Referring to the rotation speed, wind turbine concepts can be classified into fixed speed, limited variable speed and variable speed. For variable speed wind turbines, based on the rating of power converter related to the generator capacity, they can be further classified into wind generator systems with a partialscale and a full-scale power electronic converter. In addition, considering the drive train components, the
wind turbine concepts can be classified into geareddrive and direct-drive wind turbines. In geared-drive wind turbines, one conventional configuration is a
multiple-stage gear with a high-speed generator; the other one is the multibrid concept which has a singlestage gear and a low-speed generator [8]. In this section, according to contemporary wind turbine concepts, the basic configurations and characteristics of different wind generator systems are described.