The DC Voltage Balance Strategy Of The Cascaded Rectifier Based On Power Electronics Traction Transformer

The volume and weight of the traditional vehicle traction transformer are one of the reasons that restrict the weight reduction of high-speed rail way.Compared with traditional transformers,vehicle-mounted power electronic transformers have the advantages of small volume and weight which are the hot points of existing research.The complexity of power electronic transformers based on the existing industrial power devices brings about the following challenges to the front-end cascaded rectifier topologies and their control strategy and modulation strategy,which are as follows.(1)control problem;(2)capacitor voltage balance problem;(3)DC-link voltage balance problem.This thesis chooses the topology of single phase three-level neutral-point clamp cascaded rectifiers(3LNPC-CR)as the study object.Through the theoretical derivation,simulation and experiment,it mainly focus on the control strategy,inner-module voltage balance modulation strategy and inter-module voltage balance modulation strategy based on 3LNPC-CR.The main contents include:With respect to control strategy,the necessity of single phase three-level neutral-point clamp cascaded rectifier(cascaded rectifier for short)is analyzed in this paper,which is aimed at the power device selection of the vehicle-mounted PETT.Based on the thoroughly analysis of cascaded rectifier,the mathematical model of cascaded rectifier is established.According to the research of transient direct current control strategy,the control strategy of cascaded rectifier is designed.The correctness of inner-module voltage balance modulation strategy is verified by simulation and experiment.With respect to the capacitor voltage balance problem,three methods are designed to solve the problem of unbalanced capacitor voltage in the cascaded rectifier module including capacitor auxiliary circuit,inductor auxiliary circuit and software method.The correctness of the control strategy is verified by simulation and experiment.With respect to the DC-link voltage balance problem,the SVPWM with opposite vector is proposed and the voltage balance area of algorithm of it is found by finding out the area of charging zone in this algorithm.The correctness of SVPWM modulation balance strategy with reverse vector is verified by simulation and experiment.In order to avoid the problem of voltage step jump in SVPWM modulation,this thesis simplifies mathematical model of cascaded rectifier and proposes a smooth switching technique for voltage balance strategy.Based on the calculation of switching technique for voltage balance strategy,a method of calculating voltage balance capability of smooth modulation voltage balance strategy is designed.The correctness of smooth switching technique is verified by simulation and experiment.In order to simplify the modulation strategy of smooth switching technique,this thesis proposes a SPM modulation voltage balance strategy.First,based on the H-bridge cascaded rectifier,a SPM modulation voltage balance strategy is proposed.Based on this,a SPM modulation of 3LNPC-CR and the voltage balance ability of the two modulations are calculated.Finally,the correctness of SPM modulation voltage balance strategy is verified by simulation and experiment.In conclusion,it can not only ensure the normal operation of the 3LNPC-CR,but also could keep the switching frequency constant and change the switch state smoothly while balancing the DC-link voltage.The research in this paper can improve the reliability and safety of vehicle-mounted PETT and provide pre-verification for the engineering and commercialization of vehicle-mounted PETT.