PHOTO: Monica Heger

2 July 2008—Last Wednesday, American Superconductor officially commissioned the world's first high-temperature superconductor power-transmission cable system to be used in a commercial power grid. Superconductors can supply lots of energy quickly, efficiently, and unobtrusively. They conduct 150 times the electricity of similarly sized copper wires. However, because of technological difficulties, the commercial development of superconductor power-cable systems has been slow.

The system commissioned last week, part of the Long Island Power Authority's grid and funded by the Department of Energy, consists of three cables operating at 138 kilovolts. It was energized in April 2008 and has the ability to power 300 000 homes when operating at full capacity.

However, there are still some technological hurdles to overcome before superconductors replace the copper wires in our power grids. The main issue is cost. The first-generation cables, now operating successfully on Long Island, are costly, mainly because the wires are coated with silver. Testing has just begun on a second-generation cable coated with copper, which cuts four-fifths of the cost.

American Superconductor CEO Greg Yurek says that in the long run, the cost of superconductor transmission cables will be below that of adding new aboveground copper power lines. A single superconductor running underground can take the place of a nest of conventional copper lines strung overhead. The cables at the Long Island site enter the ground through a right of way not much wider than 1 meter.

Part of the cost of superconducting cables comes from the need to keep them at very low temperatures. The so-called high-temperature superconductor cables, which actually operate between about 65 and 75 kelvin (about –210 ˚C and –200 ˚C), were a breakthrough from the previous superconductors, which had to be kept at just a few K. The wires are kept at these temperatures by running liquid nitrogen through the cables.

American Superconductor is hoping to convince utilities that its technology is the future of electricity transmission.

Besides economics, another advantage the company is touting is that the cables can prevent fault currents, surges that are caused by grid-scale short circuits. Superconductors have an inherent current-limiting ability in that if the current increases past a certain threshold, they lose their superconducting abilities and become normally resistive, damping the current.

American Superconductor is working with Consolidated Edison Co. to develop a fault-current-limiting superconductor power system in New York City. The Department of Homeland Security provided a grant for that project, which is expected to be operating by 2010.

PHOTO: Monica Heger

THINNER: Slim superconductors (right) carry the current of thick copper cables (left)