Electrical conduction in impregnants for all-film power capacitors

The conduction properties of some impregnants (monodibenzyltoluene and phenylxylylethane) used in the all-film capacitors technology are investigated. The intent of this research contribution is to perform detailed investigations of these properties in a substantial range of electric fields (from 10−2 to 102 V μm−1). Measurements of the macroscopic charge transport properties (conductivity, mobility, and viscosity) at low field are reported for the temperature range 293–373 K. Evidence of ionic conduction is found from these results. It is possible to evaluate the ionic dissociation parameter of ionic species [e.g., TBAP (tetrabutylammonium picrate)] by studying solutions of known electrolytes in well-purified solvents. Combining this information with the corresponding investigation in well-purified liquids, it is found that the residual ionic conductivity of these fluids corresponds to 10−10–10−11 mol/l equivalent TBAP. In the high-field region, particular emphasis is given to the quantitative interpretation of the current (electrical losses) versus applied field characteristics. Bulk effects associated with the field-enhanced ionic dissociation (Onsager theory) are observed when the impregnant contains an ionic surfactant such as sodium di-(2-ethylhexyl) sulphosuccinate, while unipolar injection in the neighborhood of the metallic electrodes is evidenced in solutions containing small amounts of ammonium picrate electrolytes (TBAP, triisoamylammonium picrate). It is found here that the high-field conduction mechanisms in purified liquids can be attributed to a charge injection phenomena. A physical analysis that satisfactorily predicts the high-field conduction behavior of these fluids is further proposed and discussed in connection with recent numerical simulations.