A Capacity-Expandable Cascaded Multilevel Energy Storage System Based on Laminated Power Modules

In the large-scale development of centralized wind and photovoltaic (PV) power generation, addressing their randomness, volatility, and intermittency is crucial for the electrical grid. Deploying large-capacity energy storage systems is an effective solution. Current large-capacity power conversion systems (PCS) include low-voltage parallel and medium-voltage series expansion approaches. While the low-voltage parallel method is simple, it faces challenges in multi-machine parallel operation for large applications. It also requires voltage scaling with power frequency transformers, resulting in high costs, large land use, and lower efficiency. In contrast, the medium-voltage series expansion approach, mainly using Cascaded H-Bridge Energy Storage Systems (CHB-ESS), has several advantages. In this system, batteries are connected in parallel to the submodule's DC bus. The serial connection of H-bridge submodules forms a medium-voltage interface, allowing direct grid integration, segmented battery control, and transformer-less operation. However, as capacity demand increases, a single CHB-ESS unit is limited by grid voltage and individual battery capacity. To address this, this paper proposes a capacity-expandable ESS topology based on the CHB-ESS structure. The new design uses laminated power modules, each with two independent battery groups. This topology doubles the capacity of conventional CHB-ESS at the same grid voltage level. It also retains key benefits such as transformer-less operation, modularity, and scalability. The paper also proposes strategies for decoupled charging and discharging power control.