A Modified Modular Multilevel Converter with Features of Reduction of Submodule Capacitor Fluctuation and Fault-Tolerant Operation

This paper proposes a modified modular multilevel converter (MMC), where the SM capacitor voltage fluctuation is reduced and the reliability of system is enhanced. A middle submodule (SM) is placed between the upper and lower arms, which provides desirable features for the low-frequency operation of the converter under healthy and faulty conditions. During normal operation, the imbalance of low-frequency power fluctuation between both arms can be balanced by the middle SM, which leads to the reduction of SM capacitor voltage fluctuation. In the case of the fault-tolerant operation, the middle SM can be reconfigured as a healthy SM to substitute a faulty SM once the faulty SM is detected in the upper or lower arms. The feasibility of the proposed topology and control scheme has been confirmed by the simulation results.