The submitted paper describes the operation of power systems in the context of load frequency control (LFC). A state-of-the-art of LFC is first presented. Then a Matlab/SIMULINK model is proposed in order to investigate the influence of renewable energy sources on load frequency control. The created model is based on mathematical relationships describing the behavior of the elements involved in the regulation process, i.e. power systems, steam turbines, turbine speed regulators, frequency regulators and renewable energy sources (RES). A comparative analysis of the case of LFC with and without RES is then conducted out. Finally, case studies on the influence of particular types of renewable energy sources on regulation are analyzed. Final remarks are given on the impact of the limited dynamics of changes in the power generated by conventional units, the correlation between the power generated by renewable energy sources and the load power.