Thermal Failure -Overheating
Transformer losses produce heat:
. Core loss
. Winding copper loss
. Stray losses
Dielectric losses
If heat removal is insufficient, the temperature rises.
Common causes:
. Overloading·
Cooling fan failure.
Oil pump failure.
Blocked oil passages·
Radiator obstruction.
High ambient temperature.
Poor oil circulation Internal hot spots
Failure sequence:
High load - higher losses - higher temperature - acceleratedinsulation ageing - reduced mechanical strength - insulationbreakdown
A useful engineering principle is:
Overheating does not merely damage the transformer immediately;
it accelerates the ageing of the insulation system.A low-temperature thermal fault may be associated with overload orrestricted oil flow, while higher-temperature faults can result fromdefective contacts, circulating currents, and severe localized heating.The CEA manual uses DGA thermal-fault categories correspondingapproximately to below 300°C, 300-700°C, and above 700°C fordiagnostic classification.
Important correction:
These DGA temperature ranges arediagnostic fault categories, not the normal operating temperature oftransformer oil or winding insulation.