DC fault management strategy for continuous operation of HVDC grids based on customized hybrid MMC

Successful deployment of High-Voltage Direct Current (HVDC) grids necessitates effective DC fault handling strategies, which can minimize the severe consequences caused by DC faults on the AC and DC side of the HVDC grids. Therefore, this paper investigates the enhanced DC fault performance of the Customized Hybrid Modular Multilevel Converter (CH-MMC), in which a limited number of full-bridge sub-modules (FB-SMs) is added into the arms of the conventional MMC in an effort to significantly extend the timespan between fault inception and fault clearance, thus allowing the use of relatively slow and cheaper DC circuit breakers. Based on this converter, a dedicated DC fault handling strategy for CH-MMC based HVDC grids is proposed, which aims to improve the fault resiliency and security of HVDC grids for pole-to-pole faults. Moreover, the proposed DC fault management strategy guarantees the continuous operation of the grid during pole-to-ground DC faults, including full reactive power provision from the converter stations. The performance of the strategy is demonstrated using comprehensive electromagnetic transient (EMT) simulation studies conducted on an illustrative four-terminal meshed HVDC grid, which consider a range of scenarios with different fault current limiting inductors and DC circuit breaker operation times.