Control, Protection, and Integration to Electrical Systems

Olimpo Anaya-Lara | David Campos-GaonaEdgar Moreno-Goytia | Grain Adam

Offshore Wind Energy Generation

OFFSHORE WINDENERGY GENERATION

OFFSHORE WINDENERGY GENERATIONCONTROL, PROTECTION, ANDINTEGRATION TO ELECTRICALSYSTEMS

Olimpo Anaya-LaraUniversity of Strathclyde, Glasgow, UK

David Campos-GaonaMorelia Institute of Technology, Mexico

Edgar Moreno-GoytiaMorelia Institute of Technology, Mexico

Grain AdamUniversity of Strathclyde, Glasgow, UK

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Library of Congress Cataloging-in-Publication Data

Anaya-Lara, Olimpo.Offshore wind energy generation : control, protection, and integration to electrical systems / by Olimpo

Anaya-Lara, David Campos-Gaona, Edgar Lenymirko Moreno-Goytia, Grain Philip Adam.1 online resource.

Includes bibliographical references and index.Description based on print version record and CIP data provided by publisher; resource not viewed.ISBN 978-1-118-70153-9 (Adobe PDF) – ISBN 978-1-118-70171-3 (ePub) – ISBN 978-1-118-53962-0

(cloth) 1. Wind power plants. 2. Offshore electric power plants. 3. Wind energy conversion systems.I. Campos-Gaona, David. II. Moreno-Goytia, Edgar Lenymirko. III. Adam, Grain Philip. IV. Title.

TK1541621.31′213609162–dc23

2013048398

A catalogue record for this book is available from the British Library.

ISBN: 978-1-118-53962-0

Set in 10/12pt Times by Aptara Inc., New Delhi, India

1 2014

Contents

Preface xi

About the Authors xiii

Acronyms and Symbols xv

1 Offshore Wind Energy Systems 11.1 Background 11.2 Typical Subsystems 11.3 Wind Turbine Technology 4

1.3.1 Basics 41.3.2 Architectures 61.3.3 Offshore Wind Turbine Technology Status 7

1.4 Offshore Transmission Networks 81.5 Impact on Power System Operation 9

1.5.1 Power System Dynamics and Stability 101.5.2 Reactive Power and Voltage Support 101.5.3 Frequency Support 111.5.4 Wind Turbine Inertial Response 11

1.6 Grid Code Regulations for the Connection of Wind Generation 12Acknowledgements 13References 14

2 DFIG Wind Turbine 152.1 Introduction 15

2.1.1 Induction Generator (IG) 152.1.2 Back-to-Back Converter 162.1.3 Gearbox 162.1.4 Crowbar Protection 162.1.5 Turbine Transformer 17

2.2 DFIG Architecture and Mathematical Modelling 172.2.1 IG in the abc Reference Frame 172.2.2 IG in the dq0 Reference Frame 232.2.3 Mechanical System 27

vi Contents

2.2.4 Crowbar Protection 292.2.5 Modelling of the DFIG B2B Power Converter 302.2.6 Average Modelling of Power Electronic Converters 332.2.7 The dc Circuit 35

2.3 Control of the DFIG WT 362.3.1 PI Control of Rotor Speed 362.3.2 PI Control of DFIG Reactive Power 392.3.3 PI Control of Rotor Currents 412.3.4 PI Control of dc Voltage 422.3.5 PI Control of Grid-side Converter Currents 45

2.4 DFIG Dynamic Performance Assessment 472.4.1 Three-phase Fault 472.4.2 Symmetrical Voltage Dips 512.4.3 Asymmetrical Faults 532.4.4 Single-Phase-to-Ground Fault 542.4.5 Phase-to-Phase Fault 552.4.6 Torque Behaviour under Symmetrical Faults 562.4.7 Torque Behaviour under Asymmetrical Faults 582.4.8 Effects of Faults in the Reactive Power Consumption of the IG 59

2.5 Fault Ride-Through Capabilities and Grid Code Compliance 602.5.1 Advantages and Disadvantages of the Crowbar Protection 602.5.2 Effects of DFIG Variables over Its Fault Ride-Through Capabilities 61

2.6 Enhanced Control Strategies to Improve DFIG FaultRide-Through Capabilities 622.6.1 The Two Degrees of Freedom Internal Model Control (IMC) 622.6.2 IMC Controller of the Rotor Speed 652.6.3 IMC Controller of the Rotor Currents 662.6.4 IMC Controller of the dc Voltage 672.6.5 IMC Controller of the Grid-Side Converter Currents 692.6.6 DFIG IMC Controllers Tuning for Attaining Robust Control 702.6.7 The Robust Stability Theorem 70References 72

