Grid Integration of Wind Energy PDF

GRID INTEGRATION OF WIND ENERGY GRID INTEGRATION OF WIND ENERGY ONSHORE AND OFFSHORE CONVERSION SYSTEMS Third Edition Siegfried Heier Kassel University, Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) Kassel, Germany Translators: Gunther Roth Adliswil, Switz...

GRID INTEGRATION OF WIND ENERGY GRID INTEGRATION OF WIND ENERGY ONSHORE AND OFFSHORE CONVERSION SYSTEMS Third Edition Siegfried Heier Kassel University, Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) Kassel, Germany Translators: Gunther Roth Adliswil, Switzerland Rachel Waddington UK Originally published in the German language by Vieweg+Teubner, 65189 Wiesbaden, Germany, as “Siegfried Heier: Windkraftanlagen. 5. Auflage (5th Edition)” © Vieweg+Teubner | Springer Fachmedien Wiesbaden GmbH 2009 Springer Fachmedien is part of Springer Science+Business Media This edition first published 2014 © 2014, John Wiley & Sons, Ltd Registered office John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com. The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. It is sold on the understanding that the publisher is not engaged in rendering professional services and neither the publisher nor the author shall be liable for damages arising herefrom. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Library of Congress Cataloging-in-Publication Data Heier, Siegfried. [Windkraftanlagen im Netzbetrieb. English] Grid integration of wind energy / Siegfried Heier ; translated by Rachel Waddington. –Third editon. pages cm Translation of: Windkraftanlagen im Netzbetrieb. Includes bibliographical references and index. ISBN 978-1-119-96294-6 (hardback) 1. Wind power plants. 2. Wind energy conversion systems. 3. Electric power systems. I. Title. TK1541.H3513 2014 621.31′ 2136–dc23 2013041087 A catalogue record for this book is available from the British Library. ISBN: 978-1-119-96294-6 Typeset in 10/12pt Times by Laserwords Private Limited, Chennai, India 1 2014 Contents Preface xi Notation xiii 1 Wind Energy Power Plants 1 1.1 Wind Turbine Structures 1 1.2 A Brief History 4 1.3 Milestones of Development 5 1.4 Functional Structures of Wind Turbines 20 References 30 2 Wind Energy Conversion Systems 31 2.1 Drive Torque and Rotor Power 31 2.1.1 Inputs and outputs of a wind turbine 31 2.1.2 Power extraction from the airstream 32 2.1.3 Determining power or driving torque by the blade element method 34 2.1.4 Simplifying the computation method 38 2.1.5 Modeling turbine characteristics 40 2.2 Turbines 46 2.2.1 Hub and turbine design 50 2.2.2 Rotor blade geometry 51 2.3 Power Control by Turbine Manipulation 57 2.3.1 Turbine yawing 57 2.3.2 Rotor blade pitch variation 67 2.3.3 Limiting power by stall control 97 2.3.4 Power control using speed variation 100 2.4 Mechanical Drive Trains 102 2.5 System Data of a Wind Power Plant 108 2.5.1 Turbine and drive train data 108 2.5.2 Machine and tower masses 110 2.5.3 Machine costs 111 References 116 vi Contents 3 Generating Electrical Energy from Mechanical Energy 119 3.1 Constraints and Demands on the Generator 119 3.2 Energy Converter Systems 122 3.2.1 Asynchronous generator construction 125 3.2.2 Synchronous generator construction 126 3.3 Operational Ranges of Asynchronous and Synchronous Machines 126 3.4 Static and Dynamic Torque 132 3.4.1 Static torque 133 3.4.2 Dynamic torque 147 3.5 Generator Simulation 154 3.5.1 Synchronous machines 155 3.5.2 Asynchronous machines 160 3.6 Design Aspects 161 3.6.1 Asynchronous generators 162 3.6.2 Synchronous generators for gearless plants 174 3.6.3 Multi-generator concept (Dissertation A. Ezzahraoui) 187 3.6.4 Ring generator with magnetic bearings (Dissertation K. Messol) 194 3.6.5 Compact superconductive and other new generator concepts 197 3.7 Machine Data 199 3.7.1 Mass and cost relationships 200 3.7.2 Characteristic values of asynchronous machines 202 3.7.3 Characteristic values of synchronous machines 204 References 208 4 The Transfer of Electrical Energy to the Supply Grid 210 4.1 Power Conditioning and Grid Connection 210 4.1.1 Converter systems 212 4.1.2 Power semiconductors for converters 215 4.1.3 Functional characteristics of power converters 218 4.1.4 Converter designs 222 4.1.5 Indirect converter 223 4.1.6 Electromagnetic compatibility (EMC) 236 4.1.7 Protective measures during power conditioning 237 4.2 Grid Protection 238 4.2.1 Fuses and grid disconnection 239 4.2.2 Short-circuiting power 239 4.2.3 Increase of short-circuit power 242 4.2.4 Isolated operation and rapid auto-reclosure 245 4.2.5 Overvoltages in the event of grid faults 247 4.3 Grid Effects 247 4.3.1 General compatibility and interference 247 4.3.2 Output behavior of wind power plants 248 4.3.3 Voltage response in grid supply 260 Contents vii 4.3.4 Harmonics and subharmonics 271 4.3.5 Voltage faults and the fault-ride-through (FRT) 279 4.4 Resonance Effects in the Grid During Normal Operation 284 4.5 Remedial Measures against Grid Effects and Grid Resonances 290 4.5.1 Filters 290 4.5.2 Filter design 292 4.5.3 Function of harmonic absorber filters and compensation units 293 4.5.4 Grid-specific filter layout 294 4.5.5 Utilizing compensating effects 297 4.6 Grid Control and Protection 300 4.6.1 Supply by wind turbines 300 4.6.2 Grid support and grid control with wind turbines and other renewable systems 301 4.6.3 Central reactive power control 305 4.6.4 System services and operation 308 4.6.5 Connection of wind turbine to the transmission grid 310 4.7 Grid Connection Rules 311 4.8 Grid Connection in the Offshore Region 317 4.8.1 Offshore wind farm properties 317 4.8.2 Stationary and dynamic behavior of offshore wind farms 319 4.8.3 Wind farm and cluster formation at sea and grid connection 319 4.8.4 Electrical energy transmission to the mainland 323 4.8.5 Reactive power requirement and reactive power provision in the offshore grid 325 4.8.6 Flexible AC transmission systems (FACTS) 330 4.9 Integration of the Wind Energy into the Grid and Provision of Energy 333 4.9.1 Grid extension 333 4.9.2 Provision of energy 335 4.9.3 Control and reserve power 337 4.9.4 Power reserve provision with wind farms (Dissertation A. J. Gesino) 338 4.9.5 Intercontinental grid connections 346 References 346 5 Control and Supervision of Wind Turbines 355 5.1 System Requirements and Operating Modes 356 5.2 Isolated Operation of Wind Turbines 358 5.2.1 Turbines without a blade pitch adjustment mechanism 359 5.2.2 Plants with a blade pitch adjustment mechanism 360 5.2.3 Plants with load management 362 5.2.4 Turbine control by means of a bypass 362 5.3 Grid Operation of Wind Turbines 363 5.4 Control Concepts 367 5.4.1 Control in isolated operation 367 5.4.2 Regulation of variable-speed turbines 371 viii Contents 5.4.3 Regulation of variable-slip asynchronous generators 373 5.4.4 Regulation of turbines with a rigid connection to the grid 388 5.4.5 Wind turbine control using hydrodynamic variable-speed superimposing gears 390 5.5 Controller Design 390 5.5.1 Adjustment processes and torsional moments at the rotor blades 392 5.5.2 Standardizing and linearizing the variables 395 5.5.3 Control circuits and simplified dimensioning 400 5.5.4 Improving the control characteristics 404 5.5.5 Control design for wind turbines 410 5.6 Management System 411 5.6.1 Operating states 412 5.6.2 Faults 423 5.6.3 Determining the state of system components 424 5.7 Monitoring and Safety Systems 424 5.7.1 Wind measuring devices 425 5.7.2 Oscillation monitoring 425 5.7.3 Grid surveillance and lightning protection 426 5.7.4 Surveillance computer 426 5.7.5 Fault prediction 427 5.7.6 Voltage limitation 429 References 430 6 Using Wind Energy 436 6.1 Wind Conditions and Energy Yields 436 6.1.1 Global wind conditions 436 6.1.2 Local wind conditions and annual available power from the wind 438 6.1.3 Calculation of site-specific and regional turbine yields 440 6.1.4 Wind atlas methods 444 6.2 Potential and Expansion 449 6.2.1 Wind energy use on land 449 6.2.2 Offshore wind energy use 451 6.2.3 Repowering 453 6.3 Economic Considerations 455 6.3.1 Purchase and maintenance costs 457 6.3.2 Power supply and financial yields 457 6.3.3 Blue section 460 6.3.4 Commercial calculation methods 461 6.4 Legal Aspects and the Installation of Turbines 463 6.4.1 Immission protection 464 6.4.2 Nature and landscape conservation 467 6.4.3 Building laws 468 6.4.4 Planning and planning permission 469 6.4.5 Procedure for erecting a wind turbine 470 6.4.6 Offshore utilization of wind energy 472 Contents ix 6.5 Ecological Balance 474 6.5.1 Contribution to climate protection 474 6.5.2 Landscape utilization 475 6.5.3 Bird strike 475 6.5.4 Bats 475 6.5.5 Recycling of wind turbines 475 6.5.6 Energetic amortization time and harvest factor 476 References 476 Index 483 Preface In the long run, an ecologically sustainable energy supply can be guaranteed only