Abstract: Aiming at the problem of inconsistencies between the voltages of the two capacitors on the DC(Direct Current) side of the midpoint clamping (Neutral-Point-Clamped) three-level inverter and the traditional SVPWM (Space Vector Pulse Width Modulation) can solve the problem of inconsistency of the two capacitor voltages within a certain modulation system, but the midpoint potential cannot be balanced under a large modulation ratio, the control method is optimized on the basis of the traditional SVPWM and VSVPWM ( Virtual Space Vector Pulse Width Modulation). The method is based on the flow direction of current, different positive and negative small vectors using different size of balance factor, is in accordance with the midpoint potential difference value, and introduces voltage adjustment coefficient. On this basis, it is combined with the beat-free control. This closed-loop control strategy adjusts the output waveform by using the midpoint potential difference and the inverter three-phase current output value as the feedback to suppress the midpoint potential. Simulation results show that the proposed method can still maintain the balance of the midpoint potential on the DC side when the three-level inverter modulation system is high, which proves the correctness and effectiveness of the control strategy.

Key words: three-level inverter; , midpoint potential; , beat free control; , adjustment coefficient and balance factor; , virtual space vector pulse width modulation(VSVPWM)