The effect of insulator charging on breakdown and conditioning

Abstract

As part of a study on HV design concepts for microwave tubes, a number of different insulator designs have been studied. Analysis of the measured dc current, partial discharge activity and breakdown voltage shows that surface charging of insulators is a key mechanism in the breakdown process and in the conditioning process. Insulator parameters are not only the breakdown voltage, but also the conditioning speed, and the sensitivity to gas exposure or charge leakage., In all these respects insulators with a field enhancement at the anode are superior. Field enhancements at the cathode are less harmful if stepped insulator shapes are chosen. Effective conditioning requires at least a limited number of breakdowns. With sufficient conditioning breakdowns, all insulator geometries tested reached a breakdown field exceeding 12 kV/mm.

Vacuum insulator flashover mechanisms, diagnostics and design implications

“…Recent studies also show that surface charging plays an important role [15]. The conditioning speed, the conditioning stability, and the conditioning effectiveness depend drastically on the presence or absence of surface charge, and may be optimized by the choice of a proper geometry [16].…”

“…Standard electrical diagnostics for flashover studies involve measurement of the dc current, the time-resolved current, the voltage waveform and the flashover voltage [2,3,6,15,17]. Recent studies also report PD (partial discharge) measurement.…”

“…Investigations at the Eindhoven University of Technology with many different insulator designs show how surface charging influences the insulator flashover voltage as well as the conditioning process. The effect of surface charging is determined from flashover and conditioning studies in combination with a non-invasive charge removal technique [15].…”

“…With the SEM (scanning electron microscopy) mirror technique a surface is first charged with the SEM electron beam and subsequently observed with the same beam at lower energy [23,24]. An electrical method which gives indicative information on the total amount of accumulated charge makes use of the dc current and the PD behavior of a stressed insulator upon stepwise voltage changes [15]. A helpful tool in surface charge studies is the possibility of removing surface charge, without modifying the insulating structure, by admitting low pressure Nz gas [15,16].…”

“…Recent studies also report PD (partial discharge) measurement. For dc studies, PD measurements provide additional information on charging processes [15], for ac studies the analysis of phase resolved PD patterns may provide information on the presence of defects [MI.…”