Analysis of the voltage harmonic distortion at buses feeding office loads

The harmonic distortion of the phase-to-neutral voltage at low-voltage buses feeding office load is investigated for various cable configurations, lengths and cross-sections. The investigation is carried out with a software model that was validated with analytical expressions and measurements taken at a real installation. It is shown that the selection of a cable should be based not only on its ampacity and allowable voltage drop but also on its configuration, in order to avoid excessive voltage harmonic distortion. Configurations where the neutral conductor is part of a four-core cable lead to relatively small harmonic distortion, whereas cable configurations where the neutral conductor runs separately cause high-voltage harmonic distortion. In the latter case, the neutral conductor develops a significant voltage with respect to the ground. Increasing the cross-section of the neutral conductor, as the existing standards dictate, may prevent its overloading from zero-sequence harmonic currents but hardly improves the voltage harmonic distortion at the load bus.