Leg-balancing Control of Modular Multilevel Converter under Asymmetric Condition

Abstract

Modular Multilevel Converter (MMC) is widely used in high-power systems such as HVDC transmission. Improving the fault ride-through capability of MMC is very important for a power system. Aiming at the problem of unbalanced leg power under asymmetric grid conditions, an improved leg balancing control strategy based on zero-sequence voltage and dc circulating current injection of MMC is proposed. When the grid voltage is asymmetric, the proposed leg balancing control strategy can eliminate the impact of ac grid voltage asymmetry on leg power, and ensure the safe and stable operation of MMC. Finally, the effectiveness of the proposed control strategy is verified by simulation.