When the transmission system operator (TSO) Amprion (formerly RWE TSO) started planning further development of the extra-high-voltage (EHV) transmission system in the German state of Hesse, the expansion plans for the Frankfurt Airport had to be taken into account. The existing 220-kV overhead transmission lines that are routed in close proximity to the area scheduled for new runway had to be under-grounded for a distance of about 1 km (0.62 miles).

Also forming part of the power-supply system expansion in the Frankfurt region was the plan to convert the existing Kelsterbach transformer substation, situated directly alongside the Autobahn and opposite Frankfurt Airport, from 220 kV to 380 kV for integration into the existing 380-kV transmission network. Amprion had already installed the new 380-kV compact gas-insulated switchgear (GIS). The 380-kV GIS was commissioned in 2007 replacing the 220-kV outdoor installation. Interconnection to the 380-kV system is scheduled for completion by mid-2010.

Amprion, faced with the problem of under-grounding the existing 220-kV transmission line, considered two alternative solutions, conventional underground cables versus gas-insulated line (GIL). The TSO decided to install a GIL that will be connected via bushings on the transmission line terminal tower. The GIL that will be directly buried in a similar manner to installing a pipeline, terminating at the remote end with connection to 380-kV switchgear in Kelsterbach substation.

GIL Technology

The tubular conductor of a GIL, which is of coaxial construction, consists of an aluminum conductor enclosed in an aluminum tube. The tube is filled with an insulating gas mixture consisting of 80% nitrogen and 20% sulfur hexafluoride (SF6). Given their relatively high power transmission capacity compared with overhead lines, GIL's are designed so that the encapsulation does not burn through in the event of internal arcing. The superior technical characteristics for GIL's include:

  • An extremely low electromagnetic field in the external space

  • Because of their large aluminum cross-section, transmission losses are lower than for cables and overhead lines

  • Due to the gaseous dielectric, no measures are required for reactive-power compensation.

Amprion's GIL Installation

The GIL systems installed by Siemens to date have usually been laid above ground where the terminations lead to a switching station, or in a tunnel, as is the case with the system routed beneath an exhibition hall at the Palexpo in Switzerland. Additional installation standards must be applied if the tubes are laid directly in the ground in order to guarantee the long-term quality and reliability of the system. Special welding techniques enable long pipe sections to be laid without flange joints, and thus ensure strength and absolute gas leak tightness. Plastic coatings function as passive corrosion protection, and active corrosion protection is also provided. Systems for monitoring temperature and gas density are used for operational surveillance, and a measuring system for the on-demand recording of partial discharges is also provided.

Fundamentally, the characteristics of the GIL make it possible to be routed over any terrain including steep inclines and vertical sections. Through systems laid above ground or in tunnels, Siemens has been able to gather considerable experience in the connection of cavern power plants or switching stations since first deploying this technology in the 1970s.

Amprion awarded Siemens Energy the contract to commence the installation of the twin 380-kV three-phase GIL's at Frankfurt Airport in 2009. The GIL system, to be buried directly below ground level will have a continuous load-transfer capability of two times 1800 MVA (two circuits). This pilot project marks the first practical application of this technology by Amprion, the company will derive benefits from the experience of the prototype tests of this transmission technology that was undertaken by Siemens with the cooperation of E.ON Vattenfall and Amprion.

Technology for the Future

The selection of the 380-kV GIS already installed in addition to being compact was made with the longer-term objective of interconnection with the GIL's selected for this transmission line under-grounding project.


The route length of each phase extends to some 900 meters (2953 ft), so in total for the Kelsterbach system, with its two three-phase circuits, the length of buried GIL tubes will extend 5.4 km (3.4 miles). Hence, this system will represent the longest length installed to date by Siemens. Therefore, about 500 individual modules will be delivered and welded together on site in order to implement the single-phase tube length totaling 5.4 km. Construction work on installing the GIL modules below ground level along the planned route commenced in May 2009 with the completed GIL transmission circuits scheduled for interconnection to the 380-kV system in 2010.

Manufacturers and construction contractors intend to use this joint Amprion pilot project to gain experience for optimizing the laying technology for long distances, as well as for studying the short- and long-term thermal overload capacity of this technology. This GIL project is therefore a milestone in the field of gas-insulated, EHV lines. Implementation of the first buried GIL in Germany will enable both the grid operator and the manufacturer to test this transmission technology as an alternative to conventional underground cable solutions. This kind of experience is vital in light of the growing global demand for underground energy transmission systems.

The author wishes to acknowledge the support and assistance given by Dr. Ing. Stephan Pöhler, the section head of GIL at Siemens. His contributions in the preparation of this article were of great value.

Companies mentioned in this article:

Claus Neumann (claus.neumann@amprion.net) has a Dipl-Ing degree in electrical engineering by the Technical University of Aachen, following which he was engaged as a testing engineer in the high-voltage and insulating material laboratory of a switchgear manufacturer. He joined the department for high-voltage equipment of RWE in Essen, Germany, where he progressed to become head of this department. He also received his Dr.-Ing degree from the Technical University of Darmstadt. In 2001, he became honourable professor at this university, where he lectures on high-voltage switchgear and substations. Since 2005, Neumann has been responsible for the Operational Asset Management at Amprion, Dortmund, Germany. His main fields of activity are fundamental design and layout of high-voltage apparatus and asset management, including maintenance strategy, monitoring and diagnostics. He is among a member of CIGRÉ Study Committee C4, Power System Performance of CIGRÉ SC D1 Working Group 3, Gas-Insulated Systems.

Amprion www.amprion.net

E.ON Vattenfall www.vattenfall.com

RWE Transportnetz Strom www.rwe.com

Siemens www.siemens.com/energy