A generic DC grid model that is compatible with the standard AC system stability model is presented and used to analyse the interaction between the DC grid and the host AC systems.

A multi-terminal DC (MTDC) grid interconnecting multiple AC systems and offshore energy sources (e.g. wind farms) across the nations and continents would allow effective sharing of intermittent renewable resources and open market operation for secure and cost-effective supply of electricity. However, such DC grids are unprecedented with no operational experience. Despite lots of discussions and specific visions for setting up such MTDC grids particularly in Europe, none has yet been realized in practice due to two major technical barriers:

* Lack of proper understanding about the interaction between a MTDC grid and the surrounding AC systems.

* Commercial unavailability of efficient DC side fault current interruption technology for conventional voltage sourced converter systems

This book addresses the first issue in details by presenting a comprehensive modeling, analysis and control design framework. Possible methodologies for autonomous power sharing and exchange of frequency support across a MTDC grid and their impact on overall stability is covered. An overview of the state-of-the-art, challenges and on-going research and development initiatives for DC side fault current interruption is also presented.

Nilanjan Ray Chaudhuri received his Ph.D. degree from
Imperial College London, UK. His research interests include power
system dynamics and control, application of power electronics in
power systems, online system identification, FACTS, HVDC, and
renewable energy systems. He serves as an Associate Editor of the
IEEE Transactions on Power Delivery. Nilanjan is a member of the
WECC's HVDC modeling Task Force, multiple CIGRE'
subcommittees, a member of the IEEE, IEEE PES, CIGRE' and
Sigma Xi.

Balarko Chaudhuri is a Senior Lecturer in the
department of Electrical and Electronic Engineering at Imperial
College London, UK. His areas of expertise include electric power
transmission systems, control theory, smart grids and renewable
energy. He is an associate editor of the IEEE Systems Journal and
Elsevier Control Engineering Practice. He is a Senior Member of the
IEEE.

Rajat Majumder did his PhD in Power Systems at
Imperial College London, UK. He specializes in power system
analysis, modeling and control design, with special emphasis on
dynamic stability issues in large interconnected power grids
involving HVDC and FACTS. He is serving as an editorial board
member of Institute of Engineering Technology's (IET)
Proceedings of Generation, Transmission and Distribution. He is a
Senior Member of the IEEE.
Amirnaser Yazdani is an Associate Professor with
Ryerson University in Toronto, Canada. From 2006 to 2011, he was an
Assistant Professor with the University of Western Ontario in
London, Canada, and prior to that he was with Digital Predictive
Systems (DPS) Inc., Mississauga, Canada, active in the design and
production of power converters for wind energy systems. Dr. Yazdani
has extensive industry and academic experience in design, modeling,
and analysis of switching power converters and railway signaling
systems, and has served as an Associate Editor of the IEEE
Transactions on Power Delivery. He is a Senior Member of the IEEE,
a Professional Engineer in the Province of Ontario, Canada, and a
co-author of the book Voltage-Sourced Converters in Power Systems,
published by IEEE/Wiley Press, 2010.

A generic DC grid model that is compatible with the standard AC system stability model is presented and used to analyse the interaction between the DC grid and the host AC systems.

A multi-terminal DC (MTDC) grid interconnecting multiple AC systems and offshore energy sources (e.g. wind farms) across the nations and continents would allow effective sharing of intermittent renewable resources and open market operation for secure and cost-effective supply of electricity. However, such DC grids are unprecedented with no operational experience. Despite lots of discussions and specific visions for setting up such MTDC grids particularly in Europe, none has yet been realized in practice due to two major technical barriers:

• Lack of proper understanding about the interaction between a MTDC grid and the surrounding AC systems.
• Commercial unavailability of efficient DC side fault current interruption technology for conventional voltage sourced converter systems

This book addresses the first issue in details by presenting a comprehensive modeling, analysis and control design framework. Possible methodologies for autonomous power sharing and exchange of frequency support across a MTDC grid and their impact on overall stability is covered. An overview of the state-of-the-art, challenges and on-going research and development initiatives for DC side fault current interruption is also presented.

Preface xv

Acronyms xix

Symbols xxi

1 Fundamentals 1

1.1 Introduction 1

1.2 Rationale Behind MTDC Grids 5

1.3 Network Architectures of MTDC Grids 6

1.3.1 Series Architecture 6

1.3.2 Parallel Architecture 7

1.4 Enabling Technologies and Components of MTDC Grids 9

1.4.1 LCC Technology 9

1.4.1.1 Control Modes in LCC-based MTDC Grid 10

1.4.1.2 Examples of Existing LCC MTDC Systems 10

1.4.2 VSC Technology 12

1.5 Control Modes in MTDC Grid 14

1.6 Challenges for MTDC Grids 15

1.7 Configurations of MTDC Converter Stations 16

1.8 Research Initiatives on MTDC Grids 19

1.9 Focus and Scope of the Monograph 21

2 The Voltage-Sourced Converter (VSC) 23

2.1 Introduction 23

2.2 Ideal Voltage-Sourced Converter 24

2.3 Practical Voltage-Sourced Converter 28

2.3.1 Two-Level Voltage-Sourced Converter 28

2.3.2 Three-Level Voltage-Sourced Converter 31

2.3.3 Multi-Level Voltage-Sourced Converter 35

2.4 Control 38

2.4.1 Control of Real and Reactive Powers 38

2.4.2 Design and Implementation of Control 39

2.4.2.1 Space Phasors 39

2.4.2.2 Space-Phasor Representation of the AC Side 42

2.4.2.3 Current Control in the Stationary Frame 43

2.4.2.4 Current Control in a Rotating Frame 44

2.4.2.5 Phase-Locked Loop 52

2.4.3 Control of the DC-Side Voltage 56

2.4.4 Control of the AC Grid Voltage 58

2.4.5 Multi-unit Control of DC Grid Voltage and/or AC Grid Voltage 59

2.4.6 Control of Islands 61

2.5 Simulation 65

2.6 Symbols of the VSC 75

3 Modeling, Analysis, and Simulation of ACMTDC Grids 77

3.1 Introduction 77

3.2 MTDC Grid Model 78

3.2.1 Modeling Assumptions 78

3.2.2 Converter Model 81

3.2.3 Converter Controller Model 83

3.2.3.1 Outer Control Loops 83

3.2.3.2 Inner Current Control Loop 87

3.2.4 DC Network Model 87

3.2.4.1 Algebraic Equations 89

3.2.4.2 Differential Equations 91

3.2.5 State-Space Representation 91

3.2.5.1 Dynamic Equations of Converters and Controllers 92

3.2.5.2 Output Equations 93

3.2.5.3 Control Modes 93

3.2.5.4 Dynamic Equations of DC Network 95

3.2.5.5 Output Equations of DC Network 96

3.2.6 Phasor from Space Phasor 96

3.2.6.1 Base Values and Per-unit Systems 97

3.2.6.2 Phase Angle of Space Phasors 97

3.3 AC Grid Model 98

3.3.1 Generator Model 99

3.3.1.1 State-Space Representation of Synchronous Generator (SG) Model 99

3.3.1.2 Inclusion of Genera…