Research On Control Strategy For Power Electronic Transformer Based On LCL Filter

PET is a power electronic device that combines power conversion technology with high frequency transformer.Power electronic transformer can not only perform as a conventional transformer,but also with high efficiency,low volume,output voltage regulation which ensure the PET a potential alternative for the next generation traction vehicles.Aimed at the cascaded type power electronic transformer,resonance phenomenon caused by the interaction between PET and traction power supply system is discussed to design LCL-type line side filter and analysis the interaction influences.Firstly,based on the mathematical model of input-stage and middle-stage PET,as well as the topology and operation principle of active front end rectifier,the mathematical model of cascaded H-bridge rectifier based on LCL filter is derived.Then,a design method of LCL filter suitable for PET is proposed via graphic analysis.Further study on unregulated types,operation modes and DC gain characteristic of full bridge LLC resonance converter are made to obtain proper resonance network parameters.Secondly,multilevel modulation algorithm is introduced,DC side voltage unbalance mechanism is described,the DC voltage balance control algorithm based on voltage followed is adopted.The parameters of LCL filter is calculated and damping control strategy is designed.Moreover,LLC resonance converter is simulated in Saber Sketh to research on its DC gain characteristic.The simulation shows the feasibility of zero-voltage-switching(ZVS)on the primary side and zero-current-switching(ZVS)off on the secondary side.Thirdly,by discussing the resonance mechanism,the mathematical model of traction power supply system is introduced and the typical parameters of mathematical model is selected.Co-simulated model for traction network and PET is built in Matlab/Simulink platform,the resonance of traction grid is observed,the characteristic and harmonic suppression features of LCL filter is studied,and compared with L-type filter.The result proves that the LCL filter proposed in this paper can effectively attenuate the existing harmonic in the line voltage without harmonic resonance,which is significant for grid-connected PET.