Research On Modulation Strategies For Three-Level Inverters

With the advancement and development of power electronic devices and digital controllers,three-level inverters are widely used in the fields of motor drive,AC and DC power transmission systems and uninterruptible power supply.With the popularization of high-power devices and the great improved performance of microcontroller chips,inverters are the core technology devices,and their applications in various industries are increasingly widespread and developing rapidly.There are three key problems in the three-level inverter,namely the neutral point voltage fluctuation,the switching loss and common mode voltage.In different three-level inverter applications,it is necessary to comprehensively consider these three issues for the system.This paper first analyzes the development history,the main circuit topology,modulation strategy,and hotspot issues of the three-level inverter,which leads to the main problems that need to be solved in the proposed modulation strategy.For the working principle of the three-level inverter,the inverter main circuit topology,operation principle,mode switching and dead zone are analyzed in depth,and the space vector distribution of the three-level inverter is introduced in detail.Secondly,the duty cycle expression of the three-level inverter pulse width modulation strategy based on the volt-second balance principle is given.And four classical modulations(including space vector pulse width modulation,carrier pulse width modulation,virtual space vector pulse width modulation and non-continuous pulse width modulation)are deeply analyzed.The basic working principle,the vector action time or the duty cycle calculation result of the wide modulation strategy are analyzed.Theoretical analysis yields shortcomings in classical modulation strategies,such as neutral voltage fluctuations,switching losses,and common mode voltage.The theoretical analysis was further verified by experimental results.Aiming at the switching loss problem,neutral point voltage fluctuation problem and common mode voltage problem of the classical modulation strategy of three-level inverter,this paper proposes a corresponding new pulse width modulation strategy,which is the pulse width modulation strategy of three switching actions(3SNPBPWM),extended discontinuous pulse width modulation strategy(EDPWM)and discontinuous pulse width modulation strategy(RCMVDPWM)to reduce common mode voltage for different applications.Based on the above three pulse width modulation methods,the feasibility of the proposed pulse width modulation method is theoretically verified by Matlab/Simlink simulation model.Finally,the experimental results show that the proposed 3SNPBPWM strategy can effectively reduce the switching loss of the three-level inverter,and can achieve the neutral voltage balance in the full range of the modulation degree and power factor;the EDPWM method can effectively reduce the switching loss,and the neutral voltage balance can be realized within the full range of the modulation degree and power factor;the RCMVDPWM method can eliminate the DC offset of the neutral voltage and decrease of the switching loss of the system in the full range of the modulation degree and power factor while reducing the system common mode voltage.