Dielectric properties of paper insulation aged in the presence of vegetable transformer oil

The power transformer's lifetime is strongly influenced by its insulation system degradation, especially by the paper degradation. As the vegetable oil should replace mineral oil, this paper presents a study concerning the dielectric properties variation of cellulosic paper impregnated with vegetable oil as a function of the ageing duration. In order to characterize the condition of the paper the main dielectric properties have been measured: the relative permittivity and loss factor (in AC) and the electrical conductivity (in AC and DC) for vegetable oil impregnated paper aged (1500 hours) at 135°C and 155°C. A correlation between the variation of the dielectric properties (permittivity and resistivity) with the ageing duration has been found.