A steady-state simulation of ionisation phenomena in sulphur hexafluoride using a positive point/cup system

Simulations of positive-point corona in sulphur hexafluoride have been written which include the effects of space charge. The sphere/sphere electrode system used allows full advantage to be taken symmetry and has been paralleled in the laboratory by a hemisphere/hemisphere electrode system, the point/cup system described previously. As the positive DC voltage is increased (for a given pressure) the simulations predict streamer production at and slightly above a `first streamer onset voltage´ (FSOV), an avalanche-smothered region where no streamers occur, and a second streamer onset voltage (SSOV) where increasingly efficient steamer production occurs. Experimental results show corona inception voltages and breakdown voltages coincide with FSOV and SSOV for p<2 bar; breakdown voltages are close to the FSIV for p>4 bar; and that for an intermediate transition region the corona inception voltage continues to coincide with the FSOV, but that the breakdown voltage moves from SSOV to FSOV. The simulations do not allow for the probabilistic aspect of ionisation so that even where they indicate no streamers occur, it is likely that a few do and that this can account for the transition region