Average Value Modeling Method Of Cascaded H-Bridge Type Power Electronic Transformer

With the increasing depletion of traditional energy,the development and utilization of renewable energy such as wind and solar energy has been paid more and more attention.DC distribution grid provides a new way for distributed energy to connect the main grid,which conforms to the framework of the future energy internet.Power electronic transformer(PET)is an indispensable interface circuit for DC distribution grid to connect renewable source and main grid,which can effectively improve energy utilization and power quality.However,the complex connection mode between modules and small simulation step lead to low simulation efficiency.Meanwhile,with the increasing frequency of high-frequency transformer,the influence of dead-time effect is growing.Therefore,it is necessary to investigate the efficient and accurate modeling methods of PET,and take the theory of dead-time effect into account in modeling.In view of the above problems,the main work of the paper is as follows:First of all,considering parasitic parameters,the dual active bridge(DAB)converter is simplified to a first-order RL circuit.According to the waveform characteristics of voltage and current,the DAB is classified into multiple dead-time modes.The analytical expressions of the incoming and outgoing full-bridge currents in each dead-time mode are derived,and then the transmission power model is established,which can reflect the dead-time effect and parasitic loss.The change of soft switching characteristics of DAB converter after considering the dead-time effect is analyzed,and preliminary references are provided for design of the DAB circuit parameters.Secondly,a reduced-order model considering dead-time effect is proposed for cascaded H-bridge type PET with DAB structure(CHB-PET).Specifically,for the DAB unit,the equivalent current source is used to replace the full-bridge and high-frequency transformer to achieve the reduced-order average modeling.For the H-bridge rectifier unit,based on the state-space averaging method,each working condition is integrated into a unified model through duty cycle.And then equivalent sources are used to substitute for the interface circuit between H-bridge unit and AC grid or DAB.After that the average model of input-series-output-parallel(ISOP)CHB-PET is established,and the model is verified with high accuracy and operation efficiency on PSCAD/EMTDC simulation software.Finally,based on linearization and Taylor series method,a small signal model of CHB-PET is proposed,and the analytical expressions of the transfer function and the closed-loop output impedance are derived.The high accuracy of the model in the frequency domain is verified by PSCAD/EMTDC simulation software.Then,from the dimensions of parasitic parameters and electrical parameters,the influence factors of the closed-loop output impedance are analyzed,which lays the foundation for stability analysis and controller parameter setting after CHB-PET connecting to the main grid.