Effect of Insulating Oil Type on Double Electric Layer at Oil-Paper Interface

The oil-paper insulation system is the most frequently used insulation in transformers. For a long time, mineral oil has been the predominant choice due to its cost-effectiveness and commendable insulation performance, but it still has the disadvantages of low ignition point and low biodegradation rate. Vegetable insulating oil, as an environmentally friendly liquid dielectric with a high ignition point, is considered a promising substitute for mineral insulating oil. Our previous research found that the electric double layer (EDL) at the oil-paper interface hinders the charge transport and cause extra charge accumulation at the interface under dc stress, but there is still a lack of comparative studies on EDL involving various types of insulating oils. Employing Kelvin probe force microscopy (KPFM) and first-principles calculation, the charge distribution of EDL at the oil-paper interface is investigated in this work. The charge distribution of EDL at the oil-paper interface shows that negative charges gather on the insulating paper surface and positive charges diffuse in the insulating oil, and it is derived from the electron affinity discrepancy between the insulating oil and paper. In contrast to mineral oil, which is composed of diverse hydrocarbons, the molecular composition of vegetable oil closely resembles cellulose. Therefore, vegetable oil has a closer electron affinity to cellulose paper than mineral oils, which causes fewer negative charges to gather on the insulating paper surface, thereby weakening the effect of interface barrier and reducing the interface charge accumulation under dc stress, which is beneficial to the operation stability of transformers.