Fault Line Selection Principle of Single-phase-to-ground Fault Based on Improved Zero-sequence Admittance

The protection scheme of the single phase-to-ground fault based on the traditional zero-sequence admittance method does not consider the three-phases zero-sequence asymmetry in the distribution network. Therefore, the calculation of zero-sequence admittance will have a large error when a single-phase ground fault occurs, especially a high-impedance fault. For this problem, an improved zero sequence admittance calculation method and the protection scheme of the single phase-to-ground considering the three-phase zero- sequence asymmetry are proposed. Firstly, the calculation formulas of zero-sequence admittance for ungrounded and resonant grounded systems under the condition of line asymmetry are derived respectively. Secondly, according to the calculation of zero-sequence admittance, the protection scheme based on improved admittance method under two grounded systems is proposed. At the same time, the concept of phase difference coefficient based on zero-sequence admittance is proposed, and the fault feeder selection criterion is established. Finally, simulation of a single-phase-to-ground fault model for the distribution network, and field test data verified that the protection scheme based on improved zero-sequence admittance method can select the fault feeder under the condition of line asymmetry effectively.