Oxidation stability assessment of a vegetable transformer oil under thermal aging

Vegetable oil is one of the alternatives to replace mineral oils in transformers. This paper focuses on oxidation stability assessment of a vegetable transformer oil under accelerated thermal aging at 110°C with various durations up to 2500 hours. Oil samples were aged under open condition to stimulate the oxidation process, as well as under sealed condition to provide a comparison. A number of parameters including acidity, DC conductivity, dielectric dissipation factor (DDF) and viscosity were measured to see their responses to oxidation process of the vegetable oil. The results showed that viscosity responds towards oxidation more than hydrolysis whereas acidity responds strongly to both oxidation and hydrolysis, since besides oxidation, hydrolysis of the vegetable oil also generates acid byproducts. Of more importance, oxidation stability, in the term of oxidation induction time (OIT), of thermally aged vegetable oils was investigated by using Pressure Differential Scanning Calorimetry (PDSC) method. As there was no standard on PDSC method available for vegetable oils, various measuring temperatures were tried and 130°C was found to be suitable for the investigated vegetable oil. The results showed that OIT of open samples decreases obviously with aging time until reaching the bottom plateau at 1000 hours, while OIT of sealed samples stays at a high level until the end of aging at 2500 hours. Therefore, OIT measured by using PDSC is appropriate to reflect oxidation stability of the vegetable oil. This was further confirmed by the corresponding consumption of antioxidant and generation of peroxides, dissolved decay products.