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

Leon

08/02/2025

Hazards of Multi-point Core Ground Faults

A transformer core must not have multi-point grounding during normal operation. An alternating magnetic field around the winding induces parasitic capacitances between windings, core, and shell. Live windings couple through these capacitances, creating a core floating potential relative to ground. Uneven distances between components cause potential differences; when high enough, they spark. This intermittent discharge degrades transformer oil and solid insulation over time.

To prevent this, the core and shell are reliably connected to share the same potential. However, two or more core/metal component ground points form a closed loop, causing circulation and local overheating. This decomposes oil, reduces insulation performance, and in severe cases, burns core silicon steel sheets—leading to major main transformer accidents. Thus, main transformer cores must use single-point grounding.

Causes of Core Ground Faults

Key causes include: grounding plate short circuits from poor construction/design; multi-point grounding from accessories or external factors; metal debris left in the transformer; and burrs, rust, or welding slag due to shoddy core processing.

Types of Core Failures

Six common types:

Core contact with shell/clamp: e.g., unremoved transport nails, clamp limb contact with core columns, warped silicon steel sheets touching clamps, fallen cardboard between clamp feet and iron yoke, or overlong thermometer seats.
Overlong core bolt steel covers shorting with silicon steel sheets.
Tank foreign objects causing partial short circuits (e.g., a screwdriver in a 31500/110 transformer, a 120mm copper wire in a 60000/220 unit).
Damp/damaged insulation (sludge, moisture, degraded clamp/pad/iron box insulation) leading to high-resistance multi-point grounding.
Worn submersible pump bearings releasing metal powder; electromagnetic attraction forms a bridge between lower rails, pads, or tank bottom, causing multi-point grounding.
Inadequate operation, maintenance, and lack of scheduled checks.

Topics

Power Systems Electric transformer

Leon

Hello! I'm Leon, with 20 years of extensive experience in the power industry. My expertise spans from power distribution equipment to grid operations and maintenance, grounded in robust hands-on and theoretical knowledge. Currently, I focus on sharing insights into electrical equipment fault diagnosis, demystifying complex technical challenges through accessible explanations. I welcome collaboration with industry peers and enthusiasts to collectively explore the intricacies of the power sector.