Gas Insulated Switchgear Substation (GIS)

Gas Insulated Switchgear Substation (GIS)

Gas insulated substations make use of sulfur hexafluoride (SF6) gas. SF6 gas has excellent dielectric properties when used at moderate pressure for phase to phase and phase to ground insulation. The high voltage conductors, circuit breaker interrupters, current transformers, voltage transformers, switches and lightning arresters are enclosed in SF6 gas inside grounded metal enclosures.

SF6 gas has higher dielectric strength than air and the conductors and contacts are insulated by pressurized SF6 gas indicating smaller clearenes as compared to AIS substations phase to phase spacing is considerably reduced thus requiring lesser space as compared to AIS substation of same capacity. In urban environment this proves to be advantageous due to unavailability of land and their increasing prices. Thus GIS substations are installed where the land requirement for erection of substation is limited and expensive such as in large cities and towns. The overall size of each equipment and complete substation is significantly reduced to about 10 % in comparision with conventional AIS substions.

GIS substations can be a preferable choice in large cities and towns, mountains and valley regions, underground substations, off shore substations and power stations and in highly polluted and saline environment.

The compartments used in SF6 gas insulated substations and those used in conventional outdoor substations are same. The live parts are encapsulated in metal housings which are filled with SF6 gas. The cast resin insulators form the support for the live parts while some of the insulators are barriers between adjacent modules so that the gas is restricted from passing through them.

The total installation is subdivided into verious compartments. These are gas tight with respect to each other. Each compartment has independent gas monitoring system which is simple. The gas density in each compartment is monitored while the pressure of the gas is maintained at a proper value. With slight reduction in pressure, the gas is automatically trapped up low pressure alarm is sounded with leakage of gas resulting in automatic tripping.

The enclosures are made up of non-magnetic materials such as stainless steel or aluminium. The enclosures are earthed. The tightness of gas is provided by using static 'O' shaped seals which are placed between machined flanges. 

1. Advantages

The merits of gas insulated substations are as given below :

1. These are safe since the metal enclosures are earthed, the operating personnel are protected. They are reliable.

2. It takes lesser space as compared with conventional AIS substations.

3. It has long service life with less maintenance requirement.

4. It has lower weight due to aluminium enclosures.

The demerits of GIS substations are as given below :

1. It is constlier as compared to AIS installations.

2. Supply of SF6 gas at the site and its procurement is sometimes poses problems.

3. It requires separate building due to indoor nature of these substations.

4. Dust or moisture inside the compartments may cause flash overs.

5. The outage period under the event of fault is more with severe damage.

Review Question

1. State the advantages of GIS.