Study on the Influence of Natural Ester on Thermal Ageing Characteristics of Oil-paper in Power Transformer

The cellulosic insulation used in electrical transformers were aged in mineral oil and natural ester respectively to investigate the influence of oil type on thermal ageing properties of oil-paper. The accelerated ageing test was developed at three different temperatures and degree polymerization (DP) of insulation paper was measured to indicate the ageing extent of paper. Furfural concentration CF and water concentration Cw in oil were measured during the ageing processes. The kinetic models of cellulose degradation in mineral oil and natural ester were studied. The correlations between DP and furfural and water concentration were also included in our investigation. The results indicate that paper aged in vegetable oil degraded at a significantly faster rate than in natural. A leveling off degree polymerization (LODP) was observed during thermal ageing process. Once LODP was reached, the ageing rate became very slow. Comparing with mineral impregnated paper, the natural ester impregnated paper has a larger LODP, so that a relative longer time is needed to reach its end-of-life (DPsime250). In addition, the magnitude of degradation byproducts of insulation paper aged in natural ester was very difference with that aged in conventional transformer oil, thus special attention must be paid when applying the conventional ageing diagnosis criteria to the composite insulation impregnated by natural ester.