EFFECTIVE THERMAL RESISTIVITY FOR POWER-CABLES BURIED IN THERMAL BACKFILL

Existing methods for calculating cable ampacities assume that buried cables are surrounded by a single, homogeneous material. These ampacity programs have limited application when a thermal backfill material is used to increase the current carrying capability of underground cables. This paper presents a method of calculating an effective thermal resistivity for any combination of backfill materials that are placed adjacent to the cable. Results are presented for a rectangular region of backfill covered by a protection layer. The concept of an effective thermal resistivity can extend the use of existing ampacity programs to cases involving non-homogeneous soil layers. The method of calculating an effective thermal resistivity is accomplished through the use of conduction shape factors and a finite element computer program that solves for the temperature distribution around the cable. An error analysis has shown that this method gave effective thermal resistivity values within 5% of the correct value. Results are presented for two geometries: a single cable and three horizontal cables. The effective thermal resistivity for the three cable case is verified in a full-scale outdoor test using a 25kV cable. The presence of a backfill material can increase the heat dissipation from a cable installation by 50% above the case where no backfill is used for identical cable interface temperatures. The effective thermal resistivity is much more sensitive to changes in the backfill thermal resistivity when the backfill resistivity is greater than one-fifth the value of the native soil resistivity. Decreasing the backfill resistivity to less than one-fifth of the native soil resistivity cannot, in general, be justified. For high resistivity backfill materials, the protective layer can have a strong influence on the value for the effective thermal resistivity. The effective thermal resistivity is a function of the location of the cables in the backfill and how close they are placed to the protective layer. Optimum cable positions which produce the minimum value for effective thermal resistivity are identified. The optimum position produces minimum operating temperatures for a given current. © 1979 IEEE