High-Frequency Isolated DC/AC and Bidirectional DC/DC Converters for PMSG-based Wind Turbine Generation System by Xiaodong Li B.Eng., Shanghai Jiao Tong University, 1994 M.A.Sc., University of Victoria, 2004 A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY in the Department of Electrical and Computer Engineering (cid:176)c Xiaodong Li, 2009 University of Victoria All rights reserved. This dissertation may not be reproduced in whole or in part, by photocopying or other means, without the permission of the author. ii High-Frequency Isolated DC/AC and Bidirectional DC/DC Converters for PMSG-based Wind Turbine Generation System by Xiaodong Li B.Eng., Shanghai Jiao Tong University, 1994 M.A.Sc., University of Victoria, 2004 Supervisory Committee Dr. Ashoka K. S. Bhat, Supervisor (Department of Electrical and Computer Engineering) Dr. Subhasis Nandi, Departmental Member (Department of Electrical and Computer Engineering) Dr. Harry H. L. Kwok, Departmental Member (Department of Electrical and Computer Engineering) Dr. Sadik Dost, Outside Member (Department of Mechanical Engineering) iii Supervisory Committee Dr. Ashoka K. S. Bhat, Supervisor (Department of Electrical and Computer Engineering) Dr. Subhasis Nandi, Departmental Member (Department of Electrical and Computer Engineering) Dr. Harry H. L. Kwok, Departmental Member (Department of Electrical and Computer Engineering) Dr. Sadik Dost, Outside Member (Department of Mechanical Engineering) ABSTRACT In this dissertation, a high-frequency (HF) transformer isolated grid-connected power converter system with battery backup function is proposed for a small-scale wind generation system (less than 100 kW) using permanent magnet synchronous generator (PMSG). The system includes a main HF isolated DC/AC grid-connected converter and a bidirectional HF isolated DC/DC converter. Through literature survey and some comparative studies, a HF isolated DC/DC converter followed by a line connected inverter (LCI) is chosen as the grid-connected scheme. After reviewing several topologies which were used in such a DC/AC con- verter with an unfolding stage, a DC/AC grid-connected converter based on dual- bridge LCL-type resonant topology is proposed. Through the control of the phase- shift angle between the two bridges, a rectified sinusoidal dc link current can be iv modulated, which in turn can be unfolded by the LCI. This converter is analyzed with Fourier series analysis approach. It is shown that all switches in both bridges can work in zero-voltage switching (ZVS) at any phase-shift and load conditions. The redundancy of the dual-bridge structure make it easy to accommodate higher power flow. A design example of a 500 W converter is given and simulated. A prototype is built and tested in the lab to validate its performance. The simulation and exper- imental results show a reasonable match to the theoretical analysis. The expansion to three-phase grid-connection is discussed with phase-shifted parallel operation of three identical units. Input and output current harmonics of different arrangements are analyzed to search for the best choice. As the feature of a hybrid wind generation application, the battery backup func- tion is fulfilled with a bidirectional HF transformer isolated DC/DC converter. This dual-bridge series resonant converter (DBSRC) is analyzed with two ac equivalent circuit approaches for resistive load and battery load respectively, which give same results. Soft-switchingisachievedforallswitchesonbothsidesoftheHFtransformer. Test plots obtained from simulation and experiment are included for validation. v Contents Supervisory Committee ii Abstract iii Table of Contents v List of Abbreviations ix List of Symbols x List of Tables xi List of Figures xiii Acknowledgements xxxi Dedication xxxii Chapter 1 Introduction 1 1.1 History of Wind Energy Utilization . . . . . . . . . . . . . . . . . . . 1 1.2 Variable-Speed versus Constant-Speed . . . . . . . . . . . . . . . . . 3 1.3 WTGS Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3.1 Induction Generator . . . . . . . . . . . . . . . . . . . . . . . 7 1.3.2 Synchronous Generator . . . . . . . . . . . . . . . . . . . . . . 9 1.4 Motivation and Objective . . . . . . . . . . . . . . . . . . . . . . . . 11 vi 1.5 Outline of the Dissertation . . . . . . . . . . . . . . . . . . . . . . . . 16 1.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Chapter 2 ComparisonandSelectionofSuitableHFIsolatedDC/AC Grid-Connected Converter 18 2.1 High frequency converter with soft switching . . . . . . . . . . . . . . 