SF6 Gas insulated Substations (GIS) are preferred for voltage ratings of 72.5KV, 145KV, 245KV, 420KV and above. In such a situation, the various equipments like circuit breakers, busbars, isolators, and load break switches; current transformers, voltage transformers, earthing switches etc. are housed in metal enclosed modules filled with SF6 gas.
The SF6 gas provides the phase to ground insulation. As the dielectric strength of SF6 gas is higher than the air, the clearances required are smaller. Hence, the overall size of each equipment and the complete substation is reduced to about 10% of conventional air ? insulated substations. As a rule GIS are installed indoor. However outdoor GIS have also been installed earlier.
The various modules of GIS are factory assembled and are filled with SF6 gas at a pressure of about 3 Kg/ cm2. Thereafter they are taken to site for final assembly. Such substations are compact and can be installed conveniently on any floor of a multi - storied building or in an underground substation.As the units are factory assembled, the installation time is substantially reduced.
Such installation are preferred in cosmopolitan cities, industrial townships, etc. where cost of land is very high and higher cost of SF6 insulated switch gear (GIS) is justified by saving due to reduction in floor area requirement. They are also preferred in heavily polluted areas where dust, chemical fumes and salt layers can cause frequent flashovers in conventional outdoor air-insulated substations. The GIS has monitoring system.
The gas density in each compartment is monitored. If pressured drops slightly, the gas is automatically tapped up. With further gas leakage, the low-pressure alarm is sounded or automatic tripping or lock out occurs. By this method we can save the space required for the substation to build up. This means that gas-insulated substation can be built in any metropolitan city. Many experiments are going on the use of SF6 gas