Doubly fed induction generator systems for wind turbines - IEEE Industry Applications Magazine

HE AIM OF WIND TURBINE SYSTEMS DEVELOPMENT is to continuously increase output power. A few years ago, the rated output power of production-type units reached 200 kW. By 1999, the average output power of new installations climbed to 600 kW. The largest series production units today are specified to deliver 1.5-MW output power (Table 1). It is anticipated that in the near future, power rating of wind turbines will increase further, especially in offshore applications. For example, the prototype of a Nordex N80 with a rated power of 2.5 MW was installed in March 2000 near Aachen. Many low-power wind turbines built to-date were constructed according to the “Danish concept” (Fig. 1), in which wind energy is transformed into electrical energy using a simple squirrel-cage induction machine directly connected to a three-phase power grid. The rotor of the wind turbine is coupled to the generator shaft with a fixed-ratio gearbox. Some induction generators use pole-adjustable winding configurations to enable operation at different synchronous speeds. However, at 26