Analysis and control of MMC-HVDC under unbalanced voltage conditions

A vector current controller based on quasi-proportional resonant (PR) in α β stationary reference frame is used to control the positive and negative sequence currents uniformly. Through analyzing the power characteristics under unbalanced grid conditions, control strategies to suppress negative sequence current and active power ripples are realized without the additional transformation between α β and d q reference frame. A more general method is proposed to analyze the circulating current features and sequence components; then a circulating current controller is designed based on quasi-PR controller. The traditional voltage-balancing algorithm is also improved to reduce switching times by adjusting the voltage sorting frequency according to the capacitor voltage standard deviation (CVSD).