Multi-level coordinated primary frequency control strategy for battery energy storage systems integrating state of charge equalization

AbstractPrimary frequency control (PFC) is a critical functionality for grid-connected battery energy storage systems (BESS), especially for those integrating into the low-inertia power grid. However, the frequency support capability and performance of BESS is largely impacted by the equalization of cells in the pack, which can also lead to lifespan degradation or safety risks of battery. The current equalization strategies are usually reflected as a localized cell-level or pack-level task, where the system-level grid-interacting power commands of the BESS are neglected, creating possible conflicts between internal and external control objectives. Aiming at this issue, this paper proposes a multi-level PFC strategy integrating a dual-stage state of charge (SOC) equalization of cells for BESS, which efficiently coordinates the regulation capabilities across the cell, pack, and system levels. In the multi-level framework, the cell level control predicts the SOC of cells by long short-term memory (LSTM) algorithm, providing crucial input for SOC equalization. The pack level control achieves the first stage SOC equalization via power allocation among the packs, where the imbalance factor of each pack is applied to adjust the pack current in response to the PFC command of BESS. The system level performs adaptive frequency response and the second stage SOC equalization by flexibly regulating the total output power of BESS. As validated by the case study results, the proposed strategy effectively enhances comprehensive system performance. The root mean square (RMS) of frequency deviation is reduced by 38.2% compared to the scenario without BESS in the tested case. In terms of battery equalization, the inter-pack SOC standard deviation is reduced by 48.2% compared to the strategy without equalization in the tested case. Accordingly, the battery cycle life is extended by 21.6%, and the levelized cost of storage (LCOS) is reduced by 0.49% compared to the conventional adaptive PFC strategy, thereby effectively improving operational efficiency. This work provides theoretical and technical support for achieving comprehensive coordination between the internal battery state management and external grid-supporting service provision of BESS.