Distributed Control Strategy For DC Microgrids

DC microgrid has the advantages of simple structure,less conversion link and high efficiency.Compared with AC microgrid,there is no phase,frequency synchronization and reactive power loss in DC microgrid.Its control is simpler.Therefore,DC microgrid technology has received extensive attention.DC microgrid integrates distributed generation units,energy storage units and user loads.In order to ensure stable power supply,the coordinated operation of each unit is crucial.In this paper,the DC microgrid with multiple photovoltaic power generation units and energy storage units is taken as the research object.Based on the analysis of the operation characteristics of each unit in the system,the researches about the coordinated control strategy of DC microgrid and the operation control method of each unit are conducted in this thesis.In terms of system operation control,DC droop control is adopted to realize the reasonable power distribution of multiple units in the system during parallel operation.On this basis,according to the operation characteristics of each unit,the system operation is divided into five working modes with the maximum utilization of photovoltaic energy and the control of bus voltage stability as the control goal.Through DC bus voltage hierarchical control,DC bus voltage signal is used as the criterion of operation mode switching,and local control are used to realize smooth switching between each operation mode.An adaptive power control strategy is applied to PV converter,which is automatically regulates the output power of PV tracking or off the maximum power point according to the DC bus voltage and realizes the smooth switching between maximum power point tracking(MPPT)mode and off-MPPT mode.For the energy storage system control,the problems of traditional droop control in multi-energy storage system control are analyzed,and the reasons for the difference of state of charge(SOC)of multi-energy storage system are revealed.Furthermore,the defects of the existing state-of-charge equilibrium control method in the application of hierarchical control system of bus voltage are analyzed.On this basis,a hierarchical operation control strategy based on state of charge is proposed.The operation of the energy storage unit is divided into five working modes by the DC bus voltage.Meanwhile,the SOC of the energy storage unit is introduced into the voltage interval control.Each voltage interval is adjusted dynamically with the SOC in order to achieve hierarchical operation.Then,the power distribution control adjusts the droop curve automatically according to the voltage interval of each working mode of the energy storage unit.The power of each energy storage unit is distributed according to the respective SOC by the stage of power distribution control which realizes the balanced control of energy storage SOC,efficient use of energy and bus voltage support.In the end,the feasibility and effectiveness of the proposed control strategy are verified by the RTDS(Real Time Digital Simulator)hardware-in-the-loop experimental platform.The results of experiment prove that the proposed control strategy can achieve the coordinated operation of source and storage,the smooth switching between multiple modes and SOC balance of multiple energy storage without communication.