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
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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
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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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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8
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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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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19
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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. . . . . . . . . . . . . . .
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31
31
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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
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63
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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 . . . . . . . . . . . . . . . . . . .
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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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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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 . . . . . . . . . . . . . . . .
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99
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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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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159
159
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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181
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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