SOIL THERMAL PROPERTIES AND THEIR EFFECT ON THERMAL STABILITY AND THE RATING OF UNDERGROUND POWER CABLES

Accurate measurements of soil thermal properties are essential for proper rating of under- ground power cables. The two most important soil pro- perties affecting cable ampacity are,thermal resistivi- ty and thermal stability. In the cable industry the most frequently used device for measuring the thermal resistivity is the thermal probe. Resistivity measure- ments made with a thermal probe are compared with val- ues using an ASTM guarded hot plate. The comparison of the data is excellent for dry soils and for soils with high moisture contents where moisture migration caused by a temperature gradient is not a problem. The dif- ference in resistivity measurements for soil with low moisture content results from moisture migration and it points out the difficulty of making accurate thermal resistivity measurements when moisture migration is present. Thermal resistivity measurements made in the field are compared with measurements taken on recompacted samples in the laboratory. Results show that accurate soil resistivities can be measured in the laboratory as long as the field dry density and moisture content are carefully reproduced. Ampacity calculations based on moist soil condi- tions can be inadequate if moisture migration in the soil takes place. A large-scale laboratory test pro- gram was carried out to identify soil properties and cable heat flux values leading to moisture migration. Results of the test program help identify drying times for the soil adjacent to the A knowledge of the drying time as a function of heat flux generated within the cable will help the utility en- gineers to identify the potential for a thermal run- away condition. Experimental thermal stability results are compared to a new theoretical study that models the heat and moisture transport process in the soil. surface of the cable.

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