Thermal Resistivity Measurements of Soils/Rocks and Geological–Geomechanical Site Surveys for the Design of High-Voltage (HV) Cable Connections
In recent years, the construction of new transmission lines has accelerated to ensure adequate transmission capacity and grid stability. Voltage levels of cable systems are also increasing.
Globally—and increasingly in Slovenia—the share of underground high-voltage cable connections is rising. The main reasons include:
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Stricter environmental standards
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High population density in the area
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No negative visual impact on the landscape
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Resistance to weather conditions (ice storms, wind, falling trees)
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Air traffic safety
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Electromagnetic field exposure under overhead lines
The downside of HV cable systems lies in the high construction costs, which grow further with increasing voltage. However, due to the growing number of cable installations and improved construction and cable-jointing technologies, these costs have been gradually decreasing.
For the design of HV cables and to ensure their long service life, a thorough understanding of thermal conditions along the cable route and the thermal properties of surrounding materials is essential.
Thermal Behavior of HV Underground Cables
During operation, HV underground cables heat up. Overheating reduces their service life and may even cause cable failure. According to manufacturers, if the operating temperature of HV cables exceeds the design temperature by 10°C, their lifetime may be reduced by half.
Excessive cable temperatures can be prevented by effectively transferring heat to the surrounding materials, which must have high thermal conductivity and low thermal resistivity.
Geoinženiring d.o.o. Services
Geoinženiring d.o.o. performs engineering-geological and geomechanical investigations and site assessments for the planning of HV cable connections (110–420 kV). Based on terrain conditions, soil composition, and groundwater level variations, the required number of test pits is determined. In these pits, in-situ measurements of thermal resistivity/conductivity are conducted on representative materials.
Samples are also taken for laboratory analysis of moisture content and material density, as well as for future testing of thermal properties under varying temperature or humidity conditions.
Equipment Used – KD2 Pro
We use the KD2 Pro thermal property analyzer manufactured by Decagon Devices, Inc. The equipment complies with:
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IEEE Standard 442-1981 (IEEE Guide for Soil Thermal Resistivity Measurements)
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ASTM Standard D5334-08 (Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe Procedure)
KD2 Pro is a handheld device for measuring the thermal properties of fluids, solids, and insulation materials. It consists of a control unit, a sensor (needle probe), and software.
For measuring the thermal conductivity and resistivity of soils and granular materials, we use the TR-1 probe (length: 100 mm, diameter: 2.4 mm). The RK-1 probe (length: 60 mm, diameter: 3.9 mm) is used for measuring the thermal properties of rocks and concrete.