An enhanced MMC topology With DC fault clearance capability

High-voltage direct current system using modular multilevel converter (MMC-HVDC) is a potential candidate for the grid integration of renewable energy over-long distances. The dc-link fault is an issue which MMC- HVDC must deal with. This paper proposed an enhanced MMC topology with dc fault clearance capability. Adopting diode clamp double sub-modules, the freewheeling effect of diodes is eliminated by the sub-module capacitor voltages, and fault-current path is cut off and thus fault currents can be extinguished rapidly. Since the tripping of ac circuit breakers is avoided, MMC can immediately restart power transmission for non-permanent faults. The required rated voltage of additional semiconductors is half the conventional semiconductors, resulting in low extra cost. The number of sub-modules required to achieve the same dc-link rated voltage is halved, leading to simplification and cost reduction of pulse generator. Simulation results using PSCAD/EMTDC have verified the validity of the proposed topology and protection scheme. ©2014 Chin. Soc. for Elec. Eng.