The characteristic of single-phase grounding fault in neutral ungrounded system is the basic principle of small current grounding line selection. There are generally two criteria for grounding line selection device

Startup criteria: generally, there is zero sequence voltage device startup judgment. When a single-phase grounding fault occurs in the system, the open delta winding of the bus voltage transformer of the power plant (or substation) will generate zero sequence voltage. When the zero sequence voltage is greater than the setting value (generally set to 30V), the device starts, and the line selection function master station sends a signal to call the magnitude and direction of zero sequence current to all protective devices on the bus for analysis and judgment.

Line selection criteria: generally, the size and direction of zero sequence current constitute the line selection criteria. The small current grounding line selection device preliminarily judges the grounded fault line according to the collected zero sequence current of each line, and then confirms the fault line according to the zero sequence power direction of each line.

From the above characteristics of single-phase grounding fault of small current grounding system, it can be seen that the zero sequence capacitive current of fault line is the sum of zero sequence capacitive currents of all non fault lines. If there are enough lines on the same bus and the parameter difference of each line is not large, The zero sequence capacitance current of the fault line will be much greater than that of the non fault line. The fault line can be judged by using the magnitude of the zero sequence current. At the same time, the different direction of zero sequence capacitor current between fault line and non fault line is another main criterion for line selection. The zero sequence power direction of non fault line is from bus to line, while the zero sequence power direction of fault line is from line to bus. The fault line can also be judged by using the different direction of zero sequence power, Moreover, there is no action dead zone in the zero sequence power direction. If the zero sequence power direction of all lines is positive, judge that the bus is grounded.