A method of operating a wind park having a plurality of wind turbines is provided. The method includes determining a grid condition indicator indicative of the condition of a grid into which the wind park feeds; determining a first maximum power value of the wind park on the basis of the grid condition indicator; determining an operating margin on the basis of the grid condition indicator, for a grid condition indicator indicative of a weak grid; determining a second maximum power value on the basis of the operating margin; and curtailing the active power output of the wind park according to a maximum power value. A computer program product for carrying out the steps of such a method; an active power controller adapted for use in operating a wind turbine of a wind park; a wind turbine; and a wind park are also provided.