A doubly fed induction generator using back-to-back PWM converters supplying an isolated load from a variable speed wind turbine

A doubly fed induction generator (DFIG) designed to be driven at variable speed from a wind turbine and supplying an isolated load is presented. Two back-to-back PWM voltage-fed inverters connected between the stator and the rotor allow sub- and super-synchronous operation with low distortion currents. The load voltage is maintained at constant frequency and its magnitude is regulated through control of the stator flux of the generator. An auxiliary load is connected in parallel with the main load, and the auxiliary power is controlled to allow the DFIG to track the optimal wind turbine speed for maximum energy capture from the wind. An indirect stator-orientated vector control scheme is used to control the DFIG and this results in constant load voltage and frequency for variations in both load and wind speed. The techniques have been implemented and validated on a 7.5 kW experimental rig.