Offshore Wind Energy Generation
Control, Protection, and Integration to Electrical Systems

The offshore wind sector?s trend towards larger turbines, bigger wind farm projects and greater distance to shore has a critical impact on grid connection requirements for offshore wind power plants. This important reference sets out the fundamentals and latest innovations in electrical systems and control strategies deployed in offshore electricity grids for wind power integration.

Includes:

• All current and emerging technologies for offshore wind integration and trends in energy storage systems, fault limiters, superconducting cables and gas-insulated transformers
• Protection of offshore wind farms illustrating numerous system integration and protection challenges through case studies
• Modelling of doubly-fed induction generators (DFIG) and full-converter wind turbines structures together with an explanation of the smart grid concept in the context of wind farms
• Comprehensive material on power electronic equipment employed in wind turbines with emphasis on enabling technologies (HVDC, STATCOM) to facilitate the connection and compensation of large-scale onshore and offshore wind farms
• Worked examples and case studies to help understand the dynamic interaction between HVDC links and offshore wind generation
• Concise description of the voltage source converter topologies, control and operation for offshore wind farm applications
• Companion website containing simulation models of the cases discussed throughout

Equipping electrical engineers for the engineering challenges in utility-scale offshore wind farms, this is an essential resource for power system and connection code designers and pratitioners dealing with integation of wind generation and the modelling and control of wind turbines. It will also provide high-level support to academic researchers and advanced students in power and renewable energy as well as technical and research staff in transmission and distribution system operators and in wind turbine and electrical equipment manufacturers.

