This article reviews the copper cables in Detroit Edison’s Frisbie Station to be retrofit in mid-2000 with high-temperature superconductor (HTS) cables to support a major urban redevelopment project in downtown Detroit. American Superconductor’s partner, Pirelli, is participating in this project. The three HTS cables, weighing 250 pounds, will carry 100 megawatts of power, a job that nine copper cables, with a total weight of 18,000 pounds, are doing. In Germany, Siemens AG has invested hundreds of millions of dollars into researching HTS, because the capability can bring improvements across the German conglomerate’s businesses, from power to transportation to electronic devices. The principal commercial US players are American Superconductor, 3M, and Intermagnetics General. Each has organized efforts to develop commercial processes for manufacturing HTS tape based on coated conductors.

## Article

More than a decade has passed since the 1986 superconductivity milestone event, which introduced a new set of ceramic compounds that could conduct electricity without energy losses, at much higher temperatures than previously thought possible.

The discovery invigorated physicists and the engineers who apply physics to our world. Since 1911, the prospects of superconductivity had tantalized researchers, but before 1986, super conducting material required cooling by liquid helium to get down near absolute zero, or zero on the Kelvin scale, and about -273°C. With the 1986 discovery at IBM Zurich, new bismuth strontium calcium copper oxide (BSCCO, or "bis-ko" in physics slang) materials were found to superconduct at 77K, a much higher Tc' its critical temperature for superconductivity.

Already, because of the sophisticated magnets superconductors can produce, superconducting quantum interference devices (dubbed "SQUIDs") are being designed into nuclear magnetic resonance imaging equipment that has provided awe inspiring insight into biological tissue makeup. MRI equipment has used superconducting components, like current leads, for years.

Because high temperature superconductor (HTS) materials are ultrareceptive to high-frequency signals and cheap enough to cool in a remote box, superconductive communications filters are deployed in the infrastructure that carries wireless phone calls.

Now, superconductors are heading, in small steps, into the power grid. Decades of work remain to be accomplished, but the science is ready for engineering development into our daily lives.

One high-profile demonstration project has just begun in the United States. Energy Secretary Bill Richardson announced that the Department of Energy has contracted to install the world's first HTS power cable system in an electric utility network.

The project calls for the copper cables in Detroit Edison's Frisbie Station to be retrofit in mid-2000 with HTS cables to support a major urban redevelopment project in downtown Detroit. American Superconductor's partner, Pirelli, is participating in this project. The three HTS cables, weighing 250 pounds, will carry 100 megawatts of power, a job that nine copper cables, with a total weight of 18,000 pounds, are doing.

A graphical depiction of 115 kV, 400 MVA HTS cable from American Superconductor.

A graphical depiction of 115 kV, 400 MVA HTS cable from American Superconductor.

## Efficiency Out In the Cold

Bulk superconductive materials-the wire for cabling and tape for motors-promise to do for the electric industry what fiber optic cable has done for telecommunications. Electricity will be distributed faster, better, and eventually cheaper.

But it is still very cold at 77K, about -196°C, so it is only in a relative sense that the new materials came to be called high-temperature superconductors. HTS materials theoretically only require liquid nitrogen, a material in unlimited supply that costs only one-fifteenth as much as liquid helium cooling. However, engineers are finding that liquid helium in certain applications is still required, because the performance in the 20K to 30K stage is better.

According to James Daley, who manages the U.S. Department of Energy's superconductivity program, HTS equipment in general will be half the size and weight of conventional units with the same power rating and have half the efficiency losses. That is the case with both motors and transformers.

The HTS transformer has the additional advantage of eliminating problems associated with the cooling oil used in conventional transformers; several thousand gallons of oil may be used to cool large power transformers. Extensive safety precautions must be taken with conventional transformers to avoid fire and environmental damage associated with oil cooling, which HTS transformers would not need, Daley said. An advanced program for HTS transformers is that of European power giant ABB, at its power transmission and distribution lab in Raleigh, N.C.

According to Daley, "We are going through a period during the next four years that will see initial installation and test of HTS-based power cables, transformers, motors, and current controllers." The tests, by leader American Superconductor Corp. and many others, will establish the operating characteristics (including performance, reliability, and safety) that industry will need to know in order to adopt the commercial versions.

The other important consideration, of course, is cost of the commercial version relative to conventional technology, Daley notes. He expects a two- to five-year payback from efficiency improvements to be an incentive for adoption of new technology.

