256 Vdc nominal 2.04 800 800 Vrms line-neutral 0.71 277.13 277.13 Irms inverter nominal 0.71 0.18 0.18 Zbase ( ) 1 1536 1536 Pinverter 1.5 150 150 Vstorage max 1.94 760 760 Vstorage min 0.97 380 380 Vstorage center 1.53 600.83 600.83 Cdc link (mF) 16 0.0103 0.015 Cstorage (F) 16.323 0.0106 0.0116 Lchopper (mH) 0.5 768.18 660 Rchopper ( ) 0.019 28.96 14.6 Lf (mH) 0.265 407.44 420 Rf ( ) 0.01 15.36 8.2 Cf (uF) 79.58 0.052 0.050 Lt (mH) 0.159 244.46 300 Rt ( ) 0.006 9.22 7.00 Kp current regulator 1 1 1 Ki current regulator (R/L) 37.7 37.7 18 Kc (charge/discharge constant)(Watts/Joules) 0.0064 0.00647 0.00647 1/2*Cstorage (pu) 8.16 8.16 8.86 K=1/2*Cstorage (pu) Kc (Watts/Volts2) 0.052 0.052 0.057 Figure 4-2. DC link voltage re sponse for different Ki gains

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123 It can be inferred from Figures 4-1 and 42 that the DC link voltage regulator was very robust, and that large ch anges in the regulato rs gain would not affect the system performance. In general, higher proportio nal gains improved DC link voltage settling time, without causing any system overshoot as in the case of high integral gains. However, higher gains caused control actions to saturate and could have originated instabilities. Reactive Current Control The core of the Power stabilizer wa s the inner current regulators. Their performance determined the overall system re sponse. The current regulators robustness is shown in Figure 4-3 and Figure 4-4. As expected, the incr ease in the integral and/or proportional gains had no major impact on th e current response. This was due to the saturation of control actions, as seen in Figure 4-5. The original current regulator gains were designed to avoid possible saturation situations due to a st ep change in the error of 1 pu. Figure 4-6 shows the Power stabilizer current waveform during a multiple stepchange in the reactive current reference. Figure 4-3. Iqref command step change from -0.5 to 0.5 A per unit. Integral gain effect. A) Inverters reference output voltage. B) Inverter current. C) Inverters Iq. A B C

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124 Figure 4-4. Iqref command step change from -0.5 to 0.5 A per unit. Proportional gain effect Figure 4-5. Iq current regula tor output for different Kp Figure 4-6. Iqref command step change from -0.5 to 0.5 and back to -0.5 A per unit. A) Inverters reference output voltage. B) Inverter current. C) Inverters Iq. A B C

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125 The current control loop bandwidth was al so tested and compared to the ideal system. A variable frequency sinusoidal wave form was introduced into the control loop references (Iqref and Idref) to study the reduction in amplitude as the frequency of the input signal increased. The signal references can be defined as 60 2 ) ) 1 sin(( ) ) 1 cos((max max w wt h I I wt h I Iref q ref d Where h is the desired harmonic. For this te st the amplitude selected for the signal amplitude was 0.5 Amps per unit. Figure 4-7 shows the different current output amplitudes for the different harmonics. Figure 4-7. Power stabilizer harmonic in jection response for Ki=18 and Kp=1 The bandwidth of the current regulators could then be compared to the ideal system as shown in Figure 4-8. As expected the bandwidth of the real system was lower than the ideal one due to the in crease in the amount of system impe dance. However, this reduction

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126 in bandwidth did not have any impact in the system performance, since no harmonic compensation was required. Figure 4-8. Current regula tor frequency response Passive Filter Performance LCL filter design was also tested and co mpared to the ideal attenuation gain. Figure 4-9 shows the current waveforms be fore and after the LCL filter. Figure 4-9. Front-end inve rter current waveform The peak to peak ripple current was 0.06 A, and it was very similar to the expected value: 0.0572 A (0.2214pu*0.22515A=0.0572A).

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127 Figure 4-10 shows the frequency spectrum of Figure 4-9. It can be deduced that the filter attenuation gain was -19 dB. Figure 4-10. Frequency spectrum of the LCL currents Voltage Regulation The Power stabilizer did not just have th e capacity of filtering power variations, but voltage fluctuations as well. By controlling the amount of reactive current injected into the system, it was possible to modify the voltage profile at the wind-farm terminals. Figure 4-11 shows a simplified diagram of th e different impedances that played a significant role in the vo ltage regulation scheme. Lf = 720mH VinveterVsourceL=44mH Vwind Ipower stabilizer5.4% from the Wind farm point of view 1% from the Power Stabilizer point of viewP=150W P=750W V=480V17.67% from thePower Stabilizer point of viewVpcc Main Load Figure 4-11. Simplified system description

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128 If Vsource is considered to be stiff, the maximu m increment in voltage at the point of common coupling for a 150 W ra ted Power Stabilizer is V I X V V Vrms rated inverter line pcc source98 5 2 2 25515 0 2 60 044 0 2 Figure 4-12 shows the voltage at the poi nt of common coupling when the Power Stabilizer went from full i nductive to full capacitive a nd back to full inductive. It can be deduced from the previous calcula tion, that for the given power rating, the voltage regulation capabilities of the Power Stabilizer were modest (.77%). Figure 4-12. Power stabilizer vol tage regulation performance. A) Voltage at the point of common coupling. B) Inverter Iq component. System Losses Due to the size of the equipment, system losses were expected to be a significant proportion of the power ratings, compared to a full-size system. In particular, for the Power Stabilizer, the main losses were: A B

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129 Conduction losses Switching losses I2R Core losses Capacitor leakage current Because of these losses there was an exp ected asymmetric charge/discharge cycle of the energy storage system. Figure 4-13 shows this effect. To compensate for these losses the Power St abilizer was biased such that no control action from the energy storage regulator was needed. Figure 4-14 shows the compensation term in the overall control algorithm. Figure 4-13. Energy storage charge/dischar ge cycle. A) Chopper output power. B). Energy storage voltage and current. C) ESSs energy. Power Limiter ESS Voltage regulator + Losses Idref PCC Voltage regulator Iqref Current Regulators Limters and Transformations VdVq PWM Figure 4-14. Control scheme with a losses compensation term A B C

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130 The effect of introducing a loss compensati on term in the control algorithm had the same effect as modeling the Power Stabilizer as a no-static inverter/c hopper in parallel to a resistive load (Figure 4-15). The losses were estimat ed to be around 20Watts for the non-load condition. Synchronous Machine Main Load 4.5 KW Wind Farm Line Impendace Filter Losses=20Watts Power Stabilizer Figure 4-15. Power stabilizer equivalent system Power Limiter Results When trying to compensate for power fluc tuation, two different approaches were considered, tested, and compared in detail. The wind-power data used for the compar ison was 15 minutes of data from a wind farm on the big island of Hawaii. The freque ncy scan of the data was 15 minutes, and it corresponded to high wind conditions. The wi nd farm consisted of 37 Mitsubishi 250 kW wind turbines, with a total capacity close to 10 MW (Figure 4-16). Figure 4-16. Wind-power conditions under study

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131 The time response of the induction machin es power controller was low enough to be able to reproduce the changes in power usi ng step variations. However, this type of behavior was unrealistic and th erefore a linear interpolati on was used between the two consecutive scans. Power Limiter 1 (High Pass Filter) The transfer function of the high pass (HP) filter used in the design of the first power limiter is shown in Equation 4-1. s f s s Hoff cut 2 ) ( (4-1) The only parameter that needed to be determined according to the system requirements was the cut-off frequency. Figure 4-17 shows the Power Stabilizer response to wind-power fluctuation using a HP filte r with a cut off frequency of 0.005Hz. It can be concluded from Figure 4-17 that the Power Stabilizer had a very good performance smoothing wind-power fluctuati on as long there was enough energy storage available. However, such a control scheme generated abrupt changes in power when the power converter reached the energy limits. Thes e types of situations should be avoided, since the step changes in power could be ev en higher than the ones naturally produced by the wind-farm output power. Figure 4-18 is a zoom of the exact moment where the Power Stabilizer ran out of energy. When the Power Stabilizer reach ed the minimum energy limit, the power delivered to the utility did not only drop dow n to what the wind farm was producing, but it overshot due to the energy absorbed by th e Power Stabilizer (also called centering power). That power was designed to be sma ll in magnitude (proportional to the charge discharge constant Kc), but still had a negative impact on the system.

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132 Figure 4-17. Measured and modele d high pass filter results for Kc=0.0064 W/J, fcut_off=0.005 Hz. A) Power to utility. B) Power stabilizer output power. C) Power stabilizer energy level A B C

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133 Figure 4-18. Measured and modele d high pass filter results for Kc=0.0064 W/J, fcut_off=0.005 Hz (zoom in). A) Power to utility B) Power stabilizer output power. C) Power stabilizer energy level A B C

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134 In order to circumvent such types of s ituations we considered three different approaches: Increased the cut-off frequency, so the Power Stabilizer only compensated for higher frequency pow er fluctuations. Figure 4-19 shows the system response to different cut-off frequencies. Figure 4-20 confirms the re sults by using the Matlab model. It was clear that only high cut-o ff frequencies could help the system from reaching the energy limits. The drawback of higher cut-off frequencies was obvious; less power smoothing effect. Increased the size of the energy storage, so the system could face larger power fluctuations for a give n cut-off frequency. Figure 4-21 shows a comparison between a system with nominal energy and a system with an extra 66% of energy. Even with an increase of more than half in energy the power converter was unable to compensate for large swings in wi nd power without runn ing out of energy. Adaptive high pass filter. The idea behind this control strategy was to vary the cutoff frequency according to the status of th e energy storage. The closer the Power Stabilizer energy storage system was to the nominal value, the more filter was allowed (Figure 4-22). A B C Figure 4-19. Measured high pass filter perfor mance for different cut-off frequencies. System parameters Kc=0.0064 W/J. A) Power to utility. B). Power stabilizer output power. C) Power stabilizer energy level

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135 A B C Figure 4-20. Modeled high pass filter perfor mance for different cut-off frequencies. System parameters Kc=0.0064 W/J. A) Power to utility. B). Power stabilizer output power. C) Power stabilizer energy level A B C Figure 4-21. Measured high pass filter perf ormance for different energy storage sizes. System parameters, Kc=0.0064 W/J, fcut-off=0.005 Hz. A) Power to utility. B). Power stabilizer output power. C) Power stabilizer energy level

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136 f cut-offf cut-off_originEnergyEnergy nominal value Figure 4-22. Cut-off frequency trajectory of the adaptive high pass filter for a given energy deviation Power Limiter 1 (Adaptive High Pass Filter) The adaptive high pass filter control sc heme was designed to avoid saturation situations, yet allowing power fluctuation smoothing. In the adaptive filter control algorithm the number of parameters that needed to be optimized were two; fcut_off_rigin and the slope of the variable cut-off frequency. Equation 4-2 shows th e relationship between energy deviation and cut-o ff frequency used for the adaptive high pass filter. E K f ff origin off cut off cut (4-2) Figure 4-23 shows the real system response for different Kf. The fcut_off_origin was kept constant to 0.005 Hz. It can be concluded from Figure 4-23 that when high pass filter parameters were optimized the system could ride through la rge wind-power fluctuation without reaching the energy limits. Figure 4-24 shows the adaptive high pass f ilter response using an extra 66% of energy. Measured results were compared to the one obtained using nominal energy capacity. System parameters were: Kf=0.0059 Hz/J, Kc=0.0064 W/J, and a frequency of fcut_off_origin=0.005 Hz.

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137 A B Figure 4-23. Measured adaptive high pa ss filter performance for different Kfs. System parameters, Kc=0.0064 W/J, fcut-off-origin=0.005 Hz. A) Power to utility. B) Power stabilizer energy level A B Figure 4-24. Measured adaptive high pass filt er performance for different energy storage sizes. System parameters, Kf=0.0059 Hz/J, Kc=0.0064 W/J, fcut_off_origin=0.005 Hz. A) Power to utility. B) Power stabilizer energy level

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138 This approach seemed to work fairly well. However, it lacked applicability. Electric power systems can be disturbed by large pow er swings, while small power unbalances can be compensated without significant bur den. Therefore large power fluctuations should be tackled first, rather than the small ones due to its impact on the system. Thus, there was a need for a better contro l algorithm that exclusively targeted and compensated for large power variations, while avoiding compensation of less significant events. The goal was to settle down the control actions given by the synchronous machine regulator, so less stress was caused to the synchronous generator. Power Limiter 2 This approach targeted specific indexe s and therefore was more prompt to compensate for large power fluctuation ra ther than low profile power changes. The indexes considered in our study we re the ones used by HECO (Hawaiian Electric Company, Inc.), which were base d on a two-second scan sampling time. The indexes for a 10 MW wind farm were: Ramp Rate Power Fluctuation: s sMW MW RR 30=2 MW/minute where RR= Ramp Rate, may be calculated once every scan MWs-30= The instantaneous MW analog value 30 scans (60s) prior the present scan. MWs = The instantaneous MW analog value for the present scan Instantaneous Power Fluctuation: s sMW MW I 1=0.3 MW/2 sec where I= Instantaneous Power Change, calculated once every scan MWs-1 = The instantaneous MW analog value for the previous scan (2 seconds ago) MWs = The instantaneous MW analog value for the present scan Subminute Average Power Fluctuation: 3030 1 11 s s sMW MW A=1 MW/minute where Al= Subminute Average, calculated once every 30 scans MWs-1 = The instantaneous MW analog value for the previous scan MWs= The instantaneous MW analog value for the present scan

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139 These numbers had to be scaled down according to the size of the wind farm model. Thus, the actual power inde xes considered in the model were: RR=(2/10)*750=150 Watts/minute A=(0.3/10)*750=22.5 Watts/minute I=(1/10)*750=75 Watts/2seconds Note: 750 Watts is the wind-farm model rated power. Even though indexes were based on 2-second scans, the power limiter control scheme calculated such indexes several tim es per second, using a multiple-sampling control algorithm. Figure 4-25 illustrates how these i ndexes, based on 2-second scans, could be estimated more than once every two seconds. This allowed for higher refreshing times for the inverter power reference, so th at a faster and a more accurate response to wind fluctuations were achieved. Figure 4-26 demonstrates that the power lim iter injected energy when it was really needed. Moreover, the control system only re leased the necessary amount of energy to meet the indexes, so not all the energy was consumed during the large power fluctuation. It was also evident that since system losse s were not corrected 100% the power converter drew some extra power from the system to compensate for the losses. In the simulation results, only static losses (or losses in dependent from the inve rter current) were considered. Wind Farm Power 2 second scan 2 second sampling rate 2 seconds 0.5 seconds 2 second scan 0.5 second sampling rate A1 A2 Figure 4-25. Multiple sampling concept. For this example a 0.5 second sampling time was assumed

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140 A B C Figure 4-26. Measured and modele d power limiter 2 results for Kc=0.0064, RR=2 MW/minute, A=0.3 MW/minute, I=1MW/2 seconds fcut-off=0.005 Hz. A) Power to Utility. B) Power stabilizer output power. C) Power stabilizer energy level. The activity of the power indexes dur ing the 15 minute period are shown in Figure 4-27. As expected the power limiter does a very good job trying to keep the power fluctuation indexes within the limits. A significant number of vari ables could have made the power limiter behave differently. However, only a few of them we re considered for our sensibility study: Kc (charge and discharge c onstant). Nominal value Kc=0.0062 Watts/Joule Power limiter sampling frequency. Nominal value 10 Hz Power indexes. Nominal values RR=2 MW /minute, A=0.3 MW/minute, I=1 MW/2 seconds

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141 A B C D Figure 4-27. Measured power indexes activity. System parameters: Kc=0.0064 W/J, RR=2 MW/minute, A=0.3 MW/minute, I=1 MW/2 seconds, and fs=10Hz. A) Power stabilizer output power. B) Aver age power fluctuation. C) Ramp rate power change. D) Instantaneous power fluctuation. Figure 4-28 shows the effect of increasi ng the charge/discharge constant. When doing this, there was no impact on the indexes activity. However, large charge/discharge constants could have caused the activation of the power indexes, since there was power absorbed (charge) or delivered (discharge ) from or to the electric power system. Figure 4-29 shows the effect of decreasi ng the ramp rate limits, making them stricter. For this type of wi nd-power fluctuation, the ramp rate activity was the most important one, so that tighter limits could have made the Power Stabilizer reach the energy limits. In general, a ramp rate lim it of 1 MW/minute generated a good system response in terms of power fluctuation ma gnitude and energy capacity required. Lower ramp rate limits would have required larg er energy capacity, making the Power Stabilizer a non-cost effective solution.

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142 Figure 4-28. Measured power lim iter 2 response to different Kc System parameters: RR=2 MW/minute, A=0.3 MW/minute, I=1MW/2 seconds, and fs=10Hz. A) Power stabilizer output power. B) Power stabilizer energy level. Figure 4-29. Measured power limiter 2 respons e to different ramp rate limits. System parameters: Kc=0.0064 W/J, A=0.3 MW/minute, I=1 MW/2 seconds, and fs=10 Hz. A) Power to Utility. B) Powe r stabilizer output power. C) Power stabilizer energy level. A B A B C

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143 Figure 4-30 shows the effect of decreasing the average power fluctuation limit. In this case the tighter the average power limit was, the worse the response obtained became. The reason for such unusual conduct was found in the way the induction power reference was obtained. Wind-power referen ce was linearly interpolated between two consecutive data samples. Thus, a control scheme using a multisampling strategy (more than one sample per real data samples) per ceived an average power fluctuation from the original one. Figure 4-31 shows the average power fluctuation for different sampling rates. This effect made it almost impossibl e to judge the pros and cons of different average power fluctuation limits. Figure 4-30. Measured power limiter 2 respons e to different averag e power fluctuation limits. System parameters: Kc=0.0064 W/J, RR=2 MW/minute, I=1 MW/2 seconds, and fs=10 Hz. A) Power to Utility. B) Power stabilizer output power. C) Power stabilizer energy level. A B C

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144 Time (secs)Average power fluctuation (MW/minute) Time (secs)Average power fluctuation (MW/minute) Z oom in Figure 4-31. Effect of linear in terpolation on the average power fluctuation index activity. The sampling time of the original wind-power data is 2 seconds. A) Average index. B) Average index zoom in. Figure 4-32 shows the effect of reducing th e instantaneous power fluctuation limit. In general, the system responded fairly well to such limits. However, a special precaution had to be taken when reducing such an i ndex, since it could have caused the power converter to continuously compensate even for small power variations. Figure 4-32. Measured power limiter 2 res ponse to different instantaneous power fluctuation limits. System parameters: Kc=0.0064 W/J, RR=2 MW/minute, A=0.3 MW/minute, and fs=10 Hz. A) Power to Utility. B) Power stabilizer output power. C) Power stabilizer energy level. A B C A B

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145 Figure 4-33 shows the effects of decreasi ng the sampling frequency (or refreshing frequency) of the power limiter. In genera l, lower sampling freque ncies did not have a major impact on the Power Stabilizer ener gy. The advantage of using high sampling frequencies was the fact that power indexes were tracked with exactitude, allowing for a more accurate way of assuring the power fluctuations fell within the specified limits. Figure 4-33. Measured power limiter 2 res ponse to different sampling frequencies. System parameters: Kc=0.0128 W/J, RR=2 MW/minute, A=0.3 MW/minute, and I=1 MW/2 seconds. A) Power to Utility. B) Power stabilizer output power. C) Power stabilizer energy level. Power Limiters Comparison Study In determining which of the power limite rs had a more desirable impact on the electric power system, one should pay a ttention to the systems frequency. In our case, since the fre quency regulator of the sync hronous machine was tuned to instantaneously compensate for frequency deviations, no frequency variations were expected. Therefore, the only good indicator av ailable to determine the impact of the different power limiters was the control action given by the frequency regulator. A B C

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146 Windfarm power fluctuations tended to change the fre quency regulator output due to an unbalance in power. Thus, control acti ons had the tendency to displace up and down from an average value based on the mismatch between mechanical and electrical power (Figure 4-34). Figure 4-34. Measured synchronous machine out put power for the different power limiter control schemes It can be concluded from Figure 4-35 that both, the high pass filter and the adaptive filter had good response filtering small power fluctuation. However, when the Power Stabilizer capacity to absorb or supply energy was needed the most, no capacity was available. Only the power limiter 2 and the adaptive HP filter, with 66% of extra energy, were capable of injecting some energy to shave-off the surge in power. Figure 4-36 shows the frequency regulato r response for the different power limiters. This graph was a consequence of Figure 4-35 one, and gives a better picture of the benefits of each one of the different power limiters.

