- The electric power grid may be complex, but the way that transmission lines fail is simple. As the current travels through them, they heat up, their metal cores expand, and the lines sag. Utility companies monitor how much power is being sent through a line and weather conditions like air temperature, but if a line gets hot enough and sags low enough to come near the ground or vegetation, it can short-circuit.

Sagging lines helped cause the blackout last Aug. 14 that left much of the eastern United States and Ontario without power. As some lines failed, power rushed to other routes, overloading those lines in a cascading failure. Experts say that while the blackout had other causes too, reducing line sag would make the power grid more reliable. Now several companies are working to develop power lines that can handle more current without overheating and sagging. Such lines could also help reduce bottlenecks in the grid and allow electricity to be traded over a wider area without any need to build new lines. One company has won a contract to build power lines in Kansas using 21 miles of a new kind of cable. Another is testing new cables in North Dakota and Minnesota to measure their performance in various wind and icing conditions, and in Hawaii to test for resistance to corrosion from salt air. The federal Energy Department is hopeful that such cables will improve the grid's reliability and increase capacity. But for now, most utility experts are withholding judgment. The biggest commercial test has been ordered by a municipal utility in Kingman, Kan., near Wichita, which will pay $2.7 million for a 21-mile line that will use cable with a core made of a composite material. The electricity in transmission cables travels through thick aluminum strands that are twisted together like rope. But aluminum stretches easily, especially when warm, so in a standard cable, the aluminum strands are wrapped around a steel core that gives the cable strength. The cable being tested in Kansas has a core made of carbon fiber, glass and epoxy. The composite material expands very little when heated, and in fact gets stronger. The result is that the cable stretches only about a tenth as much as standard cable. That eliminates sag as a factor that will limit the amount of current that can be sent through a line. What's more, a composite core is thinner than a steel core of the same strength. So the manufacturer, the Composite Technology Corporation of Irvine, Calif., can keep the exterior diameter constant and squeeze in 28 percent more aluminum, enabling the cable to carry more electricity, said C. William Arrington, the company's president. Mr. Arrington said that eliminating the steel had the additional advantage of reducing electromagnetic fields in the cable. With its steel core surrounded by current-carrying wire, a standard cable is like an electromagnet. But since the composite core contains no metal, there is much less of an electromagnetic effect. That means less energy is dissipated during transmission, Mr. Arrington said. Composite-core cable is more expensive per foot, but because it can carry more current, the price per unit of capacity is about the same, Mr. Arrington said. And it can be easier to add capacity to a transmission system by replacing steel-core cables with composite-core ones than by building a new power line, he pointed out. To establish a new right-of-way for a line, "you can spend all the money and all the time and not have a project at the end of the day," Mr. Arrington said. By hanging the new cable from existing towers, he said, "you can get it done today, rather than in 10 years or never." Another company, 3M, has produced cable with a core of aluminum that has been reinforced with tiny ceramic fibers. Wrapped around the core are current-carrying strands made of an aluminum-zirconium alloy, which also contributes to the cable's strength, even at higher temperatures. 3M says the cable can carry two to three times the current of a conventional cable. The Energy Department is helping to sponsor tests of the cable near Fargo, N.D., this winter. A transmission expert in the department's new office of electricity reliability, Philip Overholt, said that the 3M cable was performing well there under difficult wind and ice conditions. But the companies that own transmission lines have been slow to adopt the new technology. John K. Chan, the project manager for overhead transmission cable at the Electric Power Research Institute, an industry consortium in Palo Alto, Calif., said there were several reasons. One is that conventional steel-core cable lasts "forever," he said, while no one is certain how long the newer materials will last. Mr. Chan recalled testing a steel-core cable that had been in use for 50 years. "It was all black outside," he said, "but when we tested it, it was good as new." Another problem is that the composite-core cable is stiff, which is fine when it is strung between transmission towers but makes the material difficult to wind onto spools for shipping. And metal cable can be spliced with a clamp, Mr. Chan said, but referring to the composite-core technology, he added, "You tell me how you connect plastics." Mr. Arrington said that his company had developed a compression system for splicing his composite-core cable. And a spokesman for 3M said the company had developed and tested two splicing methods for its product. But the most difficult obstacle these new cable technologies face might be the attitudes of utility officials themselves. "Typically, power utilities are very conservative," Mr. Chan said. "They don't want to try anything without a proven record. You can imagine what happens if you have a blackout."

