Research On Control Strategy For Input Stage In Cascaded Power Electronic Transformer

Power Electronic Transformer(PET)is known as Electronic Power Transformer,Flexible Transformer or Solid State Transformer,which belongs to a new type of transformer,it belongs to the combination of power electronic conversion technology and electromagnetic induction theory.It can realize many functions such as power flow control and power quality control based on the realization of conventional transformer voltage level conversion,electrical isolation and energy transmission.An important part of the three-stage cascaded power electronic transformer is the input stage,it realizes the function of converting the high AC voltage to low DC voltage.This thesis is aimed at this part,mainly contains three aspects:1)In the background of single phase cascaded power electronic transformer,the topology,working principle and working characteristics of single phase cascaded PET and its practical application are introduced.The input stage hybrid pulse width modulation strategy and voltage balancing of DC side are emphasized.Firstly,the mathematical model of the single phase cascaded power electronic transformer topology is established,and the input stage and the intermediate isolation stage are simplified.The power transmission strategy of the intermediate isolation stage is analyzed in detail.Then the network-side current following strategy of input stage current and double closed loop control of voltage is proposed by analyzing the small signal model in input stage.This control strategy is simpler and less computation compared with the traditional d-q method based on decoupling control strategy.The ideal effect can be achieved by reasonable selection of parameters.2)Secondly,the primary rectification stage of transformer is regarded as a typical object,an improved step wave level control strategy is proposed for the cascaded PET input stage multi-level synthesis,and then input rectification of the cascaded PET is studied.The hybrid pulse width modulation strategy is used to mathematically deduct the harmonic characteristics of the AC side between the carrier phase-shift pulse width modulation and the hybrid pulse width modulation,the difference between the two methods is obtained.Furthermore,a specific algorithm for the pulse width modulation strategy is proposed,which is verified by MATLAB/SIMULINK for model establishment and simulation analysis.3)Finally,the reason why the third sorting algorithm of hybrid pulse width modulation causes the DC side voltage imbalance of each submodule is analyzed.Three modulation strategies with equalizing function are proposed,and the DC side of each third submodule is analyzed.The voltage balancing strategy is to add an uncontrolled rectifier bridge to the secondary side of each transformer.This method has the ability to automatically balance intermediate DC voltage,no additional control strategy is added,simple operation,very practical,and then verify the feasibility of the above control scheme through simulation,then the simulation results show the correctness of the theory and simulation analysis of balancing strategy.