Common-Mode Resonance Quantification of 3L-NPC Converter in 3-Phase-4-Wire System and the Additional Active Circuit Based Solution

The DC bus neutral point of the three-level neutral point clamped (3L-NPC) converter is always connected to the neutral wire of the power system due to the requirement of zero-sequence output current, based on which the additional common resonance loop via the neutral wire will be introduced. The analysis of this introduced common-mode resonance is presented in this article, and the neutral point voltage fluctuation model is established considering the coupling of multi-frequency. The proposed neutral point voltage fluctuation model reveals that the output current of the 3L-NPC causes the fluctuation excitation sources of different frequencies, and the resonance will occur when the DC capacitors and the line inductance form a resonance loop at exactly this frequency. An additional extra active filter circuit is proposed in the article to suppress the fluctuation excitation sources, and the common-mode resonance won't be motivated as a matter of course. The control strategy and the stability are analyzed based on the structure of the proposed active filter circuit. Finally, simulation results verify the accuracy of the derived neutral point voltage fluctuation model and the effectiveness of the proposed active filter circuit.