Insulator-surface flashover conditioning: A streak photographic investigation

Conditioning behavior of surface flashover strength of a large-bandgap insulator (quartz) is investigated with the aid of streak photography in conjunction with electrical (voltage and current) and optical UV (phototube) diagnostics. The breakdown strength of the sample is found to be conditioned during the initial test as well as the subsequent test, performed after an overnight rest period. The results suggest that light emission during the initial test is dominated by visible light whereas during the subsequent test it is dominated by UV. The initial conditioning is attributed primarily to the desorption of absorbed molecules and rearrangement of the same by migration kinetics. Processes such as negative charge formation on the insulator surface and trapping of electrons in the deep traps are proposed to be responsible for the observed conditioning after the overnight rest period. The breakdown strength of the sample was found to degrade after experiencing breakdown at higher fields (i.e., after second day test). This deconditioning behavior is discussed in terms of evaporation of electrode material and deposition of the same on the insulator near the cathode end