nexans cable cross section

Industry readies for extreme electricity

A superconducting cable with a record-breaking performance signals a technology on the brink of commercialisation, reports E&T.

Late last year, a team of engineers, physicists and materials scientists from industry and academia gathered in a laboratory in Hanover, Germany, to set a world record. As part of the Endesa Supercable project, they had spent three years building a single phase, medium voltage superconducting cable with the intention of showing the world exactly what the technology could do. Now they wanted to put it to the test.

During ten load cycles of 24 hours, the 30m cable, alongside associated terminations and cryogenic equipment, carried an incredible 3,200A for eight hours per cycle while subjected to twice the nominal voltage of 24kV continuously. The record-breaking current intensity not only proved superconducting technology to the industry but offered a tantalising glimpse of what the power grids of tomorrow could achieve.

Dr Alvar Sanchez is one of four physicists from the Universitat Autònoma de Barcelona who worked with Spanish utility Endesa, France-based cable manufacturer Nexans and Spanish technology centre Labein Technalia to develop and construct the cable. 'We wanted to demonstrate to ourselves and the electricity companies that we could make a state-of-the-art cable with these requirements,' he says. 'It has been done and it has been tested. Only a few years ago a few scientists were convinced that this technology could be used but now the electrical companies are convinced as well.'

So why develop superconducting cables for power transmission? Numerous dramatic energy blackouts over the last decade have highlighted the ageing and inadequate transmission and distribution networks of many nations around the world. As operators upgrade electricity grids, superconducting cables could provide a means to significantly increase a grid's power potential. For example, the 'Endesa supercable' could carry five times more mega volt-amperes than a conventional underground copper cable of the same dimensions. Also, because superconducting cables can transmit the same power at a lower voltage, both the number of transformer stations and total losses in the network are minimised.

Sanchez believes the Endesa supercable could reduce energy losses by up to 70 per cent in some parts of a network, representing energy savings and reductions in carbon dioxide emissions. As he points out, Catalonia, with a population of seven million, currently consumes some 40,000GWh of energy a year, and implementing superconductor technology throughout the region would would cut annual carbon dioxide emissions by 500,000 tonnes. The region currently emits some 58 million tonnes of carbon dioxide a year.

Supercable composition

The cable employed in the Endesa Supercable project comprises concentric layers of a high temperature superconducting (HTS) wire and a dielectric insulator in a coaxial configuration. The superconducting wire, comprising filaments of bismuth strontium calcium copper oxide (BSCCO) embedded in a silver matrix, can conduct 150 times the electrical current of a copper wire with the same dimensions. Liquid nitrogen flows over and between the layers, cooling the cable to around -200C, its operating temperature, while a cable sheath protects the overall structure.

According to Sanchez, one key component of the project was to select a good quality semiconductor wire from a range of suppliers; his team settled on tape from American Superconductor. 'We needed to look at the current distribution and magnetic properties of the tape and then integrate the material into a cable design that had the required mechanical, electrical and cryogenic properties,' he explains. 'It was crucial to understand the behaviour of the semiconducting layers in the final geometry of the cable, not just as a single tape.'

Once built, the cable, its associated terminations and cryogenic equipment were tested at Nexans' Hanover laboratory in Germany. 'We actually tested beyond the required specifications,' says Sanchez. 'For example, using short, high-voltage pulses we could simulate lightning. The cable responded very well.'

Now the cable's capabilities have been demonstrated, the next step is to integrate it to the Spanish grid. According to Sanchez, Spanish utility Endesa is considering further project developments, but he can't envisage such a step being taken within a year.

'Endesa would like to invest in something it believes will work, and our project has shown the company that we can make a state-of-the-art cable with this record for current [intensity],' he says. 'This second step of grid integration now has much more of a chance, but still the decision is up to them.'

Other projects

So what will it take to get the technology out of the laboratory and into the grid? In short, cash. Looking beyond the Endesa Superconductor project, 12 other high-temperature superconductor trials are currently underway in China, Japan, Korea, Russia, Mexico and the US.

Of these, perhaps the most remarkable example lies in Long Island, New York, where the world's longest high temperature superconductor cable is installed at a major interconnection point in the New York power grid. While the 600m cable was successfully energised in April 2008, delivers power to around 300,000 homes and will now remain an integral part of the grid, it was only made economically feasible by a $23.45m cash injection from the US Department of Energy.

Outside of research and development contracts such as this, Jean-Maxime Saugrain, managing director of Nexans superconductor activity, expects we will see a truly commercial cable put to use, in around five years' time. As he points out, right now superconducting wire and cryogenic equipment is very expensive, and costs need to be cut.

But even with cost reductions, the technology is unlikely to replace overhead transmission lines or long distance underground copper cables. 'In a direct like for like comparison with conventional power cables, then superconductor cables will never seem viable,' adds Saugrain.

However the technology's advantage lies in its small footprint. Despite its cryogenic sheath, a superconducting cable will still take up less space than its copper counterpart, while providing the same power transmission capacity. What's more, the cables don't emit electromagnetic fields or heat, so they can be spaced closely together.

'The cables will be extremely cost-effective in situations where space and planning restrictions make it impossible to install conventional cables, such as densely populated urban areas,' explains Saugrain. 'Here the capability of a superconducting cable to put a lot of power through a small installation footprint could make it the only viable option.'

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