A Lightweight Hybrid Modular Multilevel Converter Topology for DC Fault Blocking

A hybrid arm-multiplexing modular multilevel converter (HAM-MMC) with dc fault clearing capability for substation miniaturization is investigated in this article. In HAM-MMC, the arm selection switches composed of series insulated gate bipolar transistors (IGBTs) are introduced to divide the phase branch of the traditional two-arm structure into three arms. Thus, the submodules (SMs) of the middle arm can be multiplexed between the upper and lower arms, which reduces 25% the number of SMs by increasing their utilization. As for dc fault blocking of this topology, FBSMs only need to be equipped in the multiplexed middle arm, which further reduces the number of devices compared with the conventional hybrid MMC with FBSMs and HBSMs. The structure of the arm selection switches and a modified capacitor voltage balance algorithm are developed. The dc fault characteristics of HAM-MMC are analyzed, and the fault blocking scheme based on flipping arm selection switches is proposed. The simulation and experimental results show that the HAM-MMC can clear dc fault currents without exceeding the limit SM voltage. With a 25% reduction in the number of capacitors and lower valve loss as well as energy storage requirement, the weight and cost of converter are both reduced.