Calculation of the current carrying capability of duct laying electric cable and analysis of the influential factors

A common characteristic of modern power transmission and distribution is the extensive use of underground electric cables. Because of tight economical constraints and limitation on space availability, public utilities around the world are striving to attain higher cable current carrying capability (ampacity), by means of improved designs and, at the same time, trying to achieve better accuracy of cable parameter values so that the simulated results would match as closely as possible the real life situation. The traditional method of computing cable ampacity is based on IEC-60287, and a much more accurate and versatile approach would use numerical method. In this paper a newer approach to duct laying cable thermal field and ampacity computation using finite element method is formulated. In this method, a temperature field distribution model was constructed to analyze the temperature distribution of duct laying electric cable area, and the linear interpolation method is employed to calculate the cable ampacity. The developed model was applied to the 8.7/15KV YJV1×400 XLPE electric cable, from the example, one can note that the duct laying cable ampacity depends on many of the installation properties and conditions, of which the soil thermal resistance, the soil temperature, the environment temperature and the external heat source exercised predominant influences. The cable ampacity increases as the soil thermal conductivity and the distance from the external heat source increases, while it decreases linearly with the increases of the environment temperature and the soil temperature.