Analysis of the Modular Multilevel Converter under Single Open-Circuit Fault in the Upper Active Switch of a Submodule

The modular multilevel converter (MMC) is one of the most promising topologies for medium-and high-voltage applications. Since the MMC is structured based on series connection of a number of identical Submodules (SMs), an open-switch fault in any of the SMs deteriorates the voltage and current. In this paper, the impacts of a single open-circuit fault in the upper active switch of a SM are focused. Under a faulty condition, the SM capacitor voltage magnitudes and arm current changing rates are analyzed. The analysis results provide design guidance to protect the MMC prior to the activation of any post-fault strategy. In addition, a current changing rate-based faulty arm locating method is proposed. Effectiveness of the proposed analysis method is investigated and verified in the PSCAD/ EMTDC software environment.