Research on mechanism of single-phase three-level half-bridge grid-tied inverter voltage unbalance of input capacitors and voltage balance control strategy

To deal with the neutral point voltage unbalance problem in the single-phase three-level half-bridge grid-tied inverter, the paper takes a current closed-loop control model as a research plant, and analyzes the half-bridge inverter neutral potential unbalance mechanisms: According to the published papers, Ttwo factors, the random initial angle of inductor current and DC component in the modulation waveform resulting from the steady error of PI controller in an AC system, contribute to the neutral point voltage imbalance. Starting from the points of view on controlling the initial inductor current angle and selecting appropriate current regulators, the paper proposed two approached to keep the neutral point voltage balanced: (1)Feedback DC component of the inductor current; (2)Employing Quasi-PR controller instead of PI controller to eliminate DC bias in the modulation wave. The simulation and experimental results show that two proposed control methods both can achieve neutral-point potential balance without additional hardware cost. © 2014 Chinese Society for Electrical Engineering.