Moisture Evacuation Procedure for Gas Insulated Transmission Bus

Gas insulated lines (GIL) are often used for longer distances than typical gas insulated substation (GIS) bus runs. GIL exit circuits from GIS stations can be several hundred meters long and stand alone GIL can be several kilometers in length. During installation the procedures used for the evacuation of air and moisture in GIS equipment are often ineffective for longer lengths of GIL. GIS bus often contains desiccant to control arc by-products created in disconnect or ground switch operation so any residual moisture in the compartment is absorbed by the desiccant. Since GIL compartments do not contain switching elements the use of desiccant is not required. The residual moisture in the GIL must be removed as completely as possible prior to filling with SF₆. As long is this is done the GIL compartment will not require drying during its service life. Experimental results are described for several different evacuation and drying methods. This is followed by sample calculations for various configurations of pumps, hoses and valves. Recommendations are given for efficient and effective moisture removal for GIL