AC Fault Current Calculation of MMC-HVDC System Via Time-Varying Capacitor Model

MMC-HVDC has become an ideal solution for the grid connection of offshore wind farms. However, MMC and wind farm have low inertia. Once a fault occurs in the offshore AC system, the fault current increases rapidly. This seriously threatens the stable operation of the system. Therefore, this paper proposes a novel method to calculate the offshore AC fault current of MMC. Firstly, a time-varying capacitor equivalent model of MMC is proposed. Then, the fault current before and after MMC blocking is calculated by the piecewise linearization method based on Nearest Level modulation (NLM) algorithm. Finally, the accuracy and efficiency of the proposed method are verified by comparing with the simulation results of PSCAD/EMTDC.