Ampacity studies on 49°C-rated transmission line

It is noted that transmission lines rated at 49°C have little thermal margin when operated in the summer. Results are given of an experimental study on four spans of an idle 115 kV three-phase transmission line. Under conditions of 33°C ambient, 2 to 4 km/hr effective wind speed, high solar input and 260 A load current, the 477 kcmil conductor temperature reached 47°C. The ampacity models of H.E. House and D.D. Tuttle (1959) and IEEE (ANSI/IEEE Standard 783-1986) provided reliable predictions of conductor temperature when the site weather data were used. Conductor temperature estimates prepared using forecast data from a weather prediction system were less certain; the differences were caused by natural scatter in the data and by reduction factors in the wind model. It is found that wind angle should be increased somewhat to model improved cooling of the conductor when wind direction is fluctuating. The change of clearance with temperature was found to be significantly larger than anticipated in short spans, because of limitations in the application of the ruling-span approximation. The physical errors are small but the line-rating process is very sensitive to these errors