A Modular Multilevel Converter Topology With Novel Middle Submodules to Reduce Capacitor Voltage Fluctuations

This letter proposes a modified topology of modular multilevel converter for medium-voltage applications, which can reduce the ac-voltage fluctuation in the submodule (SM) capacitor at low-frequency operation. A novel structure of middle SM is inserted between the upper and lower arms, which has lesser count of components compared with the existing ones, leading to the reduction in cost and power losses. At the low operating frequency, furthermore, the middle SM can attenuate the power fluctuation in arms by distributing the high-frequency power components. Therefore, the SM capacitor voltage fluctuation is significantly alleviated without imposing the common-mode voltage on the ac side. The effectiveness of the proposed topology has been verified by experimental results.