Research On Common-mode Voltage Suppression Methods Of T-type Three-level Inverter

The increasing demand for high-power supplies brings the rapid development to multilevel inverters,multilevel technology has become one of the hot spots in the field of inverter research.Among the multilevel inverters,three-level inverter（TLI）is simple in structure and widely used.In the case of high voltage,the common-mode voltage（CMV）output by the inverter produces electromagnetic interference,which affects the stable operation of the system and even threatens the equipment and personal safety.Therefore,combined with the T-type TLI,this paper studies CMV suppression methods of the TLI.The main contents are as follows:（1）The basic theory of T-type TLI is introduced,the mathematical model of T-type TLI is founded,and the working principle of T-type TLI is analyzed,respectively.（2）The space vector modulation strategy of T-type TLI is researched.To begin with,the division of the space voltage vector diagram（SVVD）and the calculation of the vector action time are simplified based on the 60° coordinate system.Secondly,the basic space vector modulation（SVM）strategy and the the virtual SVM strategy are introduced.Moreover,the effective ways to suppress CMV and eliminate low-frequency oscillation of neutral-point（NP）potential are analyzed,and a varying amplitude virtual SVM strategy for suppressing CMV is studied.By only using the basic vector of small CMV,using the idea of virtual vector,and fully considering the smooth switching of the pulse train,the wave sequence is generated,which solves the problems of the unsmooth switching of pulse trains and NP potential imbalance existing in the current SVM strategy for suppressing CMV.（3）The carrier-based pulse width modulation（CBPWM）strategy for suppression CMV of T-type TLI is researched.Firstly,the relationship between the three-phase modulated wave and the SVVD is compared,and the relationship between the duty cycle of the three-phase output level and the three-phase modulated wave is analyzed.Secondly,the harmonic characteristics of optional pulse trains in different small areas are compared,and a simplified CBPWM strategy that can suppress CMV is studied.On the premise of ensuring that the output voltage harmonic characteristics of the inverter are better,the pulse trains can be switched smoothly,and the NP potential can be balanced and controlled,the method of comparing the output level duty cycle with the upper and lower carriers to determine the output level is adopted,which solves the problem of high implementation complexity of the SVM strategy for suppressing CMV.（4）A model predictive direct power control（MPDPC）method for suppressing inverter output CMV is researched.Firstly,the basic principle of traditional three-level finite set model predictive control is introduced,and the direct power prediction model of the three-level grid-connected inverter is analyzed.Aiming at the problem of high output CMV of the three-level inverter,the control set is reduced by eliminating the basic vector with large CMV,which realizes effective power control and suppresses the output common-mode voltage of the inverter.（5）The effectiveness of methods for suppressing CMV is verified on the experimental platform of the T-type TLI.Figure [64] Table [29] Reference [61]...