To date, copper and aluminium cables carry the current into the city centres. Large substations lower the voltage to 10,000 volts and feed electricity into the distribution network. With compact high-temperature superconducting cables, this structure can be simplified. The BINE Projektinfo brochure "Superconductors for the medium-voltage network" (1/2017) describes a successful field test in Essen.
The new cable, one kilometre long, connects two substations across the city centre. It replaces a conventional 110 kV line and renders one substation in the inner city obsolete. In a test phase of the "AmpaCity" project, taking over two and a half years, it was shown that the technology is on the threshold of market maturity. The superconducting cable and the superconducting fault current limiter installed as a short-circuit protection measure work reliably. The test route has already transmitted more than 200 million kilowatt-hours.
Since superconductivity only works at very low temperatures, the cables in the Essen facility are being constantly cooled with liquid nitrogen. At the cable inlet, it has a temperature of minus 206 °C, at the cable outlet of minus 201 °C. A subcooler cools it back to the required inlet temperature via a heat exchanger. Despite the high effort involved in cooling, preliminary investigations have shown that superconducting cables are the only sensible way of avoiding high-voltage cables as well as resource and space-consuming substations in inner city areas.
The AmpaCity project is conducted under the direction of the energy suppliers innogy SE and carried out jointly with their project partners.