About the Authors xiii

Acronyms and Symbols xv

1 Offshore Wind Energy Systems 1

1.1 Background 1

1.2 Typical Subsystems 1

1.3 Wind Turbine Technology 4

1.3.1 Basics 4

1.3.2 Architectures 6

1.3.3 Offshore Wind Turbine Technology Status 7

1.4 Offshore Transmission Networks 8

1.5 Impact on Power System Operation 9

1.5.1 Power System Dynamics and Stability 10

1.5.2 Reactive Power and Voltage Support 10

1.5.3 Frequency Support 11

1.5.4 Wind Turbine Inertial Response 11

1.6 Grid Code Regulations for the Connection of Wind Generation 12

Acknowledgements 13

References 14

2 DFIG Wind Turbine 15

2.1 Introduction 15

2.1.1 Induction Generator (IG) 15

2.1.2 Back-to-Back Converter 16

2.1.3 Gearbox 16

2.1.4 Crowbar Protection 16

2.1.5 Turbine Transformer 17

2.2 DFIG Architecture and Mathematical Modelling 17

2.2.1 IG in the abc Reference Frame 17

2.2.2 IG in the dq0 Reference Frame 23

2.2.3 Mechanical System 27

2.2.4 Crowbar Protection 29

2.2.5 Modelling of the DFIG B2B Power Converter 30

2.2.6 Average Modelling of Power Electronic Converters 33

2.2.7 The dc Circuit 35

2.3 Control of the DFIG WT 36

2.3.1 PI Control of Rotor Speed 36

2.3.2 PI Control of DFIG Reactive Power 39

2.3.3 PI Control of Rotor Currents 41

2.3.4 PI Control of dc Voltage 42

2.3.5 PI Control of Grid-side Converter Currents 45

2.4 DFIG Dynamic Performance Assessment 47

2.4.1 Three-phase Fault 47

2.4.2 Symmetrical Voltage Dips 51

2.4.3 Asymmetrical Faults 53

2.4.4 Single-Phase-to-Ground Fault 54

2.4.5 Phase-to-Phase Fault 55

2.4.6 Torque Behaviour under Symmetrical Faults 56

2.4.7 Torque Behaviour under Asymmetrical Faults 58

2.4.8 Effects of Faults in the Reactive Power Consumption of the IG 59

2.5 Fault Ride-Through Capabilities and Grid Code Compliance 60

2.5.1 Advantages and Disadvantages of the Crowbar Protection 60

2.5.2 Effects of DFIG Variables over Its Fault Ride-Through Capabilities 61

2.6 Enhanced Control Strategies to Improve DFIG Fault Ride-Through Capabilities 62

2.6.1 The Two Degrees of Freedom Internal Model Control (IMC) 62

2.6.2 IMC Controller of the Rotor Speed 65

2.6.3 IMC Controller of the Rotor Currents 66

2.6.4 IMC Controller of the dc Voltage 67

2.6.5 IMC Controller of the Grid-Side Converter Currents 69

2.6.6 DFIG IMC Controllers Tuning for Attaining Robust Control 70

2.6.7 The Robust Stability Theorem 70

References 72

3 Fully-Rated Converter Wind Turbine (FRC-WT) 73

3.1 Synchronous Machine Fundamentals 73

3.1.1 Synchronous Generator Construction 73

3.1.2 The Air-Gap Magnetic Field of the Synchronous Generator 74

3.2 Synchronous Generator Modelling in the dq Frame 79

3.2.1 Steady-State Operation 81

3.2.2 Synchronous Generator with Damper Windings 82

3.3 Control of Large Synchronous Generators 85

3.3.1 Excitation Control 86

3.3.2 Prime Mover Control 87

3.4 Fully-Rated Converter Wind Turbines 88

3.5 FRC-WT with Synchronous Generator 89

3.5.1 Permanent Magnets Synchronous Generator 90

3.5.2 FRC-WT Based on Permanent Magnet Synchronous Generator 92

3.5.3 Generator-Side Converter Control 93

3.5.4 Modelling of the dc Link 96

3.5.5 Network-Side Converter Control 98

3.6 FRC-WT with Squirrel-Cage Induction Generator 100

3.6.1 Control of the FRC-IG Wind Turbine 100

3.7 FRC-WT Power System Damper 105

3.7.1 Power System Oscillations Damping Controller 105

3.7.2 Influence of Wind Generation on Network Damping 107

3.7.3 Influence of FRC-WT Damping Controller on Network Damping 108

Acknowledgements 110

References 112

4 Offshore Wind Farm Electrical Systems 113

4.1 Typical Components 113

4.2 Wind Turbines for Offshore – General Aspects 113

4.3 Electrical Collectors 115

4.3.1 Wind Farm Clusters 118

4.4 Offshore Transmission 118

4.4.1 HVAC Transmission 118

4.4.2 HVDC Transmission 120

4.4.3 CSC-HVDC Transmission 122

4.4.4 VSC-HVDC Transmission 128

4.4.5 Multi-Terminal VSC-HVDC Networks 140

4.5 Offshore Substations 141

4.6 Reactive Power Compensation Equipment 144

4.6.1 Static Var Compensator (SVC) 144

4.6.2 Static Compensator (STATCOM) 147

4.7 Subsea Cables 150

4.7.1 Ac Subsea Cables 150

4.7.2 Dc Subsea Cables 150

4.7.3 Modelling of Underground and Subsea Cables 150

Acknowledgements 151

References 151

5 Grid Integration of Offshore Wind Farms – Case Studies 155

5.1 Background 155

5.2 Offshore Wind Farm Connection Using Point-to-Point VSC-HVDC Transmission 156

5.3 Offshore Wind Farm Connection Using HVAC Transmission 159

5.4 Offshore Wind Farm Connected Using Parallel HVAC/VSC-HVDC Transmission 161

5.5 Offshore Wind Farms Connected Using a Multi-Terminal VSC-HVDC Network 164

