Causes of transformer core grounding faults

The presence of metallic foreign objects at the bottom of the oil tank is one of the main causes of multi-point grounding in the transformer core. These objects are mostly left behind during manufacturing or major overhauls. Generally, there are four types of residual metallic foreign objects:

① Granular deposits (such as welding slag) adhere to the insulation between the core laminations and structural components, forming metallic multi-point connections.

② Larger metallic objects (such as welding rod stubs, iron filings, or broken ends of wire ropes) bridge the gap between the core laminations and structural components, creating a conductive path.

③ Iron wires or sheets fall between the lower yoke of the core and the tank bottom. While they rest at the bottom without causing multi-point grounding when the transformer is de-energized, the core's magnetism attracts these objects when energized, forming a conductive channel between the lower yoke surface and the tank bottom.

④ The insulation of the core bolt is damaged, or metallic foreign objects create a connection between the bolt and the steel clamp. Additionally, metallic objects bridging the gap between the upper surface of the lower yoke and the steel clamp can also cause multi-point grounding.