New Approach to Ampacity Evaluation of Cables in Ducts Using Finite Elemet Technique

This paper presents a package of user-oriented computer programs for power cable thermal analysis using the finite element technique. Special features of the programs include: automatic grid generation routine, user friendly input and output structures, sensitivity analysis, combined heat/moisture analysis, and a new, exact method of modeling the heat flux distribution in the region between the cables and the retaining duct in the underground power transmission system. The analysis reveals that the heat flux on the inside surface of the retaining duct is not uniform, as generally assumed in cable ampacity calculations. In fact, the heat flux is maximum at the top where the convection induced flux is much stronger than at the bottom or elsewhere around the duct. Laboratory experimental verification of the heat flux model is presented. A full scale test of a 12 cable system in a concrete duct bank is described and the temperatures measured over a 42-day period are compared with those obtained using the new computer program.