A low-loss energy rebalancing control scheme of MMC under submodule fault conditions

Modular multilevel converter (MMC) has attracted great attention in the high voltage direct current (HVDC) transmission systems. Redundant submodules (SMs) are usually set to enhance the reliability of MMC. However, if some SMs break down, the arm energy is unbalanced and fundamental fluctuation appears in the DC current. This paper proposes an arm energy rebalancing control scheme to suppress the fluctuation. The proposed scheme does not increase the SM voltage if the fault number is not more than the rated number of redundant SMs, unlike the conventional schemes. Additionally, the average losses of SMs are analyzed respectively, when the proposed and conventional schemes are adopted. The simulation results of the ±350 kV/1000 MW MMC verify the correctness of the proposed energy rebalancing control scheme and its characteristic of low loss in comparison with the conventional schemes.