This disclosure is directed to a method for operating a wind farm having a plurality of wind turbines and to a control system for a wind farm. The wind farm is connected to an electrical grid. The wind turbines are operated to supply electrical power to the electrical grid in accordance with at least one setpoint value for a power related electrical variable. In the event that an available reactive power of the wind farm is smaller than a setpoint for a reactive power to be supplied by the wind farm, a required increase of reactive power is determined for each wind turbine of the wind farm.