3 Fully-Rated Converter Wind Turbine (FRC-WT) 733.1 Synchronous Machine Fundamentals 73

3.1.1 Synchronous Generator Construction 733.1.2 The Air-Gap Magnetic Field of the Synchronous Generator 74

3.2 Synchronous Generator Modelling in the dq Frame 793.2.1 Steady-State Operation 813.2.2 Synchronous Generator with Damper Windings 82

3.3 Control of Large Synchronous Generators 853.3.1 Excitation Control 863.3.2 Prime Mover Control 87

3.4 Fully-Rated Converter Wind Turbines 883.5 FRC-WT with Synchronous Generator 89

Contents vii

3.5.1 Permanent Magnets Synchronous Generator 903.5.2 FRC-WT Based on Permanent Magnet Synchronous Generator 923.5.3 Generator-Side Converter Control 933.5.4 Modelling of the dc Link 963.5.5 Network-Side Converter Control 98

3.6 FRC-WT with Squirrel-Cage Induction Generator 1003.6.1 Control of the FRC-IG Wind Turbine 100

3.7 FRC-WT Power System Damper 1053.7.1 Power System Oscillations Damping Controller 1053.7.2 Influence of Wind Generation on Network Damping 1073.7.3 Influence of FRC-WT Damping Controller on

Network Damping 108Acknowledgements 110References 112

4 Offshore Wind Farm Electrical Systems 1134.1 Typical Components 1134.2 Wind Turbines for Offshore – General Aspects 1134.3 Electrical Collectors 115

4.3.1 Wind Farm Clusters 1184.4 Offshore Transmission 118

4.4.1 HVAC Transmission 1184.4.2 HVDC Transmission 1204.4.3 CSC-HVDC Transmission 1224.4.4 VSC-HVDC Transmission 1284.4.5 Multi-Terminal VSC-HVDC Networks 140

4.5 Offshore Substations 1414.6 Reactive Power Compensation Equipment 144

4.6.1 Static Var Compensator (SVC) 1444.6.2 Static Compensator (STATCOM) 147

4.7 Subsea Cables 1504.7.1 Ac Subsea Cables 1504.7.2 Dc Subsea Cables 1504.7.3 Modelling of Underground and Subsea Cables 150Acknowledgements 151References 151

5 Grid Integration of Offshore Wind Farms – Case Studies 1555.1 Background 1555.2 Offshore Wind Farm Connection Using Point-to-Point

VSC-HVDC Transmission 1565.3 Offshore Wind Farm Connection Using HVAC Transmission 1595.4 Offshore Wind Farm Connected Using Parallel HVAC/VSC-HVDC

Transmission 161

viii Contents

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

5.6 Multi-Terminal VSC-HVDC for Connection of Inter-Regional Power Systems 168Acknowledgements 171References 171

6 Offshore Wind Farm Protection 1736.1 Protection within the Wind Farm ac Network 173

6.1.1 Wind Generator Protection Zone 1746.1.2 Feeder Protection Zone 1786.1.3 Busbar Protection Zone 1796.1.4 High-Voltage Transformer Protection Zone 180

6.2 Study of Faults in the ac Transmission Line of an Offshore DFIG Wind Farm 1806.2.1 Case Study 1 1816.2.2 Case Study 2 181

6.3 Protections for dc Connected Offshore Wind Farms 1846.3.1 VSC-HVDC Converter Protection Scheme 1846.3.2 Analysis of dc Transmission Line Fault 1856.3.3 Pole-to-Pole Faults 1866.3.4 Pole-to-Earth Fault 1876.3.5 HVDC dc Protections: Challenges and Trends 1886.3.6 Simulation Studies of Faults in the dc Transmission Line of an

Offshore DFIG Wind Farm 188Acknowledgements 192References 192

7 Emerging Technologies for Offshore Wind Integration 1937.1 Wind Turbine Advanced Control for Load Mitigation 193

7.1.1 Blade Pitch Control 1937.1.2 Blade Twist Control 1947.1.3 Variable Diameter Rotor 1947.1.4 Active Flow Control 195

7.2 Converter Interface Arrangements and Collector Design 1957.2.1 Converters on Turbine 1957.2.2 Converters on Platform 1987.2.3 Ac Collection Options: Fixed or Variable Frequency 2007.2.4 Evaluation of >Higher (>33 kV) Collection Voltage 202

