Substation in China's development, has been taking the continuous optimization of the line, the application of small current grounding line selection device, improve the accuracy of 66kV system grounding device to judge the grounding accident, greatly reduce the harm of system grounding to substation operation.

In China, small current grounding system mainly refers to the system with ungrounded neutral point or grounded by arc suppression coil. In the substation, the single-phase ground fault has a high probability, and its characteristics are shown in the following aspects:

When the single phase is not completely grounded, the voltage of the fault phase will decrease, but the voltage of the non fault phase will increase. The voltage of the non fault phase will be greater than the phase voltage and less than the line voltage, and the zero sequence voltage will increase obviously.

When the single phase is fully grounded, the fault phase voltage will be zero, the non fault phase voltage will be directly increased to line voltage, and the zero sequence voltage will be increased to about 100V. In any case, the three-phase voltage of the system can be kept symmetrical, which does not affect the power supply to the load. The system can continue to operate for a period of time, but long-term operation may break through the weak parts of the insulation and expand the accident.

In the process of improving the small current grounding line selection in substation, we can not start from a single point of view, we must fully consider some changing factors.

We should make full use of the outage time of annual inspection, and make effective inspection for all line zero sequence current transformers, especially to observe whether there are obvious changes in the transformers. For example: in the process of analysis, if the transformation ratio of the zero sequence current transformer is not uniform, it needs to be replaced, and the secondary wiring work should be effectively completed, so as to ensure the action of the small current grounding line selection device, which can greatly improve the accuracy.