IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 8, AUGUST 2009 1859 A Review of the State of the Art of Power Electronics for Wind Turbines Zhe Chen, Senior Member, IEEE, Josep M. Guerrero, Senior Member, IEEE, and Frede Blaabjerg, Fellow, IEEE Abstract—This paper reviews the power electronic applications for wind energy systems. Various wind turbine systems with differ- ent generators and power electronic converters are described, and different technical features are compared. The electrical topolo- gies of wind farms with different wind turbines are summarized and the possible uses of power electronic converters with wind farms are shown. Finally, the possible methods of using the power electronic technology for improving wind turbine performance in power systems to meet the main grid connection requirements are discussed. Index Terms—Fault ride-through, grid connection, power elec- tronics converters, reactive power compensation, wind energy con- version, wind farms, wind turbine control. I. INTRODUCTION O VER the last ten years, the global wind energy capac- ity has increased rapidly and became the fastest devel- oping renewable energy technology. By the end of 2006, the global wind electricity-generating capacity has increased to 74 223 MW from 59 091 MW in 2005. The early technol- ogy used in wind turbines was based on squirrel-cage induction generators (SCIGs) directly connected to the grid. Recently, the technology has developed toward variable speed. The control- lability of the wind turbines becomes more and more important as the power level of the turbines increases. Power electronic, being the technology of efficiently con- verting electric power, plays an important role in wind power systems. It is an essential part for integrating the variable-speed wind power generation units to achieve high efficiency and high performance in power systems. Even in a fixed-speed wind tur- bine system where wind power generators are directly connected to the grid, thyristors are used as soft-starters. The power elec- tronic converters are used to match the characteristics of wind turbines with the requirements of grid connections, including frequency, voltage, control of active and reactive power, har- monics, etc. This paper reviews the major applications of power electron- ics for wind power conversion systems, and it is organized as follows. Section II shows a brief review of the wind energy Manuscript received August 24, 2007; revised February 13, 2008. Current version published August 12, 2009. Recommended for publication by Associate Editor M. Godoy. Z. Chen and F. Blaabjerg are with the Institute of Energy Technology, Aalborg University, DK-9220 Aalborg, Denmark (e-mail: zch@iet.aau.dk; fbl@iet.aau.dk). J. M. Guerrero is with the School of Industrial Engineering of Barcelona (EUETIB), Department of Systems Engineering, Automation and Industrial In- formatics (ESAII), Technical University of Catalonia (UPC), 08036 Barcelona, Spain (e-mail: josep.m.guerrero@upc.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPEL.2009.2017082 conversion systems and modern power electronics. Then, ap- plications of power electronics for wind turbines are presented. Section III discusses the issues of operating wind turbines in power systems, and Section IV presents the power quality issues and dynamic performances. Finally, in Section V, the conclu- sions are drawn and the future trends are illustrated. A. Introduction of Wind Energy Conversion Systems The main components of a wind turbine system are illustrated in Fig. 1, including a turbine rotor, a gearbox, a generator, a power electronic system, and a transformer for grid connection. Wind turbines capture the power from wind by means of turbine blades and convert it to mechanical power. It is important to be able to control and limit the converted mechanical power during higher wind speeds. The power limitation may be done either by stall control, active stall, or pitch control whose power curves are shown in Fig. 2 [1], [2]. It can be seen that the power may be smoothly limited by rotating the blades either by pitch or active stall control while the power from a stall-controlled turbine shows a small overshoot and a lower power output for higher wind speed. The common way to convert the low-speed, high-torque me- chanical power to electrical power is using a gearbox and a gen- erator with standard speed. The gearbox adapts the low speed of the turbine rotor to the high speed of the generator, though the gearbox may not be necessary for multipole generator systems. The generator converts the mechanical power into electrical power, which being fed into a grid possibly through power elec- tronic converters, and a transformer with circuit breakers and electricity meters. The two most common types of electrical machines used in wind turbines are induction generators and synchronous generators. Induction generators with cage rotor can be used in the fixed- speed wind turbines due to the damping effect. The reactive power necessary to energize the magnetic circuits must be sup- plied from the network or parallel capacitor banks at the ma- chine terminal that may have the danger of self-excitation, when connection to the network is lost. In such a case, the terminal voltage or reactive power may not be directly controlled, and the induction generators may suffer from voltage instability prob- lem, which is becoming a significant concern with large-scale wind farm penetration. A wound rotor induction machine has a rotor with copper windings, which can be connected to an external resistor or to ac systems via power electronic systems. Such a system pro- vides a partial variable-speed operation with a small power electronic converter, and therefore increased energy capture and reduced mechanical load to the system. This type of system is an 0885-8993/$26.00 © 2009 IEEE Authorized licensed use limited to: Aalborg Universitetsbibliotek. Downloaded on September 10, 2009 at 15:03 from IEEE Xplore. Restrictions apply.