Chapter # 3                                                                                          INSULATING MATERIAL   

3.6 Insulating Gas  

          Air has been used and continues to be used as an insulating medium in electrical equipment and transmission lines covering the entire range from a low voltage (LV) to ultra-high voltage (UHV).  Compressed air has been used in air-blast circuit breakers for quenching the arc.  Air, as an insulating medium, has been well researched and documented.

          With the increase in the transmission voltage, there arose a need to make the associated switchgear equipment compact, free from environmental constraints.  Man-made sulphur hexafluoride gas with excellent dielectric properties, had been synthesized in the early part of the last century and used initially in medium & high voltage circuit breakers and subsequently in gas insulated switchgear followed by gas insulated transmission lines.   has found a mention in the Kyoto protocol and is recognized as a greenhouse gas but Sulphur hexafluoride does not deplete the Ozone layer when it is released in the atmosphere.  It does not contain Chlorine atoms as do the chlorinated fluorocarbon (CFC).  The environmental issues arising out of the use of  are its impact and contribution to ozone depletion from by-product during switching, arc quenching, and corona discharge.

          Environmental concerns have led to the usage of the gas mix (80%  and development of green gas for grid ( ) by 3M.  Gas inuslated bus, with these insulating medium, are in commercial operation.  

3.6.1   Sulphur Hexafluoride (SF6)

          Sulphur hexafluoride had been recognized for its insulating properties in the early 30s of the last century.  A manmade compound,  is non-combustible, odourless, chemically inert, and non-toxic.  Its dielectric strength and consequently, its arc quenching properties at atmospheric pressure are three times better than that of air. 

          The breakdown voltage of gas between two electrodes is a function of the pressure and the gap length and is governed by Paschen’s law. At higher pressures and gap lengths, the breakdown voltage is approximately proportional to the product of pressure and gap length.

The theoretical maximum of dielectric tress in pure  is 89 kV per / cm bar.

            Present contribution of  to global warming is less than 0.01% and is expected to reach 0.1% by the end of the century.  Nevertheless, efforts are on to reduce its leakage from containers, reclamation, re-usage, and making them neutral. It has been noted that a Mixture of  and in the ratio of 20:80 has as good a dielectric strength as .  Further, it seems to be much less susceptible to the effect of non-uniformity in the field, thus mitigating the effect of particle and surface protrusions. Hence there is a concern in the industry to optimally use the gas.  The production of  gas is about 10,000 tons per year, globally.  However, a lot of investigation needs to be carried out on its quenching properties and life before the composition of the mix can replace  in circuit breakers.

3.6.2   Green Gas for Grid (g3)

          Minnesota Mining and Manufacturing Company (3M), USA, has developed gaseous formulations that can be used instead of  .  General Electric has commissioned few installations using these as the insulating gases.  The two variations of the gases are:

3.6.3   Air

          Air is extensively used as an insulating medium in transmission lines, Switchyards, Metal enclosed Switchgear, and Metal enclosed Busbar Systems.

 

          Air is a mixture of gases, water vapor, and dust particles. It's made up of 78% nitrogen, 21% oxygen, 1% argon, and trace amounts of water vapor, carbon dioxide, and noble gases.

Paschen's law applies to Air.

Presence of moisture and dust in air, drastically affect the performance of Air as a dielectric medium.