A carrier-based pwm method with the double frequency voltage injection for three-level neutral-point clamped (NPC) converters

A well-known problem of three-level neutral point clamped (NPC) inverter is the voltage imbalance in the dc-link capacitors. This paper proposes the dc-link capacitor voltage balancing method where a common offset voltage is injected. The offset voltage consists of a harmonic voltage and a magnitude difference between the upper and the lower capacitors. Theoretical analysis is provided to balance the voltages, and it shows that both the odd and the even harmonics in the injected voltage compensates the unbalanced voltage between the capacitors. The proposed method does not require any hardware modification, so that it is easily implemented. Both the simulations and the experiments are carried out to show the excellent performance of the proposed method.