Management And Control Of Grid-Tied Modular Battery Energy Storage Systems

Energy storage has been recognized as the key technology to facilitate the large-scale penetration of wind and solar renewable energy power generation in power system.Energy storage battery has characteristics of high energy density,high flexibility and efficiency,and low requirements for environmental conditions.Moreover,in recent years,the costs of battery energy storage systems(BESSs)which are mainly composed of lithium-ion batteries have decreased and their performances have been greatly improved.Therefore,BESS has become a research and industrial application hotspot.Due to the limited voltage and capacity of a single battery stack,modular BESSs are often used in parallel or cascaded combinations in actual high-power and large-capacity applications for medium-and high-voltage grid connection.In each sub-module(SM),only a single battery module or rack is used to reduce the monitoring and balancing burden of the battery management system(BMS).This can also eliminate the circulating current and improve the efficiency.Meanwhile,the power conversion system(PCS)can use low-voltage and small-capacity devices to reduce the cost.In addition,the modular design facilitates capacity expansion and fault redundancy.But there is current coupling among SMs of the cascaded modular structure,and the management and control among SMs need to be coordinated.In order to solve the management and control problems of the cascaded grid-tied modular BESS,this thesis firstly classifies and introduces different grid-tied modular BESS structures,including DC side parallel type,AC side parallel type,DC side cascaded type and AC side cascaded type.Then related performances of each structure are compared in detail.Next,the existing management and control statuses of different grid-tied modular BESSs are reviewed and compared.On the basis of fully considering the application characteristics and requirements,three cascaded grid-tied modular BESSs with different structures are selected to carry out in-depth researches,specifically,the battery pack hot-swapping control for single-phase modular DC/DC converter(MDDC)BESS,charge throughput management for single-phase modular multilevel converter(MMC)BESS and coordinated state-of-health(SOH)balancing for three-phase MMC-BESS.Theoretical analysis,simulation and experimental prototype verification methods have been used together to obtain following research achievements:1.The traditional battery swapping station(BSS)for electric car/electric bicycle is a parallel modular BESS structure,which has the problems of low efficiency,high cost and poor stability.Therefore,this thesis proposes to apply the MDDC-BESS structure to the BSS to improve efficiency,reduce costs and enhance stability.However,the energy storage battery pack in BSS has hot-swapping requirements.Therefore,based on the analysis of traditional BSS swapping service model,a sorting selection balancing control method is proposed to realize the continuous operation for BSS when hot-swapping randomly inserted battery packs with highly different states-of-charge(SOCs).Experimental results of a 1kW/1kWh MDDC-BESS prototype with 16 SMs show that the proposed method can achieve the desired function under both charging and discharging conditions.In this way,flexible SM control is achieved.2.AC current ripples flowing into battery packs of MMC-BESS will produce harmful additional charge throughput and shorten the lifetime of energy storage batteries.This thesis analyzes and evaluates the total amount of additional charge throughput caused by current ripples in detail.Then,a variable DC-link voltage regulation method is proposed to reduce the battery total additional charge throughput in the single-phase MMC-BESS,including key technologies of switching signals redistribution and active bypassing,and gating sequences rearrangement.Experimental results of a 6.6kW/6.6kWh single-phase MMC-BESS prototype with 48 SMs show that this method can reduce the total additional charge throughput by 67.65%while the total ideal charge throughput remains unchanged,which can extend the lifetime of energy storage battery.3.In the high-voltage and large-capacity BESS,the SOH balancing within and among battery packs are difficult to be coordinated,which increases the BESS maintenance burden and has bad influence on balancing effects.This thesis conducts systematic and comparative performance analysis and evaluation for packs in MMC-BESS using different cell equalization techniques.Then a double-layer SOH balancing strategy for three-phase MMC-BESS is proposed,including the relative SOH estimation process for battery cells,the cell SOH equalization method,and the coordinated control between cell SOH equalization and pack SOH balancing.This strategy can get each cell's SOH conveniently without pack disassembling,reduce the SOH inconsistency among cells and prevent the poor cells degrade suddenly during long-term use.Finally,all cells can reach SOH lower limit and then can be replaced at the same time,decreasing the BESS maintenance burden.Simulation and experimental results of a 10kW/10kWh three-phase MMC-BESS prototype with 72 SMs show that this strategy is obviously effective in dealing with SOH inconsistencies among all cells and reducing the maintenance burden of BESS greatly.In summary,this thesis focuses on the management and control of the cascaded grid-tied modular BESS,and has carried out comprehensive and in-depth research work which can realize flexible SM control,extend the battery lifetime and facilitate the BESS maintenance.Obtained results can improve the working performance of the current grid-tied modular BESS and reduce its cost.Then it can furtherly promote the practical application of the cascaded grid-tied modular BESS greatly,which is of great significance to the further promotion and development of BESSs and corresponding technologies.