Plasma, Discharges and Processes

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

A. Beroual^a1a2 c1, M.L. Coulibaly^a3, O. Aitken^a4 and A. Girodet^a4

École Centrale de Lyon, Ampère CNRS UMR 5005, 36 avenue Guy de Collongue, 69134 Écully, France

King Saud University, Aramco Chair in Electrical Power, Riyadh, Kingdom of Saudi Arabia

ALSTOM Grid Power Transformers, TICC, 129, avenue de Paris, 91300 Massy, France

ALSTOM Grid, 130, rue Léon Blum, BP 1321, 69611 Villeurbanne Cedex, France

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 SF₆, N₂, CO₂, SF₆-N₂ and SF₆-CO₂. 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 is 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 do not always present a radial structure as reported in the literature. For given voltage and pressure, L_f is longer when the point electrode is positive than when it is 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 SF₆ than in CO₂ or N₂. On the other hand, the increase of SF₆ content in SF₆-CO₂ mixture leads to a significant decrease of L_f. Therefore, the addition of small concentration of SF₆ in a given gas mixture improves the dielectric strength of insulating structure.

(Received March 24 2011)

(Revised July 28 2011)

(Accepted August 30 2011)

(Online publication November 14 2011)

Correspondence:

^c1 e mail: abderrahmane.beroual@ec-lyon.fr