13.1 Introduction

Sulphur Hexafluoride (SF₆) has been used as an insulating medium in switchgear industries since the latter half of the last century. Its application has been extended to other areas of electrical products where there was a need to make them more compact, taking advantage of the much superior dielectric properties of this gas. This was partially driven by the need to install high voltage substations comprising gas insulated switchgear in urban load centres where there was a constraint on the availability of land. The use of this technology for the transmission of bulk power, over longer distances, was a logical extension and resulted in the development of gas insulated bus (GIB) also referred to as gas insulated lines (GIL) or gas insulated transmission lines (GITL).

In a GIS, SF₆ not only provides the desired insulation but also facilitates the quenching of arc triggered by breaking, switching, or earthing devices. In the GIB, it provides only the necessary insulation. The volume of gas required in a substation comprising GIS is far less than the volume of gas required in a GIB of any significant length. Consequently, the cost of gas as a percentage of the cost of equipment is far higher in GIB when compared to that of a GIS.

Since SF₆ has been identified as a greenhouse gas, efforts are on to reduce its usage and find alternatives. A gas insulated bus with a mixture of 80% N₂ and 20% SF₆ is considered as an environmentally friendly option. It is often referred to as second-generation gas insulated bus. This reduces the cost substantially for long-distance bulk power transmission.

Further dielectric gas (Green Gas for Grid – g³) (CF₃)₂ CFCN and (CF₃)₂ CFC(O)CF₃ with low global warming potential (GWP) and higher dielectric strength have been developed and used in GIB.

It is notable that a well-designed and operated GIB system achieves essentially zero emissions of SF₆ and consequently no significant global warming impact. Alternate options to SF₆ gas offer challenging design constraints for practical implementation under certain ambient conditions such as in outdoor installations where the temperatures are below
-25℃ (-13 ℉).

With the increase in the system voltage for substation rising to 1200 kV and the need to protect it from the vagaries of nature & environmental conditions, by installing them indoors, gas insulated switchgear has found acceptance even where the land was not at a premium.

Gas insulated bus, also called gas insulated transmission lines was a natural extension of this idea. GIBs have been manufactured and installed up to a rated voltage of 1200 kV and rated current of 8000 A. In 1975, the very first GIB rated for 400 kV was installed in a pumped storage plant in Schluchsee, Germany.