Energy Utilization Efficiency Analysis And Optimization Control In Flexible Battery Energy Storage Systems

In conventional BESS,hundreds of batteries are connected in series and parallel to attain a certain level of voltage and capacity,and then connected to the converter.This requires high consistency among batteries.However,due to different conditions during the process of manufacture and utilization,the pack always suffers from batteries,inconsistency,which decreases the available energy of the battery pack.To increase the energy utilization efficiency(EUE),flexible groups are applied in BESS which divide a large battery pack into multiple low-voltage battery packs through multilevel converters.This paper is focus on the flexible battery energy storage system.Based on the analysis of the inconsistency of the battery pack,this paper has studied the EUE of the battery pack,battery grouping optimization as well as control optimization.This paper mainly did these works:(1)Comprehensively considering the distribution of of capacities and internal resisteances,this paper analyzes the relationship between the residual available energy,energy utilization efficiency and discharge current,parameter distribution.The analysis of the energy utilization efficiency not only supports the background and significance of flexible groups,but also provides important basis for the grouping and control optimization of the system.(2)Using flexible grouping in the BESS,on one hand,there is lower requirement for the consistency of batteries,thus the energy utilization efficiency of the battery pack is increased;on the other hand,the number of converters is increased,which leads to the decrease of conveter efficiency.This paper analyzed the relationship between the EUE of the batteries and the number of groups,as well as the relationship between the converter efficiency and the number of groups.Due to the random degradation of batteris,statistics and probability methods are used for the analysis of batteries.The proposed grouping optimization model is used for figuring out the optimal number of groups among a certain number of batteries with a certain capacity distribution.(3)During the operation of the system,optimization control is also necessary.Through optimization control,the SOCs or the terminal voltages of battery modules can be kept consistent,and the battery cells can be fully charged and discharged.There are two levels of control optimization,which are the equalization of battery moduels and battery cells respectively.SOC and terminal voltage equalization are studied for battery modules.For battery cells,this paper proposed an active equalization algorithm for maximizing the available energy of the battery pack.Analysis of examples has been illustrated regarding the calculation method of EUE of the battery pack as well as grouping optimization model.Meanwhile,Matlab simulation model of flexible BESS is built using cascaded H-bridge multilevel converters,where SOC,voltage equalization strategies are verified.Finally,experiments are designed to verify the grouping optimization model,the proposed active equalization algorithm for maximizing the available energy of the battery pack and three-phase SOC equalization strategiesalization strategy for maximizing the available energy.