A Space Virtual-Vector Modulation With Voltage Balance Control for Nested Neutral-Point Clamped Converter Under Low Output Frequency Conditions

The recently-developed nested neutral-point clamped (NNPC) converter is a four-level converter for medium-voltage high-power applications. It has some merits such as operating over wide voltage ranges (2.4-7.2 kV) without switching devices in series and less diodes or capacitors when compared with classical four-level topologies. But it has severe voltage balancing problems under low output frequency conditions when conventional space vector modulation (SVM) or sinusoidal pulse width modulation (SPWM) is applied, which limits its wide applications. In order to solve this issue, a space virtual-vector modulation (SVVM) along with a voltage balance control (VBC) algorithm is proposed in this paper. It helps reduce the voltage ripples greatly and maintain them within an acceptable region under low output frequency conditions. Therefore, it guarantees even voltage stresses among switching devices and extends the NNPC converter's applications. Principles of the proposed SVVM and VBC algorithms are elaborated in detail, and the voltage balancing performance is investigated and compared with the cases where conventional SVM and SPWM with the same VBC are applied. The relationship curves between voltage ripples and flying capacitance, output power, and output frequency have been explored and compared for both conventional SVM and proposed SVVM. Simulation and experimental results are presented to verify the effectiveness of the proposed method.