Estimation of Endurance Coefficient of Oil Impregnated Kraft Papers using DEM

Kraft paper is one of the most commonly used insulation in transformers and cables. Impregnation with transformer grade mineral oil is done to improve the electrical properties of the paper. Voltage endurance coefficient (n) is used to estimate the performance characteristics of the insulation paper aging. In this paper, a comparative study is done to estimate the endurance coefficient using conventional V-t curve method and Damage Equalization Method (DEM). The endurance coefficient is also known as the power law coefficient which is used in the V-t curve method to characterize the insulation aging. The unknown power law coefficient is also obtained from known values of step durations and stresses for each sample till its breakdown under DEM. Various types of AC Breakdown tests were performed on cable insulation of different ages obtained from a local manufacturer.