by the inte- gration of renewable resources. Besides water power, which is already well established, wind energy is by far the most technically advanced of all renewable power sources, and its eco- nomic breakthrough the closest. With a few exceptions, wind power will be used mostly for generating electricity. Just three decades on from the 50 kW class machines of the mid-1980s, the development of wind turbines has led to production converters with outputs in the 3 MW range. Five to ten megawatt turbines are currently being launched in the market. In the development of these machines, successful techniques and innovations originating from small- and medium-sized turbines were carried over to larger ones, and this has led to a considerable improvement in the reliability of wind turbines. The technical availability currently achieves average values of approximately 98%. Furthermore, economic viability has increased enormously. As a result, wind energy has experienced an almost unbelievable upsurge and has already far exceeded the contributions of water power. The rapid development of wind power has awakened strong public, political and scientific interest, and has triggered widespread discussion over the past few years, much of it concerning the degree to which nature, the environment and the electricity grid can withstand the impact of wind power. If political requirements regarding the reduction of environmental pollution are to be met, long-term growth in the use of wind power must be the focus. Since obtaining electricity from the wind currently offers the most favourable technical and economic prospects of all the sources of renewable energy, it must be assigned the highest priority. Due to the fact that turbine sizes are still increasing, a high degree of grid penetration must be expected (regionally at any rate), meaning that the connection of wind turbines could come up against its technical limits. This is already the case today in some instances. The objective of a forward-looking energy supply policy must therefore be to utilize the existing grid as well as possible ones for the supply of wind power. This is made possible by the use of turbines with good grid compatibility in connection with measures for grid reinforce- ment. In assessing grid effects, control operations and the electrical engineering design of wind turbines play a significant role. The themes developed in this work are therefore particularly concerned with this topic. This edition of the book has been updated to cover important innovations in this rapidly changing technology in terms of energy converters, generators and controls, grid integration and development. Important additions were made especially in view of offshore use of wind xii Preface energy. This has resulted in special importance being paid to network connections at sea and on land. The layout of the book has also been updated to achieve a consistent format, and a number of new illustrations have been included. A great deal of new material has also been added to cover changes in legislation. This book is the result of a 37 years continuous work in research and development, especially as Head of Wind Energy Research and Professor at the University Kassel, in the Electrical Energy Supply Department of the Institut für Elektrische Energietechnik. Close cooperation with the Institut für Solare Energieversorgungstechnik (ISET) e.V. (now Fraunhofer Institut für Windenergiesysteme IWES, Kassel) brought with it a considerable broadening of the horizon of experience. My special thanks go to the founder of the ISET, Professor Dr Werner Kleinkauf. His suggestions and our technical discussions have had a considerable influence on this work. The help and support of Ms Katherina Messoll, Dr.-Ing. Alejandro Gesino, Dipl.-Ing. Christof Dziendziol. Dipl.-Ing, Adit Ezzahraoui, Dr.-Ing. Gunter Arnold, Dr Boris Valov, Dipl.-Ing. Michael Durstewitz, Dr.-Ing. Martin Hoppe-Kilpper, Dipl.-Ing.Berthold Hahn, Dipl.-Ing. Martin Kraft, Dipl.-Ing. Volker Konig, Dipi.-Ing. Werner Döring, Dipl.-Ing. Bernd Gruss, Dr-Ing. Oliver Haas, Dr.-Ing Rajeh Saiju, Mr Thomas Donbecker, Mr Bernhard Siano, Mr Martin Nagelmilller, Ms Dipl.-Des. Renate Rothkegel, Frau Melanie Schmieder, Ms Anja Clark-Carina and Ms Judith Keuch have contributed greatly to the success of this book. My grateful thanks also go to ENERCON GmbH for kindly granting permission to use the image of the wind turbine in the design of the front cover. This book is intended not only for students in technical faculties. The procedural notes and experimental results will also be of great help to engineers both in theory and practice. My special thanks must go to the publisher, John Wiley & Sons, Ltd and Laura Bell and Peter Mitchell for their readiness to publish this book and for the painstaking preparation involved. I would like to thank my wife Hannelore for her assistance as adviser for the difficult for- mulation and for her understanding that was necessary for the creation of this work. This book is dedicated to my grandchildren Serafin and Mila as well as my daughters Angela, Sandra and Tina. The issue of the fifth revision marks the third decade of my future-oriented efforts in this sector and documents the headlong development of wind energy utilization. In this scientific and energy segment with its defining technology, successes have been achieved that open up optimistic perspectives for the future of energy supply. Siegfried Heier, Kassel Notation a Constant factor related to the pivot of the profile aa Distance between point of application of lifting force and blade axis of rotation ap Distance along blade axis between the points of application of torque and gravity as Blade deflection and slewing A1 Far-upstream cross-section of flow A2 Cross-section of flow at turbine A3 Broadening downstream cross-section of flow Alt Long-term flicker factor AR Rotor swept area Ast Short-term flicker factor b Acceleration of the rotor blade centre of gravity b′ Acceleration of the rotor blade centre of gravity in the rotating coordinate system bc Coriolis acceleration of rotor blade centre of gravity bdefl Blade bending in direction of deflection bo Centripetal acceleration in the rotor head bR Centripetal acceleration arising from 𝜔R bs Bending acceleration of the rotor blade in the direction of deflection and slew bslew Blade bending in direction of slew b𝜔A Centripetal acceleration arising from 𝜔A c Magnification factor for the initial short-circuit alternating current power or the maximum possible short-circuit current ca Lift coefficient of blade profile ck Capacitor bank capacitance cm Torque coefficient of the turbine cp Performance coefficient of the turbine ct Torsional moment coefficient of blade profile (tB /4-related) cw Drag coefficient of blade profile cos 𝜑 Power factor xiv Notation cos 𝜑K Power factor in case of short-circuit C = C(k), Theodorsen function d Half-profile depth dm Average bearing diameter dAB Blade element area dFA Lift force on blade element dFAW Resultant force on blade segment from lift and drag components dFax Axial force at blade element dFt Tangential force at blade element dFW Drag force on blade element dJ̇ ax Axial momentum losses by blade streaming dJ̇ t Change of tangential momentum of angular streaming dML Moment per unit of width during blade pitch adjustment due to acceleration of air mass and due to air damping dMlift Torsional moment at blade element due to lifting forces dMT Righting moment in direction of air flow on the blade element dU Voltage deviation, voltage drop f Frequency f1 Grid frequency f2𝜈 Rotor current frequency (the 𝜈th harmonic) in asynchronous machines fG Generator frequency fL1 Bearing coefficient dependent upon bearing type and loading f𝜇 Frequency of the 𝜇th subharmonic f𝑣 Frequency of the 𝑣th harmonic Fa Axial force on bearing FN Normal force FPr Force creating propeller moment FQ Transverse force component FSt Actuating force on blade FZ Centrifugal force gL1 Bearing load direction factor iABl Transmission ratio between adjustment mechanism and blade pitch adjustment iG Total current (rotating pointer) iG1 Total current in phase 1 iG2 Total current in phase 2 iG3 Total current in phase 3 iGd Total current in longitudinal direction of field coordinates iGq Total current in transverse direction of field coordinates iMBl Transmission ratio between adjustment motor and blade rotation iMBl,rot Transmission ratio between servomotor and rotor blade adjustment iMBl,lin-rot Transmission ratio between servomotor and blade pitch adjustment in the case of direct motor drive ims Magnetizing current in the stator iR Machine-side rotor current (rotating pointer) Notation xv iR1 Machine-side rotor current in phase 1 iR2 Machine-side rotor current in phase 2 iR3 Machine-side rotor