Underground electric cables a correct evaluation of the soil thermal resistance

Abstract

Nowadays companies supplying medium/high voltage electric energy, near residential areas, tend to use underground cables laying. Hence the design engineer is required to estimate the thermal resistance around the underground cable to perform a right dimensioning of the cables; as a matter of fact what it should be avoided is the overheating determined by a bad heat dissipation due to the Joule effect.IEC rules provides a formula for the evaluation of the soil thermal resistance which is easy to apply. But from an experimental point of view, as the bibliography shows, it was discovered it tends to underestimate the problem when dealing with very dry soils in particular.Thanks to an experimental system, some useful data were collected for the validation of a 2D FEM model of an excavated area with a linear heat source reproducing the underground conduit. The numerical model presents a variation of both the geometrical parameters of the excavated area and the distance characterizing other cables (hence heat sources) located in the same site of installation examined. In this way two dimensionless coefficients, useful to correct the values of the thermal resistance furnished by the current regulations, were determined.