7.3 Dc Transmission Protection 2037.4 Energy Storage Systems (EESs) 204

7.4.1 Batteries 2057.4.2 Super-Capacitors 2057.4.3 Flywheel Storage System 2057.4.4 Pumped-Hydro Storage 2067.4.5 Compressed-Air Storage Systems 2067.4.6 Superconducting Magnetic Energy Storage (SMES) 206

Contents ix

7.5 Fault Current Limiters (FCLs) 2077.6 Sub-Sea Substations 2077.7 HTSCs, GITs and GILs 208

7.7.1 HTSCs (High-Temperature Superconducting Cables) 2087.7.2 GITs (Gas-Insulated Transformers) 2087.7.3 GILs (Gas-Insulated Lines) 209

7.8 Developments in Condition Monitoring 2097.8.1 Partial Discharge Monitoring in HV Cables 2097.8.2 Transformer Condition Monitoring 2107.8.3 Gas-Insulated Switchgear Condition Monitoring 2117.8.4 Power Electronics Condition Monitoring 211

7.9 Smart Grids for Large-Scale Offshore Wind Integration 2137.9.1 VPP Control Approach 2167.9.2 Phasor Measurement Units 217Acknowledgements 217References 218

Appendix A Voltage Source Converter Topologies 223A.1 Two-Level Converter 223

A.1.1 Operation 223A.1.2 Voltage Source Converter Square-Mode Operation 224A.1.3 Pulse Width Modulation 225

A.2 Neutral-Point Clamped Converter 240A.2.1 Selective Harmonic Elimination 242A.2.2 Sinusoidal Pulse Width Modulation 244

A.3 Flying Capacitor (FC) Multilevel Converter 247A.4 Cascaded Multilevel Converter 248A.5 Modular Multilevel Converter 249

References 258

Appendix B Worked-out Examples 271

Index 279

Preface

The motivation for this book is the rapid growth of offshore wind energy systems and theimplications this has on power system operation, control and protection. Developments onwind turbine technology and power electronic converters along with new control approacheshave enabled offshore wind energy systems performance to be improved. The authors identifiedthe need for a book that covers fundamental and up-to-date issues on this dynamic topicsuitable for beginners or advanced readership. The contents offer information on technologytrends for offshore wind energy systems, detailed modelling of variable-speed wind generatortechnologies and easy-to-use grid integration examples. The textbook is useful to final yearundergraduate and postgraduate students, and also practising engineers and scientists in thewind industry with research interest in aspects of wind generator technology and electricalsystems for grid integration.

The book is organised in seven chapters and two appendices. Chapter 1 reviews wind turbinebasics and discusses challenges of offshore wind farm connection and grid code compliance.Chapter 2 covers in detail the operation in normal and abnormal conditions of DFIG windgenerators while Chapter 3 focuses on Fully-Rated Converter technologies. In Chapter 4electrical collectors and offshore transmission schemes are covered (including multi-terminaldc transmission). Chapter 5 describes technical challenges arising as a result of integratingoffshore wind farms into the power grid and provides various case studies. Chapter 6 discussesprotection aspects of offshore wind energy systems. Chapter 7 reviews emerging technologiesfor offshore wind integration, including energy storage and condition monitoring. Topologies,control and operation of voltage source converters are covered in Appendix A. Appendix Bpresents a number of worked-out examples well suited for university students.

The text presented in this book draws together material on electrical systems of offshorewind farms from many sources such as graduate courses that the authors have taught overmany years at universities in the UK, a large number of technical papers published by theIEEE and IET, and research programmes with which they have been closely associated suchas the EPSRC-funded SUPERGEN Wind Technologies and the ETI Helmwind project. Theauthors would like to thank Professor William Leithead for his strong and continued supporton how these research programmes are conducted. Special thanks are given to Mr John O.Tande and Professor Kjetil Uhlen for their support and cooperation through the NorwegianResearch Centre for Offshore Wind Technology (NOWITECH). The authors wish to thank DrGiddani O. A. Kalcon who provided very valuable input on VSC-HVDC offshore transmissionin Chapter 4 and Dr Nolan Caliao who gave permission to include material from his PhD thesisin Chapter 3. The authors would also like to acknowledge Dr Gustavo Quinonez-Varela and

xii Preface

Dr Ryan Tumilty for the useful discussions during the preparation of the manuscript, MrWilliam Ross, Mr Alexander Giles, Mr Edward Corr and Mr Philip Morris, who assisted inproofreading the manuscript and Mr Victor Velazquez Cortes and Ms Kamila Nieradzinskawho assisted in the preparation of drawings.

