Transformerless current source converter-based battery energy storage systems

Abstract

Conventional battery energy storage systems (BESSs) connected to medium-voltage ac grids, whether integrated with renewable energy sources (RESs) or used in battery charging applications, face several challenges, including the need for bulky low-frequency transformers (LFTs) to connect to the medium-voltage ac (MVAC) grid and high current stress. These issues arise because voltage source converter-based approaches typically operate at low battery string voltages due to constraints on the series connection of them. While multilevel topologies with independent batteries, such as cascaded H-bridge and modular multilevel converters, raise voltage by stacking series-connected modules, they introduce substantial complexity because continuous energy and state of charge (SoC) balancing are required through control or added hardware. To ensure reliable and stable operation, systems often use active balancing, coordinated control, and additional balancing circuits, and often include spare modules or module overdesign to enable bypass of faulty units. These measures increase hardware count, cost, and control burden, and can still be insufficient under multiple faults or severe SoC mismatch. Moreover, the proposed LFT-less solutions for charging batteries in battery swapping stations often rely on offline balancing methods during low-demand periods, leading to unnecessary charge-discharge cycles and accelerated degradation. This work presents current source converter-based solutions for MVAC-connected applications that address these limitations without requiring LFTs, balancing controllers, or offline operation. First, a current source inverter-based BESS is proposed for the grid integration of RESs. The LFT requirement is removed, using a string of modular low-voltage battery packs to increase the voltage of the dc-link. By using the variable dc link, the system operates without SoC balancing requirements and tolerates pack bypass under faults or depletion without requiring additional modules. Second, a current source rectifier-based battery charging system is proposed for a battery swapping station, addressing key challenges of the application. Third, a current source converter-based BESS is presented to address bidirectional power flow and energy arbitrage with the grid. The systems provide regulated currents and offer inherent short-circuit protection while interfacing with the MVAC grid. The proposed system's operating principles, control strategies, and design considerations are discussed, and their effectiveness for the next generation of grid-connected battery energy storage applications is demonstrated through theoretical analysis, simulation studies, and experimental validation.