VOLTAGE BALANCING TECHNIQUES FOR FLYING CAPACITORS USED IN SOFT-SWITCHING MULTILEVEL ACTIVE POWER FILTERS

Transcription

1 VOLTAGE BALANCING TECHNIQUES FOR FLYING CAPACITORS USED IN SOFT-SWITCHING MULTILEVEL ACTIVE POWER FILTERS Byeong-Mun Song Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical Engineering Dr. Jason Lai, Chair Dr. Fred C. Lee Dr. Dan Y. Chen Dr. Werner Kohler Dr. Hugh F. VanLandingham July 24, 2001 Blacksburg, Virginia Keywords, Active Power Filter, Soft-switching Topologies, Multilevel Converters, Voltage Balancing, Flying Capacitor Copyright 2001, Byeong-Mun Song

2 Voltage Balancing Techniques for Flying Capacitors Used in Soft-switching Multilevel Active Power Filters by Byeong-Mun Song Committee Chairman: Dr. Jason Lai Department of Electrical and Computer Engineering ABSTRACT This dissertation presents voltage stabilization techniques for flying capacitors used in soft-switching multilevel active power filters. The proposed active filter has proved to be a solution for power system harmonics produced by static high power converters. However, voltage unbalance of the clamping capacitors in the active filter in practical applications was observed due to its unequal parameters. Thus, the fundamentals of flying capacitors were characterized dealing with voltage balancing between flying capacitors and dc capacitors under practical operation, rather than ideal conditions. The study of voltage balancing provides the fundamental high-level solutions to flying capacitor based multilevel converter and inverter applications without additional passive balancing circuits. The use of proposed voltage balancing techniques made it possible to have a simple structure for solving the problems associated with the conventional bulky passive

3 -iiresistors and capacitor banks. Furthermore, the proposed control algorithms can be implemented with a real time digital signal processor. It can achieve the high performance of the active filter by compensating an adaptive gain to the controller. The effectiveness of the proposed controller was confirmed through various simulations and experiments. The focus of this study is to identify and develop voltage stabilization techniques for flying capacitors used in a proposed active filter. The voltage unbalance is investigated and characterized to provide safe operations. After having defined the problems associated with the voltage unbalance, the most important voltage stabilization techniques are proposed to solve this problem, in conjunction with an instantaneous reactive power (IRP) control of an active filter. In order to reduce the switching losses and improve the efficiency of the active filter, the proposed soft-switching techniques were evaluated through simulation and experimentation. Experimental results indicate that the proposed active filter achieved zero-voltage conditions in all of the main switches and zero-current turn-off conditions to the auxiliary switches during commutation processes. Also, various studies on soft-switching techniques, multilevel inverters, control issues and dynamics of the proposed active filter are discussed and analyzed in depth.

4 To my wife Eun-Hee, daughter Ji-Hae (Diana), son In-Bum (David) And Our families in Korea

5 Acknowledgements In the name of God, I would like to thank my advisor, Dr. Jason Lai, who has been a constant source of support and guidance throughout my studies. Since September of 1996, I had gained his strong industrial background in Virginia Power Electronics Center (VPEC). I am very grateful for his training, which will be a valuable resource in my professional career. He Since I started working with PEC. I would also like to thank Dr. Fred C. Lee for his remarkable advice and concern for this topic. His strong academic background made it possible for me to gain more knowledge from the study. I also wish to thank Dr. Dan Chen, who is an excellent educator. His valuable encouragement enabled me to finish my successful study. I thank Dr. Hugh F. VanLandingham, and Dr. Werner Kohler for serving as a member of my committee. I would like to thank my former advisor, Dr. Krishnan Ramu, for his support and guidance for the 2 years that I was part of his lab, Motion Control Systems Research Group (MCSRG). At that time I had gained a lot of knowledge about motor drives. I am very grateful to all my friends in the Center for Power Electronics Systems (CPES) and Virginia Power Electronics Center (VPEC) for valuable discussions about research and our vision. I would like to thank Korea Electrotechnology Research Institute (KERI) to support my research as several national projects. Especially, I would like to thank Dr. Dong-Wook Yoo, Dr. Ki-Youn Joe, and Geun-Hee Rim. I am very grateful to my family members in Korea, who have been a great source of support and comfort. Most of all, I would like to express my deep indebtedness to my wife Eun- Hee, who has been a constant pillar of support and understanding. I owe it to her for all the sacrifices she made to care for our two children, Ji-Hae and In-Bum while I worked on this project.

