The small current grounding line selection device monitors the zero sequence voltage of each bus section and the zero sequence current of each outgoing line in real time. When the system has a grounding fault, the device calculates and analyzes the size and phase relationship between the zero sequence voltage at the PT opening triangle of the grounding bus and the zero sequence current of the outgoing line of the bus, and selects the line with the grounding fault according to the set grounding fault criteria; At the same time, relevant fault data shall be displayed, recorded and transmitted remotely to give an alarm signal, and then the cycle monitoring state shall be maintained.

Judging from the difference between the status information of single-phase grounding of small current system and that of normal operation, it seems very easy to determine the fault line, but this is not the case.

The main reasons are as follows: the current signal is too small, the interference is large, the signal-to-noise ratio is small, the influence of random factors is uncertain, and the capacitance current waveform is unstable.

The zero sequence current generated when a small current system is single-phase grounded is the system capacitance current. Its size is related to the size of the system and the type of line (cable or overhead line). The value is very small. After being compensated by the arc suppression coil connected to the neutral point, the value is smaller, and the compensation state of the arc suppression coil is different. The characteristics of the grounding fundamental wave capacitance current are opposite or the same as those when there is no arc suppression coil compensation, For the small current system with arc suppression coil, the 5th harmonic current or zero sequence current active power direction detection is adopted, and the 5th harmonic current is 20~50 times smaller than the zero sequence current.

The interference in small current system mainly includes two aspects: first, the electromagnetic interference is large at the installation site of single-phase grounding protection device in small current system of substation and power plant; Second, the zero sequence current and harmonic current caused by the unbalanced load current are large, especially when the system is small and the capacitive current to ground is small, the zero sequence current and harmonic current of the grounded circuit are even smaller than the corresponding current of the ungrounded circuit.

The distribution network in China is generally a small current system, and its operation mode changes frequently, resulting in frequent changes in the length and number of outgoing lines of substations, as well as the capacitive current and harmonic current; In addition, the bus voltage level, load current are always changing, and the grounding resistance of the fault point is uncertain. These all cause the instability of zero sequence fault capacitive current and zero sequence harmonic current.

The single-phase grounding fault of small current system is often intermittent unstable arc grounding, so the waveform of capacitive current is unstable, and the corresponding harmonic current is changing at any time.