The electricity system in the United States received renewed attention after the August 2003 blackout that affected more than 50 million customers across the Northeast United States and caused...
many directions, wind direction occurrences of 15 degrees, 30 degrees, 60 degrees, and 75 degrees were assumed to have an equal probability of 25 percent. Based on this, an "effective wind speed" distribution was created.
Figure 1 provides several important conclusions for wind conditions during the afternoon of Aug. 14, 2003:
- There was a probability of 5 percent of the effective wind being less than 0.6 ft./second in Ohio. At 0.6 ft./second, the effect of convection is equal only to the so-called natural convection, i.e. zero wind speed.
- Effective wind speed of 2 ft./second had a probability of 18 percent. The 2 ft./second value is commonly used by most utilities as a "safe" wind speed assumption.
- The median wind speed was only 4.5 ft./second, compared with summertime average median value of 12 ft./second, according to Wind Resource Atlas of United States
- The probability that the effective wind speed was less than the 3 ft./ second assumption was 30 percent.
- Some Midwest utilities assume a wind speed of 4.4 ft./second. Note that the probability that the effective wind speed was less than this value was 50 percent! Thus, this can hardly be called a "safe" assumption.
What happens if the wind speeds are less than the assumed values? This can be investigated by using a common ACSR 54/7 "Cardinal" conductor and assuming that the conductor is rated to operate at a temperature of 212 F. 1 If the ambient temperature is 90 F and if the wind speed is 3 ft./second, the line would be rated at 1326 A. If the wind speed is instead assumed to be 4.4 ft./second, the line would have a rating of 1451 A. The 1.5 knot (2.5 ft./second) assumption would result in a rating of 1278 A. The figures in Table 1 show the consequences.
Actually, conditions in the line corridors in question were most likely even more dangerous than shown by the above airport-based weather data. Transmission corridors are much more sheltered by trees and terrain than airports. Often, wind speeds in transmission line corridors are found to be only 40 to 60 percent of those at nearby airports. 2
How are the line clearances from ground affected? Assume that the lines were designed for 1,000-foot spans with final sags of 33.6 feet. If still air conditions occurred, lines designed with the 3 ft./second wind speed assumption would sag 4 feet more than assumed while lines designed with the 4.4 ft./second wind speed assumption would sag 5.2 ft. more than assumed. For lines using the PJM assumption of 1.5 knots, the sag would be 3.3 ft. more than assumed. Thus, it is very likely that many lines operated under those assumptions under the wind conditions on Aug. 14 would have been significantly below the safe clearances mandated by NESC (National Electric Safety Code), because most utilities apply safety clearance buffers of less than 3 feet nowadays.
What could the consequences have been? In an article in the Cleveland Plain Dealer, 3 a utility spokesman described the ground faults as "glitches," stating, "The ability of a line