Abstract
In large-scale 2D finite element steady-state thermal models for underground cable ampacity calculations, where the lateral and lower boundaries are adiabatic, the ground surface may be represented by isothermal Dirichlet, convective Robin, or coupled convection–radiation conditions. In such cases, the soil temperature at depths significantly below the cables tends to be equal or to exceed the temperature of reference soil or air. This paper proposes a novel approach addressing this issue by dividing the native soil into two layers, introducing an effective thermal conductivity for the upper layer, and assuming distinct reference soil temperatures at the cable depth and at the lower boundary. The upper layer spans from the ground surface to the cable installation plane (where reference soil temperature is typically measured), while the lower layer extends to the domain’s bottom. For summer periods, the effective thermal conductivity of the upper layer is calibrated to correspond to measured reference soil temperature under the most unfavorable environmental conditions. A temperature equal to the local average groundwater temperature is assigned to the lower boundary. The study focuses on AXLJ 1×1000/190 mm$^2$ 110 kV cables in trefoil formation, installed in single- and two-layered native soils. Steady-state thermal analyses are performed using COMSOL 4.3. Results show that the cable ampacity can be increased by 8.43%, a value considered safe from both scientific and engineering perspectives.