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There is a tendency to see superconducting power cables as something of a laboratory curiosity, with commercial applications some years in the future. But the reality is that high-temperature super-conductor cables, cooled by liquid nitrogen, are ready now to start playing their part in the grid.
Because of their potential for perfectly efficient conduction of large amounts of electrical power, high-temperature superconductor (HTS) cables are often imagined as a possible replacement for conventional high-voltage cables as the backbone of a transmission grid, carrying power over long distances. That could happen in perhaps the next five to 10 years. Currently, however, HTS cables are set to make the most significant impact when used in short lengths (from a few hundred metres up to a few kilometres) to carry large amounts of power through areas where space and access are at a premium.
For example, in built-up areas it is becoming increasingly difficult for utilities to obtain the rights of way to install the new overhead lines or underground cables needed to meet increasing demands for power. When operating at the same voltage, HTS cables can carry between five and 10 times as much power as conventional cables. So simply rethreading existing infrastructure with superconductors could address these bottlenecks to significantly reinforce the grid supply for power-hungry city dwellers.
This approach has been proved in a number of demonstration schemes, including the Long Island Power Authority project in the United States that started operation in 2008.
There is though an even smarter way to use superconductors. And that is to carry the same power as a conventional cable system, but reducing the operating voltage from high to medium. The smaller space needed for cables can free up distribution companies to develop simpler networks, reducing the amount of land used.
It also allows for smaller, less obtrusive substations, while studies have shown that in a typical urban network the number of transformers might be reduced from 20 to just 15. All of which starts to tip the financial balance in favour of HTS cables.
This year we are putting the lower-voltage concept to practical test in the AmpaCity project in Essen, Germany. Here Nexans, in partnership with RWE and the Karlsruhe Institute of Technology, will install a 1km underground HTS cable operating at 10kV. That’s about one tenth of RWE’s usual 110kV transmission voltage.
Grid-compatible superconductor cables are ready for commercial exploitation now. On a direct voltage and length comparison, HTS cables will appear significantly more expensive than conventional cables. However, when you consider the increased capacity, smaller footprint and reduced operating voltage, it is feasible for the total installed cost of an HTS cable system to more than match that of conventional cables.
And over the next two to three years I am confident that we will see commercial applications in which superconductor cables emerge as the only viable option.