Characteristics of Composite Insulation: Liquid-Impregnated Insulation

A description is given of composite insulation, i. e. swelling of polymers by oil, oil streaming electrification of solid dielectrics , and V-t characteristics of LN2-impregnated insulation due to partial discharge. Swelling can be accelerated by ultrasonic wave irradiation. This reduces the saturation time. Polypropy-lene-laminated paper would swell less inside its cable structure. Ac dielectric breakdown strength increases with swelling in the early stage and then decreases. Streaming electrification was found to be influenced by the oil flow rate, temperature, ac applied voltage, and the type of solid and liquid dielectrics. Surface roughness of solid dielectrics does affect the electrification greatly. Addition of benzotriazol at only ppm is effective in suppressing the electrification. V-t characteristics are divided into two regions for liquid nitrogen (LN2) impregnated insulation. The value of n in Vnt= const ranges from 50 to 100 in the first region, while it is around 5 to 10 in the second region.