Improving frequency stability in islanded urban microgrids by improved cascaded controller (1+PD-3DOF-FOPID) mobile EV energy storage

Maintaining frequency stability in islanded urban microgrids (IUMs) in the presence of disturbances from renewable energy sources such as photovoltaics and wind turbine generators, along with parameter uncertainties, constitutes a fundamental challenge. Owing to their inherent intermittency and low inertia, these sources can significantly jeopardize frequency stability in islanded operation. Mobile EV energy storage can play a pivotal role in enhancing frequency stability within IUMs. However, to ensure that such storage effectively contributes to system stability, a robust load frequency control system (LFC) is essential, one that performs reliably under both disturbances and parametric uncertainties. In this paper, a 1 + PD‑3DOF‑FOPID cascaded controller is designed for the LFC system of the IUM. In this structure, the inner (1 + PD) loop is responsible for rapid disturbance rejection, while the outer 3DOF‑FOPID controller handles fine adjustment and steady‑state error elimination. This combination effectively reduces frequency overshoot and undershoot, improves damping of oscillations, and maintains stability even under severe variations in system parameters. The coefficients of the proposed cascade controller are optimally tuned using the Harris Hawks Optimization (HHO) algorithm, which offers advantages such as avoiding local minima and achieving fast convergence. To evaluate its performance, the proposed method is compared with several existing approaches, including PD‑3DOF‑PID (COA), 3DOF‑PID (COA), and FOPID. The comparative results indicate that the proposed controller improves overshoot by 30%, undershoot by 54%, and frequency deviation settling time by 66% relative to the other methods. Moreover, the results confirm that the proposed approach exhibits strong robustness against both disturbances and parameter uncertainties, and it significantly enhances the overall frequency stability of the islanded urban microgrid.