RWE and its partners have started the work of replacing a 1 km long high voltage cable connecting two transformer stations in the Ruhr city of Essen, Germany, with a superconducting cable and fault current limiter. This will mark the longest superconductor cable installation in the world. The three-phase, concentric 10 kV cable is being produced by Nexans and is designed for a transmission capacity of 40 MW. As part of the project, known as Ampacity, the Karlsruhe Institute of Technology will analyse suitable superconducting and insulating materials. This installation will also be the first to combine a superconducting cable with a resistive superconducting fault current limiter for overload protection. The limiter will be manufactured by a specialisedsuperconductor unit of Nexans located in Huerth, Germany.

This project could presage a whole new dimension in the restructuring of inner-city networks. Following the successful completion of a two-year field test, it would be possible to install 10 kV superconducting links in large sections of the backbone of the Essen distribution network as part of efforts to reduce the numer of high-voltage installations. In the medium term, this would lead to greater efficiency as well as lower operating and maintenance costs while simultaneously reducing land use. The dismantling of numerous 110/10 kV transformer stations would help to free valuable space in inner-city areas. Owing to these factors among others the project is being supported by the energy research department of the Federal Ministry of Economics and Technology (BMWi). Research project total costs are approximately €13.5 million including government funding of €6 million.

The project was preceded by a detailed study in which a number of research institutes worked alongside the project partners Nexans and RWE to analyse the technical feasibility and economic efficiency of a superconductor solution at medium-voltage level. The study revealed that superconducting cables are the only reasonable alternative to high voltage cables in city networks and that their use would mean that resource- and land-intensive transformer stations could be demolished. Although copper medium voltage cables could also be used in inner-city areas to transmit high power, the cost efficiency of this solution would be cancelled out by the much higher ohmic (ie cable heating) losses. Furthermore, conventional medium voltage cables for the Essen project in particular would also be out of question, as they require much more routing space: instead of a single 10 kV superconductor cable, five copper cables would need to be laid in paralle, in the very limited space under the city streets.