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RESEARCH TOPICS IN WIND ENERGY 5 Ke Ma Power Electronics for the Next Generation Wind Turbine System 123 Research Topics in Wind Energy Volume 5 Series editor Joachim Peinke, University of Oldenburg, Oldenburg, Germany e-mail: [email protected] About this Series The series Research Topics in Wind Energy publishes new developments and advances in the fields of Wind Energy Research and Technology, rapidly and informally but with a high quality. Wind Energy is a new emerging research field characterized by a high degree of interdisciplinarity. The intent is to cover all the technical contents, applications, and multidisciplinary aspects of Wind Energy, embedded in the fields of Mechanical and Electrical Engineering, Physics, Turbulence, Energy Technology, Control, Meteorology and Long-Term Wind Forecasts, Wind Turbine Technology, System Integration and Energy Economics, as well as the methodologies behind them. Within the scope of the series are monographs, lecture notes, selected contributions from specialized conferences and workshops, as well as selected PhD theses. Of particular value to both the contributors and the readership are the short publication timeframe and the worldwide distribution, which enable both wide and rapid dissemination of research output. The series is promoted under the auspices of the European Academy of Wind Energy. More information about this series at http://www.springer.com/series/11859 Ke Ma Power Electronics for the Next Generation Wind Turbine System 123 Ke Ma Department of Energy Technology Aalborg University Aalborg Denmark ISSN 2196-7806 Research Topics in Wind Energy ISBN 978-3-319-21247-0 DOI 10.1007/978-3-319-21248-7 ISSN 2196-7814 (electronic) ISBN 978-3-319-21248-7 (eBook) Library of Congress Control Number: 2015944177 Springer Cham Heidelberg New York Dordrecht London © Springer International Publishing Switzerland 2015 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this Frontmatter are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. Printed on acid-free paper Springer International Publishing AG Switzerland is part of Springer Science+Business Media (www.springer.com) Preface The study for this book was carried out during my Ph.D. in the period between June 2010 and April 2013, at Department of Energy Technology in Aalborg University, Denmark. Sophisticated industry and long-term academic focus on wind power is one of the reasons I came here to do this research. After 3 years of unforgettable researches and experiences, I start to realize that the large-scale utilization of wind energy could be far more challenging than I expected. And more importantly, many of the problems as well as the technology potentials may have not been uncovered yet in this field. The purpose of this work is to study the power electronics used for the next generation wind turbine system. Some criteria and tools for evaluating and improving the critical performances of wind power converters have been proposed and established. It is the hope of the author that this book can address some emerging problems as well as possibilities for wind power conversion, and become an inspired reference for researchers in this field. I would like to show grateful thanks to Prof. Frede Blaabjerg for the impressive and fruitful discussion during this study. The constructive discussions, patient corrections, and also continuous encouragements not only contribute to this work, but also have great influences on my researching, networking, managing, and supervising. Furthermore, I would like to sincerely acknowledge Prof. Marco Liserre from Kiel University, Germany, for his inspired suggestions and invaluable help during this work. I also want to show regard to Prof. Dehong Xu from Zhejiang University, China for his supports and concerns, which are precious for my staying in Denmark. Aalborg March 2015 Ke Ma v Contents Part I Monograph 1 2 3 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 State-of-the-Art for Wind Power Generation. . . . 1.2 Development of Wind Power Technologies . . . . 1.2.1 Evolution of Wind Turbine Concepts . . . 1.2.2 Evolution of Power Electronics for Wind Turbines . . . . . . . . . . . . . . . . . . . . . . . 1.3 Emerging Challenges for Wind Power Converter 1.3.1 More Grid Supports . . . . . . . . . . . . . . . 1.3.2 Higher Reliability. . . . . . . . . . . . . . . . . 1.3.3 Special Cost Considerations. . . . . . . . . . 1.3.4 Formulation of Overall Requirements . . . 