Measurements of void gas pressure during combined thermal and partial discharge ageing of epoxy

The combined effect of partial charges and temperature on epoxy has been investigated. The void gas pressure exhibited a characteristic development with a short initial fluctuation followed by a linear decrease. The slope of the linear pressure curve was a function of temperature level and voltage stress. During ageing, the initially occurring uniform distribution of large discharges, type I pattern, was replaced by a type II pattern and eventually a type III PD pattern, displaying a few large discharges followed by many small discharges, appeared. The type III pattern possibly indicates that some discharges have changed or are about to change into swarming pulsive microdischarges. No correlation could be found between the appearance of the type III pattern and the development of void gas pressure. In the linear region, the development of void gas pressure was unaffected by any changes in PD activity. Based on this result it is suggested that even if the PD pattern changes, the total contribution to insulation degradation remains the same and hence also the degradation rate as expressed by the continuous development of void gas pressure