A Pilot Protection for LCC-HVDC Transmission Lines Based on Measured Surge Impedance at Tuning Frequency

This paper proposes a novel pilot protection scheme for LCC-HVDC transmission lines based on the measured surge impedance at tuning frequency. On the basis of amplitude-frequency characteristic of DC filter, it is found that the voltage forward traveling wave and reverse traveling wave at tuning frequency approximately satisfy total reflection at the line boundary after the internal fault. Then, combined with the traveling wave propagating processes for DC line internal faults and external faults, it is found that for internal faults, for two terminals of line, measured surge impedances at tuning frequency are both close to 0 ohm, whereas for external faults, the measured surge impedance at tuning frequency near faulty terminal equals line surge impedance, whose value is generally about 200 similar to 300 ohm, and another value at the distance faulty terminal is close to 0 ohm. Thus, this difference is utilized for fault identification. Comprehensive simulations demonstrate that the proposed protection scheme has the ability to identify DC line internal and external faults accurately. Meanwhile, the protection scheme has no requirement for two-terminal data synchronization and has low communication requirement. Besides, it can respond to the high-resistance faults sensitively.