POWER ELECTRONICCONVERTERS FORMICROGRIDS

POWER ELECTRONICCONVERTERS FORMICROGRIDS

Suleiman M. SharkhUniversity of Southampton, United Kingdom

Mohammad A. AbusaraUniversity of Exeter, United Kingdom

Georgios I. Orfanoudakis

University of Southampton, United Kingdom

Babar HussainPakistan Institute of Engineering and Applied Sciences, Pakistan

This edition first published 2014

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Contents

About the Authors xi

Preface xiii

Acknowledgments xv

1 Introduction 11.1 Modes of Operation of Microgrid Converters 2

1.1.1 Grid Connection Mode 21.1.2 Stand-Alone Mode 31.1.3 Battery Charging Mode 3

1.2 Converter Topologies 41.3 Modulation Strategies 61.4 Control and System Issues 71.5 Future Challenges and Solutions 9

References 10

2 Converter Topologies 132.1 Topologies 13

2.1.1 The Two-Level Converter 132.1.2 The NPC Converter 142.1.3 The CHB Converter 15

2.2 Pulse Width Modulation Strategies 162.2.1 Carrier-Based Strategies 172.2.2 SVM Strategies 22

2.3 Modeling 27References 28

3 DC-Link Capacitor Current and Sizing in NPC andCHB Inverters 29

3.1 Introduction 293.2 Inverter DC-Link Capacitor Sizing 303.3 Analytical Derivation of DC-Link Capacitor Current RMS Expressions 32

vi Contents

3.3.1 NPC Inverter 333.3.2 CHB Inverter 36

3.4 Analytical Derivation of DC-Link Capacitor Current Harmonics 373.4.1 NPC Inverter 383.4.2 CHB Inverter 39

3.5 Numerical Derivation of DC-Link Capacitor Current RMS Value andVoltage Ripple Amplitude 41

3.6 Simulation Results 423.7 Discussion 45

3.7.1 Comparison of Capacitor Size for theNPC and CHB Inverters 45

3.7.2 Comparison of Presented Methods for Analyzing DC-LinkCapacitor Current 46

3.7.3 Extension to Higher-Level Inverters 483.8 Conclusion 48

References 48

4 Loss Comparison of Two- and Three-Level Inverter Topologies 514.1 Introduction 514.2 Selection of IGBT-Diode Modules 534.3 Switching Losses 54

4.3.1 Switching Losses in the Two-Level Inverters 544.3.2 Switching Losses in the NPC Inverter 574.3.3 Switching Losses in the CHB Inverter 58

4.4 Conduction Losses 584.4.1 Conduction Losses in the Two-Level Inverter 604.4.2 Conduction Losses in the NPC Inverter 614.4.3 Conduction Losses in the CHB Inverter 63

4.5 DC-Link Capacitor RMS Current 654.6 Results 694.7 Conclusion 70

References 71

5 Minimization of Low-Frequency Neutral-Point VoltageOscillations in NPC Converters 73

5.1 Introduction 735.2 NPC Converter Modulation Strategies 745.3 Minimum NP Ripple Achievable by NV Strategies 77

5.3.1 Locally Averaged NP Current 785.3.2 Effect of Switching Constraints 795.3.3 Zero-Ripple Region 815.3.4 A Lower Boundary for the NP Voltage Ripple 81

5.4 Proposed Band-NV Strategies 83

Contents vii

5.4.1 Criterion Used by Conventional NV Strategies 835.4.2 Proposed Criterion 845.4.3 Regions of Operation 855.4.4 Algorithm 885.4.5 Switching Sequences – Conversion to Band-NV 90

5.5 Performance of Band-NV Strategies 915.5.1 NP Voltage Ripple 915.5.2 Effective Switching Frequency – Output Voltage Harmonic

Distortion 935.6 Simulation of Band-NV Strategies 945.7 Hybrid Modulation Strategies 100

5.7.1 Proposed Hybrid Strategies 1015.7.2 Simulation Results 102

5.8 Conclusions 106References 107

6 Digital Control of a Three-Phase Two-Level Grid-ConnectedInverter 109

6.1 Introduction 1096.2 Control Strategy 1126.3 Digital Sampling Strategy 1136.4 Effect of Time Delay on Stability 1156.5 Capacitor Current Observer 1166.6 Design of Feedback Controllers 1196.7 Simulation Results 1216.8 Experimental Results 1236.9 Conclusions 127

References 128

7 Design and Control of a Grid-Connected Interleaved Inverter 1317.1 Introduction 1317.2 Ripple Cancellation 1357.3 Hardware Design 137

7.3.1 Hardware Design Guidelines 1387.3.2 Application of the Design Guidelines 145

7.4 Controller Structure 1467.5 System Analysis 149

7.5.1 Effect of Passive Damping and Grid Impedance 1517.5.2 Effect of Computational Time Delay 1517.5.3 Grid Disturbance Rejection 154

