Superconducting cables: from the laboratory into the power grid

A 30-metre prototype had already been made. Christian-Eric Bruzek, a French engineer at the cable manufacturer Nexans, was coordinating ten European partners in the BEST PATHS demonstration “Superconducting cables for very high power transmission”, which had developed a cable system designed to carry up to 3.2 GW. No standards yet existed for the technology, but the team had set benchmarks for material performance and system safety. He spoke about what had been achieved and what remained hard.

Two advantages

The first advantage, he said, is that superconductivity means no electrical resistance and so no transmission losses; in conventional transmission those losses reach up to five per cent of the energy carried. Superconducting cables do need a constant supply of energy to run the cooling fluids that keep them below -200°C, but that is still less than what resistive cables lose.

The second is size. “Our cable design would require underground corridors that are just one metre wide. To transport the same amount of energy, conventional systems would need a corridor up to 10 metres wide with eight resistive cables installed side by side.” Superconducting cables also do not heat the ground, which makes them gentler on the environment.

What was new in BEST PATHS

Other projects were already testing superconducting cables in the grid, but all of them with alternating current. “We have for the first time designed a high-voltage cable system capable of operating in direct current,” Bruzek said. On Long Island in the United States a 0.6 GW AC cable was in operation, and in Japan a prototype AC cable able to carry 1.4 GW had been installed in a substation. Those projects rely on high-temperature superconductors made by a complex and very costly process. The BEST PATHS cable uses magnesium diboride, which is very economical to produce.

Proudest results, hardest problems

He named the wire performance first. Carrying 700 amperes, magnesium diboride proved it can handle huge currents, 500 times more than a simple copper wire, beyond the team’s expectations. The partner Columbus Superconductors had already produced hundreds of kilometres of the wire. Second was a new, safe high-voltage insulation of lapped paper impregnated with liquid nitrogen: after an electrical breakdown the nitrogen automatically fills any gap in the paper and the insulation recovers. The test results had been published in a leading scientific journal.

The hardest technical task, in his words, was managing the connection between the superconducting cable and the existing grid through high-voltage terminations, which he counted among the most important lessons of the demonstration. Other questions needed more discussion, such as how superconducting links would change the way the grid is operated.

Where they would be used first

In the short term, the best uses would exploit the small size: places where civil works are expensive, urban areas short of space, or crossings under rivers. There a short superconducting cable could act as a bridge between resistive cables or overhead lines. For that reason the team was looking closely at possible connections to the existing network.

“So to summarize, short links would be the next application, but then of course we could go further and cover long distances. For instance, we could reach remote wind farms, where several gigawatts, huge amounts of power, are generated and need to be transported to consumption centres.”

Next steps

The next step, Bruzek said, would be testing guidelines for high-voltage DC superconducting cables to secure safety and quality. A consortium of manufacturers would have to form and agree on test procedures, and transmission system operators, still cautious, would have to be involved. He called on policymakers to create economic incentives for TSOs to invest in superconducting cable systems.

New technologies often meet scepticism; as the BEST PATHS coordinator Vicente González López of Red Eléctrica had put it, people are reluctant to be first movers. Research on superconductors had advanced a long way, Bruzek said, but knowledge of it in the power grid world was still very limited.

A dissemination workshop was planned for 5 July 2018 in La Spezia, Italy, at the ASG superconducting fusion magnet factory, to show how the cable is made, followed by a presentation of the results to the consortium and Members of the European Parliament in Brussels on 19 and 20 September. “We still have quite a way to go, but I am optimistic that within the next five years we will see superconducting cables entering the real grid.”

His five-year horizon ran to 2023. The design he describes, and why DC matters for a superconducting link, are covered on the Demonstration 5 page.