Characterization of creeping discharges propagating over solid/liquid interfaces under various voltage waveforms - Relation between the materials properties and the fractal dimension.
Abstract This work deals with the electrical discharges propagating on liquid/solid interfaces generated under various voltage waveforms (lightning impulse, DC and AC) using a point-plane electrode arrangement. It's shown that the nature and the thickness of the solid insulator, the hydrostatic pressure, the shape and polarity of the applied voltage significantly influence the characteristics of creepage discharge. The final length Lf increases linearly with the applied voltage. For the same amplitude of voltage, Lf is higher under AC than under lightning impulse voltage and DC. For a given voltage, Lf increases with the dielectric constant of the solid insulator; it also increases when the thickness is reduced. The general shape of the creeping discharge is radial under lightning impulse voltage and non-radial under DC and AC voltages. The amplitude and shape of the measured currents remind those observed in large gaps of transformer oils. The current records in relative long time scale show the occurrence of a secondary discharge of an opposite polarity to that of the applied voltage. This is likely due to the space charge deposited at the solid insulating surface when the discharge propagates, resulting in a modification of the electric field. The total charge of the creeping discharge calculated using a simplified model (parallel wire-plane capacitor) is in a good accordance with that measured experimentally. The morphology of the creeping discharges is investigated using the fractal geometry. A relationship between the solid insulator characteristics and the fractal dimension D of the creeping discharges is established. We show that D depends on the thickness of the solid samples and the type of insulator. This suggests the possible implication of capacitive effects on the propagation phenomena of creeping discharges.