Mechanism analysis and control strategy of dynamic torque improvement for wind farm with squirrel cage induction generator under unbalanced voltage by STATCOM

The unbalanced grid voltage in wind farm with squirrel-cage induction generators (SCIG) can cause generator torque oscillations with double grid frequency that reduce lifetime of drive train. In this paper, the mechanism and control strategy of reducing generator torque oscillations using the static synchronous compensator (STATCOM) is investigated for wind farm with SCIG under unbalanced voltage. The torque oscillation mechanism of SCIG under the unbalanced grid voltage is analyzed based on its positive and negative equivalent circuits as well as power and torque expressions. Mathematical formula for the relationship among negative sequence voltage, STATCOM capacity, grid parameters is built up, and it sheds light on mechanism for STATCOM to reduce generator torque oscillation. In order to eliminate the generator electromagnetic torque oscillations, the positive and negative sequence control loop superimposed control strategy of STATCOM is put forward, where the positive sequence voltage control strategy controls the voltages at the point of common coupling (PCC) and capacitor to the given values and the negative sequence voltage control strategy controls negative-sequence voltage component of PCC to zero. The simulation model including wind farm with SCIG, STATCOM and grid is established in the PSCAD/EMTDC, and simulation results show that the proposed control strategy effectively compensate the negative sequence voltage and eliminate the generator electromagnetic torque oscillations so that the correctness and effectiveness of both mechanism analysis of dynamic torque improvement by STATCOM and the proposed positive and negative sequence control loop superimposed control strategy are verified.