Superconducting Transmission Grids

In this blog post, one of the world’s leading experts on superconductivity gives her insights into the developments she is working on and how they are applicable to the future of lossless electricity transmission. Some of these developments could, for example, make it possible to conduct electricity over very long distances. In particular, he talks about some of the developments for what is currently the world’s largest particle accelerator, the Large Hadron Collider (LHC), a circular accelerator with a perimeter of 27km located deep underground between France and Switzerland. Accelerators require superconductors to drive the high currents of the magnets that create strong magnetic fields to bend the trajectory of particles travelling inside them at near-light speed. Therefore, no one better than the Head of Superconductors and Superconducting Devices at CERN to explain the latest developments and their possible application to the field of electricity transmission.

In the framework of the High Luminosity Large Hadron Collider (High Luminosity-LHC) project, CERN has developed a new superconducting electrical transmission system. Following the success of the initial test with a 2×20,000 Amp demonstrator, a 60-metre long 2×110,000 Amp prototype is being built. During the testing of the model, a world record was achieved by passing a current of 20,000 Amps at a temperature of 24 Kelvin (-249° Celsius) through two 20-metre-long wires made of multiple superconducting magnesium diboride (MgB2) superconductor filaments [see Ref. 1]. This superconductor is potentially very cheap, was discovered in 2001, and has a critical temperature of 39 K, making it an attractive solution for long-distance electrical transmission.

Initially, the powder-in-tube conductor was manufactured in ribbon form because high-performance filaments with good mechanical properties, which are best suited for joining high-current cables, were not available when the CERN project started. It was necessary to develop quality cables, which had a high current density, uniform superconducting properties over long lengths and good mechanical properties adapted to the project. This was achieved through close collaboration between CERN and ASG Superconductors, which produced successive generations of cables with different architectures and performance improvements. In parallel, in a second phase with the cable already industrialised, CERN developed the high-current cables and the transmission line system, as well as the low-resistance connections to Nb-Ti’s Low Temperature Superconductors (LTS) and ReBCO’s High Temperature Superconductors (HTS), necessary for its implementation. Magnesium diboride is ideally suited to power transmission applications, where magnetic fields are relatively low (rarely exceeding 1 Tesla, equivalent to about 10,000 times the earth’s magnetic field) compared to applications such as particle accelerators, where they sometimes operate at close to 10 Tesla.

With respect to the use of magnesium diboride cables at the LHC, their utility lies in moving away the power electronics power supplies that inject current into the superconducting magnets that make up the particle beam focusing systems. A 2009 study confirmed that MgB2-based power transmission lines would be a viable and cost-effective technology, providing several advantages over the conventional Nb-Ti LTS cables used today. The advantages come from being able to operate at 20-25 Kelvin (instead of 4.5 Kelvin) and include increasing the stability of the superconductor, reducing the power consumption of the cryogenic cooling system and simplifying the cryostat. In the CERN project, the magnesium diboride conductors are cooled with helium gas, as it is available at the LHC, but the operating temperature of this superconductor makes the use of liquid hydrogen also feasible.

The demonstration tests were major steps in the development of electrical transmission systems based on the use of magnesium diboride (MgB2). Beyond the CERN initiative, superconducting technology using MgB2 was also proposed by Professor Carlo Rubbia, scientific director of the Institute for Advance Sustainability Studies (IASS) in Potsdam (Germany), for use in an innovative superconducting transmission network to carry ‘green energy’ over long distances. It proposes the use of liquid hydrogen-cooled superconducting MgB2 cables in underground electricity transmission lines with periodically distributed cryogenic cooling stations. In March 2012, a collaboration agreement was signed between CERN and IASS with the aim of demonstrating the feasibility of the technology. The aim was to test a 2×20,000 Amp DC line operated at 20 Kelvin (-253° Celsius), whose electrical requirements were close to those that CERN had for its magnet power lines. The result of the tests is the demonstration that this type of high current cables can be operated at a temperature above 20 Kelvin, proving the viability of the technology [see Ref. 2].