Aging characterization of electrical insulation papers impregnated with synthetic ester and mineral oil: Correlations between mechanical properties, depolymerization and some chemical markers

This article presents a comparative study of two types of transformer solid insulation: standard Kraft and thermally upgraded Kraft papers. The paper samples were impregnated with two different fluids, namely a synthetic dielectric ester fluid, Midel 7131, and a synthesized mineral oil, Luminol Tri. The impregnated papers were submitted to, accelerated aging at 150°C, temperature higher than the temperatures found inside transformers, for extended periods. The results show that for the first hours of aging, the cellulose depolymerization was similar for all the four studied cases. At a specific point, the paper samples aged in the ester fluid exhibited a lower depolymerization exceeding the 8000 hours of aging without reaching the levelling-off degree of polymerization (LODP). A similar trend was observed through the decrease of mechanical properties of papers in the ester-based oil. However, the difference in performance of both fluids was amplified due to the temperature used; at lower temperatures the difference should be smaller. Additionally, a relationship between the degree of polymerization and the mechanical properties accessed by the tensile testing was obtained regardless of the type of paper or oil. The total acidity in the ester fluid was higher than in mineral oil. Methanol, a chemical marker that is closely linked to the rupture of 1,4-ß-glycosidic bonds of cellulose, showed a partial sensitivity to the cellulose aging in the ester fluid. Furan (C4H4O) content was monitored during the aging and its concentration was found to be sensitive to the depolymerization along with the decrease in the mechanical properties of cellulose papers regardless of the type of paper. More importantly, its concentration was found to be dependent on the type of oil. More investigations are needed to further confirm the importance of furan as an indirect indicator of the mechanical performance along with its correlation with the depolymerization of paper aged in ester fluids.