A gas insulated substation (GIS) is a substation that uses a superior dielectric gas, sulfur hexafluoride (SF6), high-voltage. It is Conventional substations requires, small installation size, protection against atmospheric pollution and moisture, noiseless operation, nonexclusive and flame resistant, reduced maintenance, minimum radio interference, but totally enclosed substations using SF6 gas as insulation that are also known as GIS is now in widespread use in the electrical power industry .

The basic principle of gas-insulated equipment:

Is that the high-voltage current- carrying parts are within a metal enclosure and are held in a concentric configuration by cast epoxy spacer insulators.

The space between the conductor and the enclosure is filled with sulfur hexafluoride gas under moderate pressure.

Need for GIS

Non availability of sufficient space.

Difficult climatic and seismic conditions at site.

Urban site.

Reduction in length of feeders.

Improvement of the quality of voltage regulation due to short length feeder.

The total space required for GIS is 10% of the needed of conventional substation.

GIS technology can be used for installation in areas where the real estates is appreciable.

Gas Insulated Substations Safety :

The live parts and supported on at resin insulators. Some of the insulators are designed as barriers between neighboring modules such that the gas does not pass through them.

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 low-pressure alarm is sounded or automatic tripping or lock-out occurs.