Rating methods for cables installed in unventilated and partially ventilated surface troughs

Cables installed in surface troughs are a critical part of many transmission circuits. This paper presents a method for calculating the rating of such circuits using computational fluid dynamics techniques, applied to two common trough designs. One design has solid concrete lids while for the second some lids are replaced by grilles to encourage natural air circulation. For the case of covered troughs with solid concrete lids, the IEC 60287 method was found to be conservative owing to its failure to consider the thermal impact of the ground. Good agreement was observed between the CFD model and an improved analytical formulation by Anders. Modeling of partially ventilated troughs, which at present cannot be rated analytically, shows continuous rating increases of up to 31% over the covered design owing to greater air circulation.