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147 Figure 4-35. Measured synchronous machine out put power for the different power limiter control schemes. Zoom in of the largest power swing Figure 4-36. Frequency regulator output for the different power limiters. A) HP filter. B) Adaptive HP filter. C) Adaptive HP f ilter with 66% extra energy. D) Power Limier 2 In conclusion, we cannot say that there is a better or worse power control scheme, since each one of the different power limiters can serve different purposes. A B C D

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148 CHAPTER 5 SUMMARY Conclusions The purpose of our study was to develop an electronic power c onverter capable of compensating power fluctuations typical of wind farms. The proof-of-concept converter was designed using similar techniques utiliz ed in the design of medium voltage power quality products. The system design involved the mathematical description of the different equations implicated in the design of the diverse regul ators. Special attention was paid to the current controllers, for being the foundation upon which the rest of the system relies on, and to the development of new control algorithms capable of minimizing power fluctuation while optimizing the us age of the energy storage system. Two different power limiter control scheme s have been proposed and described in detail. A test-bench was developed to mimic a future scenario of an isolated power system. Tests have been conducted under realis tic modeled system conditions, in terms of wind penetration factor, and cost-effectiv e amount of energy storage. The main conclusions of the tests carried out were: The shunt connected voltage source conve rter has shown to be an excellent approach in terms of speed and flexib ility, and power fluctuation smoothing. Two main alternatives for compensati ng wind-power fluctuations using small amounts of energy; power limiters that ta rget low profile power fluctuations and power limiters that compensate according to predefined power fluctuation indexes were studied. These alternatives are usua lly designed to compensate for large power swings.

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149 High pass filters are not appropriate for power filter functions, due to their lack of continuousness when running out energy. Adaptive high pass filters have shown to be a much better alternative to high pass filters. However, behaviors similar to the simple high pass filter might occur if incorrect adaptive laws are considered. Power limiters that target specific power fluctuation indexes have a lower dutycycle than the high pass filter approach and only compensate for larger power fluctuations. Ramp-rate indexes are a good alterna tive to adaptive high pass filters. Instantaneous power fluctu ation indexes do no have as much significance as the ramp-rate indexes Average power fluctuation indexes are highly non-linear, and might create unwanted misbehaviors that are difficult to predict. Both types of power limiters have shown to be beneficial for the reduction in the control actions of the synchronous machine s governor. Thus, reducing the stress of the generator during power fluctuations. In summary, among the different power limiter regulators considered in this dissertation, the adaptive high pass filter has been shown as the most robust and reliable when designed correctly. It forces the power converter and the energy storage system to work continuously, getting the maximum effec tiveness out the system. It even helps the system ride through large power variations in a moderate way. A drawback for this approach is the wear of the energy-s torage system, due to high duty cycles. The power limiter based on power fluctuat ion indexes serves as a very specific solution to a specific problem or need. Its low duty-cycle profile in terms of real power allows for better voltage regulation or voltage flicker reduction (usi ng reactive power), in cases where system impedan ce is relatively large.

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150 Further Work There is still a lot of work to be done in the area of smoothing real power fluctuations using various t ypes of energy storage systems. The following is a summary of the different subjects that merit additional research: New control algorithms. For instance, new non-linear adaptive laws or a combination of different power limiters based on energy levels Evaluation of the benefits of real power smoothing. Evaluation of new energy storage systems Our research work has focused entirely on power fluctuations generated by wind farms. However, the same concept can be applied to other producers of real-power fluctuations, such as the continuous switchi ng of large loads, or new non-dispatchable distributed resources (wave, solar, etc)

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151 APPENDIX A MATHEMATICAL TRANSFORMATIONS In studying of power systems, mathemati cal transformations are often used to decouple variables or to help solve of difficult differentia l equations. The most common and known transformation is the method of symmetrical components developed by Fortescue in 1918 [27]. This method uses a complex transformation to represent an unbalanced system of n phasors into n system s of balanced phasors called symmetrical components of the original phasors. For a three-phase system, these symmetrical components are the positive sequence compone nt, the negative sequence component, and a zero sequence component. This re lationship can be expressed as c b a j j j j c b aX X X e e e e X X X a a a a X X X3 2 2 3 2 2 3 2 3 2 2 2 2 1 01 1 1 1 1 3 1 1 1 1 1 1 3 1 (A-1) Variable X of Equation A-1 may be currents, voltages, or fluxes. Written explicitly in terms of real and imaginary components, we have c b c b a c b a c b c b a c b aX X j X X X X X j X j X X X X j X X X X X j X j X X 2 3 2 1 2 3 21 2 3 2 1 2 3 2 1 2 3 2 1 2 3 2 12 2 1 1 (A-2)

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152 It is evident from Equation A-2 that space vectors 1X and 2X are complex conjugates of each other. For a balanced three-phase system, we have 3 4 cos 3 2 cos cos wt X X wt X X wt X Xm c m b m a (A-3) Thus, the space vectors 1X and 2X can be expressed as * 1 2 12 3 2 3 sin cos 2 3 3 2 sin sin 2 2 3 cos 2 3 3 4 cos 3 2 cos 2 3 cos 2 3 2 3 2 1 2 3 2 3 2 1jwt m jwt m m m m m m c b c b a a c b c b ae X X X e X wt j wt X wt X j wt X wt wt X j wt X X X j X X X X X X j X X X X (A-4) Definingqs ds jwt mX j X e X X the symmetrical transformation matrix equation can be written as c b aX X X a a a a X X X X2 2 2 11 1 2 3 (A-5) Sincea a* 2, row two can be eliminated without any loss of information (Equation A6). c b aX X X a a X21 3 2 (A-6)

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153 Writing the real and imaginary components in two separate rows, and adding the zero-sequence component, we obtain the gene ral real transformation (Equation A-7). c b a o qs dsX X X a a a a X X X 2 1 2 1 2 1 0 1 3 22 2 c b a o qs dsX X X X X X 2 1 2 1 2 1 2 3 2 3 0 2 1 2 1 1 3 2 (A-7) Equation A-7 is known as Clarks transfor mation, and it transforms a three-phase system to an equivalent two-phase system. Figure A-1 shows the relationships of the different axes. Figure A-2 shows the same relations hip, but in the time domain. a axisb axisc axisw=0 ds axisqs axis X f w=0 Figure A-1. Relationships among ds-qs, and abc axes Figure A-2 Stationary ds-qs co mponents in the time domain As shown by Figure A-1, ds-qs axes are stationary, while the space vector X rotates at w=2 f. To achieve high bandwidth in the control algorithm, it is necessary to

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154 transform the stationary variables dsX and qsX into drX and qrX (rotating frame reference). We can deduce that an observer moving at the same speed as X will see this space vector as a constant, un like the time variant dsX and qsX components of the stationary ds-qs axes. Figure A3 shows the geometrica l relationship of the rotating dr-qr axes with respect to the stationary ds-qs axes. qr axis ds axisqs axis X w f w=0 w fdr axis Figure A3. Relationship among ds-qs and dr-qr axes This geometrical relationsh ip can be expressed as qs ds qr drX X X X ) cos( ) sin( sin cos (A-8) The angle is the angle between th e d axis of rotating and X stationary d-q axes; it is a function of the angular speed of the rotating dr-qr axes, that is dt d wt (A-9) Figure A-4 shows the decomposition of dsX and qsX into drX andqrX The angle is the angle between the dr axis and space vector X ; it is a constant vale, and it will depend on the kinds of simplif ication or formulation best suited to a

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155 specific application. The full transformation fr om stationary reference frame to rotating reference frame, including the zero sequence, is shown in Equation A-9. qr axis ds axisqs axis X dr axis XdsXqs Xds cos ( Xqs sin ( w f w f Xds sin ( Xqs cos ( Figure A-4. Direct and quadrature components o qs ds o qr drX X X X X X 1 0 0 0 cos sin 0 sin cos (A-9) Figure A-5 shows the time domain representati on of the three different reference frames, that is abc, ds-qs, and dr-qr, for = 0.

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156 Figure A-5. Time domain represen tation of abc and d-q components In terms of the originalaX ,bX and cX components, c b a o qs ds o qr drX X X X X X X X X 2 1 2 1 2 1 2 3 2 3 0 2 1 2 1 1 1 0 0 0 cos sin 0 sin cos 3 2 1 0 0 0 cos sin 0 sin cos (A-10) c b a o qr drX X X X X X 2 1 2 1 2 1 240 sin 120 sin sin 240 cos 120 cos cos 3 2 (A-11) In short notation abc dqo dqoX T X (A-12) This transformation is called Parks transformation, and it is well-known in synchronous machine analysis. The i nverse transformation can be shown o qr dr c b aX X X X X X 1 240 sin 240 cos 1 120 sin 120 cos 1 sin cos (A-12)

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157 As it can be inferred from previous expression, 1dqo t dqoT T since the matrix dqoT is not unitary. Table A-1 summarizes all the transfor mations that used in our study. Table A-1. Mathematical transformations summary Independent variables Ma trix Transformations aX bX cX c b a o qs dsX X X X X X 2 1 2 1 2 1 2 3 2 3 0 2 1 2 1 1 3 2 dsX qsX oX o qs ds o qr drX X X X X X 1 0 0 0 cos sin 0 sin cos dsX qsX ds qs qs dsX X X X X arctan2 2 drX qrX oX c b a o qr drX X X X X X 2 1 2 1 2 1 240 sin 120 sin sin 240 cos 120 cos cos 3 2

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158 APPENDIX B MATLAB CODES Power Limiter 1 The Matlab code for the control algorithm of the high pass filters is listed below along with comments, signified by the % sign %%%%%%%%%% Inverter size %%%%%%%%%%%%%% load data.txt % load wind power files Pwind=data; % input data in MW Ts=1; % Sampling time Erangen =10; % Energy storag e positive limit-1e9 =Unlimited Erangep =10; % Energy storag e negative limit-1e9 =Unlimited INVL=2; %Inverter power limit -1e9= Unlimited wc=2*pi*0.05; % cut-off frequency %%%%%%%%% Initializatio n%%%%%%%%%%%%%%%%% nt=length(Pwind); Kc=(0.006427/tolerance); % ESS charge/discharge constant P0=Pwind(1,1); IOUTF=0; IINVRF=0; Pcomp=0; Pcomp_buffer=zeros(1,nt); Pwind_old=P0; Plosses=0.0; %%%%%%%%%% Physical syst em description %%%%%% Putility=zeros(1,nt); Energy=zeros(1,nt); Inverter=zeros(1,nt); Vstorage=zeros(1,nt); Pinverter=zeros(1,nt); RRcounter=zeros(1,nt); Icounter=zeros(1,nt); OUT_OF_ENERGY=zeros(1,nt); FULL_POWER=zeros(1,nt); %%%%%%%% System mode l %%%%%%%%%%%%%%%%%% for i=1:nt %%%%%%%%%%%%% Elect rical system%%%%%%%%%% if i==1 Energy(i)=0;

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159 else Energy(i)=Energy(i-1) -(Inverter(i-1)+Plosses)*Ts; end %%%%%%%%%%% HP filter %%%%%%%%%%%%%%%%% Pcomp=(1/(1+Ts*wc))*(Pwind(i)-Pwind_old+Pcomp); Pcomp_buffer(i)=Pcomp; Pwind_old=Pwind(i); IINVRF=-Pcomp; IOUTF=IINVRF; %%%%%%% Max Power Saturation%%%%%%%%%%%%%%% if IINVRF > INVL IOUTF=INVL; FULL_POWER(i)=1; elseif IINVRF < -INVL IOUTF=-INVL; FULL_POWER(i)=1; else IOUTF=IINVRF; end %%%%%%%% Stop inverter due to E>Elimit%%%%%%%%%%% if Energy(i)-IOUTF*Ts<-Erangen IOUTF=(Energy(i)-(sign(-IOUTF)*Erangen))/Ts; OUT_OF_ENERGY(i+1)=1*sign(-IOUTF); Pcomp=0; end if Energy(i)-IOUTF*Ts>Erangep IOUTF=(Energy(i)-(sign(-IOUTF)*Erangep))/Ts; OUT_OF_ENERGY(i+1)=1*sign(-IOUTF); Pcomp=0; end if IOUTF > INVL IOUTF=INVL; FULL_POWER(i)=1; elseif IOUTF < -INVL IOUTF=-INVL; FULL_POWER(i)=1; end %%%%%%%% Update Elec trical system %%%%%%% Inverter(i)= IOUTF+Kc*(Energy(i)-0); Pinverter(i)=IOUTF; Putility(i)=Pwind(i)+Inverter(i); Vstorage(i)=Kc*(Energy(i)-0); end

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160 Power Limiter 2 The Matlab code for the control algorith m of the power limiter 2 is listed below along with comments, signified by the % sign %%%%%%%%%%%%%% Limits Inverter size %%%%%%%%%% Ts=2 % Sampling time R=2.0 % Ramp Rate limit 2MW/min (2 sec scan) A=0.3 %Average limit; 0.3MW/min (2 sec scan) II=1.0 %Instantaneous limit 1MW/2 sec; Erange = 1e9 % Energy storag e range (MJ)-1e9 =Unlimited INVL=1e9 %Inverter power limit (MW) -1e9= Unlimited %%%%%%%%%%%%%% Data Input%%%%%%%%%%%%%% %load data.txt; ( units MW) Pwind=data; %%%%%%%%%%%%%%% Initia lization%%%%%%%%%%%% nt=length(Pwind); N=60/Ts; %samples per minute Kc=1/300; % ESS ch arge/discharge constant P0=Pwind(1,1); %%%%%%%%%%%%%%%%%% Indexes %%%%%%%%%%% J=N+1; M=0; %%%%%%%%%%%%%% Physical sy stem description %%%%%% Putility=zeros(1,nt); Energy=zeros(1,nt); Inverter=zeros(1,nt); RRcounter=zeros(1,nt); Acounter=zeros(1,nt); Icounter=zeros(1,nt); OUT_OF_ENERGY=zeros(1,nt); FULL_POWER=zeros(1,nt); %%%%%%%%%%%%%%%% Buffe rs %%%%%%%%%%%%%% IOPF=P0*ones(1,N);% 2 sec scan buffer initialization IAF=zeros(1,N); % Average buffer initialization Average=0; IOUTF=0; ISWF=0; IOWSF=0; IOWF=0; ICPAF=0; IESARF=0; INDPF=0; IAWSF=0; IAWF=0;

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161 IINVRF=0; INWS=0; INW=0; %%%%%%%%%%%%%%% System model %%%%%%%%%%%% for i=1:nt %%%%%%%%%%%%% Electrica l system %%%%%%%%%%% if i==1 Energy(i)=0; else Energy(i)=Ener gy(i-1)-Inverter(i-1)*Ts; end INWS=Kc*(Energy(i)-0)+Pwind(i); INW=Pwind(i); %%%%%%%%%%%%% Cont rol loop %%%%%%%%%%%%%% J=J-1; %%% 1 minute ago index if J<1 J=N; end M=J+1; %%%% 2 seconds ago index if M>N M=1; end %%%%%%%%%%%%% T op Limiter %% %%%%%%%%%%%%% IOWSF=INWS; %%% Ramp Rate if IOWSF-IOPF(J) > R IOWSF= IOPF(J)+R; RRcounter(i)=1; elseif IOWSF-IOPF(J) < -R IOWSF=IOPF(J)-R; RRcounter(i)=1; end %%% Average IAWSF=Average+(abs(IOPF(M)-IOWSF)-IAF(J))/N; if IAWSF > A if IOPF(M)> IOWSF IOWSF=IOPF(M)-abs((A-Average)*N+IAF(J)); Acounter(i)=1; end if IOPF(M) < IOWSF IOWSF=IOPF(M)+abs((A-Average)*N+IAF(J)); Acounter(i)=1; end end %%% Instantaneous if IOWSF-IOPF(M)> II

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162 IOWSF=IOPF(M)+II; Icounter(i)=1; elseif IOWSF-IOPF(M)< II IOWSF=IOPF(M)-II; Icounter(i)=1; end %%%%%%%%%%%%% Botto m Limiter %%%%%%%%%%%% IOWF=INW; %%% Ramp Rate if IOWF-IOPF(J) > R IOWF= IOPF(J)+R; RRcounter(i)=1; elseif IOWF-IOPF(J) < -R IOWF=IOPF(J)-R; RRcounter(i)=1; end %%% Average IAWF=Average+(abs(IOPF(M)-IOWF)-IAF(J))/N; if IAWF > A if IOPF(M)> IOWF IOWF=IOPF(M)-abs((A-Average)*N+IAF(J)); Acounter(i)=1; end if IOPF(M) < IOWF IOWF=IOPF(M)+abs((A-Average)*N+IAF(J)); Acounter(i)=1; end end %%% Instantaneous if IOWF-IOPF(M)> II IOWF=IOPF(M)+II; Icounter(i)=1; elseif IOWF-IOPF(M)< II IOWF=IOPF(M)-II; Icounter(i)=1; end %%%%%%%%%%%%%% Inve rter Power %%%%%%%%%%%%% ICPAF=IOWSF-IOWF; IESARF=-IOWF+INW; IINVRF=ICPAF-IESARF; %%%%%%%%%%%%%%% Ma x Power Saturation%%%%%%%%% if IINVRF > INVL IOUTF=INVL; FULL_POWER(i)=1; elseif IINVRF < -INVL IOUTF=-INVL;

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163 FULL_POWER(i)=1; else IOUTF=IINVRF; end %%%%%%%%%%%% Stop inverter due to E>Elimit%%%%%%%% if abs(Energy(i)-IOUTF*Ts)>Erange/2 IOUTF=(Energy( i)-(sign(-IOUTF)*Erange/2))/Ts; OUT_OF_ENERGY(i+1)=1*sign(-IOUTF); if IOUTF > INVL IOUTF=INVL; FULL_POWER(i)=1; elseif IOUTF < -INVL IOUTF=-INVL; FULL_POWER(i)=1; end end %%%%%%%%%% Update buffer data %% %%%%%%%%%%%% INDPF=IINVRF-IOUTF; Average=Average+(abs(IO PF(M)-(IOWSF-INDPF))-IAF(J))/N; IAF(J)=abs(IOPF(M)-(IOWSF-INDPF)); IOPF(J)=IOWSF-INDPF; %%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%% Electrica l system %%%%%%%%%%% Inverter(i)= IOUTF; Putility(i)=Pwind(i)+Inverter(i); end Power Limiter 3 The Matlab code for the control algorith m of the power limiter 3 is listed below along with comments, signified by the % sign %%%%%%%%%%%% Limits I nverter size %%%%%%%% Ts=2 % Sampling time R=2.0 % Ramp Rate limit 2MW/min (2 sec scan) A=0.3 %Average limit; 0.3MW/min (2 sec scan) II=1.0 %Instantaneous limit 1MW/2 sec; Erange = 1e9 % Energy storag e range (MJ)-1e9 =Unlimited INVL=1e9 %Inverter power limit -1e9= Unlimited %%%%%%%%%%%%%% Data Input%%%%%%%%% load data.txt; % Data scaled to 10 MW( units MW) Pwind=data; %%%%%%%%%%%%%%% Init ialization%%%%%%%%%% nt=length(Pwind); N=60/Ts; %samples per minute Kc=1/300; % ESS ch arge/discharge constant

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164 P0=Pwind(1,1); J=N+1; M=0; %%%%%%%%%%% Physical system description %%%%%% Putility=zeros(1,nt); Energy=zeros(1,nt); Centering=zeros(1,nt); Inverter=zeros(1,nt); Pold=P0*ones(1,N); Avg=zeros(1,N); Abuffer=zeros(1,N); Average=0; ESArp=zeros(1,nt); ESArn=zeros(1,nt); ESAr=zeros(1,nt); UR=zeros(1,nt); LR=zeros(1,nt); UA=zeros(1,nt); LA=zeros(1,nt); UI=zeros(1,nt); LI=zeros(1,nt); UT=zeros(1,nt); LT=zeros(1,nt); UW=zeros(1,nt); LW=zeros(1,nt); UC=zeros(1,nt); LC=zeros(1,nt); I_counter=zeros(1,nt); RR_counter=zeros(1,nt); A_counter=zeros(1,nt); OUT_OF_ENERGY=zeros(1,nt); FULL_POWER=zeros(1,nt); for i=1:nt %%%%%%%%%%%%% Electrical system %%%%% if i==1 Energy(i)=0; else Energy(i)=Ene rgy(i-1)-Inverter(i-1)*Ts; end %%%%%%%%%%%%% Control loop %%%%%%%%%% J=J-1; %%% 1 minute ago index if J<1 J=N; end M=J+1; %%%% 2 seconds ago index if M>N

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165 M=1; end %%%%% Ramp Rate1%%%%%% UR(i)=Pold(J)+R; LR(i)=Pold(J)-R; %%%%% Subminute1%%%%%% UA(i)=Pold(M)+(ab s(A-Average)*N+Avg(J)); LA(i)=Pold(M)-(abs(A-Average)*N+Avg(J)); %%%%% Instantaneous 1%%%%%% UI(i)=Pold(M)+II; LI(i)=Pold(M)-II; %%%%%%%%% Internal variables UT(i)=min([UR(i) UA(i) UI(i)]); LT(i)=max([LR(i) LA(i) LI(i)]); UW(i)=Pwind(i)-UT(i); LW(i)=Pwind(i)-LT(i); if -UW(i)>0 UC(i)=-UW(i); else UC(i)=0.0; end if -LW(i)<0 LC(i)=-LW(i); else LC(i)=0.0; end if UW(i)>0 ESArp(i)=UW(i); if UT(i)==UR(i) RR_counter(i)=-1; I_counter(i)=0; A_counter(i)=0; elseif UT(i)==UI(i) I_counter(i)=-1; A_counter(i)=0; RR_counter(i)=0; else A_counter(i)=-1; I_counter(i)=0; RR_counter(i)=0; end else ESArp(i)=0.0; end if LW(i)<0 ESArn(i)=LW(i);

