1. A method for operating a wind turbine when a grid fault with a voltage drop occurs, wherein the wind turbine has a rotor having at least one rotor blade with an adjustable blade pitch angle, a generator connected to the rotor for generating electric power for an electric power supply grid, and a converter connected to the generator and the electric power supply grid, the method comprising the steps of: identifying the grid fault; detecting a present value of a variable of the wind turbine representing a power; presetting a hold setpoint for the variable representing a power of the wind turbine during a hold time, wherein the hold setpoint corresponds to the present value of the variable representing a power when the grid fault arises; determining a maximum permissible setpoint for the variable of the wind turbine representing a power, wherein the maximum permissible setpoint is increased with time once the hold time has elapsed; driving the wind turbine with a setpoint preset which corresponds to the present rotational speed, wherein the setpoint preset is limited to the maximum permissible setpoint; and, ending a preset of the maximum permissible setpoint as soon as the rotational speed-dependent setpoint is less than the maximum permissible setpoint.