Excessive neutral currents in three-phase fluorescent lighting circuits

Causes for excessive neutral currents amounting to 75% and 157% of phase current values in two installations supplying predominantly fluorescent lighting circuits have been identified. These high neutral currents are accounted for by three main components, namely fundamental phase current unbalance, third-harmonic currents caused by saturation of ballasts, and more third-harmonic current caused by third-harmonic voltage waveform distortion in the supply. Solutions are presented to overcome the problem, including a scheme that cancels third-harmonic currents along a common neutral conductor by means of a six-phase system supplied by two transformers connected in antiphase