Research On Control Strategy Of The DC-link Of Power Electronic Transformer

With the continuous progress of production technology,we need more and more energy in the future urban construction,but the reserves of traditional fossil energy are not optimistic.Due to the development of new energy generation and distributed power generation,smart grid has become a research hotspot in recent years.Power Electronic Transformer(PET),as a key link and one of the emerging technologies,has also received extensive attention.As a DC link in PET,Dual Active Bridge(DAB)has been widely studied in recent years due to its small size,high transmission power,modularity,simple control and bidirectional transmission power.In this paper,the control strategy of DAB converter applied in PET is analyzed and studied,and the following work is mainly done:Based on the study of the topology structure of the dual-active-bridge converter and the basic principle and switching characteristics of the converter under single phase shift control mode,the power transfer equation of the converter is deduced.It is found that when the voltage conversion ratio k is not equal to 1,there is a problem that the switch tube cannot realize the soft switch completely,and there is also a problem that the backflow power is too large.In view of the above problems,this paper studies the multiple phase shift control mode,analyzes the principle and working mode of extended phase shift control and dual phase shift control,and deduces the power equation.However,they all have the disadvantages of insufficient control degrees of freedom and global optimization.Therefore,on this basis,a unified phase shift control strategy,namely triple phase shift control,is studied,which can realize the mutual transformation of several control modes and achieve global optimization.Then a method of minimum current stress global optimization control with less calculation is adopted on the basis of triple phase shift control,which minimizes the back-flow power of the converter,and is compared with the single phase shift control scheme to prove its effectiveness.At the same time,the soft switching behavior of the converter under the minimum current stress control mode is analyzed,and the results show that all the switches of the converter can operate in the soft switching state.Finally,the method of minimum current stress control is verified by simulation.In order to make the converter meet the requirements of power electronic transformers,the dynamic response speed of the converter should be optimized and the power balance control should be carried out on the converter modules in modular application.Based on the minimum current stress control method,a model predictive control strategy is adopted to reduce the current stress and accelerate the dynamic response speed of the converter.Then the power balance between the modules is studied,and a power self-balancing control strategy is adopted to realize the power distribution of each module.The correctness of the above model predictive control and power balance control strategy is verified by simulation.Finally,a small power DAB converter experiment platform with rated power of 250 W is designed and built,and the control strategy adopted in this paper is verified and analyzed through this platform.The experimental results show that the above control strategy is correct.