Behaviour of moist oil-impregnated paper under electric stress

The results of microscopic examination of wet paper impregnated with transformer oil and subjected to electric stress are described, together with the results of life tests upon impregnated paper samples of moisture content 1¿8% by weight, in the temperature range 20¿80° C. The times to breakdown and discharge inception have been shown to vary between a few seconds and several hundreds of hours, and to depend upon the moisture content of the samples, test temperatures, applied stress and nature of the impregnant. Measurable gas evolution from the sample has been monitored in a specially constructed cell and shown to be preceded by discharge inception. The experimentally determined discharge-inception criterion is expressed as a function of the measurable electrical properties of the samples. This criterion has been shown to correspond approximately to that derived theoretically, assuming electrochemical genesis of the initial discharge cavity, and taking account of solution losses at individual fibre ends. The practical implications of the results obtained when applied to factory testing of transformers are briefly discussed.