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166 if LT(i)==LR(i) RR_counter(i)=1; I_counter(i)=0; A_counter(i)=0; elseif LT(i)==LI(i) I_counter(i)=1; A_counter(i)=0; RR_counter(i)=0; else A_counter(i)=1; I_counter(i)=0; RR_counter(i)=0; end else ESArn(i)=0.0; end ESAr(i)=ESArp(i)+ESArn(i); Centering(i)=Kc*Energy(i); if Centering(i)>UC(i) Centering(i)=UC(i); end if Centering(i) INVL Inverter(i)=INVL; FULL_POWER(i)=1; elseif Inverter(i) < -INVL Inverter(i)=-INVL; FULL_POWER(i)=1; end %%%%%%%%%%%% Stop inverter due to E>Elimit%%%% if abs(Energy(i)-Inverter(i)*Ts)>Erange/2 Inverter(i)=(Energy(i)-(sign(-Inverter(i))*Erange/2))/Ts; OUT_OF_ENERGY (i+1)=1*sign(-Inverter(i)); if Inverter(i) > INVL Inverter(i)=INVL; FULL_POWER(i)=1; elseif Inverter(i) < -INVL Inverter(i)=-INVL; FULL_POWER(i)=1; end end Putility(i)=Pwind(i)+Inverter(i);

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167 %%%%% Update buffer%%% Average=Average+(abs(Pold(M)-Putility(i))-Avg(J))/N; Abuffer(i)=Average; Avg(J)=abs(Pold(M)-Putility(i)); Pold(J)=Putility(i); end

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168 APPENDIX C POWER STABILIZER CONTROL MODULES The following table is a summary of the main functions used in the programming of the power stabilizer. Table C-1. Control Modules File Name C code PLL.h typedef struct { _iq ds; /* Input: Vds */ _iq qs; /* Input: Vqs */ _iq e_pll; /* Variable: Error */ _iq i _pll; /* Variable: Integral output */ _iq p_pll; /*Vari able: Proportional output */ _iq pi_pll; /*Variable: P-I output */ _iq theta; /* Output: PLL output (theta) */ _iq sin_theta; /* Output: sin(theta)*/ _iq cos_theta; /* Output: cos(theta) */ _iq Kp_pll; /* Parameter: P gain */ _iq Ki_pll; /* Parameter: I gain */ _iq wdt; /* System Frequency */ _iq i_pll_out_max; /* Param: Max Int output */ _iq i_pll_out_min; /* Param: Min Int output */ _iq pi_pll_out_max; /* Param: Max PI output */ _iq pi_pll_out_min; /* Param: Min PI output */ _iq deltat; /* Parameter: Time step */ void (*calc)(); /* Pointer to calc func */ } PLL; typedef PLL *PLL_handle; void PLL_calc(PLL_handle); PLL.c #include "IQmathLib.h" /* Include header for IQmath */ #include "pll.h" void PLL_calc(PLL *v) { v->e_pll =_IQmpy(_IQcos(v->theta),v->qs)_IQmpy(_IQsin(v->theta),v->ds); v->i_pll = v->i_pll+ _IQmpy(v->Ki_pll,v->e_pll); v->i_pll = _IQsat(v->i_pll,v->i_pll_out_max,v>i_pll_out_min); v->p_pll = _IQmpy(v->e_pll,v->Kp_pll);

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169 Table C-1. Continued File Name C code PLL.c v->pi_pll= v->p_pll+v->i_pll; v->pi_pll= _IQsat(v-> pi_pll,v->pi_pll_out_max, v>pi_pll_out_min); v->theta= v->theta+_IQm py(v->deltat,v->pi_pll)+v->wdt; if (v->theta>=_IQ(3.141592654)) v->theta=v->theta-_IQ(2*3.141592654); v->sin_theta = _IQsin(v->theta); v->cos_theta = _IQcos(v->theta); } CLARKE.h typedef struct { _iq as; /* Input: phase-a variable */ _iq bs; /* Input: phase-b variable */ _iq cs; /* Input: phase-c variable */ _iq ds; /* Output: stati onary d-axis variable*/ _iq qs;/* Output: stationary q-axis variable*/ _iq os;/* Output: stationary o-axis variable*/ void (*calc)(); /* Pointer to calc func */ } CLARKE; CLARKE.c #include "IQmathLib.h" /* Include header for IQmath */ #include "clarke.h" /* 1/sqrt(3) = 0.57735026918963 */ void clarke_calc(CLARKE *v) { v->ds =_IQmpy(_IQ(0.666666666),v->as)+_IQmpy(_IQ(0.3333333),(v->bs+v->cs)); v->qs =_IQmpy(_IQ( 0.577350269),(v->bs-v->cs)); v->os =_IQmpy(_IQ(0. 333333333),(v->as+v->bs+v->cs)); } PARK.h typedef struct { _iq ds; /* Input: stationary d-axis */ _iq qs; /* Input: stationary q-axis */ _iq sin_theta; /* Input: cos(theta) */ _iq cos_theta; /* Input: sin(theta) */ _iq dr; /* Output: rotating d-axis*/ _iq qr; /* Output: rotating q-axis*/ void (*calc)(); /* Pointe r to calc function */ } PARK; typedef PARK *PARK_handle; void park_calc(PARK_handle);

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170 Table C-1. Continued File Name C code PARK.c #include "IQmathLib.h" /* Include header for IQmath*/ #include "park.h" void park_calc(PARK *v) { v->dr = _IQmpy(v->ds,v->cos_theta) + _IQmpy(v->qs,v>sin_theta); v->qr = _IQmpy(v->qs,v->cos_theta) _IQmpy(v->ds,v>sin_theta); } ICLARKE.h typedef struct { _iq ds; /* Input: ds variable */ _iq qs; /* Input: qs variable */ _iq as; /* Output: stationary a-axis */ _iq bs; /* Output: stationary b-axis */ _iq cs; /* Output: stationary c-axis */ void (*calc)(); /* Pointer to calc function */ } ICLARKE; typedef ICLARKE *ICLARKE_handle; void iclarke_calc(ICLARKE_handle); ICLARKE.c #include "IQmathLib.h" /* Include header for IQmath */ #include "iclarke.h" void iclarke_calc(ICLARKE *v) { v->as = v->ds; v->bs = _IQmpy(_IQ(-0.5), v->ds) + _IQmpy(_IQ(0.8660254038),v->qs) ; v->cs = _IQmpy(_IQ(-0.5 ), v->ds) + _IQmpy(_IQ(0.8660254038),v->qs); } IPARK.h typedef struct { _iq ds; /* Output: stationary d-axis */ _iq qs; /* Output: stationary q-axis */ _iq sin_theta; /* Input: cos(theta) */ _iq cos_theta; /* Input: sin(theta) */ _iq dr;/* Input: rotating d-axis*/ _iq qr;/* Input: rotating q-axis*/ void (*calc)();/* Pointer to calc function */ } IPARK; typedef IPARK *IPARK_handle;

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171 Table C-1. Continued File Name C code IPARK.c #include "IQmathLib.h" /* Include header for IQmath*/ #include "ipark.h" void ipark_calc(IPARK *v) { v->ds = _IQmpy(v->dr,v->cos_theta) _IQmpy(v->qr,v>sin_theta); v->qs = _IQmpy(v->qr,v->cos_theta) + _IQmpy(v->dr,v>sin_theta); } PIANTIWINDUP.h typedef struct { _iq feedback; /* Input: Feedback signal */ _iq e_pi; /* Variable: Error */ _iq i_pi; /* Variable: Integral output*/ _iq p_pi; /* Variable : Proportional output */ _iq pi_pi; /* Variable: PI output */ _iq out; /* Output: PI control action */ _iq ref; /* Parameter: Reference signal */ _iq Kp_pi; /* Paramete r: Proportional gain */ _iq Ki_pi; /* Parameter: Integral gain */ _iq i_pi_out_max; /* Parameter: Max Int */ _iq i_pi_out_min; /* Parameter: Min Int */ _iq pi_pi_out_max; /* Parameter: Max PI */ _iq pi_pi_out_min; /* Parameter: Min PI */ voi d (*calc)(); /* Pointer to calc function */ } PIANTIWINDUP; PIANTIWINDUP.c #include "IQmathLib.h" /* Include header for IQmath*/ #include "PIantiwindup.h" void PIantiwindup_calc( PIANTIWINDUP *v) { v->e_pi = v->ref-v->feedback; v->i_pi = _IQsat(v->i _pi+_IQmpy(v->Ki_pi,v->e_pi), v->i_pi_out_max,v->i_pi_out_min); v->p_pi = _IQmpy(v->e_pi,v->Kp_pi); v->pi_pi=v->p_pi+v->i_pi; v->out=_IQsat(v->p i_pi,v->pi_pi_out_max,v>pi_pi_out_min); }

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172 LIST OF REFERENCES [1] National Renewable Energy Laboratory. (2001, March). Renewable Energy: An Overview. US Department of Energy. En ergy Efficiency and Renewable Energy, Merrifield, VA. [Online]. Available:http://www.eere.energy.gov/consumerinfo/fa ctsheets/renew_energy.html Site last visited April 2005. [2] AWEA's Communications Department. (2004, June). Wind Energy Outlook 2004. American Wind Energy Association, Wa shington, DC. [Online]. Available: http://www.awea.org/pubs/documents/Outlook2004.pdf Site last visited April 2005. [3] AWEA's Communications Department. ( 2004, June). Global wind power growth continues to strengthen.[Online]. American Wind Energy Association, Washington, DC. Available:http://www.awea.org/pubs/documents/globalmarket2004.pdf Site last visited April 2005. [4] Database of State Incentive for Renewa ble Energy. (2005, April). Financial Incentives. Interstate Renewable En ergy Council, Raleigh, NC. [Online]. Available: http://www.dsireusa.org Site last visited April 2005. [5] IEEE Standard for Interconnecting Distri buted Resources with Electric Power Systems, IEEE Std. 1547, 2003. [6] Wind turbine generator systems. Part 21: Measurement and assessment of power quality characteristics of grid connected wind turbines. International Electrotechnical Commission Standard 61400-21. 1999. [7] Z. Lubosny, Wind turbine Operation in Electric Power Systems. New York: Springer, 2003. [8] B. Davidson and A. Price, Solutions to Future Market needs. The role of energy storage, in Proc. CIGRE, 2002, Paper 37-201. [9] New Energy and Industrial Technology Deve lopment Organization. (2003, April). Wind Power Stabilization Technology Development Project. NEDO, Japan. [Online]. Available:http://www.nedo.go.jp/english/a ctivities/2_sin energy/3/P0303 9e.html Site last visited April 2005.

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173 [10] VRB Power Systems Inc. (2004, March) The VRB Energy Storage System: The Multiple Benefits of Integrating the VRB-ESS with Wind Energy Producersa Case Study in MWH Applications. VRB Po wer Systems Inc., Vancouver, Canada. [Online]. Available: http://www.vrbpower.com/vrb_power.html Site last visited April 2005. [11] B. L. Norris, R. J. Parry, and R. M. Hudson, An Evaluation of Windfarm Stabilization and Load Shifting Using the Zinc-Bromide Battery (ZBB), in Proc. Windpower 2002 Portland, Oregon, 2002. [12] Z. Fengquan, G. Joos, C. Abbey, and J. Lianwei, O. T. Boon, Use of large capacity SMES to improve the power quality and stability of wind farms, in Proc. Power Engineering Society General Meeting 2004, pp. 2026. [13] T. Kinjyo, T. Senjyu, K. Uezato, H. Fujita, and T. Funabashi, Output levelling of wind energy conversion system by current source ECS, in Proc. Power Engineering Society General Meeting 2004. pp. 2007. [14] J. H. R. Enslin, J. Knijp, C. P. J. Jansen, and P. Bauer, Integrated approach to network stability and wind energy technology for on-shore and offshore applications, in Proc. Official proceedings in ternational conferences ZM Communications GmbH 2003, pp. 185. [15] F. Hardan, J. A. M. Bleijs, R. Jones, and P. Bromley, Bi-Directional Power Control for Flywheel Energy Storage Syst ems with Vector-Controlled Induction Machine Drive, IEE Power Electronics and Variable Speed Drives publ. 456, pp 477-482, September, 1998. [16] F. Hardan, J. A. M. Bleijs, R. Jones, P. Bromley, and A. J. Ruddell, Application of a Power-Controlled Flywheel Driv e for Wind Power Conditioning in a Wind/Diesel Power System, in Proc. Ninth International Conference on Electrical Machines and Drives 1999, pp 65. [17] H. Bindner and L.H. Hansen, Combini ng wind and Energy storage to improve grid integration, in presented at the IEEE Nordic Workshop on Power and Industrial Electronics, Aalborg, Danmark, 2000. [18] R. Cardenas, R. Pena, G. Asher, and J. Clare, Power smoothing in wind generation systems using a sensorless vector c ontrolled induction Machine driving a flywheel, IEEE Transactions on Energy Conversion vol. 19 pp. 206, March 2004. [19] R. Cardenas, R. Pena, and J. Glare, Control strategy for power smoothing using vector controlled induction machine and flywheel, Electronics Letters vol. 36, pp.765. 13 April 2000.

PAGE 192

174 [20] R. Cardenas, G. Asher, R. Pena, and J. Clare, Power smoothing control using sensorless flywheel drive in winddiesel generation systems, in Proc. Industrial Electronics Society 2002, 3303. [21] R. Cardenas, R. Pena, G. Asher, J. Clare, and R. Blasco-Gimenez, Control Strategies for Power Smoothing Using a Flyw heel Driven by a Sensorless VectorControlled Induction Machine Oper ating in a Wide Speed Range, IEEE Transactions on Industrial Electronics vol. 51, pp. 603, June 2004. [22] R. Cardenas, R. Pena, J. Clare, and G. Asher, Power smoothing in a variable speed wind-diesel system, in Proc. Power Electronics Specialist 2003, pp. 754. [23] I. J. Iglesias, L. Garcia-Tabares, A. Agudo, I. Cruz, and L. Arribas, Design and simulation of a stand-alone wind-diesel ge nerator with a flywheel energy storage system to supply the required active and reactive power, in Proc. Power Electronics Specialists Conference 2000, pp. 1381. [24] S. Wijnbergen, S. W. H. de Haan, and J. Slootweg, Plug 'n' play interface for renewable generators participating in gr id voltage and frequency control, in Proc. IEEE AFRICON 2002, pp. 579. [25] J. A. M. Bleijs, L .L. Freris, D. G. Infield, A. J. Ruddell, and G. A. Smith, A Wind/diesel System With Fl ywheel Energy Buffer, in Proc. International Power Conference Athens Power Tech 1993, pp. 995. [26] EPRI-DOE, EPRI-DOE Handbook of Ener gy Storage for Transmission and Distribution Applications. EPRI, Pa lo Alto, CA, Report 1001834. Dec 2003. [27] C. L. Fortescue, Method of Symmetrical Co-Ordinates Applied to the Solution of Polyphase Networks, Transactions of the American Institute of Electrical Engineers vol. 37, pt. 2, 1918. [28] N. Mohan, T, M. Undeland, and W, P. Robbins, Power electronics: converters, applications, and design New York: Wiley, 1989. [29] IEEE recommended practices and requirement s for harmonic control in electrical power systems, IEEE Std. 519-1992, 12 April 1993. [30] M. Liserre, F. Blaabjerg, and S. Hansen Design and control of an LCL-filter based three-phase ac tive rectifier, in Proc. Industry Appl ications Conference 2001, pp .299. [31] M. Prodanovic and T. C. Green, Control and filter design of th ree-phase inverters for high power quality grid connection, IEEE Transactions on Power Electronics vol. 18, pp. 373, Jan. 2003.

PAGE 193

175 [32] R. Teodorescu, F. Blaabjerg, M. Liserre, and A. Dell'Aquila, A stable three-phase LCL-filter based active rectifier without damping, in Proc. Industry Applications Conference 2003, pp.1552. [33] R. Teodorescu, and F. Blaabjerg, Flexible c ontrol of small wind turbines with grid failure detection operating in stan d-alone and grid-connected mode, IEEE Transactions on Power Electronics vol. 19, pp. 1323, Sept. 2004. [34] S. Bhattacharya, T. M. Frank, D. M. Divan, and B. Banerjee, Active filter system implementation, IEEE Industry Applic ations Magazine 1998, vol. 4, pp.47. [35] T.C.Y. Wang, Z. Ye, G. Sinha; and X. Yuan, Output filter design for a gridinterconnected three-phase inverter, in Proc. Power Electronics Specialist PESC 2003, pp. 779. [36] A. Montenegro, A. Domijan, Jr., K. E. Mattern, and C. W. Edwards, Energy Storage System for Wind Farm Applica tions: Application Methodology, presented at the 7th IASTED International Conferen ce on Power and Energy Systems, Clearwater Beach, USA, 2004. [37] O. I. Elgerd, Electric energy systems theory New York: McGraw-Hill, 1971. [38] J. F. Manwell, J. G. McGowan, and A. L. Rogers, Wind Energy Explained: Theory, Design and Application England: John Wiley & Sons Ltd, 2002. [39] A. E. Feijoo and J. Cidras, Modeling of wind farms in the load flow analysis, IEEE Transactions on Power System s, vol. 15, pp. 110 Feb. 2000. [40] Y. H. Wan, Wind Power Plant Behavior s: Analyses of Long-Term Wind Power Data, NREL, Golden, CO., Report No. CP-500-36551, September 2004. [41] Y. H. Wan and D. Bucaneg, Short-Term Power Fluctuations of Large Wind Power Plants, NREL, Golden, CO., Report No. 33842. November 2002. [42] Texas Intruments C28x IQMath Librar y A Virtual Floating Point Engine, Texas Intruments, Dallas, TX, Tech. Rep. SPRC087. March 2003. [43] G. R. Walker, "Modulation and Contro l of Multilevel Converters," PhD dissertation, University of Queen sland, Brisbane, Australia,1999.

PAGE 194

176 BIOGRAPHICAL SKETCH Alejandro Montenegro obtaine d his BS degree in Electr onic Engineering from the Polytechnic University of Valencia (UPV) in 1995 and his MS in Industrial Engineering from UPV in 2000. He has been a research assistant at the Depa rtment of Electrical Engineering (UPV) and is curr ently a research assistant and a Ph.D. candidate at the University of Florida in the Department of Electrical and Comput er Engineering. His field of interest is reliability and power quality of distribution networks using power electronic devices.


Permanent Link: http://ufdc.ufl.edu/UFE0010112/00001

Material Information

Title: Advanced power electronic for wind-power generation buffering
Physical Description: Mixed Material
Language: English
Creator: Montenegro León, Alejandro ( Dissertant )
Domijan, Alexander. ( Thesis advisor )
Ngo, Dr. ( Reviewer )
Arroyo, A. A. ( Reviewer )
Goswami, Dr. ( Reviewer )
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2005
Copyright Date: 2005

Subjects

Subjects / Keywords: Electrical and Computer Engineering thesis, Ph.D
Power electronics   ( lcsh )
Wind power   ( lcsh )
Wind power plants   ( lcsh )
Dissertations, Academic -- UF -- Electrical and Computer Engineering

Notes

Abstract: Advanced power electronic for wind power generation buffering. As the cost of installing and operating wind generators has dropped, and the cost of conventional fossil-fuel-based generation has risen, the economics and political desirability of more wind-based energy production has increased. High wind-power penetration levels are thus expected to augment in the near future raising the need for additional spinning reserve to counteract the effects of wind variations. This solution is technologically viable, but it has high associated costs. Our study presents a different solution to short-term wind-power variability, using advanced power electronic devices combined with energy-storage systems. New control schemes (designed to filter power swings with a minimum of energy) were designed, modeled and verified through experimental tests. We also determined the procedure to extract the corresponding per-unit model parameters for simulations and test purposes. We first reviewed D-Q transformations with emphasis on modeling of the system and control algorithm. System components were then designed using criteria similar to those used to design medium-voltage power products. We tested a proof-of-concept for performance of the power converter in a scaled-down isolated system using real wind-power data. Tests were conducted under realistic system conditions of wind-penetration level and energy-storage levels, to better characterized the impacts and benefits of the Power Stabilizer. We described the scaled-down isolated electric power system used in the testing. We also analyzed the performance of the wind-farm model and the synchronous machine's governor to gain an insight into the model system's limitations. Simulation results carried out in Mathematical Laboratory (MATLAB) and Power Systems Computer Aided Design (PSCAD) were compared to experimental data to verify the performance of the power converter under different system conditions and algorithms. Power limiters were also contrasted and evaluated for frequency deviations and attenuated power fluctuations. In summary we can say that, among all the power limiters considered in our study, the adaptive high pass filter presented the best performance in terms of system robustness and effectiveness.
Subject: electronics, energy, filter, inverter, isolated, Power, statcom, storage, tubine, wind, windpower, winfarm
General Note: Title from title page of source document.
General Note: Document formatted into pages; contains 194 pages.
General Note: Includes vita.
Thesis: Thesis (Ph.D.)--University of Florida, 2005.
Bibliography: Includes bibliographical references.
General Note: Text (Electronic thesis) in PDF format.

