Single-phase earth fault phase selection method and device for neutral point non-effective grounding distribution line

The invention provides a neutral point non-effective grounding distribution line single-phase grounding fault phase selection method and device, and the method comprises the steps: selecting a voltage instantaneous value in a power frequency period before and after fault triggering, and obtaining a three-phase voltage and a zero-sequence voltage before and after a fault; sorting the three-phase voltage after the fault according to the voltage amplitude from large to small, and judging whether the off-resonance degree of the neutral point arc suppression coil of the power distribution system is under-compensation or over-compensation according to the zero sequence voltage after the fault when the ratio of the intermediate voltage to the minimum voltage is greater than a threshold value; if the off-resonance degree of the neutral point arc suppression coil is under-compensation, a sampling value after a fault is taken to recalculate a three-phase voltage, the three-phase voltage is compared with the three-phase