Development of a Variable-Speed Wind Energy Conversion System Based on Doubly-Fed Induction Generator

A variable-speed constant-frequency (VSCF) doubly-fed induction generator (DFIG) system using a back-to-back PWM converter connected between the generator rotor circuit and the power grid is investigated by the authors. By using a stator-voltage oriented vector control scheme, the independent control of the active and reactive power generated by the induction machine is achieved, based on which, the control principle for DFIG is presented in detail. The bi-directional PWM converter and its controls not only decrease the current harmonics both in the rotor side and in the grid side and thus improve the power quality, but also expand the operation range and then improve the system energy efficiency. In addition, the maximum power point tracking (MPPT), power factor control in the stator side, and soft-connection to and soft-disconnection from the grid are also achieved and reported in this paper. To validate the control strategy developed in this paper, a 110kVA test system for wind-energy generation has been designed and built, on which a series of experiments have been carried out. The experimental results are presented in the paper.