Olimpo Anaya-Lara, David Campos-Gaona, Edgar Moreno-Goytia, Grain Adam2014

About the Authors

Olimpo Anaya-Lara is a Reader in the Institute for Energy and Environment at the Universityof Strathclyde, UK. Over the course of his career, he has successfully undertaken researchon power electronic equipment, control systems design, and stability and control of powersystems with increased wind energy penetration. Dr Anaya-Lara is a key participant to theWind Integration Sub-Programme of the European Energy Research Alliance (EERA) JointProgramme Wind (JP Wind), leading Strathclyde’s involvement and contribution to this Sub-Programme. He was appointed to the post of Visiting Professor in Wind Energy at NTNU,Trondheim, Norway funded by Det Norske Veritas (2010–2011). He was a member of theInternational Energy Annexes XXI Dynamic models of wind farms for power system studiesand XXIII Offshore wind energy technology development. He is currently a member ofthe IEEE and IET, and has published three technical books, as well as over 140 papers ininternational journals and conference proceedings.

David Campos-Gaona received his PhD from Instituto Tecnologico de Morelia, Mexico.He is currently a research associate in the same institution. During his academic career hehas authored and co-authored papers for refereed journals and congresses and worked as areviewer for IEEE and Wiley transaction papers. His research interests include power electronicequipment control for wind power integration and HVDC transmission systems.

Edgar Moreno-Goytia is a Professor in the Posgrado en Ingenierıa Electrica, Instituto Tec-nologico de Morelia, Mexico. His current research interests include the development of powerelectronics-based technologies applied to power grids (HVDC, FACTS and electronic powertransformers), development of dc grids, and grid integration of large wind farms system inte-gration and other renewables. He has led various research projects and has published morethan 20 papers in international journal and conference proceedings. He is a member of theIEEE and IET.

Grain Adam received first-class BSc and MSc degrees in electrical machines and power sys-tems from Sudan University of Science and Technology, Khartoum, Sudan, in 1998 and 2002,respectively, and a PhD degree in power electronics from Strathclyde University, Glasgow,UK, in 2007. He is currently with the Department of Electronic and Electrical Engineering,Strathclyde University, and his research interests are multilevel inverters, electrical machinesand power systems control and stability.

Acronyms and Symbols

ac Alternating currentAFC Active flow controlAVR Automatic voltage regulatorCB Circuit breakerCC Current controlCCC Capacitor-commutated converterCIA Constant-ignition angleCM Condition monitoringCSC Current source converterdc Direct currentDFIG Doubly-fed induction generatorDG Distributed generationEMF Electromotive forceESCR Effective short-circuit ratioESR Equivalent series resistanceFACTs Flexible alternating current transmission systemFC Flying capacitorFCL Fault-current limiterFRC Fully-rated converterFRT Fault ride-throughFSIG Fixed-speed induction generatorGIL Gas-insulated lineGIT Gas-insulated transformerGPS Global positioning systemGSC Grid-side converterGTOs Gate turn-off thyristorGW Giga-wattHP Horse powerHTS High-temperature superconductingHTSC High-temperature superconducting cablesHV High voltageHVAC High-voltage alternating currentHVDC High-voltage direct currentIG Induction generator

xvi Acronyms and Symbols

IGBT Insulated-gate bipolar transistorIMC Internal model controlIPC Individual pitch controlLCC Line-commutated convertersLVRT Low-voltage ride-throughMIMO Multiple-input multiple-outputMTDC Multi-terminal dcMVA Mega volt-ampereMW Mega wattNIST National Institute of Standards and TechnologyNSC Network-side converterODE Ordinary differential equationO&M Operation & MaintenancePCC Point-of common couplingPD Phase dispositionPDC Power system oscillations damping controllerPOD Phase opposition dispositionPI Proportional IntegralPLL Phase lock loopPM Permanent MagnetPMSG Permanent Magnet synchronous generatorPMU Phasor measurement unitPMW Pulse width modulationPSS Power system stabiliserpu Per unitRF Radio frequencyRMS Root-mean squarerpm Revolutions per minuteRSC Rotor-side converterSCADA Supervisory control and data acquisitionSCIG Squirrel-cage induction generatorSCR Silicon-controlled-rectifierSISO Single input single outputSMES Superconducting Magnetic Energy StorageSTATCOM Static synchronous compensatorSVC Static var compensatorTCR Thyristor-controlled reactorTSC Thyristor-switched capacitorTSO Transmission system operatorUHF Ultra high frequencyVAr Volt-ampere reactiveVCO Voltage-controlled oscillatorVDCOL voltage-dependent current-order limitVPP Virtual power plantVSC Voltage-source converterWAMS Wide-area measurement system

