Ion current measurements in SF6 and vacuum under high voltage DC application

Pre-breakdown current evaluation is of importance for Gas Insulated Switchgear (GIS) under DC high voltage application. Even if very small, assuming constant polarity of the applied voltage, such currents represent a source of charges which continuously accumulate on dielectric surfaces, especially if these have high resistance. This can lead to a significant change in the distribution of the electric field, possibly compromising voltage withstand. In this work, we present a measurement system realized to evaluate pre-breakdown currents and some results. The system is capable to resolve sub-pA currents and apply up to 120 kV. The influence on pre-breakdown currents of various parameters, such as surface type, humidity and presence of paint on the electrodes was evaluated and some preliminary conclusions were drawn.