Research On Common Mode Voltage Suppression And Neutral Point Voltage Balance Control Of Three Level Inverter

With the continuous development of power electronics technology,inverter technology is also changing.Multilevel inverter has become a research hotspot at home and abroad.Among all kinds of inverters,diode clamped inverter is more popular because of its superior performance.Compared with the traditional two-level inverter,the components of three-level inverter bear lower voltage and lower switching loss.In practical application,there is a common mode voltage at the output end of the diode clamped inverter.The existence of the common mode voltage will cause hidden dangers to the safety of the surrounding equipment,load and maintenance personnel.At the same time,in the process of production and manufacturing,it is impossible for the DC side capacitor to ensure that the parameters are the same,and the load cannot always be kept in a balanced state,which makes it difficult for the inverter to operate the neutral point voltage will be unbalanced,which will seriously affect the inverter performance during operation.Therefore,in order to improve the comprehensive performance of the diode clamped inverter,the common mode voltage suppression and neutral point voltage balance control of the diode clamped inverter are studied in this thesis.Firstly,the common mode voltage of diode clamped inverter is analyzed.It is found that in traditional virtual space vector modulation,the amplitude of common mode voltage is too large due to the existence of positive small vector.Based on this discovery,a new virtual space vector pulse width modulation method is proposed,which avoids using positive small vector when synthesizing virtual vector.When the basic voltage vector of quasi vector only contains zero vector,negative small vector,medium vector and large vector,the amplitude of common mode voltage is only half of that traditional virtual space vector modulation.In order to ensure that the average current generated at the midpoint of each virtual small vector is zero in a sampling period zero vector and large vector are used to synthesize virtual small vector the simulation.Results show that the analysis scheme can achieve the desired effect.Secondly,because of the topology of the diode clamped inverter,the two capacitors on the DC side cannot balance the voltage.To solve this problem,this thesis makes a quantitative analysis.When constructing the virtual vector,the corresponding virtual vectors are made according to the voltage of the upper and lower capacitors on the DC side.When the voltage of the upper capacitor is large,the energy at the midpoint is extracted to make the upper side electrified,When the voltage of the lower capacitor is higher,energy is sent to the neutral point to increase the voltage of the upper capacitor,so as to control the balance of the neutral point voltage.Finally,in order to ensure a good output voltage waveform while ensuring the neutral point voltage balance and effectively suppressing the common mode voltage,a voltage model predictive control strategy is proposed,and a control factor is added to obtain the desired results by changing the control factor.The simulation results show that the proposed scheme has good suppression effect on common mode voltage,the neutral point voltage balance effect is ideal,and the output waveform can also achieve the desired effect.