Correlation between ageing condition and flow electrification charge in transformer mineral oil

The flow electrification of mineral oil produces local electric discharges which may facilitate the damage and worsen the physico-chemical properties of the electrical power transformer insulation systems. This paper presents a study concerning the influence of the mineral oil ageing condition on the flow electrification phenomenon. The flow electrification current has been measured with a spinning disk system on unaged and thermally aged at T = 135°C mineral oil samples, for different time durations. In order to evaluate the degradation condition of the oil samples, the values of the complex permittivity components, loss factor and electric conductivity components were determined. The results have revealed the fact that the ageing of mineral oil worsens its dielectric properties and favors the flow electrification phenomenon, respectively the surface charge density at the oil-pressboard interface increases with the ageing time.