5.6 Multi-Terminal VSC-HVDC for Connection of Inter-Regional Power Systems 168

Acknowledgements 171

References 171

6 Offshore Wind Farm Protection 173

6.1 Protection within the Wind Farm ac Network 173

6.1.1 Wind Generator Protection Zone 174

6.1.2 Feeder Protection Zone 178

6.1.3 Busbar Protection Zone 179

6.1.4 High-Voltage Transformer Protection Zone 180

6.2 Study of Faults in the ac Transmission Line of an Offshore DFIG Wind Farm 180

6.2.1 Case Study 1 181

6.2.2 Case Study 2 181

6.3 Protections for dc Connected Offshore Wind Farms 184

6.3.1 VSC-HVDC Converter Protection Scheme 184

6.3.2 Analysis of dc Transmission Line Fault 185

6.3.3 Pole-to-Pole Faults 186

6.3.4 Pole-to-Earth Fault 187

6.3.5 HVDC dc Protections: Challenges and Trends 188

6.3.6 Simulation Studies of Faults in the dc Transmission Line of an Offshore DFIG Wind Farm 188

Acknowledgements 192

References 192

7 Emerging Technologies for Offshore Wind Integration 193

7.1 Wind Turbine Advanced Control for Load Mitigation 193

7.1.1 Blade Pitch Control 193

7.1.2 Blade Twist Control 194

7.1.3 Variable Diameter Rotor 194

7.1.4 Active Flow Control 195

7.2 Converter Interface Arrangements and Collector Design 195

7.2.1 Converters on Turbine 195

7.2.2 Converters on Platform 198

7.2.3 Ac Collection Options: Fixed or Variable Frequency 200

7.2.4 Evaluation of >Higher (>33 kV) Collection Voltage 202

7.3 Dc Transmission Protection 203

7.4 Energy Storage Systems (EESs) 204

7.4.1 Batteries 205

7.4.2 Super-Capacitors 205

7.4.3 Flywheel Storage System 205

7.4.4 Pumped-Hydro Storage 206

7.4.5 Compressed-Air Storage Systems 206

7.4.6 Superconducting Magnetic Energy Storage (SMES) 206

7.5 Fault Current Limiters (FCLs) 207

7.6 Sub-Sea Substations 207

7.7 HTSCs, GITs and GILs 208

7.7.1 HTSCs (High-Temperature Superconducting Cables) 208

7.7.2 GITs (Gas-Insulated Transformers) 208

7.7.3 GILs (Gas-Insulated Lines) 209

7.8 Developments in Condition Monitoring 209

7.8.1 Partial Discharge Monitoring in HV Cables 209

7.8.2 Transformer Condition Monitoring 210

7.8.3 Gas-Insulated Switchgear Condition Monitoring 211

7.8.4 Power Electronics Condition Monitoring 211

7.9 Smart Grids for Large-Scale Offshore Wind Integration 213

7.9.1 VPP Control Approach 216

7.9.2 Phasor Measurement Units 217

Acknowledgements 217

References 218

Appendix A Voltage Source Converter Topologies 223

A.1 Two-Level Converter 223

A.1.1 Operation 223

A.1.2 Voltage Source Converter Square-Mode Operation 224

A.1.3 Pulse Width Modulation 225

A.2 Neutral-Point Clamped Converter 240

A.2.1 Selective Harmonic Elimination 242

A.2.2 Sinusoidal Pulse Width Modulation 244

A.3 Flying Capacitor (FC) Multilevel Converter 247

A.4 Cascaded Multilevel Converter 248

A.5 Modular Multilevel Converter 249

References 258

Appendix B Worked-out Examples 271

Index 279

Edgar Lenymirko Moreno-Goytia,Reader,Instituto Tecnológico de Morelia, MéxicoDr Moreno-Goytia has researched power electronic-based equipment and measurement systems development. He designed and built a Thyristor Controlled Series Compensator and its control to operate in a voltage fluctuations environment, and has been involved in evaluating the impact of wind generation on the electrical grid. Dr Moreno-Goytia has published over thirty papers in international conferences and journals and is a member of IEEE and IET.

Olimpo Anaya-Lara,Senior Lecturer, Institute for Energy and Environment , University of Strathclyde, Glasgow, UKDr Anaya-Lara has researched power electronic equipment, control systems development, and stability and control of power systems with increased wind energy penetration. He has developed control strategies for Flexible Alternating Current Transmission System devices (FACTS), and designed control schemes for marine applications using advanced control techniques. He is a member of the CIGRE Working Group B4-39, two International Energy Agency Annexes, also the IEEE and IET. He has published over thirty-five journals, ninety papers and co-authored three books.

David Campos-Gaona,Research Assistant, Instituto Tecnológico de Morelia, MéxicoMr Campos-Gaona has investigated electronics-based solutions to electrical networks such as digital power meters, DSP based protection algorithms, and protection systems for wind turbines. He developed electronic equipment such as residential digital power meter with a wireless communication port. He was a research assistant with the SUPERGEN FlexNet, and is member of the IEEE. He  has published several papers and conference proceedings.