Technology Corridor

Experts say utilities are pushing transmission systems to unsafe limits.

When transmission-line rating assumptions do not match the physical realities of transmission networks, the consequences can be disastrous.

Timelines of line trips in Ohio and Indiana preceding the Aug. 14, 2003, blackout indicate that many lines failed because of phase-to-ground faults. Such faults can occur only if the lines are operated well above their design temperatures-due to inadequate clearances to trees and other objects under the line-or most likely, because of a combination of both conditions.

A news release of Aug. 22, 2003, from American Electric Power (AEP) reveals important information. It shows that at least four, and more likely, six transmission lines had phase-to-ground faults between 3:41 and 3:51 p.m., while the line loadings were substantially below assumed safe ratings. A later article in the identifies a total of 64 breaker actions in Ohio between noon and 4 p.m. on Aug. 14.

We believe that AEP bases its rating assumptions on actual ambient temperature, full sun, and a 3 ft./second wind speed. Compared with those assumptions, ambient temperatures in the area were slightly lower than assumed, but the sky was generally clear. PJM uses ambient-adjusted emergency ratings, with a wind speed of 1.5 knots (2.5 ft./second), while some utilities in the Midwest use wind speed assumptions up to 4.4 ft./second. Thus, an important question: Were these assumptions valid at the time of the emergency loads?