7.6 Controller Design 1547.7 Simulation and Practical Results 1587.8 Conclusions 167

viii Contents

References 167

8 Repetitive Current Control of an Interleaved Grid-ConnectedInverter 171

8.1 Introduction 1718.2 Proposed Controller and System Modeling 1728.3 System Analysis and Controller Design 1758.4 Simulation Results 1788.5 Experimental Results 1798.6 Conclusions 182

References 182

9 Line Interactive UPS 1859.1 Introduction 1859.2 System Overview 1889.3 Core Controller 192

9.3.1 Virtual Impedance and Grid Harmonics Rejection 1939.4 Power Flow Controller 195

9.4.1 Drooping Control Equations 1959.4.2 Small Signal Analysis 1969.4.3 Stability Analysis and Drooping Coefficients Selection 200

9.5 DC Link Voltage Controller 2069.6 Experimental Results 2099.7 Conclusions 217

References 218

10 Microgrid Protection 22110.1 Introduction 22110.2 Key Protection Challenges 221

10.2.1 Fault Current Level Modification 22110.2.2 Device Discrimination 22310.2.3 Reduction in Reach of Impedance Relays 22310.2.4 Bidirectionality and Voltage Profile Change 22410.2.5 Sympathetic Tripping 22410.2.6 Islanding 22410.2.7 Effect on Feeder Reclosure 224

10.3 Possible Solutions to Key Protection Challenges 22510.3.1 Possible Solutions to Key Protection Challenges for an

Islanded Microgrid Having IIDG Units 22510.4 Case Study 229

10.4.1 Fault Level Modification 23110.4.2 Blinding of Protection 23210.4.3 Sympathetic Tripping 233

Contents ix

10.4.4 Reduction in Reach of Distance Relay 23310.4.5 Discussion 234

10.5 Conclusions 235References 236

11 An Adaptive Relaying Scheme for Fuse Saving 23911.1 Introduction 239

11.1.1 Preventive Solutions Proposed in the Literature 24011.1.2 Remedial Solutions Proposed in the Literature 24111.1.3 Contributions of the Chapter 242

11.2 Case Study 24211.3 Simulation Results and Discussion 24511.4 Fuse Saving Strategy 247

11.4.1 Options and Considerations for the Selection ofIpickup of the 50 Element 249

11.4.2 Adaptive Algorithm 25111.5 How Reclosing Will Be Applied 25211.6 Observations 25511.7 Conclusions 257

References 257

Appendix A SVM for the NPC Converter–MATLAB®-Simulink Models 261

A.1 Calculation of Duty Cycles for Nearest Space Vectors 261A.2 Symmetric Modulation Strategy 262A.3 MATLAB®-Simulink Models 263

References 279

Appendix B DC-Link Capacitor Current Numerical Calculation 281

Index 285

About the Authors

Suleiman M. Sharkh obtained his BEng and PhD

degrees in Electrical Engineering from the University

of Southampton in 1990 and 1994, respectively. He is

currently the Head of the Electro-Mechanical Research

Group at the University of Southampton. He is also the

Managing Director of HiT Systems Ltd, and a visit-

ing Professor at the Beijing Institute of Technology and

Beijing Jiaotong University.

He has 20 years research experience in the field of

electrical and electromagnetic systems, including electric

switches, power electronics, electrical machines, con-

trol systems, and characterization and management of

advanced batteries. To date he has published about 150 publications. He has obtained

research grant income of about £2M from the Research Councils and industry since

1998. He has supervised 11 PhD students to completion and is currently supervising 5

PhD students. He is an established doctoral external examiner in the UK and abroad,

including Europe, China, and Australia. His research has contributed to the develop-

ment of a number of commercial products, including rim drivenmarine thrusters (TSL

Technology Ltd), down-hole submersible motors for drilling and pumping oil wells

(TSL Technology Ltd), sensorless brushless DCmotor controllers (TSL Technology),

power electronic converters for microgrids (Bowman Power Systems and TSL Tech-

nology), high-speed PM alternators for Rankine cycle and gas microturbine energy

recovery systems (TSL Technology, Bowman Power Systems, and Freepower), and

battery management systems (Reap Systems Ltd).

He was the winner of The Engineer Energy Innovation and Technology Award that

was presented at the Royal Society London in October 2008 for his work on novel

rim driven marine thrusters and turbine generators, which are produced commercially

under licence by TSL Technology Ltd. He was also awarded the Faraday SPARKS

award in 2002. He is a past committee member of the UK Magnetics Society, a

member of the IET and a Chartered Engineer.

xii About the Authors

Mohammad A. Abusara received the BEng degree fromBirzeit University, Palestine, in 2000, and the PhD degreefrom the University of Southampton, UK, in 2004, bothin electrical engineering. From 2003 to 2010, he waswith Bowman Power Group, Southampton, UK, respon-sible for research and development of digital control ofpower electronics for distributed energy resources, hybridvehicles, and machines and drives. He is currently aSenior Lecturer in Renewable Energy at the University ofExeter, UK.

