| In recent years,in order to manage problems about environmental pollution and reduction of fossil fuels.Distributed power generation(DGs),renewable energy(RES)and energy storage systems(ESS)have been widely used and developed.In order to coordinate the unbalance between large power grids and distributed power sources,and to fully tap the value and benefits of distributed power sources,the concept of DC microgrid was generated.This paper regareds the DC microgrid system as the research object.To improve the power sharing and bus voltage fluctuation of the DC microgrid,the BUCK,BOOST converter and ZIP loads are selected as the control object.Also the mathematical model is established,theoretical derivation,simulation model construction,and experimental verification are used to ensure the stability of the DC microgrid.Besides,the DC microgrid system simulation model is built in the PLECS/MATLAB.The specific research content includes the following aspects:(1)Firstly,the DC microgrid model is studied,including the small-signal models of BUCK and BOOST converters derived from the source-side converter according to the state-space averaging method,which provides a basis for analyzing the stability of the proposed scheme and analysis of related parameters.Then,the traditional method based on the mix potential theory is used to analyze the large-signal stability of the studied system,and the relationship between the stability criterion obtained and the conservativeness is studied.Then,in the BUCK cascade system,the stability of the large signal is analyzed,the nature and hazards of the conservativeness are analyzed,and the influence of the parasitic parameters on the conservativeness is considered.This paper ultlizes an improved modeling method,which reveals the the ideal response characteristics of the load converter,and greatly reduces the conservative domain.Finally,the idealized model was improved,and a complete ZIP load model was established.(2)Secondly,to solve the problem of unbalanced power sharing caused by different line impedances in the DC microgrid,the trade-off problem of traditional droop control is analyzed,and then a new hierarchical control scheme is proposed.Furthermore,the small-signal model of the DC microgrid is established to derive it and the transfer function is used to select appropriate control parameters.Finally,6 parallel conververs in DC microgrid system simulation models are built.The complex working conditions of ZIP load are verified in the simulation model,including control parameters of the control strategy,the response speed,and dynamic performance analysis.In addition,the system has completed the experimental verification under different control parameters.The DC bus voltage can be restored to the reference value of 60 V,and the current sharing of each microgrid has realized current sharing(1:1:1)and different ratios(2: 3: 4),also the different line impedance problem could be corrected by the droop coefficient method,which ensure accurate power sharing.(3)Thirdly,to solve the problem of negative incremental impedance and positive dynamic transient impedance caused by constant power load(CPL),a hierarchical control scheme based on passive integration on the basis of droop coefficient correction control was used.An output current feedback control loop is introduced to ensure the passivity of the output impedance.In addition,the large-signal control law of the interface converter system is established to comprehensively study the influence of control parameters on the passivity of the converter.Moreover,a simulation model with six parallel converters is implemented in PLECS.Also,the constant power load is considered with dual closed-loop control and passive integral control to verify the effectiveness of the proposed large-signal stabilization under five conditions.Compared with the double closed-loop control,the current spike of the passive integral control method is reduced from 5.5 A to 4.6 A.The bus voltage fluctuation is reduced from 16 V to 2.8 V under the test of the reference voltage change.The current spike in the passive integral control method is reduced from 5 A to 3.6 A in plug and play under the test.The simulation results indicates that the control effect based on the passive integral hierarchical control method is better than the double closed-loop control method.(4)Finally,the distributed hierarchical control method is studied based on dynamic consensus algorithm(DCA)and droop coefficient correction control.To achieve accurate current sharing and maintain bus voltage,doop coefficient correction loop,primary voltage and current control loop,secondary voltage and current control loop control loop are included in hierarchical control.In addition,a 6 parallel BUCK converters was built with maximum power level of the system is 720 W.In addition,the effect of the convergence speed of the DCA and the dynamic characteristics of the system is studied.Also,the system load change,topology change,communication time change,adjust the secondary control parameters and other working conditions are simulated in the simulation model.Compared with the traditional master-slave control method,the system is more interactive,which reduce oscillation and improvethe dynamic response of the system. |
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