This paper proposes a simulation model of Virtual Power Plants (VPP) and analysis the impact of the proposed model in frequency control. This paper, also, analyses the impact of reduced inertia on the hydro power plants stability. The simulations were done considering an increased share of renewable energy sources (RES) in electrical power production and a reduction of system inertia. The simulations were performed in Matlab/Simulink on the Kundur two-area frequency model which contains both primary and secondary frequency control levels. The simulation time was considered equal to 900 seconds, equivalent to 15 minutes, this being a reference value for secondary frequency control. The virtual power plant is composed of hydro power plants, renewable sources and energy storage systems and is controlled by a hybrid controller meaning that the VPP can contribute to both primary and secondary frequency control.