Conference paper in the framework of IEEE PES ISGT Europe 2023 conference under the title “A Hybrid Energy Storage System for flexibility provision: Modeling and Control design”. The manuscript has been published in the proceedings of the conference. Abstract: The increasing power demand and the high penetration of renewable energy sources (RES), among with grid security, and stability make the need of flexibility a major challenge. Different methods have been identified to meet this need, of which the integration of the power sources with storage provides one of the most prominent ones. Moreover, hybridization of storage systems is a new trend with guaranteed benefits. In this paper, the potential of a proposed hybrid energy storage system (HESS) and its control scheme regarding the RES integration and grid stability, is under investigation. The HESS consisting of supercapacitor (SC) cells and a battery array that can interchange flexibility between RES units and the utility grid. Power electronic devices act as interfaces that aiming to meet the flexibility needs in real-time and guaranteeing the balancing between RES production, load, and grid fluctuations. Control structure based on hierarchical and cascaded control are applied. The obtained closed-loop HESS is evaluated under varying conditions and the results fully verify that the flexibility is activated in real-time transients, once the high-performance control schemes are applied such that the frequency and dc and ac voltage are regulated quickly and smoothly.