A Neutral-Point-Potential Balancing Strategy of Three-Level NPC Inverters by Injecting Absolute Value of Modulation Signal

This paper presents a strategy for neutral potential (NPP) balancing in a three-level neutral point clamped (NPC) inverter by injecting the absolute value of the modulating signal. The injection involves the zero-sequence component and the 2nd harmonic component. When the power factor is large, the injected zero-sequence component maintains the balance of the neutral potential on the DC side. Besides, the injection of the 2nd harmonic component also achieves the objective of NPP balancing and the control effect is independent of the power factor. The analysis shows that when an absolute value of the modulating signal is injected, both the zero-sequence and the 2nd harmonic component lead to voltage balancing of the DC side and can be scaled adaptively to the path as the power factor changes. The NPP balancing strategy is verified by simulation from the voltage balancing effect and adaptive reduction of harmonic components.