Extending methods to determine the life remaining of transformer paper insulation

Efforts have been underway to estimate the life remaining of the paper insulation used in power transformers, and thus estimate the remaining life of the whole unit. However, one problem encountered when estimating remaining life is that the current state of the paper is not often exactly known. One method to determine the condition of paper is to measure the furan content of the oil to estimate the degree of polymerization of the paper. However, one problem with this technique is that paper does not age uniformly within a transformer. In particular, paper usually degrades fastest at the top of the winding where the temperature is highest. We therefore set out to determine whether a correlation exists between the condition of the paper at the top of the winding and that inferred by the furan content of the oil. Samples of paper from sixteen decommissioned transformers were used to investigate the rates at which paper degrades at the top of the winding. These results imply that the rate of ageing of paper in this location did not exceed double that calculated from using oil furan content.