The gas-insulated transmission line (GIL) was invented in 1974 to connect the electrical generator of a hydro pump storage plant in Schluchsee, Germany. The GIL went into service in 1975 and has remained in service without interruption since then, delivering peak energy into the southwestern 420 kV network in Germany.

With 700 m of system length running through a tunnel in the mountain, this GIL is still the longest application at this voltage level in the world. Today, at high-voltage levels ranging from 135 to 550 kV, a total of more than 100 km of GILs have been installed worldwide in a variety of applications, e.g., inside high-voltage substations or power plants or in areas with severe environmental conditions.

Typical applications of GIL today include links within power plants to connect high-voltage transformers with high-voltage switchgear, links within cavern power plants to connect high-voltage transformers in the cavern with overhead lines on the outside, links to connect gas-insulated substations (GIS) with overhead lines, and service as a bus duct within gas-insulated substations.

The applications are carried out under a wide range of climate conditions, from low-temperature applications in Canada, to the high ambient temperatures of Saudi Arabia or Singapore, to the severe conditions in Europe or in South Africa.

The GIL transmission system is independent of environmental conditions because the highvoltage system is completely sealed inside a metallic enclosure. The GIL technology has proved its technical reliability in more than 2500 km years of operation without a major failure.

This high system reliability is due to the simplicity of the transmission system, where only aluminum pipes for conductor and enclosure are used, and the insulating medium is a gas that resists aging. The high cost of GILs has restricted their use to special applications.

However, with the second generation GIL, a total cost reduction of 50% has made the GIL economical enough for application over long distances. The breakthrough in cost reduction is achieved by using highly standardized GIL units combined with the efficiencies of automated orbital-welding machines and modern pipeline laying methods.

This considerably reduces the time required to lay the GIL, and angle units can be avoided by using the elastic bending of the aluminum pipes to follow the contours of the landscape or the tunnel. 

This breakthrough in cost and the use of N2/SF6 gas mixtures have made possible what is now called second-generation GIL, and it is a very interesting transmission system for high-power transmission over long distances, especially if high power ratings are needed.

The second-generation GIL was first built for eos (energie ouest suisse) at the PALEXPO exhibition area, close to the Geneva Airport in Switzerland. Since January 2001, this GIL has been in operation as part of the overhead line connecting France with Switzerland. 

The success of this project has demonstrated that the new laying techniques are suitable for building very long GIL transmission links of 100 kilometers or more within an acceptable time schedule.

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