Research On Modeling And Stability Analysis Method Of Cascaded Power Electronic Transformer

Under the position of traditional energy shortage and progressively serious ecological difficulty,scattered new energy has many advantages such as extendable,hygienic and low-carbon,strong randomness and low mislaying,which can ease the deficiency of resources and the force of power grating.As the core gadget of connecting extendable energy such as wind energy to the power grating,power electronic transformer(PET)is a new type of intelligent transformer composed of power electronic gadget and converters.It can make up for the shortcomings of conventional power transformers,effectively reduce the volume and weight of transformers,and realize basic functions such as voltage level conversion and electrical isolation.Moreover,power electronic transformers also have the characteristics of high power density,excellent performance and rich power supply ports.Therefore,they can be widely used in the fields of energy Internet,locomotive traction,power router and so on in the future.Starting from the suppression of bus voltage fluctuation and stability of power electronic transformer when power changes,the power electronic transformer composed of cascaded H-bridge(CHB)and dual active bridge(DAB)converters is studied.The research contents are as follows:(1)Firstly,this paper summarizes the evolution of PET,and focuses on the analysis of several typical power electronic transformer topologies in each period.The modulation mode,mathematical modeling and related control strategies of input rectifier stage and isolation stage DC-DC converter are analyzed.Aiming at the unevenness of DC capacitor voltage and transfer power in PET,the vital causes of PET voltage and power unevenness are analyzed,and the corresponding control strategies are put forward to further enhance the undertaking security of power electronic transformer.(2)Secondly,the bus voltage of PET system cannot be balanced quickly in a short time and the dynamic regulation ability is weak when load switching occurs.In this paper,a load current feedforward control method based on the overall structure of PET is proposed.The strategy is composed of load current feedforward of DAB level and CHB level.By feeding the load current directly to CHB and DAB,the tracking error of CHB and DAB DC bus voltage is greatly reduced,and the dynamic following and anti-interference performance of the two-stage converter are improved.The voltage dynamic response performance of PET under different working conditions such as traditional control and feedforward control is compared and analyzed through simulation and experimental results.(3)Finally,the interaction among the three subsystems of rectifier stage,intermediate isolation stage and output stage in power electronic transformer may cause the problem of system instability.A widespread three-stage manner for analyzing the overall firmness of PET is proposed in this paper.In this method,the PET system is divided into two modules: cascade H-bridge PET and inverter,which correspond to their output and input impedance respectively.Using the impedance ratio stability criterion,the influence of controller parameters on the stability of PET connected to power grid system is analyzed.The simulation results show that the change of inverter proportional coefficient will affect the input impedance frequency characteristic curve and the stability of the two-stage converter system,which provides a theoretical basis for setting a reasonable stability margin.