6 TABLE OF CONTENTS 1. INTRODUCTION OVERVIEW STATE OF THE ART Power Quality and Active Filters Multilevel Active Power Filters Soft-Switching Techniques Control Issues of Active Filter Fundamentals Issues of Flying Capacitors SCOPE DISSERTATION OUTLINE SOFT-SWITCHING MULTILEVEL INVERTER WITH FLYING CAPACITORS FOR ACTIVE POWER FILTER APPLICATIONS INTRODUCTION REVIEWS OF SOFT-SWITCHING INVERTERS PROPOSED SOFT-SWITCHING INVERTER TOPOLOGY CHARACTERISTICS OF SOFT-SWITCHING CIRCUITARY Coupled Inductor Characteristics Reset Mechanism with Saturable Inductor Soft-switching Control SIMULATION AND EXPERIMENTAL RESULTS CONCLUSION 44

7 3. FUNDAMENTALS OF VOLTAGE STABILIZATION FOR FLYING CAPACITORS INTRODUCTION FUNDAMENTAL ISSUES OF VOLTAGE STABILIZATION Flying Capacitors DC Link Split Capacitors Voltage Synthesizing ANALYSIS OF MAIN CIRCUIT AND ITS MODULATION Circuit Analysis Voltage Synthesizing Modulations CHARATERISTICS OF FLYING CAPACITORS CHARATERISTICS OF SNUBBING CAPACITORS CHARATERISTICS OF DC LINK CAPACITORS INTERACTION BETWEEN FLYING CAPACITORS AND DC CAPACITOR CONCLUSION CONTROL FOR VOLTAGE STABILIZATION INTRODUCTION INSTANTANEOUS REACTIVE POWER CONTROL CURRENT CONTROL DC BUS CAPACITOR VOLTAGE CONTROL FLYING CAPACITOR VOLTAGE CONTROL Analysis of Voltage Variations Voltage Stabilizer DISCUSSION 103

8 5. SIMULATION AND EXPERIMENTAL RESULTS SIMULATION RESULTS Control Models Performance Evaluation Conclusion EXPERIMENTAL RESULTS Test Descriptions Experimental Verification Dynamic Responses CONCLUSION CONCLUSION SUMMARY FURTHER RECOMMENDATIONS..121 REFERENCES.122 APPENDIX: A: A NOVEL TWO-QUADRANT SOFT-SWITCHING CONVERTER WITH ONLY AUXILARY SWITCH FOR HIGH POWER APPLICATIONS..135 B: SWITCHING CHARACTERISTICS OF NPT- AND PT IGBTS UNDER ZERO-VOLTAGE SWITCHING CONDITIONS.158 VITA.177

