## 6 Conclusions  
This paper presents a coordinated active power control scheme for a BESS participating in frequency containment and restoration, along with other regulating units. A comprehensive approach is introduced, comprising the economic dispatch schedule, AGC system and real-time local battery controller. A typical AGC, without any major deviation from systems implemented in practice, regulated deployment of secondary reserves, while BESS-level control ensures that SOC remains within acceptable limits, even following severe contingencies. For this purpose, a SOC controller is implemented locally, while BESS aFRR provision is gradually reduced when the SOC approaches its variation limits, re-dispatching aFRR from the batteries to other units under AGC control. All units are subject to economic dispatch scheduling, which is responsible for SOC restoration, via appropriate charge/discharge dispatch orders, ensuring that this process does not deplete the operating reserves of the system.  
Simulation results from the application of the proposed BESS participation concept in a non-interconnected island system demonstrate its effectiveness. The batteries provide frequency restoration reserves in parallel with other regulating units, along with FCR and inertial response. When the SOC approaches its variation limits, BESS participation in LFC is gradually reduced, with secondary regulation undertaken by the other generating  
Figure 14: Frequency quality metrics, for different SCADA communication delays. **(a)** Frequency range, **(b)** frequency standard deviation, **(c)** time outside 49.95–50.05 Hz.  
Figure 13: Response following a diesel unit outage at 150 s and synchronization of another at 900 s, for different supervisory control and data acquisition (SCADA) communication delays. **(a)** Frequency, **(b)** BESS active output power (positive: injection, negative: absorption).  
units in a smooth and well-coordinated manner via the central AGC system. The SOC is regulated back to its reference value when conditions allow, through a process managed by the system's real-time economic dispatch process, rather than locally at the BESS level. In comparison to the other power control schemes evaluated in the paper, the main advantages of the proposed method lie in allowing the batteries to participate in the AGC and ED process on equal terms with other regulating units, whilst effectively managing energy reserve depletion. The robustness of the method is evaluated against various parameters. The proposed method is easily applicable in large-scale interconnected power systems and in the presence of multiple independent BESSs.  
With storage taking up the role of traditional dispatchable generation in decarbonized power systems, further work on energy reserve management and prevention of depletion to ensure effective provision of balancing services is of critical importance. Future extensions of this work could include BESS portfolios' secondary frequency regulation, RES participation in reserve provision and therefore storage charge management, adaptation of the proposed method for systems operating without conventional units, i.e., relying on RESs and storage, and expansion of the economic dispatch module to accommodate more sophisticated market functionalities.  
Conceptualization, A.G.P.; methodology, A.G.P.; software, A.G.P.; validation, A.G.P. and S.A.P.; writing--original draft preparation, A.G.P.; writing--review and editing, S.A.P.; visualization, A.G.P. and S.A.P.; supervision, S.A.P.; funding acquisition, A.G.P. All authors have read and agreed to the published version of the manuscript.  
The work of [PERSON] is co-financed by Greece and the European Union (European Social Fund--ESF) through the Operational Programme \"Human Resources Development, Education and Lifelong Learning\" in the context of the project \"Strengthening Human Resources Research Potential via Doctorate Research\" (MIS-5000432), implemented by the State Scholarships Foundation (IKY).  
The authors declare no conflict of interest.