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

Heat, water and oxygen accelerate the degradation of cellulose insulation in electrical transformers. Their effects on insulation have been studied over 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, on ageing in oil in a three-parameter, two-level, partial factorial experiment. Ageing was measured in terms of change of degree of polymerisation of the paper and analysed according to recently developed models. In addition, the concentrations of furfural-based degradation products in the oil were measured. Water and temperature are most effective in accelerating ageing, with oxygen about one-third as efficient. There is a strong synergistic effect between temperature and water, a weaker synergism between temperature and oxygen, but, importantly, an apparent antagonistic effect between water and oxygen, at low water levels. If correct, the antagonism implies that the effectiveness of water, as an accelerator of ageing, decreases with increasing levels of oxygen in the oil at low water levels, which could explain why the insulation in scrapped transformers is sometimes found to be in very good condition.