A method for controlling a power plant which delivers electrical power to an electrical grid. The grid has a grid frequency which fluctuates around a nominal frequency. A power output of the power plant is controlled as a function of a control frequency, in such a manner that the power output is increased when the control frequency decreases below the nominal frequency. On the other hand the power output is decreased when the control frequency rises above the nominal frequency. The grid frequency is continuously measured. The measured grid frequency is averaged to give, as a moving average, a slowly varying averaged trend frequency which is characteristic of the long term behavior of the grid frequency. The averaged grid frequency is used as the control frequency, if the measured grid frequency lies within a predetermined band around the averaged grid frequency. The measured grid frequency is used as the control frequency, if the measured grid frequency lies outside the predetermined band.