Wide Frequency Operation Control Method of DFIG Connected to DC Grid with Constant Slip

Doubly-fed induction generator (DFIG) connected to DC microgrid is different from that connected to AC power grid. It is not necessary to limit the frequency of power generation to fundamental frequency. In order to improve the efficiency of power generation, we can broaden frequency range of DFIG and always make it operate at super-synchronous state within a certain range of wind speed. The topology of DFIG-DC system in this paper is to connect an uncontrolled rectifier on the stator winding and a small capacity full-controlled converter on the rotor side of DFIG. Double closed loop control method is adopted to control frequency with magnetic linkage and realize the Maximum Power Point Tracking (MPPT) with stator power. By means of the vector control, a smaller constant slip is maintained so that the rotating speed of stator magnetic field is somewhat less than the rotor speed to reduce the cost of rotor side converter (RSC). The simulation results show that the constant slip control strategy can be used in the wide frequency operation of the DFIG-DC system.