Fault Current Calculation of MMC-HVDC With Varying Capacitance Equivalents

Flexible high-voltage DC (HVDC) transmission system composed of half-bridge modular multilevel converters (MMCs) becomes more and more popular in China. As the most serious fault in MMC-HVDCs, short-circuit fault on transmission lines should be discussed carefully to improve the setting and performance of fault current controls and protections. Generally, the post-fault equivalent of short-circuit fault of half-bridge MMC is a RLC discharging circuit. The effect due to AC system is ignored. But the AC system continues charging capacitors before IGBT blocks. In this paper, to reduce the errors of fault current calculation, the AC charging of capacitors is considered by replacing the capacitance of each arm with varying capacitances. Simulations have been done to verify the calculation results. The comparisons show that the proposed method could be more accurate than the RLC equivalent method.