Temperature Evaluation of Protection Pipe with Diurnal Load Variation

The allowable current of an underground power cable system is evaluated on the basis of the temperature rise of a conductor. Generally, the steady allowable current is evaluated on the basis of saturated temperature rise. Because there is a large difference between the time constant of heat conduction in a cable and that in soil, loss factor (Lf) is generally used. However, when an allowable current is calculated using Lf, there is a possibility of overestimation, such as in a shallow laying case. We previously developed programs for evaluating the transient temperature rise of power cables in a protection pipe with actual transient load change. Using these programs, we can evaluate the overestimation due to Lf in this study. Among the cases we assumed to be normal, there occurred 5°C of overheating of a protection pipe and 3°C of overheating of a conductor.