How to judge whether the temperature change of the transformer is normal or abnormal?

1. How to Judge Whether the Temperature Change of a Transformer is Normal or Abnormal

During operation, losses in the transformer’s core and windings are converted into heat, causing temperature rise in various parts. This heat is dissipated through radiation, conduction, and other means. When heat generation and dissipation reach equilibrium, the temperature of each part stabilizes. Iron losses remain nearly constant, while copper losses vary with load.

When inspecting a transformer, record the ambient temperature, top oil temperature, load, and oil level, and compare these values with historical data to assess whether the transformer is operating normally.

If, under the same operating conditions, the oil temperature is more than 10°C higher than usual, or if the load remains constant but the temperature continues to rise despite normal operation of the cooling system, an internal fault may be present (and the possibility of thermometer error or malfunction should be checked).

Typically, the main insulation of a transformer (winding insulation) is Class A (paper-based), with a maximum allowable operating temperature of 105°C. The winding temperature is generally 10–15°C higher than the top oil temperature. For example, if the top oil temperature is 85°C, the winding temperature may reach 95–100°C.

2. Causes of Abnormal Transformer Temperature

(1)Internal Faults Leading to Abnormal Temperature

Internal faults such as inter-turn or inter-layer short circuits, discharge from windings to nearby shielding, overheating at internal lead connections, multi-point grounding of the core causing increased eddy currents and overheating, or stray flux from zero-sequence unbalanced current forming a loop with the tank and generating heat—can all cause abnormal temperature rise. Such faults are often accompanied by operation of gas or differential protection. In severe cases, the explosion-proof pipe or pressure relief device may discharge oil. In such situations, the transformer should be taken out of service for inspection.

(2)Abnormal Temperature Due to Cooler Malfunction

Abnormal temperature may result from improper operation or failure of the cooling system, such as submersible pump shutdown, fan damage, fouling in cooling pipes, reduced cooling efficiency, or failure of radiator valves to open. Timely maintenance or flushing of the cooling system should be performed, or a backup cooler should be activated. Otherwise, the transformer load must be reduced.

(3)Temperature Indicator Errors
If the temperature indication is inaccurate or the instrument is malfunctioning, the thermometer should be replaced.