Influence of thermal ageing on transformer oil properties

In this paper, we present the electrical and physic-chemical behaviours of oil in high voltage transformers, under thermal ageing. Continuous thermal ageing tests were made on transformer oil BORAK22 during 5000 hours at two different temperatures 80degC and 120degC. Here, every 500 hours, the dielectric losses factor, the relative permittivity and the resistivity were measured. Other properties such as the breakdown voltage, the acidity degree, the water content, the colour and the cinematic viscosity are measured before and after ageing. The obtained results of this investigation show that the thermal ageing affects greatly the properties of transformer oil.