| In recent years,the continuous increase of distributed energy permeability has brought new challenges to the safe and stable operation of traditional AC grids.Microgrid that integrates distributed energy,energy storage and load is considered as an effective way to solve the problem of high permeability of distributed energy.In the microgrid,there are DC power sources such as photovoltaics and energy storage,and DC power equipment is also increasing,which makes the DC microgrid widely concerned by domestic and foreign scholars.Compared with the AC microgrid,the DC microgrid reduces the intermediate link of energy change and improves the energy utilization efficiency.In addition,it also has the characteristics of simple structure,good controllability and high power quality.Therefore,the research on DC microgrid has theoretical significance and application value.In this thesis,the photovoltaic and energy storage are taken as the basic units to construct the photovoltaic and energy storage system in DC microgrid.The numerical model of each unit,basic operation control,coordinated operation control and improved droop control of DC microgrid are studied.The research contents are as follows:First of all,the basic structure of the photovoltaic and energy storage system in DC microgrid and the working status of each unit are analyzed.On this basis,topological structures of photovoltaic power generation unit,energy storage unit and AC power grid interface converter are expounded.The basic control method of each unit is determined.The parameters of each converter are analyzed and calculated.Second,the photovoltaic power generation unit,hybrid energy storage unit and AC power grid in the microgrid system are modeled.The output characteristics of each unit are analyzed.Combined with the interface converter of each unit,the basic control of each micro-source is completed.The effectiveness of models built and control methods designed is verified.In view of the high and low frequency power fluctuations in the microgrid when the load is abrupt,the droop control method of the virtual capacitor and resistor was adopted by the hybrid energy storage unit.The droop control method makes the super capacitor compensate the high frequency power component,and the battery provide continuous power component.The effectiveness of the proposed control method is verified by simulating.Then,the basic principle of photovoltaic and energy storage system in DC microgrid is given.On the basis of satisfying the constraints of each unit,the DC bus signaling(DBS)is used as the basis for the switching of different modes.And the coordinated control strategy of the whole system is proposed.Based on DBS,three operation modes was established including off-grid energy storage operation control,off-grid photovoltaic operation control and gridconnected operation control.The system can automatically switch between different modes according to the bus voltage.Based on the simulation model of each unit,DC microgrid simulation system is constructed,and the proposed coordinated control strategy is verified.Finally,the problems of traditional droop control in bus voltage deviation and current sharing are analyzed in detail.In order to reduce the bus voltage deviation and balance the state of charge,the fuzzy droop control of the photovoltaic unit and the energy storage unit are proposed.The proposed fuzzy droop control shifts the droop curve according to the bus voltage deviation to restore the bus voltage level,and adjusts the droop coefficient according to the state of charge to avoid deep charge and discharge of the battery.The traditional droop control and the proposed fuzzy droop control are simulated and compared,and the effectiveness of the proposed fuzzy droop control in bus voltage recovery and state of charge balance is verified. |
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