by Adela Marian
n the present article, our author explains to us how magnesium diboride superconductors were used in a technology demonstration of a DC superconducting electricity transportation line. Magnesium diboride is a conductor that can operate at high temperatures (-250ºC), in comparison to other superconductors, and its manufacture is inexpensive. It is expected to have a great potential for this kind of applications, as stated by Amalia Ballarino from the CERN in a previous post. This time, Adela Marian, Senior Research Associate at IASS Postdam, explains to us a project where a HVDC (High Voltage Direct Current) link was operated in a field test, reaching 320kV and 10kA, equivalent to 3GW of power. Superconductors have losses when they operate in AC, so they must operate in DC, reason why their future integration in electrical grids will be through HVDC connections.
Best Paths was the largest research project in the field of energy financed under the European Union’s 7th Framework Programme for Research, Technological Development and Demonstration. The project ran for four years between October 2014 and September 2018 and focused on developing novel grid technologies to increase the European transmission capacity and electricity system flexibility. Best Paths was coordinated by Red Eléctrica de España (REE) and encompassed 38 leading organizations from science and industry in 11 European countries. These experts were grouped around five large-scale demonstrations ranging from connecting offshore wind parks, to developing standardized solutions for multivendor converter technologies, upgrading multi-terminal high-voltage DC (HVDC) links, and repowering of existing AC corridors. The fifth demonstrator – Demo 5 – focused on validating HVDC superconducting links capable of transporting several gigawatts of electricity.
Why is it necessary to employ such high capacity links in the future European grid? Recent grid studies, such as the e-HighWay2050 project, have shown that bulk power transmission over several hundred kilometers is necessary to bring the electricity produced by remote renewable energy farms to the consumption centers. In this context, superconducting cable systems have generated a lot of interest lately due to their low transmission losses, compact size and reduced environmental impact.
Demo 5 showcased a full-scale 3-GW-class HVDC superconducting cable system operating at 320 kV and 10 kA. The system is based on the magnesium diboride (MgB2) superconductor and consists of the following main elements (see cable sketch in the image below):
- the cable conductor consisting of 18 MgB2 wires wound around a copper core,
- the cryogenic envelope,
- the cooling fluid(s), stably maintained at the adequate cryogenic temperature by the cooling system,
- the high-voltage insulation,
- the terminations providing the connection to the electricity grid
Image – Main components of a superconductive cable system. Note that the electrical terminations used to provide the connection to the grid are not represented here due to their large size
These main components of the cable system were specified, designed, developed and optimized at industrial scale during the project, which brought together 12 partners from 5 countries. The Demo 5 partners included transmission system operators as well as industry and research organisations from the fields of material sciences, cryogenics, energy systems, and electrical engineering: CERN, Columbus Superconductors, ESPCI Paris, IASS Potsdam, Karlsruhe Institute of Technology (KIT), Nexans France, Nexans Germany, Nexans Switzerland, Ricerca sul Sistema Energetico (RSE), Réseau de Transport d’Électricité (RTE), Technische Universität Dresden, and Universidad Politécnica de Madrid. Throughout the project, the actual needs of the system operators were taken into account, while also heeding the overall cost, reliability, and environmental impact of the cable system. Thus, the input data provided by the French transmission system operator RTE was essential for the proper design of the cable conductor, as it specified the expected performance and behavior in the electricity grid, particularly under transient conditions. In a different instance, due to the lack of references for testing HVDC superconducting cable systems, the high-voltage testing program was based on the combination of two sets of established international practices (the Cigré technical brochure no. 496 and the IEC standard 62895). Also in this case, the resulting protocol for the HVDC tests was shared with and accepted by the transmission system operators who were partners of the Best Paths project.
Some of the key achievements of Demo 5 are:
- Manufacturing MgB2 wires and the cable conductor in a robust and reproducible industrial process
- Investigating the nominal and transient behavior of an MgB2 component embedded in the grid
- Manufacturing the HVDC electrical cable insulation, which proved its very high electrical performance and reliability in detailed subsequent tests
- Designing and constructing high-voltage electrical terminations using an innovative modular concept that can be easily adapted to various grid voltage and current values
- Investigating the behavior of the grid with an operating superconducting cable
- Exploring long-length superconducting links that can be used in the long term to transport large amounts of electricity
The decisive final tests took place at the end of the project at a dedicated industrial test platform on the premises of Nexans Germany in Hannover. They involved a demonstrator consisting of a 30-meter-long superconducting cable prototype connected to two full-size terminations. The cable system was subjected to pure DC high-voltage tests as well as overvoltage steps, polarity reversals, and several sequences of superimposed switching and lightning impulses. Overall, the tests were intended to simulate as thoroughly as possible the various grid events that can affect a DC cable system. The full testing program was successfully completed in 1.5 months, resulting in the first demonstration of a superconducting cable system operating in the 320 kV DC voltage class. These pioneering tests thus represent an important step towards establishing a standard for testing superconducting HVDC cables before their installation in the grid.
Links to two relevant publications aimed at a general audience:
A. Marian and C. E. Bruzek: “Advancing superconducting links for very high power transmission”, IASS Brochure, September 2018.
A. Chervyakov, M. Ferrari, A. Marian, S. Stückrad, H. Thomas: “Superconducting Electric Lines”, IASS Fact Sheet, November 2015.
Links to two IASS press releases related to Best Paths: