Real-Time Ampacity Model for Overhead Lines

A thermal model is formulated that has the capability of predicting the temperature of overhead transmission lines. The thermal model is formulated into a computer program that can calculate both steady-state and transient temperatures of a conductor. The transient portion of the program allows the simulation of ``real-time´´ line operation and provides a method to predict instantaneous conductor temperatures. In addition to predicting the temperature and the ultimate sag of the line, the program can be used to predict the operating conditions of lines in the design stage. The program is also useful for designing new transmission spans and for predicting contingency ratings during emergency conditions. The model includes energy generated in the conductor, incident solar energy, emitted radiant energy, convection from the surface of the conductor, and energy stored within the conductor. Both free and forced convection modes are considered. The radiation model includes contributions due to direct and diffuse solar energy. The resulting differential equation for the conductor temperature is solved by the computer using a numerical technique. The program is capable of predicting the real-time line temperature for all conductor designs and any variation in weather conditions and line current. Coupled with a sag program, this type of information will permit the determination of instantaneous ground clearances of overhead conductors. Results are presented for the time constant of several conductors subject to step changes in current. The influence of weather conditions on conductor temperature are examined.