INTRODUCTION
            A gas insulated substation is an electrical substation in which the major structures are contained in a sealed environment with sulphur hexafluoride gas as the insulating medium. The use of SF6 gas as an insulating medium in switchgear reduces the clearance distance between active and non-active parts of a switchgear facilitating the following advantages of gas insulated applications compared to air insulated applications:
l  Less space requirements - especially in congested city areas.
l  Less sensitivity to pollution, as well as salt, sand or even large amounts of snow
l  Less operation & maintenance costs
            Industries and Utilities are under extreme pressure to meet consumer and regulatory demand for high quality of supply at competitive prices while lowering environmental impact. Therefore, substation construction requires an overall responsible partner for the system, the products and integration to the grid.
            Reliable and economical power transmission and distribution are key functions for the future electric power supply. Gas insulated switchgear is used in industrial areas to fulfil high-energy demands by space saving design with a minimum of cost. SF6 insulated switchgear is able to fulfil these requirements.

GIS Vs AIS
            The advantages of gas-insulated switchgear are its compact design and the modular system (Fig-1). The standardized modular structure is made to match the various customers specifications and allows to realize almost all substation configurations in compliance to them.
            GIS installations tend to be much more expensive that air-insulated installations with the same rating. The additional capital cost is justified based on the reduced cost of real estate, the ability to provide environmental containment, or the fact that the substation is totally shielded from lightning, something that is not practical with air insulated technology.

EXAMPLE OF REPLACEMENT OF AIS WITH GIS
Fig-1 (1) :Existing AIS substation (72kV-1200kA, 2 X 15MVA)

Fig-1 (2) :After replacement (Gas Insulated Substation)

HYBRID SUBSTATION
            In most cases, the circuit breakers in gas insulated substations employ SF6 as the interrupting medium as well as the insulating medium, but there are hybrid installations (especially at lower voltages) in which breakers use vacuum interruption. The gas pressure required for SF6 to serve as an interruption medium is much greater than the pressure required for it to be an insulation medium. In early generation SF6 installations, a dual-pressure approach was required - one gas pressure in those areas where SF6 is only an insulating medium, and a higher pressure inside the breaker interrupting compartment. That was complicated and expensive and required high maintenance. Modern equipment uses a 'puffer' technique in which SF6 gas is forced into the space between breaker contacts as they are opening, increasing local gas pressure enough to support interruption.
Fig-3 :Mitsubishi Electric Power Products, Inc. (MEPPI) now introduces its latest evolution in circuit breaker technology, the Gas Switch and Circuit Breaker (GSB). The primary advantage of the GSB over the GCB is the space savings at the substation. The GSB uses the same proven dead-tank circuit breaker, but replaces the switches using now with ones insulated by SF6. The space required for the breaker maintenance ground switches and disconnect switches is reduced dramatically. The resulting product replaces the standard combination of GCB and air switches and places the same functions into one-third the length.


VERY FAST TRANSIENT OVERVOLTAGES (VFTO)
            Very Fast Transient Over Voltages (VFTO) are caused by disconnector switching operations within a Gas Insulated Substation. The very fast transient overvoltages contains very high frequency components in the order of several 10's of MHz and may cause high frequency oscillations in the transformer windings, leading to possible overstressing of the transformer insulation.
            For certain switching conditions where GIS disconnectors are operated following circuit breaker operation it is possible to generate VFTO, which act as travelling waves and propagate within and external to the GIS. The source of these overvoltages is due to the re-strikes or pre-strikes during disconnector opening or closing operations.                                         
            The maximum magnitude of the re-strike voltage is 2.0pu. This value is largely dependent upon the level of trapped charge on the GIS busbar existing at the time of the re-strike. The amplitude of trapped charge is strongly influenced by the asymmetry of the      intercontact  breakdown voltage occurring on the fixed and moving contacts of the    disconnector. This is dependent upon the dimensional differences between the contacts. Trapped charge is also influenced by the operating speed of the disconnector during its opening cycle.

PROPERTIES OF SF6
  1. SF6 does not harm to the ozone layer.
  2. Sf6 gas is chemically stable
  3. Non poisonous
  4. Colourless & heavier than air
  5. Almost water insoluble
  6. Non inflammable

GAS INSULATED TRANSFORMER
l  Uses SF6 Gas as the insulating and cooling medium instead of insulating oil.
l  First units produced in 1967.
l  Transformer applications: Distribution class units up to 400 MVA, 345 kV. Primarily used in substations located in urban areas (including inside buildings, underground) due to safety benefits.

LAYOUT EXAMPLE OF GAS INSULATED SUBSTATION
            Since gas insulated transformers do not require a conservator, the height of the transformer room can be reduced. In addition, the characteristics of non-flammability and reduced risk of tank explosion mean fire-fighting equipment can be removed from the transformer room. As a result, the gas insulated transformer, gas insulated shunt reactor, GIS and control panels can be all housed in one room.
Fig-4 :Layout Example of 300kV Insulated Substation


Fig-5 :330kV Gas Insulated Underground Substation

            Due to the inherent safety and compact layout of a insulated substation, it is the best solution for substations which are to be built in restricted underground spaces. Gas insulated substations are particularly suitable for underground construction beneath public buildings.

APPLICATIONS
The main applications for gas insulated substations today are:
1.    High voltage installations (usually 115kV and above although some manufacturers offer equipment with voltage ratings down to 20kV). The higher the voltage, the more favorable gas insulated technology becomes. The footprint of 765kV conventional substation is enormous, and GIS technology allows a significant size reduction.
2.    Urban installations. Usually, but not always, GIS technology is used for installations in areas where the cost of real estate is a significant consideration.
3.    Indoor installations (which are more common in urban areas for aesthetic reasons). It is generally not practical to build an air-insulated substation inside a building, but GIS can easily go inside buildings.
4.    Other environmentally sensitive installations. GIS technology is popular in desert and arctic areas because it can be enclosed in a building with some environmental control.
5.    Gas insulated substations also contain the electrical components within a Faraday cage and are therefore totally shielded from lightning.

ADVANTAGES
  1. Gis have no risks for fire & explosion due to leakage of oil
  2. They generate no noise & have no radio interference
  3. Can Located closure to load centers.
  4. It offer solutions including
-In industrial areas where space & pollution problems
-Mountain areas where ice & snow are major problems
5.    Less space requirements - especially in congested city areas
6.    Less sensitivity to pollution, as well as salt, sand or even large amount of snow
7.    Less operation & maintenance costs
8.    Reliable, experienced flexible and space saving GIS products in advanced technology  
           
DISADVANTAGES
  1. Although much smaller in size, GIS installations tend to be much more expensive than air-insulated installations with the same rating. The gas itself can be very expensive if something like Sulphur Hexafluoride is used.
  2. VFTO (Very Fast Transient Overvoltage) during switching operations or earth faults and transient enclosure voltages and particle contamination.
  3. Maintenance is difficult for GIS.
CONCLUSION
            GIS are necessary for EHV & UHV and some important areas to be studied include more conservative designs better particle control & improved gas handling & decomposition product management techniques. Achieving & maintaining high levels of availability requires a more integrated approach to quality control by both users and manufactures.
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