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Abstract

To effectively incorporate renewable energies, the electrical power grid requires the extension or construction of numerous transmission lines. In addition to the traditional overhead lines, an increasing number of grid expansions are being carried out with underground power cables.

The heat that is produced during the operation of the cables has to be dissipated to the environment. Thus, the current carrying capacity of an underground power cable system depends strongly on the thermal properties of the surrounding bedding material and soil. In addition, local drying-out may occur in the vicinity of the cable. As a result, power cable rating models often employ a two-zone approach for the consideration of partial soil drying-out, dividing the cable bedding into two distinct areas (wet and dry). In this model approach, each zone is assigned with a constant thermal conductivity value. However, in reality, the process is more complex. The apparent thermal conductivity varies and depends on the moisture content of the backfill and soil materials, which, in turn, is influenced by temperature and other hydraulic boundary conditions.

This paper compared cable rating results obtained from the two-zone model, IEC 60287-1-1, and a fully coupled model that considers moisture-heat transfer. The results obtained from IEC 60287-1-1 and the two-zone model were similar, while the fully coupled moisture-heat transfer model showed a 30% increase.