Thermal Instability in Buried Cable Backfills

Abstract

Buried high voltage power cables generate heat when a current is passed through the conductor, and failure of the cable may occur if the conductor or insulation temperature exceeds the design limit. Thermal instability, a moisture migration phenomenon causing a decrease in soil thermal conductivity, has caused thermally induced failure of cables and may be a problem in the case of other buried heat sources as well. Thus accurate methods for predicting thermal instability are needed. Until recently, such problems have been mainly in the domain of electrical and mechanical engineers; however, geotechnical engineers are now frequently involved in the study of this problem. The Hartley and Black method, based on the assumption that the ratio of the time to instability to the diameter squared of the heat source is a constant for a given heat input rate, is investigated. Laboratory tests, scaled model tests, and field tests were used to provide a wide range in the diameter of the tubular heaters employed (1.6 to 406 mm). Based on data collected from scaled model tests, thermal instability was accurately predicted for a field test of three simulated cables. The influence of cyclic versus constant thermal loading on the thermal stability of sand was also investigated, and it was found that the time to instability was not a function of the method of loading for cycle times that are short relative to the time to instability.