19 2.2 ComparisonandselectionofHFisolatedDC/ACgridconnectionschemes 22 2.2.1 Stage 1: DC to HF AC . . . . . . . . . . . . . . . . . . . . . . 22 2.2.2 Stage 2: HF AC to LF AC . . . . . . . . . . . . . . . . . . . . 24 2.3 Topologies of HF isolated DC-to-LFAC converter including an unfold- ing LCI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.3.1 Topology 1: Variable frequency sinusoidally-modulated PWM converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.3.2 Topology 2: Fixed frequency sinusoidally-modulated LCL-type SRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 2.3.3 Topology 3: Fixed frequency sinusoidally-modulated parallel dual SRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 2.3.4 Topology4: Fixedfrequencysinusoidally-modulatedseriesdual SRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 2.3.5 Pros and cons of the four topologies . . . . . . . . . . . . . . . 42 2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Chapter 3 A Phase-Modulated High-Frequency Dual-Bridge LCL DC/AC Resonant Converter 60 3.1 Principle of a HF Isolated Dual-Bridge LCL Resonant Converter . . . 61 3.2 Fourier Series Analysis of the Proposed Converter . . . . . . . . . . . 67 3.3 Design Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 vii 3.4 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 3.5 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Chapter 4 Multi-cell Operation of HF Isolated Dual LCL Reso- nant Converter 116 4.1 Multi-cell operation of high power single-phase grid connection . . . . 117 4.1.1 Multi-cell operation of high power three-phase grid connection 118 4.1.2 Input dc current harmonics . . . . . . . . . . . . . . . . . . . 118 4.1.3 Y connection of three cells to three-phase grid . . . . . . . . . 120 4.1.4 ∆ connection of three cells to three-phase grid . . . . . . . . 121 4.2 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 4.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Chapter 5 A Bidirectional HF Isolated Dual-bridge SRC for Bat- tery Charging 129 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 5.2 Principle of the Proposed Bidirectional Battery Charger . . . . . . . 131 5.2.1 Charging (Controlled Rectifier) Mode . . . . . . . . . . . . . . 133 5.2.2 Discharging (Regeneration) Mode . . . . . . . . . . . . . . . . 137 5.3 AC Equivalent Circuit Analysis for DBSRC . . . . . . . . . . . . . . 137 5.3.1 Normalization and Definitions . . . . . . . . . . . . . . . . . . 139 5.3.2 Method I for Voltage Source Load . . . . . . . . . . . . . . . 140 5.3.3 Method II for Resistive Load . . . . . . . . . . . . . . . . . . . 142 5.4 Design Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 5.5 Simulation and experiment results . . . . . . . . . . . . . . . . . . . . 156 5.5.1 Simulation results . . . . . . . . . . . . . . . . . . . . . . . . . 156 viii 5.5.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . 157 5.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Chapter 6 Conclusions 170 6.1 Summary of Work Done . . . . . . . . . . . . . . . . . . . . . . . . . 170 6.2 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 6.3 Suggestions for Future Work . . . . . . . . . . . . . . . . . . . . . . . 173 Appendix A Circuit layout of simulations in Chapter 2 186 Appendix B Implementation of control circuit of Chapter 3 191 Appendix C Phasor domain analysis of the dual-bridge LCL converter194 Appendix D Simulation layout of Chapter 4 199 Appendix E Design of RC snubber 200 Appendix F More Simulation Results of DBSRC in Chapter 5 202 ix List of Abbreviations ac, AC alternative current BJT bipolar junction transistor DBSRC dual-bridge series resonant converter dc, DC direct current DFIG doubly-fed induction generator EMI electro-magnetic interference FFT fast fourier transformation HF high frequency IG induction generator IGBT insulated-gate bipolar transistor MOSFET metal-oxide-semiconductor field-effect transistor MPPT maximum power point tracking PFC power factor correction PMSG permanent magnet synchronous generator SCIG squirrel cage induction generator SCR silicon controlled rectifier SG synchronous generator THD total harmonics distortion WRIG wound rotor induction generator WRSG wound rotor synchronous generator WTGS wind turbine generation system ZCS zero-current switching ZVS zero-voltage switching x List of Symbols α, β, θ, φ angles ω angular frequency c, C capacitance d, D diodes f, F frequency i, I current J normalized current l, L inductance M voltage gain n, n , n , n transformer ratio p s t P power r, R resistance s, S switches t time v, V voltage X reactance Z impeadance