Modulation Strategy For Common Mode Voltage Suppression Of NPC Three-level Inverters

Compared with the traditional two-level inverter control system,the NPC three-level inverter with the advantages of low switching frequency,high quality of output voltage waveform,low stress on power switch device become one of the most widely used multi-level converter in the high-voltage high-power applications such as modern industry,power system,urban rail transit system.However,the NPC three-level inverter easily lead to the DC side of the two capacitor charging and discharging imbalance,resulting in the midpoint voltage imbalance,under the role of small vectors and middle vectors,which will bring the inverter control system many adverse effects.Simultaneously,the inverter generate common-mode voltage at output,which will cause great harm on the motor and the surrounding electrical equipment.Therefore,it is very meaningful for in-depth studying on NPC three-level inverter.Aiming at the problem of midpoint voltage imbalance and common-mode voltage in NPC three-level inverter,this paper proposes an improved virtual space vector modulation strategy which it is based on the traditional strategy of virtual space vector modulation.The new vector is synthesized by re-selecting the basic vector(large vector,middle vector and zero vector).each new synthesis of the virtual vector generate the current zero of the midpoint,and thus achieve the midpoint voltage balance control.In addition,the maximum value of the common-mode voltage generated by the newly synthesized virtual vector is Udc/6,which is half of the common-mode voltage value(Udc/3)generated by the conventional virtual space vector method,so that it achieve an effective suppression of common-mode voltage.In the newly synthesized virtual middle vector,the distribution coefficient k2 is introduced,and the distribution coefficient ?2 of the virtual middle vector is selected as the control variable.When the midpoint voltage fluctuates greatly,the midpoint current is controlled by adjusting the value of the distribution coefficient k2,So that the midpoint current flows in the direction of decreasing the fluctuation of the midpoint voltage,and thus achieve the suppression of the mid-point potential fluctuations.At the same time,the common mode-voltage generated at the inverter output terminal can be prevented from increasing.In this paper,the traditional virtual space vector modulation strategy and the strategy proposed in this paper were introduced,and analyze the principle of the control point voltage.A simulation model is built on Matlab/Simulink to verify the effect of the two methods on the common-mode voltage suppression and the mid-point voltage balance control,and make a comparative analysis,and then propose using the distribution coefficient ?2 to suppress the fluctuation of mid-point voltage.Finally,the traditional virtual space vector modulation strategy and the modulation strategy proposed in this paper are experimentally validated on NPC three-level inverter experimental system.The results show that the proposed modulation strategy can effectively suppress the common-mode voltage and mid-point voltage fluctuations.