current in phase 3 iRd Machine-side rotor current in longitudinal direction of field coordinates iRd act Actual value of iRd iRd des Desired value of iRd iRN Grid-side rotor current (rotating pointer) iRq Machine-side rotor current in phase 1 iRq act Actual value of iRq iRq des Desired value of iRq I0 No-load current in one machine phase I1 Stator current I1 Effective value of fundamental component current I2′ Rotor phase current acting on stator side Ian Starting current of asynchronous machines IE Exciter current IFe Iron loss current in one machine phase ISt Electric current or hydraulic flow for blade pitch positioning IZ Current of reactive power compensation system I𝜇 Magnetizing current in one machine phase I𝜈 Effective value of the 𝜈th harmonic current JB Moment of inertia of blade during rotation around the hub JBl Moment of inertia of rotor blade when turned about its longitudinal axis JBl(A) Moment of inertia of rotor blade taken at the drive motor side JG Moment of inertia of generator rotor JLB Equivalent moment of inertia due to accelerated air mass JMot Moment of inertia of the drive motor JMot(Bl) Moment of inertia of the drive motor acting on the rotor blades JR Moment of inertia of all rotating masses Jtot Total moment of inertia of blade pitch adjustment system Jtot(A) Total moment of inertia of the entire blade pitch adjustment system taken from the drive side Jtot(Bl) Total moment of inertia of the entire blade pitch adjustment system taken from the blade side Jtrans Moment of inertia of transmission elements such as gears, couplings, etc., between drive motor and blade turning mechanism kA Rate of change factor of the rotor displacement angle after falling out of step kd Characteristic damping kDB Coefficient of structural and aerodynamic damping kDK Coefficient of damping for the drive train kRL Coefficient of friction for bearing friction at rotor blade during blade pitch adjustment kt Ratio of the acceleration moments of the drive-train component to the entire rotor system (MBT ∕MBR ) kTHD Total harmonic distortion xvi Notation kTHD0 Grid-state-dependent and grid short-circuit power-dependent output value of total harmonic distortion kTHD1 Gradient of relative harmonic content kTHD2 Elongation factor of relative harmonic content kTS Torsional stiffness of the drive train ku Harmonic distortion of voltage kU Factor for the maximum magnification of generator moment m Number of phases of three-phase current windings mB Mass of a rotor blade mdyn = MKD ∕MKS max , dynamic increase in moment M Moment M Torque MA Driving torque Mact Actual value of moment MAG Driving torque at generator MAM Motor start-up torque MAn External torque of blade pitch adjustment drive taking into account spring and damping characteristics MAT Driving torque of drive train including losses MAV Internal torque of blade pitch adjustment drive MAW Wind turbine driving moment Mbend Torsional moment at rotor blade due to bending MBG Acceleration moment at the generator MBl Rotor blade torsional moment during turning about blade longitudinal axis MBl max Maximum blade torsional moment in extreme situations MBln Blade torsional moment in normal operation MBR Acceleration moment in rotor system MBT Acceleration moment in drive train MBW Acceleration moment on wind turbine MCz Coriolis moment in relation to the z axis MD Damping moment of the synchronous machine Mdes Desired value of torque Mfrict Moment of friction of all blade bearings during blade pitch adjustment MK Breakdown torque of asynchronous machine MK Pull-out torque of synchronous machine MKD Dynamic breakdown or pull-out torque MKG Generator breakdown or pull-out torque MKM Motor breakdown or pull-out torque MK max Maximum breakdown or pull-out torque MK′′ max Maximum moment of synchronous machines due to subtransient short-circuit currents in the damping winding MK′ max Maximum value of pulsating short-circuit moment of synchronous machines due to transient currents MKS Static breakdown or pull-out torque MKS max Static breakdown or pull-out torque at maximum excitation MKS min Static breakdown or pull-out torque with no-load excitation Notation xvii MKu Coupling torque at generator ML Moment with blade pitch adjustment by acceleration of air masses and air damping MLB Moment with blade pitch adjustment due to acceleration of air masses MLD Moment with blade pitch adjustment due to air damping Mlift Torsional moment at rotor blade due to lift forces Mmax Maximum moment MN Nominal moment MNG Generator nominal moment MNM Motor nominal moment MPr Propeller moment Mres Reserve moment during acceleration of the blade pitch adjustment mechanism MRL Load-dependent moment of friction of a bearing MRLk Load-dependent moment of friction of bearing k Ms Steady-state torque MS Pull-up torque MSM Pull-up torque of a motor (asynchronous machine) MSt Moment exerted upon blade by actuator MT Righting moment in direction of air flow on blade profile MTD Damping component of drive train moment Mteeter Torsional moment at the blade due to teetering of the rotor MTT Torsionally elastic component of drive-train moment MW Load torque MWG (Electrical) load torque of the generator n Rotational speed n1 Speed of rotating field or synchronous speed nA Number of turbines nact Actual value of rotational speed nAV Rotational speed of blade pitch adjustment drive ndes Desired value for speed nKG Generator breakdown-torque speed (asynchronous machine) nKM Motor breakdown-torque speed (asynchronous machine) nNG Generator nominal speed (asynchronous machine) nNM Motor nominal speed (asynchronous machine) n𝜈 Speed of the harmonic field of ordinal number 𝜈 p1 Number of pairs of poles in the stator p2 Number of pairs of poles in the rotor P Average value of power PE Power of producer in the grid Pel Electrical generator power PG Total active power in rotor and stator PL Power of the load in the grid PL0 Equivalent static bearing loading Pmech Mechanical input power of generator PN Nominal power PO Moving air mass power xviii Notation PStn Power for normal positioning procedures PSts Power for fast positioning procedures PT act Actual value of total active power PT des Desired value of total power PW Wind turbine power PW max Maximum wind turbine power P𝛿 Air gap power of an electrical machine P𝜎 Standard deviation of power QC Compensation reactive power QG Total reactive power in rotor and stator QT act Actual value of total reactive power QT des Desired value of total reactive power r Radius of a blade element r′ Radius of the rotor blade centre of mass ro Distance between yaw and rotor blade fulcrums R1 Stator resistance of one machine phase R′2 Rotor resistance of one phase of an asynchronous machine transformed on the stator side Ra Outer radius of rotor blade Rgrid Resistance of the connection elements between higher grid and point of common coupling Ri Inner radius of rotor blade RL+T Ohmic resistance of lines and transformers RL+T Resistance between wind turbine and point of common coupling s Slip (of an asynchronous machine) sK Breakdown slip (asynchronous machine) sN Nominal slip (asynchronous machine) s𝜈 Slip of the 𝜈th harmonic (asynchronous machine) Sgrid Grid apparent power Sk Grid short-circuit power Sk′′ Initial value of alternating current short-circuit power Sload Load apparent power Sp Centre of gravity SrG Generator rated apparent power SrT Transformer rated apparent power Ssupply Supply apparent power t Time t0 Time for rotor blade adjustment into a safe operating state t0b Time for rotor blade adjustment into a safe operating state with pure acceleration processes tAf Acceleration time of blade positioning drive in the case of fast positioning procedures tAPD Acceleration time of blade positioning drive system tAPDd Acceleration time of direct-driven blades tAPDz Acceleration time of z rotor blades adjusted by positioning drive system Notation xix tB Blade thickness tf Duration of secondary effect of flicker tv Delay time tv Rotor blade adjustment time at constant speed TD Time constant of damping of torque oscillation TE Time constant of the exciter circuit TG Generator acceleration time constant Tn = 1∕𝜔0 = 1∕Ao , time constant of the rotation speed integrator TR Rotor system acceleration time constant TV Time constant for the decaying dynamic pull-out torque to its steady-state value TW Wind turbine acceleration time constant T𝜀 = p∕𝜔0 , time constant of integrator for the determination of the angle of torsion (generator side) uG Total voltage (rotating pointer) corresponds to stator voltage (uT = uS ) uGq Total voltage in quadrature-axis direction of the field coordinates (uTq = uSq ) ukASM Magnification factor of the short-circuit power of asynchronous machines ukSM Magnification factor of the short-circuit power of synchronous machines uN Nominal