Superconductors are materials which generate minimal energy losses when transmitting electrical power. Superconductivity occurs at extremely low temperatures and superconducting cables use liquid nitrogen as a coolant. The experts involved are dealing with temperatures ranging from 65 K to 80 K (-208 °C to -193 °C).
Superconducting cables are able to transmit large quantities of electrical power in a highly energy-efficient way. They have therefore become an attractive option to replace conventional cables. They are also lighter and more compact, making them easier to install. One of the drawbacks is that these cables cannot be used over long distances because of the problems implicit in supplying the nitrogen they require for cooling purposes. They are also more expensive to manufacture than conventional cables.
On the other hand, their smaller dimensions and energy-efficient properties make them ideally suited for high-load centres, such as cities or dense business areas. A number of demonstration projects have already been successfully set up, in China, Japan, South Korea, Germany, the Netherlands, Russia and the US. But commercial implementation is lagging behind, mainly because of the cost of the technology.
A requirement for standards
International standards can help to lower the cost of superconducting cables by establishing benchmarks that can be used by the cable industry anywhere around the world. Guidelines would help to streamline manufacturing procedures making the whole production process less expensive.The IEC recently published IEC 63075, Superconducting alternating current (AC) power cables and their accessories for rated voltages from 6 kV to 500 kV- test methods and requirements. The 54-page document is a first in this area and is the result of the work of experts from two IEC technical committees, IEC TC 20: Electric cables, and IEC TC 90: Superconductivity. “Several members of the project team which developed the standard were also active in different TC 90 working groups. TC 90 took the initiative to develop the standard and TC 20 then took over as the standard is specifically related to cables,” explains Dr Mark Stemmle, who heads the IEC TC 20 project team.
IEC 63075 is based on the work of the International Council for Large Electric Systems (CIGRE). “CIGRE technical brochure (TB) 538 was the basis for discussions in the project team. Most of the test recommendations of the TB were transferred to IEC 63075 and we also introduced additional tests which were missing in the CIGRE work,” said Stemmle.
IEC 63075 defines a wide number of tests for cables both before and after installation.They include voltage, bending and thermal cycle tests and heat invasion of cryostat. According to Stemmle, future editions of the standard could also include testing of the liquid nitrogen cooling systems. “There are more and more superconducting cable projects coming up and we will be monitoring the progress of these projects. Depending on requirements, we could include these additional tests in a new edition.”
Stemmle does not think that superconducting cable technology will be used over long distances in the near future, as major technology hurdles still have to be overcome for that scenario to become viable. He agrees, however, that city centres are where superconducting cables could initially meet commercial requirements. “They are ideally suited for areas where there is not much space for installation. An example is the Ampacity project in Essen, Germany, where a medium voltage superconducting cable was used”.
The project linked two Essen substations with a 1 km-long cable. After operating for three years, the analysis of the trial was published in a report issued during the summer of 2017. One of its main findings was that the technology was mature enough to operate under real grid conditions.
The business case for superconducting cables is gradually being made. IEC 63075 is helping by anticipating market requirements. “With this standard, there is now a unified approach to testing superconducting cables which should have a positive impact on costs,” Stemmle concludes.