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

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 s), using a point-plane electrode arrangement. The fillers the authors investigated are MoS₂, Al₂CoO₄, SiO₂ and CaF₂. The gases and mixture the authors considered are SF₆, CO₂ and SF₆-CO₂. It is shown that the stopping length of discharges L_f increases quasi-linearly with the voltage; L_f is shorter in SF₆ than in CO₂ 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 MoS₂ or Al₂CoO₄. Thus, insulators with these fillers appear as the best insulators.