voltage uR1 Machine-side rotor voltage in phase 1 uR2 Machine-side rotor voltage in phase 2 uR3 Machine-side rotor voltage in phase 3 uRd Machine-side rotor voltage in direct-axis direction of the field coordinates uRq Machine-side rotor voltage in quadrature direction of the field coordinates uS1 Stator voltage in phase 1 uS2 Stator voltage in phase 2 uS3 Stator voltage in phase 3 uSq Stator voltage in quadrature direction of the field coordinates u𝜇VT Compatibility level of the 𝜇th subharmonic specific to the fundamental component u𝜈 Harmonic voltage of the 𝜈th-order specific to the fundamental component u𝜈VT Compatibility level of the 𝜈th harmonic specific to the fundamental component U0 Direct voltage component U1 Effective value of fundamental component voltage, grid voltage U1 Grid voltage UC Reference conductor voltage of capacitor banks Udi Ideal direct voltage Ug Direct current link voltage UGen Generator voltage UGrid Grid voltage Ui Induced machine voltage UMot Motor voltage Up Rotary-field (internal) voltage of a synchronous machine UR max Maximum rotor voltage U𝜈 Effective value of the 𝜈th harmonic voltage UZ Ignition impulse voltage xx Notation 𝑣 Wind speed 𝑣 Average wind speed 𝑣0 Rotational speed of the rotor head 𝑣1 Undisturbed far-upstream wind speed 𝑣2 Wind speed at the turbine 𝑣2ax Axial component of the decelerated wind speed at the rotor blade 𝑣2t Tangential component of the decelerated wind speed at the rotor blade 𝑣3 Decelerated wind speed far downstream of the turbine 𝑣r Resultant wind speed 𝑣u Peripheral speed 𝑣w Local wind speed 𝑣𝜎 Standard deviation of wind speed Va Airstream volume element WW Energy drawn by wind turbine Xd′′ Subtransient reactance of a synchronous machine X1𝜎 Leakage reactance of one stator phase ′ X2𝜎 Leakage reactance of one rotor phase specific to the stator Xd Synchronous direct-axis reactance of a synchronous machine Xd′ Transient reactance of a synchronous machine XG Leakage reactance from stator and rotor of an asynchronous machine Xgrid Reactance of the connection elements between grid and point of common coupling Xh Main reactance of one machine phase XL+T Reactance between wind turbine and point of common coupling XL+T Reactance of lines and transformers Xq Synchronous quadrature reactance of a synchronous machine Xu Reactance of the frequency converter valves z Number of rotor blades za Number of driven rotor blades ZC Impedance of the capacitor bank Zk Grid impedance (in short-circuit) Zload Load impedance 𝛼 Local profile flow angle at the rotor blade 𝛼 Trigger or firing angle of thyristors 𝛼0 Ignition angle output value 𝛼max Maximum thyristor trigger angle/thyristor inverter stability limit 𝛼max 15 Maximum thyristor trigger angle/thyristor inverter stability limit at 15% voltage drop 𝛽 Rotor blade pitch 𝛽̇ Adjustment speed of the rotor blade 𝛽̇n Normal adjustment speed of the rotor blade 𝛽̇s Fast adjustment speed of the rotor blade 𝛽̈ Adjustment acceleration of the rotor blade 𝛾 Cone angle 𝜈 Ordinal number of one harmonic (integer) Notation xxi 𝛿 Angle between plane of rotation and resultant air flow velocity Δn Adjustment range of rotation speed ΔP Power fluctuation range ΔU Voltage drop ΔUr Voltage rise ΔUr perm Permissible voltage rise Δ𝑣 Fluctuation range of wind velocity Δ𝜀 Angle of torsion 𝜀G Generator rotor angle of rotation 𝜀̇ G Angular velocity of the generator rotor 𝜀W Wind turbine angle of rotation 𝜀̇ W Angular velocity of the turbine 𝜂ABl Transfer efficiency between positioning system and rotor blade 𝜃 Rotor displacement angle (electrical) of a synchronous machine 𝜃 Angle (mechanical) between plane of rotation and chord 𝜃0.7 Angle between plane of rotation and chord at the rotor blade given at 0.7 times the radius 𝜃B Angle between 𝜃0.7 and the chord of the rotor blade 𝜆 Load angle (electrical) in asynchronous machines between fixed grid slip voltage and load-dependent slip voltage 𝜆 Line angle 𝜆 Tip speed ratio (mechanical) of the rotor blade tip speed to wind velocity 𝜆N Tip speed ratio in nominal operating state 𝜇 Noninteger factor of subharmonics 𝜈 Ordinal number of harmonics 𝜌 Air density 𝜑Gen Phase angle of generator current (input angle) 𝜑Mot Phase angle of motor current 𝜑𝜈 Phase displacement angle of the 𝜈th harmonic 𝜓 Position of rotor in relation to tower 𝜓Bl Rotor blade position in relation to tower 𝜔 Resultant angular velocity from azimuth yaw and turbine rotation 𝜔0 Steady-state grid (angular) frequency 𝜔1 Angular velocity of the stator rotating field in two-pole winding design (p = 1) 𝜔2 Angular velocity of the rotor rotating field in two-pole winding design (p = 1) 𝜔A Yaw control angular velocity 𝜔Bl Angular velocity of blade rotation about its longitudinal axis 𝜔BV Design value for angular velocity of blade pitch adjustment system 𝜔G Angular velocity of the generator rotor 𝜔mech Angular velocity of the mechanical rotation of the generator rotor 𝜔Mot Angular velocity of the servomotor 𝜔N Nominal angular velocity 𝜔R Angular velocity of the turbine rotor (vector quantity) 𝜔st Angular velocity of actuator 𝜔W Angular velocity of wind turbine 𝜔𝜈 Angular frequency of the 𝜈th harmonic 1 Wind Energy Power Plants Rising pollution levels and worrying changes in climate, arising in great part from energy-producing processes, demand the reduction of ever-increasing environmentally damaging emissions. The generation of electricity – particularly by the use of renewable resources – offers considerable scope for the reduction of such emissions. In this context, the immense potentials of solar and wind energy, in addition to the worldwide use of hydropower, are of great importance. Their potential is, however, subject to transient processes of nature. Following intensive development work and introductory steps, the conversion systems needed to exploit these power sources are still in the primary phase of large-scale technical application. For example, in Germany around 8% of electricity is already being provided by wind turbines. However in the German provinces Mecklenburg-Western-Pomerania, Schleswig-Holstein, Brandenburg and Saxony-Anhalt there are about 50% wind power feed in. In Germany more power is supplied by wind energy than by hydroelectric plants. These environmentally friendly technologies in particular require a suitable development period to establish themselves in a marketplace of high technical standards. The worldwide potential of wind power means that its contribution to electricity production can be of significant proportions. In many countries, the technical potential and – once established – the economically usable potential of wind power far exceeds electricity con- sumption. Good prospects and economically attractive expectations for the use of wind power are, however, inextricably linked to the incorporation of this weather-dependent power source into existing power supply structures, or the modification of such structures to take account of changed supply conditions. 1.1 Wind Turbine Structures In the case of hydro, gas or steam, and diesel power stations (among others) the delivery of energy can be regulated and adjusted to match demand by end users (Figure 1.1(a)). In con- trast, the conversion system of a wind turbine is subject to external forces (Figure 1.1(b)). The delivery of energy can be affected by changes in wind speed, by machine-dependent factors such as disruption of the airstream around the tower or by load variations on the consumer side in weak grids. Grid Integration of Wind Energy: Onshore and Offshore Conversion Systems, Third Edition. Siegfried Heier. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd. 2 Grid Integration of Wind Energy Energy supply Energy distribution (a) Effective power Control rpm Voltage Reactive power Energy input (fuel) G Grid (b) Effective power Control rpm Voltage Reactive power Energy input (wind) G Grid Figure 1.1 Energy delivery and control in electrical supply systems: (a) diesel generators, etc., and (b) wind turbines The principal components of a modern wind turbine are the tower, the rotor, the nacelle (which accommodates the transmission mechanisms and the generator) and – for horizontal-axis devices – the yaw systems for steering in response to changes in wind direc- tion. Switchgear and protection systems, lines, and maybe also transformers and grids, are required for supplying end users or power storage systems. In response to external influences, a unit for operational control and regulation must adapt the flow of energy in the system to the demands placed upon it. The next two figures show the arrangement of the components in the nacelle and the differences between mechanical–electrical converters in the modern form of wind turbines. Figure 1.2 shows the conventional drive train design in the form of a geared transmission with a high-speed generator. Figure 1.3, by contrast, shows the gearless variant with the generator being driven directly from the turbine. These pictures represent the basis for the functional relationships and considerations of the system. Wind Energy Power Plants 3 Control cabinet Rotor Rotor hub Hood Rotor shaft Oil cooler Gearbox Clutch Heat exchanger Blade pitch adjustment drive Bearing block Azimuth drive Noise decoupler Generator Base frame Ventilation Figure 1.2 Nacelle of a wind turbine with a gearbox and high-speed 1.5 MW generator (TW 1.5 GE/Tacke). Reproduced by permission of Tacke Windenergie Figure 1.3 Schematic structure of a gearless wind turbine (Enercon E66, 70 m rotor diameter, 1.8/2 MW nominal output). Reproduced by permission of Enercon 4 Grid Integration of Wind Energy Following a brief glance back into history, developmental stages and different wind turbine designs and systems will be briefly highlighted and the processes of mechanical–electrical power conversion explained. Moreover, particular importance is assigned to the interconnec- tion of wind turbines to form wind farms and their combined effect in grid connection. 