A Modular Multilevel Converter Submodule Based on Series-connected Power Devices for Medium Voltage Distribution Grid Applications

Modular multilevel converters (MMCs) are used to interconnect medium voltage DC and AC grids. This paper employs series-connected power devices (SPD) to reduce the number of submodules, which can reduce the MMC footprint, complexity and assembly cost. The key problem of SPD is the voltage imbalance between the devices. To address this problem, an active clamping module (ACM) is paralleled with each main power device. Each ACM consists of a clamping capacitor and an auxiliary switch. The operating principle, balancing process and control method of ACM are introduced. Simulation and experimental results are provided to verify the effectiveness of the proposed method. The voltages of SPD are well balanced and the application of SPD does not impact the normal operation of MMC.