Common Issues with Transformer Cores

I. Hazards, Causes and Types of Multi-point Grounding Faults in Transformer Core

1. Hazards of Multi-point Grounding Faults in Transformer Core

When a transformer is in normal operation, multi-point grounding of the core is not allowed. This is because the alternating magnetic field around the windings induces parasitic capacitances between the high-voltage and low-voltage windings, between the low-voltage windings and the core, and between the core and the casing. The charged windings will cause a floating potential on the core relative to the ground through the coupling effect of these parasitic capacitances. Since the distances between the core and other metal components from the windings are unequal, potential differences exist between these components. When the potential difference between two points is high enough to break down the insulation, spark discharge occurs. This discharge is intermittent, and over time, it adversely affects the transformer oil and solid insulation.

To eliminate this phenomenon, the core is reliably connected to the casing to make it equipotential with the casing. However, when the core or other metal components have two or more grounding points, a closed loop is formed, causing circulation current, leading to local overheating, decomposition of the oil, and degradation of insulation performance. In severe cases, the silicon steel sheets of the core may burn out, causing major accidents in the main transformer. Therefore, the core of the main transformer should only be grounded at one point.

2. Causes of Core Grounding Faults

Grounding faults in transformer cores mainly include:
(1)Short circuits of grounding plates caused by poor construction technology or design;
(2)Multi-point grounding caused by accessories or external factors;
(3)Grounding caused by metal foreign objects left inside the main transformer, as well as burrs, rust, welding slag, and other defects from poor core processing.

3. Types of Core Faults

Common fault types of transformer cores are as follows:
(1)Core touching the casing or clamping pieces. After installation, due to negligence, the transportation stabilizing (positioning) nails on the tank cover are not flipped over or removed, causing the core to touch the tank; the clamping piece limbs touch the core columns; warped silicon steel sheets touch the clamping piece limbs; the padding feet of the lower clamping piece of the core fall off from the cardboard between the yoke, making the padding feet touch the silicon steel sheets; the thermometer sleeve is too long and touches the clamping pieces, yoke, or core columns, etc.
(2)The steel bushing of the through-core bolt is too long, short-circuiting with the silicon steel sheets.
(3)Foreign objects in the tank cause local short circuits of silicon steel sheets. For example, in a 31500/110 kV power transformer in a substation in Shanxi, a multi-point grounding fault occurred in the core. When the cover was lifted, a screwdriver with a handle was found between the clamping piece and the yoke. In another 60000/220 kV power transformer in a substation, a 120 mm long copper wire was found after lifting the cover. In a 120000/220 kV power transformer in another substation, an iron block was found between the lower clamping piece and the yoke. Additionally, in a large power transformer in a substation in Northeast China with a core multi-point grounding fault, three segments of 曲折型 (zigzag) steel wires with a diameter of about 0.31 mm and lengths of 25 mm, 28 mm, and 31 mm respectively were found at the bottom of the tank after lifting the cover.
(4)Moisture or damage to core insulation, such as sedimentation of oil sludge and water at the bottom, decreased insulation resistance, and moisture or damage to the insulation of clamping pieces, padding irons, and iron boxes (cardboard or wood blocks), leading to high-resistance multi-point grounding of the core.
(5)Wear of the submersible pump bearings, causing metal powder to enter the tank and accumulate at the bottom. Under the action of electromagnetic attraction, a bridge is formed, connecting the lower rail with the padding feet or the tank bottom, resulting in multi-point grounding.
(6)Poor operation and maintenance, such as failing to carry out regular inspections.