A survey of neutral currents in three-phase computer power systems

Neutral current in three-phase power systems is often thought to be only the result of the imbalance of the phase currents. With computer systems, very high neutral currents have been observed even when the phase currents are balanced. Measurements from a sample of computer power systems in the United States are presented to determine the extent of the neutral current problem. The author explores the cause of high neutral currents in three-phase computer power systems, potential problems, and recommended remedies. It is found that, while very high neutral currents are possible due to the additive nature of triple harmonic currents, a low percentage of data-processing sites in the US are actually experiencing neutral currents in excess of the rated phase current. However, recent trends in computer systems make high harmonic neutral currents more likely. Power system problems associated with high harmonic neutral currents include overloaded transformers, voltage distortion, and common-mode noise. It is suggested that whenever three-phase, wye power systems are used to supply power to computer systems or other similar electronic loads, the power system design should allow for the possibility of high harmonic neutral current to avoid potential problems