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This paper develops a multi-objective co-design optimization framework for the optimal sizing and selection of battery and power electronics in hybrid battery energy storage systems (HBESSs) connected to the grid. The co-design optimization approach is crucial for such a complex system with coupled subcomponents. To this end, a nondominated sorting genetic algorithm (NSGA-II) is used for optimal sizing and selection of technologies in the design of the HBESS, considering design parameters such as cost, efficiency, and lifetime. The interoperable framework is applied considering three first-life battery cells and one second-life battery cell for forming two independent battery packs as a hybrid battery unit and considers two power conversion architectures for interfacing the hybrid battery unit to the grid with different power stages and levels of modularity. Finally, the globally best HBESS system obtained as the output of the framework is made up of LTO first-life and LFP second-life cells and enables a total cost of ownership (TCO) reduction of 29.6% compared to the baseline.

Recently, multilevel converters (MLCs) have gained significant attention for stationary applications, including static compensators, industrial drives, and utility-grid interfaces for renewable energy sources. Compared to two-level voltage-source inverters (VSI) MLCs feature high-quality AC voltage with reduced harmonic content despite the lower switching frequency of the semiconductor devices. On the DC side, MLCs can integrate multiple isolated/non-isolated battery modules instead of a single battery pack. This helps to keep the system in service in case of a malfunction of one or more battery modules, as well as active balancing among the modules, a feature not possible with two-level VSI. In general, MLCs can be classified into two types: (i) two-port MLCs, which provide a single interface to connect with the battery pack, and (ii) multiport MLCs, which provide multiple interfaces to allow connection at the module or cell level. The classical topologies of both MLC types (e.g., neutral point clamped, flying capacitor, cascaded bridge) face limitations due to the high switch count. Consequently, many hybrid and reduced-switch topologies are reported in the literature. This paper presents a critical overview of both classical and recently reported MLC topologies and offers a better insight of MLC operation for grid-connected and standalone applications. In addition, the analysis thoroughly assesses various high-level control and modulation strategies while considering active balancing among the battery modules. Other salient features such as balancing speed during offtake/grid-injection mode and fault-ride-through capability are also incorporated. In conclusion, the key findings are summarized for a better understanding of the present and future integration of battery systems in stationary applications.

Emerging wide bandgap (WBG) semiconductors, such as silicon carbide (SiC), will enable chargers to operate at higher switching frequencies, which grants the ability to deliver high power and enhances efficiency. This paper addresses the modeling of a double-sided cooling (DSC) SiC technology-based off-board charger for battery electric buses (BEBs) and the design of its control and real-time (RT) implementation. A three-phase active front-end (AFE) rectifier topology is considered in the modeling and control system design for the active part of the DC off-board charger. The control system consists of a dual-loop voltage–current controller and is used to ensure AC to DC power conversion for charging and to achieve the targeted grid current total harmonic distortion (THD) and unity power factor (PF). Linear and nonlinear simulation models are developed in MATLAB/Simulink for optimum control design and to validate the voltage and current control performances. Four types of controllers (i.e., proportional–integral (PI), lead–lag, proportional–resonant (PR), and modified proportional–resonant (MPR)) are designed as current controllers, and a comparative analysis is conducted on the simulation model. In addition, the final design of the dual-loop controller is implemented on the RT–FPGA platform of dSpace MicroLabBox. It is then tested with the charger to validate the control performance with experimental data. The simulation and experimental results demonstrate the correct operation of the converter control performance by tracking the reference commands.