The share of energy produced by RES tends to increase during the last decade. New types of generators were introduced and the situation in terms of power system frequency control has changed. The modern wind turbines and PV inverters have functions for power output correction during significant frequency excursions. This contribution presents developed by the authors structure model of the electric power system which is appropriate for assessment of the RES impact on the frequency regulation during large disturbances. The paper also presents a methodology for optimal tuning of the wind turbine frequency responsive controllers. The test case results show that generators based on RES have big potential for contribution to the frequency control. It is also shown that optimizing their controller parameters can significantly improve the overall system response.