A Brief Analysis of Transformer Temperature Rise and Its Permissible Range

A transformer’s temperature rise is the difference between its temperature and the surrounding air temperature. Among the many factors that affect a transformer’s service life, temperature causes insulation aging and has the greatest impact on service life. Heat is not distributed evenly inside a transformer, so temperatures vary considerably from one part to another. For a transformer under rated load, the allowable temperature increase of each part is specified; this is the transformer’s permissible temperature rise.

Permissible Temperature-Rise Range

Oil-immersed transformers generally use Class A insulation, with a maximum permissible temperature of 105°C. The permissible temperature rise for each part is as follows: the permissible temperature rise of the winding is 65°C. Based on the Class A insulation limit of 105°C, when the ambient temperature is 40°C, 105°C − 40°C = 65°C. Since the transformer temperature is generally 10°C lower than the winding temperature, the permissible temperature rise of the transformer oil is 55°C. To prevent oil aging, the temperature rise of the top oil layer must not exceed 45°C. Regardless of changes in the surrounding air temperature, as long as the temperature rise does not exceed the permissible value, the transformer can operate safely throughout its specified service life.

The main insulation in a typical transformer is Class A, with a specified maximum operating temperature of 105 degrees. During operation, the winding temperature is 10–15 degrees higher than the top-oil temperature.

If the top-oil temperature of a transformer in service remains around 80–90 degrees, the windings are often around 95–105 degrees. Long-term operation at high temperatures shortens the life of the internal insulating pressboard, making it brittle and prone to cracking and loss of its intended insulating function, which can lead to breakdown and other accidents. Severe aging of the transformer winding insulation also accelerates deterioration of the insulating oil and affects the transformer’s service life. Therefore, operation at high temperatures should be avoided, especially for extended periods. The insulation class indicates the heat-resistance rating of the insulation materials used. Dry-type transformers are classified into seven insulation classes: (A, E, B, F, H, N, C).

Diagram of temperature-rise limits. The temperature-rise limit indicates the heat resistance of the insulation material. For example, A—105°C means that the transformer’s own temperature plus the ambient temperature on that day must not exceed 105°C during operation; the other classes follow the same principle. China’s national standard Dry-Type Transformers GB6450-1986 also specifies the temperature-rise limits for dry-type transformers.