Discussion on Fault Clear Strategies for Multi-Terminal UHVDC Transmission System

For DC circuit breaker has not been applied in UHVDC project, the fast-isolating switches are used to isolate faults of the converter station or transmission line in the multi-terminal UHVDC transmission system. New problems which are not existed in two-terminal HVDC system and DC power grid but faced by the multi-terminal UHVDC transmission system are analyzed. Especially, the fault clear problems of pole protection action, valve protection action, DC line fault restart, metallic line fault restart and pole balance. On the basis of those analysis, fault clear strategies to solve these problems are put forward exploringly.