In the small current grounding system, single-phase grounding is a common fault in power system, which means that one of the three-phase system is short circuited with the earth. After single-phase grounding fault occurs in 10kV distribution line, the voltage transformer on 10kV bus bar of substation detects zero sequence current and generates zero sequence voltage on the open triangle. The core of voltage transformer is saturated. If it runs for a long time, the voltage transformer will be burned.

The characteristics of single phase to ground fault in small current grounding system are as follows

When one phase (e.g. phase a) is not completely grounded, i.e. through high resistance or arc grounding, the voltage of fault phase decreases, and the voltage of non fault phase increases. They are greater than phase voltage, but can not reach line voltage. When the voltage of the opening triangle of the voltage transformer reaches the setting value, the voltage relay acts and sends out the grounding signal.

If phase A is fully grounded, the voltage of the faulty phase drops to zero and the voltage of the non faulty phase increases to the line voltage. At this time, three times of the original phase voltage appears at the opening triangle of the voltage transformer, and the voltage relay acts and sends out the grounding signal.

When one phase (phase a) is broken or the fuse is fused at the high voltage side of the voltage transformer, the indication of the fault phase is not zero. This is because the phase voltmeter forms a series circuit through the transformer coil and other two-phase voltmeters in the secondary circuit, and the relatively small voltage indication appears, but it is not the actual voltage of the phase, and the non fault phase is still the phase voltage. The voltage value of about 35V will appear at the triangle opening of the transformer, and the relay will be started to send out the grounding signal.

Due to the existence of elements with good capacitance and inductive parameters, especially the ferromagnetic inductor with iron core, when the parameter combination does not match, it will cause ferromagnetic resonance, and the relay will act and send out grounding signal.

No load bus false grounding phenomenon. In the no-load operation of the bus, the three-phase voltage unbalance may occur and the grounding signal will be sent out. But when a line is sent, the grounding phenomenon will disappear.

 In order to reduce the adverse impact of single-phase grounding fault on the operation of power grid, it is not only required that the personnel on duty are familiar with the relevant operation regulations, understand the operation status of the equipment, constantly summarize experience in practice, and improve the ability to deal with problems, but also install small current grounding line selection device in the system, so as to shorten the time of finding and handling grounding fault, and to prevent non fault phase current The fault line must be determined and removed as soon as possible.