Presently, the preferred circuit of PWM (pulse width modulation) converters has the open-wye 3-phase connection, in which the zero sequence does not exist. Single-line-to-ground fault studies have shown that the grounded-wye connection is better from protection and security consideration. However, extensive investigation is required to evaluate exhaustively the comparative merits of the grounded-wye connection. This paper initiates the research of the zero sequence in PWM converters by presenting a fundamental study based on analysis using the switch function concept. The switch function analysis shows: how the zero-sequence current leads to DC capacitor voltage unbalance; how the dc voltage unbalance becomes a source of zero sequence voltages on the AC-side; how the zero sequence current can lead the AC circuit to resonate with the DC circuit; and how such resonance can be eliminated electronically through feedback.