The SF6 Gas and MV Switchgear

Since 1960, SF6 has been a revolution in Switchgear Field. Its typical electrical, chemical and physical properties have brought a sea of change in the Electrical Industry. Substations are becoming more and more compact and easy to handle . All these factors are providing us an impetus for us to knock on UHV transmission volatges of 800/1200 kV .

SF6 Gas has almost entirely replaced Vacuum as Arc Extinguishing Medium in EHV and UHV applications. In MV applications up to 36 kV, Vacuum and Air are still predominant arc extinguishing medium while use of SF6 as extinguishing medium is limited by guidelines of Kyoto Pr otocol. However, recent developments in sealing techniques provide us an opportunity to use SF6 as insulating or switching media in MV Switchgear.The following products are a result of such a development.

1) SF6 Load Break Switch - In this case, SF6 is used as switching medium. SF6 gas is enclosed in a sealed tank as shown below .

The switching takes place in SF6 tank, while protection is provided by means of HT fuses. Such a system is suitable for Switching and protection of loads and Transformers upto 1 MVA in 12/24kV Systems. Such a system is called half-insulated system. (as Air is Insulating while SF6 is Switching medium)
In case multiple loads need to be fed, the switches can be arranged in a Ring Main configuration. Below photo shows a configuration with 2I/C and 3 O/G feeders 

In special case to make the system even more compact, SF6 gas can be used as both insulating as well switching medium. Such a system is called fully insulated system. Some configurations of this type can be seen as below 

2) Gas Insulated VCB - Vacuum Circuit Breakers are the most predominant switchgear in MV range.By using SF6 as an insulating medium, VCBs can be made compact thru reduced clearances in SF6. Such a configuration is called Hybrid Gas Insulated Switchgear (GIS)
Many manufacturers have developed their own designs and models for GIS. GIS are popular in places wherein space is at a premium and are an integral part of Compact Secondary Substations (CSS). Some models of GIS can be observed as below 

Using SF6 gas in MV switchgear can provide us with a lot of advantages as below ;
1) Very limited space required for installation.
2) Favourable ergonomics due to reduced size and weight.
3) Can be installed in any type of climate (Sea Shore, hills, flooded areas, polluted industrial areas).
4) Low risk of fire as SF6 is non-inflammable. Also due to sealed enclosures, no toxic hazards are possible (as per IEC 61634).

All the above features also contribute to higher service life, better continuity as well as good aesthetics. It results in efficient design of the power supply system. In fact the location of the equipment optimally with respect to the local demand, reduces the losses of energy with a double result: better technical and economical performance of the network and less energy generation with its environmental side-effects.

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