Calculation of the ampacity of high voltage cables laid in free air

The paper presents various approaches for determining the current rating of three 110 kV cables in trefoil formation laid in free air including the developed finite element method. All models are possible to consider all circumstances affecting the ampacity of cables including the heat transfer to the surrounding ambient by radiation and convection, as well as the effect of solar heating. It has been shown that all available approaches have provided results that match with a reasonable accuracy. The permissible current rating for the three AC cables in trefoil formation laid in free air are calculated in summer for both, cables protected from solar radiation and directly exposed to solar radiation. Also, the time constants for heating-cooling cycle for the considered bunch of cables are calculated. It is important to note that the ampacity of three cables in trefoil formation, by all methods, determined by omitting the cooling by wind in order to obtain results for the worst conditions