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

Evidence for transformer failures related to insulation failure indicates t hat the primary cause is normally mechanical failure/loss of integrity due to loss of mechanical strength as a result of degradation. The paper invest igates the primary causes of loss of strength of paper during ageing under accelerated conditions in insulating oil. The latest mathematical models ar e used to relate change of tensile strength to ageing time and to degree of polymerisation (DP). Comparison of measurements using wide-span and zero-s pan tensometers suggests that the primary loss of strength results from los s of fibre strength, but that failure ultimately occurs due to loss of inte r-fibre: strength. This remains constant until a DP of about 200, then rapi dly 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 probabilit y of loss of inter-fibre strength would provide a better end-of-life criter ion for predicting insulation life than those currently used.