Kamran Sharifabadi, Power Grid & Regulatory Affairs, Statoil, Norway Kamran has twenty-five years of international experience in the field of HVDC technology projects. He started out as a research engineer in ABB and Siemen, worked as a consultant for five years, then became a manager at the Norwegian TSO. He is currently a senior technology advisor for Statoil`s HVDC projects, a guest lecturer in the topics of VSC HVDC, Wind power generation technologies at NTNU and at various different universities in central Europe. Kamran is an active member of the Cigre B4 (HVDC) working group and the leader of the steering committee for a European research project on DC grids.

Remus Teodorescu, Aalborg University, Denmark Remus is an Associate Professor at the Institute of Technology, teaching courses in power electronics and electrical energy system control. He has authored over 80 journal and conference papers and two books. He is the founder and coordinator of the Green Power Laboratory at Aalborg University, and is co-recipient of the Technical Committee Prize Paper Award at IEEE Optim 2002.

Hans Peter Nee, KTH, Sweden Hans is Professor of Power Electronics in the Department of Electrical Engineering. He has supervised and examined ten finalized doctor's projects, and was awarded the Elforsk Scholarship in 1997. He has served on the board of the IEEE Sweden Section for many years and was Chairman during 2002 and 2003. He is also a member of EPE and serves in the Executive Council and in the International Steering Committee.

Lennart Harnefors, ABB, Västerås, Sweden Lennart is currently with ABB Power Systems - HVDC, Ludvika, Sweden as an R&D Project Manager and Principal Engineer, and with KTH as an Adjunct Professor of power electronics. Between 2001 and 2005, he was a part-time Visiting Professor of electrical drives with Chalmers University of Technology, Sweden. He is an Associate Editor of the IEEE Transactions on Industrial Electronics, on the Editorial Board of IET Electric Power Applications, and a member of the Executive Council and the International Scienti¿c Committee of the European Power Electronics and Drives Association.

Staffan Norrga, KTH, Sweden Between 1994 and 2011, Staffan worked as a Development Engineer at ABB in Västerås, Sweden, in various power-electronics-related areas such as railway traction systems and converters for HVDC power transmission systems. In 2000, he returned to the Department of Electric Machines and Power Electronics of the Royal Institute of Technology, where he is an associate professor. He is the inventor or co-inventor of 11 granted patents and 14 patents pending and has authored more than 35 scientific papers.

Preface xiii

Acknowledgements xv

About the Companion Website xvii

Nomenclature xix

Introduction 1

1 Introduction to Modular Multilevel Converters 7

1.1 Introduction 7

1.2 The Two-Level Voltage Source Converter 9

1.3 Benefits of Multilevel Converters 15

1.4 Early Multilevel Converters 17

1.5 Cascaded Multilevel Converters 23

1.6 Summary 57

References 58

2 Main-Circuit Design 60

2.1 Introduction 60

2.2 Properties and Design Choices of Power Semiconductor Devices for High-Power Applications 61

2.3 Medium-Voltage Capacitors for Submodules 92

2.4 Arm Inductors 96

2.5 Submodule Configurations 98

2.6 Choice of Main-Circuit Parameters 112

2.7 Handling of Redundant and Faulty Submodules 118

2.8 Auxiliary Power Supplies for Submodules 121

2.9 Start-Up Procedures 126

2.10 Summary 126

References 127

3 Dynamics and Control 133

3.1 Introduction 133

3.2 Fundamentals 134

3.3 Converter Operating Principle and Averaged Dynamic Model 137

3.4 Per-Phase Output-Current Control 148

3.5 Arm-Balancing (Internal) Control 161

3.6 Three-Phase Systems 175

3.7 Vector Output-Current Control 184

3.8 Higher-Level Control 192

3.9 Control Architectures 207

3.10 Summary 212

References 212

4 Control under Unbalanced Grid Conditions 214

4.1 Introduction 214

4.2 Grid Requirements 214

4.3 Shortcomings of Conventional Vector Control 215

4.4 Positive/Negative-Sequence Extraction 219

4.5 Injection Reference Strategy 223

4.6 Component-Based Vector Output-Current Control 226

4.7 Summary 228

References 231

5 Modulation and Submodule Energy Balancing 232

5.1 Introduction 232

5.2 Fundamentals of Pulse-Width Modulation 233

5.3 Carrier-Based Modulation Methods 236

5.4 Multilevel Carrier-Based Modulation 243

5.5 Nearest-Level Control 252

5.6 Submodule Energy Balancing Methods 256

5.7 Summary 270

References 271

6 Modeling and Simulation 272

6.1 Introduction 272

6.2 Leg-Level Averaged (LLA) Model 274

6.3 Arm-Level Averaged (ALA) Model 275

6.4 Submodule-Level Averaged (SLA) Model 278

6.5 Submodule-Level Switched (SLS) Model 280

6.6 Summary 281

References 282

7 Design and Optimization of MMC-HVDC Schemes for Offshore Wind-Power Plant Application 283

7.1 Introduction 283

7.2 The Influence of Regulatory Frameworks on the Development Strategies for Offshore HVDC Schemes 284

7.3 Impact of Regulatory Frameworks on the Functional Requirements and Design of Offshore HVDC Terminals 286

7.4 Components of an Offshore MMC-HVDC Converter 287

7.5 Offshore Platform Concepts 294

7.6 Onshore HVDC Converter 295

7.7 Recommended System Studies for the Development and Integration of an Offshore HVDC Link to a WPP 298

7.8 Summary 303

References 303

8 MMC-HVDC Standards and Commissioning Procedures 305

8.1 Introduction 305

8.2 CIGRE and IEC Activities for the Standardization of MMC-HVDC Technology 306

8.3 MMC-HVDC Commissioning and Factory and Site Acceptance Tests 309

8.4 Summary 317

References 317

9 Control and Protection of MMC-HVDC under AC and DC Network Fault Contingencies 318

9.1 Introduction 318

9.2 Two-Level VSC-HVDC Fault Characteristics under Unbalanced AC Network Contingency 319

9.3 MMC-HVDC Fault Characteristics under Unbalanced AC Network Contingency 322

9.4 dc Pole-to-Ground Short-Circuit Fault Characteristics of the Half-Bridge Mmc-hvdc 325

9.5 MMC-HVDC Component Failures 327

9.6 MMC-HVDC Protection Systems 329

9.7 Summary 333

References 334

10 MMC-HVDC Transmission Technology and MTDC Networks 336

10.1 Introduction 336

10.2 LCC-HVDC Transmission Technology 336

10.3 Two-Level VSC-HVDC Transmission Technology 338

10.4 Modular Multilevel HVDC Transmission Technology 339

10.5 The European HVDC Projects and MTDC Network Perspectives 343

10.6 Multi-Terminal HVDC Configurations 345

10.7 dc Load Flow Control in MTdc Networks 348

10.8 dc Grid Control Strategies 349

10.9 dc Fault Detection and Protection in MTdc Networks 355

10.10 Fault-Detection Methods in MTDC 357

10.11 dc Circuit Breaker Technologies 362

10.12 Fault-Current Limiters 367

10.13 The Influence of Grounding Strategy on Fault Currents 369

10.14 dc Supergrids of the Future 370

10.15 Summary 371

References 371

Index 373