The power conversion system (PCS) is the interface between the battery and the power grid or AC load. It not only determines the power quality and dynamic characteristics of the battery energy storage system's output, but also significantly impacts the battery's safety and lifespan. Based on circuit topology and transformer configuration, the basic types of PCS can be divided into power-frequency step-up type and high-voltage direct-connection type, as shown in the figure.
Currently, the voltage level of conventional battery clusters does not exceed 1500V, and there is a certain fluctuation range depending on the state of charge (SOC). Therefore, to adapt to the voltage requirements of different power grids or loads, a power frequency transformer is often configured on the AC side of the PCS (Power Conversion System). This not only achieves AC voltage boosting or regulation, but also allows for the creation of a three-phase four-wire system in off-grid systems to supply single-phase loads. Furthermore, it improves the protection and electromagnetic compatibility suppression of the energy storage system.
Based on the number of stages, power frequency step-up type PCS can be divided into single-stage and double-stage topologies.
The power-frequency step-up single-stage PCS offers high efficiency and a simple structure; however, it suffers from low battery capacity and limited flexibility in voltage selection. Furthermore, a short-circuit fault on the DC side of the PCS can easily lead to a large current surge in the battery pack, posing a significant risk. Single-stage PCS can also be classified into two-level, three-level, or multi-level systems based on the output voltage level. As the number of levels increases, the DC voltage level and output power quality of the PCS can be further improved, as shown in the figure.
The power-frequency boost-type two-stage PCS, as shown in Figure 2-22, is configured with a bidirectional DC/DC converter at the battery input terminal, which increases the battery pack capacity and enhances the flexibility of voltage selection, and can achieve independent control of multiple battery packs. However, it has high cost, relatively complex control, and low efficiency. Based on the different structures of the DC/DC converter, the two-stage PCS can be divided into non-isolated and isolated types. The isolated two-stage PCS can further improve the voltage transformation ratio and has wider battery voltage adaptability, but the design of a large-capacity isolated high-boost ratio bidirectional DC/DC converter presents significant technical challenges. The main difficulties include high-voltage transformer design, system insulation, phase-shift or series resonant soft switching, and high power density design.
For lithium-ion batteries, which are commonly used in large-capacity energy storage systems, the output voltage does not vary significantly when the state of charge (SOC) is within the range of 15% to 85%. Therefore, most large-capacity energy storage systems currently used in my country employ a single-stage power conversion system (PCS). However, as the DC voltage approaches 1500V, three-level topology structures will be increasingly adopted. A 1500V battery energy storage system reduces the required footprint and the use of electrical equipment such as switch boxes and DC cables, thus lowering system costs to some extent. However, due to the short distance between the battery and the PCS, it does not offer the significant reduction in DC transmission losses seen in large-scale photovoltaic power plants. Furthermore, it places higher performance demands on components such as bidirectional DC circuit breakers and bidirectional DC contactors. The electrical safety and protection design of the DC circuit is a core challenge in the implementation of this system.
To enable the application of ultra-large-scale battery energy storage power stations, and to avoid the parallel connection of too many battery packs, as well as to avoid the losses caused by power frequency transformers and reduce costs, high-voltage direct-connected PCS with a modular cascaded structure has become a major research direction. Similar to power frequency step-up PCS, high-voltage direct-connected PCS can also be divided into single-stage and two-stage topologies according to the number of power conversion stages.
The cascaded single-stage PCS can output high voltage without a power frequency transformer, directly connecting to the high-voltage power grid, making it suitable for building ultra-large-scale energy storage systems. The cascaded structure achieves multi-level output, ensuring low output voltage harmonics even with low switching frequencies in individual modules, thus reducing switching losses. However, the cascaded single-stage PCS requires mutual insulation on the DC side, resulting in high insulation stress for low output voltages, necessitating special design. There are common-mode current paths between each battery pack and the ground, requiring solutions for common-mode current suppression.
The charging and discharging currents of the battery packs contain second-harmonic ripple, which negatively impacts the battery's current path and increases costs. Cascaded single-stage PCSs can be mainly divided into H-bridge cascaded and modular multilevel converter (MMC) cascaded types, as shown in the figure.
Overall, high-voltage direct-connection PCS (Power Conversion System) is a key solution to address the safety and efficiency challenges brought about by the ultra-large capacity of energy storage systems. However, it places high insulation requirements on both the battery pack and the isolated DC/DC converter, which limits its widespread adoption and application. Furthermore, there are challenges in the concentrated stacking, electrical connection, and safety design of ultra-large capacity battery systems.