The present invention relates to a power system having a plurality wind turbine generators and a power plant controller arranged to communicate with the plurality of wind turbines generators, where each of the plurality of wind turbine generator being related to a wind turbine controller, the wind turbine controller being arranged to control an active power output in its related wind turbine generator according to an active power set point received from the wind power plant controller; a first subset of wind turbine generators operating at an active power output unrestricted of the active power set point; and a second subset of wind turbine generators operating according to an active power set point; and wherein the wind power plant controller communicates the active power set point, in accordance with the active power output of the first subset of the plurality of wind turbines generators, so as to reduce active power fluctuation of the aggregated active power output of the first and second subset of the plurality of wind turbines generators. The present invention also relates to method accordingly.