Research On SOC-based Droop Control Strategy In Photovoltaic-storage-DC Microgrids

The distributive re-generated electricity technology is vastly promoted under the background of severe deterioration of traditional resource and the worsen ecological environment problems.Compared with traditional energy,the new energy is widely used because of its large reserves,small environment pollution and high economy.Whereas,the new energy electricity power technology is featured with the intermittent and volatile in electric generation,affecting the steady operation of the grid,therefore,the microgrid system is made up with multiple distributed power source and energy storage system,which usually includes three types: the direct current(DC)microgrid,alternating current(AC)microgrid and the combination of DC and AC microgrid.Among them,the DC microgrid achieves rich attentions because of its preference such as high efficiency,convenient control,no harmonics and so on.The research object is on independent optical storage DC microgrid and is aimed at improving the stability and economy of the system,specifically,being centered on the control strategy of microgrid and the system reconcile control strategy.At first,this paper introduces the system structure and composition of DC microgrid,induces in-detail the mathematical mode of photo-voltaic cell,and analyses its outputting characteristics and control mode;through the equivalent example of the mathematical mode of storage battery,this paper introduces the estimating method of battery's State of Charge(SOC)and controlling methods of charge and discharge;and it briefly tells the topological structure and working principles of voltage transmitter applied by each kind of micro energy source and builds the simulation model and testify it in the Matlab/Simulink environment.Secondly,this paper researches the droop controlling strategy based on the improvement of energy reserving system and ensures the practicability and stability of parallel energy storing units.It analyses the principles and limitations of the droop controlling strategy and puts forward to an modified droop controlling strategy according to its shortcomings.This strategy mainly built the functional relationship between the droop coefficient and the SOC of storage battery to make more discharge and less incharge for higher SOC in storage battery and less discharge and more charge for lower SOC in storage battery.In order to accelerate the equalization speed of SOC,a variable speed-factor control strategy is adopted to avoid the occurrence of excessive charging and discharging of the battery.So as to achieve the stability of the system and improve the superiority of droop controlling system through simulation model.Finally,the mode of optical DC microgrid system is built to research the coherence controlling strategy of the system,so the scheduling function of the system is realized by maintaining the balance between the busbar voltage stability and SOC of storage battery.According to the present condition of the micro source in the microgrid,the grid is divided into four working mode to study on the controlling strategies under the mode.The cooperative controlling strategy reaches the maximum usage of the micro source,entertains the energy balance in the microgrid system,and efficiently improves the economic efficiency of the system operation.The effectiveness of the above cooperative controlling strategy is testified by the simulation mode built in the Matlab/Simulink environment.