Research On The Coordinated Suppression Strategy Of Neutral Point Potential And Common Mode Voltage For Three-level Inverter

Compared with the two-level inverter,the three-level inverter has the advantages of small output voltage harmonics,low switch tube withstand voltage and high operating efficiency,and is widely used in high-voltage and high-power fields.However,the commonly used NPC type three-level inverter has problems such as midpoint potential fluctuations and common-mode voltage interference,which affect the safe and reliable operation of electric drives and grid systems.Therefore,it is necessary to conduct in-depth research on the neutral point potential balance and common-mode voltage suppression of the three-level inverter.Firstly,the topological structure and control strategy of the three-level inverter and its latest research status are compared and analyzed by investigating domestic and foreign literature.Taking the T-type three-level inverter as an example,the working principle,the causes of midpoint potential fluctuation and common-mode voltage interference are explained.Taking Space Vector Pulse Width Modulation(SVPWM)as an example,the method of synthesizing space vectors is analyzed and verified by MATLAB/Simulink simulation.Secondly,the basic principles of correction factor algorithm and virtual space vector modulation strategy(Virtual Space Vector Pulse Width Modulation,VSVPWM)are introduced.Aiming at the problem of large fluctuations in the midpoint potential of the three-level inverter,the correction factor algorithm is combined with SVPWM and VSVPWM respectively,and the control effects of the two methods on the midpoint potential fluctuation are compared through simulation.Then,in order to suppress the midpoint potential fluctuation and the common-mode voltage at the same time,a cooperative suppression virtual space vector algorithm is proposed,that is,the mid vector is used to synthesize a virtual small vector to reduce the common-mode voltage,and a correction factor is added to the action time of the mid vector to strengthen the control of the midpoint potential fluctuations.By comparing the simulation results,it is proved that the cooperative suppression virtual space vector modulation strategy can synergistically suppress the midpoint potential and the common-mode voltage in the case of full modulation ratio,and can quickly control the initial offset of the midpoint potential,with good control.effect.Finally,an experimental system composed of the main circuit,drive circuit,control circuit and voltage and current sampling circuit was designed and built,and the PWM program was written with DSP as the control chip.Through prototype experiments,the space vector algorithm with correction factor,the virtual space vector algorithm with correction factor and the cooperative suppression virtual space vector algorithm are experimentally verified.The comparative experimental results show that the cooperative suppression virtual space vector modulation strategy proposed in this paper can effectively suppress the common-mode voltage and the fluctuation of the midpoint potential at the same time,which has a good control effect.