Hybrid space vector PWM strategy for three-level NPC inverters with optimal extension mode

Abstract

Neutral-point (NP) voltage oscillation amplitude, THD of output voltage and the switching frequency of the inverter are the three key performance indices in three-level neutral-point-clamped (NPC) inverter. Every element connects with each other and restricts each other, which indicates that to keep the three key performance indices balanced is the research focus of the modulation strategy for three-level NPC inverter. Hybrid space vector PWM strategy, which is proposed in this paper, combines NV and non-NV, synthesizes the advantages of the two methods, reduces NP low-frequency oscillation and also decreases switching frequency and output voltage THD. Taking NTV (the most typical method of NV) and NTV (the most typical method of non-NV) as examples, A new hybrid SVPWM method, based on the NP current maximum minimum (MM) method is proposed in this paper. This new hybrid space vector PWM method applies NTV to uncontrolled balancing area and minimizes the switching frequency and THD of output voltage within the range of the allowable NP oscillation amplitude. Meanwhile, the impact on the output voltage produced by NP oscillation is researched in this paper. The experimental results show the validity of the proposed strategy.