Acronyms and Symbols xvii

WT Wind turbineWTG Wind turbine generatorXLPE Cross-linked polyethylene

Symbols Used in Chapter 1

Pair Power in the airflow𝜌 Air densityA Swept area of rotor, m2

𝜐 Upwind free wind speed, m/sCp Power coefficientPwind turbine Power transferred to the wind turbine rotor𝜆 Tip-speed ratio𝜔 Rotational speed of rotorR Radius to tip of rotorVm Mean annual site wind speedVdc Direct voltage

Symbols Used in Chapter 2

vas, vbs, vcs Stator a b c voltagesras, rbs, rcs Stator a b c windings resistanceias, ibs, ics Stator a b c currents𝜓as,𝜓bs,𝜓cs Stator a b c fluxesvar, vbr, vcr Rotor a b c voltagesrarrbr, rcr Rotor a b c windings resistanceiar, ibr, icr Rotor a b c currents𝜓ar,𝜓br,𝜓cr Rotor a b c fluxes[LIG

]Induction generator inductance matrix

Lms Stator magnetising inductanceLls Stator leakage inductanceLm Magnetising inductanceLlr Rotor leakage inductanceNs Effective stator windings turnsNr Effective rotor windings turns𝜇0 Permeability of free spacera Radius of the induction generator air gap annulusl Effective length of the machine (i.e. the effective length of the pole

area)𝜃dq Angle between the d axis of the rotating frame and stator phase a of

the induction generator𝜔dq Angular speed of the dq0 rotating frameids, iqs, i0s d q 0 components of stator currentvds, vqs, v0s d q 0 components of stator voltage𝜓ds, 𝜓qs,𝜓0s d q 0 components of stator flux

xviii Acronyms and Symbols

idr, iqr, i0r d q 0 components of rotor currentvdr, vqr, v0r d q 0 components of rotor voltage𝜓dr, 𝜓qr,𝜓0r d q 0 components of rotor fluxSdq0 Induction generator instantaneous power in d q 0Te Electromagnetic torqueTshaft Torque from the shaft of the mechanical systemJg Generator mass moment of inertia𝜔t Turbine angular speedJt Turbine mass moment of inertiaTtorsion Shaft elasticityTdamping Shaft damping torque𝜃t Turbine rotor angle𝜃r Induction generator rotor angleKtot Shaft torsion constantD Shaft damping constantSB2B Back-to-back converter power ratingvdc Back-to-back converter dc voltage levelrcb Crowbar resistanceQ1, Q2, Q3, Q4, Q5, Q6 VSC IGBTsd1, d2, d3, d4, d5, d6 VSC DiodesQs Induction generator reactive power𝜔s Synchronous speed𝜔r Rotor speedJ Turbine and generator added moment of inertiaC dc capacitance′ Quantity referred to the statorr Resistance between the VSC and the gridL Inductance between the VSC and the gridpwm PMW signalda, db, dc a, b, c, duty cyclema, mb, mc a, b, c, modulator signalsW Capacitor energyLs Stator inductanceLr Rotor inductancePgsc Grid-side converter active powerQgsc Grid-side converter reactive powerPx(s) PlantKx(s) PI controllerKpx, Kix Proportional and integral gains𝓁x(s) Open loop gainBx(s) Closed-loop transfer function𝜏x Closed-loop time constantGx Inner feedback loop gainMx(s) Plant with inner feedback loop gain

Space vector notationTs Stator time constant

Acronyms and Symbols xix

Tr Rotor time constantu Voltage dip magnitudet−0 , t+0 Instant before and after the fault happeningv1, v2, v0 Positive, negative and zero sequence components of ac voltages+𝜔, s−𝜔 Positive and negative sequence slip speedT+

e , T−e Positive and negative sequence electromagnetic torque

s𝜔max Induction generator maximum speedPr Rotor active powerPag Power across the airgapP′(s) Plant modelG(s) IMC controllerL(s) IMC filterMx(s) Model of plant with inner feedback loop gain𝛼x IMC Filter bandwidthFx(s) Two-degrees-of-freedom IMC controller configured as a classical

controllerlm Model uncertainty𝜂 Controller performance𝜛 Normalised control system input

