Investigation on creeping discharges propagating over epoxy resin and glass insulators in the presence of different gases and mixtures

Abstract

This paper deals with the experimental characterization of discharges propagating over insulators of epoxy and glass, immersed in a gas or a gaseous mixture, under lightning impulse voltages (1.2/50 µs), using a point-plane electrode arrangement. The gases and mixtures we considered are SF 6 , N 2 , CO 2 , SF 6-N 2 and SF 6-CO 2. The morphology of creeping discharges and their final lengths are investigated versus the kind of insulator material, the amplitude and polarity of the voltage, the type of the gas (resp. mixture) and its pressure. It's shown that the shape of discharges and their final (stopping) lengths L f depend significantly on the solid insulator and the type of gas. For given solid and gas, L f increases quasi-linearly with the voltage and decreases when the gas pressure increases. The discharges don't always present a radial structure as reported in literature. For given voltage and pressure, L f is longer when the point electrode is positive than when it's negative while the initiation voltage of discharges is higher with a negative point than with a positive one; and L f is longer with glass than with epoxy. L f is shorter in SF 6 than in CO 2 or N 2. On the other hand, the increase of SF 6 content in SF 6-CO 2 mixture leads to a significant decrease of L f. Therefore the addition of small concentration of SF 6 in a given gas mixture improve the dielectric strength of insulating structure.

Effect of micro-fillers in polytetrafluoroethylene insulators on the characteristics of surface discharges in presence of SF6, CO2 and SF6-CO2 mixture

“…Note that by extrapolating the L f (U) curves (straight lines), one can deduce the threshold voltage U 0 of creeping discharges for given solid/gas (mixture) interface and pressure. As we already reported [21], the extrapolated value of U 0 is very close to that corresponding to the appearance of the first current pulse and the smallest emitted light detected by the CCD camera we measured. U 0 over which a discharge is initiated varies with the pressure and the solid/gas system; it is all the higher as the pressure is high [21].…”

“…The experimental setup is similar to that we used in previous work [21]. We recall here the main features (Fig.…”

“…This is due to the electro negativity of SF 6 which constitute a major factor influencing the electric field at the head of discharge channels. And the higher the gas pressure the weaker the charge carriers are and the shorter the discharge channels are [21].…”

AC creeping discharges propagating over solid–gas interfaces

“…Similar results have been obtained under standard lightning impulse voltage with insulators made of glass and resin epoxy [14,15]. This is due to the excellent insulating properties of SF 6 with respect to that of N 2 and CO 2 .…”

“…We observe that, for a given gas pressure, L f increases quasi-linearly with the voltage. Such a variation has been already reported for lighting impulse [14,15] and DC voltages [16] when testing the same type of insulators. Note that each value of L f reported in figures represents an average value on ten tests; and this value corresponds to the most extended discharge L f independently of the polarity of alternation (positive or negative).…”