How long would modern society cope with a power outage, what would happen to vital systems that we today take for granted in modern society. The Nordic electricity grid is facing a major shift where electricity production from non-renewable sources are to be replaced increasingly by renewable sources. By increasing the penetration of wind and solar power the electric power system might be exposed to disturbances due to decreasing inertia as a result of the electricity transition. Currently the electric power system has different reserves to use to maintain frequency stability but there are other reserves that could help further in the fight for the balance between electricity production and consumption. This project examines whether the new reserve service, fast frequency reserve (FFR), can help the existing frequency containment reserve for disturbed (FCR-D) operation. Therefore, two experiments were conducted using the simulation tool ARISTO, addressing relevant issues involving frequency stability. Motivation for the hypothesis was to investigate if FFR could reduce the frequency transients and improve frequency variations by developing various setups and cases when inertia was retained and when the system inertia was reduced at different stages. The results of the experiments showed that the global minimum frequency, nadir, had increased for all test cases compared to the reference case when using FFR, this proved that the FFR in fact help reducing frequency transients. The results showed furthermore that when the FFR had a duration time of 30 seconds compared to only 5 seconds, the frequency variations could be improved for certain setups for experiment 2.