Gas Insulated Substation uses sulfur hexafluoride (SF6) gas which has superior dielectric properties used at moderate pressure for phase to phase and phase to ground insulation.
Characteristics of SF6 gas :
1. SF6 is colorless, odorless, and a Chemical neutral gas.
2. SF6 is 5 times heavier than air, is non-toxic, and has no dangerous components inside.
3. SF6 is no hazardous material
4. SF6 has no dangerous components inside, has no impact on the ozonosphere is a potent greenhouse gas.
Importance of Gas Insulated Substations :
Gas Insulated Substations are used where there is space for providing the substation is expensive in large cities and towns.
In a normal substation, the clearances between the phase to phase and phase to the ground are very large. Due to this, large space is required for the normal or Air Insulated Substation (AIS).
But the dielectric strength of SF6 gas is higher compared to the air, the clearances required for phase to phase and phase to ground for all equipment are quite lower.
Hence, the overall size of each piece of equipment and the complete substation is reduced to about 10% of the conventional air-insulated substation.
Locations of Gas Insulated Substation :
1. Large cities and towns.
2. Underground stations.
3. Highly polluted and saline environment Indoor GIS occupies very little space.
4. Substations and power stations located Offshore.
5. Mountains and valley regions.
1. Busbar
2. Disconnector
3. Maintenance Earthing Switch
4. Current Transformer
5. Circuit Breaker
6. Current Transformer
7. Maintenance Earthing Switch
8. Diconnectos
9. Earthing Switch
10. Voltage Transformer
11. Bushing
SF6 Gas Insulated Substation (GIS) contains the same compartments as in the conventional outdoor substations.
All 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 the gas does not pass through them.
The entire installation is subdivided into compartments that are gas-tight with respect to each other.
Thereby the gas monitoring system of each compartment can be independent and simpler.
The enclosures are of non-magnetic materials such as aluminum or stainless steel and are earthed.
The gas tightness is provided with static 'O' seals placed between the machined flanges.
The 'O-rings are placed in the grooves such that after assembly, the 'O'rings are get squeezed by about 20%.
Quality of the materials, dimension of grooves, and O-seals are important to ensure
gas-tight performance of Gas Insulated Substation.
Gas Insulated Substation has a gas monitoring system. Gas inside each compartment should have a pressure of about 3kg/cm2. The gas density in each compartment is monitored. If the pressure drops slightly, the gas is automatically trapped up. With further gas leakage, the low-pressure alarm is sounded or automatic tripping or lock-out occurs.
Advantages of SF6 Gas Insulated Substation:
1. Safe :
Gas-insulated Substations are very safe and operating personnel are protected by the earthed metal enclosures. While the Substation is in operating condition the Operating personnel can touch the compartment.
2. Reliable :
The complete enclosure of all live parts guards against any impairment of the insulation system.
3. Space Saving:
SF6 switchgear installations take up only 10% of the space required for conventional installations.
4. Economical :
Initial high investment is required for installation but the cost can be comparable for the less maintenance, reliable, safe operation against conventional substation.
5. Maintenance Free :
An extremely careful selection of materials, an expedient design, and a high standard of manufacturing quality assure long service life with practically no maintenance requirement.
6. Low Weight :
Low weight due to the aluminum enclosure, corresponds to low-cost foundations and buildings.
7. Shop assembled :
Quick site assembly ensured by an extensive pre-assembly and testing of complete feeders or large units in the factory.
Disadvantages of Gas Insulated Substation:
(1) Cost is higher compared to Air Insulated Substation or conventional substation.
(2) Procurement of SF6 gas and supply of gas to the site is problematic.
(3) Normally these types of substations are indoor types and require separate buildings.
(4) Maintaining Cleanliness is very important. Dust or moisture inside the compartment
causes flashovers.
(5) When a fault occurs internally, the outage period will be very long. The damage effect
will also be severe.