Study on Ageing Characteristics of Insulating Pressboard Impregnated by Mineral-Vegetable Oil

Mineral oil is an excellent dielectric liquid with high coolant capability and low cost, playing important roles as both insulators and coolant liquid in oil-immersed power transformer. However, its flash point is too low, and it is poorly biodegradable once the oil spills or leakage occurs. In this work, pressboards impregnated by mixed oil containing pure mineral oil (80 vol%) and vegetable oil (20 vol%) are prepared, and their thermal ageing characteristics are studied. It was found that the degree of polymerization (DP) of the mixed-oil pressboard sample A was decreased more slowly than that of the pure mineral oil-pressboard sample B with the aging time. Dielectric properties of the samples were measured in the frequency range 10(-1) Hz-10(6) Hz. It was found that the dielectric loss tan. of sample A increased more slowly than that of sample B with the thermal ageing. The breakdown field of the two samples keeps at nearly the same level in the thermal ageing process. It was believed that the mixture oil-pressboard insulation may be a potential alternative to the traditional mineral oil-pressboard insulation used in the power transformer.