State-of-Charge Balancing of Distributed Energy Storage Units to Support Aggregated Dispatch of Primary Frequency Control

The increasing penetration of renewable generation presents challenges to maintaining the supply-demand balance in power systems. Consequently, energy storage units will be a critical source of flexibility, particularly through the dispatch of primary frequency control (PFC). However, without an appropriate energy regulation strategy, the activation of PFC may be truncated due to intrinsic energy constraints, effectively reducing the available reserve power for frequency support. To address this limitation, this paper proposes a coordinated control strategy based on state-of-charge (SoC) balancing for a fleet of distributed energy storage units. The proposed control law introduces an energy-sharing droop gain and SoC equalization power term as complementary mechanisms to steer the units towards a common target SoC value. Since both mechanisms depend on information aggregated from all storage units, a peer-to-peer communication network is assumed to enable the implementation of distributed consensus-based algorithms. Simulation results demonstrate that the proposed strategy enhances energy utilization across the storage fleet, thereby extending the PFC activation and unlocking the full power capacity for frequency control.