Waveform distortion due to switch mode power supplies on a large rotary UPS system

This paper covers an analysis of waveform distortion on a UPS feed at a major telecommunications installation. High levels of distortion had been blamed for mal-operation of sensitive test equipment and other problems. UPS systems have a significantly higher impedance than the grid and it was initially suspected that high harmonic currents were resulting in significant voltage distortion. Current waveform distortion of extremely high levels was found in several branches of the network although the overall distortion was relatively low. This was traced to the use of switchmode or capacitor input type power supplies in the areas with the worst problems. It was found that harmonic currents flowing in the neutral conductor were causing significant neutral-earth potentials and that voltage waveform distortion was not significant. Several different means were considered in an attempt to reduce the current in the neutral conductor in these areas to acceptable levels. The ultimate solution adopted involved a total replanning of the station power layout to solve the distortion problem and allow further expansion of the UPS feed