Electrical breakdown of SF6 at high pressure

This project was conducted to assist the upgrade of the Z-machine at Sandia National Laboratories. At Sandia, the 5.5 Megavolt Laser Triggered Gas Switch (LTGS) developed surface flashover within the PMMA envelope. This surface flashover occurs initially during the regular cascade switching process without affecting the switch’s electrical performance. However, the switch will pre-fire in the successive shot. The damage to the envelope has to be repaired before normal switch operation can resume. One of the more probable failure modes is that the existing electric field on the surface of the envelope with the addition of the UV from the volume breakdown event initiates surface flashover. A 340 kV system was designed which replicated conditions found in the LTGS between two adjacent backbone electrodes. Besides measurement of flashover delay times for different dielectric materials (Lexan, Acrylic, Epoxies, etc.), emphasis was put on the detailed characterization of the flashover phenomenology. Hence, in addition to voltage and current diagnostics, fast optical imaging of the flashover along with the acquisition of optical emission spectra (~200 nm to 700 nm) was employed. This paper will investigate individually the two events that are occurring within the chamber, surface flashover and volume breakdown. The conclusions will serve to lay the groundwork for experiments to follow this paper which will directly investigate the impact of volume breakdown on flashover across surfaces in the vicinity of the volume breakdown in high pressure SF6. Understanding the fundamental physical mechanisms and the extent to which these processes influence discharge behavior is vital to characterizing and modeling electrical breakdown in high pressure SF6.