Characteristics of residual charge distribution on insulator surface under DC voltage in vacuum

For dielectric-vacuum compound insulation systems, the discharges usually occur across the insulator surface with an onset voltage much lower than the breakdown voltage for vacuum gap of the same length. It is no wonder that the surface charge accumulated on the insulator plays an important role in the flashover process in vacuum, especially under DC voltage. In this paper, the off-line observation of residual surface charge on PTFE under DC voltage in vacuum was carried out by a Kelvin electrostatic probe. It indicated that surface charging behaviors had close correlation with electric field distribution. For the geometry without back electrode, i.e., normal electric field was not so strong, surface charging at the first stage was the results of injection of charge carriers from electrode into sample surface layer under strong electric field. For higher excitation, it supported the secondary electron emission (SEE) as a mechanism of surface charging of an insulator in vacuum. However, under geometry with back electrode, i.e., normal electric field was strong enough, dielectric surface would be charged with the same polarity of applied voltage, indicating the injection of carriers into surface states becomes the domain factor in deciding surface charging. This research might be of great help to make a better understanding of surface charging in flashover process, and probably to put forward new measures for increasing the surface flashover voltage.