DESIGN OF POWER ELECTRONIC INVERTER FOR ACTIVE POWER REDISTRIBUTION IN AN UNBALANCED THREE PHASE SYSTEM M. T.K.De Silva (PG/EE/22/99 ) thesis submitted to the Faculty of Engineering, University of Moratuwa in partial fulfillment of the requirement for the degree of Master of Engineering In Electrical Engineering Under the supervision of Dr. J.P.Karunadasa Faculty of Engineering University of Moratuwa 2004 80500 Abstract This project presents a scheme to balance the active power consumed by unbalanced loads in three-phase four wire system. Power systems are generally unbalanced due to asymmetry of the load applied and different time operations by consumers. Unbalanced operating conditions cause lot of problems to the power system. There are schemes for power balancing in three phase three wire systems, but this project illustrates power balancing in three-phase four wire systems. It can be shown that a power electronic converter based on the generalized instantaneous power theory, can redistribute active power among phases. The generalized instantaneous power theory can be used in both three-wire and four-wire three-phase systems. Power electronic circuit takes power from the phase that delivers low power and feeds to the phase that delivers high power. Therefore load as viewed by the power source becomes balanced without negative and zero sequence components although unbalanced power is still supplied to the load. The load current and the voltage are measured continuously and the instantaneous power is calculated. Reference current wave for the hysteresis current controller is calculated using the-control strategy. Power electronic inverter is controlled by hysteresis current controller and it redistributes the active power in the phases. There are no external power sources used and the inverter is driven by a capacitor. Therefore power source supplies balanced power to an unbalanced source. The simulation studies of the project is done by MatLab software and results show that the source current becomes balanced after connecting to the power electronic converter The rating of the power electronic converter is decided on the basis of the phase unbalance rather than the rated load power. DECLARATION I hereby declare that this submission is my own work and that, to the best of my knowledge and behalf, it contains no material previously published or ;- wntten by another person nor material, which to substantial extent, has been accepted for the award of any other academic qualification of an university or mstitute of higher learning except where acknowledgement is made in the text. ~ {'T t-t.. ~ '"~ ~1.T.K.De Silva ~ebruary 2004 ~ Dr. J.P.Karunadasa ProJect Supervisor February 2004 ~-~- .t ~ ~ "'-- :. ..... __,- / Acknowledgements · First l!>fall, thanks to Dr. J. P. Kanmadasa for his guidance and advice throughout my research as the supervisor. 1 am also ~:,rrateful to all the stafr led by Prof J.R. Lucas of the Department of Electrical Engineering, University ofMoratm\-a lor their help\\ ith this research. \1} special thanks also go to all the previous researchers\\ ho contributed to the development or knowledge in this area of concern wh1ch 1mmensely help me throughout this research work .. Last, but by no means least, a special mention ior my daughter, ~ethmi and my husband \1uditha who have provided the love and support required for such a work as this to be completed. Thank you to eveJ)'One.for all the help. support and encouragement. If tf~- ..... ~ ,.._ .; ,. / 11 Acknowledgement Abstract CHAPTER I INTRODUCTION CHAPTER 2 Contents ACTIVE POWER UNBALANCE CHAPTER 3 LiTERATURE REVIEW j 3.1 Phase Power Balancing of a Diesel Generator Using a Bi-Directional PWM Inverter 3.2 Instantaneous Reactive Power Compensators Comprising Switching Devices without Energy Storage Components 3.3 Generalized Instantaneous Power Theory for Three-Phase Power Systems & Reactive Power Compensation CHAPTER 4 PRINCIPLE OF SOLUTION ;.' -{' 4.1 Principle Description 4.2 Symmetrical Components of Current Waveforms CHAPTER 5 MEHODOLOGY / ~-""" -;.. .t .. -;· Page ii Ill 3 6 6 <) 13 17 17 20 22 5.1 Circuit Diagram / 22 5.2 Three-Phase PWM Current Controlled Voltage Source Inverter 24 5.3 Hysteresis current controller 26 11 5.4 Control Strategy CHAPTER 6 OBSERVATION & RESULTS CHAPTER 7 CONCLUSION REFERENCES ANNEX I Static, Thyristor Controlled Shunt Compensator ANNEX 2 l A Review of Active Filters for Power Quality Improvements ANNEX 3 4.1 Mat Lab- Math Works Product Family 4.2 Mat Lab - Model & Simulate Power System ~~!>· - "' Ill 29 32 37 38 39 40 42 44 ..... ~,"- :.. ~ .. ., / Table of Figures I. Figure 2.1 Voltage and Current wavefom1s of Balanced Resistive Load 2. Figure 2.2 Three-phase Load Currents in Balanced Case 3. Figure 2.3 Instantaneous Active Power in Balanced Condition 4. Figure 2.4 Three-phase Load Currents in Unbalanced Case 5. Figure 2.5 Instantaneous Active Power in Unbalanced Condition 6. Figure 3.1 Single Line Diagram of the System 7. Figure 3.2 Inverter Unit of Each Phase 8. Figure 3.3 Phasor diagram: Inverter mode l 9. Figure 3.4 Phasor diagram: Rectifier mode 10. Figure 3.5 a. - ~ coordination transformation 11. Figure 3.6 Instantaneous Space Vectors 12. Figure 3.7 Reactive Power Compensator System · 13. F:gure 3.8 Control Circuit 14. Figure 3.10 Three phase Coordination Page 3 4 4 5 5 6 7 8 8 9 10 I I II 13 15. Figure 3.11 System Configuration oflnstantaneous Reactive Power Compensation 15 16. Figure 4.1 Power Redistribution in an Unbalanced Three-phase Four-wire System 17 17. Figure 5.1 Schematics of Power Redistribution System 22 18. Figure 5.2 Circuit Diagram 23 19. Figure 5.3 Three-Phase PWM Current Controlled Voltage Source Inverter 25 20. Figure 5.4 Hysteresis cun·ent controller , .. " -~ 21. Figure 5.5 Hysteresis current controller 22. Figure 5.6 Reference current wave generation control strategy 23. Figure 6.1 lnstantaneous Voltage and Current Waveforms 24. Figure 6.2 Unbalanced Load Current Wavelonns 25. Figure 6.3 Instantaneous Active power · 26. Figure 6.4 Reference Current Waveforms 27. Figure 6.5 Hysteresis Current Controller Output Wa\'eforms 28. Figure 6.6 Capacitor Charging & Discharging \Vavefom1 IV ~ ~"-4 M.t .. vi "' / 27 28 31 33 33 34 34 35 35 29. Figure 6.8 Inverter Current Waveforms 36 30. Figure 6.9 Source Balanced Current Waveforms 36 l ~~" - "' ..... ~;.- :.. ~ ... .; / v