Investigation of Different Trench Geometries for Optimized Bedding of Buried Power Cables

For an efficient integration of renewable energies, many transmission lines of the electrical power grid have to be extended or newly built. Besides the common overhead transmission lines, an increasing proportion of these grid expansions is conducted using underground power cables.

During the operation of buried cable systems, the mechanical and thermal properties of the cable’s surroundings need to meet certain requirements. To avoid insulation faults in the cables due to overheating, the ampacity is limited by specific conductor temperatures and the thermal energy resulting from the electric losses during transmission needs to be reliably dissipated. Thus, the actual performance of a buried power cable system depends strongly on the thermal properties of the cable bedding materials and soil.

In practice, buried power cable lines typically require the use of cable trenches. The pre-existing soil from the cable trench is usually replaced by sand or artificial fluidized backfill materials with well-known material properties, which may differ from the properties of the surrounding soil. Thus, heterogeneous structures are created in the shallow subsurface, which affect the heat and water transport around the power cables. With an installation depth of 0.5 - 2.5 m, the cables are typically located in the vadose zone, where the thermal properties of the bedding are affected by the varying water content by up to one order of magnitude. Therefore, precise knowledge of the influence of size and geometry of the cable trench on the water distribution around the cable is crucial for an adequate assessment of the cable’s ampacity ratings.

Within the scope of our research, the influence of cable trench geometry and size on heat and mass transfer around buried power cables were investigated with a coupled approach of laboratory experiments and numerical modeling.