Method to estimate the impact of harmonic and unbalanced currents on the ampacity of concentric neutral cables

摘要

Concentric neutral cables are commonly used in medium voltage power distribution systems. At present, the cable ampacity calculation methods can take into account only the fundamental frequency current and the harmonic currents in the phase conductors. In reality, the cable neutral can carry both fundamental frequency and harmonic currents (induced and unbalanced), leading to a further increase of cable temperature. In view of the increased harmonics and load imbalance in power distribution systems and the need to determine their impact on concentric neutral cables, this paper presents a method to estimate the ampacity of concentric neutral cables, especially for multi-grounded neutral systems. The proposed cable derating factor can be used to determine the increase in cable loading caused by both unbalanced load and harmonics. A chart is also presented as a simplified method for cable ampacity and cable loading estimation, for which only the total harmonic distortion (THD) and zero sequence current ratio of the phase current are needed.