Research On Neutral Point Potential Balance Of NPC Three-level Inverter

With the development of power electronics technology,multilevel inverter is becoming the most important choice to realize high voltage and large capacity power conversion.In the diversified topology of multilevel inverter,the neutral point claped(NPC)three-level inverter has been widely used because of its low voltage requirement,high equivalent switching frequency and good output waveform.However,the imbalance of neutral point potential in NPC three-level inverter leads to the serious distortion of output voltage waveform and the switching device life shortening and other adverse effects.Based on the study of literature,this thesis takes NPC three-level inverter as the research object,and the space vector pulse width modulation(SVPWM)algorithm is used to analyze and simulate it.Firstly,this thesis introduces the common topology of three-level inverter and their advantages and disadvantages,and expounds the working principle and characteristics of NPC three-level inverter.Secondly,this thesis analyzes the principle of SVPWM algorithm,and aims at the defects of the traditional SVPWM algorithm,such as the complexity of regional judgment and the large amount of time calculation of the vector action.The SVPWM algorithm in 60°coordinate system is adopted,which reduces the trigonometric function operation and improves the system operation speed.Then,the causes and adverse effects of the neutral point potential imbalance are analyzed in detail,and a hardware and two software control methods are given.The hardware control method is to restrain the neutral point voltage fluctuation by changing the circuit structure of the three-level inverter.The software control method is that the potential balance of the control point can be achieved by rationally allocating the action time of positive and negative small vectors.The other method need to construct virtual space vector by using small and medium vectors so that small and medium vectors can participate in the control of midpoint voltage so as to obtain better control effect.In this thesis,the simulation model of SVPWM control method and neutral point potential balance control method for three-level inverter is established in MATLAB/Simulink simulation environment.The output voltage waveforms and the midpoint voltage waveforms of the inverter are obtained under different simulation conditions.The hardware experiment platform of three-level inverter control system is built with TMS320F28335 DSP as the control core,and the experiment is carried out.The correctness and validity of the proposed control method are verified by simulation and hardware experiments.