Gas Insulated Substations (GIS) differ from Air Insulated Substations (substations we usually see) such that all the substation equipment such as busbars, circuit breakers, current transformer, potential transformers and other substation equipment are placed inside SF6 modules filled with SF6 gas. The SF6 gas which has high dielectric property acts as an insulating medium to the High Voltage or Extra High Voltage substation equipment. Generally, Gas Insulated Substations (GIS) are indoor type as it requires 1/10th of the space required for conventional substations. Some of the advantages and disadvantages of Gas Insulated Substations (GIS) are:
Advantages of GIS Substation
- It occupies very less space (1/10th) compared to ordinary substations. Hence these Gas Insulated Substations (GIS) are most preferred where area for substation is small (eg: Cities)
- Most reliable compared to Air Insulated Substations, number of outages due to the fault is less
- Maintenance Free
- Can be assembled at the shop and modules can be commissioned in the plant easily
Disadvantages of GIS Substation
- Cost is higher compared to Ordinary Conventional Substations
- Care should be taken that no dust particles enter into the live compartments which result in flashovers
- When fault occurs internally, diagnosis of the fault and rectifying this takes very long time (outage time is high)
- SF6 gas pressure must be monitored in each compartment, reduction in the pressure of the SF6 gas in any module results in flashovers and faults.
Locations where Gas Insulated Substation is preferred
Gas Insulated Substations are preferred in the places where the land requirement for the substation is difficult such as in populated areas and highly polluted areas where outdoor switchyard is not preferred. In Gas Insulated Substations all the switching electrical equipment such as circuit breakers, isolators, earth switches, and busbars are completely enclosed inside modules which are filled with SF6 gas. These modules are factory made and site assembled. Hence takes very little time for commissioning compared to air-insulated substations.
The main advantage of this Gas Insulated Substation is because of its compact size due to a high dielectric strength of the SF6 gas. And the availability and reliability of the GIS substations are more compared to air-insulated substations (Faults occurring chances of Air insulated substation or conventional substations are more. Hence GIS substations are provided where the high reliability of electric power is required such as nuclear plants and and other important facilities where uninterruption of power is more required.
Some of the places where Gas insulated substations are preferred are:
- Large towns where space available is limited
- Industrial complexes where uninterruption of power is necessarily
- Mountain regions and valleys
- Underground substations
- Off-shore (On sea or lake) substations
- HVDC transmission system terminal substations
In large cities area for commissioning an ordinary Air insulated substation is difficult because of the lack of space available. In such conditions GIS substations out rule the ordinary substation. However, it is to be considered that GIS substation requires high capital cost compared to an ordinary substation.
Offshore and highly polluted areas where equipment present in the outdoor will be corroded due to the pollution and saline environment. Under such case, indoor GIS substation is very much helpful.