Three-level NPC Inverter Mid-point Voltage Balanced Hybrid Modulation Strategy

Three-level inverter has gradually replaced two-level inverter to be widely used in high-voltage and high-power occasions due to its advantages of low harmonic content of output waveform and low voltage stress of power tube.Among them,the neutral point clamped(NPC)three-level inverter has the problem of mid-point voltage deviation and the common mode voltage generated by the output side of the inverter,which limits its application and development,and also becomes the key research direction of three-level inverter.By improving the space vector modulation strategy,a hybrid modulation strategy is proposed to reduce the common mode voltage while achieving the midpoint voltage balance.Firstly,this paper discusses the topology and working principle of NPC type three-level inverter,constructs its mathematical model,and fundamentally analyzes the causes of mid-point voltage deviation and common mode voltage.Secondly,the realization process of the traditional space vector and virtual space vector modulation is studied,and the 60 coordinate system is introduced to construct the rotation model and the vector standardization method to simplify the algorithm.At the same time,the virtual small vector and medium vector are redefined,PI controller is introduced and control factor is introduced to reduce the common mode voltage and balance the medium point voltage when the virtual vector is synthesized.Finally,based on the different control ability of the two strategies for the mid-point voltage at different modulation ratios,a hybrid improved modulation strategy is proposed.The Matlab/Simulin simulation model was built to compare the capability of the proposed hybrid modulation strategy,the two modulation strategies and the traditional hybrid modulation strategy to balance the midpoint voltage and reduce the common mode voltage at different modulation ratios.On this basis,a three-level experiment platform is built to further verify the effectiveness of the hybrid control strategy proposed in this paper.The experimental and simulation results show that the hybrid control strategy proposed in this paper can better balance the midpoint voltage and effectively reduce the common mode voltage.