A review of paper aging in power transformers

As part of a comprehensive study of transformer behaviour, the factors which control the aging processes in transformer paper have been reviewed experimentally. Three different types of paper have been used: 60/40 manilla/wood, all-wood and thermally upgraded paper. Aging has been done in vitro under oil in atmospheres of nitrogen and air with controlled water contents. The aging temperatures were in the range 110-140°C with aging periods up to 1.5 years. Paper condition was monitored by measuring its degree of polymerisation (DP) checked by burst strength. In most of the experiments normal BS 148 oil was used, but some experiments were done to examine the effect of oil preservatives on paper aging. Opportunity was taken to compare the furfuraldehyde contents of oils from the aging cells with paper condition. It is confirmed that paper aging is highly dependent on temperature and the presence of water and oxygen, although the aging of the thermally upgraded paper examined is virtually unaffected by water and only slightly by oxygen.