1.4 Scopes of the Book. . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 5 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8 8 11 13 14 15 16 Promising Topologies and Power Devices for Wind Power Converter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Promising Converter Topologies . . . . . . . . . . . . . . . . 2.1.1 Traditional Two-Level Converters . . . . . . . . . 2.1.2 Multilevel Converters . . . . . . . . . . . . . . . . . . 2.1.3 Multi-cell Converters . . . . . . . . . . . . . . . . . . 2.2 Potential Power Semiconductor Devices . . . . . . . . . . 2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 19 19 21 23 26 27 27 Criteria and Tools for Evaluating Wind Power Converter . 3.1 Importance of Thermal Stress in Wind Power Converter 3.1.1 Thermal Stress Versus Reliability. . . . . . . . . . . 3.1.2 Thermal Stress Versus Cost. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 31 32 34 vii viii Contents 3.2 Classification and Approach for the Thermal Stress Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Classification of Thermal Stress in Wind Power Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Methods and Models for Stress Analysis. . . . . . 3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5 ..... 37 . . . . . . . . . . . . . . . . . . . . 37 38 42 42 . . . . . . . . . . . . . . . . . . . . . . . . 45 45 47 47 50 51 .... 51 . . . . . . . . . . . . . . . . 53 55 55 56 . . . . . . . . . . . . . . . . 57 57 60 61 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 63 67 67 70 71 71 74 77 ...... 79 Thermal Stress of 10-MW Wind Power Converter Under Normal Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Requirements and Conditions Under Normal Operation . . 4.2 Stress of Converter Imposed by Wind Speeds . . . . . . . . 4.2.1 Thermal Stress Under Steady-State Wind Speeds . 4.2.2 Thermal Stress Under Wind Speed Variations . . . 4.3 Stress of Converter Imposed by Grid Codes . . . . . . . . . . 4.3.1 Converter Efficiency Considering Reactive Power Demands by Grid Codes . . . . . . . . . . . . . 4.3.2 Thermal Stress Considering Reactive Power Demands by Grid Codes . . . . . . . . . . . . . . . . . . 4.4 A Thermal Control Method Utilizing Reactive Power . . . 4.4.1 Control Idea and Diagram . . . . . . . . . . . . . . . . . 4.4.2 Idea to Overcome the Reactive Power Limits . . . 4.4.3 Thermal Stress Considering Extended Q Ranges in Paralleled Converters . . . . . . . . . . . . . . . . . . 4.4.4 Thermal Control Results . . . . . . . . . . . . . . . . . . 4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stress Analysis of 3L-NPC Wind Power Converter Under Fault Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Requirements and Conditions Under Fault Operation. . 5.2 Stress Analysis of Converter Under LVRT. . . . . . . . . 5.2.1 Electrical Behaviors . . . . . . . . . . . . . . . . . . . 5.2.2 Thermal Behaviors . . . . . . . . . . . . . . . . . . . . 5.3 Thermal Redistributed Modulations Under LVRT . . . . 5.3.1 Basic Idea . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 A Group of Modulation Methods . . . . . . . . . . 5.3.3 Loss and Thermal Improvements . . . . . . . . . . 5.3.4 Neutral Point Potential Control and Total Harmonic Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Contents ix 5.4 New Power Control Methods Under Unbalanced AC Source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Applicable Conditions and Control Structure . 5.4.2 Control Ideas and Methods . . . . . . . . . . . . . 5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 81 82 91 92 6 Conclusions and Future Works . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Proposals for Future Research Topics . . . . . . . . . . . . . . . . . . 95 95 97 7 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Used Models for Analysis . . . . . . . . . . . 7.1.1 Wind Speed Generator . . . . . . . . 7.1.2 Wind Turbine Model . . . . . . . . . 7.1.3 Generator Model . . . . . . . . . . . . 7.1.4 Parameter for Thermal Impedance of Used IGCT . . . . . . . . . . . . . . 7.2 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 99 99 99 101 ............... ............... 101 103 Part II Specially Selected Topics 8 9 The Impacts of Power Switching Devices to the Thermal Performances of 10 MW Wind Power NPC Converter . . 8.1 Wind Power Converter for Case Study . . . . . . . . . . 8.2 Thermal-Related Characteristics of Different Power Switching Devices . . . . . . . . . . . . . . . . . . . 8.2.1 Switching Loss . . . . . . . . . . . . . . . . . . . . . 