A novel circulating current suppressor for modular multilevel converters based on quasi-proportional-resonant control

The inner circulating current in the modular multilevel converter (MMC) is briefly analyzed and the necessity to restrain it is pointed out. A novel circulating current suppressor (NCCS) for MMC based on the direct feedback control principle is designed. The NCCS adopts notch filters to extract the secondary component in the inner circulating current, and then compensates for it with the modulation signals of sub-modules by the quasi-proportional-resonant (quasi-PR) controller. The proposed method is simple in principle, easy in implementation, and suitable for both single-phase and three-phase power systems. Finally, the correctness and feasibility of the proposed control strategy are verified by simulation results.