Evaluation of impregnants designed for all-film power capacitors engineering: Electric conduction and breakdown

An experimental study of conduction and breakdown phenomena of M/DBT (monodibenzyltoulene) being used as polypropylene impregnant in the all-film capacitor technology is reported. The liquid electric losses exhibit a strong interfacial dependence when the liquid is in contact with a metallic or a polymer, (either impregnated or dry) interface. Charge-carrier injection in the neighborhood of the interface is the dominant contribution of high-field liquid conductivity. The breakdown phenomena are shown to be related either to high-field conduction or to the presence of particles or to local field enhancements. The dielectric properties of inhomogeneous media constituted by a glassy semicrystalline polymer (polypropylene) impregnated by M/DBT are explored. The measured permittivity is compared to various forms of mixture equations. The closest agreement is obtained with the Lichteneker's logarithmic mixing expression.