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.

I. Introduction

In recent years, the installed capacity and the share of electricity generated from wind and solar power in the electric power system have significantly increased [1]. The International Energy Agency predicts that the global installed capacity of renewable energy is expected to increase by nearly 2400 GW, an almost 75% rise, from 2022 to 2027. In response to the large volume of intermittent renewable energy sources being integrated into the grid, energy storage systems are increasingly being deployed at renewable energy bases [2], [3].