SOC- and RoCoF-Aware Bounded Adaptive Droop Control of Battery Energy Storage for Fast Frequency Response in Islanded Renewable Energy Systems

Abstract

High penetration of converter-interfaced photovoltaic and wind generation reduces the effective inertia of islanded renewable energy systems and makes frequency stability more sensitive to load and renewable-power disturbances. This paper proposes a bounded adaptive droop framework for battery energy storage system (BESS) fast frequency response under low-inertia conditions, built around a bounded coordination mechanism. The framework coordinates frequency deviation, filtered rate of change of frequency (RoCoF), renewable-power fluctuation, and state of charge (SOC) to unify frequency-support capability with SOC availability and converter operating constraints. SOC-dependent available power limits, rated converter saturation, a ramp-rate limiter, RoCoF filtering, and measurement delay translate support demand into feasible power commands. A unit-consistent per-unit frequency-response model is combined with kW-level BESS power commands in MATLAB simulations. Compared with Fixed Droop, the proposed method reduces Max |Δf| and RMS Δf by 31.61% and 31.06%, respectively, on average across five deterministic cases, including four main disturbance cases and one charging case. In 30 Monte Carlo validation trials, the mean Max |Δf| decreases from 0.586 Hz to 0.330 Hz (43.66%), and the mean RMS Δf decreases from 0.416 Hz to 0.234 Hz (43.71%), with no SOC violation. Charging-protection tests further show that a fully charged BESS cannot absorb surplus renewable power, indicating the need for coordination with renewable curtailment, dump loads, secondary control, or reserve-SOC management.