| In the modern power grid,with the large-scale access of distributed energy,energy storage system and electric vehicles,the DC distribution network will play an increasingly important role.As the core control equipment of DC distribution network,DC transformer based on DC/DC converter has become a research hotspot.Dual Active Bridge(DAB)converter has become one of the most suitable dc converters for dc transformer construction due to its advantages such as high power density,fast regulation speed and bidirectional energy flow.In order to comprehensively improve the performance of DAB converter,it is necessary to maximize its operating efficiency under the premise of achieving accurate control of its output voltage.This paper has carried out in-depth research on the performance optimization of DAB converter.In this paper,the working principle of DAB converter is analyzed firstly.The switching state of the converter is analyzed in detail.and the expressions of transmitted power,current stress and back-flow power of the converter under shift control are derived.The soft switching characteristics of double phase shift converter are studied and analyzed.Then,in order to realize the precise control of output voltage of DAB converter,this paper proposes a large signal mathematical model based on direct power control.The closedloop control system based on this model can control the output voltage quickly and precisely by directly controlling the standard transmitted power.According to the standard transmitted power determined in the process of voltage regulation,it can provide the choice of multiple group shift comparison combination,which lays a foundation for the design of multiobjective optimization strategy.Thirdly,in order to optimize the converter efficiency,this paper studies the efficiency optimization control strategy based on the current stress minimization and the efficiency optimization control strategy based on the reflux power minimization respectively.These two optimal control strategies consider all operating modes of the converter and obtain the global optimal solution of the optimization problem.Among them,two soft switching modes are considered in the study of current stress minimization control: one method makes all switching tubes work in ZVS state,and the other method makes all switching tubes work naturally in ZVS or ZCS state.Combined with the loss analysis of DAB converter,this paper proves the effectiveness of the two optimization control strategies.However,after comparative analysis,it is found that it is not necessary to ensure that all switching tubes work in ZVS,and the current stress minimization control based on natural soft switching can achieve the highest efficiency of the converter.Finally,the experimental platform of DAB converter is built,and the current stress minimization control strategy and reflux power minimization control strategy are respectively verified and compared.The experimental results show that the efficiency of the reflux power minimization control is not as good as that of the current stress minimization control.For the current stress minimization control,although the full ZVS mode ensures the ZVS of all switching tubes,it cannot achieve the highest efficiency.The current stress minimization control based on natural soft switch realizes the minimum value of current stress and thus achieves the highest efficiency.This paper compares several efficiency optimization methods.It is proved that the current stress minimization control based on natural soft switch can achieve the highest efficiency.The synthesis optimization method proposed in this paper achieves excellent performance and efficiency of the converter.The research results of this paper will provide a theoretical basis for the design of DAB converter with high performance,high efficiency and high power density.It is of great significance for the engineering application of dc transformer in dc distribution network. |
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