## A 100-Fold Increase in Capacity

The incentive for putting bulk superconductors into the power grid is efficiency. A thin-film superconductive tape could have a carrying capacity of electricity 100 times greater than the copper and aluminum wire it would replace. The development is considered critical because the demand for electricity is expected to double during the next 30 years, according to 3M of St. Paul, Minn. The 3M R&D lab is in the midst of studying the feasibility of manufacturing a second generation of superconductive tape in longer lengths.

The most widely used superconducting wires are made from niobium with titanium or tin in a copper matrix. These materials are low-temperature superconductors (LTS) because their critical temperatures are less than 25K (or -248°C).

Most HTS cable uses filaments of bismuth, lead, strontium, calcium, copper, and oxygen in a metal matrix, according to BICC Superconductors of Wrexham, Wales. This material, (Bi,Pb)2Sr2Ca2Cu3O10, is nicknamed Bismuth-2223 and has a T c of 110K. Like all HTS materials, the lattice structure consists of planes of copper-oxygen ions sandwiched between blocks of insulating ions.

BICC Superconductors is focusing on long-length HTS tapes, which are marketed as Cryobicc, and HTS current leads and cables, according to Wolf gang Blend!, manager of superconductivity at BICC.

BICC's tapes can be made to meet specific customer requirements, the company said. Tapes for sale have a typical length of 200 meters, but any length up to 1.2 kilometers can be produced on request.

BICC also markets HTS current leads, which are trademarked as Econex. The new current leads have been designed for reduced heat leak in cryofree and vapor-cooled superconductive magnet systems. Econex current leads are produced using the Cryobicc tapes with a proprietary low thermal conductivity metal matrix surrounding the superconducting filaments. The current leads are designed for operating temperatures of 77K and below.

In Germany, Siemens AG has invested hundreds of millions of dollars into researching high-temperature semiconductors, because the capability can bring improvements across the German conglomerate's businesses, from power to transportation to electronic devices. Research occurs mainly in Erlangen, and a pilot wire and cable production line is ramping up at the company's Vacuumsclunelze subsidiary in eastern Germany.

The 200-hp superconducting synchronous motor that Reliance Electric, a subsidiary of Rockwell Automation, demonstrated in 1996. The Cleveland engineering team is now working on a 1,OOO-hp motor.

The 200-hp superconducting synchronous motor that Reliance Electric, a subsidiary of Rockwell Automation, demonstrated in 1996. The Cleveland engineering team is now working on a 1,OOO-hp motor.

The Drive for Super Motors

Integrating superconductors in electric motors can yield hefty benefits, says R.D. Blaugher of the National Renewable Energy Laboratory in Golden, Colo.

According to Blaugher's estimate, all electric motors account for about 64 percent of demand side electrical consumption in the United States, which translates to about 6.4 quad. One quad of energy is about 1015 British thermal units-a measure of energy consumption worth about $20 billion in retail sales at 7 cents per kilowatt hour. Electric motors consume about$128 billion worth of energy every year, with one-half of that amount used by large motors, defined as those greater than 1,000 horsepower.

Assuming substantial introduction of large superconductor motors, Blaugher estimates that the users would save about $2 billion a year in efficiency improvements and a second$2 billion a year could be saved at the utility or electric power producer.

Blaugher notes the $500,000 to$750,000 current market price for a conventional 5,OOO-hp motor combined with an adjustable speed drive (ASD). If superconductor motor producers can compete with that cost, including ASD and cryogenic support, Blaugher believes there would surely be a market for many hundreds of SC motors.

Reliance Electric, the Rockwell Automation company based in Cleveland, has a small but ambitious HTS motor team, led by David I. Driscoll. Driscoll is superconducting motors manager in the Advanced Technology unit of Rockwell Auto's Euclid Labs.

Driscoll and his team are currently in the second phase of a multiyear collaboration with American Superconductor. The collaboration is one of the Department of Energy's Superconductivity Partnership Initiatives, which support much of the work here in the United States. The first phase was construction and load testing of a 125-hp motor, which performed well over 200 hp from an inverter. One of the key accomplishments of that project was finding that the performance of the HTS coil did not degrade under the mechanical stresses associated with rotational force.

The second phase is construction and testing of 1,000- and 5,OOO-hp motors. The 1,OOO-hp design is complete and under construction.

A recent paper by Driscoll and his team summarized that effort. The 1,OOO-hp motor is designed for operation from an adjustable speed drive using the same topology as the previously demonstrated 200-hp HTS motor, which was an air-core ac synchronous motor with HTS field-winding operating at 1,800 rpm. The ASD is a conventional rectifier/ inverter system as used with present ac induction motor products, Rockwell said.

