Common Fault Types of Transformer Cores

1) Core touching the tank or clamping parts. After installation, due to negligence, the transport locking pins (or positioning nails) on the tank cover were not flipped back or removed, causing the core to touch the tank shell. Other causes include: core clamping limbs touching the core limbs/columns; warped silicon steel sheets touching the clamping limbs; cardboard spacers between the lower core clamp feet and the iron yoke falling off, causing the clamp feet to touch the silicon steel sheets; or the thermometer socket being too long and touching the clamps, yoke, or core column.

2) The steel sleeve of the through-core bolt being too long, short-circuiting the silicon steel sheets.

3) Foreign objects inside the oil tank causing local short circuits in the silicon steel sheets. For example, a 31500/110 power transformer at a substation in Shanxi experienced multi-point core grounding; upon tank removal inspection, a screwdriver was found between the clamp and the iron yoke. At another substation, a 120mm long copper wire was found during the inspection of a 60000/220 power transformer. In another case, a pot shard was found between the lower clamp and the iron yoke of a 120000/220 power transformer after tank removal. Additionally, a large power transformer at a substation in Northeast China suffered from multi-point core grounding; inspection revealed three pieces of zigzag steel wire at the bottom of the tank, with a diameter of about 0.31mm and lengths of 25mm, 28mm, and 31mm respectively.

4) Moisture or damage to core insulation. This includes the accumulation of oil sludge and moisture at the bottom, a drop in insulation resistance, and moisture or damage to the clamp insulation, shim insulation, and core box insulation (cardboard or wooden blocks), leading to high-resistance multi-point core grounding.

5) Wear and tear of oil pump bearings. Metal powder enters the oil tank, accumulates at the bottom, and forms a conductive bridge under electromagnetic attraction, connecting the lower core rail with the clamp foot or tank bottom, resulting in multi-point grounding.

6) Poor operation and maintenance, and failure to perform scheduled overhauls.