1.2 A Brief History For thousands of years, mankind has been fascinated by the challenge of mastering the wind. The dream of defying Aeolos1 and taming the might of the storm held generations of inventors under its spell. To attain limitless mobility by using the forces of Nature, thereby expanding the horizons of the then known world, was a challenge even in antiquity. Thus, sailing and shipbuilding were constantly pursued and developed despite doldrum, hurricane, tornado and shipwreck. Progress could only be achieved by employing innovative technologies. These, together with an unbridled lust for voyages of discovery, built up in the minds of sovereigns and scholars a mosaic of the world, the contours of which became ever more enclosed as time went by. With wind-harnessing technology on land and on the sea, potentials could be realized and works undertaken that far outpaced any previously imagined bounds. For example, using only the power of animals and of the human arm, it would never have been possible for the Nether- lands to achieve the drainage that it has through wind-powered pumping and land reclamation. Archaeological discoveries relating to the use of wind energy predate the beginning of the modern era. Their origins lay in the Near and Middle East. Definite indications of windmills and their use, however, date only from the tenth century, in Persia [1.1]. The constructional techniques of the time made use of vertical axes to apply the drag principle of wind energy capture (Figure 1.4). Such mills were mostly found in the Arab countries. Presumably, news Figure 1.4 Persian windmill (model) 1 Aeolos: Greek god of the winds. Wind Energy Power Plants 5 Figure 1.5 Sail windmill of these machines reached Europe as a result of the Crusades. Here, however, horizontal-axis mills with tilted wings or sails (Figure 1.5) made their appearance in the early Middle Ages. The use of wind energy in Western Europe on a large scale began predominantly in England and Holland in the Middle Ages. Technically mature post mills (Figure 1.6) and Dutch wind- mills (Figure 1.7) were used mostly for pumping water and for grinding. More than 200 000 (two hundred thousand) of these wooden machines were built throughout North-West Europe, representing by far the greatest proportion of energy capture by technical means in this region. At the beginning of the twentieth century, some 20 000 (twenty thousand) windmills were still in use in Germany. From the nineteenth century onwards, mostly in the USA, the so-called ‘western wheel’ type of turbine became widespread (Figure 1.8). These multibladed fans were built of sheet steel, with around 20 blades, and were used mostly for irrigation. By the end of the 1930s, some 8 million units had been built and installed, representing an enormous economic potential. 1.3 Milestones of Development The first attempt to use a wind turbine with aerodynamically formed rotor blades to generate electricity was made over half a century ago. Since then, besides the design and construction of large projects in the 1940s by the German engineers Kleinhenz [1.2] and Honnef [1.3], the pilot 6 Grid Integration of Wind Energy Figure 1.6 Post mill projects of the American Smith-Putnam (1250 kW nominal output, 53 m rotor diameter, 1941), the Gedser wind turbine in Denmark (200 kW nominal output, 24 m rotor diameter, 1957) and the technically trail-blazing Hütter W34 turbine (100 kW nominal output, 34 m rotor diameter, 1958) are worthy of mention (Figure 1.9). The German constructor Allgaier started the first mass production of wind power plants in the early 1950s. They were designed to supply electricity to farmsteads lying far from the public grid. In coastal areas these turbines drove 10 kW generators; inland they were fitted with 6 kW units. Their aerodynamically formed blades of 10 m diameter could be pitched about the longitudinal axis so as to regulate the power taken from the wind. Even today, some of these Wind Energy Power Plants 7 Figure 1.7 Dutch windmill turbines (see Figure 1.10) are in operation with full functionality, after more than 50 years of service. After the 1960s, cheaper fossil fuels made wind energy technology economically uninter- esting, and it was only in the 1970s that it returned to the spotlight due to rising fuel prices. Some states then developed experimental plants in various output classes. In particular in the USA, Sweden and the Federal Republic of Germany, turbines with outputs in the megawatt class have attracted most attention. Here, with the exception of the American MOD-2 (Figure 1.11) with five units and the Swedish–American WTS-4 (Figure 1.12) with five or two units, large converters such as the German GROWIAN (Figure 1.13), the Swedish WTS-75 AEOLUS model, the Danish Tvind turbine and the US MOD-5B variants in Hawaii were all one-offs. Despite many and varied teething troubles with the pilot installations, it was 8 Grid Integration of Wind Energy Figure 1.8 American wind turbine clear even then that technical solutions could be expected in the foreseeable future that would permit the reliable operation of large-scale wind turbines. Second-generation megawatt-class systems such as the WKA 60 (Figure 1.14) and the Aeolus II (Figure 1.15) have confirmed this expectation. Mainly in the US state of California, but also in Denmark, Holland and the Federal Republic of Germany, considerable efforts were being made, independently of the development of large turbines, to use wind power to supply energy to the grid on a large scale. In the 1980s, wind turbines with total capacity of around 1500 MW were installed in California alone. In the initial phases, turbines of the 50 kW categories were used (Figure 1.16). Scaling-up the systems that were successful through the 100, 150 and 250 kW classes (Figures 1.17 and 1.19) and the 500/600 kW order of magnitude (Figures 1.18 and 1.20) has led to wind farms with turbines in the megawatt range (Figure 1.21). Wind Energy Power Plants 9 Figure 1.9 Hütter W 34 turbine This development has made the mass production of wind turbines possible. A considerable improvement of performance can thus be achieved. Progressively increasing turbine size (see Figures 1.22 to 1.25) using designs of widely differing types and costs has led to the devel- opment of machines in the 500 kW and megawatt classes that are remarkable for their high availability and good return-on-investment potential. The individual manufacturers have chosen very different routes to market success in relation to this trend. NEG Micon has retained the classic Danish stall-regulated turbines with an asyn- chronous generator rigidly coupled to the grid in the power classes up to 1.5 MW (Figure 1.22). Bonus (Figure 1.23), Nordex (Figure 1.24) and Vestas (Figure 1.25) as well as GE/Tacke (Figure 1.26) have altered their turbine configuration in the different size classes, particu- larly with regard to the turbine regulation (stall or pitch) and generator systems (fixed-speed or variable-speed with a thyristor/ IGBT frequency converter). Currently 3 to 5 MW systems from all well-known manufacturers are being operated as prototypes or are available on the market. One new development has been the trend towards gearless wind turbines. Several attempts have been made to introduce and establish in the market small, high-speed, horizontal-axis 10 Grid Integration of Wind Energy Figure 1.10 Allgaier turbine turbines with direct-drive generators. Up until now these attempts have met with limited suc- cess. Microturbines (Figure 1.27) with a permanent-magnet synchronous generator driven directly from the turbine are usually used as battery chargers. The success