DC creeping discharges over insulating surfaces in different gases and mixtures

This paper deals with the propagation of discharges over insulators of Bakelite and glass, in gases and their mixtures (namely N2, SF6, CO2, SF6 - N2 and SF6 - CO2) under DC voltage in a point - plane electrode arrangement. The shape and stopping length, Lf, of these discharges are investigated versus the type of insulator material and its thickness, the amplitude of applied voltage, the type of gas/mixture and pressure. The obtained results show that the discharges do not always propagate radially; this depends on the type of solid/gas interface. However, as we reported elsewhere for lightning impulse voltage, Lf increases quasi-linearly with the voltage; it decreases when the gas/mixture pressure increases. Lf is shorter in SF6 than in CO2 or N2; and the increase of SF6 content in the investigated mixtures leads to a significant decrease of Lf. Also, for a given gas, Lf is slightly longer with glass than with Bakelite indicating that the higher the dielectric constant, the longer the discharge is; the dielectric constant of glass and Bakelite being εG = 5.0 and εB = 4.8, respectively. Thus the density of the discharge branches and Lf reduce when the thickness of insulators increases and/or their dielectric constant decreases indicating that the capacitive charge effect and electric field influence the development of creeping discharges.