Internal Corona Testing of Casting Resins

The causes of corona in cast transformers are briefly discussed. Dissections of failed units and other observations have led to the conclusion that cracks along projecting margins of interlayer materials are sites of the most damaging corona. A test specimen is described which is intended to simulate corona in such cracks. Internal corona life data obtained with the new test specimen at 60 vpm and at test frequencies of 60 Hz and 1440 Hz are given for a number of formulations. Breakdown is shown to involve two or more phases, usually cavity erosion followed by "treeing". The 1440-Hz accelerated tests were found to give grossly overestimated 60-Hz lives for some materials; a possible explanation is presented. Test data are also included on the effectiveness of various barrier films in preventing internal breakdown.