Power frequency zero-sequence directional element for parallel transmission lines

The zero-sequence mutual inductance between parallel transmission lines can weaken the reliability of protective relay. As a novel zero-sequence directional element is proposed based on the model recognition theory, an inductance model is established for the internal fault. The model error is small for an internal fault while large and oscillating for an external fault. To solve the problem on the reliability and the sensitivity of the proposed method, caused by harmonics in the transient waveform, power frequency component is extracted by Fourier algorithm, least squares algorithm and fast phasor extracting algorithm separately in this paper. A 500kV parallel transmission-line model with zero-sequence mutual inductance is established in EMTP to verify performance of the proposed protective relay based on these algorithms. Each algorithm is studied by different data windows (5ms, 10ms and 20ms). Simulation result verifies the fast phasor extracting algorithm can accurately extract the power frequency component even with a short data window. And the proposed protective relay based on the fast phasor extracting algorithm has the best performance.