AC breakdown strength of a natural ester transformer liquid under accelerated thermal ageing

Abstract

In recent years, there is an increasing interest in applying natural esters as the alternative to mineral oils in large power transformers. But the low resistance to oxidation/gelling process poses a challenge for applying natural esters in freebreathing transformers. Some previous studies presented the gelling/oxidation behaviour of natural esters in terms of physical and chemical properties. There is a lack of study considering the electrical performance of natural ester during the gelling process. In this paper, the AC breakdown strength of a natural ester from liquid state to totally gelled state was determined. Test cells with a 1 mm sphere-sphere electrode gap were used to hold ester samples. The ester samples and the test cell were aged together in an air-circulating oven at 120 ˚C. Dynamic viscosity of the ester samples was also measured to monitor the oxidation/gelling process. Results showed that dynamic viscosity of the natural ester increases more than 100 times during the gelling process. But unlike the rapidly changing physical parameter, AC breakdown strength of the natural ester remains stable during the whole ageing process till the natural ester turns into totally gelled form.