Investigation of interfacial surface creep breakdown at oil-pressboard interfaces in natural ester liquid and mineral oil

Natural ester liquids are increasingly established in power transformers, especially if the water protection and prevention of fire hazard in operation region of transformer are in focus. They show a comparable dielectric strength to mineral oil in homogeneous and slightly inhomogeneous fields. Nevertheless, differences exist between natural ester liquids and mineral oils in terms of electrical properties. The different ratios of liquid and solid insulation materials permittivities at interfaces cause a shift in electric field distribution within the insulation system of a transformer. Therefore, it is important to explore the effects of interfacial surface creep breakdown strength at high stressed oil-paper interfaces inside power transformers. This will be called simply creep breakdown. This study investigates the creep breakdown along an oil-paper interface at defined oil gap in natural ester liquid and mineral oil. A high-density pressboard sheet is impregnated and prepared giving a vertical electrode gap for AC creep breakdown test setup. Pressboard is fixed within one rounded electrode at the upper end and within one rounded electrode at the lower end. Special care is taken in the electrode design to avoid / minimize possible oil wedges in the setup. Tests are conducted with dry oil at a relative moisture content of less than 12 % of saturation. Breakdown mean values, 2 % withstand values and standard deviations are compared. The empirical breakdown results and the measured differences between the insulation liquids are used to interpret the effects of electric stress shift in natural ester liquids on transformer design rules.