How to detect and eliminate a transformer core grounding fault?

1. Elimination of Multi-Point Grounding Faults

(1) Temporary mitigation 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 grounding wire can be temporarily disconnected. However, monitoring must be intensified to prevent the core from developing a floating potential if the fault point disappears.

② If the multi-point grounding fault is intermittent or unstable, a slide-wire resistor can be connected in series with the working grounding wire to limit the current to below 1 A. The slide-wire resistor should be selected by dividing the voltage measured across the open working grounding 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, if it cannot be repaired immediately, the normal core grounding strap can be relocated to the same position as the fault point. This will significantly reduce the circulating current.

(2) Thorough maintenance measures. Once monitoring indicates a multi-point grounding fault, transformers that can be taken out of service should be de-energized promptly so the fault can be completely eliminated. Depending on the type and cause of the multi-point grounding, appropriate maintenance measures should be taken. However, in some cases, the fault point cannot be found even after the transformer is de-energized and the core is lifted. In such situations, the following methods can be used on-site to accurately locate the grounding point:

① DC Method: Disconnect the grounding strap between the core and the clamping structure. Apply a 6 V DC voltage across the silicon steel sheets on both sides of the yoke. Then, use a DC voltmeter to measure the voltage between adjacent silicon steel sheets sequentially. When the voltage reads zero or the meter indicates a reverse polarity, that location can be considered the fault grounding point.

② AC Method: Connect the low-voltage winding of the transformer to an AC voltage source of 220–380 V to establish magnetic flux in the core. If a multi-point grounding fault exists, a current will be detected by a milliammeter (provided the grounding strap between the core and the clamping structure is disconnected). Measure point by point along the various levels of the core yoke using the milliammeter. When the milliammeter reads zero, that location is the fault point.

2. Abnormal Phenomena Caused by Multi-Point Grounding

(1) Eddy currents are generated within the core, leading to increased iron losses and localized overheating of the core.

(2) If severe multi-point grounding remains unresolved for an extended period, continuous operation will cause the oil and windings to overheat, gradually aging the oil-paper insulation. This can cause the insulation coating between core laminations to age and flake off, resulting in even greater core overheating and potentially burning out the core.

(3) Prolonged multi-point grounding degrades the oil in oil-immersed transformers, generating combustible gases and triggering the gas relay.

(4) Core overheating can cause the carbonization of wooden spacer blocks and clamping structures 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 cause discharge phenomena.

3. Reasons Why the Core Requires Single-Point Grounding During Normal Operation:

During normal operation, an electric field exists between the energized windings and the transformer tank, with the core and other metallic components situated within this field. Due to uneven capacitance distribution and varying field strengths, if the core is not reliably grounded, charging and discharging phenomena will occur. This can degrade the solid insulation and reduce the dielectric strength of the oil; therefore, the core must have a reliable single-point ground.

The core is composed of silicon steel sheets. To minimize eddy currents, there is a certain level of insulation resistance between the sheets (typically ranging from a few ohms to several tens of ohms). Because the inter-sheet capacitance is extremely large, it can be considered a conductive path in an alternating electric field. Thus, grounding the core at a single point is sufficient to clamp the potential of the entire stacked core laminations to ground potential.

If the core or its metallic components are grounded at two or more points, a closed loop will be formed between the grounding points. This loop links a portion of the magnetic flux, inducing an electromotive force (EMF) and creating a circulating current. This leads to localized overheating and can even burn out the core.

A single-point ground is the only acceptable and normal grounding condition for a transformer core. In other words, the core must be grounded, and it must be grounded at exactly one point.

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