Aging Evaluation of Dry-Type Transformer Insulating Systems [includes discussion]

The aging performance of dry-type transformer insulating systems is shown to be dependent not only on the materials used but to a large extent on the gas medium in which they are operated. This gas medium may retard or accelerate aging because of its chemical reaction with the base materials and their impregnants. Data are presented which show the effect of gas mediums such as air and nitrogen. The classification of insulations by chemical content with the designations A, B, and H are shown to be no longer significant; the assignment of a temperature limit to an insulating system is to be preferred. Also, the aging effect of periodically introducing specified quantities of dry air or high humidity air into insulating systems operated in a nitrogen atmosphere is shown to have a very minor influence on insulation aging. For the first time, extensive data are presented on silicone insulations subjected to thermal aging and periodically tested at high dielectric stresses. Previous data on thermal aging of silicones have been limited to tests on motors which for the most part operate at low dielectric stresses.