Research On Coordinated Control Of Cascaded Power Electronic Transformer

As the most important device in the power system,the traditional power transformer can not meet the requirements of new energy power generation because of its single function and many shortcomings.Power electronic transformer not only has the functions of electrical isolation,voltage level transformation and energy transmission of traditional power transformer,but also has high controllability of voltage and current.In this paper,the control strategy of cascaded power electronic transformer is deeply studied to solve the DC side voltage imbalance of cascaded H–bridge,suppress the DC side voltage secondary ripple and improve the power density of power system.The research content mainly includes three aspects:(1)Modeling and control of input stage of cascaded power electronic transformer.The basic unit of cascaded H–bridge,single–phase pulse width modulation rectifier,is introduced,and its modeling process is given.Then it is extended to three–phase cascaded H–bridge,and the average mathematical model of the input stage of cascaded power electronic transformer is given.Several control methods for the DC side voltage imbalance between the phases of star cascaded H bridge are introduced.In view of the shortcomings of the existing methods,a DC side voltage balance control method based on injecting zero sequence current is proposed,and the simulation verification of star cascaded H bridge startup with or without injecting zero sequence current is given.(2)Operation characteristics,modeling and control of intermediate stage of cascaded power electronic transformer.The basic topology of the intermediate stage dual active bridge is introduced.On this basis,the working modes of the dual active bridge are analyzed,and the law of its power flow direction is obtained.According to this,the transmission power characteristics,current stress characteristics,double power frequency ripple characteristics and soft switching operation range of the dual active bridge are analyzed,and the simulation verification of the dual active bridge under different voltage transmission ratio is given.Based on the generalized state space average method,the dual active bridge is modeled,and the transfer function under the full–order model of the dual active bridge is obtained.In order to reduce the double power frequency ripple in the dual active bridge and minimize the filter capacitance,a control strategy based on proportional integral resonant controller is proposed,and the simulation verification compared with the traditional proportional integral control is given.(3)The coordinated control strategy of cascaded power electronic transformer is proposed.The ripple characteristics of the interphase DC side of the input stage of the star cascaded power electronic transformer are analyzed.Combined with the control strategy of the voltage imbalance of the input stage and the suppression strategy of the secondary ripple of the intermediate stage,a coordinated control strategy is proposed.The input stage adopts positive and negative sequence decoupling control,which is only used to obtain the modulation wave,and the intermediate stage is used to obtain and correct the shift ratio to maintain the interphase DC side voltage at the command value.The design process of the main device parameters of the circuit input stage and intermediate stage is further analyzed,and the simulation and physical verification are carried out.This research provides a reference for the control of cascaded power electronic transformer and is verified on the hardware in the loop simulation platform.The proposed strategy can balance the interphase DC side voltage of the input stage of cascaded power electronic transformer and suppress the double power frequency ripple of the intermediate stage dual active bridge,which has reference value for relevant research work.