Surface charge-induced field distortion on GIS spacers

Circuit breakers (CB) in transmission substations are generally flanked on each side by disconnect switches (DS) which allow the CB to be isolated for maintenance. DS are operated after the CB is open and need interrupt only the capacitive load of the bus between the DS and CB. In the case of SF₆ gas-insulated substations (GIS), operation of a DS can leave a substantial dc voltage “trapped” on the section of bus between the DS and CB.The present contribution discusses the implications of an ion stream impinging on a support insulator. It is expected that a point ion source above a GIS spacer would result in a line of charge on the space at most a few mm wide and with a charge density related to the applied field (napped charge). The effect of such a line charge distribution is minimal at the electrodes assuming that the apex between the spacer and electrode is well shielded, as is normally the case. If, as computations of the Brownian motion suggest, the charge distribution stops within a fraction of a mm, the component of the field along the spacer surface caused by the charge distribution could be in the range of 5 to 10 kV/mm and would be in a direction opposing the component of the applied field along the spacer surface. However, when the disconnector was reclosed, the transient field resulting from initial intercontact breakdown would add to the field along the surface and could precipitate a surface flashover. One obvious way to eliminate such possibilities is to manufacture the support insulators with a controlled surface resistivity as is required for any de application. The push toward dc applications of SF₆ insulation may eventually lead to the development of such spacers