LAUSR

Compared with synchronous generators, modular multilevel converter-based high voltage direct current (MMC-HVdc) stations have unique fault behaviors, potentially impeding the correct operations of distance relays of lines connecting MMC-HVdc stations. To minimize the adverse effect caused by the fault resistance and MMC-HVdc stations, a new zone 1 protection criterion based on projected reference impedance is presented. According to the polarity of direction factor and the amplitude comparison of projected reference and measured impedances, the internal and external faults can be accurately distinguished. To shorten the response time of distance relays for the faults occurring in transmission lines emanating from MMC-HVdc stations, especially for the faults in the protection deadzone of zone 1, an accelerated distance protection based on the new zone 1 protection criterion is presented, ensuring that distance relays at two ends can clear the internal faults at high speed. Furthermore, the proposed protection scheme remains unaffected by fault resistance and operates well under different fault locations. The performance of the proposed scheme is assessed by more than 2100 digital simulations.