Ampacity Studies on 490c-Rated Transmission Line

Many older transmission lines were designed and constructed with a maximum conductor temperature of 49-55° C (120-1 30° F). The low thermal limit was selected at that time to prevent annealing of aluminum. Unfortunately, on warm days with bright sunshine and low wind speeds, the conductors on these lines can reach the thermal limit without any additional contribution from resistive 12R heating. Annealing temperature limits are now known to be around 1 00° C. However, safe and economical operation of 49° C lines at higher temperatures is still limited by overhead clearance requirements. In Ontario, there are over 1000 km of 11 5-kV and 230-kV lines now operated to this thermal limit. An informal survey has shown that these problem lines exist in most major power utilities. A comprehensive test program was initiated to establish the true thermal capacity of 49° C lines. Four spans of an idle 11 5-kV three-phase transmission line with 477-kcmil conductors were heated by circulating a current of 260 A. This was slighty above the summer-limit current established for Canadian Standards Association clearance requirements. The wind speed, direction, ambient temperature and solar radiation were measured by instruments mounted on the conductor. The resulting conductor temperature and ground clearance were also recorded. The meteorological readings were supplemented with observations from the ground and from nearby weather stations. In two summers of observations, the highest conductor temperature observed was 47° C.