How to Handle Transformer Core Grounding

1) Temporary Troubleshooting Methods (When the Transformer Cannot Be Shut Down):

① Temporarily disconnecting the ground wire: If an external grounding lead is present and the fault current is relatively large, the transformer can be operated with the ground wire temporarily disconnected. However, monitoring must be strengthened to prevent the core from developing a floating potential if the fault point disappears.

② Inserting a rheostat (sliding resistor): If the multi-point grounding fault is of the unstable type, a rheostat can be connected in series with the working ground wire to limit the current to below 1A. The appropriate resistance value is determined by dividing the measured voltage (obtained when the normal ground wire is opened) by the current on the ground wire.

③ Gas chromatography monitoring: Dissolved gas analysis (DGA) must be used to monitor the gas generation rate at the fault point.

④ Relocating the grounding shunt: Once the exact fault point is located through measurement but cannot be repaired immediately, the normal core grounding shunt can be relocated to the exact same position as the fault point to significantly reduce the circulating current.

(2) Thorough Maintenance Measures:
Once monitoring indicates a multi-point grounding fault, transformers that can be taken offline should be shut down promptly to completely eliminate the fault. The specific maintenance measures should be tailored to the type and cause of the multi-point grounding. However, in some cases, the fault point cannot be found even after the core is lifted during a power outage. To accurately locate the grounding point, the following methods can be used on-site:

① DC Method: Disconnect the connection strap between the core and the clamps, and apply 6V DC across the silicon steel sheets on both sides of the yoke. Then, use a DC voltmeter to measure the voltage between the silicon steel sheets at each stage sequentially. When the voltage reads zero or the meter indicates a reverse polarity, that specific location can be identified as the fault grounding point.

② AC Method: Connect the low-voltage winding of the transformer to a 220~380V AC power supply to establish magnetic flux in the core (ensure the connection strap between the core and clamps is opened). If a multi-point grounding fault exists, a current will be detected using a milliammeter. Move the milliammeter along the yoke at various stages for point-by-point measurement; when the current reads zero on the milliammeter, that location is the fault point.