An example of hidden DC ground fault detection (via contact grounding)

First, the phenomenon of failure
A 10kV busbar section was repaired and repaired. After the maintenance was completed, the low-voltage plant-use variable load switch was pushed into the restoration process. A secondary plug was inserted. After sending 110V DC control power, a DC grounding alarm was issued (positive earth resistance was 0, positive bus voltage to ground). 0).
The 10kV switchgear uses Shanghai General Electric's complete set of VB2-12 vacuum circuit breakers and is equipped with Beijing Sifang CSC-241C digital plant (owned) variable protection monitoring and control device.
Unplug the secondary plug, send 110V control power, no grounding, 110V DC power supply are 999 positive and negative insulation.
OK: The ground resistance is 0 and it should be dead ground .
Second, fault finding
1. Initial judgment is that the internal secondary wire of the switch is grounded, but the use of a 1000V oscillating meter is to shake and insulate all the secondary plugs of the secondary plug. There is no grounding phenomenon.
2. Then suspect the grounding point in the wiring outside the switch, use a multimeter to measure the resistance of the positive and negative power supply parts of the cabinet to the ground ∞, use the 1000V meter to measure the positive and negative power supply ground insulation of the cabinet is about 0.5MΩ.
3, re-into the switch cabinet, plug the secondary plug, the ground resistance is zero, replace the other interval switch, the same is 0.
4, so determine the grounding point outside the switch wiring, but the external wiring power supply insulation is 1000M 1000M rocking table 0.5MΩ (do not match the dead ground phenomenon).
5. Turn off the positive and negative power lines on the secondary terminals of the switchgear cabinet and shake them one by one. It is found that the CSC-241C comprehensive protection device has a negative common terminal of 0.5MΩ, and other circuits are normal. In order to avoid interference with the entire loop search by opening the negative terminal, the negative common terminal opened is thrown away, the secondary plug is reinserted, and the connection is restored, and the insulation resistance is still 0 for the positive ground.
6. Keep the plug of the switchgear cabinet plugged in, turn off all the positive power circuits, and check the insulation one by one. Find that the line resistance to the secondary plug C12 is 0, pull the secondary plug out and check from the terminal to the secondary. The entire plug is complete and well insulated.
(Question: Where is the grounding point?)
7. Check that C12 is a spring-less energy storage signal. The switch has not yet stored energy when it was inserted. It is suspected that this signal communicates with the opposite end of line D12. Finally, check that D12 has been worn out through the perforation at the upper part of the switch cabinet to the middle of the switch cabinet. .
8. Why didn't the switch check all power circuits? There is also a 0.5MΩ? As shown below:
Equivalent to the negative segment resistance grounded by the diode string, so the measurement is not dead ground, but the "spring is not stored" signal communicates positive ground.

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