An Improved Droop Control Strategy for Primary Frequency Regulation in Islanded Microgrids Using Distributed Energy Storage

The large-scale integration of renewable energy generation into the grid has led to increased frequency fluctuations in the power grid. Distributed energy storage systems, with their rapid response times, serve as a critical resource for primary frequency regulation. Traditional constant-droop control cannot adjust the droop coefficient based on the state of charge (SOC) of the energy storage module, nor can it coordinate the SOC balance among multiple units. To address these limitations, this paper proposes a primary frequency regulation strategy integrating elliptic variable-droop control with SOC balancing. First, an elliptic variable-K droop control law is formulated, which dynamically adjusts the power control coefficient based on the energy storage SOC range to prevent overcharging and over-discharging while extending service life. Building on this, a balancing control factor is introduced to achieve coordinated power allocation among multiple energy storage modules, ensuring SOC consistency across all units during primary frequency regulation. MATLAB/Simulink (MathWorks, Natick, MA, USA) simulations and RT-LAB hardware-in-the-loop (HIL) experiments validate the proposed strategy. The results demonstrate that the proposed method effectively suppresses secondary frequency drops, mitigates frequency fluctuations under both step and continuous disturbances, and maintains synchronized SOC convergence among multiple energy storage modules. Compared with the case without energy storage, the maximum frequency deviation is reduced by 38.89%, while the maximum SOC deviation among energy storage units is decreased by 5.34%. Moreover, the RT-LAB HIL experimental results show good agreement with the MATLAB/Simulink simulation results, demonstrating the effectiveness and practical applicability of the proposed strategy. These results provide a theoretical and technical reference for the coordinated participation of distributed energy storage clusters in primary frequency regulation of power systems.