On the Economic Selection of Medium Voltage Cable Sizes in Nonsinusoial Conditions

Selection of cable size in nonsinusoidal conditions is based only 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 be high, the selection of a cross-section higher than required for ampacity considerations can result in large cost reductions. This article proposes a method that allows the optimal economic selection of medium voltage cables in nonsinusoidal operating conditions; it takes into account the initial investment costs and the cost in Joule losses, including the additional costs due to current harmonics. It employs simplified expressions similar to those adopted by IEC Standard in sinusoidal conditions, taking into account the harmonic presence 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.