Oil electrification measured on a pressboard coming from a damaged power transformer

In high power transformers, pressboard and oil are required for electrical insulation. On the other hand, the oil is also used as a coolant of the system. The flow electrification process occurring at oil-pressboard interface seems to be initiating on electrical discharge incidents. Our recent investigations of electrification generated by an oil flow through different pressboards showed an influence of the pressboard degradation by artificial electrical discharges. Consequently the goal of this experimental study is to measure and to compare the convected space charge density value obtained with a brand new pressboard sample and with a pressboard coming from a damaged power transformer. The influence of an additive (BTA benzotriazol 1-2-3) in oil is also analyzed. The results show that the pressboard sample obtained from the damaged power transformer induces a high increase of flow electrification due to a modification of the exchange process at the pressboard-oil interface. On the other hand, an addition of BTA permits to reduce significantly flow electrification.