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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?
Traditionally, the engineering and planning functions of the transmission owners have determined thermal line rating assumptions. Unfortunately, these persons often have little or no understanding of meteorology, and they generally base their rating assumptions on weather data from National Weather Service sites. A quick review of weather data from National Weather Atlas shows that the median summer wind speed in Ohio averages about 12 ft./second and that the probability of wind speeds of less than 2.5 knots is only 2 to 5 percent. To a layman, this certainly would seem to indicate that the aforementioned wind speed assumptions are quite conservative.
But are they?
The National Weather Service sites in the area are generally located at airports and have anemometers with stall speeds of about 2.5 knots, or about 4 ft./second. Thus, winds speeds of the critical range of less than 2.5 knots are recorded as zeros. To make this data useful for estimation of low wind speeds, all of the data from the sites shown in Table 1 (see next page) for the period of noon to 5 p.m. were first aggregated, and the zero wind speed events were distributed based on Rayleigh distribution. Of the "zero readings," 55 percent were assigned a value of 2 knots (3.4 ft./second), 30 percent a value of 1 knot (1.7 ft./second), and 15 percent a value of 0 knots.
Next, the effect of the wind direction should be considered. A parallel wind has approximately 42 percent of the cooling effect of a perpendicular wind, while winds at intermittent angles have cooling effects between these two values. Because the transmission lines have many angles and go in