Hybrid varied virtual vector modulation method of parallel three-level inverter considering neutral point potential and phase current balance

The shift and low-frequency oscillations of neutral point potential are key issues in the neutral point clamped (NPC) inverter. The space vector pulse width modulation (SVPWM) algorithm has limited voltage balancing ability at high modulation levels and low power factors. The virtual SVPWM can achieve midpoint potential balance across the entire range, but it has the problem of multiple switching times and poor harmonic characteristics. Parallel three-level NPC inverters can increase the power rating and efficiency of systems, thus being suitable for high-power applications. However, their parallel topology also brings current sharing issues. This paper first proposes several methods of combining varied virtual middle vectors based on five-level vectors, which control the midpoint current by adjusting the action time of the basic vector. Second, different virtual vector combinations are used for voltage commands with different modulation indexes to reduce the switching frequency of the inverter and the synthesis vector error. The proposed hybrid method has fewer switching times, reduces the amplitude of midpoint voltage fluctuation, and has good harmonic characteristics of output current. Finally, the feasibility and effectiveness of the hybrid method are verified through simulation and experiments.