The voltage-source converter (VSC)-based high-voltage direct-current (HVDC) grid is an effective means for large-scale integration of renewable energies. This paper proposes a pilot protection scheme as the backup protection for transmission lines in the VSC-HVDC grid. The traveling wave (TW) propagating processes and waveform characteristics at both ends of the transmission line are analyzed under different fault conditions. Based on the different characteristics between internal and external fault conditions, the protection is formulated utilizing similarity measure of TW waveforms. A four-terminal VSC-HVDC grid is modeled on PSCAD/EMTDC platform to test the performance of the proposed protection. Various simulation results demonstrate that the proposed protection is able to accurately discriminate internal faults from external faults and identify the faulty pole. The protection performance analysis shows that the protection is insensitive to the fault resistance, the protection sampling rate, and the inductance value of the reactors at the end of lines. The protection algorithm is simple with small calculation amount and does not require data synchronism. Furthermore, the proposed protection has superiority in the fault resistance withstand ability compared with other backup protections.
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