Unified Impedance Modeling And Analysis Of Cascaded DC Transformer Based On Boost+LLC

In recent years,large-scale development and utilization of renewable energy has become a new trend in future energy development.With the help of DC microgrid technology,the realization of AC-DC hybrid distribution network has become an important direction for the development of smart grids in the future.Among them,the DC transformer plays an important role as a key device in the DC microgrid.At the same time,in high-power systems,single-stage DC transformer can no longer meet the needs of applications,and the research of two-stage DC transformer has gained widespread attention from scholars at home and abroad.This paper proposes a unified impedance modeling method for cascaded DC transformer based on input series output parallel(ISOP)connection.Firstly,the topology of Boost+LLC cascaded DC transformer is introduced.Secondly,in order to solve the problem of the DC transformer system modeling,which constructed with the two-stage converter coupling.Combined with the LLC resonant converter function of the voltage clamping,it can approximate equivalent constant gain in the resonance mode.The two-stage coordinated control of Boost?LLC cascaded DC transformer can merge into single-stage unified control.On this basis,combined with the reduced-order small-signal model of LLC resonant converter and the input stage and the output stage average model of DC transformer under fundamental approximation method,the system model of Boost+LLC DC transformer is derived.And then the input and output impedance model is derived based on the system model.The problem of impedance modeling of DC transformer under multi-level power conversion,high frequency isolation and nonlinear coupling is solved.The overall analysis of system impedance characteristics is realized.Finally,a simulation model of Boost+LLC is built in MATLAB/Simulink to verify the correctness of the modeling method.At the same time,the stability of the low-voltage DC network based on the DC transformer is analyzed,that is,the DC transformer cascade Buck converter system stability.The specific research contents are as follows:Firstly,a topology for DC transformer which connected medium-voltage DC network and low-voltage DC network is introduced.The basic principle of DC transformer is introduced in detail.In order to derive the system model of the DC transformer,the small-signal equivalent circuits of the DC transformer input stage Boost converter,the intermediate isolation stage LLC resonant converter,and the low-voltage DC output stage are established by using the state space averaging method and the fundamental wave approximation method,which lay the foundation for the derivation of the system model.Secondly,combined with the LLC small-signal model and the average model of the input and output sides of the DC transformer,the small signal models are simplified.A small-signal average model only including the input and output sides of the DC transformer can be obtained.Combined with the single-stage unified control strategy of the system,a unified impedance model of the modular DC transformer can be obtained furthermore,that is,the input impedance model of the medium-voltage DC side and the output impedance model of the low-voltage DC side.The simulation model of the modular DC transformer is built.Based on the parameters of the simulation system,the theoretical Bode diagram is obtained in MATLAB.The voltage and the current disturbance injection method are used to inject the disturbance voltage and disturbance current into the simulation system.The Bode diagram which simulation measurement can be obtained.The comparison of Bode diagrams can verify the correctness of the unified impedance modeling method proposed in this paper.Finally,the stability of the low-voltage DC network based on the DC transformer is analyzed,and the influence of the system passive parameter on the output impedance of the DC transformer is analyzed by the Bode diagram.At the same time,when the Buck converter cascade the DC transformer,according to the simulation model of the cascaded system,the corresponding simulation curve can be obtained,and combined with the cascaded system impedance Bode diagram and Nyquist curve diagram,the DC grid stability is analyzed.