Gas Insulated Substation (GIS) Advantages - Disadvantages
Gas Insulated Substation uses sulfur hexafluoride (SF6) gas which has a superior dielectric properties used at moderate pressure for phase to phase and phase to ground insulation. In Gas Insulated Substation the high-voltage conductors, circuit breaker interrupters, switches, current transformers, voltage transformers and lightning arresters are encapsulated in SF6 gas inside grounded metal enclosures.
Where and Why Gas Insualted Substations are Used
Gas Insulated Substations are used where there is space for providing the substation is expensive in large cities and towns. In normal substation the clearances between the phase to phase and phase to ground is 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 equipments are quite lower. Hence, the overall size of each equipment and the complete substation is reduced to about 10% of the conventional air insulated substation.
Locations where Gas Insulated Substation is preferred:
- Large cities and towns
- Underground stations
- Highly polluted and saline environment Indoor GIS occupies very little space
- Substations and power stations located Off shore
- Mountains and valley regions
The 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 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 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 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.
Gas Insulated Substation Questions
Merits of SF6 Gas Insulated Substation:
Gas insulated Substations are very safe and operating personnel are protected by the earthed metal enclosures. While the Substation in operating condition the Operating personnel can touch the compartment.
The complete enclosure of all live parts guards against any impairment of the insulation system.
SF6 switchgear installations take up only 10% of the space required for the conventional installations.
Initial high investment is required for installation but the cost can be comparable for the less maintenance, reliable, safe operation against conventional substation.
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.
Low weight due to aluminum enclosure, corresponds to low cost foundations and buildings.
Quick site assembly ensured by extensive pre assembly and testing of complete feeders or large units in the factory.
Demerits Of Gas Insulated Substation:
- Cost is higher compared to Air Insulated Substation or conventional substation.
- Procurement of SF6 gas and supply of gas to the site is problematic
- Normally this type of substations are indoor type and requires separate building
- Maintaining Cleanliness is very important. Dust or moisture inside the compartment causes the flash overs
- When fault occurs internally, the outage period will be very long. The damage effect will also be severe.