In October of this year, the US Department of Energy announced a $5.5 million contract to install the world’s first high temperature superconductor (HTS) power cable system in an electric utility network. The HTS cables will triple the power throughput of the copper cables they will replace in the Frisbie Station in Detroit. Detroit Edison will begin using the cable in mid 2000. If this application proves to be a success, the HTS cable will be made commercially available by 2001. The Frisbie project is particularly relevant for utilities with congested power delivery infrastructures.

This will be the first demonstration of the use of HTS cables in a live utility grid. Dr Paul Grant of EPRI said lengths of up to 50 yards of the cable have already been tested. These tests were carried out between March and August 1998, using 115 kV single phase prototype HTS cable with a joint and terminations.

The cable for the Detroit contract will be manufactured and installed by Pirelli Cables and Systems using HTS wire produced by American Superconductor. Lotepro Corporation, a subsidiary of Linde AG, will develop the cable cooling system. EPRI will partially fund and help manage the cable project.

“The Detroit Edison cable project is the first of several we expect to undertake in the next few years, as the market for high capacity HTS cables grows,” said Walter Alessandrini, CEO of Pirelli Cables and Systems North America.

The project value is $5.5 million. The US Energy Department, through its Office of Energy Efficiency and Renewable Energy, is providing $2.75 million, the remainder being funded by the other participants.

Nine of the present four inch diameter copper power lines connecting a transformer station with the distribution grid will be replaced by three superconducting lines chilled with liquid nitrogen circulating through the cable cores.

One advantage is that dispersal of waste heat is a much less serious problem for a superconducting cable than for a conventional copper line. Therefore, trenches dug for new lines can be narrower than those for ordinary conductors. This will reduce the right of way utilities must acquire. Each superconducting four inch cable will carry 2400 A at 2400 V.

The HTS cable is much lighter than copper cable. Only 250 lbs of HTS cable wll be needed to replace over 18,000 lbs of copper wire currently in the Frisbie Station. This reduction in weight is attainable because HTS wires can carry over 100 times the amount of electricity carried by copper wires of a similar size. In addition, HTS wires have a far lower resistance to the flow of electric current, making them more efficient in transmitting electricity.

Superconducting cable generates only limited amounts of heat. As a result, it can be retrofitted into existing ducts.

Existing cable will be retrofitted with the 400 foot superconducting cable in existing conduits, avoiding disruption and damage caused by additional digging. The HTS cable system will carry three times the power carried by the conventional copper cables.

This project will test the practicability of large-scale superconducting power transmission, and could be the first of many such projects. Announcing Federal support for the project, Energy Secretary Bill Richardson predicted that power lines of this nature could save $6 billion a year due to greater transmission efficiency.

American Superconductor and Pirelli are jointly developing two types of High temperature superconductor (HTS) cable.

  • Retrofit cables can be drawn into existing conduits as worn out copper cables are extracted. Because HTS retrofit cables can carry five times as much power as copper cables, this addresses the need to send more power through existing rights of way. This is the same strategy used by the telecommunications industry with optical fibres.

  • Coaxial cables intended primarily for underground installations. Projections indicate the capacity of coaxial HTS cables will be six times that of traditional underground copper cables. Because a single HTS coaxial cable will carry more power than a copper cable, the ultimate cost will be less. HTS coaxial cables offer savings when switching from overhead to underground cables.

    For installations where fitting an HTS cable into existing ductwork is not an important consideration, a coaxial configuration could be used. Such coaxial HTS cables would be able to carry several times the current of unshielded HTS cables. For long distances, the use of DC rather than AC must be considered. Because cables act as capacitors, the amount of AC power required to ‘charge-up’ a 30-mile conventional high-pressure, fluid-filled cable is about the same as the power that can be delivered to produce useful work.

    Using DC power would eliminate this problem – an option that will become increasingly attractive as the cost of converting AC to DC and back again decreases. Application of HTS cables would mean lower DC voltages and higher currents could be used, further reducing the cost of power conversion. It has been estimated that a coaxial HTS cable using DC power at 50 kV will be able to carry 5000 MWe. Such a development would make underground transmission systems competitive with overhead lines for long distances.

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