Symbols Used in Chapter 3

Over _ Per unit quantity

b Base quantity𝜙s Stator magnetic field𝜙r Rotor magnetic fieldids, iqs Stator currents in d and q-axisvds, vqs Stator voltages in d and q-axis𝜓ds, 𝜓qs Stator flux linkage in d and q-axisTe Electromagnetic torqueTm Mechanical torquePe Electrical powerPm Mechanical powerQ Reactive power𝜔b Base synchronous speed𝜔s Synchronous speed𝜔r Rotor speedJ Inertia constantH Per unit inertia constantK Shaft stiffnessf System frequencyC Capacitanceif Field currentikd, ikq1, ikq2 Damper winding d and q-axis currentsLlkd, Llkq Leakage inductance of damper windings in d and q-axis

xx Acronyms and Symbols

Lmd, Lmq Mutual inductance in d and q-axisLlf Leakage inductance of the field coilLls Leakage inductance of the stator coilrs Stator resistancerf Field winding resistancerkd, rkq1, rkq2 Resistance of damper d and q-axis coilsvfd Field voltagevkd, vkq1, vkq2 Damper winding voltages in d and q-axis𝜓f Field flux linkage𝜓kd, 𝜓kq1, 𝜓kq2 Damper winding flux linkage in d and q-axis𝛿r Rotor angleCs Synchronising power coefficientCd Damping power coefficientidr, iqr Rotor currents in d and q-axisvdr, vqr Rotor voltages in d and q-axis𝜓dr,𝜓qr Rotor flux linkage in d and q-axised, eq Voltage behind the reactance in d and q-axisLm Mutual inductance between stator and rotor windingsXm Magnetising reactanceLr, Ls Rotor and stator self inductanceXr, Xs Rotor and stator reactanceLlr Rotor leakage inductanceLls Stator leakage inductancerr Rotor resistancers Stator resistances Slip of an induction generatorp Number of poles

Symbols Used in Chapter 4

S Three-phase symmetrical short-circuit levelPdc Rated dc powerQC Three-phase fundamental MVAr at rated Pdc𝛾 Extinction angle𝛼 Delay angleIm Current margin𝜏 Dc capacitor time constantC CapacitanceXT Transformer leakage reactanceRT Transformer winding resistanceXF Smoothing reactor reactanceVdc VSC converter dc voltageRdc dc link resistanceLdc dc link inductance𝛿 Phase angle difference between the ac system and the VSC converter

Acronyms and Symbols xxi

𝜃s Phase angle difference between the ac system voltage and the ac current flowingfrom or into the converter

𝜃c Phase angle difference between the VSC converter voltage and the accurrent

x1 Sending endx2 Receiving endP Active Powervsa,vsb,vsc ac source three phase voltagesisa,isb, isc Three phase currents flowing between the AC source and the VSCvca,vcb, vcc Three phase voltages at the VSC terminalsica,icb, icc Three phase currents at the VSC terminalsLt Inductance of the coupling transformerRF Resistance of the coupling reactorLF Inductance of the coupling reactorIdc dc link currentisd,isq d and q currents flowing between the AC source and the VSCvsd,vsq d and q AC source voltages𝜔 Synchronous speedM Modulation index𝜃 Converter phase angleQ Reactive powerB TCR susceptanceQSVC SVC power firing angleXSVC SVC effective reactanceXTCR TCR fundamental frequency equivalent reactanceXL TCR inductor reactance𝜎 TCR Conduction angleVVSC Statcom voltage𝛿 Statcom angle

Symbols Used in Chapter 5

GS-VSC Grid-side converterBG ac voltage at PCCBWF1 Wind farm ac voltagePg Wind farm active powerQg Wind farm reactive power

P∗g Wind farm active power reference

Q∗g Reactive power reference

G ac gridWF-VSC Wind farm-side converterBWF1 Wind farm ac voltageVSC Voltage source converterVdc VSC converter dc voltageP VSC active power

xxii Acronyms and Symbols

Symbols Used in Chapter 6

Ipickup Over current protection pickup currentImax Maximum load currentIscmin Smallest short circuit currentvdc sc Equivalent dc voltage in a HVDC link during a pole-to-pole faultidc sc Equivalent dc current in a HVDC link during a pole-to-pole faultC, Ldc, rdc Equivalent capacitance, inductance and resistance in a HVDC link during a pole

to pole faultv′dc sc Equivalent dc voltage in a HVDC link during a pole-to-earth fault

i′dc sc Equivalent dc current in a HVDC link during a pole-to-earth fault

C′, L′dc, r′dc Equivalent capacitance, inductance and resistance in a HVDC link during a

pole-to-earth faultrf Fault resistance in a HVDC link during a pole-to-earth fault

1Offshore Wind Energy Systems

1.1 Background

With construction restrictions inhibiting the deployment of wind turbines onshore, offshoreinstallations are more attractive (e.g. in the UK) (The Crown State, 2011). By mid-2012,offshore wind power installed globally was 4620 MW, representing about 2% of the totalinstalled wind power capacity. Over 90% of the offshore wind turbines currently installedacross the globe are situated in the North, Baltic and Irish Seas, along with the EnglishChannel. Most of the rest is in two demonstration projects off China’s coast. According tothe more ambitious projections, a total of 80 GW of offshore wind power could be installedworldwide by 2020, with three quarters of this in Europe (GWEC, 2013).

