Non characteristic line harmonics of PWM AC-DC converters

This paper uses frequency domain analysis to compare a PWM voltage source rectifier with a PWM current source rectifier when with unbalanced AC supply and with reduced DC side filtering. It shows that noncharacteristic 3rd and supposedly eliminated harmonics can reappear and cause a parallel resonance between the input filter capacitor and the source impedance. The procedures are first validated on a six-pulse converter as the special case for a PWM current source, and on a six-step inverter as the special case for a PWM voltage source. A selection of waveforms is given for a 5% unbalance using specific PWM strategies