An Improved Repetitive Control of DFIG-DC System for Torque Ripple Suppression

In order to obtain smooth torque and output power of the doubly fed induction generator (DFIG) dc system in which an uncontrollable rectifier is used to connect the stator winding and dc grid, an improved repetitive control (RC) regulator is proposed for the rotor-side converter to suppress the torque ripple in this paper. The torque control of the DFIG is divided into the average torque control and the torque ripple control. For the average torque control, a novel stator frequency estimation method is proposed to acquire the frequency and angle of the stator flux linkage. Then, the stator flux linkage orientation is applied for the decoupling control of the stator frequency and average torque. For the torque ripple control, an improved RC regulator is designed and directly employed for suppressing the torque ripple completely, which can avoid the sequence decomposition and rotor harmonic current calculation. The detailed design of the improved RC regulator is presented considering the control bandwidth, the fractional delay, and suppressing of the infinite dc gain. Finally, the proposed control method is validated by the experiment results.