With the increasing need of transmitting electrical energy over very long distances, combined with necessary space saving installations, the focus is more and more shifted to DC operated gas insulated systems. Their insulation system is nearly uniform, hence well-known problems, like space charge accumulation and metallic particles become more critical using SF6 in comparison to non-uniform air insulations. As soon as the applied voltage exceeds the inception voltage at the particle tips, the particle will produce charge carriers. These space charges will accumulate on the insulator and possibly influence the flashover behaviour. For a systematic study, variable parameters as the size of the particle and its position relative to the high voltage electrode, were chosen. In this paper, the flashover behaviour of epoxy resin insulators with adhesive metallic particles under both positive and negative DC voltage stress in gas insulated systems was investigated.
Particle-initiated breakdown characteristics in SF6 gas are investigated. Breakdown voltages are greatly influenced by particle diameter especially for AC voltages, and for very fine metallic wire particles are much higher than those for moderately thick ones. The breakdown stress of a gas gap is obtained as a function of particle length and can be estimated fairly well by assuming a corona stabilizing zone. For a spacer, the electric stress which is parallel to its surface dominates surface breakdowns and the relation between the stress and particle length is experimentally determined. In order to increase breakdown voltages, the effects of both the dielectric coating on a high-voltage electrode and spacer shape are examined. Finally, the effectiveness of the dielectric coating on a grounded electrode is demonstrated to prevent particle levitation.
We prove that groups acting geometrically on delta-quasiconvex spaces contain no essential Baumslag-Solitar quotients as subgroups. This implies that they are translation discrete, meaning that the translation numbers of their nontorsion elements are bounded away from zero.