How do electricity transmission lines withstand a lifetime of exposure to the elements?

How do electricity transmission lines withstand a lifetime of exposure to the elements?
The poles holding them up, and the insulators keeping their current contained, are more likely to fail before the lines do…

 

As long as electrical transmission lines are kept clean, they last up to 100 years—at least a lifetime, says John Kassakian, professor of electrical engineering. Designed to hold up in adverse weather conditions, the transmission lines primarily use ACSR conductors: aluminum cable wrapped around steel-reinforced cores. The aluminum conducts the electricity; the steel adds strength to prevent the line from sagging.

There are two main kinds of electrical lines: transmission lines that conduct power from a generating source to substations, and distribution lines that send power on to individual homes and businesses. The lines are one part of a three-part system that makes up our power “grid.” First, there are the towers, which are made of wood or steel; then the insulating structures, which are ceramic or porcelain; then the lines themselves.

The lines are so sturdy, Kassakian says, that the electrical towers and insulating systems are more likely to fail over time than the lines themselves. Wooden towers begin to rot after about 40 years, and steel towers can corrode. Excessive dirt can degrade the insulating structures (though rain usually keeps them sufficiently clean). But the lines “are generally not replaced,” Kassakian says; the exception is when electrical voltage levels in a certain transmission area need to be increased.

The only effect temperature has on power lines is additional electrical demand; the extra electricity flowing through the lines generates extra heat. “The line will not break or fall down,” Kassakian says. “But if you’ve got a high ambient temperature, and there’s a heavy electrical load because everyone’s got their air conditioners on, a line could sag and could come in contact with, say, a tree.” If that scenario occurs, some the electricity flowing through the line would go through the tree to the ground. This would trip a breaker in a power distribution station, and power would be shut off. “As soon as the line cools down,” Kassakian adds, “you’re back in business.”

Even though power lines last “essentially forever,” says Kassakian, the increased demand for electricity coming from new devices, appliances, and equipment in our homes gives the question of their durability a new dimension. “People are being hit with new electrical costs,” he says. “They’re saying, ‘Gee we’re spending all this money on new transmission lines. How long are these things going to last?’ Not an unreasonable question.” — Jennifer Sutton

 

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April 26, 2010