Review of the State of the Art of Power Electronics for Wind Turbines

This paper reviews the power electronic applications for wind energy systems. Various wind turbine systems with different generators and power electronic converters are described, and different technical features are compared. The electrical topologies 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.

OVER the last ten years, the global wind energy capacity has increased rapidly and became the fastest developing 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 technology 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 controllability of the wind turbines becomes more and more important as the power level of the turbines increases. Power electronic, being the technology of efficiently converting 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 turbine system where wind power generators are directly connected to the grid, thyristors are used as soft-starters. The power electronic 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, harmonics, etc. This paper reviews the major applications of power electronics for wind power conversion systems, and it is organized as follows. Section II shows a brief review of the wind energy conversion systems and modern power electronics. Then, applications 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 conclusions 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 mechanical power to electrical power is using a gearbox and a generator 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 electronic 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

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