An improved zero-sequence directional element for parallel lines based on fast phasor extracting algorithm

The zero-sequence directional protection on parallel lines will mal-operate for zero-sequence mutual inductance. An inductance model for an internal fault is established and a model error function is defined on model recognition theory by analyzing the fault equivalent circuit. The model error is small for an internal fault while large and oscillating for an external fault. To solve the problem caused by the high-frequency components in the transient waveform on the set threshold value of the proposed method, the power frequency component is used for the model error function in this paper. A new phasor extracting method named fast phasor extracting algorithm is introduced to extract the power frequency component during the transient period. Simulation results from EMTP prove the high quality of the improved zero-sequence directional element and its validity to pick up speed to remove the fault.