The Analysis And Design Of Large Scale Battery Energy Storage System For Secondary Battery

Battery energy storage systems(BESSs)with secondary batteries could have the utmost potential in the development of smart grids for the future power systems.The BESS constitutes of two key components,the power conversion system(PCS)and the battery management system(BMS).During the selection of PCS structure,the efficiency and reliability of a power converter must be considered.For BESS,the commonly used converter structures are 2-level and3-level voltage source converters(VSCs).Nevertheless,the modular multilevel converter(MMC)is selected as a PCS structure for BESS with secondary batteries in this thesis,due to the reasons that MMC delivers numerous advantages in comparison with other VSCs,like;scalability and modularity,low total harmonic distortion(THD),reliability,and excellent efficiency.The large scale BESS implementation is limited because of battery technologies and their cost.Therefore,the retired batteries from electric cars overcome this drawback and gives the opportunity to implement low cost BESS.However,these batteries suffer from the issue of different state of health(SOH)so,their integration for a large scale BESS is quite challenging.In this regard,the MMC offers a solution by integrating small battery packs in its submodules(SMs)due to its modular structure.All the above mentioned characteristics of MMC and the modularity offered by its structure makes it an ideal candidate for BESS.In this thesis,the MMC structure for BESS is briefly introduced including the SM topologies.Due to the utilization of secondary batteries,the selection of a suitable SM is very important.Several SM structures with different output voltage levels have been reported in literature for MMC.The SM topology analysis has been done for BESS while keeping in mind the inconsistency issue of secondary batteries.While,selecting a SM for BESS,the state of charge(SOC)control complexity is a key concern which increases with the increase in voltage levels.However,the half-bridge SM and full-bridge SM provide the simplest SOC control along with other advantages among all 24 SM topologies.Depending on SMs analysis,the MMC-BESS based on half-bridge SM integrated with secondary batteries is investigated by splitting it into AC and DC equivalent circuits.According to the AC and DC equivalent circuits,the corresponding mathematical models were developed.On the basis of these models,the AC and DC side power controls were acquired along with the SOC control considering batteries inconsistency.Afterward,a comparative study is done between the MMC-HVDC and MMC-BESS to understand the dynamics of different controls according to the structural differences of both applications and their impact on performance of the system.At the end,a main controller is designed for the practical implementation of MMC-BESS with secondary batteries.The schematic and PCB design of main controller along with the voltage sensing circuits,current sensing circuits,and analog signal conditioning circuits are described in detail.The preliminary experimentation tests were carried out on the laboratory prototype which proved the effectiveness of the main controller and its peripheral circuits.