| Energy storage systems are becoming indispensable to stabilize the power grid, among of which batteries are treated as suitable candidate and have been implemented broadly. Rechargeable battery energy storage has a long history of the energy storage technology with its high energy density and energy conversion efficiency in electrochemical energy storage. With the battery manufacturing technology progress, the improvement of the performance of the battery improves continuously, and rechargeable batteries for mass storage can have appeared. Storage battery system can have broad prospects for development in the on-grid or independent operation of the distributed power station. The used batteries can be used into the storage system since the quality of batteries implemented in grid is not sensitive as those used in electric vehicles, which also reduces the construction cost of massive battery energy storage systems in grid.For now, neither battery management system nor grid-tied inverter could be used to regulate the SOH of battery stacks during operation. But fortunately, the recently proposed modular multilevel converter battery energy storage system provides such control flexibility. Then the SOH of integrated battery stacks can be considered in the control algorithm during operation to maximize the lifetime of whole system.For energy storage system and three-phase grid inverter interface, modular multilevel converter, with its flexible charge and discharge control, is particularly suitable for battery storage system as a new voltage source inverter. In addition, the redundant structure can not only ensure the large capacity, but also maintain a relatively stable operation.This paper proposes multiple time scale operation principle for MMC battery system in renewable applications to optimize the battery lifetime and operation capacity of whole system. The main contents of the paper are as follows:1) Backgrounds of the research and its purpose.2) Analysis on MMC inverter topologies and basic control strategies, including the working principle of the converter and main control targets.3) Battery parameters, battery model and battery life influence factors. It includes the main parameters of the battery and their estimation, like voltage and internal resistance, battery state of charge, battery state of health, several battery model, battery management system and battery aging process.4) The proposed operational principle defines the control objectives under three different time-scales. The operational principle under the long time scale defines the wanted SOC variation profile according to the measured SOC variation rate in operation. The operational principle under the medium time scale will use the unchanged SOC value to command the real-time controller. The operational principle under the short time scale is to control the circulating current and output current of MMC in real-time. Doing so, the total charging/discharging power will be properly distributed according to the defined operational principle and charge/discharge times among battery stacks can then be optimized. Matlab simulations and experimental results verified the performance of proposed operation strategy at last.5) The conclusion is summary and prospect, which briefly summarizes the research work of this paper, and the prospect of life optimization of modular multilevel battery energy storage system is the next step. |
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