Gas Breakdown in Case of Steep-Fronted Pulses and Insulator Interfaces

Two fundamental factors influencing the insulating characteristics of compressed gases are steep-fronted overvoltages and interfaces between gas and solid insulators. In order to estimate the influence of steep-fronted overvoltages it is necessary to know the limiting voltage-time-characteristics, that is the formative time lag. Especially in electronegative gases it is necessary to separate the statistical time lag from the formative time lag as the first time interval may be many orders of magnitude higher than the second. It will be shown that especially in SF6 the formative time lag may be either extremely short or rather long, mainly influenced by the field distribution. In compressed gas insulation the breakdown voltage is reduced by even ¿clean¿ insulator surfaces. In this case breakdown is induced by one or several micro-discharges caused by local field enhancement at the insulator surface. Further electron multiplication must be caused by photoionization. Breakdown occurs if several micro-discharges develop and grow together.