Theory of electrical corona in SF6

The dominant physical properties of SF6 are reviewed, and it is shown that the very high electron attachment coefficient in SF6 leads to a very different form of electrical corona in SF6, compared to that in air. The key to understanding these differences is the characteristic attachment time, τ, for the formation of negative ions which is 0.06 ns for typical SF6 conditions, and 143 ns for typical conditions in air. As a consequence of the short attachment time, the field in an SF6 streamer channel is limited to E≥E∗, greatly limiting the range of the streamers. (E is the electric field, and E∗ is the field for which the ionisation equals attachment.) In air such restrictions do not apply, due to the slower attachment rate, and streamers have a much greater range. The development of discharges (from the first electron through to fully developed discharges) is outlined for parallel plate electrodes, and point-plane electrodes in SF6 at atmospheric pressure with dc and impulse applied voltages. The theories are developed by solving the simultaneous continuity equations for electrons, positive ions, and negative ions, coupled with Poisson's equation which describes the space-charge dominated electric field. The effects of ionisation, attachment, recombination and photoionisation are included. The occurrence of stepped leaders in SF6 (similar to those occurring during lightning) is discussed, and preliminary results are presented which reveal a likely mechanism for streamer channel heating which causes leader formation in SF6. Results for the effects of pressure and of additives on corona formation in SF6 are also discussed. Finally, a practical solution is proposed for the problem of the failure of the corona fine tuning of the accelerator voltage in contaminated SF6.