In February 2000, Southwire Company announced that it had dedicated a high temperature superconductor (HTS) power delivery system, the first in the world to supply power to an industrial load. Just seven months on, in August 2000, that system reached 2000 hours of operation at 100 per cent load.

This milestone is an important one for Southwire and its partners. It is the culmination of several years’ work and goes a long way to demonstrating the commercial viability of HTS cable. The $15.3 million project—one of several under the US Department of Energy’s (DOE) Superconductivity Partnership Initiative (SPI)—will close in early 2001 after a year of operation and testing.

The potential benefits of HTS technology are significant. The DOE estimates that a US electric utility system that used HTS materials in its motors, generators and transmission cables would reduce losses in this equipment by up to 50 per cent and save $5 billion per year. Globally, it is projected that the market for HTS products will be worth around $150-200 billion by 2020.

Superconductors discovered

Superconductivity— the ability of materials to carry power with zero electrical resistance— was discovered in the early 20th century by Dutch physicist Kamerlingh Onnes. Certain materials become superconducting when they are cooled to around 0 Kelvin (K), or -273°C, and are known as low temperature superconductors (LTS).

In 1986 came the discovery of high temperature superconductors, which can function at temperatures of 140 K (-133°C). These materials, which are based on ceramic compounds, can be cooled more economically and efficiently than LTS, which are metallic or semi-metallic.

The first major commercial application of superconductivity was clinical magnetic resonance imaging (MRI), but the discovery of HTS opened the possibility of applying this technology to electric power devices such as transformers, motors, generators and transmission cables. The main advantage of HTS over LTS is that with HTS materials, the superconducting state can be reached with the use of liquid nitrogen— far less expensive than the liquid helium required by LTS. The reduced cooling needs of HTS also offer performance advantages to electric power devices that did not exist with LTS.

The development of HTS technology has concentrated on producing long lengths of high performing superconducting tapes and wires which are flexible enough to be wound into transmission cables and coils for other devices. Several projects have been able to demonstrate a number of devices, and further development is now focussed on reducing the cost of producing HTS-based devices and commercialising the technology, as well as on the development of ‘second generation’ HTS, which is thought to have better performance characteristics.

In the USA, much of this development work has been supported by the DOE. In 1988, the DOE created the Superconductivity for Electric Systems Program which aims to help industry develop and commercialize the electric power applications of HTS. SPI forms part of this.

The SPI programme supports applied research into superconductivity and is designed to accelerate the introduction of HTS electric power devices to the market. Several device-specific projects are underway as part of SPI, including Southwire’s cable demonstration project.

The Southwire project


Improving the manufacturing process could be one way of reducing the cost of HTS technology
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Southwire, a leading wire and cable manufacturer, embarked on the HTS cable demonstration in February 1998. Its goal was to complete the development, installation and testing of a 30.48 m, three-phase HTS power cable operating at 12.5 kV and supplying 1250 A of current. Its partners in the project have included the Argonne National Laboratory, Georgia transmission, Intermagnetics General Corp., Oak Ridge National Laboratory, Plastronics Inc., and Southern Company Services. Around 47 per cent of the funding for the $15.3 million project was provided by the DOE.

The HTS power cable has been installed at Southwire’s Carrollton, Georgia, headquarters, and is supplying power to three of the company’s manufacturing plants. Southwire began installing the cable in late August 1999. Installation, which included the construction of terminations at each end of the cable and a liquid nitrogen cooling system, was completed in early 2000. Connection to a live load was achieved in February 2000.

“This one is the first [HTS cable] in the world that has been put in to feed an industrial load,” said Southwire project manager R. L. Hughey. “We bypassed the cable that was feeding three of our manufacturing plants and were able to switch off the overhead line and make [the power] flow through these cables. So … our three operating plants here are getting power through this link, which is about 30 meters long.”

According to Hughey, two of these manufacturing plants are wire mills making utility-type products as well as building wire products. The third factory is a large machine shop.

The two National Laboratories— Oak Ridge in Tennessee and Argonne in Illinois— carried out a lot of the design and testing work for the project. Southwire’s other partners also include a number of utilities who ultimately will be the end users of this type of technology. Their role in the project has largely been an advisory one, according to Hughey.

The cable at Southwire’s facility uses HTS wire designed and fabricated by Intermagnetics General Corporation (IGC), a leading developer and manufacturer of superconducting materials and devices. It is a first generation HTS material known as BSCCO 2223 powder-in-tube, composed of bismuth, strontium, copper, calcium and oxygen. Within the cable are two layers of the HTS wire, or ‘tape’, which are cooled using liquid nitrogen which flows down the centre of the cable.

In the centre of the cable is a hollow pipe which conducts the liquid nitrogen. The pipe has a ‘leaky’ design where small holes in the pipe allow the nitrogen to escape out into the cable. Wrapped around the central pipe is the first layer of HTS wire in flat tape form which carries the electric current. Around the superconducting layer is a layer of insulation— the dielectric— which consists of a polymer tape approximately 25.4 mm wide built up into a thick layer. This, too, operates at cryogenic temperatures.

