Abstract
In analytical and numerical calculations of the underground cable ampacities in accordance with the relevant IEC standards and the IEC technical report, it is usually assumed that the native soil, cable bedding, and other materials used in and around cable trenches are homogeneous, although this is not the case in reality. This paper, however, assumes that the native soil consists of two horizontal layers with different thermal conductivities. The upper native soil layer includes soil material as well as shrinkage cracks between the ground surface and the horizontal plane where the reference soil temperature is measured, whereas the lower native soil layer includes only soil material below the upper layer, without any porosity. The concept of effective thermal conductivity is applied to the upper layer of native soil. It is also assumed that the effective thermal conductivity of the upper native soil layer in the summer and winter periods corresponds to the most unfavorable summer and most common winter conditions, respectively. In this regard, the aim is to determine the seasonal thermal behavior of three 110 kV power cables installed in trefoil touching formation, directly in two-layered native soil or in four different cable trenches surrounded by the same two-layered native soil. Ampacities are determined analytically in MATLAB, using a standard IEC-based procedure for cables in single-layered native soil, as well as numerically in COMSOL Multiphysics 4.3, using FEM-based steady-state thermal analysis for cables in single- and two-layered native soils. The IEC-based ampacities are used as base values. It is determined that the summer and winter ampacities of the 110 kV cables, compared to the corresponding base ampacities, can be increased by up to 28.46% under the most unfavorable summer conditions and by up to 31.92% under the most common winter conditions, respectively.