How to Diagnose and Eliminate Transformer Core Grounding Faults

1. Elimination of Multi-Point Grounding Faults

(1) Temporary Remedial Measures When the Transformer Cannot Be Shut Down:
① If there is an external grounding lead and the fault current is relatively high, the ground wire can be temporarily disconnected to allow continued operation. However, enhanced monitoring is mandatory to prevent the iron core from developing a floating potential once the fault point disappears.
② If the multi-point grounding fault is unstable, a rheostat (sliding resistor) can be connected in series with the working ground wire to limit the current to below 1A. The rheostat should be selected by dividing the measured voltage across the open working ground wire by the current flowing through it.
③ Dissolved Gas Analysis (DGA) must be used to monitor the gas generation rate at the fault point.
④ Once the exact fault location is identified through measurement but cannot be immediately repaired, the normal core grounding plate can be relocated to the same position as the fault point. This will significantly reduce the circulating current.

(2) Thorough Maintenance Measures:
Once a multi-point grounding fault is detected, transformers that can be taken out of service should be shut down promptly so the fault can be completely eliminated. Specific maintenance actions should be tailored to the type and cause of the multi-point grounding. In some cases, no fault point can be found even after the transformer is de-energized and the core is lifted. To accurately locate the grounding point on-site, the following methods can be employed:
① DC Method: Disconnect the connection link between the iron core and the clamps. Apply 6V DC across the silicon steel sheets on both sides of the yoke. Use a DC voltmeter to sequentially measure the voltage between adjacent silicon steel sheets. When the voltage reads zero or the meter indicates a reverse 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, establishing magnetic flux within the iron core. With the connection link between the core and clamps opened, use a milliammeter to check for current. Measure point-by-point along each level of the yoke using the milliammeter; when the current reading drops to zero, that location is the fault point.

2. Abnormal Phenomena Caused by Multi-Point Grounding
(1) Eddy currents are generated within the iron core, increasing core losses and causing localized overheating.
(2) In severe cases of multi-point grounding left untreated for extended periods during continuous operation, the oil and windings will also overheat, accelerating the aging of oil-paper insulation. This causes the interlaminar insulation of the core laminations to age and peel off, leading to further core overheating and potentially burning out the iron core.
(3) Prolonged multi-point grounding degrades the insulating oil in oil-immersed transformers, generating combustible gases and triggering the Buchholz relay (gas relay).
(4) Overheating of the iron core causes carbonization of wooden spacer blocks and clamps inside the active part.
(5) Severe multi-point grounding can burn out the grounding wire, causing the transformer to lose its normal single-point ground, leading to catastrophic consequences.
(6) Multi-point grounding can also induce discharge phenomena.

3. Reasons Why the Iron Core Requires Single-Point Grounding Under Normal Conditions:
During normal operation, an electric field exists between the energized windings and the oil tank. The iron core and other metallic components reside within this electric field. Due to uneven capacitance distribution, the electric field strength varies across different areas. If the iron core is not reliably grounded, charging and discharging phenomena will occur, damaging solid insulation and reducing the dielectric strength of the oil. Therefore, the iron core must have a reliable single-point ground.

Transformer cores are composed of silicon steel sheets. To minimize eddy currents, there is a certain level of insulation resistance between the sheets (typically ranging from just a few ohms to several tens of ohms). Because the interlaminar capacitance is extremely high, these gaps act as electrical pathways under an alternating electric field. Thus, grounding the core at only one point is sufficient to clamp the potential of the entire stacked assembly to ground potential.

If the iron core or its metallic components are grounded at two or more points, a closed loop forms between the grounding points. This loop links part of the magnetic flux, inducing an electromotive force (EMF) and creating a circulating current, which leads to localized overheating and may even burn out the iron core. A single-point ground is the only acceptable and normal grounding configuration for a transformer core. In short, the iron core must be grounded, and strictly at one point only.

Core faults are primarily caused by two factors: poor construction techniques resulting in short circuits, and multi-point grounding triggered by accessories or external environmental factors.