The influence of surface charge upon flash-over of particle-contaminated insulators in SF6 under impulse-voltage conditions

This paper presents an experimental investigation of the effect of surface charge upon the impulse flash-over of metallic-particle-contaminated insulators, which were clamped between two parallel planar electrodes, in SF6 at 1.0 bar. The results show that the accumulated surface charge greatly affects the impulse flash-over strength of insulators. The influence of the surface charge is closely dependent upon its polarity and density, as well as upon the electrode configuration. For a PTFE cylindrical insulator with a metallic particle in contact with the anode, a 25% reduction in the 50% impulse flash-over voltage occurs when negative charge of peak density about 59 mu C m(-2) is present. However, with positive charge, the impulse flash-over voltage is not decreased, but increased instead, the enhancement in impulse strength being 21% at 49 mu C m(-2). When metal inserts are used to shield the electrode triple junction, charge accumulation on the insulator surface will be predominantly determined by the field normal to the surface. In this case, the accumulated surface charge appears to have little influence upon the impulse flash-over voltage.