A wind power turbine with a wind rotor which operates at variable speeds, a generator that can operate at a constant speed and an electric machine that can operate at a variable speeds, which are each in driving connection with one another by way of a superimposition transmission, such that as a power control device for regulating the speed of the generator, the electric machine is connected to a power grid and operates as a generator and motor. A controllable brake, for braking the generator, is arranged in the force flow between the transmission and the generator to extend the operating range of the wind power turbine down to lower wind speeds. When operating the turbine at low wind speeds, the brake is applied, the generator is deactivated, and electric current is produced by operating the electric machine as a generator, which is fed to the power grid.