<P> Today the frequency changer used in applications up to few tens of megawatts consists of two back to back connected IGBT inverters . </P> <P> Several brushless concepts have also been developed in order to get rid of the slip rings that require maintenance . </P> <P> DFIG for Double Fed Induction Generator, a generating principle widely used in wind turbines . It is based on an induction generator with a multiphase wound rotor and a multiphase slip ring assembly with brushes for access to the rotor windings . It is possible to avoid the multiphase slip ring assembly (see brushless doubly - fed electric machines), but there are problems with efficiency, cost and size . A better alternative is a brushless wound - rotor doubly - fed electric machine . </P> <P> The principle of the DFIG is that rotor windings are connected to the grid via slip rings and back - to - back voltage source converter that controls both the rotor and the grid currents . Thus rotor frequency can freely differ from the grid frequency (50 or 60 Hz). By using the converter to control the rotor currents, it is possible to adjust the active and reactive power fed to the grid from the stator independently of the generator's turning speed . The control principle used is either the two - axis current vector control or direct torque control (DTC). DTC has turned out to have better stability than current vector control especially when high reactive currents are required from the generator . </P>

Doubly fed induction generator in a wind turbine