Remote, efficient and high-capacity power transmission
By Amalia Ballarino
In this blog entry, one of the most renowed global experts on superconductivity offers us her view on the developments she is working at and how they can be applied to the future of efficient power transmission. We would like to thank Amalia Ballarino for her explanations and collaboration with our blog.
In the framework of the High-Luminosity LHC project, CERN has developed a novel superconducting electrical power transmission system, and following the successful test of a 2×20 kA demonstrator a 60 m long prototype rated for than 110 kA is being constructed. In the demonstrator a world record current of 20 kA at 24 K (about -250°C) was passed through two 20-metre long cables made of multifilamentary magnesium diboride (MgB2) superconductor [Ref. 1]. This potentially inexpensive superconductor with a critical temperature of 39 K, discovered in 2001, makes the use of such technology an attractive solution for long-distance power transmission.
Initially, the powder-in-tube conductor was produced in the form of tape: suitable high-performance reacted round wire with robust mechanical properties, more appropriate for assembling into high-current cables, was not available when the CERN project started. It was necessary to develop quality wires, with high current density, uniform superconducting properties over long unit lengths and good mechanical properties adapted for use in such a project. This was accomplished via a close collaboration between CERN and ASG Superconductors, which manufactured successive generations of wires with different architectures and with improved performance. In parallel, CERN developed the high-current cables – in a second phase industrialized – and the transmission line system, and the low resistance joints to Nb-Ti and HTS (ReBCO) cables necessary for its implementation. The material is perfectly adapted for power transmission applications, where the associated magnetic fields are relatively low (~ 1 T).
Tests at CERN
In the HL-LHC project, i.e. an upgraded configuration of the LHC final beam focus systems, the power converters supplying current to the superconducting magnets are moved from their present locations in the LHC tunnel to radiation-free underground areas, to be connected to the magnets via a new cold powering system. A dedicated study in 2009 confirmed that electrical transfer lines based on the use of MgB2 would be a viable and economical technology, bringing several advantages with respect to the conventional Nb-Ti cable used today. The advantages derived from operation at 20 K – 25 K (instead of 4.5 K) include increased stability of the superconductor, reduced power consumption of the cryogenic cooling system and simplification of the cryostat. In the CERN project, helium gas, available at the LHC, is used for cooling, but the operating temperature of this superconductor is also well matched to the use of liquid hydrogen.
The demonstrator tests were important steps in the development of cold electrical power transmission systems based on the use of MgB2. Further to the CERN initiative, MgB2 superconducting technology was also proposed by Prof. Carlo Rubbia, scientific director of the Institute for Advanced Sustainability Studies (IASS) in Potsdam, for an innovative transmission line for long-distance transport of “green power”.
MgB2 superconducting cables cooled by liquid hydrogen are proposed for use in underground power transmission lines, with periodically spaced cryogenic cooling stations. A collaboration agreement between CERN and IASS was signed in March 2012 with the objective of proving the feasibility of the technology. The development was aimed at testing a 2×20 kA DC line operated at 20 K (-253 °C), which was also conveniently close to the CERN requirement for powering the magnets. The result of the tests was a demonstration that such high-current cables can be operated at and above the temperature of 20 K, and that the basic related technology is proven [Ref. 2].
Amalia Ballarino is Head of the Superconductors and Superconducting Devices unit at CERN, and initiator of the project.