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 003322409
System ID: UFE0010112:00001

Permanent Link: http://ufdc.ufl.edu/UFE0010112/00001

Material Information

Title: Advanced power electronic for wind-power generation buffering
Physical Description: Mixed Material
Language: English
Creator: Montenegro León, Alejandro ( Dissertant )
Domijan, Alexander. ( Thesis advisor )
Ngo, Dr. ( Reviewer )
Arroyo, A. A. ( Reviewer )
Goswami, Dr. ( Reviewer )
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2005
Copyright Date: 2005

Subjects

Subjects / Keywords: Electrical and Computer Engineering thesis, Ph.D
Power electronics   ( lcsh )
Wind power   ( lcsh )
Wind power plants   ( lcsh )
Dissertations, Academic -- UF -- Electrical and Computer Engineering

Notes

Abstract: Advanced power electronic for wind power generation buffering. As the cost of installing and operating wind generators has dropped, and the cost of conventional fossil-fuel-based generation has risen, the economics and political desirability of more wind-based energy production has increased. High wind-power penetration levels are thus expected to augment in the near future raising the need for additional spinning reserve to counteract the effects of wind variations. This solution is technologically viable, but it has high associated costs. Our study presents a different solution to short-term wind-power variability, using advanced power electronic devices combined with energy-storage systems. New control schemes (designed to filter power swings with a minimum of energy) were designed, modeled and verified through experimental tests. We also determined the procedure to extract the corresponding per-unit model parameters for simulations and test purposes. We first reviewed D-Q transformations with emphasis on modeling of the system and control algorithm. System components were then designed using criteria similar to those used to design medium-voltage power products. We tested a proof-of-concept for performance of the power converter in a scaled-down isolated system using real wind-power data. Tests were conducted under realistic system conditions of wind-penetration level and energy-storage levels, to better characterized the impacts and benefits of the Power Stabilizer. We described the scaled-down isolated electric power system used in the testing. We also analyzed the performance of the wind-farm model and the synchronous machine's governor to gain an insight into the model system's limitations. Simulation results carried out in Mathematical Laboratory (MATLAB) and Power Systems Computer Aided Design (PSCAD) were compared to experimental data to verify the performance of the power converter under different system conditions and algorithms. Power limiters were also contrasted and evaluated for frequency deviations and attenuated power fluctuations. In summary we can say that, among all the power limiters considered in our study, the adaptive high pass filter presented the best performance in terms of system robustness and effectiveness.
Subject: electronics, energy, filter, inverter, isolated, Power, statcom, storage, tubine, wind, windpower, winfarm
General Note: Title from title page of source document.
General Note: Document formatted into pages; contains 194 pages.
General Note: Includes vita.
Thesis: Thesis (Ph.D.)--University of Florida, 2005.
Bibliography: Includes bibliographical references.
General Note: Text (Electronic thesis) in PDF format.

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 003322409
System ID: UFE0010112:00001


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ADVANCED POWER ELECTRONIC FOR
WIND-POWER GENERATION BUFFERING
















By

ALEJANDRO MONTENEGRO LEON


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2005
































Copyright 2005

by

Alej andro Montenegro Le6n


































To my brother
















ACKNOWLEDGMENTS

I would like to Birst express my gratitude to Charles Edwards, the principle

engineer at S&C Electric Co. (Chicago, 1L) for his patience and the knowledge he shared

throughout the proj ect. I would also like to acknowledge Kenneth Mattern (manager at

S&C Electric Co., Power Quality Division) for his constant encouragement and

confidence in my ability. I am grateful to S&C Electric Company in general for all of

their contribution and concern.

Additionally, I would like to thank Alexander Domij an (my supervisory committee

chair) for his funding during my graduate studies. My gratitude also goes to my

supervisory committee (Dr. Ngo, Dr. Arroyo, and Dr. Goswami) for all of their time and

effort.

I would furthermore like to acknowledge my family in Spain, for supporting me

and believing in me throughout my stay in the United States. I would Einally like to

express my love and gratitude to my girlfriend, Andrea Victoriano, for her help with the

proofreading and for always being the shoulder I could lean on throughout the proj ect



















TABLE OF CONTENTS


page

ACKNOWLEDGMENT S .............. .................... iv


LI ST OF T ABLE S ................. ................. viii............


LIST OF FIGURES .............. .................... ix


AB STRAC T ................ .............. xvii


CHAPTER


1 INTRODUCTION ................. ...............1.......... ......


Wind-Ener gy Outlook .............. ...............1.....
Electrical Issues .............. .... ........ ..............
Solutions to Wind-Power Fluctuations ................. ...............9................
State of the Art. ................ .......... .................. .........................9

Obj ective ................. ...............11.................

2 SY STEM DESIGN ................. ...............14........... ....


Introducti on ................. ...............14.................
Control Scheme ................ ...... ...............1
Positive Sequence Calculation .............. .... ...............14.
Real Power Calculation Using dq Components .............. .....................2
Phase Locked Loop .............. ...............21....
Control Al gorithm Design ................. ...............26........... ....
Inner regulators .............. ...............27....
Outer regulators ................. ...............35.................
Per-Unit System Model .............. ...............56....
Inverter Output-Filter Design ................. ...............56........... ....
Harmonic content ................. ...............57.......... .....
Switching frequency ................. ...............60.................
Passive filter design............... ...............61.
Passive filter damping .............. ...............65....
Direct-Current Link Capacitor Design .............. ...............68....
Energy Storage Design ................. ...............69........... ....
Chopper Inductor Design .............. ...............71....
Per-Unit System Model Summary............... ...............72
Simulated Model ................. ...............73.................












3 SY STEM DESCRIPTION ................. ...............78........... ....


Sy stem Overview............... ...............78
Electrical Network Model ................. ...............80........... ....

Synchronous Machine .............. ...............80....
Voltage regulation .............. ...............8 1....
Prim e M over ................... ........ ....... .... ...............8 1.

Synchronous Machine Control Algorithm .............. ...............83....
Wind-Farm Model .............. .... ...............87..
Wind-Farm Control Al gorithm............... ...............9
Wind-Farm Power-Factor Correction............... ...............9
Wind-Farm Soft-Start System .............. ...............94....
Power Stabilizer................... ... .. .........9
Power-Stabilizer Hardware Description............... ..............9
Interface board............... ...............99.

Digital signal processor .....__.....___ ..........._ ............10
Field-programmable gate array .............. ...............106....
Intelligent power module .............. ...............107....
Isolation interface circuit............... ...............108
Power Stabilizer Software Description .............. ...............108....
Description of DSP program ............_.. ....__.....__ ...........10
FPGA program description ................. ...............114................


4 SY STEM PERFORMANCE ................. ......... ...............121 .....


System D ata ............... .. .......... ...............121......
Power Stabilizer Transient Response .............. ...............121....
Direct-Current Link Voltage Control .............. ...............121....
Reactive Current Control ................. ...............123................
Passive Filter Performance .............. ...............126....
Voltage Regulation ................ ...............127................
System Losses................. ...............12
Power Limiter Results .............. ........... ..............13
Power Limiter 1 (High Pass Filter) ............... ...............131...
Power Limiter 1 (Adaptive High Pass Filter) ................. .........................136
Power Limiter 2 ................... .......... ...............138......
Power Limiters Comparison Study ................. ...............145...............


5 SUMMARY ................. ...............148................


Conclusions............... ..............14
Further W ork .............. ...............150....


APPENDIX


A MATHEMATICAL TRANSFORMATIONS ................. .............................151


B MATLAB CODES .............. ...............158....












C POWER STABILIZER CONTROL MODULES .............. ...............168....


LIST OF REFERENCES ................. ...............172................


BIOGRAPHICAL SKETCH ................. ...............176......... ......



















LIST OF TABLES


Table pg


1-1 Technical specifications oflIEC and IEEE ......____ ..... ... .__ ........_........

1-2 Wind-farm output-power requirements .....__.....___ ..........._ .............

1-3 Large-scale wind-power output-leveling proj ects ...._ ................ .. .............10

1-4 Conceptual wind-power filtering proj ects ................. ........._ ...... 12___ ...

1-5 Basic system configurations ............ .....___ ...............13..

2-1 Outer regulator assignation .............. ...............35....

2-2 Rate-of-change limits or PPA for a 10 MW wind farm ................... ...............4

2-3 Generalized Harmonics of line-to-line voltage .............. ...............59....

2-4 L filter vs. LCL filter............... ...............61.

2-5 LC L filter design .............. ...............64....

2-6 LCL equivalent impedance with damping resistance .............. ....................6

2-7 Per-unit sy stem ............ ..... .._ ...............65...

2-8 Per-unit system parameters .............. ...............73....

2-9 Designed system results and simulated system results comparison ......................76

3-1 Synchronous machine output voltage profile at rated speed ............... .........._....82

3-2 Alternatives for the power stabilizer controller............... ..............10

3-3 FPGA code words .............. ...............120....

4-1 System parameters............... ..............12

A-1 Mathematical transformations summary ................. ...............................157

C-1 Control Modules............... ...............168


















LIST OF FIGURES


Figure pg

1-1 Wind-power output for two wind farms during one month. .................. ...............5

1-2 Power fluctuation comparison............... ...............

1-3 Typical power curve of a wind turbine. ............. ...............6.....

1-4 Wind-farm output power vs system frequency. ............. ...............7.....

1-5 Control strategies along the power curve ................. ...............8......._._. ..

1-6 Wind-farm generation buffering concept .................._...... .........._. .......1

2-1 Unbalanced system............... ...............15.

2-2 Space vector trajectory of an unbalanced system in the d-q-o plane ...................... 16

2-3 Space vector traj ectory proj section over the d-q plane ........._._ ...... .._............16

2-4 Direct and quadrature components of an unbalanced system .............. ..................17

2-5 Representation of an unbalanced system in the frequency domain......................... 17

2-6 Positive-sequence extraction algorithm .............. ...............19....

2-7 Voltage waveforms for an unbalanced fault event ................. ....._._ ............19

2-8 Response of the positive-sequence extraction algorithm ................. ................. .20

2-9 Distortion of phase angle due to a negative sequence component ................... .......22

2-10 PLL diagram............... ...............23

2-11 PLL simplified model ................. ...............24................

2-12 PLL system step response .............. ...............25....

2-13 Root locus for two different regulator gains .............. ...............25....

2-14 PLL system response to an unbalanced system condition ................... ...............26











2-15 PLL system response to a frequency excursion .............. ..... ............... 2

2-16 System description .............. ...............27....

2-17 Simplified system model ........._.._.. ...._... ...............28...

2-18 Electrical representation of the dq components ........._.._.. ......._ ........._.....30

2-19 System model block diagram .............. ...............30....

2-20 Inverter current regulator-system model block diagram ................ ................ ...31

2-21 Inverter current regulator-system model simplified block diagram ................... .....32

2-22 Simplified current control diagram .............. ...............32....

2-23 Current regulator step response ......... ........_____ ..... ........ .......33

2-24 Chopper equivalent system .............. ...............34....

2-25 Chopper current controller .............. ...............35....

2-26 Powers' definition ............ __.. ......... ...............36....

2-27 System model .............. ...............37....

2-28 DC link equivalent system block diagram .............. ...............37....

2-29 DC link simplified system block diagram ................. ....___ .............. .....3

2-30 DC link voltage regulator step response .............. ...............38....

2-31 Simplified system model ................. ...............40.____.....

2-32 Source impedance voltage drop .............. ...............41....

2-33 Transfer functions of inverter' s quadrature current component. ................... ..........42

2-34 Transfer functions of inverter' s direct current component ................. ................ .42

2-35 Voltage regulator system block diagram ................. ...............44........... ..

2-36 Positive sequence extraction algorithm equivalent system ................ ................ .44

2- 37 Voltage regulator detailed block diagram .............. ...............45....

2- 38 Voltage regulator simplified control diagram .............. ..... ............... 4

2- 39 System response to a 5% change in voltage reference ................. .....................45











2-40 Adaptive control scheme .........__.._ ....._... ...............46.....

2-41 Power Regulator general control scheme ........._..._.. ....._.._ ......._._. .......4

2-42 Power limiter 1. Control block diagram ........._..._.. ....._.._ ..................4

2-43 Power limiter 1. Performance using different cut-off frequencies (unlimited
power and energy) ........._..._. ....._... ...............49.....

2-44 Power limiter 1. Performance using different cut-off frequencies (Pinverter=1.0O
MW and Einverter=+8.5 MJ) .............. ...............49....

2-45 Power limiter 2. Limiters details ....__. ...._.._.._ ......._.... ..........5

2-46 Power limiter 2. Control block diagram ........._.._.. ....._.. ......._.._........5

2-47 Power limiter 2. Compensation performance............... ..............5

2-48 Power limiter 2. Inverter response for a sampling time of 2 seconds .....................52

2-49 Power limiter 2. Inverter response for different power ratings. Sampling time 2
second s .............. ...............53....

2-50 Power limiter 2. Inverter response for different ESS sizes. Sampling time 2
second s .............. ...............53....

2-51 Power limiter 3. Control block diagram ........._.._.. ....._.. .......__. .......5

2-52 Power limiter 3. Compensation performance............... ..............5

2-53 Power limiter 3. Inverter response for a sampling time of 2 seconds............._.._. ...55

2-54 Inverter topology .............. ...............57....

2-55 Line-to-line and line-to-neutral voltage of a three phase inverter...........................57

2-56 RMS Line-to-line voltage harmonic spectrum ................. .......... ................5 8

2-57 Static Synchronous Generator diagram ................. ...............59........... ...

2-58 LC L filter topology .............. ...............61....

2-59 LCL equivalent block diagram ................. ...............62........... ...

2-60 Single phase equivalent filter model at the fundamental frequency .......................62

2-61 Single phase equivalent filter model at the hth harmonic .............. ....................63

2-62 LCL equivalent impedance with damping resistance .............. .....................6











2-63 Single phase harmonic generator equivalent circuits ................. ......................66

2-64 LCL gain frequency response .............. ...............67....

2-65 Inverter frequency analysis .............. ...............67....

2-66 Capacitor Voltage vs. Energy Storage .............. ...............70....

2-67 ES S-Chopper topology ................. ...............71................

2-68 Equivalent circuit for maximum current ripple calculation .................. ...............72

2-69 System overview .............. ...............74....

2-70 Per-unit electric system model .............. ...............74....

2-71 Power Stabilizer Control Scheme .............. ...............75....


3-1 Equivalent system model .............. ...............79....

3-2 DC gen-set............... ...............83

3-3 Two single quadrant chopper circuit ................. ...............83...............

3-4 Synchronous generator control system .............. ...............84....

3-5 Frequency deviation .............. ...............85....

3-6 DC-GEN set control scheme ................ ...............85........... ...


3-7 System frequency response for Af=-1Hz ................. ...............86........... ..

3-8 Frequency control equivalent system ................. ...............87........... ...

3-9 Equivalent model frequency response for Af= 0.01666 pu ................. ...............88

3-10 Dynamic model used for transient studies .............. ...............88....

3-11 Static model used for steady-state studies............... ...............88

3-12 Wind-farm model .............. ...............89....

3-13 Wind-farm controller ................. ...............90........... ....


3-14 Wind-farm power regulator & current regulator step response (AP=100%) ..........91

3-15 Induction generator PQ curve .............. ...............92....

3-16 Wind-farm PF correction capacitor bank ................. ...............93...............











3-17 PF correction capacitor bank current waveforms ................. ................ ...._.93

3-18 Capacitor bank impedance frequency scan .............. ...............94....

3-19 Machine control scheme operating states ................. ...............................95

3-20 Electric power system start-up .............. ...............96....

3-21 Detail of the transition from start-up mode to run mode ................. ................ ..96

3-22 Power Stabilizer system overview .............. ...............97....

3-23 Interface board overview............... ...............10

3 -24 AC voltage scaling circuit (input [-1000+1000V], output [0 +3V]) ................... ..101

3-25 DC voltage scaling circuit (input [0 +1000V], output [0 +3V]) ................... ........101

3-26 CT current scaling circuit (input [-5 +5A], output [0 +3V]) ................. ...............101

3-27 LEM current scaling circuit (input [-0.36 +0.36A], output [0 +3V])....................101

3-28 Power supplies' voltage monitoring ................. ...............102............

3-29 System' s critical signals during turn on ......___ ..... .._.. ....._._........10

3-30 System' s critical signals during turn off ........._._ ...... .__ .. ...._._.......10

3-31 Darlington drivers .............. ...............104....

3-32 IPM status signals interface circuitry ..............._ ......... ........... .........0

3-33 DAC circuit ................. ...............105...............

3-34 DSP built-in PWM output performance vs. FPGA ................. ............ .........107

3-35 IMP power circuit configuration .....__.....___ ..........._ ...........10

3-36 Isolated interface board ............ ..... .__ ...............109..

3-38 Power stabilizer control algorithm sampling rates ......____ ..... ...___...........110

3-37 Interconnections between the different sub-systems of the power stabilizer........ 111

3-39 Power stabilizer control stages ................. ...............113........... ...

3- 40 Power stabilizer start-up sequence ................. ...............113..............

3-41 FPGA system overview ................. ...............116...............











3 -42 Up/Down counter ................. ...............117...............

3-43 PWM generator ................. ...............117...............

3 -44 One phase dead-time generator detailed diagram ................. .......................119

3-45 Dead-time generator' s waveforms ................. ...............119........... ...

3-46 W watchdog logic .............. ...............120....

4-1 DC link voltage response for different Kp gains............... ...............121.

4-2 DC link voltage response for different Ki gains .............. .....................2

4-3 Iqrer command step change from -0.5 to 0.5 A per unit. Integral gain effect ........123

4-4 Iqrer command step change from -0.5 to 0.5 A per unit. Proportional gain
effect ................. ...............124................

4-5 Iq current regulator output for different Kp .............. ...............124....

4-6 Iqref command step change from -0.5 to 0.5 and back to -0.5 A per unit ............124

4-7 Power stabilizer harmonic inj section response for Ki=18 and Kp=1 ................... ..125

4-8 Current regulator frequency response .............. ...............126....

4-9 Front-end inverter current waveform ................. ...............126........... ...

4-10 Frequency spectrum of the LCL currents ........._.._.. ....._.. ........._.._.......2

4-11 Simplified system description ...._.._.._ ..... .._._. ...._.._ ...........2

4-12 Power stabilizer voltage regulation performance ........._._. .........._._............128

4-13 Energy storage charge/discharge cycle .............. ...............129....

4-14 Control scheme with a losses compensation term ................. ........... ...........129

4-15 Power stabilizer equivalent system .............. ...............130....

4-16 Wind-power conditions under study .............. ...............130....

4-17 Measured and modeled high pass filter results for Ke=0.0064 W/J,
fat off-0.005 Hz ............ ..... .._ ...............132..

4-19 Measured high pass filter performance for different cut-off frequencies. System
parameters Ke=0.0064 W/J............... ...............134..











4-20 Modeled high pass filter performance for different cut-off frequencies. System
parameters Ke=0.0064 W/J............... ...............135..

4-21 Measured high pass filter performance for different energy storage sizes.
System parameters, Ke=0.0064 W/J, fcut-off-0.005 Hz. ............. .....................13

4-22 Cut-off frequency traj ectory of the adaptive high pass filter for a given energy
deviation ................. ...............136................

4-23 Measured adaptive high pass filter performance for different Kf' s. System
parameters, Ke=0.0064 W/J, fcut-off-origin=0.005 Hz..........._.._.. .......__. ..........137

4-24 Measured adaptive high pass filter performance for different energy storage
sizes. ............. ...............137....

4-25 Multiple sampling concept. .............. ...............139....

4-26. Measured and modeled power limiter 2 results for Ke=0.0064, RR=2
MW/minute, A=0.3 MW/minute, I=1MW/2 seconds fcut-on-0.005 Hz. ................140

4-27 Measured power indexes activity. System parameters: Ke=0.0064 W/J, RR=2
MW/minute, A=0.3 MW/minute, I=1 MW/2 seconds, and fs=10Hz. ................... .141

4-28 Measured power limiter 2 response to different K, System parameters: RR=2
MW/minute, A=0.3 MW/minute, I=1MW/2 seconds, and fs=10Hz ................... ...142

4-29 Measured power limiter 2 response to different ramp rate limits. ........................142

4-30 Measured power limiter 2 response to different average power fluctuation
lim its. ........... ........... ...............143....

4-31 Effect of linear interpolation on the average power fluctuation index activity.
The sampling time of the original wind-power data is 2 seconds ................... .......144

4-32 Measured power limiter 2 response to different instantaneous power fluctuation
lim its ................. ...............144................

4-33 Measured power limiter 2 response to different sampling frequencies.................145

4-34 Measured synchronous machine output power for the different power limiter
control schemes ................. ...............146......... ......

4-35 Measured synchronous machine output power for the different power limiter
control schemes. ............. ...............147....

4-36 Frequency regulator output for the different power limiters. .............. .... ...........147

A-1 Relationships among ds-qs, and abc axes .............. ...............153....











A-2 Stationary ds-qs components in the time domain ........................... ...............153

A- 3 Relationship among ds-qs and d,-gr axes .............. ...............154....

A-4 Direct and quadrature components ................. ...............155........... ...

A-5 Time domain representation of abc and d-q components .............. ................... 156
















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

ADVANCED POWER ELECTRONIC FOR
WIND-POWER GENERATION BUFFERING

By

Alej andro Montenegro Le6n

May 2005

Chair: Alexander Domijan, Jr
Major Department: Electrical and Computer Engineering

As the cost of installing and operating wind generators has dropped, and the cost of

conventional fossil-fuel-based generation has risen, the economics and political

desirability of more wind-based energy production has increased. High wind-power

penetration levels are thus expected to augment in the near future raising the need for

additional spinning reserve to counteract the effects of wind variations. This solution is

technologically viable, but it has high associated costs. Our study presents a different

solution to short-term wind-power variability, using advanced power electronic devices

combined with energy-storage systems. New control schemes (designed to filter power

swings with a minimum of energy) were designed, modeled and verified through

experimental tests. We also determined the procedure to extract the corresponding per-

unit model parameters for simulations and test purposes.










We first reviewed D-Q transformations with emphasis on modeling of the system

and control algorithm. System components were then designed using criteria similar to

those used to design medium-voltage power products.

We tested a proof-of-concept for performance of the power converter in a scaled-

down isolated system using real wind-power data. Tests were conducted under realistic

system conditions of wind-penetration level and energy-storage levels, to better

characterized the impacts and benefits of the Power Stabilizer. We described the scaled-

down isolated electric power system used in the testing. We also analyzed the

performance of the wind-farm model and the synchronous machine' s governor to gain an

insight into the model system's limitations.

Simulation results carried out in Mathematical Laboratory (MATLAB) and Power

Systems Computer Aided Design (PSCAD) were compared to experimental data to verify

the performance of the power converter under different system conditions and algorithms.

Power limiters were also contrasted and evaluated for frequency deviations and

attenuated power fluctuations.

In summary we can say that, among all the power limiters considered in our study,

the adaptive high pass filter presented the best performance in terms of system robustness

and effectiveness.


XV111















CHAPTER 1
INTTRODUCTION

Wind-Energy Outlook

Wind power has been used for at least 3000 years, mainly for milling grain,

pumping water, or driving various types of machines. However, the first attempt to use

wind turbines for producing electricity date back to the 19th century. In 1891, Poul La

Cour in Demark built an experimental wind turbine driving a dynamo. The oil crisis of

the 1970s revived interest in wind turbines. Nowadays, the power is the fastest growing

source of energy in the world and its growth rates have exceeded 30% annually over the

past decade [1]. Cumulative global wind-energy generating capacity approached 40,000

MW by the end of 2003 [2]-[3]. The main drivers for developing of the wind industry in

the United States are

* Federal Renewable Energy Policies, particularly the Production Tax Credit (PTC)
that provides a 1.5 cent per kilowatt-hour credit for electricity produced from a
wind farm during the first 10 years of operation. This wind energy PTC expired
December 3 1, 2003 but will be reinstated through 2005 as part of a maj or tax
package (H.R. 1308).

* State-level renewable energy initiatives, such as the Renewable Portfolio Standard,
or green pncmig.

The Database of State Incentive for Renewable Energy [4] gives more information

on incentives. These government initiatives, together with technological advances, plus

the need for a new source of energy capable of meeting the world' s growing power

demand and the rising prices of conventional fossil fuel-based generation, make the wind

power one of the most promising industries in the future.










According to the European Wind Energy Association and Greenpeace, no barriers

exist for wind to provide 12% of the world' s electricity by 2020. The American Wind

Energy Association forecasts that wind power will provide 6% of the US's electricity by

2020 if the wind industry maintains an annual growth rate of 18%.

The positive effects of using such types of renewable resources are well known.

However, wind-power plants, like all other energy technology, have some drawbacks that

should be mentioned. These problems can be divided into maj or groups: environmental

issues and interconnection issues.

Environmental issues. Most significant among these are the following:

* Sound from turbines: Some wind turbines built in the early 1980s were very
noisy. However, manufactures have been working on making the turbines quieter.
Today, an operating wind farm at a distance of 750 to 1,000 feet is no noisier than a
moderately quiet room. Research in aero-acoustics is still being carried out to
further reduce noise from wind on the blades.

* Bird death: Wind turbines are often mentioned as a risk to birds, and several
international tests have been performed. The general conclusion is that birds are
seldom bothered by wind turbines. Studies show that for example, overhead power
pole lines are far more hazardous for birds than wind turbines [2].

* Wind-tower shadow effect: Wind turbines, like other tall structures cast a shadow
on the neighboring area when the sun is visible. It may be irritating if the rotor
blades chop the sunlight, causing a flickering effect while the rotor is in motion,
especially when the sun is low in the sky.

Interconnection issues. Connecting wind turbine to operate in parallel with the

electric power system influences the system operating point (load flow, nodal voltages,

power losses, etc). These changes in the electric power system state bring up new system-

integration issues that system operators and power quality engineers must take into

account. These interconnection issues can be divided into operational issues and electrical

issues.

* Operational Issues: These include unit commitment and spinning reserve.










-The unit commitment problem is to schedule specific or available
generators (on or off) on the utility system to meet the required loads at a
minimum cost, subj ect to system constraints. The most conservative
approach to unit commitment and economic dispatch is to discount any
contribution from interconnected wind resources because of wind
variability.

-Operating reserve is further defined to be a spinning or non- spinning
reserve. Any probable load or generation variations that cannot be
forecasted, such as wind power, have to be considered when determining
the amount of operating reserve to carry out.

* Electrical issues: These factors are considered in the next section.

Electrical Issues

Wind-turbine generator-system operation has some negative influence on power

systems. This influence on the electric power system depends on wind variations and on

wind-turbine technology. Impacts on the electric power system can be grouped as

follows:

* Power quality: Voltage variations, flicker, harmonics, power-flow variations
* Voltage and angle stability
* Protection and control

The IEEE 1547 [5] and the IEC 61400-21 [6] standards are the bases to evaluating

the impact of such wind-turbine generation systems on the electric power system.

According to the IEEE 1547 [5, page 2] abstract,

This standard focuses on the technical specifications for, and testing of, the
interconnection itself. It provides requirements relevant to the performance,
operation, testing, safety considerations, and maintenance of the interconnection. It
includes general requirements, response to abnormal conditions, power quality,
islanding, and test specifications and requirements for design, production,
installation evaluation, commissioning, and periodic tests. The stated requirements
are universally needed for interconnection of distributed resources (DR), including
synchronous machines, induction machines, or power inverters/converters and will
be sufficient for most installations. The criteria and requirements are applicable to
all DR technologies, with aggregate capacity of 10 MVA or less at the point of
common coupling, interconnected to electric power systems at typical primary
and/or secondary distribution voltages.


























































MW, and the large one has a capacity of 150 MW.


Switching operation
Harmonics


According to the IEC 61400-21 [6, page 9] abstract,

The purpose of this part of IEC 61400 is to provide a uniform methodology that
will ensure consistency and accuracy in the measurement and assessment of power
quality characteristics of grid connected wind turbines (WTs). In this respect the
term power quality includes those electric characteristics of the WT that influence
the voltage quality of the grid to which the WT is connected.

This standard provides recommendations for preparing the measurements and
assessment of power quality characteristics of grid connected WTs.

Table 1-1 shows technical specifications for interconnection and power assessment

covered in both standards.


Table 1-1. Tec


chnical specifications of IEC and IEEE
Interconnection system
respnseoexursonsPower quality assessment
IEEE Voltage
Frequency
IEC Voltage Voltage fluctuations:
Frequency Continuous operation


As shown in Table 1-1, both standards overlooked one of the most significant

characteristics of wind farms: its variability (i.e., power fluctuations) [7], the most

important ones being

* Gusty wind variations having a spectrum of frequencies from 1-10 Hz.

* Shadow effect having a spectrum of frequencies from 1-2 Hz and producing torque
variations up to 30%.

* Complex oscillations of the turbine tower, rotor shaft, gear box, and blades with
spectrum frequencies from 2-100 Hz, and creating torque variations up to 10%.

Figure 1-1 shows actual output power data collected by NREL from two large

wind-power plants in the United States. The small wind farm has a capacity of about 35

























0 0.5 1 1.5 2 2.53
Time (s) x 10

Fiue150 idpwrotu o w idfam uigoemnh(a 03.A










coeu Figure 1-1. Wi nd-oe soututfo two wginud ofarm es drn poner month (ay200).A






Wind turbine manufactures usually provide power curves (Figure 1-3) to

developers to determine the amount of power that will be transferred into the grid for a

single turbine, given the wind speed. However, those figures represent only the mean

values, since a series of stochastic values cannot be controlled, and create additional

power fluctuations.

Wind-output power fluctuations can have different effects on the electric power

system, but the most significant ones are voltage variation and frequency variation in

small or isolated systems.

















0 A
0 1 2 3 4 5 6 7 8
Tim (s) Xl 44









0 1 2 3 4 5 6 7 8
Tim (s) x 104

Figure 1-2. Power fluctuation comparison. A) Nominal capacity 35 MW. B) Nominal
capacity 150 MW.

600






0-+
10 in, avraevaue

Mea vau sre n
4 1 2 4 16 1
v (mis
Fiue13 yia oe creo idtrie

As the owrfutaeteratvpoereurdbthtubnscngss


well,~~~ an terfre olag vritln.ion r xetd seill hntewn ami










located at weak points in the system. To compensate for such voltage variations and keep

the voltage close to its rated value, several solutions are available: simple capacitor

banks, static voltage compensator (SVC), or static compensators (STATCOM).

A different approach must be taken for frequency variations due to power

fluctuations. Normally, wind farms connected to big systems do not present a maj or

problem in terms of frequency variations, because of the stiffness of the system.

However, with small or isolated systems that contain slow or no automatic generation

controls, a mismatch between generated and absorbed power can significantly affect

system frequency unless spinning reserves are significant. Figure 1-4 shows the effect of

wind-power fluctuation on an isolated system with a wind penetration level of 1%.

To counter these negative effects, countries and small isolated systems with high

wind-penetration factors developed special purchase power agreement (PPA)

requirements or indexes for wind-farm developers (Table 1-2).

4.10 1 MA;

.Lnr'rd Poweri~











D ~ D i 020 250
secnc
Fiue -.I Windfr oupu poesssemfeuny










Table 1-2. Wind-farm output-power requirements
Average (max
Ramp Rate dP/dt Instantaneous
variation)
Netherlands <12 MW per min
Denmark <0.1-Pnom per min <0.05 Pnom per
60 sec period
Hawaii <2 MW per min 1 MW change <0.3MW per 60
per 2 sec sec period
scan
Germany <0.1-Pnom per mi
Scotland No limit for Pnom<15 MW/min
Pnom/15 for Pnom= [15-150] MW/min
10 MW for Pnom>150 MW per min

These power requirements guarantee minimum impact on system voltage and

frequency control. However, today's wind farms have limited capacity to reduce the rate

of change of power, especially the down ramp rate.

At high wind speeds (above the rated wind speed), active and stalled pitch controls,

among other strategies, can help keep the output power under control. However, modern

wind turbines are designed to obtain as much power as possible at low wind speeds

(Figure 1-5), making them very vulnerable to wind variations.


Figure 1-5. Control strategies along the power curve









Solutions to Wind-Power Fluctuations

To reduce the effects of wind-power variations and meet the PPA requirements for

electric utilities, two solutions can be considered:

* Higher spinning reserves
* Wind farm buffer

Increasing spinning reserves is a costly solution. A better approach would be to use

an energy-storage system that could deliver the required power when needed.

Work has been done in developing large-scale energy storage systems that have

overcome these issues by absorbing undesirable power fluctuations and providing firm,

dependable peaking capacity [8]. However, a less costly solution should be explored

based exclusively on power-fluctuation indexes (such as ramp rate indexes or

instantaneous fluctuation indexes).

State of the Art

Storing wind power is not a new concept; in fact, back in 1900, the father of the

modern wind turbine, Poul La Cour, tackled for the first time the problem of energy

storage. He used the electricity from the wind turbines for electrolysis and to store energy

in the form of hydrogen. However, with time, system requirements, energy storage

systems, and wind turbine ratings have changed.

Nowadays, the average wind turbine installed is around 1 MW, according to the

European Wind Energy Association, and wind-power farms usually consists of ten to

several tens of wind-turbine generators of rated power up to 2 MW. Thus, the amount of

energy storage needed to stabilize the power output change in the short term has

increased. Table 1-3 shows some recent projects dealing with output leveling of wind-

energy conversion.
















Table 1-3. Large-scale wind-power output-leveling proj ects
Prj c nm Wn fr. sz Active power reference
Project ame md arm ize Energy storage system
control scheme


Moving Average of wind farm output
determined as


Subaru Proj ect [9].
Tomamae wind-power
station.





King Island [10]. Energy-
storage system provided
by Pinnacle VRB



Oki proj ect by Fuji
Electric


1.65 MW *16
(Vestas).
1.5 MW 5
(Enercon) .

Total Capacity
30.6 MW
250 kW*3
850 kW*2

Total Capacity
2.45 MW

600 kW *3

Total Capacity
1.8 MW


Vanadium-Redox Flow Battery

PVRB nominal =4.000kW
EVRB-6.000kWh
S inverter=6.000kVA


Vanadium-Redox Flow Battery

PVRB nominal=200kW
PVRB short-term ( 5 minutes)=300~kW
PVRB short-term (10 seconds)=400kW
EVRB =1100kWh
Flywheel

E flywheel = 100 kW 90 sec
P inverter flywheel side- 1 10kVA
P inverter nower system side- 150~kVA


Pbattery=Pwind average (t-At)-Pwind(t)
(for At=8 seconds to 8 hours)


Isochronous frequency mode over the VRB
power range.
Speed droop characteristic during instantaneous
and short-term load (>+ 200 kW).


Power ramp rate limiting










However, small-scale concepts and technical/economic feasibility studies have

been proposed (Table 1-4). Each of these proj ects has a different obj ective (frequency

control, power smoothing, load leveling, etc.). However, they all end up using one of the

topologies and energy-storage systems shown in Table 1-5, where the flywheel or

capacitors may be replaced by some other energy-storage medium. Tables 1-3 and 1-4

show that the amount of energy needed for wind-power balancing using current

technology and current pricing is so significant, that a more flexible and integrated

approach is needed.

Our study focused on developing new power smoothing control algorithms. The

new integrated approach used a shunt-connected voltage-source converter with added

storage included on the DC link bus. The system can

* Exchange active power with the system.
* Regulate voltage at the point of common coupling
* Increase power quality and system stability

Objective

Our purpose was to develop, simulate, and implement a proof-of-concept prototype

advanced-power electronic device capable of controlling and smoothing the power

fluctuations of a wind farm using an optimal amount of energy. The wind-power

generation buffering concept is shown in Figure 1-6. The Power Stabilizer was designed

to store excess power during periods of increased wind-power generation and release

stored energy during periods of decreased generation due to wind fluctuations.

We tested the performance of the advanced electronic device on

* DC-synchronous machine set
* Passive load
* DC-asynchronous machine set
* Wind-farm buffer or also called Power Stabilizer





Table 1-4. Conceptual wind-power filtering projects
Wind farm size Energy storage system
20 MW Zinc-bromide battery

PZBB nominal (charge) =-750kW
PZBB nominal (discharge)=1500kW
EZBB-=1500 kWh
Maximum power Super-conducting magnetic
oscillation 2.5 MW energy storage (SMES)
300kW Electric double layer
capacitor
P ECs =100 kW
E Ecs =1.1 kWh


Active power reference control scheme
Limiting instantaneous power fluctuations based
on a 2 seconds interval
APinstantaneou (At=2 seconds)- 11.3 MW
Average power levels over a 2 hour window
Paverage(t- At=2hours)= 200 kW
Active power reference is chosen to control
system frequency
ESS active power reference is determined by
detection power oscillation components using a
high pass filter


Comments
Technical and economical
feasibility evaluation
[l l]


Simulation study [12]

Simulation study [13]


6GW



45KW


55kW


Redox-flow battery
(Regenesys )
E=62004 MWh
P=255MW
Flywheel
E flywheel- 12MJ
P drive=45kW
Lead Acid Battery
E battery=35SkWh
P conver-ter-50kVA


Power balancing


Feasibility study [14]


The active power demand is extracted via a 2nd
order Butterworth high pass filter, with a 5mHz
bandwidth
Power smoothing


Practical results [15]-[16]


Practical results [17]














Wind% powbr+ Win farm buffer Power















Figure 1-6. Wind-farm generation buffering concept

Table 1-5. Basic system configurations
Voltage source inverter
ESS connected at the DC link side [19]-[21]-[24]






ESS (flywuheel)
ESS connected at the AC side [18]-[20]-[22]-[23]-[25]

Wind Turbine Electric System
System configuration -6i


Voltage Surce
-ESS (flywuheel)
Current source inverter (shunt connected) [13]

Wind TrbineElectric System



Current Source
Inetr ESS (capacitors)


Available options [26]
*Compressed air energy storage
*Battery storage
Energy storage system Electro-chemical flow cell systems
*Fuel cell/electrolyser/hydrogen systems
*Kinetic energy (flywheel) storage
*Pumping water















CHAPTER 2
SYSTEM DESIGN

Introduction

One of the most difficult tasks when designing a control algorithm for a power

electronic converter is to calculate the regulators' gains. Determination of the controllers'

parameters is based on the electric power system they are connected to, and also on the

power electronic converter topology. This chapter details the design of the different

regulators involved in the control of the Power Stabilizer and also the design of the

different components that define its topology.

The system design was carried out per unit, so results can be extrapolated to any

system size, to facilitate implementation of the control scheme in a Eixed-point digital

signal processor. The system design was also compared to simulation results to assure the

correctness of the design methodology used

Control Scheme

Positive Sequence Calculation

Three-phase systems are not always balanced, especially during fault conditions,

and it is expected to have positive, negative and even zero sequence components.

However, for voltage regulation purposes, only the positive sequence component is of

importance.

Before going into detail on the positive extraction algorithm description, we will

explain first where the transformations given in Appendix A fail in coupling the different

symmetrical components. Consider the following set of phasors











Va = 0.5 0-

V, = 1.0 -1200 (2-1)
V =1.0
-2400


Figure 2-1 shows the time domain representation of this three-phase unbalanced system.






0




-08



U 0 002 0.004 0 006 0 008 0 01 0 012 0 014 0.016 0 018 0.02
Time (sec)


Figure 2-1. Unbalanced system

If we now calculate the symmetrical components of this unbalanced system, we obtain


V, = 0.833,o-

V2 L0= 0.167/1o (2-2)

Vo = 0. 167,,so-


The symmetrical components transformation is a good tool to determine the type of

distortion or asymmetry the system has. However, it has the drawback of having to use

phasors as input instead of time domain signals. Therefore a different transformation was

needed in order to extract the positive sequence component out of the rotating space

vector.


Figure 2-2 shows the traj ectory followed by the rotating space vector of the

unbalanced system in the d-q-o plane using Clarke's transformation. This traj ectory is







16


clearly distorted from the ideal one, and the space vector no longer follows a circular path


(Figure 2-3).








0.1 -






-0.1 1::; ~ .::::

-0.2
1 0

1 -1
Vds
Vqs


Figure 2-2. Space vector traj ectory of an unbalanced system in the d-q-o plane









/1


0ji.5


0.5


-0.5


Vds


Vqs


Figure 2-3. Space vector traj ectory proj section over the d-q plane

Figure 2-4 shows the Vd, and Vqr components (Park' s transformation) of the

unbalanced system in the time domain for 6 = 00.































































,-w dc w_



-2w -4v dc


06 -


Vqr


-0.20' 0 002 0 004 0.006 0 008 0.01 0.012 0 014 0 016 0.018 0 02
Time (sec)


Figure 2-4. Direct and quadrature components of an unbalanced system

It is clear that the Vd, component is not constant any more, and it contains a 2nd

harmonic due to the negative sequence. This effect can also be explained in the frequency

domain as shown in Figure 2-5. The rotating reference frame aligns with the fundamental


frequency, w=2xnf, and therefore

* a negative sequence (-w) appears as a 2nd harmonic
* a dc component appears as a 1st harmonic
* a positive sequence (w) has a constant value.


acaxis


0.833


0.167


Figure 2-5. Representation of an unbalanced system in the frequency domain









Thus, it can be concluded that Clarke's and Park' s transformations do not provide

suitable components that can be used in a voltage regulation control algorithm. It is

therefore necessary then to redefine the transformations in order to extract the desired

components.

