Test transformers often experience faults during on-site testing, and a common fault is caused by core short-circuit. The main causes of core short-circuit are:
(1) There are conductive suspended substances inside the transformer, which form conductive small bridges under the action of electromagnetic fields, causing the core to short-circuit with the tank wall or the bottom of the tank.
(2) When manufacturing transformers or replacing the core during overhaul, the quality of the selected silicon steel sheets may be problematic. For example, the surface of the silicon steel sheets is rough and not smooth; the insulating paint film coated on hot-rolled silicon steel sheets may peel off; the insulating oxide film of cold-rolled silicon steel sheets has poor adhesion and may also peel off. The above situations can cause inter-laminar short-circuits, leading to multi-point grounding.
(3) During the manufacturing process of the transformer tank and radiator, welding slag and other debris are not thoroughly cleaned. During long-term forced oil circulation, they are gradually carried out by the oil flow, short-circuiting the core and the tank wall.
(4) The core processing technology is unreasonable. For example, burrs exceed the standard, the sheets are not placed flat during cutting, and fine metal particles or hard non-metallic particles are mixed in, pressing small pits on one side of the laminations and small protrusions on the other side. After stacking, the insulation layer is also damaged, causing inter-laminar short-circuits.
(5) Improper lamination. The lamination factor is set too high, causing excessive pressure and damaging the inter-laminar insulation.
(6) Improper operation and maintenance. Long-term operation of the transformer beyond the nameplate capacity causes aging of the inter-laminar insulation; insufficient routine inspection and detection lead to severe local overheating of the core, damaging the inter-laminar insulation and causing multi-point grounding. In addition, during manufacturing or overhaul, broken strands of steel brush wires, steel wire ropes used for lifting, and tiny metal wires may stand up under the action of electromagnetic fields, causing short-circuits between the core and the bottom of the tank.
(7) Water ingress into the transformer, causing the insulation pads at the bottom of the core to become damp or the insulation of the through-core bolts to be damaged, leading to a sharp decline in core insulation and resulting in multi-point grounding.
