An improved arctic puffin optimization-based multi-objective approach with novel cascade dual FOPID control for frequency regulation in standalone microgrids

Modern standalone microgrids face significant frequency stability challenges due to renewable intermittency, reduced inertia, and load fluctuations. To address this, we propose a novel cascade dual fractional-order proportional(P)–integral(I)–derivative(D) (FOPID) controller optimized using an improved Arctic Puffin optimization (APO) algorithm with a multi-objective criterion. The proposed approach aims to enhance load frequency regulation in standalone microgrid systems integrated with a battery energy storage system (BESS) operating under a droop control strategy. Unlike conventional single-loop FOPID schemes, the proposed controller employs a fully fractional cascade architecture with two serial loops, enabling finer control granularity, enhanced tuning flexibility, and stronger robustness against nonlinearities, communication delays, uncertainties, and disturbances. A modified performance index is further introduced by integrating frequency deviation and control effort into a single time-weighted criterion, allowing simultaneous improvement of stability and efficiency. Additionly, the improved APO (IAPO) algorithm incorporates structural enhancements to accelerate convergence and better balance exploration and exploitation. It is used not only to optimize controller parameters but also to co-optimize the energy capacity of the BESS under droop control. The proposed method is validated on a multi-source standalone microgrid, demonstrating up to 75 % reduction in frequency deviation, around 20 s faster settling time, and lower control effort compared with PID and single-loop FOPID controllers. Moreover, comparisons with PSO, HBA, and the original APO show that the proposed IAPO achieves up to 23 % smaller overshoot and 21 % smaller undershoot, reduces the multi-objective optimization index by about 6 %, and narrows the control bounds by up to 55 %, confirming its superior convergence, improved stability margins, and stronger disturbance resilience. These results establish the IAPO-based cascade dual FOPID as a robust and scalable solution for frequency regulation in a hybrid microgrid with high renewable penetration.