In this work, a battery-ultracapacitor based Hybrid Energy Storage System (HESS) is proposed to attain a constant output power on a 10kVA Virtual Power Plant (VPP) with three days autonomy. Firstly, the importance of a HESS is remarked and different topologies of battery-ultracapacitor based HESS are analyzed. Thus, the VPP scheme based on a photovoltaic farm, a wind energy conversion system and the proposed HESS is described. Furthermore, in order to achieve a suitable energy conversion, Sliding-Mode Control (SMC) on a cascade scheme is designed by using the space state representation of the DC-DC power converters. Finally, the system performance is presented remarking proper dynamic and steady-state responses under different solar irradiation, wind speed and load profiles.