Hazards, causes, and types of multi-point ground faults in transformer core

Hazards of Multi-Point Grounding Faults in Transformer Cores

During normal operation, transformer cores must not be grounded at multiple points. The windings of an operating transformer are surrounded by an alternating magnetic field. Due to electromagnetic induction, stray 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 stray capacitances, causing the core to develop a floating potential relative to ground. Because the distances between the core, other metallic components, and the windings are unequal, potential differences arise among these components. When the potential difference between two points reaches a level sufficient to break down the insulation between them, intermittent spark discharges occur. These discharges can gradually degrade the transformer oil and solid insulation over time.

To eliminate this phenomenon, the core and tank are reliably bonded to maintain the same electrical potential. However, if the core or other metallic components become grounded at two or more points, a closed loop is formed between the grounding points, causing circulating currents. This leads to localized overheating, decomposition of insulating oil, and degradation of insulation performance. In severe cases, the core's silicon steel laminations can be burned out, resulting in a major failure of the main transformer. Therefore, the core of the main transformer must be, and can only be, grounded at a single point.

Causes of Core Grounding Faults

Transformer core grounding faults mainly include: short circuits of the grounding plate due to poor construction techniques or design; multi-point grounding caused by accessories or external factors; and grounding caused by metallic foreign objects (such as burrs, rust, welding slag) left inside the main transformer or by deficiencies in core manufacturing processes.

Types of Core Failures

There are six common types of transformer core failures:

• Core contacting the tank or clamping structure. During installation, due to oversight, transport positioning pins on the tank cover were not flipped or removed, causing the core to contact the tank shell; clamping structure limbs touching the core column; warped silicon steel laminations touching the clamping limbs; insulating cardboard between the core feet and yoke falling off, causing the feet to contact the laminations; thermometer housing being too long and contacting the clamping structure, yoke, or core column, etc.
• The steel bushing of the core bolt is too long, causing a short circuit with the silicon steel laminations.
• Foreign objects in the tank cause local short circuits in the silicon steel laminations. For example, in a 31500/110 power transformer at a substation in Shanxi, multi-point core grounding was discovered, and a screwdriver with a plastic handle was found between the clamp and the yoke; in another substation, a 60000/220 power transformer was found, during a cover-lifting inspection, to have a 120mm-long copper wire.
• Core insulation is damp or damaged, such as sludge and moisture accumulating at the bottom, reducing insulation resistance; damp or damaged insulation of clamps, support pads, or tank insulation (cardboard or wooden blocks), resulting in high-resistance multi-point grounding of the core.
• Bearings of submersible oil pumps wear out, allowing metal powder to enter the tank and accumulate at the bottom. Under electromagnetic attraction, this powder forms a conductive bridge connecting the lower yoke to the support pads or tank bottom, causing multi-point grounding.
• Poor operation and maintenance, with no scheduled maintenance performed.