Improved-Variable Virtual Space Vector Modulation Scheme for Three-Level Inverter in Unbalanced Neutral-Point Voltage Conditions

Abstract

This study proposes an improved-variable virtual space vector modulation scheme to enhance the neutral-point voltage (NPV) balancing capability at a high modulation index for a three-level inverter with unbalanced neutral-point voltage conditions. First, to simplify the calculation, the 60° g-h coordinate system is used and the spatial coordinates of each vector are given when the NPV is unbalanced; the imbalance factor $\zeta$ is also introduced to make the voltage synthesis more accurate and ensure a good quality of output current. Subsequently, by analyzing the relationship between the space vectors, a virtual vector of equal size and direction is found for the basic center vector, which can form a pair of redundant states with the basic center vector. The NPV can be adjusted by introducing a medium-vector balance factor to regulate the action time of the pair of redundant states. The proposed method simultaneously considers the small vector balance factor, variable virtual vector adjustment factor, and newly proposed medium vector balance factor to accelerate the NPV adjustment speed, eliminate periodic fluctuations caused by over-adjustment, and ensure a stable output current under NPV imbalance conditions. Simulation and experimental results validate the feasibility of this approach.