Study on neutral-point voltage balancing control in three-level grid-connected photovoltaic inverter

Three-level photovoltaic grid-connected inverters are widely used in the photovoltaic grid-connected systems because of their high efficiency and low harmonic characteristics. However, the major problem of the three-level inverter has always been its core challenge, significantly affecting its system reliability and performance. This study reviews the causes of neutral-point voltage imbalance, discusses three typical three-level inverter topologies, including neutral-point-clamped inverter, flying capacitor inverter, and cascaded H-bridge inverter, and compares their application effectiveness in neutral-point voltage balancing control. Then, it analyzes several common control methods, such as the modulation method based on space vector, model predictive control, and relatively new techniques for controlling virtual neutral-point voltage. The comparative analysis results show that space vector is easy to implement, but the neutral-point voltage fluctuates markedly; model predictive control exhibits strong dynamic response performance but has a high computational complexity; virtual neutral-point voltage does not rely on the real-time feedback of actual neutral-point voltage. In conclusion, this study makes a prospect for the development trends of neutral-point voltage balance control technologies and proposes possible development directions for further improving the performance of grid-connected photovoltaic inverters.