8.2.2 Conduction Voltage and Loss . . . . . . . . . . . 8.2.3 Thermal Resistance . . . . . . . . . . . . . . . . . . 8.3 Thermal Analysis of Different Device Solutions . . . . 8.3.1 Normal Operation . . . . . . . . . . . . . . . . . . . 8.3.2 Low-Voltage-Ride-Through Operation . . . . . 8.3.3 Wind Gust Operation . . . . . . . . . . . . . . . . . 8.3.4 Summary of Thermal Performances Under Different Operation Modes . . . . . . . . 8.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....... ....... 107 107 . . . . . . . . . . . . . . . . 108 109 110 112 112 113 115 120 ....... ....... ....... 121 121 122 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reliability-Cost Models for the Power Switching Devices of Wind Power Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Loss Model with Chip Number Information . . . . . . . . . . . . . . 9.2 Thermal Impedance Model with Chip Number Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 124 129 x Contents 9.3 Analytical Solution of Junction Temperature with Chip Number Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 137 138 10 Electro-Thermal Model of Power Semiconductors Dedicated for Both Case and Junction Temperature Estimation . . . . . . . . . . 10.1 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 143 143 11 Reactive Power Influence on the Thermal Cycling of Multi-MW Wind Power Inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Effect of Reactive Power in Case of Single Converter . . . . . . 11.2 Effect of Reactive Power in Case of Paralleled Converters . . . 11.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 146 152 157 157 12 Thermal Loading of Several Multilevel Converter Topologies for 10 MW Wind Turbines Under Low Voltage Ride Through. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1 Promising Topologies and Basic Design . . . . . . . . . . . . . 12.2 Operation Status Under Balanced LVRT . . . . . . . . . . . . . 12.3 Loss Distribution Under Balanced LVRT. . . . . . . . . . . . . 12.4 Thermal Distribution Under Balanced LVRT . . . . . . . . . . 12.5 Unbalanced LVRT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 159 161 164 166 171 179 13 Another Groups of Thermal Optimized Modulation Methods of Three-Level Neutral-Point-Clamped Inverter Under Low Voltage Ride Through . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1 Basic Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 Neutral Point Potential Control Method . . . . . . . . . . . . . . 13.3 Loss and Thermal Performances . . . . . . . . . . . . . . . . . . . 13.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 181 183 185 187 14 Limits of the Power Controllability of Three-Phase Converter with Unbalanced AC Source . . . . . . . . . . . . . . . . . . . . 14.1 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 196 Abstract Wind power generation has been steadily growing both for the total installed capacity and for the individual turbine size. Due to much more significant impacts on the power grid, the power electronics, which can change the behavior of wind turbines from an unregulated power source to an active generation unit, are becoming crucial in the wind turbine system. The objective of this project is to study the power electronics technology used for next generation wind turbines. Some emerging challenges as well as potentials like the cost of energy and reliability are addressed. First, several potential converter topologies and power semiconductor devices for the future wind power applications are presented in respect to the advantages/drawbacks. Then the criteria for evaluating the wind power converter are generally discussed, where the importance of thermal stress in the power semiconductors is emphasized and a multidisciplinary approach for stress analysis is introduced. Based on the proposed criteria and tools, the electrical and thermal behaviors of wind power converters are investigated under both normal and fault conditions, where the factors of wind speeds, grid codes, converter controls, and grid conditions are taken into account. In order to relieve the electrical and thermal stress of the converter in wind turbine system, some new control methods and concepts are thereby proposed. In Chap. 4 a thermal control concept which utilizes the reactive power is used to stabilize the thermal excursion under wind gust. In Chap. 5 a series of special modulation methods which can achieve better thermal loading of power devices under grid faults are introduced. Also in Chap. 5 a series of power control