Decay of surface charges on insulators in SF6

The decay of surface charges deposited by corona on insulators made of epoxy-resin (quartz-filled and unfilled) is measured in SF6-gas at a pressure of just below 100 kPa. In these experiments, the effect of surface conduction is found to be negligible compared to charge neutralization by charged particles that are produced by ionization due to natural background radiation. Background radiation accounts for a decay rate in the order of 10 nC per day inside a measuring volume of 3.5×10su−2 m³ SF6-gas at 98 kPa. This is sufficient to remove a localized charge distribution on an insulator with a peak value of 60 µCm⁻² within a day. A model that describes the decay of surface charge due to conduction and interaction with the gas is presented. It is shown that losses towards the gas explain the experimental results.