Thermal ageing effects on the dielectric properties and moisture content of vegetable and mineral oil used in power transformers

The operational safety of transmission and distribution grids depends essentially on the power transformers´ state. Lately, there is a tendency to replace mineral oils with vegetable oils because vegetable oils provide a higher explosion safety aspect for transformers due to their higher fire points. Further, vegetable oils are favorable for the environment due to their higher biodegradability. The focus of the present paper is to study the properties of mineral and vegetable after these oils are exposed to accelerated thermal ageing. Vegetable and mineral oil samples were subjected to accelerated thermal ageing (with and without cellulosic paper) at three different temperatures: 135 °C, 155 °C and 175 °C. The dielectric properties of the samples (relative permittivity, loss factor and resistivity) and their water content were determined for different ageing durations. The results show that in the case of mineral oil samples, there is a direct relation between the water content values and the dielectric properties. However, the obtained results indicate that the water content does not fully characterize the ageing condition of vegetable oil.