Nowadays, most of the ancillary services such as reserve capacity, inertia and frequency control relies on large conventional power plants. Approaching future power systems with high penetration of renewable energy sources (RES) has resulted in imperative need for the evaluation of ancillary services. This research focuses on the frequency stability which must be ensured in order to maintain the grid stability against imbalances between generation and load. This large conventional power plants that provide ancillary services are called “must-run” units. These facilities are generation power plants necessary during certain operating conditions and they are responsible for providing enough ancillary services to ensure a reliable operation of power systems. Given a high RES penetration in the future, must-run units are expected to be reduced or totally decommissioned reducing the power system inertia. This may result in insecure operation threatening the reliability of the power supply. This project investigates the frequency stability support from renewable energy generation such as wind power plants (WPPs) and solar photovoltaic systems (SPVSs) in future power systems with high penetration of RES and without must-run units. Sensitivity studies for frequency stability are performed on a simulated 2030 scenario for western Denmark (DK1) power system. The objective of this master thesis is to study the DK1 power system to analyse the ability of modern controllable WPPs to provide frequency stability without must-run units in a future scenario dominated by RES generation. This project examines the primary frequency control in DK1 simulating an overfrequency event islanding DK1 from the CE power system with high wind forecast. The main results of this project reveal that the fast deploy of active power by the RES generation counterbalances the reduced inertia in the power system, which can operate without a lack of stability of the power supply for overfrequency events without must-run units. However, there are technical capabilities and limitations that curtail the RES penetration. Recommendations on the parameters of the WPPs frequency control are made according to the droop, the ramp rate and the RES penetration. The virtual inertia is recommended for frequency control of WPPs and increases its importance when the RES penetration is high. The support of HVDC interconnections is an interesting facility to increase the RES penetration allowing the power system to operate with even less inertia online maintaining a stable supply. Although, the measurement and communication delay by the frequency controllers increases its importance when increasing the RES penetration as faster power deploy is needed.