Research On Hierarchical Control Strategy Of DC Microgrid

With the rise of electric vehicles and the increasing number of DC loads in life,the DC micro grid has gradually entered people’s vision.In recent years,the research on the related technologies of DC micro-grid is not mature enough in many countries,and there are many key technical problems that need to be solved further.How to make the DC micro grid run safely and steadily has been the research hotpots of the scientists.Therefore,it is of great significance to coordinate and control the stable operation of each micro element in the DC micro-grid and to manage to its energy.The mathematical modeling of each micro-element photo-voltaic cell,battery,supercapacitor,Boost converter,bidirectional DC/DC converter and converter is carried out and the topology of each element is analyzed,and the topologys that is easy to control are selected.According to the analysis of the mathematical model of each micro element,the control strategy of each micro element is analyzed in detail.Photo-voltaic cells adopt the perturbation method of adaptive variable step,so that Photo-voltaic cells can output the maximum power and fluctuate less at any time.The grid connected converter DC/AC uses decoupling control to prevent coupling between axis and shaft,and is easy to control.Based on the hybrid energy storage system of battery and super capacitor,a new control strategy is proposed in this paper.In the traditional control strategy,when the system is disturbed,the battery can not quickly response,thus affecting the stability of the system,and the improved control strategy,current reference value calculated by the super capacitor power battery without compensation,so as to effectively compensate for the deficiencies of the traditional control strategy,enhance the dynamic response of the hybrid energy storage systemIn this paper,the hierarchical control strategy is proposed because of the instability of traditional peer control and master control strategy.In this paper,control of dc micro grid can be divided into two layers,namely the device control layer and bus control layer.The control strategy does not need communication equipment to realize the information transmission between the various elements,and the anti-interference ability is stronger.The bus control layer is divided into 6 operation modes according to the DC bus voltage fluctuation interval and the remaining capacity SOC of the battery.In each mode,each element corresponds to the corresponding control strategy.In order to make the dc micro-grid to switch smoothly in each operation mode,the droop control strategy based on battery residual power and bus voltage is introduced in the bus-bar layer.The device control layer is controlled by the photovoltaic,mixed energy storage and grid converter unit in micro-grid.The simulation results of the designed DC system show that the DC system can work more safely and reliably under the hierarchical control strategy.