| Aiming at the problem of power fluctuation and unbalance in DC microgrid,this paper proposes a coordinated control strategy for the hybrid energy storage system(HESS)to stabilize and balance the DC bus voltages in bipolar-type DC microgrids.In recent years,the microgrid has become a useful supplement to the large power grid.DC load is increasing gradually.Moreover,Compared with the AC microgrid,the DC microgrid has no complex parameters such as frequency,phase and reactive power,and has a more flexible and simple control mode.Therefore,the DC microgrid becomes one of the main forms of network expansion and interconnection in the future.Based on the development of single DC bus,the bipolar-type DC micro grid has wider applicability.However,the research on its voltage regulation is still in the early stage.Because of the intermittent and random nature of the distributed generation,the power quality of the micro grid deteriorates after the integration of a large number of intermittent energy sources,and the voltage fluctuates with uncertainty.Supporting the efficient energy storage system is the important method to solve the distributed power supply and stable microgrid operation.At present,the hybrid energy storage system(HESS)composed of lithium battery and super capacitor for their complementary performances of high energy density and high power density can not only improve the dynamic response speed,but also prolong the service life.However,with the diversification of DC distribution form,the advantages of hybrid energy storage system are not fully utilized,and the application of hybrid energy storage system in the bipolar-type DC microgrid is lack of in-depth study.The main purpose of this paper is to stabilize the DC bus voltage and optimize the hybrid energy storage control.Through the analysis of DC microgrid architecture and hybrid energy storage coordination control,a hybrid energy storage coordinated control strategy for bipolar-type DC microgrid is proposed.And the feasibility of the control strategy is verified on the Matlab/Simulink simulation platform and the experimental platform based on DSP(Digital Signal Processor).The main contents are as follows:Firstly,in order to solve the stability problem of DC microgrid,the working mechanism of energy storage system based on DC bus voltage is studied by deriving the relationship between DC bus voltage and power flow in microgrid.According to the complementary advantages of lithium batteries and super capacitors,ahe structure and energy flow of the hybrid energy storage system are analyzed.Besides,the traditional filter control method and the novel cascade control method of hybrid energy storage are studied.On the basis of traditional filter control,the coordinated control strategy of voltage dividing droop control and virtual voltage increment is designed.In the cascade control mode,the output of super capacitor is taken as the pre control,and the control of the lithium battery can be used as the secondary control.Secondly,according to the characteristics of the bipolar-type DC bus,a coordinated control strategy for the hybrid energy storage system in bipolar-type DC microgrids is designed.The hybrid energy storage system consists of lithium battery and super capacitor,and voltage balancer is also used to coordinate with hybrid energy storage to stabilize different bus voltage in bipolar-type DC micro-grid.In order to smooth the working current of the lithium battery and avoid the high-frequency harmonic interaction between different buses through voltage balancer,the output of the lithium battery and working state of voltage balancer are separately controlled by measuring the relationship between the two sets of super capacitor.Meanwhile,the adaptive droop control method is designed by setting voltage reference values of DC buses.It achieves the correct response of the lithium battery and voltage balancer to the super capacitor.The effectiveness of the proposed control strategy is preliminarily verified by Matlab/Simulink simulation platform.Finally,By building an experimental platform based on DSP,this paper designs the experimental scheme and analyzes the experimental results.The experimental results show that due to the super capacitor as an energy buffer,the current of the lithium battery can be smoothed out and the high-frequency harmonic interaction between different buses through voltage balancer can be avoided.In addition,the proposed control strategy achieves the correct response of the lithium battery and voltage balancer to the super capacitor.Therefore,the proposed control strategy can effectively regulate the bus voltage and guarantee the power balance of the bipolar-type DC microgrids. |
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