On top of the electrical insulation layer are four more layers of superconducting tape wrapped around in a spiral. The final, outer layer is a double-walled stainless steel pipe. Between the two walls of the pipe is a vacuum, allowing the pipe to insulate the superconductor from the outside temperature.

This cable design is known as ‘shielded’ as it uses two layers of HTS tape. As the dielectric is sandwiched between two superconducting layers, it operates at cryogenic temperatures and the cable is therefore a ‘cold’ or ‘cryogenic dielectric’ type. An alternative HTS cable design uses only one layer of HTS tape with the dielectric insulation layer forming the outermost layer of the cable; this is known as a ‘warm’ design.

The liquid nitrogen, or cryogenic, system consists of a refrigerator and a device to control the pressure of the nitrogen. The liquid nitrogen is pumped down the centre of the cable and then recirculates along the outside of the outer superconducting layer. “[The liquid nitrogen] goes down the centre of that pipe, and when it gets to the termination at the other end it turns and comes back over the outside of the cable,” explains Hughey. “There is one path of nitrogen cooling the inner superconductor and another cooling the outer superconductor.”


A cable cross-section of the Cryogenic dielectric cable design
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Such cables are designed to run underground— this is seen to be one of the advantages HTS power cable has over conventional overhead transmission lines. The Southwire cable, however, is above ground and raised on a series of pedestals. This, says, Hughey, is so that it can be seen: “We are bringing our utility customers and superconducting partners in from around the world.”

Testing

Southwire plans to operate and test the cable for one year in total, running the system until the end of 2000. If additional testing is required, then this period will be extended by up to six months. According to Hughey, Southwire is currently evaluating the market potential of the product and the value of this project and will decide by the end of the year whether to continue and take the project to the next stage, or draw it to a close.

If Southwire decides to continue with the HTS cable project, the next step would be the installation of a second cable. What the company would like to see, says Hughey, is a HTS power cable operating in a ‘real’ utility environment, i.e. in a utility grid. While the current cable is operating in an industrial environment, it is still essentially an ‘in-house’ project. A second cable would also have a higher current, higher voltage and a longer length than the current cable.


Line diagram of the superconducting power delivery system
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Such goals raise some issues for the design and operation of HTS cable, in particular the additional length and how the liquid nitrogen would be pumped through. “Ideally it would achieve all of those: more current, more voltage and more length,” said Hughey.

“We are talking to people, looking at some sites, and we are also having some outside people evaluate the value of the technology, what the market looks like, and what the competition looks like,” commented Hughey. “We’ll do a business plan and by the end of 2000 we’ll have all these studies done and will make the decision of ‘go’ or ‘not go’.”

Stepping forward

One of the biggest challenges in commercializing first generation HTS technology is in the reduction of cost. One area where this could be achieved is in the materials used in the HTS tape, which is currently composed of around 70 per cent silver. “If you could get [the silver content] down to around 40 to 50 per cent, this would be a great price driver,” said Hughey. However, this will not be an easy task.

An alternative to reducing the silver content in the tape is to increase its current-carrying capacity. Over the years of development, this has been constantly increasing so bringing the technology closer to commercialization. According to Hughey, five years ago, a 1 m length of tape would have been capable of carrying 10 A. Just a few years later, this rose to 25 A and then 50 A, and now HTS tape can carry 100 A.

At the same time, the cost of producing 1 m of tape has fallen from around $60 to less than $50. “Getting better and better, and getting cheaper and cheaper per meter is bringing that cost down; that’s what we’ve seen during this project,” commented Hughey.

A new generation

While work on commercializing first generation HTS technology continues, the development of second generation HTS is picking up speed. Although in the early stages of development, second generation HTS technology is expected to have a higher capacity than first generation technology and also to be less expensive to manufacture.


The 30 m-long HTS cable at Southwire’s manufacturing facility is raised above ground on pedestals
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IGC, through its IGC-SuperPower subsidiary, is one company which is focussing on the development of second generation HTS tape, which are coated conductors based on a YBCO (yttrium, barium, copper oxide) compound. Although only short (around 1 m) lengths of second generation HTS tape have been manufactured and demonstrated, IGC believes that commercialization of this technology will be reached within four years, and that ultimately, second generation products will replace first generation products.

“The second generation, coated conductor HTS tape would be composed of less expensive materials and produced through a less labour-intensive process than current HTS material,” said Glenn H. Epstein, president and CEO of IGC.

“Despite more than ten years of research and development efforts, including some very solid advances, the current first-generation powder-in-tube process has not come close to reaching the cost threshold we believe is necessary for commercial viability.”

IGC and the DOE recently signed a contract for the first phase of a three-year,

$4.5 million project aimed at commercializing the manufacturing process for second generation HTS materials. Working with Argonne and Los Alamos National Laboratories, IGC will develop and scale up a new manufacturing process that will help reduce the cost of second generation technology.

Nevertheless, the commercialization of second generation HTS will depend on the successful development and testing of first generation materials and HTS devices. IGC anticipates that prototypes developed with first generation HTS will enable the cryogenic system development and proof-of-feasibility of HTS devices needed to enter the commercial market. When better performing, less expensive commercially viable second generation material becomes available, it will replace the first generation material without the need to go through extensive prototyping. In other words, one can move directly to commercialization based on the results achieved with first generation prototypes.