Modeling And Control Of Buck/Boost And Full-bridge Bidirectional DC/DC Cascade System

With the rapid development of renewable energy,the bidirectional DC/DC converter has been widely used in energy storage systems.Dual active bridge(DAB)bidirectional DC/DC converter is considered to be the most promising bidirectional DC/DC converter.However,because of the backflow power problem,it cannot be used in wide voltage regulation applications,and cannot achieve large-scale application in energy storage systems.Therefore,it is important to do research on DAB converter which fits for wide input range.In this thesis,the bidirectional DC/DC converter for energy storage system is studied.Then the topology and control strategy of DAB converter with wide input range are analyzed.And research scheme of Buck/Boost cascade DAB converter is determined,which combines the advantages of Buck/Boost and DAB,realizes electrical isolation and expands the input voltage range of DAB converter.Firstly,the states during a switching cycle of Buck/Boost and DAB are analyzed.And the mathematical models of Buck/Boost and DAB are established.On this basis,the mathematical model of cascade system is proposed.Secondly,according to the equivalent circuit models,the open-loop input impedance and output impedance of each DC/DC converter in bidirectional operation are obtained.Based on the impedance stability criterion of cascade system,the influence of closed-loop parameters on input and output impedances with single-loop voltage control is analyzed.It is concluded that the higher loop gain is,the smaller closed-loop output impedance and the larger input impedance are.A cooperative control strategy of cascade system is proposed.Through collaborative control of Buck/Boost double closed-loop system and DAB single-loop system,the voltage matching between the transformer terminals is realized under the static operation state of DAB,thereby reducing power backflow and improving the efficiency under wide input range.In this thesis,a hardware-in-loop simulation platform is built.The circuit model and the controller model of cascade system are designed.The cascade circuit model is simulated in MATLAB,and the controller algorithm is implemented on DSP.The effectiveness of the proposed cooperative control strategy is verified by hardware-in-loop simulation.