of such systems is rooted in their attractive design and low price as well as in the modern worldwide sales concept and the simple installation of the plants. To some degree, companies that have entered into the production of wind generators at a later stage have been able to draw upon existing developments and techniques, thus allowing their first efforts to overtake the systems of established manufacturers. DeWind started its devel- opment (Figure 1.28) with a pitch-regulated 600 kW turbine and a variable-speed generator system (double-fed asynchronous machine), which could not have been produced at an eco- nomical cost a few years previously and which is currently favoured by most manufacturers. Then 1 and 2 MW systems of the same design followed. Wind Energy Power Plants 11 Figure 1.11 MOD 2 in the Goodnoe Hills (USA): 2.5 MW nominal output, 91 m rotor diameter, 61 m hub height Figure 1.12 WTS-4 turbine in Medicine Bow, USA.: 4 MW nominal output, 78 m rotor diameter, 80 m tower height 12 Grid Integration of Wind Energy Figure 1.13 GROWIAN by Brunsbüttel/Dithmarschen, 3 MW capacity, 100 m rotor diameter, 100 m hub height The development of wind power systems has largely been carried out by medium-sized companies. Smaller manufacturers, however, face financial limits in the development of MW systems. The 1.5 MW turbine MD 70/MD 77 (Figure 1.29), again with the double-fed asyn- chronous generator design, which was developed by pro + pro for the manufacturers BWU, Fuhrländer, Jacobs Energie (now REpower Systems) and Südwind / Nordex is opening up new developmental and market opportunities for smaller companies in the field of large-scale plants. Vertical-axis rotors, so-called Darrieus turbines, are enchantingly simple in structure. In their basic form they have up until now mostly been built with gearing and generators at base level Wind Energy Power Plants 13 Figure 1.14 WKA 60 in Kaiser-Wilhelm-Koog: 1.2 MW nominal output, 60 m rotor diameter, 50 m tower height (Figure 1.30). Variants in the form of so-called H-Darrieus gearless turbines in the 300 kW class were first designed with rotating towers and large multiple generators at ground level (Figure 1.31(a)). Further development led to machines with fixed tripods and annular gen- erators in the head (Figure 1.31(b)). These variants have not, however, been successful in establishing themselves widely in the wind power market. The Enercon E 40 horizontal-axis turbine was the first system in the 500 kW class with a direct-drive generator to establish itself in the market with great success in a very short time. Figure 1.32 shows the schematic construction of the nacelle. The generator, specially developed for this model, connects directly to the turbine and needs no independent bearings. In this way, wear on mechanical components running at high speed is reduced to a minimum. Operational run times of 180 000 hours have been quoted for many years. 14 Grid Integration of Wind Energy Figure 1.15 AEOLUS II near Wilhelmshaven: 3 MW nominal output, 80 m rotor diameter, 88 m tower height The gearless E 30, E 40, E 58, E 66 and E 112/E 126 models from Enercon were produced as a development of the stall-regulated geared models E15/E16 and E17/E18, by way of the E 32/E 33 variable-pitch turbines (Figure 1.33). In parallel, but with a slight delay, the con- version from thyristors to pulse inverters was accomplished. This configuration thus unites the advantages of variable speeds (and the associated reduction in drive-train loading) with those of a grid supply having substantially lower harmonic feedback. In comparison to the gearless designs with electrically excited synchronous generators, as shown in Figure 1.33(d) to (h), permanent-magnet machines permit the arrangement of higher numbers of poles around the rotor or stator. By using high-quality permanently magnetic Wind Energy Power Plants 15 Figure 1.16 Wind farm in California with turbines in the 50/100 kW class Figure 1.17 Wind farm in California with turbines in the 250 kW class 16 Grid Integration of Wind Energy Figure 1.18 Wind farm in Wyoming with turbines in the 600 kW class Figure 1.19 Wind farm in North Friesland with turbines of the 250 kW class Wind Energy Power Plants 17 Figure 1.20 Wind farm on Fehmarn Island with turbines of the 500 kW class (a) On land (b) At sea. Reproduced by permission of GE Wind Energy Figure 1.21 Wind farm with 1.5 MW turbines 18 Grid Integration of Wind Energy (a) NTK 150/25 (b) NTK 300/31 (c) NTK 600–180/43 (d) NEG 1500/60 Figure 1.22 Size progression of stall-regulated turbines of the same design (fixed-speed, fixed-pitch machines) from NEG Micon / Nordtank. Reproduced by kind permission of NEG Micon (a) 300 kW / 33-2 (b) 600 kW / 44-3 (c) 1 MW / 54 (d) 2 MW Figure 1.23 Size progression of Bonus turbines: (a,b) fixed-speed, stall-controlled turbines; (c,d) active (combi-)stall turbines with a slight blade pitch adjustment Wind Energy Power Plants 19 (a) N 27/29 (b) N 43 (600 kW) (c) N 62 (1300 kW) (d) N 80/90 (2500 kW). (150/250 kW ) Reproduced by permission of Nordex Figure 1.24 Size progression of Nordex turbines: (a,b,c) fixed-speed, fixed-pitch machines; (d) a large-scale, variable-speed, variable-pitch unit materials, relatively favourable construction sizes can thus be achieved (Figure 1.34) and very high efficiencies attained, particularly in the partial load range. Such a plant configuration of the 600 kW class (Figure 1.34(a)) has been able to achieve excellent returns over several years of fault-free operation. A 2 MW unit with such a generator design (Figure 1.34(b)) was designed with a medium-voltage generator of 4 kV system voltage. A further possibility, which has been considered for large, slow-running turbines in par- ticular, is the combination of a low-speed generator and a turbine-side gearbox, as shown in Figure 1.35. The single-stage gearbox turns the generator shaft at around eight times the tur- bine speed of approximately 100 revolutions per minute. Thus, even for units in the 5 MW range, generators in compact and technically favourable construction sizes of approximately 3 m diameter can be used. Further large-scale turbines in the 5MW class with a rotor diameter of over 125m are REpower 5 M and 6 M and Siemens SWT 6-154 (Fig. 1.36). A double-fed asynchronous generator with medium-voltage isolation in the low-voltage range (950 V stator-side or 690 V rotor-side) is used in the Repower system. The Siemens turbine has a direct drive permanent excited synchronous generator. In the following we consider various real operational situations, the essential differences between the systems involved and the resulting effects on supply to the grid, taking as a basis the functional structure of wind power machines and their influences. 20 Grid Integration of Wind Energy (a) V 17 (55 kW) (b) V 25 (225 kW) (c) V 39 (500 kW) (d) V 44 (600 kW) (e) V 66 (1650 kW) (f) V 112 (3 MW) Figure 1.25 Size progression of Vestas turbines: (a) small, fixed-speed, fixed-pitch machine; (b,c,d) larger variable-pitch units; (d,e) machines with speed elasticity; or double-fed asynchronous generators; (f) machines with permanent excited synchronous generators. 1.4 Functional Structures of Wind Turbines For the following consideration, which is mainly concerned with the mechanical interaction of electrical components and with interventions to modify output, we will draw upon the nacelle layout shown in Figure 1.2. With the correct design, the influences of the tower and of steer- ing in response to changes in wind direction can be handled separately (Section 2.2.1) or treated as changes in wind velocity. The block diagram shown in Figure 1.37 (see page 28), Wind Energy Power Plants 21 (a) TW 80 (80 kW, (b) TW 250 (250 kW, (c) TW 300 (300 kW) 21 m rotor diameter) 26 m rotor diameter) (d) TW 600 (600 kW, (e) TW 1.5/1.5S (1500 kW, (f) GE 3.6 (3.6 MW, 100 m rotor 43 m rotor diameter) 65/70 m rotor diameter) diameter). Reproduced by permission of GE Wind Energy Figure 1.26 Size progression of turbines from GE / Tacke: first (a,b) and second (c,d,e,f) generation machines, from fixed-speed, fixed-pitch turbines (a to d) to large-scale, pitch-controlled, variable-speed turbines (e,f) 22 Grid Integration of Wind Energy Figure 1.27 Small system-compatible turbine from aerosmart. Reproduced by permission of Aerodyn Energiesystems GmbH (a) DeWind 4 (600 kW, (b) DeWind 6 (1000/ 1250kW, 46/48 m rotor diameter) 60/62/64 m rotor diameter) Figure 1.28 DeWind 4 (600 kW, 46/48 m rotor diameter). Reproduced by permission of DeWind Wind Energy Power Plants 23 Figure 1.29 Joint development of the 1.5 MW MD 70/MD 77 turbine (70/77 m rotor diameter) Figure 1.30 Fixed-speed 300 kW Darrieus unit with gearing and a conventional generator 24 Grid Integration of Wind Energy (a) annular generator (b) annular generator at ground level in head Figure 1.31 Variable-speed 300 kW gearless H-Darrieus unit which illustrates the links between the most important components and the associated energy conversion stages, may serve as the basis for later detailed observations. This diagram also gives an idea of how operation can be influenced by control and supervisory processes. Fur- thermore, the central position occupied by the generator is made particularly clear. The following pages therefore explain the physical behavior of a wind energy extraction system and the conversion of this mechanical energy to electrical energy by means of genera- tors. We examine how mechanical moments are handled in the drive unit when the generator is connected to the grid, the design of generators suitable for wind turbines and the combined effects of turbines and power supply grids, as well as the regulation of turbines in isolation and in grid operation, bearing in mind the conditions imposed by the grid and the consumer. From Figure 1.37 (see page 28), the functional structures for entire wind energy conversion systems, or for particular types of wind energy converter as shown in Figure 1.38(a) and (b) (see page 29), can be further developed. Such simplified block diagrams can help us to under- stand how the principal components of pitch- or stall-regulated horizontal-axis wind energy converters work and interact. Wind energy converters with variable-blade pitch (Figure 1.38(a)) allow direct control of the turbine. Figure 2.61 shows that by varying the blade pitch it is possible, firstly, to influence the power input or torque of the rotor, with a smaller blade pitch angle 𝛽 (or greater 𝜗) leading to a lower turbine output and a greater 𝛽 leading to a higher turbine output (pitch regulation). Secondly, by a few degrees adjustment of the rotor blades, the profile can be brought more fully into stall when 𝛽 is greater (active stall regulation) and the turbine power falls. A slight Wind Energy Power Plants 25 Blade pitch adjustment drive Rotor blade bearing Locking brakes Axle journal Rotor blade Service crane Generator stator Generator rotor Wind direction indicator Main bearing Hub Figure 1.32 Schematic layout of the Enercon E 40 gearless turbine. Reproduced by kind permission of Enercon reduction to the blade pitch angle, on the other hand, guides the rotor out of stall and power increases until laminar flow is achieved on the blade profiles. In this way, the speed of rotation, determined by integration of the difference between turbine torque and the generator’s load torque, taking rotating masses (or mechanical time constants) into account, can be influenced at all performance levels – insofar as sufficient energy is available. The pitch control of a wind turbine therefore makes it possible to regulate energy extraction. In this way, adaptation to user needs (e.g. in standalone operation) can be achieved, as well as a measure of protection in storm conditions. 26 Grid Integration of Wind Energy (a) E 15/16 (55 kW) (b) E 17/18 (80 kW) (c) E 32/33 (300 kW) (d) E 30 (200 kW) (e) E 40 (500 kW) (f) E 58 (850 kW / (g) E 66 (1500 kW / (h) E 112 /E 126 (4,5 1000 kW) 1800 kW) MW/6 MW/7,5 MW) Figure 1.33 Enercon turbines from variable-speed geared models with thyristor inverters (a,b,c) to gearless configurations with pulse inverters (d,e,f,g,h); (a,b) with fixed and (c,d,e,f,g,h) with variable pitch. Reproduced by kind permission of Enercon In (passive) stall-controlled converters (Figure 1.38(b)), the rotor speed is kept at an almost constant speed by the load torque of a rigidly coupled asynchronous (mains) generator, usually of large dimensions. When wind strength rises above nominal levels, the flow over the rotor blades achieves partial or even total stall – whence the so-called ‘stall regulation’. The power take-up of the turbine is thereby passively (i.e. design-dependently) limited under full loading Wind Energy Power Plants 27 (a) Genesys 600 (b) Zephyros Z72 (70.65 m rotor diameter and 2 MW nominal output) with medium-voltage generator. Reproduced by permission of Harakosan Figure 1.34 Gearless wind turbines with permanent-magnet synchronous generator (46 m rotor diameter, 600 kW nominal output) Figure 1.35 Nacelle of the large-scale Multibrid N 5000 (5 MW, 116 m rotor diameter) with single-stage gearing, integral hub and low-speed synchronous generator. Reproduced by permission of Multibrid Entwicklungsgesellschaft GmbH 28 Grid Integration of Wind Energy (a) (b) Figure 1.36 Offshore turbines: (a) Repower offshore and onshore turbine 5M/6M, 5 MW/6 MW nom- inal output power, 126,5 m rotor diameter. Source: Repower; (b) Siemens offshore turbine SWT 6-154, 6 MW nominal output power, 154 m rotor diameter. Source: Siemens Wind energy Torque and Mechanical–electrical converter speed converter converter Rotor blades Wind speed V Switching and protective equipment Generator Mechanical Transformer, Flow energy drive train power lines, of the air (gearbox, etc.) mains Control and Consumers, supervision storage Kinetic energy Mechanical energy Electrical energy (active power) Figure 1.37 Functional chain and conversion stages of a wind energy converter to values such that under operational wind speed conditions the nominal output of the generator is not significantly exceeded. The use of variable-speed generators in both regulation systems allows the reduction of sud- den load surges, and considerably extends the range of operation. The optimal power can be produced by adjusting the speed of the rotor to the desired speed. For example, it is also Wind Energy Power Plants 29 v Wind energy v  n v  n n  v Blade pitch 𝛽 Driving Driving adjustment torque torque Super- MA + Mechanical MA Super- + vision energy vision and Mechanical n n Mechanical and control drive train drive train control MW – – MW n n Generator Generator (electrical) (electrical) Electrical energy Main Main consumer consumer Electrical data Electrical data (a) Pitch regulation (b) Stall regulation MA driving torque of wind turbine n rotation speed of the rotor MW load torque of wind turbine ν wind speed (velocity)  blade position 𝛽 blade pitch Figure 1.38 Functional structure of a wind energy converter possible, in cases where partially increased transitional loads must be handled, to influence the drive torque of stall-regulated turbines by varying the rotational speed of the generator. A detailed treatment of the generator and associated discussions on the theme of turbine regulation require knowledge of the physical processes and a review of the mathematical laws governing the entire converter system. The following text should encompass this, insofar as is necessary. More detailed studies are also necessary regarding the combined effects of wind turbines working together with existing grid systems and the measures that must be applied to control these effects throughout the entire system. 30 Grid Integration of Wind Energy The following chapters summarize the results of years of research and development. Through practical references achieved from completed projects, particular weight has been given to the usefulness of the results for plant conception and design. References [1.1] Meyers Enzyklopädisches Lexikon, Vol. 25.9, Completely Revised Edn, Bibliographisches Institut AG, Mannheim, 1979. [1.2] Kleinhenz, F., Projekt eines Großwindkraftwerkes, Der Bauingenieur, 1942, 23/24. [1.3] Honnef, H., Windkraftwerke, Vieweg, Braunschweig, 1932. [1.4] Zephyros brochure, Permanent Performance. 2 Wind Energy Conversion Systems As shown in Figure 1.37, the blades of a wind turbine rotor extract some of the flow energy from moving air, convert it into rotational energy and then deliver it via a mechanical drive unit (shafts, clutches and gears), as shown in Figures 1.2 and 1.3 respectively, to the rotor of a generator and thence to the stator of the same by mechanical–electrical conversion. The electrical energy from the generator is fed via a system of switching and protection devices, lines and if necessary transformers to the grid, consumers or an energy storage device. In the further discussion of the relevant system components, special attention must be given to the drive torque or performance properties, the structures based thereon and the actions necessary to limit turbine speed, together with reaction effects of the transmission on the turbine. 2.1 Drive Torque and Rotor Power In contrast to the windmills of yesteryear, modern wind turbines used for generating electricity have relatively fast-running rotors. A few blades with high lift-to-drag ratio profiles that utilize lift attain much higher levels of efficiency than drag-type rotors. They also make it possible to alter the power of the turbine more quickly. Such turbines can, given sufficient wind, attain operating conditions akin to those of conventional power stations. In examining the operational characteristics of a wind energy converter we must determine the variables influencing the turbine, beginning with the forces on the rotor blades or on small areas thereof, and thence derive the resulting drive torque and corresponding output power. 