In this topology, the HTS coils are located on the rotor and the stator is composed of more conventional copper winding.

"Most of the technology is really in the rotor," Driscoll said. To accommodate the side effects of an HTS rotor, like the very high magnetic field created, the stator is water-cooled and Litz wire is used to reduce eddy current losses.

The coils are mounted on their own support structure, which houses a conduction-cooling apparatus: Rockwell is using a commercial reverse Brayton cycle helium refrigeration system for initial component and system testing, but is looking at other options.

The wire being wound to make the motor coils is BSCCO, which is superconductive at 77K and so only requires liquid nitrogen for cooling. However, the motor application requires helium for cooling to 20K to 30K, Driscoll said, to keep the superconductors behaving as desired.

Much of Reliance Electric's work today is in preparation for more progress on the materials front. "All of the engineering we're doing today to get a motor at 1,000 hp at 33K will only help us in the future, when we have wire that will actually be performing at 77K," Driscoll said.

These motors will go into the standard large-motor applications , such as centrifugal compressors, boiler feed pumps, induced and force draft fans, and industrial scrubbers, Driscoll said.

The efficiency losses of an HTS motor, Rockwell expects, will be half those of an energy-efficient ac induction motor. Also, the motor has an active volume that is 55 percent of that for a 1,800-rpm, 5,000-hp, high-efficiency induction motor. This leads to reductions in friction and windage, core, stray load, and armature 12R loss.

Meanwhile, the Naval Research Lab (NRL) pursues superconducting motors for ship propulsion, Donald Gubser, superintendent of the materials science and technology division at the NRL in Washington, noted.

"The Navy has had a superconducting motor program for almost 30 years for ship propulsion," Gubser said. "We've recently been attempting to ascertain the viability of converting one of our low T c motors to high T c'" he said, referring in shorthand to the very first superconducting materials, those of niobium titanium, and their replacement high-temperature ceramic counterparts.

The NRL's demonstrator superconducting motor operates at the low, liquid helium temperature of 4.2K at 300 hp. During 1998, researchers operated it off a cryo-cooler at 27K, which allowed for a 2oo-hp motor. Gubser noted that this is a homopolar motor; it is completely direct current, which requires brushes to make contact with the superconducting stator windings. Improving the brushes is currently one of the things NRL is working on, Gubser said.

## Triple-Bonded in Silver

Siemens believes that the most promising route for cable in transmission and distribution is silver tape conductors with cores of triple-bonded bismuth. Multicore conductors used in this way, according to Klaus Meyer, a project leader in Erlangen, could be produced in considerable lengths using extruding machines, followed by stretching and rolling.

Siemens is developing a concept that each of three power cable cores contains coaxially arranged outward and return lines. This configuration would ensure that no alternating magnetic fields occur in the area surrounding the cable. In a recent demonstration, a cable carried 3,000 amps, and this was only limited by the power supply available at the time, Meyer said.

At Vacuum schmelze, schn1elze, superconductive 55-filament standard wires with a current carrying capacity of 20 to 22 kA per square centimeter can be manufactured in lengths of 400 to 600 meters. Tens of kilo meters of that material have been produced. Also, Siemens' Power Cables division has already begun work on adapting production facilities at its Berlin factory for HTS cable manufacture. Siemens predicted it will have cable ready for commercial use in 2005.

Superconducting Technologies, a Carteret, N.J., division of U.K. instrument leader Oxford Instruments, produces hundreds of tons of low-temperature superconducting materials every year for wire and cable applications. These are applied in MRI equipment and very large magnets.

An old-line engineering house, Illinois Tool Works of Glenview, Ill., has made a move into the field. In November 1998, NKT Cables of Denmark was acquired by ITW, which is known to many for products like Devcon adhesives; the NKT buy is regarded as a strategic move. NKT has a collaborative agreement with Atlanta cablemaker Southwire Co., which in recent years has significantly bolstered its own superconductivity resources.

3M has begun a $3 million "coated conductor" program with the DOE. Above is 3M's depiction of a transformer that would use such second-generation material. Elimination of oil cooling that comes with HTS materials is key. 3M has begun a$3 million "coated conductor" program with the DOE. Above is 3M's depiction of a transformer that would use such second-generation material. Elimination of oil cooling that comes with HTS materials is key.

## Super Transformers

An advanced program for HTS transformers is under way at ABB Power T&D Co. in Raleigh, N.C. The T&D lab work is done with American Superconductor supplying the cable.

