Electrostatic charging tendency and correlation analysis of mineral insulation oils under thermal aging

In order to investigate the electrostatic charging tendency (ECT) of transformer oil under thermal aging and understand its correlations to the other insulation parameters, a series of accelerated aging tests were carried out on two types of mineral insulation oils, A and B, both of which were mainly used in 500 kV power transformers in China. Oil samples were pre-processed by degassing and dehydrating, and then aged at 120°C for various periods up till to 675 h, meanwhile, different contents of oxygen and copper/pressboard/silicon steel sheet were also added into the oil samples for sensitive study. The results indicate that the ECT of oil A increases notably with aging time, the ECT of oil B however does not change so much during the whole aging period. Injection of oxygen in the samples enhances the ECT of oil A, such a promotion effect is minor for oil B. The correlations between the ECT and other parameters, such as tanδ, volume resistivity (ρv) and acid number were analyzed as well. There are linear correlations between ECT and tanδ, acid number and log (ρv) of oil A, of which ECT and tanδ has the highest correlation coefficient of 0.881. Regarding oil B, there is only a positive linear correlation between ECT and acid number.