2.1.1 Inputs and outputs of a wind turbine To model the power or torque properties of a wind turbine, we can consider the block labelled ‘driving torque’ in Figure 1.38(a) as a single structure having inputs and outputs as shown in Figure 2.1. These can be subdivided as follows: the independent input quantity ‘wind speed’, which determines the energy input but which can act at the same time as an interference quantity; Grid Integration of Wind Energy: Onshore and Offshore Conversion Systems, Third Edition. Siegfried Heier. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd. 32 Grid Integration of Wind Energy machine-specific input quantities, arising particularly from rotor geometry and arrange- ment and the state variables ‘turbine speed’, ‘rotor blade position’ and ‘rotor blade angle’ arising from the transmission system of the complete wind-power unit, and with the aid of which turbine output quantities, such as ‘power’ or ‘drive torque’, may be controlled. The power or torque of a wind turbine may be determined by several means. The best results might be obtained by considering the axial and radial momentum of the airstream, or of small-diameter infinite flow-tubes impinging on the rotor blades, or of elements thereof, thus allowing us to determine local flow conditions and the resulting forces or rotational action on the turbine blades. The following considers this briefly. Calculations based on circulation and vortex distribution over aerofoils [2.1], which allow the derivation of solutions by the Biot–Savart theorem, are not taken into consideration here. 2.1.2 Power extraction from the airstream Within its effective region, the rotor of a wind turbine absorbs energy from the airstream, and can therefore influence its velocity. Figure 2.2 represents the flow that develops around a con- verter in an unrestricted airstream in response to prevailing transmission conditions, whereby the airstream is decelerated axially and deviated tangentially in the opposite direction to the rotation of the rotor. From references [2.2, 2.3] and [2.4], the energy absorbed from an air volume Va of cross-section A1 and swirl-free speed of flow 𝑣1 far upstream of the turbine, which results Energy input / disturbance factor wind speed V ωR,β, MAW Machine state variables Wind turbine Turbine output quantities PW ϑB(r), tB, ca, cw Rotor geometry machine-specific input quantities Figure 2.1 Wind turbine inputs and outputs Wind Energy Conversion Systems 33 Va Va v2 v v2ax 2t v1 v3 dAR dr A1 A2 A3 Figure 2.2 Airstream around the turbine in a downstream reduction of flow speed to 𝑣3 with a corresponding broadening of the cross-sectional area (wake decay) to A3 , can be expressed as 𝜌( ) Ww = Va 𝑣21 − 𝑣23 (2.1) 2 The wind turbine power may therefore be expressed as ( )) 𝜌( 2 dWw Va2 𝑣1 − 𝑣2 3 Pw = =d. (2.2) dt dt An air volume flow in the rotor area (A2 = AR ) of dVa = AR 𝑣2 (2.3) dt yields, in the quasi-steady state, 𝜌( 2 ) PW = AR 𝑣1 − 𝑣23 𝑣2. (2.4) 2 The power absorption and operating conditions of a turbine are therefore determined by the effective area AR , by the wind speed and by the changes occurring to these quantities in the field of flow of the rotor. The power of the turbine can thus be influenced by varying the flow cross-sectional area and by changing flow conditions at the rotor system. According to Betz [2.2], the maximum wind turbine power output 16 𝜌 3 PWmax = A 𝑣 (2.5) 27 R 2 1 is obtained when 2 1 𝑣2 = 𝑣 and 𝑣3 = 𝑣. (2.6) 3 1 3 1 34 Grid Integration of Wind Energy Under normal operating conditions up to the nominal output capacity, this reduction of wind speed is approached. When the rotor is idling, or running under light load, the value of 𝑣2 approaches that of 𝑣1. The ratio of the power PW absorbed by the turbine to that of the moving air mass 𝜌 P0 = AR 𝑣31 (2.7) 2 under smooth flow conditions at the turbine defines the dimensionless performance coefficient PW cp =. (2.8) P0 The above expression is based upon the assumption that tubular axial air mass transport only occurs from the leading side of the entry area A1 to the exit area A3. A more detailed examination of the turbine or rotor blades can be carried out using the modified blade element theory, by introducing a radial wind speed gradient and by taking into account any angular movement of the airstream. 2.1.3 Determining power or driving torque by the blade element method If, instead of a circular section, we consider an annular section of radius r, width dr and area at the turbine of dAR = 2𝜋rdr, (2.9) then the following is valid for the mass flow rate dṁ in front of, at and behind the rotor in a quasi-steady state: dṁ 1 = dṁ 2 = dṁ 3 (2.10) or 𝜌 dA1 𝑣1ax = 𝜌 dA2ax = 𝜌 dA3 𝑣3ax. (2.11) The force that brakes the air axially from 𝑣1ax to 𝑣3ax may be derived from the loss of momen- tum from entry to exit by ( ) dJ̇ ax = dFax = dṁ 𝑣1ax − 𝑣3ax. (2.12) In the rotor area, by application of Froude’s theorem, 𝑣1ax + 𝑣3ax ( ) 𝑣2ax = or 𝑣1ax − 𝑣3ax = 2 𝑣1ax − 𝑣2ax (2.13) 2 and the thrust of the air tubes ( ) dFax = 4𝜋𝜌 dr 𝑣2ax 𝑣1ax − 𝑣2ax (2.14) can be obtained as a function of the axial wind speed on the rotor (to be determined). The tangential change of momentum may be determined in the same fashion: dJ̇ t = 𝑣1t dṁ 1 − 𝑣3t dṁ 3. (2.15) Wind Energy Conversion Systems 35 When the air entering the flow tube is swirl-free, no other moment is applied to it. The tangen- tial force, which brings the air into rotational flow, is derived as dFt = dJ̇ t = −𝑣3t dṁ 3 = −𝑣2t dṁ 2 (2.16) or, in applications, dFt = −2𝜋𝜌r dr 𝑣2t 𝑣2ax. (2.17) The force thus depends on the radius and the axial and tangential airstream at the turbine. The air, according to equations (2.14) and (2.17), exerts identical forces on the rotor blades. For the sake of clarity, the physical processes will be shown for a single rotor blade. Multi- blade arrangements for fast-running turbines (e.g. with z = 2, 3 or 4 lift-type blades) can be handled by extension of this system, considering conditions at a single blade of z-fold depth. Depending on blade radius, Figure 2.2 shows that there is different flow behavior at the profile for different blade angles (Figure 2.3). The combined effect of velocity components and the resultant forces are shown for a single blade element in Figure 2.4. Total values (forces, moments, power) are obtained by the inte- gration of the corresponding values over the blade radius, or by summation of the components of individual blade sections. ϑ 0,7 β V1ax ϑ 0,7 0,7·R ϑB β V1ax Rotor axis ωR Plane of rotation Figure 2.3 Definition of blade pitch angle 36 Grid Integration of Wind Energy ϑ 2t – R ω × ϑ dFW r δ α dFt ϑr ϑr dFA dFAW β dFax ϑ2ax ωR ωR Rotor axis Rotor axis Plane of Plane of rotation rotation Figure 2.4 Airflow and forces on a rotor blade segment A segment at radius r of a blade rotating with angular velocity 𝜔R experiences two airflows: that due to the wind deceleration across the swept area, v2 = v2ax + v2t (2.18) and that due to the speed of the rotating element at the given radius, v = −𝜔R × r. (2.19) If we disregard the cone angle then, in the direction of the resultant velocity component, √ ( )2 𝑣r = 𝑣22ax 𝜔R r + 𝑣2t , (2.20) a drag of 𝜌 2 dFW = t 𝑣 c (𝛼) dr (2.21) 2B r w is exerted, acting against the movement of the blade, while an orthogonally directed lift of 𝜌 2 dFA = t 𝑣 c (𝛼) dr (2.22) 2B r a Wind Energy Conversion Systems 37 exerts a propulsive component. The force on the blade segment resulting from the lift and drag components is dFAW = dFA + dFW. (2.23) Separating this into axial and tangential components leads, for z blades, to a drive torque-generating value of 𝜌 ( ) dFt = z tB 𝑣2r ca sin 𝛿 − cw cos 𝛿 dr (2.24) 2 and an axial thrust on a rotor blade and on the hub of 𝜌 ( ) dFax = z tB 𝑣2r ca cos 𝛿 − cw sin 𝛿 dr. (2.25) 2 According to this, and for the respective wind and blade tip velocities, these forces are essen- tially dependent on: the pitch angle 𝜗 of the blade element with respect to the plane of rotation or 𝛽 with respect to the rotor axis; the local angle of attack 𝛼 between the resultant wind velocity and chord of the aerofoil; the lift and drag coefficients (ca , cw ) for the blade profile (Figure 2.5), the Reynolds number and the roughness of the blade surface, which should be ignored here, as should skewed airflow impingement due to the cone angle, etc. The drive torque of the wind turbine is therefore Ra Ra 𝜌 ( ) MAW = r dFt rz tB 𝑣2r ca sin 𝛿 − cw cos 𝛿 dr. (2.26) ∫Ri ∫Ri 2 2.0