The cellulosic insulation used in electrical transformers was aged in mineral oil and natural ester to investigate the influence of oil type on the thermal aging properties of oil–paper. Accelerated aging tests were conducted at three different temperatures and the degree of polymerization (DP) of insulation paper was measured to indicate the extent of aging of the paper. Various aging characteristics in oil, i.e., furfural, CO, CO2, water concentration, and oil acidity, were measured during the aging processes. Kinetic models of cellulose degradation in mineral oil and natural ester were established. The correlations between DP and various aging by‐products were also included in our investigation. The results indicate that paper degraded at a significantly faster rate in mineral oil than in natural ester. A leveling off degree of polymerization (LODP) was observed during the thermal aging process. Once the LODP was reached, the aging rate became very slow. Paper impregnated with natural ester has a higher LODP than paper impregnated with mineral oil, so that a longer time is needed to reach its end‐of‐life (DP ≈ 250). In addition, the content of degradation by‐products of insulation paper aged in natural ester was very different from that aged in conventional transformer oil. Thus special attention must be paid when applying conventional diagnostic criteria to aging of composite insulation impregnated with natural ester.
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Yang et al. (2009) studied this question.
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