## 1 Introduction
Although effective, this approach is not applicable to standard central AGC systems, is complex in its practical implementation and it is non-scalable to large-scale interconnected systems. Moreover, in such a scheme the batteries do not participate on equal terms with other units--via a market-based mechanism--in the secondary regulation of the system. Control methods with significantly higher complexity addressing SOC regulation and participation of energy storage in LFC are also available in the literature, e.g., \(\text{H}_{\infty}\) control [54], whose implementation in real-world power systems is rather challenging. In this paper, an active power control scheme is proposed for batteries in island systems to provide secondary frequency regulation, along with inertia emulation and droop response. SOC regulation is implemented through a local controller, continuously supervising SOC to ensure it remains within acceptable limits, with its restoration back to its reference being effected within the real-time economic dispatch process. To achieve frequency restoration, the BESS is integrated in a centralized AGC system that dynamically redistributes the total regulation signal to all load-following units, whilst considering participation factors coming from the electricity market, as well as the SOC of the BESS. The proposed method allows batteries to participate in secondary regulation delivered through a centralized AGC system and driven by economic dispatch, while incorporating charge management functionality to prevent battery energy reserve depletion, particularly in challenging situations, when energy reserves are depleted while BESSs are providing secondary control. The questions addressed include how to minimize the impact on secondary regulation, as well as how to prevent SOC restoration from placing an excessive burden on system reserves provided by other regulating units. Building upon existing work on BESS participation in secondary regulation, this paper presents a complete solution in the time scale of seconds to minutes, consisting of an economic dispatch module, executed every 15 min to derive optimal loading base points for all units, a centralized AGC system generating regulating signals for load-following units every few seconds and continuously acting local BESS controllers to execute AGC signals while providing primary frequency control and charge management. Unlike other schemes presented in the literature, the proposed method is simple, scalable and easy to apply in real-life systems, its tuning is undemanding and it is compatible with different market and regulation environments. The introduction of such a method constitutes the novelty and the main contribution of this paper, which is significant and timely given the momentum of grid storage and the anticipation that battery systems will fully participate in regulation and control. To validate its performance and demonstrate its advantages, the proposed method is compared with three alternative regulation approaches, using as a study case a small island system. The paper is organized as follows: The proposed method, along with three alternative regulation methods, are described in Section 2. Section 3 presents the real-time economic dispatch problem. The study case island grid is described in Section 4. In Section 5, simulations results are presented and discussed, leading to the conclusions summarized in Section 6.