GAS INSULATED SUBSTATION (GIS)



DISCRIPTION:-

Gas Insulated Switchgear (GIS) refers to the general equipment of a substation in a metal enclosure filled with high-insulation SF6 gas and connected to the ground, which includes a circuit breaker, disconnecting switches, earthing switches, and peripheral devices (e.g., current transformer, voltage transformer, and lightning arrester). It can be installed in a small space in cities or buildings. Ensuring reliable and stable operation and good maintainability, it is especially suited for polluted and/or salty seashore areas.

Gas Insulated Substations(GIS) is a compact, multi component assembly enclosed in a ground metallic housing which the primary insulating medium is compressed sulphur hexafluoride(SF6) gas. It generally consists components Of
a) Bus bars
b) Circuit Breakers
c) Disconnecting switches
d) Earthing switches
e) Current transformers
f) Voltage transformers
g) Cable and boxes
h) Gas supply and gas monitoring equipment
i) Densimeters and
j) Local control

Locations where Gas Insulated Substation is preferred:


· Large cities and towns
· Under ground stations
· Highly polluted and saline environment Indoor GIS occupies very little space
· Substations and power stations located Off shore Mountains and valley regions


Merits of SF6 Gas Insulated Substation:


Safe:
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.

Reliable:
The complete enclosure of all live parts guards against any impairment of the insulation system.

Space Saving:
SF6 switchgear installations take up only 10% of the space required for the conventional installations.

Economical:
Initial high investment is required for installation but the cost can be comparable for the less maintenance, reliable, safe operation against conventional substation.

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.

Low Weight:
Low weight due to aluminum enclosure , corresponds to low cost foundations and buildings.

Shop assembled:
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.

GIS Disadvantages:

  • Excessive damage in case of internal fault. Long outage periods as the repair of damaged part at site may be difficult.
  • Requirement of cleanliness is very stringent. Dust or moisture can cause internal flashovers.
  • Such substations are generally indoor, so they need a separate building. This is generally not required for conventional outdoor substations.
  • Procurement of gas and supply of gas to site is problematic, adequate stock of gas must be maintained.
  • Project needs almost total imports including SF6Gas. Spares conventional substation is totally indigenous up to 400 kV.

GIS Applications:


Some possible applications of these very high voltage metal-enclosed stations:

1 .Main distribution stations inside cities.

2. Main distribution stations for important customers.

3. Main distribution stations in zones with pollution, salt, or risk of explosion.

4. Main distribution stations with special characteristics (underground stations, shelters of reinforced concrete, etc.).

5. Classic installation expansion, in case of reduced space.

6. Mobile transformation stations.

Economics of GIS:


The equipment cost of GIS is naturally higher than that of AIS due to the grounded metal enclosure, the provision of a local control cabinet, and the high degree of factory assembly. A GIS is less expensive to install than an AIS. The site development costs for a GIS will be much lower than for an AIS because of the much smaller area required for the GIS. The site development advantage of GIS increases as the system voltage increases because high voltage AIS take very large areas because of the long insulating distances in atmospheric air. Cost comparisons in the early days of GIS projected that, on a total installed cost basis, GIS costs would equal AIS costs at 345 kV. For higher voltages, GIS was expected to cost less than AIS. However, the cost of AIS has been reduced significantly by technical and manufacturing advances (especially for circuit breakers) over the last 30 years, but GIS equipment has not shown any cost reduction until very recently. Therefore, although GIS has been a well established technology for a long time, with a proven high reliability and almost no need for maintenance, it is presently perceived as costing too much and is only applicable in special cases where space is the most important factor.

Locations where Gas Insulated Substation is preferred:


  • Large cities and towns
  • Under ground stations
  • Highly polluted and saline environment Indoor GIS occupies very little space
  • Substations and power stations located Off shore
  • Mountains and valley regions


Why need GIS?


  • GIS has small ground space requirements.
  • Gas insulated Substations have easy maintenance (nearly zero Maintenance
  • Less field erection time & less erection cost.
  • For underground powerhouse of Hydro electric power project where
  • Space constraint is a major issue.
  • For Fast Growing Major Cities where land availability is costlier.
  • Non-Flammability & Non-Explosive, Oil-free & Less Pollution

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