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The superconducting cable ends in a U-bend at the substations, which compensates for the temperature-related changes in length. The cable can extend or contract up to three metres.
© innogy SE
High-temperature superconducter
Projektinfo 01/2017

Structure of the superconducting cable. The liquid nitrogen supply is at the centre. The superconducting strips for the three phases are surrounded by the copper neutral conductor. The nitrogen return cools from the outside.
© innogy SE

Laying the superconducting cable in the inner city of Essen.
© innogy SE
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Superconductors for the medium-voltage grid

Scientists are testing the longest high-temperature superconducting cable in the world under real conditions in Essen. One kilometre long, it connects two substations in the inner city. It replaces a conventional 110 kV line and renders one substation in the inner city obsolete. After two years of testing, it has passed the field test. It could be a blueprint for the future power supply system in urban areas.

Demand for electricity in these areas is growing. At the same time, Germany's power generation structure is shifting away from large power stations towards a large number of decentralised producers. The grid, originally conceived as a one-way street, has to adapt to these developments: electricity from power stations reaches consumers via high-voltage, medium-voltage and low-voltage grids. For some time now, renewable energy sources, however, have been feeding into the medium-voltage grid. This is why many researchers agree that the energy transition is taking place in the medium-voltage grid.

In particular in the Ruhr area, these new challenges are superimposed by a structural change in the form of a sharp decline in the heavy industry. Here, the changing requirements to capacity and spatial distribution of the grids present special opportunities to test new grid structures and technologies. In the pilot project AmpaCity, scientists from innogy SE and their project partners are investigating how high-temperature superconducting cables (HTS) could be used to banish transformer substations and high-voltage cables from inner cities. As an additional new component of the medium-voltage grid, they employ a superconducting fault current limiter that protects the HTS cable during short circuits.

The test helps to estimate the application potential of these superconducting components and to better evaluate economic aspects. In over two and a half years of operation, the superconducting system in the city of Essen has shown that the technology is now on the threshold of market maturity. The superconducting cable and the superconducting fault current limiter installed for short-circuit protection work reliably. The technology can be used, for example, in inner city areas in place of expensive high-voltage components, thereby making distribution grids more efficient and economical.


Current instead of voltage

In conventional power supply, cables with copper or aluminium conductors carry the current into the city centres at high voltages in order to minimise transmission losses. Here, large substations lower the voltage to 10,000 volts and feed electricity into the distribution grid. Smaller stations then transform the supply voltage for customers to 400 volts.

With compact superconducting cables, this structure can be simplified. This was confirmed in a feasibility study conducted by the Karlsruhe Institute of Technology, which preceded the pilot project. High-voltage lines and substations could be dismantled step by step by using the new cables in inner city areas, since HTS cables carry large amounts of current over long distances at 10,000 volts, almost without energy losses. Theoretically, high-capacity transmission at the medium-voltage level would also be possible using copper cables, but the energy losses and the demand for transmission routes would be far higher. In Essen, for example, five parallel copper cables would have to be laid underground instead of just one HTS cable.

The conclusion of preliminary investigations is that superconducting cables are the only sensible way to avoid inner city high-voltage cables and to lower the number of resource and area-consuming substations. For a 40-year service life, their economic efficiency versus conventional high-voltage cables ought to be higher as well – despite cooling costs. This superconductor technology has the potential to considerably lower the overall costs created by the future expansion of the grid.

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