The Effect of DC Prestress on Impulse Flashover of Insulators in Vacuum

The results of an experimental investigation of the surface flashover of high density alumina spacers in vacuum are presented. The insulators were subjected to a combination of a dc or 60-Hz prestress and a 1-50-us impulse stress. It is shown that the overall flashover voltage is strongly dependent upon the magnitude and polarity of the prestress when the spacers make a butt contact with the electrodes. Recessing the solid-electrode junction below the plane of the electrode at the impulse cathode makes the overall flashover voltage independent of the prestress. The effect of the prestress is shown to disappear within 50 us of the removal of the stress. The results are explained in terms of a charge injection model.