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 v ery different form of electrical corona in SF6, compared to that in ai r. The key to understanding these differences is the characteristic at tachment time, tau, for the formation of negative ions which is 0.06 n s 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 s treamer channel is limited to E greater than or equal to E, greatly l imiting the range of the streamers. (E is the electric field, and E i s the field for which the ionisation equals attachment.) In air such r estrictions do not apply, due to the slower attachment rate, and strea mers have a much greater range. The development of discharges (from th e first electron through to fully developed discharges) is outlined fo r parallel plate electrodes, and point-plane electrodes in SF6 at atmo spheric pressure with dc and impulse applied voltages. The theories ar e developed by solving the simultaneous continuity equations for elect rons, positive ions, and negative ions, coupled with Poisson's equatio n which describes the space-charge dominated electric field. The effec ts of ionisation, attachment, recombination and photoionisation are in cluded. The occurrence of stepped leaders in SF6 (similar to those occ urring during lightning) is discussed, and preliminary results are pre sented which reveal a likely mechanism for streamer channel heating wh ich causes leader formation in SF6. Results for the effects of pressur e and of additives on corona formation in SF6 are also discussed. Fina lly, a practical solution is proposed for the problem of the failure o f the corona fine tuning of the accelerator voltage in contaminated SF 6.