Transformer core grounding method

There are three methods for grounding the transformer core:

① Both the core and the metallic structural components are reliably grounded through the oil tank.

② The core is reliably grounded via a bushing led out from the top of the oil tank, while the structural components are reliably grounded through the oil tank.

③ Both the core and the structural components are reliably grounded via separate bushings led out from the top of the oil tank (with insulating paperboard laid between the core pedestal feet and the tank bottom).

The specific grounding method adopted for each transformer is determined by standards and the requirements of the operating department. Generally, the third method is typically used for high-voltage, large-capacity transformers; the second method for those with lower voltage and smaller capacity; and the first method for small-capacity, low-voltage units.

With the first grounding method, it is impossible to externally inspect the oil tank for multi-point grounding faults in the core. It can only be diagnosed through long-term no-load operation combined with dissolved gas analysis (DGA) of the oil.

With the second grounding method, the insulation resistance between the core and the oil tank can be measured via the grounding bushing lead (after disconnecting the grounding wire) to determine if there is multi-point grounding in the core. However, it cannot determine whether multi-point grounding exists between the core and the structural components.

With the third grounding method, the insulation resistance between the core and the structural components (as well as to the oil tank) can be measured separately via their respective grounding bushing leads. This allows for the detection of any electrical connection (short circuit) between the core and the steel clamps.