Let’s start with some definitions. AIS stands for air insulated switchgear and GIS stands for gas insulated switchgear. So far we can see that both are some sort of insulated switchgear like this.
So the remaining difference is air vs gas. These are the actual insulators in the switchgear. In air insulated switchgear the arc between the contacts is extinguished by the air. In the gas switchgear it is extinguished by the gas.
The differences are that GIS is more compact as the gas, usually sulfur hexafloruide - SF6 has a higher dielectric breakdown voltage than air. So less gas is needed to extinguish the arc. However the gas insulation raises costs. Therefore it tends to be used in areas where space is a premium such as in cities.
Air Insulated Switchgear
Gas Insulated Switchgear
However it should be noted that SF6 is a potent greenhouse gas. It is 23,500 times more potent than carbon dioxide, and can persist in the atmosphere for 1000 years. As the deployment of renewables increases so does the usage of SF6 gas. There are attempts being made to use alternatives to the gas whether its a different kind of gas or a combination of clean air and vacuum technology.
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