On the economic selection of medium voltage cable sizes in nonsinusoidal conditions

Selection of cable size in the nonsinusoidal conditions is only based on ampacity considerations without any attention to the cost of the losses that will be suffered in the cable life. Since the cost of these losses (fundamental plus harmonics) can assume significant values, the selection of a cross section higher than required for ampacity considerations can result in a large reduction of cost. This paper proposes a method which allows the optimal economic selection of medium-voltage cables in nonsinusoidal operating conditions; it takes into account the initial investment costs and the Joule losses costs, including the additional costs due to current harmonics. It employs simplified expressions similar to those adopted by the IEC Standard in sinusoidal conditions, being the harmonic presence taken into account by a proper definition of a harmonic loss factor and by the introduction of harmonic coefficients to be predicted. Numerical applications to medium-voltage cables are developed and discussed in order to show the sensitivity of the cable optimum size to variations in the coefficients that characterize the harmonic presence.