Protection of full bridge MMC DC grid employing mechanical DC circuit breakers

This article studies the protection of DC grid based on full bridge (FB) MMC and mechanical DC circuit breakers. A four terminal DC grid cropped from the CIGRE B4 DC grid test system is used as the studied system. Three level cascaded controllers are installed at each FB MMC, along with central DC grid dispatcher. In the proposed approach FB MMCs are operated through DC faults exploiting FB MMC capability to control DC fault currents. Mechanical DC CB are selected because of their low costs and since speed of protection operation is not critical with FB MMC. Differential DC grid protection method is adopted for its good robustness and suitability for wide range of fault impedances. EMTP-RV simulation of complete system under DC faults is provided to confirm validity of the proposed method.