Degradation of cellulosic insulation in power transformers. Part 3: Effects of oxygen and water on ageing in oil

Heat, water and oxygen accelerate the degradation of cellulose insulation i n electrical transformers. Their effects on insulation have been studied ov er a number of years using degree of polymerisation (DP) of the paper as a measure of ageing. Results are reported of a systemic study to measure the relative effectiveness of each component individually and in combination, o n ageing in oil in a three-parameter, two-level, partial factorial experime nt. Ageing was measured in terms of change of degree of polymerisation of t he paper and analysed according to recently developed models. In addition, the concentrations of furfural-based degradation products in the oil were m easured. Water and temperature are most effective in accelerating ageing, w ith oxygen about one-third as efficient. There is a strong synergistic effe ct between temperature and water, a weaker synergism between temperature an d oxygen, but, importantly, an apparent antagonistic effect between water a nd oxygen, at low water levels. If correct, the antagonism implies that the effectiveness of water, as an accelerator of ageing decreases with increas ing levels of oxygen in the oil at low water levels, which could explain wh y the insulation in scrapped transformers is sometimes found to be in very good condition.