Migration Characteristics of Moisture in RIP Materials and Its Influence on Surface Insulation Characteristics

At present, the wall bushings and converter transformer bushings in domestic HVDC projects mostly use epoxy resin impregnated paper (RIP) dry bushings. The RIP material has a certain degree of water absorption, so the surface of the RIP core is coated with a two-component insulating varnish made of epoxy resin and modified polyamide resin to prevent moisture. This paper studies the surface insulation strength and moisture transfer law of materials subjected to moisture. It is found that the DC flashover field strengths of the coated samples before being wetted are higher than those of the uncoated samples by 5.7%. After the sample was exposed to moisture for 24 hours and 72 hours, the flashover voltage decreased by 31.6% and 47.1% respectively compared with the flashover voltage without moisture. The water absorption rate of RIP material decreased by 39.9% after being coated. However, after coating, the surface layer of the material has high humidity due to the easy water absorption of polyamide, so the insulation performance along the surface of the coated sample is still significantly reduced after being exposed to moisture for a long time. Based on the research results, a vacuum high-temperature drying process that can effectively improve the DC flash insulation properties of damp specimens is proposed, which provides reference and support for the method of core moisture resistance and improving its insulation performance along the surface.