Steinar J. Dale, a department manager in the Electric Systems Technology Institute at ABB, sees significant advantages in superconductive transformers, including their size, weight, oil-free insulation, and cooling, but also in the possibility that they can be built as low-impedance transformers and yet serve as fault current limiters when need arises.

If an HTS transformer is driven out of its superconducting state-if temperature or current exceeds a critical level, for instance-it becomes an effective resistor. As ABB sees it, a superconductive transformer will react to a power spike by replacing superconductivity with resistance. The low-impedance operation under normal conditions will allow for better voltage regulation to the power grid. Overall, power quality improves.

Meanwhile, Siemens believes that the losses of present HTS cables are still too high in the typical power transformer application (50 Hz alternating field at medium magnetic field strengths of about 0.3 Tesla) . Siemens believes the materials are not ready yet.

ABB and American Superconductor encountered the same problem of losses that were too great in their 1997 Geneva demonstration of what ABB believes to be the first HTS transformer connected to a power grid-a 630-kVA, three-phase, 18-kV transformer.

The BSCCO wire, Dale said, was not specifically tailored for transformers, but was the best available at the time.

A program among the two companies and Électricité de France (EDF) aims to develop wire specifically suited for transformers, with low ac losses. ABB and EDF are designing and building a new 10-MVA, 69-kV, three-phase superconductor transformer, Dale said.

ABB sees commercial transformer products coming to the fore somewhere between 2005 and 2010.

Initial application efforts indicate that the industry is counting on improvements in the material characteristics. Universities and national labs are pressing toward a second generation of high temperature semiconductor materials, especially for electron devices like the SQUIDs and telecom filters, and toward greater theoretical understanding.

The second generation, based on a mixture of yttrium barium copper and oxygen, are called YBCO (pronounced "yib-co") for short and sometimes are referred to as "coated conductors."

At the forefront is work spearheaded by Los Alamos National Laboratory, under Dean Peterson. The leader of Los Alamos' Superconductivity Technology Center, Peterson and his team have shown significant improvements in achievable lengths of YBCO tape, something the OEMs have been craving.

"We've got one-meter-length tapes. In terms of achieving the desired properties in long lengths, we've made significant progress. We're approaching one million amps per meter at liquid nitrogen temperatures. So that's pretty exciting stuff," Peterson said.

A side-by-side graphical comparison of the first- and second-generation technology for HTS wire. Source: American Superconductor Corp.

A side-by-side graphical comparison of the first- and second-generation technology for HTS wire. Source: American Superconductor Corp.

## Tape Players

The principal commercial US. players are American Superconductor, 3M, and Intermagnetics General, Peterson said. Each has organized efforts to develop commercial processes for manufacturing HTS tape based on coated conductors, Peterson said. They are each collaborating with the national laboratories to accelerate the tape development. Meanwhile, Japan and Germany both have allocated substantial resources to develop HTS coated conductors. However, all of these efforts are kept quiet, behind laboratory doors.

A joint European effort is taking place under the name READY (derived from "refrigerated, efficient, ac conductor by chemical vapor deposition of YBCO") . It intends to identify the best manufacturing route for YBCO coated conductors that offers economic benefits for the construction of power devices. In the second half of the project, a practical superconducting transformer will be constructed. Participating in the four-year project coordinated by Oxford Instruments are three academic institutions and six commercial entities.

The European Union is funding READY through its Brite Euram program, similar to a US. Department of Energy setup. Oxford leads the program because its principal interest is higher field magnets than are possible today.

Last fall, 3M, an extrusion and tape manufacturing world leader, signed an 18-month, $3 million agreement with the US. Department of Energy to explore the feasibility of YBCO-based tape. Arnie Funkenbusch leads the 3M team in St. PauL The effort is part of the DOE's national Superconductivity Program for Electric Power Systems. Partners in the project with 3M include Southwire Co. of Atlanta and two DOE laboratories: Los Alamos, which is managed by the University of California, and Oak Ridge National Lab, managed by Lockheed Martin Energy Research Corp. According to 3M, market research points to a potential market in the United States, Japan, and Europe for superconductor products and services reaching$122 . billion by the year 2020.

An advanced cryogenic system used in American Superconductor's power quality products. Currently, all superconducting systems still require cooling by liquid helium or liquid nitrogen.

An advanced cryogenic system used in American Superconductor's power quality products. Currently, all superconducting systems still require cooling by liquid helium or liquid nitrogen.