Calculation of ampacity of underground cables under humidity migration conditions

Abstract

This paper presents a new approach for the determination of buried cable ampacity, considering the humidity migration in soil caused by the temperature rising around cables, and compares calculated results with migrationless considerations. To optimize the utilization of current carrying capacity, accurate calculation of real-time ampacity is needed in case of buried cable transmission systems, which is dependent on analysis of thermal field under application environment. The heat generated from the long-running buried cables can possibly drive away the moisture contained in the soil in the direction of temperature gradient. What's more, the decrease of moisture content could reduce the thermal conductivity of the soil around cables, which distinctively lessens the current carrying capacity of cables. Hence, the ampacity of buried cables could drop with time thanks to the coupling of heat and moisture in soil. This paper presents a new model to calculate the ampacity of buried cables, which has taken the humidity migration in soil into account. Complementary laboratory measurements show that the calculation model proposed in the paper (Model II) is more refined, compared with IEC standards (Model I). When factors like depth of laying, surface wind velocity and environment temperature varies, the calculation results of Model I is smaller and its trends distinctively differ from Model II, which matches the practice better.