Assuming the three-phase electric system has positive and negative sequence

components

Va = Vp cos(wt)+V -, cos(-wt)
2xi 2xi
Vb =~ Vcos(wt- )+V -,cos(-wt +- (2-3)
hp3 3
4xi 4xi
Vc = Vp -cos(wt )+V -,cos(-wt +-
3 3

Clarke's transformation can be used to obtain

Vds = Vp cos(wt) +V -, cos(wt) = Vds +Vyds
(2-4)
V=~ V sin(wt)-V -nsin(wt) = V +V

where Vds and V,,z are the d-q components of the positive sequence, while V, and V,

are the d-q components of the negative sequence.

If we now assume that the symmetrical components remained constant for at least a

quarter of cycle, the equations can be rewritten as


Es. (t) = -Vd t)- Vat



(2-5)


V ,s(t)= dst-- -Vqjt







19


These components can now be transformed using the rotating reference frame in

order to obtain the positive sequence component. Figure 2-6 shows the block diagram of


the algorithm used to extract the positive-sequence component. The same concept could


be used if the negative sequence magnitude is needed.





dq d~q, V










Figure 2-6. Positive-sequence extraction algorithm

Figure 2-8 shows the algorithm performance when an unbalanced fault condition

takes place at t=0.02 sec (Figure 2-7). The data used for this example is given by


Equation 2-2.




0. /


/ik, i




0 03 0.035 0.04


-06



-0.8


0 0 005 0.01 0 015 0 02 0.025
Time (sec)


Figure 2-7. Voltage waveforms for an unbalanced fault event





nR~


1

08
3
4
U) 06
a,
n,
a
0 04


1,,3 ..:


1 ****: ***)6


Time (sec)


111.
Time (sec)


Figure 2-8. Response of the positive-sequence extraction algorithm. A) Positive sequence
using V/2 and 1 cycle filters. B) Positive sequence using Vdr With 1 cycle filter

The meaning of the different plotted variables is the following:


* Vpositive-sequence magnitude is the output of the positive-sequence extraction algorithm. As
expected, its time response is only one quarter of a cycle. However, the transient
response is very abrupt an uneven.


* Vpositive-sequence magnitude (1/2 cycle filter) is the filtered signal of Vpositive-sequence magnitude USing
a half cycle sliding window filter.


* Vpositive-sequence magnitude (1 cycle filter) iS the filtered signal of Vpositive-sequence magnitude USing a
one-cycle sliding window filter. Its transient response is the slowest but at the same
time the smoothest among the three signals.


* Vdr filtered is the filtered signal of Vdr The one cycle sliding window filter (also
called moving average) rej ects all harmonics. Therefore there is no need to use the
Vds+ and Vqs+ calculator to extract the positive sequence. However its transient
response is not as smooth as the Vpositive-sequence magnitude (1 cycle filter) One.

Real Power Calculation Using dq Components


As shown in Appendix A Park' s transformation matrix is not unitary


(~, t [ dqo ) and therefore is not power invariant.


The total instantaneous power in abc quantities can be transformed into q-d-o


quantities as shown in Equation 2-6.









This relationship between dqo quantities and the instantaneous power is later used

in the control system to determine the amount of direct-current component (Idr ) needed

to meet the power fluctuation requirements.



Pabc = Va la +Vyb .b +V -e Ic = V bI b~,I

Vd I,





V I


Vdr V, V, Tdqo -13 q 1 q

2 Idr

01 I

3 / 1 I,+C+-1 (2-6)
=2 dr dr+3 7,+ V l

Phase Locked Loop

The phase angle of the utility voltage (6) is of vital importance for the operation of

most of the advanced power electronic devices connected to the electric utility, since it

has a direct effect on their control algorithms.

A simple and fast method to obtain the phase angle of the utility voltage is to use

Clarke's transformation as shown in Equation 2-7.


1X 1 1 artnX 27
Xds 2 2 a X
rX, 3 X:11 X
X,










However, this approach is not robust since it is very sensitive to system

disturbances. The phase angle 6 distorts as the utility's voltage becomes affected by

different power quality events, such as voltage unbalance, voltage sags, frequency

variations, etc.

Figure 2-9 shows the voltage's phase angle under unbalanced conditions using

Equation 2-7. The angle distortion is due to the negative sequence component of the

unbalanced three-phase system.



Phase Alng e idea"










0 0005 0.01 0.015 0 02 0.025 0.03 O 035 0.04
Time (sec)

Figure 2-9. Distortion of phase angle due to a negative sequence component

In order to lock the phase angle of the utility voltage in a robust way, a phase

locked loop (PLL) was used.

Assuming a balanced three phase system, the control model of the PLL was

obtained using Park' s transformation as shown in Equation 2-8.


6, cos(8*) cos(8* -1200) cos(6* 2400) -
V,=- sin(8*) sin(8* -1200) sin(6* 2400) yay Irb

(2-8)
cos(6 ) cos(6* -1200) cos(6* 2400) V cos(wt)
---sin(8 ) sin(8* -1200) sin(6* 2400) V co(wt -12400)
V co(wt 2400)










Where 8* is the PLL phase angle output, 6 is the utility' s phase angle, and w = dBO

Thus, if 8(t=0)=0, we can substitute wt for 8(t) and obtain

V,.cos(6 ) cos(6* -1200) cos(6* 2400) -. Vcos(0)
V,,. -sin(8 ) sin(8* -1200) sin(6* 2400) V cos(6 -12400)(29
V, _Vcos(8 240") 2


Using trigonometric identities, Equation 2-9 results in

V,. cos(8* 8) cos(AO)

V,=V in0-6 V si(O) (2-10)


Where AO is the error between the utility angle and the PLL output. If the AO is set to

zero, Vdr=V and Vq,=0. Therefore, it is possible to lock the utility angle by regulating Vq,

to zero without needing any information regarding the magnitude of the utility voltage.

Figure 2-10 shows the details of the PLL algorithm used in our study. The limits of

the controller integrator and the limiter were 30 rad/sec. Thus, the PLL was able to track

the system frequency as long as this was within 22n60130 rad/sec or 55 to 65 Hz range.

To use linear control techniques for the design and tuning of PLL controller, it was

assumed that:

*For small values of AO, the term sin (AO) behaved linearly, i.e., sin(AO) A O.
*Wrer was assumed to be a constant perturbation.
*Limiters behave linearly for small control actions, and therefore can be removed.




v v v


Wref=271f


Figure 2-10. PLL diagram










Figure 2-11 shows the PLL control loop after eliminating the non-lineal terms.

PLL controller Plant transfer
Function








Figure 2-11. PLL simplified model

The closed loop transfer function of Figure 2-11 determines the dynamic

characteristics and stability of the system, and can be expressed as

*K~s +K
H : (2-11)
B s2 +Kps +K,

The control system (Kp and Ki) was designed to satisfy two performance

obj ectives

* < 10% overshoot
* Settling time inside the 2% band error lower than 2 secs

The criterion to select the settling time was a tradeoff between high distortion

rej section and tracking of normal system frequency variations.

The PLL closed loop transfer function was compared to a standard second order

transfer function to determine the regulator' s gains. The obtained values were

r = 0.7(for 5% overshoot)
ts = 2 sec
4 4
K, =i,, m.72 8. 1

K, =2-(-m~ = 2-0.7-2.85 =4

Figure 2-12 shows the system's closed-loop step response for two different PI

regulators.









The originally designed regulator did not meet the system requirements due to the

effect of the zero introduced by the PLL regulator. This additional zero increased the

overshoot, but it had very little influence on the settling time. Thus, it was necessary to

tune the original regulator gains in order to meet the system requirements.




















Figure 2-12. PLL system step response

Figure 2-13 shows the root locus of the single-input single output PLL system for

the two regulators.


Figure 2-13. Root locus for two different regulator gains











Figures 2-14 and 2-15 show the PLL system response to a negative sequence

condition (V2=16.6%) and a system frequency excursion (w=2xn60+30 rad/sec).

PLL response


PLL response


000 ooos ob 15 002 0025 003 0035
Time(sec)


Figure 2-14. PLL system response to an unbalanced system condition


Ae (phase angle error)


002 004 006 008 01 012 014 016 018 02
Time(sec)


0 01 02 03 04 05 OB 07 08 09 1
Time (sec)


Figure 2-15. PLL system response to a frequency excursion. A) Angle. B) PLL error.

Control Algorithm Design

Park' s transformation was used to model the system's equations to facilitate the


design of the control system. The usage of a rotating reference frame had the following


advantages:


* Improvement of the steady-state performance of the current controllers:
Sinusoidal signals were transformed into dc components, and accordingly it is
possible to achieve small signal errors.

* High bandwidth current controllers: Feedback signals and reference signals
were not sinusoidal, but dc.










*Decoupling of active and reactive power: This was very useful when trying to
control voltage at the point of coupling while meeting the system requirements in
terms of power fluctuations.

Figure 2-16 shows the overall system topology as well as the sign notation that was

used in the control system design. In general, power flowing out of the inverter will be

considered to be positive. The obj ective was to smooth out wind-power fluctuations using

the power stabilizer as a buffer. The energy-storage voltage was expected to change in

order to accommodate for those changes in wind power.

WIND UTILITY
FARM SYSTEM
Transformer
Inverter DC link bus
equivalent impedance Chopper



LnnVr L Cde ~ ~ chopper

c, 1,ny vn Vchopper 5Vstorage



Filter ESS
P+

Figure 2-16. System description

Inner regulators

Inverter system model. For the following set of equations, it was assumed that the

inverter behaved as an ideal controllable voltage source, neglecting the effects of the

current harmonics. System's non-linearities, such as saturation or dead-time effects were

taken into consideration later on in the design.

The capacitor filter was neglected in the analysis, since the filter current

represented a small portion of the inverter' s current.

The system can then be represented as shown in Figure 2-17.













Vpoc0 li aP Cr ,, Viny c DC LINK




Figure 2-17. Simplified system model
The system equations for the simplified model are

ylv =.I IbIL In Vcc
Ia Jla ro pccaI (-2


Applying Park' s transformation we get

RT I I + Ld dq -1Td (2-13)
dqo d Inqr do tq dt o invqr pccqr



nvdr invdr tvdr Ypccdr

]bo I II V 214


lr,, 1I,, +Il r 1 I I1 V
~nvdr twvdr 1nvdr twvdr pccdr
R-I +L- Tdq d TdqoTdq -1 dl + V (2-15)
~,,I,, I,, I,, Vc
Invov invov twovd invo pccov

Ivdr twdr~vq Snvdr twdrv + pccdr
vq tqro dt nvr dt q pcr
I ~- I. I V 2-6
vo two two, twopcc


V-ny -1 -a











imdr tw~dr dE.p~ 1 Invdr invdr pccdr
l: R R I'P +L- Tdq dTdqo + dl + (2-7)
wqr tqr odt In""" dt Invqr pccqr
Io, invo o twno pcco

Where


1 1- cos(8 ") sin(B 1 0) 1 sin(B 10)-snB20) cos(B-40) 0 d
d[Tdqo d d6H)sn~
cs(8- 100) sn(8- 100) 1 = sn(8- 100) cos(8 1200) 0
dt dt dt
cos( 2400) sin( 2400) 1 -si(-24)-co(-20)0
(2-18)
dB


It can be shown that


[dq dq -

cos(6 ) cos(6* -1200) cos(6* 2400) -_ sin(8) cos(8) 0-(-9
= m--sin(8 ) sin(8 1200) sin(8 2400)1 _sin( 1200) cos( 1200) 0
3 -sin(8 2400) cos( 2400) 0

0 -1 00 -m 0
=m- I1 0 OO =
00 00 00


Thus, the equations for the simplified model in the d-q plane are


Invdr twvdr invdr invdr pccdr
ylvvIR-lIln +L-m~i 0 0I + I +V 2-0
nvqrtwqrinvqr dt invqr pccqr
nvo, Invo, inv tw ,,,J o pcco

The zero-sequence component can be removed, since the system is a three-phase

three-wire inverter with the DC link bus isolated from the AC side (the DC link mid-

point will not be tapped to neutral). Removing the zero sequence we obtain

dl~nd
yn =R-Ilvd +L- + -La-
(2-21)
dllnq
ynq R- Iln +L- "' +V +L-a-~I
nvqrtwqrdt pccqr twvdr










Equation 2-21 can be represented as a coupled electrical system as shown in Figure


2-18.


d iny qr


Vpoo







Vpcc


Viny dr


V
iny qr

B


Figure 2-18. Electrical representation of the dq components. A) Direct circuit. B)
Quadrature circuit.

Using Laplace's transformation we can re-write the equations as Equation 2-22.

Vnvd rr (S.) = (R + Ls) Invdr ~j(S) +Vpccdr (S.) L m Invqr y(S.)
(2-22)
V,,,, (s) = (R + Ls) I'"nvqr (S)+ +VpCCqr (S) +L m, Invdr (S)

Thus, the block diagram of the system is represented in Figure 2-19.


I
iny dr


Figure 2-19. System model block diagram


L liny dr










The inverter' s critical control variable was the inverter' s current. This was due to

the fact the outer control loops, such voltage regulators, power regulators, etc, were based

on the inner current regulators. That was why the current controllers were designed to

meet two basic requirements, which were high accuracy and high bandwidth.

The inverter' s terminal-voltage needed to generate the desired inverter current can

be determined as

dl
nVd =R- Iv +L- +V -L-aI AV+V-La-
nvdrtwdrdt pccdr invqr dropd, pccdr invqr
(2-23)
dl
V RI +L- """' +V +L-a-~I = AV +V +L-a-Il
Inv qr tvqr dt pccqr twvdr drope pccqr twvdr


The voltage drop due to the filter inductance was compensated using a PI

controller. Figure 2-20 shows the inveter' s current controller implementation for the

system given in Equation 2-23.


Ipd Oj III

my,,,t dr re/ AtVdrop dr +~ / Vlnyd t u d I I~ny dr

I I wt


IK I II
IV"et II mL |t~doq t


t I I I
I. II Ipeq
I; I II

CURRENT REGULATORS SYSTEM MODEL


Figure 2-20. Inverter current regulator-system model block diagram

The character ^` over a constant or variable indicates that the quantity is estimated,

and therefore subj ect to measurement errors.







32


To design the current regulator gains, cross-coupling factors were assumed to

cancel each other out. Under these conditions, the simplified current regulator block

diagram is shown in Figure 2-21.

II II I
II III



II II R sII


II II I
II II
II II II
II I
II (______________31



Fiur 2-21 shw ht
Th sytmbhvs liealy an hrfr iercnrl ehiuscnb sdt
dtrine the reuaos'gis
*I Bot reuaor rdetcl




Fimpdacere n-1 I reeddt esg h current regulator.sse oe ipife lc iga


ofFigure 2-22 i shown inEqatio -4


Figure 2-22. Simplified current control diagram










I Kp s+ K
H(s) = (2-24)
Ie Ls2 +(R+Ks+K


Using the following system data, the transfer function is given in Equation 2-25.

*X=Xtransfomer+Xfilter-5%+10% = 0.15 R 0 L= 400 CIH
*X/R=10 -R=0.015 R
Note: More on the system parameters can be found in the per-unit mode section.


H(s) = 000s +(05Ky:K (2-25)


The Figure 2-23 shows the system step response for two different current regulator

gamns.





O, Sytern closed loop output
Kp=1 Ki=36




Kp=1 Ki=36




Figure 2-23. Current regulator step response

Even though the current regulator with the highest gains had a faster settling time,

the control action required to obtain such a response doubled the regulator with the

lowest gains. To avoid possible system saturations the control action was kept below 1

pu.

The best PI controller performance was achieved when the plant' s dominant pole

was cancelled by the controller (Equation 2-26). Thus, the zero at KI was assigned to
K,


the time constant of the plant, which was, K( =R
K, L











K s+ KK

PI =K, + ; (2-26)
s s

The synthesis was done by selecting the integral time constant of the PI equal to

that of the load. For our study the selected values were


K, 37.5

K, = 1

Chopper system model. The analysis of the chopper system was less complex than

the inverter one, since no transformations were involved. Again, it was assumed that the

chopper behaved as an ideal controllable voltage source and therefore the effects of the

current harmonics were neglected.


Chopper



Vchppe Vto rag e


Figure 2-24. Chopper equivalent system

The system equations for the chopper equivalent circuit (Figure 2-24) are given in

Equaion 2-27.

dl
V = L chopper +V
storage dt chopper
(2-27)
dl
V =~ V -L chopper
chopper storage dt

The chopper's terminals voltage needed to generate the desired chopper current

can be determined as


chopper = storage drop AV (2-28)










The voltage drop due to the chopper inductance was compensated using a simple P

controller. The gain of the controller was found by converting the continuous system into

discrete time system as shown in Equation 2-29.

MJhpe (Ichpe~ Ichpe )
V, = V L chopper = L chper copr
chope storage dt soaedt
(2-29)
Ychopper storage L .chopper LhreK


Where KL is the regulator' s gain and At is half of the sampling time period.

Figure 2-25 shows the implementation of the chopper' s current regulator.

Chopper ref + _V hpe




Chopper Vstorage


Figure 2-25. Chopper current controller

Outer regulators

There were a total of three controllable currents, which consisted of Ichopper ref,

linvdr~ref, and linvrrf~.. However, there were four variables that needed to be controlled,

which were voltage at the de link bus, voltage at the point of common coupling, voltage

at the energy storage system, and wind farm power fluctuation. Table 2-1 shows how

these variables were assigned to the respective current regulators.

Table 2-1. Outer regullator assignation
Inner current Variable to be
Comments
regulator controlled
liny dr ref Vstorage, Pawind The direct current component will be responsible
for controlling the state of charge of the ESS
and for smoothing the wind farm output power
liny qr ref Vec The quadrature current component will be
deployed for voltage regulation purposes
Ichopper ref Vdc link The chopper current will regulate the DC link bus
voltage.










DC link Voltage regulator. The DC link bus was the bridge between the energy

storage system (chopper) and the inverter. Therefore, it was a critical variable in the

overall system. Poor DC voltage regulation could bring the system down, since the

inverter and chopper would not be able to meet their respective voltage requirements.

The DC link system can be modeled as shown in Figure 2-26.













choppe dchn losse zpou
Pdcnk cope Vstrag out osse DCLN "p

Asu ing ise 0 we hav




(Vtoag I We can re-rie heeqatona




Vdu link,, PS) =n 2Clstorag choppe (S)



Figure 22-27 Poeshow dnthebockdarmo h ytmmdl











out




+ 7
Ichopper .. storage




Figure 2-27. System model

Considering 4%,~ as a disturbance, the transfer function of the system is


1 d(V ) 1
2 dc dt chopper storage 2 dc-( s chopperS)-Vstrg

V (s) 2 soae(2-3 3)
Ichopper (S) Cdc S

The system model and the DC link voltage regulator can be represented in the form of a

block diagram as shown in Figure 2-28.



PStoragetog

Yd Imkref + ~chopper 2 storaged n
Cdchnk *S



DC LINK VOLTAGE REGULATOR SYSTEM MODEL


Figure 2-28. DC link equivalent system block diagram

Pou
Under ideal conditions the terms o cancel each other out, resulting in a
storage


simplified block diagram (Figure 2-29).

















K





DC LINK VOLTAGE REGULATOR SYSTEM MODEL



Figure 2-29. DC link simplified system block diagram

The closed-loop transfer function of the simplified DC link system is:


2-V ,, -(K.s+K,)
H(s)=strg (2-34)
dchlnk S storage -Ks+I-2-Vtrg


Figure 2-30 shows the system step response for two different regulator gains, using


the following system data:


*Cdc link =15700C1F
*Vstorage nominal =1.533 p.u.


18}
1.6

14- System closed loop output slmcoe opotu
Kp=2l Ki=22
a, 1.2 ~K= i2

1


0.6C -Q System closed loop control action
Kp=2 Ki=22
0 4 ::\. System closed loop control action
\ Kp=1 KIl=11
0.2

U 0 005 a 01 .015 0 02 0 025 0 03 0 035 0 04 0 045 0 05
Time (sec)


Figure 2-30. DC link voltage regulator step response

To avoid a possible saturation of the DC link voltage regulator, the controller with


lower gains was chosen. In this case, the control action was the chopper current, and it


was designed to always be below 1.pu.










Point of common coupling voltage regulator. The voltage support capability of

the inverter depended on the available line impedance back to the utility source voltage,

and its dynamics response was directly affected by the line parameters.

The regulation of the voltage at the point of common coupling was accomplished

by changing the amount of reactive current generated / absorbed (lin q) by the inverter. It

was also possible to improve the voltage regulation controlling the real current

component. However, as it will be shown, the voltage regulation range was significantly

reduced.

The system model used in our study (Figure 2-3 1) was a simplified version of the

actual system. It consisted of the source (modeled as an infinite bus with a series

impedance), and the inverter (modeled as a controllable current source). System non-

liberalities, such as switching of the semiconductor devices, transformer saturation, etc,

were neglected.

