Characteristics analysis and improved arm control of modular multilevel converter under asymmetric operation conditions

This paper proposes a general analysis model and an improved arm control of modular multilevel converter (MMC) under asymmetric operation conditions. This general analysis model is established as the arm voltage equations with equivalent impedance parameters, and is applicable for characteristics analysis under both submodule (SM) fault condition and asymmetric arm parameter condition. Based on the analysis model, the equivalent circuits of the circulating current and the ac side current are obtained, and then the asymmetric characteristics are performed. According to the analysis results, an improved arm control of MMC is proposed, which is aimed at balancing the dc components in upper and lower arm currents, as well as suppressing the fundamental and double frequency circulating currents. Besides, the dynamic response of the proposed arm control is significantly improved by embedding an arm voltage inertial feedback control. Simulations on PSCAD/EMTDC verify the correctness of the theoretical analysis and the validity of the proposed control strategy.