The invention relates to a method for operating a thermal power plant, which includes a gas turbine and a generator driven directly by the gas turbine by means of a shaft and being connected to an electrical grid having a grid frequency (FG) via an electronic decoupling apparatus and a step-up transformer. A synthetic inertia response is achieved by said method includes the steps of: sensing said grid frequency (FG); detecting if in case of an excursion of said grid frequency (FG) additional inertial power is required or not; if inertial power is required, calculating the magnitude and duration of the additional inertial power; and releasing additional inertial power to said electrical grid in accordance with said calculations via said electronic decoupling apparatus.