A New Differential Protection of Transmission Line Based on Equivalent Travelling Wave

Differential protection is widely used for transmission lines, but its performance is affected by distributed capacitance current and current-transformer (CT) saturation. Travelling-wave (TW) differential protection is immune to these factors and can achieve ultra-high-speed operation. These characteristics are essential for long-distance extra/ultra-high-voltage transmission-line protection. However, the contradiction between sampling frequency and communication traffic makes it hard to ensure high sensitivity and high reliability at the same time. To solve this problem, a new TW differential protection based on equivalent travelling wave (ETW) is proposed in this paper. Wavelet transform is adopted to extract wavelet-transform modulus maxima (WTMM) of TW, which are used to reconstruct ETW. Only several WTMMs need to be exchanged with the opposite terminal, so the amount of communication is largely reduced. Current energy ratio is defined and used for operation criterion to enhance the sensitivity during faults with large fault resistance and a small fault inception angle. Detailed discussions on the selection of setting value ensure the reliability of the proposed protection scheme. Extensive simulations are conducted to test the performance of the protection. Simulation results verify that the protection can correctly discriminate between internal and external faults with high sensitivity and reliability.