Experiment on DC-fault Ride Through of MMC Using a Half-Voltage Clamp Submodule

High-voltage, direct current systems using modular multilevel converters (MMCs) could be widely used in bulky energy transmission. Dc-fault ride through, including fault clearance and converter restart, is an issue that MMC must deal with. Conventional MMC topologies with dc-fault ride through capability involve a relatively high-power semiconductor cost and converter loss, and experimental verification is insufficient. This paper presents an MMC topology that employs a half-voltage clamp submodule. The proposed topology utilizes the submodule capacitor voltages to eliminate the freewheeling effect of diodes actively and achieve fast fault clearance and converter restart. The proposed topology also entails low additional cost. A 20 kW/1000 V laboratory prototype is built. Experiments are conducted to investigate dc-fault clearance and converter restart and verify the dc-fault ride through capability of the proposed topology.