All current offshore wind installations are relatively close to shore, using well-knownonshore wind turbine technology. However, new offshore wind sites located far from shorehave been identified, with clusters of wind farms appearing at favourable locations for windpower extraction, like in the UK Dogger Bank and German Bight (Figure 1.1) (EuropeanUnion, 2011). The depths of the waters at these sites are in excess of 30 m.

1.2 Typical Subsystems

The typical subsystems in an offshore wind farm are shown in Figure 1.2. At first glance, itcomprises the same elements of an onshore wind farm. However, the environment in which aturbine operates allows a distinction to be made. Considering that the nature of the sea state willact to prohibit accessibility of wind turbines for repair, there is a greater need for offshore windturbines to be reliable and not require regular repair. This requirement means that the designsand controllers of offshore wind turbines differ from those seen with onshore wind turbines.This is to ensure that performance is maximised whilst minimising cost (German EnergyAgency, 2010).

Offshore Wind Energy Generation: Control, Protection, and Integration to Electrical Systems, First Edition.Olimpo Anaya-Lara, David Campos-Gaona, Edgar Moreno-Goytia and Grain Adam.© 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.Companion Website: www.wiley.com/go/offshore wind energy generation

2 Offshore Wind Energy Generation

Figure 1.1 Europe’s offshore wind farms in operation, construction and planning (Source:www.4coffshore.com/offshorewind).

Power train

Turbine control

Central controller

Wind farm control

Wind turbinelevel

Windfarmlevel

EnvironmentMechanical

(load mitigation)

Electrical(power production)

Waves&

Sea currents

Electrical losses

Wake effects

WindWind shearturbulence

Minimise

Offshore transmission network

Onshore network

(TSO)

PitchYaw

Tower

OWF-1

OWF-2

Offshore substation(s) Onshore substation(s)

Figure 1.2 Subsystems of an offshore wind farm installation (Anaya-Lara et al., 2013).

Offshore Wind Energy Systems 3

Structural Dynamics

Lightning

Operational & accidental loads

icing

Ship impact (breaking) ice

Marine growthscour

Foundation behaviour

(breaking) waves

Current tides

Earthquake

Wake turbulence

Salinity, humidity & temperature

Ambient turbulence

Grid interaction

Figure 1.3 Impacts on a bottom-fixed wind turbine (Fischer, 2006).

In the offshore environment, loads are induced by wind, waves, sea currents, and in somecases, floating ice (Figure 1.3), introducing new and difficult challenges for offshore windturbine design and analysis. Accurate estimation and proper combination of these loads areessential to the turbine and associated controllers design process. Offshore wind turbines havedifferent foundations to onshore wind turbines. The foundations are subjected to hydrody-namic loads. This hydrodynamic loading will inevitably exhibit some form of coupling to theaerodynamic loading seen by the rotor, nacelle and tower. This is an additional problem thatmust be considered when designing offshore wind turbines. Ideally, the total system composedof rotor/nacelle, tower, substructure and foundation should be analysed using an integratedmodel (Nielsen, 2006). Development of novel wind turbine concepts optimised for operationin rough offshore conditions along with better O&M strategies is crucial. In addition, turbinecontrol philosophy must be consistent and address the turbine as a whole dynamic element,bearing in mind trade-offs in terms of mechanical performance and power output efficiency(Anaya-Lara et al., 2013).

At the wind farm level, the array layout and electrical collectors must be designed on asite-specific basis to achieve a good balance between electrical losses and wake effects. Forpower system studies, it is typical to represent the wind farm by an aggregated machine model(and controller). However, more detailed wind farm representations are required to take fulladvantage of control capabilities, exploring further coordinated turbine control and operationto achieve a better array design. Full exploitation of the great potential offered by offshore windfarms will require the development of reliable and cost-effective offshore grids for collectionof power, and its transmission and connection to the onshore network whilst complying withthe grid codes. It is anticipated that power electronic equipment (e.g. HVDC and FACTs), andtheir enhanced control features, will be fundamental in addressing these objectives.

4 Offshore Wind Energy Generation

1.3 Wind Turbine Technology

1.3.1 Basics

Wind turbines produce electricity by using the power of the wind to drive an electrical generator(Fox et al., 2007; Anaya-Lara et al., 2009). Wind passes over the blades generating lift andexerting a turning force. The rotating blades turn a shaft that goes into a gearbox, whichincreases the rotational speed to that which is appropriate for the generator. The generator usesmagnetic fields to convert the rotational energy into electrical energy. The power output goesto a transformer, which steps up the generator terminal voltage to the appropriate voltage levelfor the power collection system.

