The inventive protection arrangement of an electric power system comprises a relay arrangement (12) for measuring current, voltage or both of an electric grid (15). The relay arrangement is adapted to detect over-current, under-voltage or both. The protection arrangement comprises also a speed/load controller (16) that is in connection with the relay arrangement (12). The speed/load controller is arranged to drive a prime mover (11) of a generator (10) set as a response for the said detection. The speed/load controller (16) is connectable with said prime mover (11). In more detail the controller (16) is arranged to drive power output of the prime mover (11) to a lower level when over-current, under-voltage or both is detected.