Influence of AC Conditioning Treatment on Subsequent Vacuum Surface Flashover Process under Impulse Voltage Application

For high-voltage application of vacuum circuit breakers, it is crucial to suppress a surface discharge along the insulator. Vacuum surface discharge is primarily initiated by field electron emission from the cathode and secondary electron avalanche on the insulator. In this study, we used a cylindrical sample composed of metal electrodes and a ceramic cylinder, which were sealed in a high vacuum of less than 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> Pa. Two samples were prepared: one treated with AC pre-conditioning and one untreated. We investigated the influence of AC preconditioning on surface flashover characteristics along the insulator surface by measuring voltage and current waveforms under impulse voltage application and studying the flashover history. Measured results indicated that, the AC preconditioning sample exhibited a pre-flashover current of –10 A. The reduction in the pre-flashover current is attributed to the AC pre-conditioning, which decreased microscopic protrusions on the electrode surface, thus suppressing initial electron emission from the electrode. Furthermore, the initial flashover voltages were −75 kV for the untreated sample and −125 kV for the AC pre-conditioning sample, indicating that AC preconditioning improved the flashover voltage (FOV). However, the increasing rate in impulse FOV for the AC pre-conditioning sample was slower. This slower increase was because of the contamination of the insulator surface by the electrode material during AC pre-conditioning, reducing the surface resistivity. As a result, the secondary electron emission avalanche (SEEA) process along the insulator changed, leading to differences in the pre-flashover current waveform.