A wind turbine extracts kinetic energy from the swept area of the blades (Figure 1.4).The power in the airflow is given by (Burton et al., 2001; Manwell et al., 2002):

Pair =12𝜌A𝜐3 (1.1)

where 𝜌 is the air density, A is the swept area of the rotor in m2, and 𝜐 is the upwind freewind speed in m/s. The power transferred to the wind turbine rotor is reduced by the powercoefficient, Cp:

Pwind turbine = CpPair =12𝜌A𝜐3Cp (1.2)

A maximum value of Cp is defined by the Betz limit, which states that a turbine can neverextract more than 59.3% of the power from an air stream. In practice, wind turbine rotors havemaximum Cp values in the range 25–45%. It is also conventional to define a tip-speed ratio,𝜆, as

𝜆 = 𝜔R𝜐

(1.3)

where 𝜔 is the rotational speed of the rotor and R is the radius to tip of the rotor.

Wind

Figure 1.4 Horizontal-axis wind turbine.

Offshore Wind Energy Systems 5

0 5 10 15

0.1

0.2

0.3

0.4

0.5

Pow

er c

oef

fici

ent

Tip speed ratio

Figure 1.5 Illustration of power coefficient/tip-speed ratio curve, Cp∕𝜆.

The tip-speed ratio, 𝜆, and the power coefficient, Cp, are dimensionless and so can be used todescribe the performance of any size of wind turbine rotor. Figure 1.5 shows that the maximumpower coefficient is only achieved at a single tip-speed ratio. The implication of this is thatfixed rotational speed wind turbines could only operate at maximum efficiency for one windspeed. Therefore, one argument for operating a wind turbine at variable rotational speed isthat it is possible to operate at maximum Cp over a range of wind speeds.

The power output of a wind turbine at various wind speeds is conventionally described byits power curve. The power curve gives the steady-state electrical power output as a functionof the wind speed at the hub height. An example of a power curve for a 2-MW wind turbineis given in Figure 1.6.

The power curve has three key points on the velocity scale:

� Cut-in wind speed – the minimum wind speed at which the machine will deliver usefulpower.

� Rated wind speed – the wind speed at which rated power is obtained.� Cut-out wind speed – the maximum wind speed at which the turbine is allowed to deliver

power (usually limited by engineering loads and safety constraints).

0.5

1

1.5

2

2.5

0 5 10 15 20 25 30

Wind speed [m/s]

Ele

ctri

cal

pow

er [

MW

]

Rated wind speed

Cut-out wind speed

Cut-in wind speed

Figure 1.6 Power curve for a 2-MW wind turbine.

6 Offshore Wind Energy Generation

Below the cut-in speed of about 4–5 m/s, the wind speed is too low for useful energyproduction, so the wind turbine remains shut down. When the wind speed is above this value,the wind turbine begins to produce energy; the power output increases following a broadlycubic relationship with wind speed (although modified by the variation in Cp) until ratedwind speed is reached at about 11–12 m/s. Above rated wind speed, the aerodynamic rotor isarranged to limit the mechanical power extracted from the wind and so reduce the mechanicalloads on the drive train. Then at very high wind speeds, typically above 25 m/s, the turbineis shut down. The choice of cut-in, rated and cut-out wind speed is made by the wind turbinedesigner who, for typical wind conditions, will try to balance obtaining maximum energyextraction with controlling the mechanical loads (Anaya-Lara et al., 2009).

1.3.2 Architectures

Figure 1.7 shows the main wind turbine generator concepts which are divided into fixed-speedwind turbines (type A), and variable-speed wind turbines (types B, C and D) (Tande et al.,2007; Fox et al., 2007).

1.3.2.1 Fixed-Speed Wind Turbines

A fixed-speed wind turbine (Type A in Figure 1.7) employs a three-phase squirrel-cageinduction generator (SCIG) driven by the turbine via a gearbox and directly connected tothe grid through a step-up transformer. Thus, the induction generator will provide an almost

Control system

IG

Control system

Gearbox

Controlsystem

Controlsystem

G

Type A: Fixed speed Type B: Variable slip

Type C: Doubly-fed IG Type D: Full-rated converter (IG/PM/SG)

Gearbox

DFIG

acac

acac

Gearbox Gearbox

Figure 1.7 Overview of wind turbine concepts (Tande et al., 2007).