Utilities Find the Hidden Capacity in Transmission Lines

Tenessee Valley Authority (TVA), Public Service of New Mexico, Long Island Power and several other major U.S. utilities are installing a device that will allow them to identify the hidden capacity in existing transmission lines and increase power flows significantly.

EPRI's video sagometer, a real-time transmission line sag monitor, developed by EPRI and the EPRIsolutions Engineering & Test Center (Lenox, Massachusetts, U.S.), overcomes problems inherent in traditional static rating of transmission lines. Normally, when utilities calculate static rating, they must assume the worst ambient conditions — usually the highest summertime temperatures when transmission lines heat up and sag under high electrical loads. This can result in ratings that are much lower than the true power capacity of the line most of the time.

As a result, transmission lines often have significant “hidden” capacity that could be used if a reliable means of monitoring sag on a real-time basis (dynamic rating) were available. According to EPRI studies, most transmission lines would be able to carry from 5% to 20% more power than they do now under present limitations.

TVA has installed two sagometers on 161-kV lines coming out of a large fossil plant in its western service area. “During the peak summer load, TVA has been limited by the line configuration and ratings in getting power out of this fossil plant,” says Fisher Campbell, Jr., TVA project manager. “However, by using the video sagometer for real-time sag monitoring, TVA engineers determined that we can increase loading on the two lines by up to 10%.”

Commonwealth Edison has had a similar experience. “The EPRI video sagometer prototypes we have on our 345-kV transmission lines have given us a great deal of information on conductor movement in a variety of conditions,” says Brian S. Cramer, of ComEd Technical Services. “And they provide additional security because now we know from direct measurement that the lines are operating safely, with adequate clearance, even under heavy loads.”

Further, says Cramer, “Installation of the sagometer was simple, requiring no outage. Since the initial installation and subsequent adjustment, the units have operated continuously without maintenance.”

“The accuracy of the video sagometer is truly amazing,” says Bernie Clairmont, the EPRIsolutions project manager who led the development effort. “It allows utilities to monitor sag in real time and with quarter-inch accuracy, which in turn permits utilities to increase power flows — and revenues — considerably. Worst-case assumptions no longer rule. We're going to see these things all over the place.”
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