Experts say that while the blackout had other causes too, reducing line sag would make the power grid more reliable.

Now several companies are working to develop power lines that can handle more current without overheating and sagging. Such lines could also help reduce bottlenecks in the grid and allow electricity to be traded over a wider area without any need to build new lines.

One company has won a contract to build power lines in Kansas using 21 miles of a new kind of cable. Another is testing new cables in North Dakota and Minnesota to measure their performance in various wind and icing conditions, and in Hawaii to test for resistance to corrosion from salt air. The federal Energy Department is hopeful that such cables will improve the grid's reliability and increase capacity. But for now, most utility experts are withholding judgment.

The biggest commercial test has been ordered by a municipal utility in Kingman, Kan., near Wichita, which will pay $2.7 million for a 21-mile line that will use cable with a core made of a composite material.

The electricity in transmission cables travels through thick aluminum strands that are twisted together like rope. But aluminum stretches easily, especially when warm, so in a standard cable, the aluminum strands are wrapped around a steel core that gives the cable strength.

The cable being tested in Kansas has a core made of carbon fiber, glass and epoxy. The composite material expands very little when heated, and in fact gets stronger. The result is that the cable stretches only about a tenth as much as standard cable. That eliminates sag as a factor that will limit the amount of current that can be sent through a line.

What's more, a composite core is thinner than a steel core of the same strength. So the manufacturer, the Composite Technology Corporation of Irvine, Calif., can keep the exterior diameter constant and squeeze in 28 percent more aluminum, enabling the cable to carry more electricity, said C. William Arrington, the company's president.

Mr. Arrington said that eliminating the steel had the additional advantage of reducing electromagnetic fields in the cable. With its steel core surrounded by current-carrying wire, a standard cable is like an electromagnet. But since the composite core contains no metal, there is much less of an electromagnetic effect. That means less energy is dissipated during transmission, Mr. Arrington said.

Composite-core cable is more expensive per foot, but because it can carry more current, the price per unit of capacity is about the same, Mr. Arrington said. And it can be easier to add capacity to a transmission system by replacing steel-core cables with composite-core ones than by building a new power line, he pointed out.

To establish a new right-of-way for a line, "you can spend all the money and all the time and not have a project at the end of the day," Mr. Arrington said. By hanging the new cable from existing towers, he said, "you can get it done today, rather than in 10 years or never."

Another company, 3M, has produced cable with a core of aluminum that has been reinforced with tiny ceramic fibers. Wrapped around the core are current-carrying strands made of an aluminum-zirconium alloy, which also contributes to the cable's strength, even at higher temperatures. 3M says the cable can carry two to three times the current of a conventional cable.

The Energy Department is helping to sponsor tests of the cable near Fargo, N.D., this winter. A transmission expert in the department's new office of electricity reliability, Philip Overholt, said that the 3M cable was performing well there under difficult wind and ice conditions.

But the companies that own transmission lines have been slow to adopt the new technology. John K. Chan, the project manager for overhead transmission cable at the Electric Power Research Institute, an industry consortium in Palo Alto, Calif., said there were several reasons.

One is that conventional steel-core cable lasts "forever," he said, while no one is certain how long the newer materials will last.

Mr. Chan recalled testing a steel-core cable that had been in use for 50 years.

"It was all black outside," he said, "but when we tested it, it was good as new."

Another problem is that the composite-core cable is stiff, which is fine when it is strung between transmission towers but makes the material difficult to wind onto spools for shipping. And metal cable can be spliced with a clamp, Mr. Chan said, but referring to the composite-core technology, he added, "You tell me how you connect plastics."

Mr. Arrington said that his company had developed a compression system for splicing his composite-core cable. And a spokesman for 3M said the company had developed and tested two splicing methods for its product.

But the most difficult obstacle these new cable technologies face might be the attitudes of utility officials themselves. "Typically, power utilities are very conservative," Mr. Chan said. "They don't want to try anything without a proven record. You can imagine what happens if you have a blackout."