A technique for measuring the DC stress distribution across an insulator

Opening a disconnector in a gas insulated substation (GIS) on an AC transmission system may leave a trapped charge of up to 1 pu on the isolated section of busbar, thereby stressing the solid support barriers by a DC voltage. This condition may persist for several hours or days, because it is not normal practice to earth the EHV conductor and drain the charge unless the chamber is to be opened for some purpose. Barriers for use on an AC system are also designed to withstand a high DC voltage, mainly to allow any cables feeding the GIS to be tested. Nevertheless it is suspected that under some conditions trapped charge might possibly lead to the barrier flashing over spontaneously. Small scale laboratory tests have been undertaken to investigate the mechanism by which this might occur, and as part of these the device described here was developed