The system of the equations for Figure 2-3 1 is


V~:pc II' dtI V,"Y,,b( 5



pcca inva r oa orce
dl
y =Rouc I + L I 4 + yoVcq ,,, (2-35)Ilvd
pcccv invv inc ure











EQUIVALENT
SYSTEM MODEL

'LRL
V 0 ce lInva Vpocab

V as L, ce Inyb Vpcc b













I pee lInv a V nv a
Vpee Inyb Vlnyb I
Vpee lIny c VIny c








Figure 2-31. Simplified system model

Using Laplace's transformation and re-organizing the terms, we obtain the transfer

functions shown in Equation 2-37

R Itd 1

sIl~d (s) = R oreI (s) w Il q (s) + V s ()
source source

(s + 1;r, -,,,, V s)- s)

invdr v(S) = SUe sourceVdYS)

souvcesource


(s 1-V s -V (s)+
invdr Lpcqouer
twqr v(S) = source
S+ source


r (S)=source










Defining the voltage drop, AV, as the voltage across the source impedance (Figure

2-32), it was possible to find the amount of current needed to obtain the desired voltage

drop (Equation 2-3 8).

AVL





Figur 2-32. Sourc imeac volag dro
'LL

Iznvdr~ ~ ~ ~ ~ ~ ~~~n bS = o V, s ore 2 bd S

Lsourr R~~crce LSOourcline s +Rsorc
'LL
Figur 2-3. Sorce mpedace vsourcero
-w 1.
/L /L
Iznvqr R -(S) = SOYC sou e -yd (S) + source V, s
s + Rsorc 2 auc
Lsource,, ) Lsorc s + Rsour

/L /L




The Bode plots of the Equation 2-39 for a system with a source impedance of 10%,

and X/R=10 are shown in Figure 2-33 and Figure 2-34.

Even thought the Bode plots of I iny dr and I iny, qr 0k very similar, the effect on the

amount of voltage drop for a given source impedance were significantly different.

There are two ways of controlling the amount of voltage drop at the source

impedance; regulating AV, (s) and/orB Ad(S). However, in order to increase system

stability and gain robustness, the phase shift between the utility voltage and the voltage at







42


the point of common coupling must be as small as possible. Therefore, it was preferable

to regulate Vec by controlling AT1,, (s) exclusively.


Bode Diagram
















Frequency (Hz)


Figure 2-33. Transfer functions of inverter' s quadrature current component


Bode Diagram


Frequency (Hz)


Figure 2-34. Transfer functions of inverter' s direct current component

Comparing Figure 2-33 to Figure 2-34, it is clear that, in the low frequency range,

the cross coupling between I,,,q z, nd AT is much greater than the direct gain between










Irwdr and Aydr This means that the voltage can be regulated by controlling only the

quadrature current.

Thus, for instance, the steady-state bus voltage of a system with a source

impedance of 10%, and X/R=10 in terms of Iznvdr and I,,,,, is

R I td 1 (, r,,.(,
0= SOUrCe I (s)+w-II q (s)+ V (sV s)
L 2nd wr L pcd ouer
soure sorce(2-40)
R I t 1," ,,, S)
L Invqrrd L pcr ucq
source source

Forllnvdr = 0,


V a= V 2 V~i 2 = V iad-w I~r~al )2 + Vrar +R uI e)2 (2-41)


FrIznvqr = 0,


Vyc = IVpcdr Vpccdr Vsourcer +Rsorcezd) sucqr+~orzd (2-42)


If Iraqr = -1pu (capacitive),Vsourcedr = 1pu, Vsourceqr = 0 pu, Xsource = 0.102, and


source w


The amount of Irwdr needed to obtained the same voltage would be


V =1.1 (V +R )2+ V +wL I ) ->InV = 3.67pu


This proves that for a system where the ratio X/R>1, the PCC bus voltage can be

regulated in an efficient way by inj ecting only quadrature current.

The design of the voltage regulator requires the knowledge of the source

impedance. However, this impedance varies with time and on online estimation can be

very complex if transient situations are present in the system.











For steady-state conditions the transfer function between AV and I inyqrca b


reduced to just the source impedance of value Xsource-wLsource Therefore, the control


block diagram of the voltage regulator can be interpreted as shown in Figure 2-3 5.


I I ource dr


CON IO~pERIMPEDANCE

POSITIVE |
SEQUENCE
EXTRACTION
L________________________,L_______________I___
VOLTAGE REGULATOR SYSTEM MODEL


Figure 2-35. Voltage regulator system block diagram

The current controller is represented as a second order transfer function in Equation

2-43.


K -s+K
H (s) = ~pcurrent regulator I current regulator 2-3
current regulator 0.004s + (.05 K +
pcurrent regulator I current regulator


For Kp=1 and Ki=36 the current controller transfer function is

I s+36
H,,, euao (s) = /M
curet eglaor I 0.0004s2 +1.015s + 36
invref


The positive sequence extraction transfer function can be modeled in continuous

time as shown in Figure 2-36.


Integrator






Td
Transport
delay


Figure 2-36. Positive sequence extraction algorithm equivalent system































































sore1%Xsource=20%

Xgurce 5 6





// Xsource 1%b


For the simplified voltage regulator system, there was not need for any


transformation. Only the modeling of the positive sequence 1 cycle sliding window filter


(Td=16.666msec) was required (Figure 2- 37).


VOLTAGE REGULATOR SYSTEM MODEL


Figure 2- 37. Voltage regulator detailed block diagram

The system was further simplified assuming that the current regulator time


response was much faster than voltage regulator time response (Figure 2- 38).











VOLTAGE REGULATOR SYSTEM MODEL


Figure 2- 38. Voltage regulator simplified control diagram


Figure 2- 39 shows the system step response for a given voltage regulator under


different system conditions.


Xur=1%Xsource=20%


/Xo urce 59


Tine(sec) A ,,m.-, B


Figure 2- 39. System response to a 5% change in voltage reference for Kp=2 Ki=250. A)
With saturation. B) Without saturation










The settling time was a function of the system impedance, and therefore it was not

possible to predict the system response without knowing the source impedance. One

solution was to use an adaptive parameter tuner capable of adjusting the regulator gains

according to the identified plant dynamics. The block diagram of the adaptive control

scheme is shown in Figure 2-40.


Adaptive
Parameter Tuner



VPccre; poserve seqe~ zn rr LN



/d +






Figure 2-40. Adaptive control scheme

However, due to the difficulty in distinguishing between changes in the system

impedance, load variations, and utility voltage, a simpler but robust solution was adopted.

It consisted of a classic PI regulator, with gains that were tuned in the field. The

drawback was a slower response that could occur for any given condition.

Power regulator. The power regulator required to control the power fluctuations of

a wind farm was the most complicated control scheme among all the described so far. It

involved non-linear algorithms which made the system very sensitive to instabilities due

to non forecasted conditions.

The basic idea behind the power regulator was to determine the amount of the real

power required by the inverter in order to meet the utility's power fluctuation limits.

A generic power regulator control scheme is shown in Figure 2-41.











Iwinda
Vo X Rampnat vae Is Vstorage

Iwindb Vn
Vpccb X Pur+Limiters I toret r
Iwindc
Vpccc- X+
ESArequired
-|- power
Allowedccentenngpower

Powerlnverter
Reference



Figure 2-41. Power Regulator general control scheme

First the wind farm power was calculated and the compared to rate-of change limits


(Table 2-2). If limits were exceeded, the difference would be compensated by the

inverter.


The centering algorithm was a control scheme used to hold the energy storage near

its nominal value, to be ready for the next supply or absorption cycle. If the wind farm


power was causing the limiters to activate, this centering action would not take place.

That way a higher priority was given to the power limiters.

Table 2-2. Rate-of-change limits or PPA for a 10 MW wind farm
Parameter Value
Instantaneous 1 MW change per 2 second scan
Sub minute average average of 0.3MW change per 2 second
scan for any 60 second period
Ramp rate 2 MW per minute up, and down when
operationally possible

The first proposed control scheme of the power limiter consisted of a high pass


(HP) filter which canceled the high frequency power fluctuations independently of the

rate-of-change limits. Figure 2-42 shows the HP filter control block diagram. A small

bias power was added to assure the charging of the energy storage system.

This approach had three maj or drawbacks:











* Rate-of-change limits might not meet unless inverter' s power and energy
requirements were increased.

* Optimal cut-off frequency design was unknown.

* Inverter duty cycle was higher than the next approaches.

Wind
Farm
Output HI igh Pass
Filter

Storage
Nominal

State of Charge Centering
Storage I, Charge/ +_ Inverter
(Integrator) I V~D discharge Centering ~V I Power
Constant



Figure 2-42. Power limiter 1. Control block diagram

Wind farm power data records were used to test the power limiter control scheme

under different system conditions. Figure 2-43 and Figure 2-44 show the inverter


requirements as well as the system performance for different cut-off frequencies.

Note: Inverter size requirements cannot be extrapolated from the 15 minutes


simulation. A more detailed study must be performed using long wind-power data

records (perhaps years). It was also not cost effective to correct every possible scenario.


Therefore, the number of times and/or amount that the wind farm may exceed the power

index limits, with a Power Stabilizer installed, needs to be determined, when traded off


against inverter and storage ratings.

The main advantages of the HP power limiter were its simplicity and its stability


under unexpected power fluctuations.

The control scheme was implemented in MATLAB in order to test the power


limiter performance under different system conditions. Appendix B gives more

information on the MATLAB code.











10F



06


1=0 005Hz


~1



-.2
O

20




-60 _
0


Time secss)


f =0 oos~iz
, 0'


100 200 300 400 500
Time secss)


600 700 800 900 1000


Sf=0 005Hz


100 200 300 400 500
Time secss)


600 700 800 900 1000


Figure 2-43. Power limiter 1. Performance using different cut-off frequencies (unlimited
power and energy). A) Power to utility. B) Power-stabilizer output power. C)
Power stabilizer's energy storage.


10r





5-
O


~1



-12

O i
20 -
S10-

c 1


d farrr power 'A


100 200 300 400 500
Time secss)


600 700 800 900


B


I I I I


100 200 300 400 500
Time secss)


600 700 800 900


-- =0 005Hz


100 200 300 400 500
Time secss)


600 700 800 900


Figure 2-44. Power limiter 1. Performance using different cut-off frequencies (Pinverter
MW and Einverter=+8.5 MJ). A) Power to utility. B) Power-stabilizer output
power. C) Power stabilizer's energy storage.










The second proposed control scheme of the power limiter consisted of a power

limiter with the three rate-of-change limiters in cascade (Figure 2-45). The ramp limit

was first applied, followed by the sub minute limit, and finally the scan-to-scan limit.


Input+R +s Scan-to- Output
a ~Scan
-R -sJ ~ Limiter
R S
Subminute
Limit
Calculator






Figure 2-45. Power limiter 2. Limiters details

As mentioned earlier, the "centering" of the energy storage energy was needed so

that it could supply or absorb power from its nominal state. Therefore this energy must be

taken into account when calculating the rate-of-change limits, since it was real power

being interchanged with the system. Thus, the power limiter control scheme had two

limiters in parallel (Figure 2-46); one limiter acted upon the wind farm output only,

another limiter acted on the wind farm power plus the desired centering power.

If the inverter were big enough to supply or absorb the excess power and energy

from the wind farm, the power limiter would keep the power within that allowed by the

rate-of change limits. The problem occurred when the power or energy storage is beyond

the rating of the inverter, since the history of what is actually delivered to the utility could

be wrong. Thus, a saturation limiter was needed in order to adjust the buffer input data.

The control scheme was implemented in MATLAB in order to test the power

limiter performance under different system conditions. Appendix B gives mode

information on the MATLAB code.








51




Desired Power
(Wind+Storage Centering) Lmte

Last thing to be
updatedlevaluated
r----~;i=------1+-
Previous scansI
(BUFFER)
Wind r---~~---

Output
aLimiter
Storage
Nominal

Centering State of Centering
Charge/ Charge ES eurdPower
DiscargeSupply/Absorb Allowed
Constant








SInverter



Power Limiter

Figure 2-46. Power limiter 2. Control block diagram


Wind farm power data stored on a 2 second basis was used to test and size the


power limiter control scheme under different scenarios. The following figure shows the


system performance for a period of 15 minutes.


a VT\ Wind farm power

5.0 100 200 300 400 500 600 700 800
Times secss) ,
Zoom in





Figure 2-47. Power limiter 2. Compensation performance


' Wind farrn power
450 500 550
Times secss)







52


The inverter power and energy required to meet the rate-of-change limits for the

15- minute simulation is shown in Figure 2-48.


L O





C -4 5





UJ -


100 200 300 400 500 600 700 800 900
Time secss)




-n~tn inrl


0O 100 200 300 400 500 600
Time secss)


700 800 900


1000


Figure 2-48. Power limiter 2. Inverter response for a sampling time of 2 seconds. A)
Power-stabilizer output power. B) Power stabilizer's energy storage.

To test the stability of the control algorithm, saturation effects were taken into

account. Figure 2-49 and 2-50 show the system performance for different under-rated

inverters. Rate-of-change limits were not met, but the system was stable.

It is very difficult anticipate all of the types of misbehavior that might occur in the

system, and that there could be unusual power fluctuations from the wind farm could get

the inverter into a mode where it would continue to swing the power around in an

undesirable manner. Therefore it was recommended to include some type of

"misbehavior detector" in the power limiter control scheme to protect the inverter and the

sy stem.





















0 100 200 300 400 500 600 700 800 900 1000
Time secss)







u. -4 R


-5
0 100 200 300 400 500 600
Time secss)


700 800 900 1000


Figure 2-49. Power limiter 2. Inverter response for different power ratings. Sampling
time 2 seconds .A) Power-stabilizer output power. B) Power stabilizer's
energy storage.


Zoom in


F
r
o
a,
5
0-0.5
a


II


I I I I


100 200 300 400 500
Time (sec


600 700 800 900 1000


\,- :- -:'
r~5ld;itia;=i- I-13'i
'i- I;li;r nd*= i- 4 :d',"
1.
Zii-rei nir= urii-i:~d

r' B


100 200 300 400 500 600
Time secss)


700 800 900 1000


Figure 2-50. Power limiter 2. Inverter response for different ESS sizes. Sampling time 2
seconds. A) Power-stabilizer output power. B) Power stabilizer' s energy
storage.









The third control scheme considered for the power limiter consisted of a power

limiter with the three rate-of-change limiters in parallel (Figure 2-51). The limiter' s input

was the power out to the utility instead of the wind power, plus the centering power, for a

more accurate control of the power fluctuations seen by the utility.

Each limiter determined the maximum and minimum amount of power allowed

changing per scan. Then the absolute maximum and minimum were calculated in order to

establish the centering power limits and the required power from the inverter.


Figure 2-51. Power limiter 3. Control block diagram

Figure 2-52 shows the response of the power limiter 2 using the same wind-power

data records used previously.

It can be concluded from Figure 2-47 and Figure 2-52 that both power limiters have

the same response under normal conditions.












9.5


9-








O~ 7-




5 .5




c1


07


O -.5-



0 100 200 300 400 500 600 700 800 900 1000
Time secss)











C- -0.


0 100 200 300 400 500 600 700 800 900 1000
Time secss)


Fiue25.Pwrlmtr3 netrrspnefrasmln ieo eod.A
Poe-taiie oupu poer B)Poer stabilizrseegysoae












Per-Unit System Model

The reasons why to convert system variables into per unit are:

*System easily scalable
*Facilitate fixed point operations
*Power system components can be treated uniformly no matter what voltage level

The two variables selected as based values are

Vbase = Va n-eta p V

Ibase = max line = 1pu (A)

Thus, the rest of variables can be calculated as

VRSlne neutral V
Base Impedance: Zbas I/S ae ase2
I I
RM/S line base
*Base Power (3 phase):

Phse= inererraepower = RM/S line neutral RM/S line = JZ 3 = .5

Inverter Output-Filter Design

The purpose of inverter filter was to attenuate the high frequency switching

harmonics produced by the inverter in order to avoid disturbing other EMI sensitive

equipment on the grid.

Its optimal design is very complex and it involves coupled design constraints and

non-linear equations.

The inverter topology for which the filter would be designed was a 6-pulse 3-wire

inverter, without DC bus mid-point tapped to neutral (Figure 2-54), where power

semiconductors were considered as ideal switches.














Iny a Vlny a
Iny b Vlny b
myv c Vlny a







Figure 2-54.Inverter topology

Harmonic content

The typical line-to-neutral and line-to-line voltage of a three phase inverter using a

PWM strategy is shown in Figure 2-55.











Time secss) Time secss)


Figure 2-55. Line-to-line and line-to-neutral voltage of a three phase inverter

Figure 2-56 shows the harmonic spectrum of the line-to-line voltage under the

following conditions:

*fsw = 4860 Hz
*fi=60 Hz
switching fr~equency f,,
*Frequency Modulation, mf *81
Sfundamentaldd~~~dd~~~ddd~~ frequency f,
*Vdc = 2.04 pu
*Vsource max line-to-neutral= 1 pu
Y.a ele
Peak amplitude of the control signal = a eie

*Amplitude of the triangular signal = 1







58


peak amplitud'e of the control signal
Amplitude modulation, m =1
aamplitude of the triangular signal


1.4
mf-81
h= 1
1.2 V-=1i.24





I




0 00 40 00 80 00 20

Freqenc(Hz
Fiue25.RSLiet-ievlae amncsetu

Th anhroi opnnt fteln-oln upu otg eecluae













(SSG)r capabl ofS pr-oducing et vofg adusablei volctags hc myb opldt na

power sytm texhamnge independently o t lnetrollabe realu and ractie pwer. This wase

acomlihe b the usulte age of 28 a synhoosidco hc ikdteivreuptt



th a sauppl sid e (Figuren 2-57).cn efon n ale23










Table 2-3. Generalized Harmonics of line-to-line voltage for a large and odd mf that is a
multiple of 3
K (Generalized Harmonics of Vrms 1-1)
ma
h 0.2 0.4 0.6 0.8 1
1 0. 122 0.245 0.367 0.490 0.612
mfd 2 0.010 0.037 0.080 0. 135 0. 195
mfd 4 0.005 0.011
2mfd 1 0. 116 0.200 0.227 0. 192 0. 111
2mfd 5 0.008 0. 020
3mfd 2 0. 027 0.085 0. 124 0. 108 0.038
3mfd 4 0.007 0. 029 0. 064 0.096
4med 1 0. 100 0.096 0. 005 0.064 0. 042
4mfd 5 0. 021 0.051 0.073
4mfd 7 0.010 0.030


synchronous -
Inductor Ivre







SSG


Figure 2-57. Static Synchronous Generator diagram

Thus, the inverter voltage harmonics would generate current harmonics, which

amplitude would not only be a function of the inverter' s mf and ma, but the synchronous

inductor as well. The inverter current harmonics can be calculated as:

*For h=1 (fundamental frequency):
~netv m 1-Vs,, Ys1 1
I (h = vers re:-2 ourc re:-2(2-45)
res~24 h -L


*For h>1 (assuming no harmonics are present in the utility bus voltage):
nvrerrs-2() 1 Vdc -K 1
I nl(h)= I~~re
res 24 -h-L 24c -h- L


(2-46)


Where fl is the fundamental frequency.









The inverter current harmonics must be attenuated in order to avoid interference

with communication circuits and other types of equipment, the increase of system losses,

resonance conditions, and malfunction of power electronic devices.

The IEEE 519 Standard [29] is a recommended practice to be used for guidance in

the design of power systems with nonlinear loads and therefore should be taken into

account on the design of the switching ripple filter. The worst case scenario is for general

distribution systems (120V through 69000V) with a TDD < 5% for current harmonics

below the 50th. TDD is the total demand distortion and is defined as




TDD(%) = h=2 100 (2-47)


The maximum demand load, which is IL, can be estimated from data used to size

the inverter isolation transformer.

Switching frequency

The selection of the switching frequency was based on the recommendations given

by [28], which stated that:

* Because of the relative ease in filtering harmonic voltages at high frequencies, it is
always desirable to use as high a switching frequency as possible.

* In most applications, the switching frequency is selected to be either less than 6
k
* In order to avoid sub-harmonics, synchronous PWM must be used. Synchronous
PWM requires that me be an integer.

* In the 3-wire three-phase inverters, only the harmonics in the line-to-line voltage
are of concern, and only the odd harmonics exit as sidebands, centered around mf
and its multiples, provided mf is odd.

* If mf is chosen to be an odd multiple of 3 the most dominant harmonics in the line-
to-line voltage (even harmonics of mf) will be cancelled out.





Characteri sti cs L filter LCL filter
Control method Hysteresis controllers Fixed switching frequency
control methods
Attenuation above 20dB 60 dB
resonance frequency ( first order system) (third order system)

Igrzdte (S>

Total line filter inductance High line inductance, and Low line inductance, and
for a given grid current therefore poor transient fast transient performance
ripple magnitude performance


*For high power applications (kVA) where switching losses play a maj or role in the
overall system design, the switching frequency is usually selected between 3 k and 5 kHz.

Taking all these elements into consideration, the optimal switching frequency

selected for the inverter and for the chopper was f,, = J; mf = 60 81 = 4860H:

Passive filter design

The switching ripple filter topology selected for the inverter filter was based on a

LCL network as shown in Figure 2-58.


Isolation transformer Filter or synchronous
equivalent impedance Inductor
I~ .


Inverter


sorc

I


LCL switching ripple filter

Figure 2-58. LCL filter topology


The main advantages of the LCL filter compare to the L filter are summarized in

Table 2-4.


