Transformer mineral oil electrification

The static electrification phenomenon is a result of the mineral oil flow through the solid insulating components and cooling ducts. The contact of streaming high resistivity mineral oil with the solid surface of the paper insulation generates electrical charge accumulation, which can produce electrical discharge that affects the insulation system properties. Many experimental results indicate that the static electrification phenomenon depends on the system´s dynamic and also on the physico-chemical characteristics of dielectric materials. This paper presents a study concerning the electrification phenomenon in a system consisting of a mineral oil metallic cylindrical cell and a pressboard disk that rotates inside the oil. The current generated by the charge concentration gradient, at the pressboard-oil interface is measured for different pressboard disk diameters and several rotation speeds. The influence of the oil thermal aging on the currents is also investigated.