Non-Unit Ultra-High-Speed DC Line Protection Method for HVDC Grids Using First Peak Time of Voltage

The high-speed and selective dc line protection method is required for the safe operation of HVDC grids. In this paper, the expressions of the line-mode fault component voltage at the line end as well as its first peak time (FPT) when different internal and external faults occur are derived respectively. The conclusion proves that the FPT of line-mode fault component voltage for internal faults is smaller than that for backward and forward external faults, which is independent of fault impedance and fault type as well. Then a FPT-based non-unit ultra-high-speed dc line protection method for HVDC grids is proposed. The algorithm is verified by a four terminal ±400 kV MMC-HVDC grid PSCAD test model as well as implemented in the developed protection device and validated by the Zhangjiakou-Beijing MMC-HVDC grid RTDS test system. The test results show that i) the FPT-based protection algorithm is independent of fault impedance and fault type, ii) the proposed method can fast and correctly identify dc line faults in the HVDC grids, even with 500 Ω fault impedance, and iii) the sensitivity and fault identification time of the proposed method meet the engineering requirement of MMC-HVDC grids for dc line protection.