A post-fault strategy to control the AC-AC modular multilevel converter under input-side line-to-ground fault

Abstract

The AC-AC Modular Multilevel Converter (MMC) is a promising candidate for medium-voltage motor drive applications. In this paper, a post-fault strategy to control the AC-AC MMC under input-side line-to-ground fault condition is proposed. Since the line-to-ground fault results in negative-sequence components of input-side voltages, input-side current regulation and capacitor voltage balancing methods need to be redesigned to maintain power transfer capability and keep capacitor voltages of all clusters balanced. Besides, an inherent problem of the AC-AC MMC is large capacitor voltage fluctuations when outputside frequency is close to input-side frequency. To overcome the problem under the line-to-ground fault, a capacitor voltage fluctuation mitigation method is developed in this paper. The proposed post-fault strategy achieves seamless transition from normal operation to the post-fault operation. Effectiveness of the proposed post-fault strategy is verified by simulation results in the PSCAD/EMTDC software environment.