Research On Zero-sequence Circulating Control Strategy Of Converters Based On PWM Modulations

With the shortage of energy resource, new energy based distributed power systemsdeveloped rapidly. As an important technology of distributed power systems, theparallel technology of power converters can raise the power level and the redundancy ofsystem without affecting the power semiconductor devices’ stress. So it grows fast andbecomes more and more popular. However, the parallel of power converters will bringus circulating current which will decrease the efficiency of system and make thesystem’s load current out of balance. The analysis in this paper is mainly focused on theissue of parallel circulating current.Firstly, two kinds of topological structures of paralleled three-phase three-wireconverter are analyzed. Based on this, the paralleled small signal model and paralleledcirculation model are set up. And a ideal working state of the paralleled converters areanalyzed in the view of circulating current.Secondly, from the point of generation principle and control target, the advantagesand disadvantages of the converter’s PWM modulation algorithms which are based on adifferent numbers of carriers is analyzed. What’s more, the unified expression of PWMduty cycles and realization method of PWM is obtained. Then, PWM methods basedcirculating current are analyzed in nature, and some factors associated with the amountof circulating current and the components of circulation are obtained. And theadaptability of SPWM and HIPWM modulation methods for converters’ parallel isanalyzed in the angle of zero-sequence circulation.Besides, the needed quantity of the parallel zero-sequence circulation controller,the phase that should be controlled and the zero-sequence voltage components ofthree-phase three-wire converter based on SVPWM are analyzed in detail. And it alsoanalysis two kinds of parallel zero-sequence circulation control strategies which containzero vector distribution and master-slave zero vector replacement. The characteristics ofthem are obtained. Based on the analysis of PWM methods, this paper proposes azero-sequence circulating current control strategy for parallel converters, which is basedon a dual-carrier SVPWM method. Finally, by using the Matlab software, the three-phase three-wire converter parallelsimulation is used for verifying the deduction and control strategy this paper proposed.The simulation results show that the carrier-based PWM algorithm based zero-sequencecirculation control strategy can suppress the converter parallel zero-sequence circulationeffectively.