Influence of dry ambient conditions on performance of underground medium-voltage DC cables

Highlights: • The critical temperature for drying is calculated for a field in a semi-arid region. • Heat-transfer simulations are performed for a system of two medium-voltage DC cables. • Day-to-day variations in field conditions may significantly affect ampacity. -- Abstract: In its publication IEC 60287, the International Electrotechnical Commission recommends procedures for thermal analysis of power cables in the presence of a dry-out zone in the surrounding medium. It is well known, that the ampacity of cables depends strongly on the so-called critical temperature at which the surrounding medium dries out. However, research on how to estimate this critical temperature has been limited. In semi-arid Mediterranean climate regions, dry summers typically lead to a decrease of the critical temperature. In this paper, the critical temperature is calculated for a field in Riverside, California under various ambient conditions. This is done from the numerical solution of the differential equation for moisture content in the planar case of steady-state coupled heat and mass transfer in porous media. Finite-difference simulations of a system of two medium-voltage DC cables in this field are performed with SHEMAT-Suite (Simulator of HEat and MAss Transport) to compute the maximum temperature for daily cases of calculated critical temperature and compared to a prediction from IEC 60287. It is found that large day-to-day variations in field conditions in a semi-arid area may significantly affect the maximum allowable current. Our results emphasize the importance of ambient conditions for the cable performance.