The characteristic of single-phase grounding fault in ungrounded neutral system is the basic principle of small current grounding line selection

Starting criterion: generally, zero sequence voltage setting device is used as starting criterion. When single-phase ground fault occurs in the system, the open delta winding of bus voltage transformer in power plant or substation will generate zero sequence voltage. When the zero sequence voltage is greater than the setting value (generally set to 10-20v), the device will act, and the main station of line selection function will send out a signal, and all the protective devices on the bus will call the direction of zero sequence current for analysis and judgment.

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

The characteristic of single-phase grounding fault in small current grounding system is that the zero sequence capacitive current of fault line is the sum of the zero sequence capacitive currents of all non fault lines. If there are enough lines on the same bus and the parameters of each line are different, the zero sequence capacitive current of fault line will be far greater than that of non fault line, The zero sequence current can be used to judge the fault line. At the same time, the difference of zero sequence capacitance current direction 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 power direction of fault line is from line to bus, And there is no dead time in the direction of zero sequence power. If the zero sequence power direction of all lines is positive, it is judged that the bus is grounded.