Research On PWM Improvement And Power Balance Control Technology Of CHB Inverters

With the continuous improvement of the functional and performance requirements of inverters in the field of high voltage and high power conversion,multi-level inverters and their control technology have developed rapidly.According to the development trend of symmetrical CHB and asymmetrical CHB multi-level inverters,and in view of their practical problems,this paper conducts a comprehensive and in-depth research on the PWM optimization and power balance control technology,which has certain theoretical significance and engineering practical value for high-voltage and high-power conversion technology.The innovative achievements of this paper are summarized as follows:Firstly,aiming at the problems of poor voltage harmonic performance of carrier phase shifting(PS-PWM)output line and the inability of self-equalization of output power of all levels of interconnection units by carrier space overlap(LS-PWM)in multi-carrier modulation technology of symmetrical CHB multi-level inverters,two optimized PWM modulation techniques and multi-level hybrid optimal modulation techniques are proposed by optimizing multi-carrier PWM modulation technology.By improving and reconstructing the carrier wave,the output voltage of CHB multilevel inverters has the advantages of PS-PWM and IPD-PWM modulation,which not only ensures the output voltage of high quality,but also realizes the output power self-balancing control of all levels of interconnection units.Secondly,for the characteristic harmonic elimination(SHEPWM)modulation technology of symmetrical CHB multilevel inverters,there are some problems,such as limited modulation degree and inability to calculate the switching angle online,too strong dependence on the initial value of the switching angle of the harmonic elimination model solved by traditional numerical algorithm,adverse effects of the fluctuation of the DC-side power supply of the inverters on the harmonic elimination effect,and unbalanced output power of each cascade unit.Based on Wlash function transformation multi-band SHEPWM,multi-population genetic algorithm SHEPWM,considering DC-side power supply voltage fluctuation SHEPWM,power balancing SHEPWM and other optimization control technologies,the multi-band SHEPWM non-linear equations of symmetric CHB multi-level inverters are transformed into piecewise linear equations in Walsh function domain to solvethem respectively,and the degree of modulation is extended to a certain extent.It solves the problem that it is impossible to calculate the switching angle on-line;without the initial value of the switching angle,the switching angle can be solved by multi-population intelligent genetic algorithm directly;by introducing the voltage fluctuation factor of DC-side power supply into the SHEPWM harmonic elimination model,it can provide reference for choosing the harmonic elimination angle in the case of irregular DC-side voltage;and by choosing the harmonic elimination side.The expression of fundamental wave amplitude equation in the system is split equivalently to achieve optimal control such as output power equalization of interconnected units at all levels.Secondly,according to the characteristics of the topology of type II and III asymmetric CHB multi-level inverters,the improved hybrid frequency modulation strategies are proposed to solve the current and power back-filling problems of type II asymmetric CHB inverters,as well as the problem of large low-frequency harmonic content of type III asymmetric CHB inverters.The optimal control of hybrid frequency modulation technology is realized.Finally,based on the analysis of the mathematical model of type II ACHB multi-level inverters and the output power balance conditions of each cascade unit,a new power balance control strategy is proposed,which is aimed at the unbalanced output power of the power unit in the topology of type II asymmetric CHB multi-level inverters.According to the power balance constraint equation,the conduction angle of the output voltage of the high voltage unit under different modulation degrees is calculated.Real-time dynamic adjustment makes the fundamental amplitude of the output voltage of the high-voltage unit and the low-voltage unit keep a ratio of 2:1 all the time in the full modulation range,thus realizing the power balance control under the full modulation.On the basis of guaranteeing the output power balance of each unit,the strategy also keeps the fundamental amplitude of the output voltage linearly changing,thus avoiding the non-linear increase of the output voltage of the type II asymmetric CHB inverter without the power balance control and the work existing in the low modulation degree.The extremely unbalanced rate distribution greatly improves the performance and stability of the system.