strategies utilizing the zero sequence current are presented to achieve better control performance under the unbalanced AC source. It is concluded that power electronics will play a more important role and regulate all the generated power in the next generation wind turbine system. In this case, the stress in the converter components becomes more critical because power conversion is pushed to multi-MW level with high power density requirement. It has also been revealed that thermal stress in power semiconductors is closely related xi xii Abstract to many determining factors in wind power applications such as reliability, cost, power density, etc., therefore it is an important performance for the next generation wind power converter. It is found that the thermal behaviors of wind power converter could be rather adverse under some required operating conditions. On the other hand it is also possible to improve the thermal behaviors by many aspects like smart control, special modulation, advanced modeling, as well as new converter designs. Outlines of this Book The book consists of two parts—the general monograph in Part I and the specially selected topics in Part II. The monograph is divided into 7 chapters, and 7 special topics are attached to detail and back up the analysis. The structure of the book is organized as follows: Chapter 1 presents the introduction and motivation of the whole work, where the background, objectives, and structure are addressed. In Chap. 2 several promising converter topologies for the next generation wind power converter are first presented and discussed in respect to the advantages/drawbacks. Afterwards, three potential power semiconductor devices for wind power application are highlighted and evaluated. Chapter 3 discusses the criteria for evaluating the next generation wind power converter. The importance of thermal stress in power semiconductors is emphasized by relating it with the reliability and cost of converter. Then a multidisciplinary approach for stress analysis of wind power converter is introduced, in which the factors of converter design, converter control, wind speed, and grid codes are taken into account. Chapter 4 gives the stress analysis of wind power converter under normal operation based on a 10 MW wind turbine. The junction temperature profiles in the power semiconductors are presented under both steady-state wind speeds and speed variations. Then the converter efficiency and thermal distribution modified by grid codes are investigated. Finally, a thermal control concept which utilizes the reactive power circuited among paralleled converters is proposed to relieve the thermal excursion in power devices under wind gust. Chapter 5 investigates the thermal stress of wind power converter when suffering grid faults. Comprehensive analysis for electrical and thermal loading of power semiconductor devices is conducted on the three-level Neutral-Point-Clamped (3L-NPC) wind power converter undergoing various grid faults. Afterwards a series of thermal-redistributed modulation methods which can achieve better thermal loading of power devices under this extreme operation are proposed. Finally, a new power control strategy which utilizes the zero sequence current is presented to xiii xiv Outlines of this Book achieve better control performance and current loading under the unbalanced AC source condition. In Chap. 6 the conclusions and contributions of this work and some potential proposals for the future research are discussed. The titles of the 7 special topics are listed as follows: Chapter 8: “The Impact of Power Switching Devices on the Thermal Performance of a 10 MW Wind Power NPC Converter” Chapter 9: “Reliability-Cost Models for the Power Switching Devices of Wind Power Converters” Chapter 10: “Electro-Thermal Model of Power Semiconductors Dedicated for both Case and Junction Temperature Estimation” Chapter 11: “Reactive Power Influence on the Thermal Cycling of Multi-MW Wind Power Inverter” Chapter 12: “Thermal Loading of Several Multilevel Converter Topologies for 10 MW Wind Turbines Under Low Voltage Ride Through” Chapter 13: “Another Groups of Thermal Optimized Modulation Methods of Three-Level Neutral-Point-Clamped Inverter Under Low Voltage Ride Through” Chapter 14: “Limits of the Power Controllability of Three-Phase Converter with Unbalanced AC Source” Part I Monograph Chapter 1 Introduction This chapter gives the background, motivation, and organization of this work. The state-of-the-art for wind power generation, development of power electronic technology, as well as some emerging challenges for the next generation wind power converters are presented. Then the objectives and structure of this book are outlined. 