The impact of partly liquefied SF6 on the dielectric performance of SF6-insulated switchgear

When gas insulated switchgear (GIS) is operated at temperatures below the condensation temperature of the SF
₆
used as the insulating medium, liquid droplets or puddles can form and influence the electric field distribution. For the surface conditions and electric field strengths and distributions relevant for GIS, the instability of liquid droplets represents the worst case. Finite element method calculations were carried out to model the interaction between the electric field and unstable liquid droplets, both for water (in air) and liquid SF
₆
. The model accurately reproduced experimental data available for water in the literature. The model was then used to assess how critical the presence of liquid SF
₆
droplets is for the dielectric performance of GIS. It was found that SF
₆
droplets are much less critical than small (0.5 mm) metallic particles, the presence of which is assumed and taken into account in the design of GIS. Water droplets, on the other hand, can lead to high field enhancement and a corresponding reduction in the breakdown electric field strength.