Effects of environment on transient thermal performance of underground cables

Two forms of cable installation are considered, direct burial and integral cooling. Existing methods for the calculation of transient temperatures in directly buried underground cables with changes in load assume a homogeneous environment of soil. In the present work, the effects of trench backfills of different thermal resistivities from the existing soil are investigated using a numerical method and a computer. The relative effects of the backfill and the soil on the transient temperatures are discussed. For times of up to 100 h after the application of a step function of the load current, the effective thermal resistivity of the cable environment approximates to that of the backfill. Using this value, established analytical techniques to represent the soil may be applied with reasonable accuracy. For times greater than this, the soil becomes increasingly important, and the full 2-dimensional heat-flow approach is necessary. Temperatures in integrally cooled cables are readily calculable, but if, owing to a fault in the cooling system, the water flow ceases, new thermal models must be developed to facilitate the calculation of the subsequent cable-temperature rises. By means of practical experimentation, an approximate but simple representation for this condition, using a thermal network, is obtained, and is used in conjunction with the cable thermal network to compute conductor temperatures.