In this paper, a novel technique which can suppress the common-mode voltage (CMV) of the three-level neutral-point clamped (3L-NPC) inverter is proposed, where an auxiliary NPC leg is added to produce the compensating voltage through a single-phase four-winding transformer. In this method, the utilization of the DC-link voltage is kept unchanged and the saturation of the transformer is avoided owing to the half-bridge structure of the proposed circuit even though the asymmetrical operating condition exists. With the proposed method, the CMV of the system has been reduced to about 10% in RMS compared with that of the regular modulation method. The validity of the proposed method has been verified by the experimental results for the proto-type system.