Modeling And Distributed Secondary Control Strategy Of DC Microgrids

With the exhaustion of non-renewable energy,the development of renewable energy technology has attracted much attention.DC microgrids,which integrate renewable energy sources,energy storage systems and DC loads,provide a new way to support the normal operation of power grids,improve energy utilization efficiency and reduce the dependence on non-renewable energy.The DC electronic converters,with good dynamic characteristics,are often used as the interface between power sources,loads and DC bus.However,when they are connected to the grids as constant power loads,the negative impedance characteristics will lead to instability of the system.On the other hand,in order to fulfill the cooperative control among the units in the DC microgrid system,the appropriate distributed control strategy should be adopted in addition to relying on the coordination ability of the power supply itself.To this end,this thesis takes the DC microgrids with multiple DC converters as the research object,with the aim of realizing bus voltage stability and distributed consistent control.The detailed contents are listed below:First,the mathematical model of DC microgrid system with constant power loads is established.The stability issue is also discussed.Based on the control strategy of droop control and double closed-loop control,the linearized small signal model of DC boost converters is derived.With this model,the distribution of characteristic roots is analyzed.The stability and sensitivity of the system are performed in terms of the changes of capacitance,inductance and constant power load parameters.In addition,the occurance of the phenomenon of Hopf bifurcation when the system changes from stable to unstable state is also investigated.Second,in order to maintain the voltage stability of DC bus and achieve good dynamic performance,a fixed-time prescribed performance controller is designed for the DC converter system.After transforming the boost converter mathematical model into a standard form,the fixed-time extended state observer is presented to estimate the uncertainty and disturbance of the system.Then,based on the estimated total disturbance,a fixed-time prescribed performance controller is developed.And the fixed-time convergence of errors and the desired tracking performance are achieved.Simulation results verify the effectiveness and superiority of the proposed control strategy.Finally,a distributed predefined-time secondary controller is proposed for DC microgrids with multiple converters,which is used for voltage regulation and current distribution of distributed units.Based on the multi-agent theory,each converter unit is regarded as an agent,and the communication among agents is described by algebraic graph theory.In order to restore the bus voltage drop and load proportional distribution caused by droop control,a predefined-time voltage controller and a predefined-time current controller are designed respectively to make the followers follow the state of the leader in a predefined time.Based on the Lyapunov theory,the rigorous proof that the system error converges to zero in a predefined time is given,and the dynamic performance,plug and play performance and link fault recovery ability of the designed controller are further verified by simulation.