This work reports on an experimental study of the aging behavior of thin films of different classes of natural ester dielectric liquids. Thin film samples of three different natural ester liquids were prepared for the investigations and the aging experiments were performed in a medium of dry air at different temperatures (23°C and 65°C) and durations (up to around 100 days). The degradation of the samples was then evaluated in terms of changes in the dynamic viscosity of the liquid. Further analyses were also performed on the aged liquids through visual inspection, Infrared (IR) spectroscopy analysis and Oxidative Induction Time (OIT) measurements. Results show that the aging behaviors of natural ester dielectric liquids may greatly vary depending on the base chemistry of the liquid. Based on these results, it is suggested that natural ester dielectric liquids be classified into different categories considering their oleic acid content.

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