Research on Operation of Modular Multilevel Converter under Asymmetric Grid Conditions

An asymmetric AC grid fault will lead to a zero sequence component of double-frequency appeared in the circulating current of modular multilevel converter (MMC). Since the zero sequence component cannot be counteracted between three phases, it will flow into the DC side, generating voltage and current fluctuations with double-frequency, which affects the stability of the system. In this paper, the mechanism of DC side fluctuations caused by the asymmetric fault on AC grid are illustrated. And three major types of asymmetric faults including single-phase grounding, two-phase grounding and two-phase short-circuit faults are compared to study the effect of different faults on the DC side fluctuation of MMC. Furthermore, a decoupling control strategy of MMC is adopted, which can independently control the AC and DC loops thus easier to suppress the fluctuation in DC voltage and current. Finally, Correctness of theoretical analysis and the control strategy in this paper are both verified by simulation and experimental results.