Gas Insulated Switchgear

SF6 Gas Insulated Switchgear (GIS) are preferred for voltage rating of 36 kV, 72.5 kV, 145 kV, 245 kV, 420 kV and above. In such a substation, the various equipments like Circuit Breakers, Busbars, Isolators, 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 and phase to phase insulation. As the dielectric strength of SF6 gas is higher than air, the clearance required are smaller. The overall size of each equipment and the complete substation is reduced to about 10% of convention air-insulated substation. GIS are installed indoor.

The various modules of GIS are factory assembled and are filled with SF6 gas at a pressure of about 3kgf/cm2 .  Thereafter, thy are taken to site for final assembly. Such substations are compact and can be installed conveniently on any floor of a multistoried building or in an underground substation. The installation time s substantially reduced. Such installation are preferred in cosmopolitan cities , industrial townships, hilly terrains etc., where cost of land is very high and higher cost of  gas insulated switchgear is justified by saving due to reduction in floor area requirement. They are also preferred in heavily pollutes areas where dust, chemical fumes and salt layers cause frequent flashovers in conventional outdoor air-insulated switchgear.

The SF6 Gas Insulated Switchgear (GIS) contain the same component as the conventional outdoor substations. All live parts are enclosed in metal housings filled with SF6 gas. The live parts are enclosed in metal housings filled with SF6 gas. The live parts are supported on cast resin insulators. Some of the insulators are designed as barriers between neighboring modules such that gas do not pass through them. The entire installation is sub-divided into compartments which are gas tight with respect to each other. Thereby the gas monitoring system of each compartment can be independent and simpler.

The enclosures are non-magnetic material such as aluminum or stainless steel and are earthed. The gas tightness si provided by static ‘O’ seals placed between machined flanges. The ‘O’ rings are placed in the grooves such that after assembly, the O ring get squeezed by about 20%. Quality of material and dimension of grooves and O-seals are important to ensure gas tight performance.

The GIS has gas monitoring system. The gas density in each compartment is monitored. If pressure drops slightly, the gas is automatically tapped up. With further gas leakage, the lo pressure alarm is sounded or automatic tripping or lock out occurs.