Thermal ageing assessment of synthetic ester and mineral oil for transformer applications

Synthetic ester (SE) insulating oil provides better fire safety features compared with conventional mineral oil (MO). However, the long‑term performance of SE under thermal stress is not yet fully understood, and therefore, in this study, the performance of SE and MO subjected to accelerated thermal ageing was compared. Accelerated thermal ageing tests were conducted using samples consisting of the insulating oil (MO or SE), cellulose-based materials (kraft paper and pressboard), and metal catalysts enclosed in sealed glass bottles. The samples were thermally aged at 130 °C for 250, 500, and 750 h. The physicochemical properties (moisture content, interfacial tension, and acidity) were evaluated for both SE and MO. The results showed that SE exhibited higher moisture content than MO throughout the ageing duration, indicating that SE more effectively extracted moisture from the kraft paper. Interfacial tension measurements revealed that SE maintained better stability compared with MO. In addition, SE demonstrated higher acidity that was consistent with ester hydrolysis, whereas MO maintained a relatively stable acidity profile.