Charge behavior in Palm Fatty Acid Ester Oil (PFAE) / pressboard composite insulation system under voltage application

Palm Fatty Acid Ester Oil (PFAE) is one of the most promising environment-friendly biodegradable insulating oils for transformers. PFAE has significant advantages compared to other vegetable origin insulating oils and mineral oil, e.g. not only good insulation performance, high permittivity and high moisture tolerance, but also excellent cooling ability due to low kinematic viscosity, high thermal and chemical stability and high productivity. However, since the molecular content of PFAE is quite different from mineral oil, the fundamental charge behavior in PFAE has not been clarified yet. To clarify the temporal and spatial charge behavior in PFAE, we directly measured the electric field in oil duct model with PFAE / pressboard (PB) composite insulation system under dc voltage application and charged flow condition by using Kerr electrooptic field measurement technique. By the comparison of electric field and charge behavior characteristics with fresh and degraded mineral oil, we revealed that the essential mechanism of charge behavior is similar, but the difference in temporal and spatial characteristics is attributed to the higher intrinsic ion density and higher permittivity of PFAE than that of mineral oil.