Life Testing of Electronic Power Transformers-Part II

This paper summarizes the results of a functional life testing program conducted over a period of several years on low voltage resin encapsulated electronic transformers. The program was designed to establish how much environmental factors, such as thermal cycling, humidity, vibration, cold shock, overvoltage, and method of heating the units, affected the life of the units in thermally accelerated aging tests. It was concluded that thermal cycling was the most important factor influencing the life, which was usually terminated by a turn or layer-to-layer short circuit. It was further concluded that oven heating gave very similar results to internal circulating current heating of the windings. An important result of these tests was that this system, which incorporated a cellulose paper layer insulation, usually considered 110°C class layer insulation proved to have life values corresponding to a 130 to 155°C class system. This reemphasizes that temperature ratings of apparatus with composite insulation should be made on the basis of system performance rather than individual material performance. The life of the transformers followed the Arrhenius log life versus reciprocal Kelvin temperature relation.