Research On Charinand Discharging Controlled Technology Of Seiral Batteires In Photovoltaic Micro-Gird System

With problems of environmental pollution and resource shortage, more and morepeople pay attention to renewable energy resources. Photovoltaic micro-grid system isan independent controlled unit off the grid and able to work in stand-alone model orgrid-connected model. The negative effect on grid greatly reduces due to its ability ofself-regulation and equalization. The photovoltaic micro-grid system consists of solararray, micro-grid converter, storage device, load and grid.Storage device is an indispensable part of photovoltaic micro-grid system. It iscapable of storing or discharging energy to adjust the electrical need betweenphotovoltaic array and grid. Thus, the utilization ration of solar energy is improved.Valve-regulated lead-acid batteries are widely used in solar system because of its lowcost and stable characteristic. Nevertheless, lifetime of batteries in photovoltaic systemare much shorter than other applications due to the random change of solar energy. Onemethod to solve this problem is to choose proper battery charging and dischargingcontrolled technology for prolong batteries’ lifetime.The paper will search optimalbatteries charging and discharging controlled technology for micro-grid system basedon analyzing that technology in stand-alone system. Main work has been done asfollow:First, by analyzing the structure of photovoltaic system and the principle of systemdesign, batteries capacity and type is determined based on load power when grid is infault. Traditional lead batteries equal models in solar system are also been discussed andcompared. Finally, the dynamic voltage model is improved by adopting capacitorenergy computation equation due to the problem about the determination of batteries’equivalent capacity.Secondly, by analyzing the principle of current decreased charging method andshortage, the current decreased based on batteries’ SOC is proposed. This methoddetermines the reduction coefficient through Mass’ law and choose different chargingrate according to batteries’ SOC. Finally, stop charging by detecting the maximumcharging voltage. It has the advantage of preventing batteries overcharging. During thebatteries discharging, the method of maintaining different DC bus voltage is usedaccording to the state of grid.Thirdly, according to the state space average model of bidirectional DC/DCconverter and voltage current dual loop PI controller is adopted for batteries chargingand discharging. The system model with controller is simulated inMATLAB/SIMULINK software, either in the case when the solar array output isenough or the grid is in fault, and the result of simulation is analyzed.Fourthly, parts of hardware circuit and software flowchart have been designed forbatteries charging and discharging controlled system, which could realize somefunctions, such as switching modes between charging and discharging, choosing proper charging and discharging method and batteries’ SOC estimation.The main work in this paper provides those who engaged in integrated grid withimportant reference value.