Experimental determination of soil thermal conductivity for design of underground cables

The dimensioning of underground extra-high voltage direct current transmission lines (HVDC) requires detailed knowledge of soil properties, particularly thermal conductivity, to optimise cable spacing and minimise project costs and environmental impact. Given the absence of an internationally standardised procedure for testing soil thermal conductivity, a novel approach was developed as part of the SuedLink project - a 700 km long underground power line which will transmit renewable wind energy between northern and southern Germany. The unique aspect of this methodology lies in its ability to deliver reproducible results of the thermal conductivity of a soil across the entire water content spectrum. This allows the use of realistic thermal conductivity values, as opposed to the current use of overly conservative values in the design of underground power lines. Additionally, the highly efficient testing configuration enables the test to effectively be rolled out on a large scale as was shown with over 6000 tests done on the SuedLink project. This article explains the thermal conductivity test procedure and the evaluation of the raw-data obtained from these tests.