| In order to meet the growing demand for electricity of users and promote thesustainable development of human beings, research of micro grid which mainlyconsists of renewable energy has spread rapidly. As an important component ofmicro grid, energy storage unit can ensure the stability, reliability and security ofmicro grid. In this paper, an energy storage unit that will be applied to PV microgrid is studied to achieve different requirements of charging and discharging for PVmicro grid.According to the energy demand of PV microgrid and the structure features ofAC bus, the VRLA battery has been selected to be energy storage device of theenergy storage unit. The charging and discharging controller has selected thebidirectional Buck/Boost converter and the Full-bridge converter to realize thetwo-way flows of energy.Three different working modes of the energy storage,which consists of stand-alone discharging, grid-tied discharging and grid-tiedcharging, has been proposed. Based on it, the small signal model of the bidirectionalBuck/Boost converter and the Full-bridge converter have been respectivelyestablished. The transfer function of duty cycle to the current of the inductance hasbeen deduced, and provided theoretical basis for the design of the inner current loopand the outer voltage loop.Combined with the working characteristics of PV microgid, the controlstrategy in the different work mode was proposed. On the basis of small establishedsignal model, the inner current loop and the outer voltage loop of energy storageunit in the different control strategies have been designed. In MATLAB/Simulinkenvironment, simulation model of the energy storage unit is built, and thesimulation results have proved the effectiveness of the control strategy.Now,it has already finished to design the software and hardware of the energystorage unit. The experimental platform is built based on STM32F103VBT6.Theexperimental results demonstrate that control strategy of the energy storage unit iscorrect, and show that the energy storage unit can realize three different chargingand discharging functions as well. |
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