Fast and Accurate Neutral-Point Voltage Control under Balanced and Unbalanced Neutral-Point Voltage for Coupled Ten-Switch Three-Phase Three-Level Inverter

Virtual space vector modulation (VSVM) is capable of suppressing neutral-point voltage (NPV) ripples in a coupled ten-switch three-phase three-level inverter. However, it presents a low NPV control (NPVC) speed. In this article, an enhanced VSVM (EVSVM) with the improved NPVC speed for balanced and unbalanced NPVs is proposed. By analyzing the features of the defined virtual vectors under the unbalanced NPV, the region division and duty ratio calculation are properly modified. Two coefficients are configured in the EVSVM. One distributes the duty ratios of two virtual small vectors to improve the NPVC speed, whereas another adjusts two pairs of redundant small vectors considering the NP current polarity to achieve accurate active NPVC. The highlight of this method is the improved NPVC speed and zero NP current in each switching cycle in a steady state. Theoretical analysis shows that the EVSVM increases the available NP current by 4.5% at least under a higher modulation index. Besides, the EVSVM could regulate the NPV without obvious ac ripples and dc offset under a full range of modulation index and power factor for both balanced and unbalanced NPVs. The feasibility and performance of the proposed modulation and control strategies are verified by a 3-kVA experimental prototype.