Determination of Thermal Life Expectancy of Overhead Distribution Transformers

The calculated thermal life expectancy of distribution transformers is used by the Southern California Edison Company as part of an evaluation program to develop a comparative basis for the economic purchase of annual transformer requirements. The test procedure utilized in accelerating the thermal aging of the transformers, the pre-test measurements, the test measurements, and the mathematical derivation of the transformer aging rate as demonstrated by its performance on the accelerated test are described. In addition to the test procedure, a simulated residential loading schedule has been developed and is used to calculate the expected life of each test transformer. A tabulation is used to demonstrate how the calculations are made and to provide a comparison of the life expectancy of two typical transformers.