Directly buried gas-insulated transmission lines (GIL)

Regarding the increasing interest in underground transmission systems, gas-insulated lines (GIL) represent an attractive alternative to cable solutions. Changes in the regulatory framework with respect to planned extensions of extra high voltage (EHV) systems have intensified the demand for underground transmission lines in general. GIL are utilized mainly above ground (e.g. in substations) or in underground tunnels. However, the option of a directly buried GIL which does not need the expenditure of a tunnel has not so far been put into effect in the EHV range, although it exhibits a number of advantages such as high current carrying capacity, low losses, low charging capacity etc. Some years ago a test installation of directly buried GIL proved that the operation of such a system is favourable in many aspects. For this reason, it was decided in 2008 to set up a system for commercial use in the network of German network operator Amprion. In the area of Frankfurt airport, a double system of a directly buried GIL with a total length of about one km has been erected to ensure the safety distance between overhead transmission line (OHL) and the new runway. The GIL containing an N₂/SF₆ gas mixture is at one end directly connected to a 400 kV gas-insulated substation (GIS). At the other end the GIL joins the incoming OHL via gas-insulated bushings.