Research On Design And Coordinated Control Of Bi-directional DC/DC Converter For Hybrid Energy Storage System In Microgrid

Hybrid energy storage technology(HEST)is able to overcome various shortcomings of single energy storage technology(EST),especially the HEST composed of high energy density of the battery and high power density of the super-capacitor.The HEST helps microgrid absorb solar energy,wind energy and other renewable energy sources(RES)to a greater extent in microgrid,which is conducive to the development of environment-friendly society.In this thesis,the bi-directional DC/DC converter(BDC),which is the key equipment in this field,is studied based on the battery super-capacitor hybrid energy storage system(HESS)in microgrid.BDC is an important link of the energy interaction between energy storage units and the DC bus.The study on the BDC is of great theoretical and practical significance.Firstly,the mainstream topology forms suitable for the HESS are described in this thesis.The important components of HESS,dividing into isolated BDC and non-isolated BDC,full bridge BDC and half bridge BDC,and interleaved parallel BDC,are compared and introduced.The BDC of HESS in microgrid studied in this thesis is mainly aimed at half bridge BDC and interleaved parallel BDC in non-isolated BDC.Secondly,a novel BDC suitable for the battery super-capacitor HESS is proposed according to the structure and operation characteristics of the existing HESS.The control strategy and steady-state performance of the converter are analyzed,including the operation principle,three operation modes(Super-capacitor pre-charging cold stand-by mode,Boost mode and Buck mode),the voltage gain,switching device voltage stress.Meanwhile,the model is built with Matlab/Simulink to verify its effectiveness.The simulation results show that the novel BDC for the battery super-capacitor HESS reduces the voltage stress of switching devices and improves the performance of energy storage units while ensuring the voltage gain of the converter.Then,a Z-source two-phase interleaved parallel improved BDC is proposed for energy storage system(ESS)aiming at the characteristics of low voltage gain and high voltage stress of switching devices of existing BDC.The control strategy and steady-state performance of the converter are analyzed,including the operation principle,the voltage gain,voltage stress of switching devices and self current sharing among energy storage inductors of each phase.Meanwhile,the model is built with Matlab/Simulink to verify its effectiveness.The simulation results show that the Z-source two-phase interleaving improved BDC for ESS realizes the complementary advantages of the two-phase interleaving converter and the Z-source network.The improved BDC has the characteristics of increasing the voltage gain of the converter,reducing the voltage stress of switching devices and improving the performance of the energy storage units.Then,the coordinated control strategy of the proposed Z-source BDC in the microgrid is analyzed using the low-pass filtering method(LPF)to realize the energy distribution in the micro-grid.A simple microgrid simulation model is built in Matlab/Simulink for verification.The results show that the coordinated control strategy based on LPF makes the battery super-capacitor HESS play the characteristics of high energy density of battery and high power density of super-capacitor,and maintains the voltage stability of DC bus.Finally,the photovoltaic grid connected system based on the battery super-capacitor HESS is briefly introduced,and the basic topological structure of the system is briefly described.The energy flow direction of photovoltaic power supply module,DC/AC converter module,battery super-capacitor hybrid energy storage module and the photovoltaic grid connected system based on the battery super-capacitor hybrid energy storage are analyzed in detail.Simultaneously,the mathematical model of three-phase full control bridge converter with constant voltage SVPWM control strategy is established.Meanwhile,the feasibility of each module is verified with the help of the battery super-capacitor hybrid energy storage photovoltaic grid-connected system built with Matlab/Simulink.The simulation results show that the battery super-capacitor HESS helps micro-grid absorb distributed RES,and maintains the voltage stability of DC bus.