The influence of charge on surface flashover

The presence of surface charge can significantly affect the properties of surface discharges such as the initiation voltage, the appearance and the geometrical structure. The charge may be produced through injection, from the electrodes, at triple junction sites, from contact and tribo-charging, or from gaseous ionisation processes. The pre-charging of a dielectric surface has been used in the generation of long, surface discharges. This has enabled surface discharges of up 5 metres in length to be generated and investigated using modest voltages in the range 10-30 kV. The generation of such discharges has facilitated a detailed study into surface discharge characteristics. The system used to generate the surface discharges is described along with results which indicate the propagation velocity and the factors which affect this. Variations in the discharge behaviour with voltage magnitude and polarity are described. A study of the emission spectra from discharges in SF₆ and air is presented and the effect of surface charge on the spectral content is shown