Impact of local overheating and electrical discharge on the streaming electrification of transformer oil

There is a general agreement that in service conditions the quality of mineral insulating oils gradually deteriorates under the impact of electrical, thermal and chemical stresses. It is also well established that incipient electrical failures such as hot spots and partial discharges are responsible for the deterioration and gassing of oil. This article intends to show how the chemical composition of hydrocarbon blend and the oil born decay products are also contributing factors to the streaming electrification of the oil. The effects of different stresses on the streaming electrification of transformer oil have been investigated using some key testing methods. In this regard, various scenarios were considered. The study was carried on mineral oil samples submitted to electrical discharge according to ASTM D6180 or local overheating and a combination of both stresses. This paper provides experimental evidence that the formation in service conditions of undetected oil-born decay products is a contributing factor to the electrostatic charging tendency of oil.