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

Abstract

This study is devoted to the optical and electrical characterisation of discharges propagating over insulators made of polytetrafluoroethylene (PTFE) filled with different kinds of micro-mineral fillers immersed in gas or gaseous mixture, under lightning impulse voltage (1.2/50 ms), using a point-plane electrode arrangement. The fillers the authors investigated are MoS 2 , Al 2 CoO 4 , SiO 2 and CaF 2 . The gases and mixture the authors considered are SF 6 , CO 2 and SF 6 -CO 2 . It is shown that the stopping length of discharges L f increases quasi-linearly with the voltage; L f is shorter in SF 6 than in CO 2 and it is higher 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. The discharges do not always present a radial structure as reported in the literature. Also, the type of filler greatly influences the characteristics of creeping discharges. In a given gas or mixture, the shortest L f is obtained with PTFE filled with MoS 2 or Al 2 CoO 4 . Thus, insulators with these fillers appear as the best insulators.

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).…”

“…This phenomenon has also been observed with solid-gas interface under lightning impulse [14,15] and DC voltages [16] as well as with solid-liquid interfaces [22,23].…”

AC Volume Breakdown and Surface Flashover of a 4% NovecTM 4710/96% CO2 Gas Mixture Compared to CO2 in Highly Nonhomogeneous Fields

“…The polarity of flashover for all used insulators is identical to breakdown in gas gap alone independent of the gas type, except for the Epoxy insulator. Based on this observation, it can be said that the gas pressure plays an important role on the discharge propagation causing the discharge to stick to the insulator surface as the pressure increases as observed by [33,34]. It was found that, depending on the polarity, the efficiency of the charge injection from the solid insulator or the attachment/recombination process will govern the propagation path.…”

“…This results in a reduction of the surface flashover voltage. On the other hand, it was observed that the surface discharge trends to stick the insulator surface with the increasing of the gas pressure [33,34,37,38]. This can explain the relative stability of the surface flashover voltage for high pressures: the main electrical charges are collected from insulator caused by the increase of the attachment process with the gas pressure rising.…”