Evaluation of in-service oxidative stability and antioxidant additive consumption in corn oil based natural ester insulating fluid

The Rancimat method was used to evaluate the consumption of antioxidant additive in a natural ester insulating fluid (NEI). NEI samples were aged in the laboratory with and without oxygen bubbling. NEI samples were also collected from a transformer in operation. Samples of NEI (unused, aged in the laboratory and collected from transformer in operation) were analyzed by Fourier-transform infrared spectroscopy (FTIR) and by the Rancimat method for induction time (IT) measurement. The results showed that it is possible to evaluate the consumption of antioxidant additive in NEI in service by the Rancimat method. Laboratory tests were also performed to extend the lifetime of NEI in service by adding new loads of antioxidant additive. The results showed that the addition of new antioxidant additive loads when the IT of in-service NEI (determined by Rancimat method) reaches about 3 h extends the lifetime of this oil.