Installing residual current monitoring (protection) devices is an effective measure to prevent personal electric shock, electrical fires and damage to electrical equipment. Before installing residual current monitoring devices, it is first necessary to investigate the grounding mode of the station AC power supply system. To cooperate with the investigation work on the grounding mode of the station AC power supply system organized by the Equipment Department of State Grid Corporation of China, the DC Working Group and the Secondary Maintenance Team of Wuhan Haomai Zhengwei carried out a professional investigation on the 220kV substation in XX City.

Investigation Process

1. Check the drawings to clarify the wiring mode of the station low-voltage AC power supply system and each outgoing line.
2. Conduct on-site verification to ensure the correctness of the wiring mode, and grasp the power supply status of the neutral line busbar grounding, circuit breakers, contactors, switching devices, as well as the common neutral line circuit of dual power supplies.
3. Test the residual current of all outgoing lines to determine whether there is multi-point grounding in the system. If the measured residual current of the circuit is ＜150mA (for the common neutral line circuit of dual power supplies, disconnect the neutral line of the standby power supply circuit), it is considered that there is no repeated grounding of the neutral line.
4. If the measured residual current of the non-common neutral line circuit is ≥150mA (for the common neutral line circuit of dual power supplies, disconnect the neutral line of the standby power supply circuit), cut off the power supply of the outgoing line circuit pole by pole, and further verify whether there is repeated grounding of the neutral line of the circuit through testing the insulation of the neutral line to the ground.

Investigation of Grounding Mode

According to the drawings of the station AC power supply system, conduct on-site point-by-point verification of the grounding points of the station AC power supply system.

△ Verify Whether There is a Common Neutral Line in the Dual Power Supply Circuit

△ Common Neutral Line Without Grounding

For Single Power Supply, Dual Power Supply Without Common Neutral Line Circuits (and Outgoing Circuits After Switching of Dual Power Supply Common Neutral Line Circuits)

If the measured residual current of the circuit does not exceed 150mA, it can be considered that there is no repeated grounding of the neutral line, and the residual current during operation will not have an adverse impact on the residual current protection and monitoring.

If the measured residual current of the circuit reaches 150mA or more, cut off the power supply of the outgoing line circuit pole by pole, and further check whether the neutral line of the circuit cable and electrical equipment is repeatedly grounded through the neutral line to ground insulation test.

For Dual Power Supply Common Neutral Line Circuits

After outgoing from the Section I and Section II busbars of the central power distribution panel, a three-pole switching device is adopted in the load-side distribution box, and the neutral lines of the two power supplies share one neutral line busbar on the load side. The three-phase unbalanced current will shunt, resulting in the measured current value failing to truly reflect the actual residual current of the outgoing line, and the measured current is larger than the actual residual current. One power supply should be shut down and its neutral line should be disconnected before measurement. If the measured value is ＜150mA, it can be considered that there is no repeated grounding.

Investigation Results

Through the investigation, it was found that there are 14 groups of dual power supply circuits in the station AC power supply system of the substation, among which 12 groups have common neutral lines without grounding; the power supplies of No. 1 DC charging panel and No. 2 DC charging panel have no common neutral lines and no grounding, and the corresponding circuit breakers and switching device air switches are all three-pole.

This investigation clarified the operation status of the station AC system of the substation, laying a solid foundation for the next work such as the installation of residual current monitoring (protection) devices.