Characterization of creeping discharges propagating over gas/solid interfaces – Relation between materials properties and fractal dimension ; Caractérisation des décharges électriques se propageant aux interfaces gaz/solide – Relation entre propriétés des matériaux et dimension fractale

This work deals with the study of morphology and final length Lf of creeping discharges propagating over different kinds of solid dielectrics (PTFE with various fillers, Epoxy resin and glass) in presence of SF6, N2 and CO2 gases as well as SF6 - N2 and SF6 - CO2 mixtures, under lightning impulse voltage (1.2/50 µs) using a point - plane electrode arrangement. The materials we considered were selected for their wide use in high voltage apparatus such as GIS and circuit breakers. The characteristics of creeping discharges are analyzed in function of the amplitude and polarity of voltage, the kind and thickness of solid insulator, the type of gas (or mixtures) and pressure as well as the concentration of constituents of mixtures. It's shown that the morphological of creeping discharges depends on the components of solid/gas interface, the amplitude and polarity of voltage as well as the gas pressure. In presence of a given gas (mixture), Lf increases quasi-linearly with the voltage and decreases when the pressure and/or the thickness of insulator and/or the rate of SF6 in a given mixture increase. And for given voltage and pressure, Lf is higher when the point electrode is positive whereas the initiation threshold voltage of discharges is higher with a negative point. The results obtained with nitrogen and SF6 - N2 gas mixture are very scattered. The computation of electric field using the finite elements method (Flux 2D/3D codes) shows that the electric field is enhanced in the vicinity of the point; it's more important as the difference between the dielectric constants of gas and solid material is high. The recordings of currents associated to discharges revealed the existence of a secondary discharge of opposed sign to the applied voltage. This phenomenon is due to the accumulation of charges at the surface of solid insulator; some materials accumulate less surface charges and the final length of the discharges which develop over them is shorter. A relation between the type of gas (mixture) and its pressure, ...