DC Line Fault Identification Based on Pulse Injection From Hybrid HVDC Breaker

The half bridge modular multilevel converter (MMC) coordinated with dc circuit breakers (DCCBs) is one of the most promising solutions for the future multi-terminal HVdc system protection. After the isolation of dc line fault, the electrical quantities of the dc system remain stable and no coupling relationship exists between healthy and faulty poles. Thus, ac adaptive reclose scheme is difficult to be applied in dc transmission systems since the information of the adjacent lines cannot be used to assess whether the line fault is cleared or not. An innovative adaptive reclose scheme applied to the MMC-HVdc system is proposed in this paper by injecting voltage pulses to the faulty line. When the breaker is tripped, the converter and dc faulty line are connected by the main breaker of the hybrid HVdc breaker. On the basis of switching control of solid-state devices in the main breaker, the voltage pulse injection to the dc faulty line is realized. Then, the fault characteristic is identified based on the traveling-wave-based fault location algorithm. A fast restart scheme for the MMC-based bipolar HVdc transmission system is also achieved by this method. The simulation results verify the effectiveness of the proposed method.