Harmonic Characteristic Analysis And Modulation Strategy Improvement Of Three-level Converters

With the development of economy,the voltage level of converter application is higher and higher,and the power level is bigger and bigger.The application of multi-level converter comes into being.Among them,the three-level converter is the most widely studied because of its simple topology and control algorithm.In the control of three-level converter,the modulation algorithm has always been a research hotspot.The sinusoidal pulse width modulation(SPWM)based on carrier disposition(PD)and carrier alternate phase opposition disposition(APOD)are the most mature technologies in three-level converters.For the two modulation technologies,the traditional research only focuses on the harmonic characteristics of the converter output voltage and the filter current when passing through the low-order filter.However,with the increasing demand for power quality in society,a large number of high-order filters with inherent resonance points have been widely applied in converter systems.The output voltage harmonics of the converter may form a large resonant current after passing through the filter,which greatly reduces the quality of the output power and the stability of the whole converter system.Therefore,it is necessary to deeply analyze the characteristics of the two modulation modes when the three-level converter is connected with a high-order filter.This thesis firstly analyzes the basic principle of carrier modulation,based on this,mathematical modeling of three-level converter output voltage harmonic spectrum for carrier phase disposition sinusoidal pulse width modulation(PD&SPWM)and carrier alternate phase opposition disposition sinusoidal pulse width modulation(APOD&SPWM)for three-level converters.Secondly,the harmonic characteristics of the output voltage of the converter arm of the three-level converter under two modulation modes are compared in detail through theoretical analysis and strict mathematical derivation.In addition,combining this harmonic characteristics with the filtering characteristics of LCL high-order filters,the advantages and disadvantages of the two modulation modes are comprehensively analyzed.Through comprehensive comparison,it is found that harmonic distribution of APOD&SPWM modulated transmission voltage is concentrated when the three-level converter is connected to a high-order filter such as LCL,which can avoid the resonant points of high-order filters but required larger filter volume;Although the output voltage harmonic of PD&SPWM modulation is small,its harmonic distribution is dispersed.When the switching frequency is low,the dispersed voltage harmonic may fall within the resonant bandwidth of the high-order filter,resulting in poor output power quality and affecting the stability of the system.In order to reduce the total inductance requirement of APOD while retain the advantage of concentrated harmonic distribution of APOD modulation,an improved APOD modulation strategy based on the optimal third harmonic injection is proposed.The influences of different third harmonic injections on the voltage harmonic distribution and the total inductance requirement are analyzed.The calculation method of the optimal third harmonic injection value aiming at the minimum total inductance requirement is presented,which could automatically calculate the injected optimum amplitude of the third harmonic according to the current modulation degree,and realize the online injection of the optimum third harmonic.Finally,the simulation and experimental results show that the proposed strategy can not only effectively avoid the resonant points of high-order filters,but also reduce the amplitude of the dominant harmonic voltage.Therefore,the required total inductance could be effectively reduced.