Georgios I. Orfanoudakis received his MEng in Electri-cal Engineering and Computer Science from the NationalTechnical University of Athens (NTUA), Greece, in 2007,and his MSc in Sustainable Energy Technologies from theUniversity of Southampton, UK, in 2008. He then joinedthe Electro-Mechanical Research Group at the Univer-sity of Southampton and obtained his PhD in 2013. Hisresearch focused on the modulation and DC-link capaci-tor sizing of three-level inverters. SinceOctober 2012 he isworking as a Research Associate in a Knowledge Trans-fer Partnership (KTP) with the University of Southamp-ton and TSL technology Ltd., performing R&D work on

inverters for motor drive applications. Dr Orfanoudakis is a member of the IEEEPower Electronics Society.

BabarHussain received the BSc degree in electrical engi-neering from the University of Engineering and Tech-nology, Taxila, Pakistan, in 1995 and the PhD degree inelectrical engineering from the University of Southamp-ton, Southampton, UK, in 2011. He has more than 10years experience in the electric power sector. His majorresearch interests include protection of distribution net-works with distributed generation, power quality, and con-trol of grid-connected inverters.

Preface

Microgrids and distributed generation (DG), including renewable sources and energy

storage, can help overcome power system capacity limitations, improve efficiency,

reduce emissions, and manage the variability of renewable sources. A key component

of such a system is the power electronic interface between a generator or an energy

storage system, and the grid. Such an interface needs to be capable of performing

several functions, including injection of high quality current into the grid to meet

national standards; charging and discharging energy storage systems in a controlled

manner; anti-islanding protection to disconnect from the grid when the mains are lost;

and continuing to supply critical loads when the grid is lost.

The aim of this book is to provide an in-depth coverage of specific topics related to

power electronic converters for microgrids, focusing on three-phase converters in the

range 50–250 kW. It also discusses the important problem of protection of distribu-

tion networks, including microgrids and DG. The book is intended as a textbook for

graduating students with an electrical engineering background who wish to work or

do research in this field.

Chapter 1 presents a review of the state of the art and future challenges of power elec-

tronic converters used in microgrids. Chapter 2 describes the structure andmodulation

strategies of the conventional two-level and three-level neutral point clamped (NPC)

and cascaded H-Bridge (CHB) converter topologies. Chapter 3 discusses the sizing of

DC-link capacitors in two-level and three-level inverters, based on expressions for the

rms values and the harmonic spectrum of the capacitor current. Chapter 4 investigates

semiconductor and DC-link capacitor losses in two-level and three-level inverters,

and presents a comparison between the different topologies. Chapter 5 investigates

the problem of low-frequency voltage oscillations that appear at the neutral point

of an NPC converter, and proposes an algorithm for minimizing these oscillations.

Chapter 6 discusses the design and practical implementation of a digital current

controller for a three-phase two-level voltage source grid-connected inverter with an

LCL output filter. Chapter 7 discusses the design and control of a three-phase voltage

source grid-connected interleaved inverter and describes its practical implementation.

Chapter 8 discusses the design and practical implementation of a repetitive controller

for an interleaved grid-connected inverter. Chapter 9 discusses the design and practical

implementation of a line interactive UPS (uninterruptible power supply) system

xiv Preface

capable of seamlessly transferring between grid-connected to stand-alone modes inparallel with other sources, as well as managing the charging and discharging ofthe battery. Chapter 10 discusses protection issues and challenges arising from theintegration of microgrids and DG into the grid. Chapter 11 discusses the problemof recloser–fuse coordination in a distribution network including microgrids andDG, and proposes an adaptive fuse saving scheme that takes into account the statusof the DG. There are also two appendices. Appendix A gives some backgroundmaterial on SVM (space vector modulation) for NPC converters and describes theMATLAB®/Simulink models and programs used to carry out the simulations inChapter 5. Appendix B includes the MATLAB® code used to numerically calculatethe DC-link capacitor rms current and voltage ripple.

Acknowledgments

The authors would like to thank and acknowledge the valuable support of TSL Tech-nology Ltd and Bowman Power Group Ltd who funded some of the research includedin this book. In particular they wish to thank Dr Mike Yuratich at TSL and Mr JohnLyons at Bowman for their support and help over the last 15 years. They also wishto acknowledge the contributions of Dr Zahrul Faizi Hussain and Dr Mohsin Jamil tosome of thematerial presented in this book. Thanks are also due to theUKEngineeringand Physical Science Research Council (EPSRC) for supporting Dr Orfanoudakis’sPhD. We wish also to express special appreciation to the staff at Wiley, especiallyJames Murphy, Clarissa Lim, and Shelley Chow for their support, patience, and trustin this project.