An AC Current-Limiting Strategy for the Improved Modular Multilevel Converter

Abstract

The improved modular multilevel converter (MMC) based on half-bridge submodules (HBSMs) and H- bridge circuits can block the de fault current with a small number of extra semiconductor devices. Besides, the loss of the improved MMC is close to the loss of HB-MMC if ultra-fast mechanical disconnectors (UFDs) are employed to consist of direction switches (DSs). However, an ac short circuit fault will occur during the opening period of the UFDs. The ac currents will increase rapidly, threatening the system's safety and disturbing the ac grid. An ac current-limiting strategy is proposed in this paper to solve this problem. By blocking the other five arms, de inductors can limit ac currents during the UFD opening period. By inserting reverse-biased voltages (RBVs) provided by the current transfer arm (CTA) early according to the actual breakdown voltage of UFDs, ac currents can be limited by RBVs during the opening period of the UFD. Thus, the safety of the system can be assured. Besides, the requirement for the opening time of UFDs can be reduced, which means less cost.