What are the reasons for transformer overheating?

Transformers can get heated due to a variety of reasons.  Excessive heating of the transformer increases transformer losses, weakens the insulation and may result in reduced transformer life.  The principal reasons for transformer overheating are

Overload

Excessive loading can cause overheating.  Transformers are rated in kVA.  This kVA rating is dependent on temperature.  Some transformers have two kVA ratings.  One rating is for below a specific ambient temperature, say 45 degrees while another rating is above the ambient temperature.

Excess current in the neutral of the transformer

Excess current in the neutral of the transformer is usually caused by high zero sequence harmonic components.  This can be resolved by proper grounding usually through a zig zag transformer (See article).

Problems in the Cooling system

Malfunctioning of the cooling equipment such as blockages in the cooling oil circuit.  Failure of the cooling fans to operate in the case of Forced Air cooling systems.

High Harmonic content in the power supply

High harmonic loads can also cause heating in the transformer.  These loads need to be identified and suitable remedial measures such as harmonic filters can be implemented.

Sustained Overvoltages

Overvoltages which exist for a long period of time can overexcite the transformer and cause overheating.

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