Table 2-4. L filter vs. LCL filter


I
S Invre


Filter
-capacitor









The LCL inverter filter equations are the following:

dl

dl
V V = L, grid (2-48)

in e te ri d t



Applying Laplace's transform, the LCL inverter filter can be modeled as shown in

Figure 2-59.








Irx




Figure 2-59. LCL equivalent block diagram

The inverter filter was divided into two different equivalent filter models based on

the frequency under study. Thus, we have:

Equivalent filter circuit configuration at fundamental frequency. Under these

conditions the inverter was considered as an ideal sinusoidal voltage source. This was the

lineal inverter model valid for the design of the system controllers. Figure 2-60 shows the

filter equivalent system at fundamental frequency.


r Vsou~rce(l Lt L, ~ y(i





Figure 2-60. Single phase equivalent filter model at the fundamental frequency










Equivalent filter circuit configuration for the h harmonic (for h>1). At high

frequencies the converter was considered to be a harmonic generator, while the grid can

be considered short-circuited.



Vsourc (hfl)= 0fYT

Sgrld(hfl) I nverter (hfl)


Figure 2-61. Single phase equivalent filter model at the hth harmonic

Thus, the current ripple attenuation, passing from the inverter side to the grid side

can be calculated as

Igrzd rms(S) 1
nverter, msIn(s) s3 Lt -Cf-Lf + s -(Lt + Lf)


5,,nvter res(s) s42 Ci Lr +1 (-9
nveterms -ns)3 L, Cr ,-L + s -(L, + Lr)


Igrzd rms(S) 1
Iznverteru re(s) s2 C, Lt +1

There are different ways of designing the LCL inverter filter, as well as different

specifications or constrains. Table 2-5 is a summary of the most common parameter used

to design the inverter filter.

It can be inferred from Table 2-5 that there is no a unique approach or limit when

designing the LCL filter.

The LCL parameters selected for the inverter filter design are

X,, = 10% -> L, = 265.25 pH
Xof = 3333.33% -> C, = 79.577pFF
X,, = 5% -> L, = 132.62plI (typical transformer equivalent impedance)












Table 2-5. LCL filter design
Parameter Description Eauations Limits
Current Maximum Peak to Peak For ma < 1 Peak to Peak value:
ripple value m, yd 15%-25% of rated current [35]
nr =Msi m2 37 31% of rated current [32]
Note: Maximum r',,te ma 4 -L, f, 4 -L, f,
current ripple at
Vsource(t)=0 differs For ma =
from Vsource(t)= Vmax Vdc
ernveter repple max 7 Li fSn
Most significant For ma =1 Most significant harmonic component
harmonic components Islefrmph=m 2"Vde0 1 (mf+t2)
'" 27rf h Lf 10% of rated current [30]
*1.6% of rated current [31]
Attenuation Laplace domain Igradrms(S) 1 0.2 attenuation [30]
of I,;,, ,(s) s2 -Cf -L+ 0.5 attenuation [32]
harmonic
content Frequency domain Igrrd rms h = mf 2) Zc,~ (A h)
I~mterem h= y -) IZc, (A-;h)+ Z, Cf; -h)
Voltage drop across the filter during AllmaxL IlnverterI ma1x 21f, Lf Total value of inductance should be
normal operation lower than 10% to limit the voltage
drop and the de link voltage[30],[33]
*1.7% on the inverter kVA base [34]
Filter resonant frequency 1 Resonance frequency between 10 times
2reso I =2x C (L, //L, the line frequency and half of the
switching frequency[30][33]
Filter capacitor reactive power 1 Lc,i (%) Pinveter ratedi power Qf
C, = 2 *<5%[30]-[33]-[34]
100 3-2-z- f,suc -V, I
ourc res-" *15% [35]










The electrical characteristics of the LCL filter for the system parameters given

Table 2-7 in are summarized in Table 2-6.

Table 2-6. LCL equivalent impedance with damping resistance

Parameter Equation Stiff system

Current ripple At twerter current ripple max i 0.2264 pu (App)
(peak to peak) rwLf ,
Current ripple
(most i,,,~ i(fiM -2-) p Vdc 0.2- 1 0.00003 pu
significant J5 21(fsw -2- fl). Lf (Arms)
harmonic)
Harmonic Igrzd rms fsw 2 -f) ZC(, /w- 2 -fl)
attenuation r,,nverter rm(fs 2 f-lzc ZC/s 2 fl)+ ZLt (sw 2 fl)-85d
Max Voltage 0 p Vls
Amax L Inverter max 2x fl LI. u Vls
drop
Filter resonant 1
fresonant = 1897 Hz
frequency 2.2 C (Lt//L )
Filter capacitor c,32r-f V 3.0% VAr
reactive power COc, (%) = Pnveter rated on ev ms:- lo (Icf, = 0.03 pu )


Table 2-7. Per-unit system
Variable Per unit
M4nX line-neutra 1.0
V 1.22474
RM/S line-lne
YRM/S lne-neutral .07
I 1.0
M4X Inverter nommnal
Im wrernma 0.70711

Zbase1.
nverter 1.5
V, 2.0412


Passive filter damping

To determine the system stability, the LCL inverter filter damping resistances must

be taken into account when calculating the system attenuation at resonance frequency.

The system resistors are given in Figure 2-62.










VInveter


Vsource


Figure 2-62. LCL equivalent impedance with damping resistance

Using a X/R=10 for all inductors, the damping resistances of the LCL filter are

X
X, = 10% -> = 10 -> R, = 0.0 la


X
'= 10 -> R, = 0.0050
R,


X,, = 5% ->


The LCL inverter filter could resonate due to harmonics generated either from the

source or from the inverter. The two equivalent circuits are


vorc


Rt Vpacltr __L^ Rf Vlne


Figure 2-63. Single phase harmonic generator equivalent circuits. A) Inverter as a
harmonic generator. B) Source as a harmonic generator


Thus, the apa o transfer functions are given in Equations 2-50 and 2-51:


(2-50)


caa~cro,


(2- 51)


capacrto, 1
~nverter 1 1 ~


1+Z, -+







67



The Bode frequency response of both models is given in Figure 2-64.


Bode Diagram Vaa ol ule















Frequency (Hz)


Figure 2-64. LCL gain frequency response

It can be deduced from Figure 2-64 that there is a significant gain at the resonance


frequency (small system damping resistance), and therefore harmonics close to this


frequency could be amplified by the LCL filter.

From the inverter point of view there are two sources of disturbances:


1. Voltage harmonics due to the PWM
2. Disturbances amplified by the current regulator


LCL (Vcapacitor/Vinverter)
40 Current regulator
-Vlnvelter I-n harmonics












102 103 104
Frequency


Figure 2-65. Inverter frequency analysis










Figure 2-65 shows the current regulator frequency response, the filter frequency

response, and the inverter line-to-neutral harmonic spectrum. It can be inferred from

Figure 2-65 that the current regulator attenuates any signal with a frequency > 400Hz

(cut-off frequency), and the inverter voltage harmonics do not make the LCL filter

resonate.

From the point of view of the voltage source there are two sources of disturbances:

* Large infrequent transient, such as capacitor bank switching. This type of
disturbance may ring the filter, but it will damp out in a few cycles.

* System harmonics. A detail study of the system it is required to determine if it is
likely .

Direct-Current Link Capacitor Design

The DC link bus voltage had the following constrains:

* IPM Max voltage 1200V.

* Line to line voltage 480 V. This would allow the use standard isolation
transformers

* Minimum DC link voltage = 1.1 Vmax rlne ro -ane = 1.1 .480 J = 750 V Minimnum
voltage to guarantee system controllability.

* IPM trip level = 900V. Capacitor switching voltage transients tend to raise the DC
link voltage and could damage the IGBT's. A trip level of 900 V allows riding-
through the maj ority of the capacitor switching transients.

* Low DC-link voltage was desirable in order to reduce the switching losses.

Given these system restrictions the selected DC-link voltage was 800V. In per unit

800
Vdc-lnk =J2 2.0412 pu




The dimensioning of the DC link capacitor was determined by the following

constramns:










* Maximum permissible current stress for a required working life (current ripple)
* Existence of any zero sequence component
* System controllability ( avoid large gains)
* Max ripple voltage 10%

For a traditional STATCOM configuration, the DC-link capacitor is necessary for

an unbalanced system operation and harmonic absorption. For a configuration with

energy storage, the DC link capacitor main function is to reduce the DC current ripple

from/into the ESS and therefore a smaller DC-link capacitor could be used.

DC link Energy
The time constant selected for our study was = 21.5 msec Thus,
InverterPower


the DC-link capacitor in per-unit model is

DC -link Energy = 2msc=DC -link Energy,,
Inverter Power n'erter pu


DC-link Energy,
DC-link Energy ,, = 0.033J
1.5W


DC link Energy, = -t Cdc-hnk 1 -hnk, d c-hnk 15700 p F


Energy Storage Design

The Energy Storage System (ESS) design parameters were

* The voltage at the energy storage system (ESS) was designed to vary from 95% to
0.95*"50% of the DC link voltage.

Total Energy in the ESS=204se
Inverter Power

Note: More on the design and size of the Power Stabilizer energy storage system

can be found in [36].

The center voltage of the ESS can be calculated as









-1 (1 1 1 (1 2
2 storage storage max storage cetr 2 storage storage center storage m
(2-52)
V,, 1 V2+V


Thus, for a system with a Vdcnominal = 2.0412pu the ESS nominal voltage was


storage max0.95 2.0412 = 1.9391nu
ysyg m=0.95 0.50 2.0412 = 0.9695 pu





Figure 2-66 shows the relationship between the capacitor voltage and the energy

storage.

The capacitance of the ESS in per-unit can be calculated from the time constant

Total Energy in the ESS
as
Inverter Power

Max Energy Max Energyp
S20.45 sec -> Max Energy, = 30.67J
Power nverter pu

Max~~ Enrg, storage storage ma storage =1.


Figure 2-66. Capacitor Voltage vs. Energy Storage










Chopper Inductor Design

The purpose of chopper inductor was to reduce the current ripple produced by the

chopper in order to guarantee the ESS working life (Figure 2-67).

The current ripple current selected for this application was 30%. Under this

condition the chopper inductance in per unit is calculated as shown in Equation 2-53.


A chopper = Lhopper dchtye ,w eeA chopper = e Vd-nk storage (-3







dc-link Ichopper Lehopper
d c-link V



-I ~~storage trg




Figure 2-67. ESS-Chopper topology

In the worst case scenario the DC-link voltage is at its nominal value, while the

voltage at the ESS is at its minimum. Thus, the maximum voltage drop across the

chopper inductor is

chopper max = dc-knk nommal storage mm r .42-099 .77p

Discretization of the chopper inductor voltage drop differential equation yields the

Equation 2-54.

AI
AV = L chopper (2-54)
chopper chopper a

1 1
Where Alhope is the ripple current, and At = as shown in Figure 2-68.
chopper 2 fr











T,


Triangular
waveform







I
chopper


t l


Time


Vdc-link=2.0412 pu


Time


Figure 2-68. Equivalent circuit for maximum current ripple calculation

Thus, the chopper inductor in per unit is:


1
1.0717 -2-46

chopper


A chopper max At

chopper


L
chopper


chope = 0.03 Icopr ae (30% ripple)


SLehoppe = 500plI


Pinveternomal
I
chopper rated V
dc-hnk


1.5W
2.0412


0.73 52 pu (A)


Per-Unit System Model Summary

Table 2-8 is a summary of the per-unit system parameters used in the control and

modeling of the system.


Al
chopper
( ,,,,V,,=.65p
Lehopper











Specs


Table 2-8. Per-unit system parameters
Variable Per-unit Model
yM4X kne-neutral 1O
YRM/S kne-hne1.22474V
RM/S kne-neutral 0.70677V
I 1.0A
M4X Inverter nommnal
IIms Inverter nommnal 0771
Base 10
~nverter 1. 5W
V 2.0412V
dc nommnal
V 1.533V
storage center
Cdc knk 15700 IF
Cstrg 16.31F

Lehopper 500.01H
Rchpe 0.0188490
L, 265.251H
R, 0.010
C, 79.57 IF
L, 132.631H
R, 0.050


21.5 msec time constant

20.45 sec time constant (at
maximum ESS voltage)
30% current ripple
X/R=10

10% Impedance
X/R=10

3% VArs (3333.3% Impedance)
5% Impedance
X/R=10


Simulated Model

Power Systems Computer Aided Design (PSCAD) was used for the modeling and

simulation of the power stabilizer. The PSCAD model was based on the per-unit system,

so that the system performance could be compared to any given unit size. Figure 2-69

shows the main components of the system.

The PSCAD model can be divided into two main subsystems; the electric system

(Figure 2-70) and the control algorithm (Figure 2-71).










The maj ority of components used in the modeling were part of the PSCAD library.

Only the power limiter 2 had to be implemented in FORTRAN and linked to PSCAD

given the complexity of its design.

UTILITY
SYSTEM



XsourceChopper



WIND
FARM LCL INVERTER ESS


Figure 2-69. System overview








I I t t II I I I
I~ II I I II

LCL filter Inverter DC link bus Chopper

Figure 2-70. Per-unit electric system model

Table 2-9 shows the model performance as well as the designed specifications for

comparison. It can be observed that designed and simulated system closely agree.






















I

In e',~;~;~,~,d"'

Sve Irt rl"


I--------n


II-t I~Vr,,, Irr-





-I III


I I I I



Currentregulator Limiters & Transformations IT~t"


Frame transformation & positive seq uence calculation Flat-top


I


CarKV1 PLL
aarK 1 ParK 1
L k
~-~--~~ abe /
.,,,,,,,,,
4v. 4 me, I ~,...
ParKV1 L /d,-q, 'eguafor
V. rorurrent 41. R
snonDtar ,onnotar
u. ;rl~c~,,
'eguafor
,r

I "'"' ,, ,.,...~..,.....

""''
,,,,,,,...~..,..... ,~~~~~~~~~~~~ rg
"'"'"
"'"' g
snrtnDtar rosonDtar
r------------------------------~
P"o""'"'
v,

~ ~'t~,3~0 Y^". ~~~~;3~0
'. n f --
,,
,,..,,,.,.
-
~

L______________________________r

Chopper Control Scheme



Figure 2-71. Power Stabilizer Control Scheme


Rotating Reference


Power Limiter








76



Table 2-9. Designed system results and simulated system results comparison. System
conditions: V de link =2.04 pu, V source, ma=1 pu, stiff system


Parameter
Inverter
current

ripple


Model response


Design system/Comments


4-L,-f~ 2


4
-
"------=


Marnwessy tem_= Marx(0 198, O 2241)= 0 2241 pu (App)


Isvr or res _(h =m + 2)4 V 0 2 1
1 2nf -h -L,
Isaver,res_(h =mi &2)4 002986 pu(A rms)


Frequency(Hz)


Harmonic
attenuati
OH


I nnn(frw
I,,,,.(f,
Ignd rs(fr
Isanrd nr (f m


-2.f) Zc,1-2-f
- 2 ) Z ,(s ) z /w-2-
-2-,)
= 18 5bB
2 )
2- )=0O1188-0O02986= 00035pu(Arms)


0 005


3500 4000 4500 5000 5500 6000 6500
Frequency(Hz)


Current

regulator
step
response


Iqref step change[ 1
(from capacitive to inductive)
No PCC voltage regulator
Stiff system
Limiters: Vmax=1.15, Vflat=1


05


0 002 0 004 0 006 0 008 0 01 0 012 0 014 0 016 0 018 0 02
Time(sec)


Dc link

step
response


Vdc ref Step change [2.0412 2.3]
Stiff system


20L 0 01 0 02 0 03 0 04 0 05 0 06 0 07 0 08 0 09 0 1
Time secss)






















































SI4loit nllnilodi
i


:o lio :a -1 :n a: ia: n


Table 2-9. Continued

Parameter Model response
Voltage ls, ,
regulation xsource-20%


Design system/Comments
Vpecrerstep change [1.0 1.05]
Variable line impedance:
[1% 2% 4% 5% 10% 20% ]


Current
regulator
bandwidth


I,,er =0.2sin (wt)
Idref Iess+0.2cos (wt)
f= [120 300 420 540 660


Vdc-link=2.0412pu (V)
Vsouce max 1-n= 1 pu (V)
Stiff system


Theortic
current regulator
frequency response
PSCAO model
current regulator
frequency response


Frequency (Hz)

Cut-off frequency 400 Hz


Power
filtering


Power limiter 2 simulation
results for a sampling time of 2
seconds















CHAPTER 3
SYSTEM DESCRIPTION

System Overview

The performance of the power advanced electronic device was tested in a test

bench based on:

* DC motor synchronous machine set
* Passive load
* DC motor asynchronous machine set
* Wind farm buffer

The basic idea was to reproduce the basic electrical components of a small isolated

system in order to asses the benefits of smoothing wind-power fluctuations. Figure 3-1

shows this main idea.

Bulk generation. The system model's bulk generation was represented with a

single synchronous machine, which the main function was to control the system

frequency and voltage.

Load. The power system's load composition was strongly dependent on the time of

day, month, and season, but also on weather. A typical load profile was studied in [37],

and can have the following approximate composition:

* Induction motors, 60 per cent
* Synchronous motors, 20 per cent
* Other ingredients (passive load, electronics...), 20 per cent









Wind Farm model
OCMotor I\rynohronour
Lne

1





.~c ~t~t






L:


Electric Network Model


Figure 3-1. Equivalent system model

Dynamic loads usually consume between 60 to 70 % of the total power system

energy. However, their dynamics are of special importance for voltage stability studies

due to their reactive power requirements. Thus, since only real power fluctuations were

of interest in our study, the system' s load was reduced to a one three-phase passive load.

Renewable Resources. Renewable resources are growing faster than traditional

energy sources, with the fastest growth being in wind and solar energy. It is expected that

in the near future, they will play a significant role in the generation mix.

The system's renewable resources were modeled using a single induction generator

that would represent 15% of the system capacity. This number was very conservative

compared to other grids such as Western Denmark with a penetration level of 63% of

peak load and the Island of Crete, where wind power has a penetration level close to

40%.


Pil SLW


~--sunohronou
~ ..,,,..


I


II.1LL

t'"~i ":"~"
Power Stabilizer










Power Quality Devices. Because of wind power' s high penetration factor in the

near future, new advanced power electronic devices as well as grid operation procedures

have to emerge to minimize the impact of non-dispatchable wind power.

In modeling the system, only a proof of concept wind farm buffer was considered

to study different control schemes that could reduce wind-power fluctuations.

Electrical Network Model

Synchronous Machine

The first requirement of a reliable service is to have the synchronous generators

with adequate capacity to meet the load demand. Any unbalance between the generation

and load initiates a transient that causes the synchronous machine to accelerate or

decelerate due to the appearance of net torques on the rotor.

It can be shown that the interconnection ofj finite machines with inertia constants

Mj can be reduced to a single finite machine with inertia H, where H can be calculated as

shown in Equation 3-1.


H = (3-1)
11 1
-+-+...+-
H, H2 H~

The synchronous machine selected for modeling the electrical system is a three-

phase, brushless, self excited, externally regulated, AC generator.

The ratings of the synchronous machine were

* Rated Power 7.0 kW intermittent, 5.4 kW Continuous
* Rated Voltage 240/480 3ph 60 Hz
* Rated Speed 1800 rpm










The system voltage selected for the model is 480V; therefore the synchronous

machine's coils were connected in a high series Y configuration.

Voltage regulation

Load voltage regulation was mainly carried out by the generator's exciter using an

external voltage regulator. The automatic voltage regulator received both its input power

and voltage sensing from the generator' s output terminals. The DC output voltage of the

exciter field required to maintain constant the generator' s terminal voltage was

automatically changed by the voltage regulator, which had a voltage regulation accuracy

of 1%. The voltage regulator set point was 480V, line to line.

Due to synchronous machine imperfections and asymmetries, output voltage was

not an ideal sinusoidal waveform, as shown in Table 3-1. The most significant distortions

were the second, third, fourth, and fifth harmonics, with an unbalance of approximately

1%. Such types of distortions were not very common in electric systems and may have an

impact on the control system. Simple sliding windows were used to filter/reduce their

impact.

Prime Mover

The prime movers of large generators are principally hydraulic turbines, steam

turbines, and combustion turbines. In our model the prime mover that was used to

produce the mechanical torque was a DC machine with the following specs:

* Rated Power: 7.5 HP
* Armature Voltage 240 V dc
* Field Voltage 150 V dc
* Rated Speed 1750 rpm










Both machines were connected in cascade through their shaft, so power could be

transferred from one machine to another. Figure 3-2 shows the system configuration as

well as the variables used in the control.

Table 3-1. Synchronous machine output voltage profie at rated speed
Synchronous Machine Voltage Synchronous Machine Voltage
Features
profie for unloaded condition profie for unloaded condition
Waveform so


FFT


Frequency(Hz)


Unbalance


A single quadrant chopper was used for speed control of the DC machine. Chopper

circuit specs are shown in Figure 3-3.

Note: Figure 3-3 shows that the DC power supply was used by the two choppers

required in the model. One was for the prime mover of the synchronous machine, and the

other one a different DC machine that would represent wind speed variations.