1.1 State-of-the-Art for Wind Power Generation Wind Turbine System (WTS) is still the most promising renewable energy technology. It started in the 1980s with a few tens of kW power production per unit, while nowadays multi-MW wind turbines are being installed. There is a wide-spread use of wind turbines in the distribution networks and more and more wind farms start to be connected with the transmission networks [1]. The cumulative wind power capacity from 1996 to 2012 is shown in Fig. 1.1; it can be seen that the wind power has grown fast to a capacity of 282 GW with around 45 GW installed only in 2012—this is more than any other renewable energy sources [2]. In 2011, the global electric power installation was around 208 GW; this number indicates that the wind power is really an important player in the modern energy supply system. As an extreme example, Denmark has a high penetration by wind power, and today more than 30 % of the electric power consumption is covered by wind. This country even has the ambition to achieve 100 % non-fossil-based power supply by 2050 [3]. Regarding to the markets and manufacturers of wind power, China has the largest market with over 17.6 GW capacity installed in 2011, together with the EU (9.6 GW) and USA (6.8 GW) sharing around 85 % of the global market. The Danish company Vestas was still on the top position among the largest manufacturers, closely followed by the GE and Goldwind. Figure 1.2 summarizes the worldwide top suppliers of wind turbines in 2011. It is interesting to see that there are four Chinese companies in the Top 10 manufacturers with total market share of 26 % [2]. © Springer International Publishing Switzerland 2015 K. Ma, Power Electronics for the Next Generation Wind Turbine System, Research Topics in Wind Energy 5, DOI 10.1007/978-3-319-21248-7_1 3 4 1 Introduction Worldwide wind power capacity (Giga Watts) Fig. 1.1 Global cumulative installed wind power capacity from 1996 to 2012 Fig. 1.2 Distribution of wind turbine market share by manufacturers in 2011 [2] Besides the quick growth in the total installed capacity, the size of individual wind turbine is also increasing dramatically in order to reduce the price per generated kWh. In 2011, the average turbine size delivered to the market is 1.7 MW, among which the average offshore turbine size achieves 3.6 MW. The growing trends of emerging turbine size between 1980 and 2018 are shown in Fig. 1.3; it is noted that the cutting-edge 8 MW wind turbines with diameter of 164 m have already shown up in 2012 [4]. Right now most manufacturers are developing products in the range of 4.5–8 MW, and it is expected that more and more large wind turbines with multi-MW power level, even up to 10 MW, will be present in the next decade—driven mainly by the considerations to lower down the cost of energy [5]. 1.2 Development of Wind Power Technologies 5 7~8 MW D 164 m 10 MW D 190 m 5 MW D 124 m 2 MW D 80 m 100 kW D 20 m 50 kW D 15 m 500 kW D 40 m 600 kW D 50 m 1980 1985 1990 Rating : 0% Power Electronics Role : Soft starter 1995 2000 2005 10% 30% Rotor Rotor resistance power 2011 2018 (E) 100% Full generator power Fig. 1.3 Evolution of wind turbine size and the power electronics seen from 1980 to 2018 (Estimated), blue circle indicates the power coverage by power electronics 1.2 Development of Wind Power Technologies 1.2.1 Evolution of Wind Turbine Concepts The technologies used for Wind Turbine System (WTS) have also changed dramatically for the last 30 years with four to five generations emerged [6–8]. Until now the existed or existing wind turbine configurations can be generally categorized into four concepts [8]. The main differences between these concepts locate on the types of generator, power electronics, speed controllability, and the way in which the aerodynamic power is limited. A. Fixed Speed Wind Turbines (Type A) As shown in Fig. 1.4, this configuration corresponds to the so called “Danish concept” that was very popular in 80s. The wind turbine is equipped with Bypass switch Squirrel Cage Induction Generator Transformer Grid Gear Soft starter Capacitor bank Fig. 1.4 Fixed speed wind turbine with direct grid connection 6 1 Introduction Bypass switch Wound Rotor Induction Generator Transformer Grid Gear Rotor resistance Soft starter Capacitor bank Fig. 1.5 