Transmission Brains

Feb 27, 2009 - Kate Rowland - energycentral.com

Transmission wires have always been the transportation workhorses of the electric grid. Directing electrical current from generation to substation, these aging electricity highways have traditionally lacked intelligence.

But faced with the daunting task of increasing power delivery across the grid in a reliable and secure fashion -- not one wrought with congestion and inefficiency -- today's transmission wire manufacturers are looking at new solutions to transmission issues.

A smart grid solution that focuses only on information and communications infrastructure and smart metering activities, said Jack McCall, director of transmission and distribution systems for American Superconductor Corporation, is "looking only at the brain, and not at the brawn."

The brawn, in this case, is the superhighway of wires that transport the electricity. With this in mind, the U.S. Department began shoring up the maturing brain of the modern grid ideal with some super-powered brawn. With substantial federal investment in play, it has partnered with industry to introduce new technology into the power delivery infrastructure by sponsoring projects showcasing the use of high-temperature superconducting cables in modernizing electricity transmission and distribution systems.

One of these, by the superconducting association and the Long Island Power Authority (LIPA), is a 2,000-foot-long, 138-kilovolt high-temperature superconducting cable system capable of transmitting 574 megawatts. The world's first transmission voltage superconductor cable in operation, the LIPA project was energized last April. The Energy Department funded $27.5 million of the $58.5 million total project cost.

The hair-thin wires produced are able to conduct 150 times the electricity of similar-sized copper wires. This power-density advantage means transmission-voltage high-temp superconducting cables can use far less wire to conduct up to five times more power -- in a smaller right of way -- than traditional copper-based cables.

The second-generation high-temp "wire is the underlying smart material," McCall said. A perfect conductor, the wire is able to instantly switch to resistive mode when encountering a surge in current, automatically and instantly suppressing power surges.

Another project currently under way using such cables is being partially funded by the U.S. Department of Homeland Security Science and Technology Directorate. The $39-million project will be operated by Consolidated Edison Company of New York. Homeland Security is providing up to $25 million for the project.

This "will enable us to parallel the substations, improve reliability and resiliency, and connect much more substantial power in the urban grid," McCall said. This will better enable the large-scale introduction of plug-in hybrid vehicles without severely straining the downtown grid as commuters drive in to work in the morning and plug in their cars.

Grid Enhancement

The second-generation high-temp superconducting cable also provides the ability to bring electricity into urban areas from more remote generation. The wire is easy to site in a limited right-of-way because it has no electrical or magnetic fields associated with it, and it can also conduct electricity with virtually no electrical losses.

But superconducting alternating current (AC) transmission solutions are also getting some assistance from the direct current (DC) quarter. ABB's high-voltage direct current cable, developed in the 1990s, is gaining some major ground in projects both in the United States and around the world. With overhead, underground and submarine applications, it is being used both in environmentally sensitive areas and in applications where the generated electricity has to travel exceptionally long distances.

One example, the Cross Sound Cable, is the high voltage direct current underwater cable link between Connecticut and Long Island, N.Y. Running for nearly 25 miles underwater, this line is buried under the seabed, with a converter station at New Haven, Conn., and Shoreham on Long Island. The roughly150 kilovolt line has a power rating of 330 megawatts, and was commissioned in 2002.

"One of the biggest advantages of direct current in general is that it is controllable, it's dispatchable," said Mike Bahrman, products marketing manager for ABB. "You can control the voltage." He also noted that DC, unlike AC, does not draw capacitive current, which becomes a limiting factor as distance increases. "This becomes significant over 25 to 30 miles."

Of course, the marriage of superconducting cable and high-voltage direct current cable might be the perfect combination of capabilities and economics. It's something the Electric Power Research Institute is exploring in its current Superconductivity Program, looking at superconducting DC cable for long transmission.

The research into all types of grid advancement is well-underway. Ultimately, a more intelligent utility will benefit all of society by making energy production more efficient and more reliable.