Super conducting

A European consortium is currently working on a project to develop a High Temperature Superconducting energy cable. As part of that consortium, Nexans updates us on ‘Super3C’, a project that represents the next step in the development of HTS wires.

Jean-Maxime Saugrain, Nexans

It is nearly a century since the Dutch scientist, Heike Kammerlingh Onnes, cooled mercury down to -269à‚°C and found that below certain critical values of temperature, magnetic field and current density, it became a superconductor – losing all electrical resistance. Almost from that moment, the power utilities have been seeking ways to use this phenomenon to create a new generation of compact, high-efficiency cables, capable of carrying high currents. But until recently, the need to cool superconducting materials down to the extremely low temperatures at which helium becomes liquid made them both impractical and expensive. Indeed, the commercial uses of low-temperature superconductors have been largely restricted to applications where there is no alternative, such as in MRI (magnetic resonance imaging) machines.

The situation changed in the mid-80s, with the discovery of new high temperature superconductors (HTSs). These operate at around -200à‚°C, the temperature of liquid nitrogen, so cryogenic costs are much lower. Until now, all HTS cables have been manufactured with bismuth-based multifilament wires as the current carrying elements. However, these wires are expected to be replaced in the future by a second generation of HTS tapes, the YBa2Cu3O7-x (YBCO) Coated Conductors (CC) which, as the result of a lower production cost, will allow for a wider use of HTS cables.

Figure 1. Utilities are keen to exploit the advantages of superconducting technology
Click here to enlarge image

In June 2004 the European Community commissioned a project, within the Sixth Framework Programme for research and technological development, that aims to establish the feasibility of a low-loss HTS AC cable using CC tapes as current-carrying elements. It comprises the development, manufacturing and testing of a functional model consisting of a one-phase, 30 m-long, 10 kV, 1 kA cable with its terminations.

Apart from Nexans, the project team encompasses partners in Germany (European High Temperature Superconductors, E.ON Energie, E.ON Engineering, Centre for Functional Materials (ZFW) in Gàƒ¶ttingen), Spain (Barcelona Institute of Materials Sciences (CSIC) and Labein), Finland (Tampere University of Technology), France (Air Liquide) and Slovakia (Bratislava Institute of Electrical Engineering). The European Commission is funding about half of the projected total €4.47 million ($5.43 million) cost of this three-year project.

Key issues

The main scientific and technological issues of the Super3C project are:

  • The development of an optimal CC tape cross-sectional architecture leading to a critical current (Ic) of 400 A/cm-width on short IBAD/HR-PLD tape length (1 m)
  • The establishment of an up-scaled processing technique that allows for the manufacturing of CC tapes in 100 m unit lengths with a critical current of 75 A for 4 mm-wide tapes
  • The development of a cable design and a cable manufacturing process compatible with CC tapes
  • The development of 10 kV outdoor terminations for CC cables
  • The fabrication and successful testing of a functional model which will consist of a 30 m long, 10 kV, 1 kA one-phase CC cable with its terminations, and which is expected to require the manufacture of about 2 km of CC tapes
  • The establishment of the suitability of IBAD/HR-PLD CC tapes for AC cables
  • The establishment of the suitability of a chemically deposited YBCO layer for AC cables.

HTS cable design

The HTS cable is of the cold dielectric type that requires cooling with pressurised liquid nitrogen. The core is composed of a flexible support surrounded by a HTS conductor separated through a lapped dielectric insulation from an HTS screen. Such a design creates a flow of current in the screen which will be opposite and equal in ampacity to the one in the conductor. As a consequence, the cable will not generate any significant magnetic field outside. The individual phases of such a cable system are electrically and thermally independent. The development and test of a one-phase functional model designed for AC applications will therefore provide the same output data, at a lower project cost, as the test of an AC cable with three independent phases.

For the CC tape fabrication, the work is focusing on the following technological routes:

  • Use of low-cost stainless steel as a substrate tape
  • Ion-Beam Assisted Deposition (IBAD) of a biaxially textured buffer layer
  • High-Rate Pulsed Laser Deposition (HR-PLD) of YBCO film
  • Physical vapour deposition (PVD) of the shunt/protecting layers.

IBAD and HR-PLD are well-established technologies that have to be scaled up to deliver the tape length demanded for the manufacturing of the functional model. The selection of these two processes aims at securing the CC tape delivery as well as possible. However, chemical deposition techniques may, in future, become attractive alternatives, in particular for the deposition of the YBCO layer. In an attempt to find a good compromise between securing the project and preparing the future in case chemical processes become competitive, the plan is to employ the dip-coating process developed at Nexans Super Conductors in Germany within the framework of the European project Solsulet to deposit a YBCO film on 20 per cent of the IBAD-buffered substrate fabricated for the functional model.

A first objective will be to demonstrate the suitability of a chemically deposited YBCO layer for AC cables. A second will be to set the basis for future developments aimed at defining the best combination of processes to further reduce the cost of CC tapes and, consequently, of CC cables.

Project progress

At the halfway stage, the Super3C project is on target. The design of the cable itself has been completed, as has the design of the termination. We are now starting to order the major termination components, such as the cryostat, while EHTS is just about to start manufacturing the HTS tapes. This is both the most critical part of the project and one of the parts where we will really be breaking new ground, as there is no previous experience of creating such a long length (2 km) of HTS tape.

Before we produce the actual cable we will be producing a 30 m dummy cable, with only a few HTS tapes, for initial voltage and current tests. This will go on test in the first quarter of 2006.

Figure 2. To help with crossing obstacles the HTS cable could be carried by existing structures
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The Super3C is just one of a number of projects across the world that are currently aiming to develop practical experience of HTS cables in power transmission applications. For example, in Albany, New York, SuperPower is carrying out a $26 million dollar project (with the backing of the New York State Energy Research and Development Authority and the Department of Energy’s superconductivity research initiative) to install a superconducting distribution cable.

Nexans is also involved, with our partners American Superconductor, Air Liquide, and the Long Island Power Authority (LIPA), and with the support of the US Department of Energy, in the longest HTS cable project worldwide, a 610 m link with a capacity of almost 600 MVA at 138 kV. This will be commissioned in 2006.

HTS cable applications

Because of the high efficiency and current carrying capabilities of superconductors there is a tendency to think of HTS cables as a potential replacement for conventional HV cables as the backbone of a transmission grid, carrying power over long distances. Perhaps that will come true eventually, however in the short to medium term, HTS cables can make a very significant impact when used in short lengths (from a few hundred metres up to a few kilometres), to carry large amounts of power through areas where space and access are at a premium.

For example, utilities frequently encounter problems with obtaining rights of way in built-up areas to install the new cables needed to meet increasing demands for power. Replacing the existing conventional cables with an HTS link could enable several times the power to be carried by a system with the same footprint.

The compact nature of HTS cables could also help when it comes to crossing obstacles such as rivers. So instead of directional drilling, or constructing a dedicated cable tunnel, the HTS cable could be carried by existing structures, such as a bridge or tunnel.

On a direct comparison basis then, length for length, the added manufacturing complexity of HTS cable will always make them significantly more expensive than conventional cables. However, when you look at the increased capacity and small footprint of HTS cables then it is entirely feasible for the total installed cost of the complete cable system to more than match that of conventional cables.

The question always asked about HTS cables is ‘when will they be commercially available?’ Assuming reasonable success with the current projects then five to seven years is feasible, and possibly a little earlier for short links. There is no doubt that Super3C is a very challenging project, at the cutting edge of the technology, but as the progress we have already made is showing, the team has the right people and the relevant expertise to make it a success.

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