Partial variable speed wind turbine with variable rotor resistance asynchronous Squirrel Cage Induction Generator (SCIG), and smoother grid connection can be achieved by incorporating a soft-starter. The disadvantages of this early concept are as follows: a reactive power compensator (e.g., capacitor bank) is required to compensate the reactive power demand by the asynchronous generator. Because the rotational speed is fixed without any controllability, the mechanical parts must be strong enough to withstand adverse mechanical torque, and the wind speed fluctuations are directly transferred into the electrical power pulsations which could yield to instable output voltage in case of week power grid. B. Partial Variable Speed Wind Turbine with Variable Rotor Resistance (Type B) As presented in Fig. 1.5, this concept is also known as OptiSlip (VestasTM) emerged in the mid 1990s [9]. It introduces the variable rotor resistance and thus limited speed controllability of wind turbines. The Wound Rotor Induction Generator (WRIG) and corresponding capacitor compensator are typically used, and the generator is directly connected to the grid by a soft-starter. A technology improvement of this concept is that the rotational speed of the wind turbine can be partially adjusted by altering the rotor resistance. This feature will contribute to the mechanical stress relief and make a more smooth electrical power output. However, the power loss dissipating constantly in the rotor resistors is a significant drawback for this concept. C. Variable Speed Wind Turbine with Partial-Scale Power Converter (Type C) This concept is the most established solution nowadays and it has been used since 2000s. As shown in Fig. 1.6, a back-to-back power electronics converter is adopted in conjunction with the Doubly-Fed Induction Generator (DFIG). The stator windings of DFIG are directly connected to the power grid, while the rotor windings are connected to the power grid by the power electronics converter with normally 30 % capacity of the wind turbine [10, 11]. By the use of power electronic converter, the frequency and current in the rotor can be flexibly regulated and thus the variable speed range can be further extended to a satisfactory level. Meanwhile the power converter can partially regulate the 1.2 Development of Wind Power Technologies 7 Transformer Double-fed induction generator Grid Gear AC DC DC AC Filter 1/3 scale power converter Fig. 1.6 Variable speed wind turbine with partial-scale power converter output power of the generator, improving the power quality and providing limited grid support. The smaller converter capacity makes this concept attractive from a cost point of view. However, its main drawbacks are the use of slip rings and the insufficient power controllability in case of grid faults—these disadvantages may comprise the reliability performance and are hard to satisfy the future grid requirements as claimed in [12, 13]. D. Variable Speed Wind Turbine with Full-Scale Power Converter (Type D) Another promising concept that is becoming popular for the newly installed wind turbines is shown in Fig. 1.7. It introduces a full-scale power converter to interconnect the power grid and stator windings of generator; thus all the generated power by the wind turbine can be regulated. The asynchronous generator, Wound Rotor Synchronous Generator (WRSG), or Permanent Magnet Synchronous Generator (PMSG) have been reported to be used in this concept. The elimination of slip rings, simpler or even eliminated gearbox, full power and speed controllability as well as better grid support ability are the main advantages of this solution compared to the DFIG-based concept. Thanks to the use of full-scale power converter, the voltage level of the power conversion stage can be rather flexible; in the future the voltage might be high enough to directly connect to the power grid without the bulky low-frequency-transformer, which is an attractive feature for the future wind turbine system. However, more stressed and expensive power electronics are expected and the price for Permanent Magnet (PM) materials may raise some uncertainties for this concept to be further commercialized— leading to the development of generator technology with less or even no PM in the future. Transformer AC Filter Gear Asynchronous/ Synchronous generator DC DC Grid AC Full scale power converter Fig. 1.7 Variable speed wind turbine with full-scale power converter Filter
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