Hybrid Energy Storage Control And System Hierarchical Coordinated Control Strategy For DC Microgrids

As the new thing in resent years, the microgrid is used to integrate all kindsof distributed resources, improve the utilization of the renewable energy, andreduce the bad effects of distributed resources when they are connected into thepower system. The DC microgrids is classed as the important developmentdirection of the microgrid. In the DC microgrid, energy conversion is less,system control is easier and system loss is lower compared with AC microgrids.The study of DC microgrids is still in early stages at home and abroad by far. Inthis paper, the system operation and control of the DC microgrid includinghybrid energy storage system are researched. This paper has got the support ofInternational S&T Cooperation Program of China the cooperation research onmirogrid operation and optimal control with distributed generations and so on.First, the mathematical model, output features, and simulation curves of PVare analyzed. According to system operation require, the control method of PVcontains maximum power point track (MPPT) control and constant volatage(CV) control, and the variable step incremental conductance method is used torealize the MPPT control. The circuit construction is designed, parameters are selected, and the simulation model of PV unit is set up in Matlab/Simulink,and the MPPT and CV control method are tested in all kinds of experimentalconditions. The simulation result shows the PV unit can work steadily in the twocontrol strategy.Second, the operating principle, output features, and mathematical modelard studied, and this paper places a great emphasis on the circuit struction andparameters of bi-directional DC/DC converter of hybrid energy storage system(HESS). The improved control strategy of HESS is proposed. The strategycould make the advantages of high power density of super capacitor (SC) andhigh energy density of lithium battery complementary to each other, optimizetheir output features, and improve dynamic flexibility and operating life of theHESS. With the coordinate work of SC and lithium battery in correspondingoperation layers, the target of adjusting the dc link voltage and balancing systempower is realized.Third, this paper studies the sruction, power flow, and control method of theDC microgrid. The hierarchical coordinated control by the variation of the dcvoltage is proposed. The system operation is divided into five different layers bythe proposed control strategy. With the coordination of the MPPT and CVcontrol, the voltage droop control, and the control of dc load, the dc link voltageis adjusted automatically, the different layers is switched smoothly, and the DCmicrogrid could work well in all kinds of operating condition.At last, the DC microgrid experimental platform including of the PV unit, HESS and dc load is constructed. The hardware structure and software design ofthe platform are discussed in detail. The strategy of HESS control and systemhierarchical coordinated control are verified by five experiments. Experimentalresults show that the HESS can adjust dc link voltage and balance system powerwell, and that the dc link voltage is regulated automatically, the different layersis switched smoothly and the DC microgrid can operate safely and steadily bythe system coordinate control.