Research On Improved Decentralized Control Strategy Of DC Micro-grid

Recently,the main form of microgrid is AC microgrid.However,with the increasing application of photovoltaic cells(PV),batteries and DC loads in modern power grid,DC microgrid shows a more prominent advantage.The evaluation index of DC microgrid control strategies mainly focuses on one aspect of power quality or control cost,but there are few studies can be done on both sides.Medium and small DC microgrid will develop dramatically in the future,so the control strategies that considered both power quality and cost are in great request.Based on the analysis of the traditional control strategies,this dissertation proposes an improved decentralized control strategy,which can take into account both of the high power quality and low cost.The structure of DC microgrid in this dissertation is similar to the traditional decentralized structure.The main difference is that this structure needs one communication line between battery controller and grid-converter(G-VSC).In order to meet the requirements of high power quality,the DC microgrid control strategies are redesigned.Aiming at decreasing the fluctuation,DC bus voltage is controlled by the energy storage units which include super capacitor(SC)and battery.When DC microgrid works in grid-connected mode,high frequency power is regulated by SC and low frequency power is regulated by both battery and G-VSC.When DC microgrid works in islanded mode,high frequency power is regulated by SC and low frequency power is regulated by battery.The system mainly consists of three subsystems: PV power generation unit,energy storage unit and G-VSC unit.PV power generation unit mainly works in MPPT mode and constant-voltage-droop mode.Energy storage unit is made up of battery and SC.G-VSC has four operating modes.According to the state of charge and power of the battery,it can adjust its reference current to ensure the power balance of microgrid system.A DC micro-grid simulation model is built in the MATLAB and the simulation results show that the DC micro-grid system can work well by using the proposed strategy,no matter in grid-connected mode or islanded mode.