Troubleshooting of multiple-grounding faults in transformer cores

(1) Temporary troubleshooting methods when the transformer cannot be taken out of service:

① If there is an external grounding wire and the fault current is relatively high, the ground connection can be temporarily opened for operation. However, enhanced monitoring is mandatory to prevent a floating potential from developing on the core if the fault point disappears.

② If the multiple-grounding fault is intermittent (unstable), a sliding rheostat can be connected in series with the operational grounding wire to limit the circulating current to below 1A. The required resistance value of the rheostat is determined by dividing the open-circuit voltage measured across the normal grounding wire by the current flowing through it.

③ Dissolved Gas Analysis (DGA) must be utilized to monitor the gas generation rate at the fault point.

④ Once the exact fault location is identified through measurements but cannot be immediately repaired, the normal core grounding strap can be relocated to the same position as the fault point. This will significantly reduce the circulating current.

(2) Comprehensive maintenance measures.
After monitoring indicates a multiple-grounding fault in a transformer, if the unit can be taken out of service, it should be shut down promptly to thoroughly eliminate the fault. The specific maintenance actions should be tailored to the type and root cause of the multiple-grounding issue. However, in some cases, the fault point may not be found even after de-energization and core inspection. To accurately locate the grounding point under these circumstances, the following field methods can be applied:

① DC Method: Disconnect the bonding strap between the core and the structural clamps. Apply a 6V DC supply across the silicon steel sheets on both sides of the yoke. Then, sequentially measure the voltage between each stage of silicon steel sheets using a DC voltmeter. When the voltage reads zero or the meter shows a reversed polarity, that specific location can be considered the fault grounding point.

② AC Method: Connect an AC voltage of 220–380V to the low-voltage winding of the transformer to establish magnetic flux within the core. Ensure the bonding strap between the core and the clamps is disconnected. If a multiple-grounding fault exists, a current will be detected when measuring with a milliammeter. Measure point-by-point along the stages of the iron yoke using the milliammeter; the location where the current drops to zero is the fault point.