The sixth topic in our solar series explores how to prevent premature failures to newly installed underground powers cables.
Prior to the design and installation of underground electrical cables on new solar schemes, it is crucial that you understand the thermal properties of the in-situ soil or made ground.
It is important to ensure that the heat produced by current flowing through an underground power cable is properly dissipated to avoid premature failures. Often estimations are made, but where conditions are uncertain or variable along a cable route, it is essential that a proper quantitative assessment is made of the ground.
The Method: Soil Resistivity Testing
Soil thermal resistivity testing measures the capacity of the ground to conduct or dissipate heat. The thermal resistivity of the soil will determine whether a buried power cable remains stable or overheats. A build-up of heat around the cable can reduce transmission efficiency, or in the worst case cause it to melt.
The testing procedure involves taking measurements along the cable routes at different locations using a needle probe, which is inserted in a pit or an open trench at the proposed cable depth.
Above : A cable route showing 5 locations where the soil thermal properties require to be measured.
Above :In-situ thermal resistivity testing with a needle probe.
Thermal resistivity readings can then be recorded and presented in a tabulated format to be used for calculations by the installation engineers ahead of the solar panel installation.
Above : Example results of thermal resistivity testing at 3 locations.
Using this data, potential problems can be identified and remedial measures taken, including changing the capacity and insulation of the cables, or installing corrective thermal backfills in the cable trench.