Computation of the electric field at a solid/gas interface in the presence of surface and volume charges

Abstract

Enhancement of the electric field at a solid insulator/electrode/gas triple junction and on the surface of an insulator, due to imperfect contacts, surface irregularities and contamination, reduces the withstand voltage of the gas-only system. The charges deposited on the surface and in the bulk of the insulator, when stressed under high voltage, further modify the electric field. In the paper, the influence of accumulated static charges on the electric field distribution associated with a cylindrical insulator held between plane electrodes in atmospheric air is investigated. The electric field along the solid dielectric/air interface is calculated for different surface and volume charge distributions by using the finite-difference method based on Gauss's law. Derivation of the iteration equations are relatively easy, and, unlike most methods, there is no need for external imposition of solid dielectric/gas boundary conditions. The results indicate that, for some surface and volume charge distributions, field enhancement occurs at the triple junction, and this could initiate a surface flashover.List of symbols and units V = electric potential, V <p = electric flux, C E = electric field strength, Vm ~1 E n = normal electric field strength, Vm" 1 E t = tangential electric field strength, Vm" D = electric flux density, C m " 2 h = grid length, m r, z = cylindrical co-ordinates, m R = distance from axis of symmetry, m e 0 = permittivity of free space, Fm " l 6; = permittivity of an octant, Fm " l s r = relative permittivity Paper 4949A (S2, S3, S8), first

Time dependence of spacer charging in SF<inf>6</inf> under DC stress

“…This can lead to considerable charging and a change in the dielectric stress of the insulation [7]. The neuralgic part of the insulation seems to be the surface of the insulators interfacing SF6 and solid insulation, as well as the triple junctions on the insulators [8]. Especially breakdown voltages in situations of polarity reversal may be reduced under DC conditions due to charge accumulation [9].…”

Micro discharges in HVDC gas insulated systems

“…They can reduce the voltage withstand capability and even lead to a breakdown of the insulation system [3]. The most critical parts in SF 6 gas insulated DC equipment are the surface of the insulators interfacing SF 6 and the solid, and triple points [4,5]. In this article, we investigate the effect of charges arising from the gaseous phase on the solid insulator.…”