Research On Three-level Inverter Carrier PWM Method

In recent years, three-level inverter has been widely used in high power AC frequency conversation for speed adjustment, mainly because of its advantages such as its large output capacity, high output voltage and small current harmonic content. This thesis mainly researches the topology structure and control methods of three-level inverter deeply and carries on theoretical and simulation study on carrier PWM methods of NPC three-level inverter in detail.Firstly, the thesis summarizes the research signification and actuality development of multi-level inverter. Secondly, it researches three kinds of basic topology structure of three-level inverter and discusses working principle and the advantages and disadvantages of all kinds of structure. Based on this, the thesis also researches PWM control methods of multi-level inverter and introduces the carrier PWM method of three-level inverter in detail.The thesis proposes three kinds of new carrier PWM methods based on the idea of control degrees freedom combination and the several existing carrier PWM methods. MATLAB/Simulink simulation model is structured. The simulation results show that, the utilization of DC voltage of the phase opposition carrier overlapping switching frequency optimal PWM method proposed in this thesis is bigger compared with traditional carrier PWM methods. However, the disadvantage of this method is big harmonic.Besides, based on the basic topology structure of NPC three-level inverter, this thesis analyzes the reasons of neutral-point potential unbalance and carries on simulation research on carrier PWM methods. The simulation results show that, the neutral-point voltage offset of phase opposition carrier overlapping switching frequency optimal PWM method is rather small, having little difference with other methods.Finally, the thesis analyzes the reasons of common-mode voltage in three-level inverter and the influence of carrier PWM methods on common-mode voltage. The simulation results show that phase opposition carrier overlapping switching frequency optimal PWM method can reduce common-mode voltage. In order to eliminate the influence of common-mode voltage, the thesis also proposes a new PWM method to eliminate common-mode voltage and carries simulation verification about it.