Rating of bare overhead conductors for intermittent and cyclic currents

Equations are derived and solutions given for determining the variation with time of the temperature of bare overhead conductors carrying intermittent and cyclic currents. The temperature variation depends on the rate of heat input, the heat capacity of the conductor and the rate of total heat transfer. The latter is proportional to (temperature rise)q. If the heat transfer by radiation can be neglected, q=1 for forced convection and approximately 1.25 for natural convection. If radiation is not negligible, q is greater than 1, the actual value being obtained from calculations or steady-state measurements. Thermal time constants can only be used when q=1, and they are then only approximations. Measured heating- and cooling-time constants agree reasonably well with calculated values. The significance of the difference between heating- and cooling-time constants is discussed. The importance of the heat source or sink formed by the core in steel-cored conductors is shown, and calculated values of the radial thermal conductivity are similar to those found from previous steady-state measurements. Calculated temperature/time characteristics for conductors in still air are in good agreement with measured characteristics.