Research On Multi-source Coordinated Control Strategy Of Isolated DC Microgrid With Hybrid Energy Storage System

As a way to integrate distributed power generation,DC microgrid are more conducive to the access of distributed power sources and DC loads than AC microgrid.During the operation and control of the DC microgrid,the energy storage system has played an important role in the system’s internal power balance and stable operation.Therefore,how to achieve coordinated control among multiple power sources in the system and maintain the stability of the bus voltage together becomes a key issue in the research of DC microgrid.Aiming at the problems in the current multi-source coordinated control of DC microgrid,this paper systematically studies the control method of DC microgrid with hybrid energy storage system(HESS).Aiming at the problems of state of charge(SOC)balance and dynamic load power allocation during charging and discharging of multiple energy storage units,an improved droop control method based on SOC of energy storage units is proposed.This method reduces the influence of line impedance on load current sharing by setting an equalization factor in the droop coefficient.In addition,an acceleration factor is introduced to improve the balancing speed and accuracy in the SOC and load power balancing processes,so as to avoid that some energy storage units are prematurely withdrawn from operation due to overcharge or overdischarge.At the same time,a secondary control link is introduced in the droop control to compensate the DC bus voltage deviation caused by the droop control and improve the quality of the bus voltage.A photovoltaic-storage simulation model with multiple batteries was built in MATLAB / Simulink,and the utility of this proposed control method was tested and verified by simulation.In order to give full play to the complementary advantages of supercapacitors and batteries,and to extend the overall operating life of HESS,a coordinated control strategy for HESS in DC microgrid based on the power difference of source-load was proposed.This strategy sets a dynamic power-stratification point at the power-distribution layer based on the current remaining capacity of the energy storage systems as the basis for switching between supercapacitor and battery.That is,the supercapacitor and the battery work preferentially when the power difference of source-load is small and large to avoid frequent charge-discharge switching of the battery near the balance point of source-load power.Furthermore,the DC microgrid system is divided into 7 working modes based on the power difference between source and load and the dynamic power-stratification point,which realizes the coordinated control of photovoltaic units,hybrid energy storage and load power requirements in the system.An independent DC microgrid simulation model containing multiple groups of photovoltaic-hybrid energy storage units was built in MATLAB / Simulink.The simulation results show that the proposed coordinated control strategy can run stably under various working modes.Aiming at the problem of cooperative control in HESS,an improved droop control method is adopted at the power-balance layer and a distributed cooperative control method based on a consensus algorithm is used to achieve coordinated control between hybrid energy storage units.By constructing voltage observers and SOC observers,the method enables the local energy storage unit controller to communicate with neighboring node controllers to achieve the average of the actual global state variables such as voltage and SOC.It can avoid the problem that the entire system cannot run due to a single point of failure in centralized control.Simulation proves that the strategy can make the load power proportional to the SOC of each energy storage unit,and effectively improve the quality of the public DC bus voltage.At the same time,it has strong robustness against communication failures,which improves the reliability of system operation.