Gelling behaviour of natural ester transformer liquid under thermal ageing

There have been increasing interests on natural esters as potential substitutes to conventional mineral oils in transformers, due to their less-flammable, non-toxic and environmentally friendly properties. The application has been successful in traction and distribution transformers with sealed type. But the susceptibility to oxidation and hence the suspected gelling behaviour prevent the application of natural esters in free-breathing transformers. This paper presents an experimental study of gelling/oxidation behaviour of thin films of a natural ester under thermal ageing. Ageing experiments were carried out with presence of air at different temperatures of 120 °C and 20 °C and durations up to 2 years. The gelling/oxidation behaviour of natural ester was evaluated in terms of changes in viscosity and acidity. Results showed that both viscosity and acidity can be used to assess gelling/oxidation behaviour of natural ester from initial liquid phase to eventually gelled form. In addition to temperature, the thickness of the thin film samples has a significant effect on the gelling speed, i.e. the thinner of the film, the faster of the gelling speed.