Theoretical Analysis of Dielectric Recovery in SF6 Gas-Blast Arcs

The dielectric recovery experiments of Schade and Ragaller [1] in SF6 gas-blast arcs are re-examined in light of the recent temperature calculations of Mitchell et al. [2]. The streamer criterion is used to compute theoretical characteristics for both the reference and enlarged geometries of [1]. It is shown that the theoretical recovery characteristic is relatively insensitive to the computed steady-state arc temperature. The streamer theory predictions agree well with experiment for the first two stages ("fast-slow") of dielectric recovery. However, they do not indicate the final "fast" stage, nor do they account for the large experimental scatter in the smaller geometry. It is suggested that both of these phenomena may be understood in terms of a postulated critical volume of about 1 mm3. If the volume of residual hot gas in which breakdown can originate becomes smaller than this critical volume, the hot-gas volume may not contain the free electron(s) required to initiate the breakdown process. Consequently, breakdown may be delayed until later during the voltage pulse, leading to scatter in the experimental results, or it may be suppressed altogether, leading to the final fast stage of recovery.