9 LIST OF FIGURES Fig. 2.1 Basic block diagram of active harmonic compensation.18 Fig. 2.2 Sample current waveforms of a three-phase active power filter.19 Fig. 2.3 Reactive power compensation of a three-phase system.19 Fig. 2.4 Configuration of a single-phase three-level inverter circuit.22 with flying capacitors Fig. 2.5 Proposed soft-switching three-level inverter with one phase leg.22 Fig. 2.6 Switching sequences and corresponding voltage and current waveforms of the multilevel inverter..23 Fig. 2.7 Corresponding voltage and current waveforms during.24 Fig. 2.8 Fig. 2.8(a) Fig. 2.8(b) Fig. 2.8(c) Fig. 2.8(d) Fig. 2.8(e) Fig. 2.8(f) Fig. 2.8(g) Fig. 2.8(h) Fig. 2.8(i) Fig. 2.8(j) Fig. 2.8(k) Fig. 2.8(l) Fig. 2.9 Fig. 2.9(a) Fig. 2.9(b) Fig. 2.9(c) Fig Fig. 2.10(a) Fig. 2.10(b) Fig commutations for S 1 and S 4. Operational modes of the commutation sequence (a) Mode 0 [t 0 t 1 ] (b) Mode 0 [t 1 t 2 ] (c) Mode 0 [t 2 t 3 ] (d) Mode 0 [t 3 t 4 ] (e) Mode 0 [t 4 t 5 ] (f) Mode 0 [t 5 t 6 ] (g) Mode 0 [t 6 t 7 ] (h) Mode 0 [t 7 t 8 ] (i) Mode 0 [t 8 t 9 ] (j) Mode 0 [t 9 t 10 ] (k) Mode 0 [t 10 t 11 ] (l) Mode 0 [t 11 t 12 ] Reset mechanism of the coupled inductor with a saturable inductor (a) Reset mode (b) Blocking function (c) Voltage sharing Control block diagram for soft-switching control (a) Soft-switching control scheme (b) Gate signals Simulated output voltage and current waveforms of a singlephase three-level inverter with flying capacitors Fig Simulated voltage and current waveforms of the soft-switching inverter with fixed time of T D. Fig Experimental current waveforms of the inverter under softswitching operations Fig Experimental voltage and current waveforms of S x1 under soft

10 Fig. 2.14(a) Fig. 2.14(b) Fig. 2.14(c) Fig. 2.14(d) Fig. 2.14(e) Fig. 2.14(f) switching operations (a) Auxiliary switch and resonant inductor (b) Resonant branch (c) Coupled inductor and its blocking voltage (d) Saturable inductor and its blocking voltage (e) Current sharing between coupled windings (f) Auxiliary switch and diode currents Fig. 3.1 Unbalanced flying capacitor waveforms under soft-switching.46 operations Fig. 3.2 Self-balanced waveforms of a dc link capacitor under PWM.47 switching Fig. 3.3 Configuration of a half-bridge 3-level inverter with a flying.51 capacitor Fig. 3.4 Fig. 3.4(a) Fig. 3.4(b) Simulated capacitor voltages at start-up (a) Numerical calculation (b) P-spice simulation Fig. 3.5 Fig. 3.5(a) Fig. 3.5(b) Fig. 3.5(c) Bode plots of the flying capacitor voltage with open-loop (a) C f = 1000µF and I L = 1A (b) C f = 470µF and I L = 1A (c) C f = 470µF and I L = 20A Fig. 3.6 Fig. 3.6(a) Fig. 3.6(b) Fig. 3.6(c) Experimental waveforms of the flying capacitor at start-up conditions (c) Balanced case at a load current of 10A (V Cf = Vs/2) (d) Unbalanced case of a load current of 20A (V Cf > Vs/2) (c) Voltage imbalance waveforms of the main switch Fig. 3.7 Single-phase 3-level active power filter with flying capacitors.58 Fig. 3.8 Normalized capacitor current vs. duty ratio at a load frequency.58 of 500 Hz Fig. 3.9 Fig. 3.9(a) Fig. 3.9(b) Fig. 3.9(c) Current and voltage waveforms of clamping capacitor (a) Load current of 80A (b) Capacitor ripples voltage vs. duty ratio at a load current 80A (c) Ripple voltage of the clamping capacitor under PWM Fig Fig. 3.10(a) Fig. 3.10(b) Fig. 3.10(c) Fig Fig. 3.11(a) Fig. 3.11(b) switching Relationship between the ripple voltage and modulation index (a) C f = 200 µf (b) C f = 1000 µf (c) Ripple voltage versus modulation index Experimental waveforms of the current and voltage of IGBTs under hard-switching (a) Hard-switching: I sw = 300 A (b) Hard-switching: I sw = 600 A

