Thermal aspects of changes in the environment of underground cables

Abstract
The thermal field established by a trench containing three closely spaced single-core cables in horizontal formation, bedding sand, concrete slabs and backfill has been determined by the use of an electrolytic tank, and the effects of variation in the backfill thermal resistivity obtained for bedding-sand and soil resistivities of 150 and 120cmdegC/W, respectively. The effective thermal resistances from the cable outer surface to the ambient surroundings are compared with those obtained by the analytical method of Schmill, which applies to only two different resistivities, backfill and soil. The effect of changes in the bedding-sand resistivity are summarised in a simple equivalent thermal circuit. Using this, the cable-surface-temperature-rise/cable-loss curves are obtained for sand-resistivity/temperature relationships which are assumed to represent the moisture-migration process; this applies for moisture migration in the bedding sand alone. It is seen that the temperature dependence of the resistivity of the material immediately around the cables is decisive when determining thermal runaway. The cable-surface temperatures are determined with artificial cooling using external water pipes, and include the effects of coolant-temperature rise and bedding sand. It is shown that the thermal time constants obtained for a particular environment and cooling scheme may be approximately adjusted to represent others, provided that the burial depth is the same. The changes in conductor temperature with an interruption in coolant supply are shown.

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