How to Judge, Detect and Troubleshoot Transformer Core Faults

1.1 Hazards of Multi-Point Grounding Faults in the Core

Under normal operation, a transformer core must be grounded at only one point. During operation, alternating magnetic fields surround the windings. Due to electromagnetic induction, parasitic capacitances exist between the high-voltage and low-voltage windings, between the low-voltage winding and the core, and between the core and the tank. The energized windings couple through these parasitic capacitances, causing the core to develop a floating potential relative to ground. Since the distances between the core (and other metal parts) and the windings are unequal, potential differences arise among components. When the potential difference between two points exceeds the dielectric strength of the insulation between them, spark discharges occur. These discharges are intermittent and, over time, degrade both the transformer oil and solid insulation.

1.2 Causes of Core Grounding Faults
Common causes include:

• Short circuits due to poor construction techniques or design flaws in grounding straps;
• Multi-point grounding caused by accessories or external factors;
• Metallic foreign objects left inside the transformer during assembly, or burrs, rust, and welding slag from poor core manufacturing processes.

1.3 Types of Core Faults
Common types of transformer core faults include the following six categories:

• Core contacting tank or clamping structures:
  During installation, transport bolts on the tank cover may not be flipped or removed, causing the core to touch the tank. Other instances include clamping limb plates contacting core limbs, warped silicon steel sheets touching clamping plates, fallen paper insulation between lower clamp feet and yoke allowing contact with laminations, or overly long thermometer bushings contacting clamps, yokes, or core columns.Excessively long steel sleeves on through-core bolts shorting to silicon steel sheets.
• Foreign objects in the tank causing localized short circuits in the core:For example, a 31,500/110 kV power transformer at a substation in Shanxi was found to have a screwdriver handle lodged between the clamp and yoke during hood lifting. Another 60,000/220 kV transformer was found to contain a 120 mm copper wire.
• Moisture or damage to core insulation:Accumulated sludge and moisture at the bottom reduce insulation resistance. Deterioration or moisture ingress in clamp insulation, footpad insulation, or core box insulation (paperboard or wood blocks) can lead to high-resistance multi-point grounding.
• Worn bearings in oil-immersed pumps:Metallic particles enter the tank, settle at the bottom, and—under electromagnetic forces—form conductive bridges between the lower core yoke and footpads or tank bottom, causing multi-point grounding.
• Poor operation and maintenance, such as failure to perform scheduled inspections.

2.1Testing Methods for Core Faults

• If resistance is 200–400 Ω: high-resistance grounding exists; the transformer requires repair.
• If resistance >1,000 Ω: grounding current is small and hard to eliminate; the unit may continue operating with periodic online monitoring (clamp meter or DGA).
• If resistance is 1–2 Ω: metallic grounding is confirmed; immediate corrective action is mandatory.

2.2 Treatment Methods for Multi-Point Grounding

• For transformers with external core grounding leads, a resistor can be inserted in series in the grounding circuit to limit fault current—this is only an emergency temporary measure.
• If the fault is caused by metallic foreign objects, hood lifting inspection usually identifies the issue.
• For faults caused by burrs or accumulated metal powder, effective remediation methods include capacitor discharge impulse, AC arc, or high-current impulse techniques.

• The core shall be flat, with intact insulation coating, tightly stacked laminations, and no翘起 (lifting) or waviness at edges. Surfaces must be free of oil residue and contaminants; no inter-lamination short circuits or bridging; joint gaps must meet specifications.
• The core must maintain good insulation from upper/lower clamps, square irons, pressure plates, and base plates.
• A uniform and visible gap must exist between steel pressure plates and the core. Insulating pressure plates must be intact—without cracks or damage—and properly tightened.
• Steel pressure plates must not form a closed loop and must have exactly one grounding point.
• After disconnecting the link between the upper clamp and core, and between the steel pressure plate and upper clamp, measure insulation resistance between core/clamps and core/pressure plates. Results should show no significant change compared to historical data.
• Bolts must be tight; positive/negative pressure studs and locking nuts on clamps must be secure, in good contact with insulating washers, and show no signs of discharge or burning. Negative studs must maintain sufficient clearance from the upper clamp.
• Through-core bolts must be tight, with insulation resistance consistent with historical test results.
• Oil passages must be unobstructed; oil duct spacers must be neatly arranged, without falling off or blocking flow.
• The core must have only one grounding point. The grounding strap shall be made of purple copper, 0.5 mm thick and ≥30 mm wide, inserted into 3–4 core laminations. For large transformers, insertion depth must be ≥80 mm. Exposed portions must be insulated to prevent core shorting.
• The grounding structure must be mechanically robust, well-insulated, non-looping, and not in contact with the core.
• Insulation must be sound, and grounding reliable.