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Gas Insulated Substation
Gas Insulated Substations(GIS) is a compact, multicomponent assembly enclosed in a ground metallic housing which the primary insulating medium is compressed sulphur hexaflouride(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
Why we need GIS1) Gas insulated Substations have found a broad range applications in power systems over the last three decades because of their high reliability Easy maintenance, small ground space requirements etc.,. 2) In our country also few GIS units have been in operation and a large number of units are under various stages of installation. 3) Although GIS has been in operation in several years, some of the problems are needful attention4) These problems include VFTO during switching operations or earth faults and transient enclosure voltages and particle contamination.
Advantages of GIS:
a) Because of the entire equipment being enclosed in enclosures, filled with pressurized SF6 gas, installation is not subject to environmental pollutions, as experienced along coastal areas or certain types of industries. b) Such installations are preferred in cosmopolitan cities, industrial townships, etc., where cost of land isveryhighand higher cost of SF6 insulated switchgear is justified by saving due to reduction in floor area requirement. It is not necessary that high voltage or extra high voltage switchgear to be installed out doors. c) Since most of the construction is modular and the assembly is done in the works, one site erection time both for supporting structures and switchgear is greatly reduced.

Disadvantages with Air Insulated Substations:
a) It requires huge amount of area .
b) Each and every component of substation is exposed to air and pollution.
c) Particularly in coastal area all the insulators are exposed to air and saline contamination.
d) Frequent flashovers and breakdown occurs.
e) Maintenance cost is more.
f) Installation time is also more.
g) More concrete work is necessary.

A) The necessity of this study :
Extremely high dielectric properties of SF6 have long been recognized. Compressed SF6 has been used as an insulating medium as well as arc quenching medium in electrical apparatus in a wide range of voltages.
Gas Insulated Substations (GIS) can be used for longer times without any periodical inspections. Conducting contamination (i.e. aluminum, copper and silver particles) could, however, seriously reduce the dielectric strength of gas-insulated system
B) The origin of these particles :
Metallic particles in GIS have their origin mainly from the manufacturing process or they may originate from moving parts of the system, such as breakers and disconnectors.
Metallic particles can be either free to move in the GIS or they may be stuck either to an energized electrode or to an insulator surface (spacer, bushing etc.). 1) Different metallic contamination viz Cu, Al and Ag have been considered for the above study. Typically a GIB of inner and outer dia 102/292mm have been considered for a 245KV system. used for simulation. Co-efficient of restitution and pressure have been varied from 0.1 to 0.9 and 0.1Mpa to 1Mpa respectively. 3) Typical results show that for a variation of copper particles of radii from 0.2 to 5mm, the maximum radial movement reduces from 6mm to 1mm. 4) Whereas in the case of variation of length from 1mm to 16mm the radial movement increases from 1mm to 9.8mm. With the change of pressure the movement is found to be almost same for majority of particles consider If a metallic particle crosses the gap and comes into contact with the inner electrode or if a metallic particle adheres to the inner conductor, the particle will act as protrusion on the surface of the electrode, and the voltage required for breakdown of the GIS will be dramatically decreased.
A metallic particle stuck on an insulator surface in a GIS will also cause a significant reduction of the breakdown voltage
Depending on the shape of the particles, as well as the geometry and voltage levels of the system, the particles get more or less influenced by the electric field which, in turn, makes them hazardous to the electrical system, in terms of partial discharges and breakdown.
1. The introduction of SF6 gas has revolutionized not only the technology of circuit breakers but also the layout of substations. 2. The dielectric strength of SF6 gas at atmospheric pressure is approximately three times that of air. 3. It is incombustible, non toxic, colourless and chemically inert. 4. It has arc-quenching properties 3 to 4 times better than air at equal pressure. 5. This enables the substation to be laid in a much lesser space.
6. Space requirement is only 10 to 25 percent of what is required is a conventional substation. 7. Thus where creation of a substation becomes necessary but adequate space is not available, such as in big cities or otherwise highly populated areas, GIS is the solution

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reference:
ayyarao.blog.co.in
google.com
wikipedia.com