Degradation of cellulosic insulationin power transformers.Part 4: Effects of ageing on the tensile strength of paper

Evidence for transformer failures related to insulation failure indicates that the primary cause is normally mechanical failure/loss of integrity due to loss of mechanical strength as a result of degradation. The paper investigates the primary causes of loss of strength of paper during ageing under accelerated conditions in insulating oil. The latest mathematical models are used to relate change of tensile strength to ageing time and to degree of polymerisation (DP). Comparison of measurements using wide-span and zero-span tensometers suggests that the primary loss of strength results from loss of fibre strength, but that failure ultimately occurs due to loss of inter-fibre strength. This remains constant until a DP of about 200, then rapidly falls to zero, at the same time as the furan levels in the oil increase. It is suggested that a better understanding of the statistical probability of loss of inter-fibre strength would provide a better end-of-life criterion for predicting insulation life than those currently used.