A New Insight on Four-Level Neutral Point Clamped Converters: Four-Level = Two-Level + Three-Level

This article proposes a new insight on the four-level neutral point clamped converters: Four-level = two-level + three-level. With the proposed idea, every four-level neutral point clamped converter topology can be regarded as a hybrid combination of a two-level converter and a three-level converter. Thus, the common and complex capacitor voltage imbalance issue of four-level neutral point clamped converters can be simplified into closed loop control of the two-level converter unit and simple voltage balancing control of the three-level converter unit. Specifically, the middle capacitor voltage balancing control is realized based on the two-level converter unit using a variable-reference modulation scheme, while the upper and lower capacitor voltages are balanced based on the three-level converter unit using zero sequence voltage injection methods. Simulation and experimental results have been conducted on a four-level active neutral point clamped converter as an example to verify the proposed idea and the proposed voltage balancing control methods.