A Modified Topology of Modular Multilevel Converters for Low Frequency Operation

This paper proposes a modified topology of modular multilevel converter (MMC) for medium-voltage applications, where the low-frequency voltage fluctuation on submodule (SM) capacitor is alleviated. A proposed middle SM is placed between the upper and lower arms, producing a compensation voltage to cancel the injected voltage component in each arm, by which the low-frequency power fluctuations are mitigated. In addition, the power balance between in upper and lower arms can be achieved through the middle SM. Consequently, the SM capacitor voltage fluctuation is mitigated without imposing the common-mode voltage on the AC-side. The feasibility of the proposed topology has been confirmed through simulation results.