11 Fig Fig. 3.12(a) Fig. 3.12(a) Fig Soft-switching characteristics of IGBT under different snubbing capacitors (a) C r = 0.14 µf (b) C r = 0.28 µf Turn-off dv/dt comparisons corresponding to snubbing capacitors Fig Turn-off switching energy comparisons under hard- and softswitching operations Fig. 3.14(a) (a) T j = 25 C.70 Fig. 3.14(b) (b) T j = 100 C.70 Fig Unbalanced voltage waveforms of clamping capacitor.71 Fig Power flow of the capacitors during reactive power compensation (a) Charging mode for C f1 (b) Charging mode for C f2 (c) Flying mode (d) Dc voltage mode Fig (a) Fig (b) Fig (c) Fig. 3.16(d) Fig. 4.1 Proposed three-level active power filter with flying capacitors.81 Fig. 4.2 Definition of active and reactive power with α-β coordinates.83 Fig. 4.3 Fig. 4.3(a) Fig. 4.3(b) p-q algorithm for the current reference extraction (a) Block diagram of control logic (b) Implementation of control logic Fig. 4.4 A current control loop for active filter.86 Fig. 4.5 Fig. 4.5(a) Fig. 4.5(b) Bode plots of an open-loop current control system with proportional-integral control (a) Proportional-integral (PI) controller (b) Open-loop system Fig. 4.6 Fig. 4.6(a) Fig. 4.6(b) Voltage control loop for dc bus capacitor (a) Overall voltage control loop (b) Simplified voltage control loop Fig. 4.7 Voltage control for balancing.93 Fig. 4.8 Fig. 4.8(a) Fig. 4.8(b) Fig. 4.9 Fig. 4.9(a) Fig. 4.9(b) Fig. 4.9(c) Fig Fig. 4.10(a) Fig. 4.10(b) Estimated voltage imbalance of the clamping capacitor during inverter operation (a) C f = 1000µF (b) C f = 200µF Proposed voltage controller for flying capacitor (a) Diverse capacitor voltage waveforms of the inverter (b) Feedback control scheme of the voltage stabilization (c) Voltage stabilizer with a PI controller Proposed voltage controller for flying capacitor with PI control (a) Voltage stabilizer for flying capacitor with a PI controller (b) Simplified voltage control loop

12 Fig Step response of the close-loop system for a flying capacitor.100 voltage loop Fig Fig. 4.12(a) Fig. 4.12(b) Bode plots of a voltage loop transfer function with proportional-integral control (a) Proportional-integral (PI) controller (b) Open-loop system Fig. 5.1 Calculation of source voltage and load current with α-β.105 orthogonal coordinates Fig. 5.2 Calculation of current commands from instantaneous reactive.106 power Fig. 5.3 Compensated source current and control signals of the active.107 filter Fig. 5.4 Simulated results of a compensated source current.107 Fig. 5.5 Compensated harmonic spectrum under frequency domain.109 Fig. 5.6 THD comparison under active filtering.109 Fig. 5.7 Fig. 5.7(a) Fig. 5.7(b) Fig. 5.7(c) Simulation results of an active filter with/and without a proportional-integral (PI) voltage balancing controller (a) Overall simulation results of the controller (b) Dynamic responses of a controller with voltage balancing gains (c) Dynamic responses of a controller without voltage balancing gains Fig. 5.8 Experimental results of the source voltage and the load.113 currents without an active filter Fig. 5.9 Experimental results of the source voltage and load currents.113 with an active filter Fig Compensated source voltage and current waveforms under.114 high impedance load conditions Fig Compensated source voltage and currents under unbalanced.115 load conditions Fig DC link voltage transitions while the load is suddenly.116 connected to the source Fig DC link voltage transitions while the load is suddenly.116 disconnected from the source Fig DC link voltage transitions during start-up.117

13 LIST OF TABLES Table 3-1 Switching states of a single-phase three-level inverter.57 Table 3-2 Possible switching states of a single-phase 3-level active filter.58 Table 3-3 Voltage ripple comparison under different flying capacitors.63 Table 3-4 Power flows between flying capacitors and dc capacitor during.75 reactive compensation Table 5-1 System parameters of the experimental prototype...112