Wind turbine and method for operating same

1. A wind turbine comprising:

an asynchronous generator having a rotor and a stator;

means for determining an actual torque-speed curve of said generator;

means for determining an actual grid frequency;

means for determining a shifted torque-speed curve of said generator to the determined actual grid frequency if the determined actual grid frequency is higher than a predetermined upper frequency limit or lower than a predetermined lower frequency limit, wherein said means for determining a shifted torque-speed curve is further adapted to determine the shifted torque-speed curve such that an increase in the rotor voltage of said generator rotor is counteracted when shifting the actual torque-speed curve to the shifted torque-speed curve; and

means for shifting the actual torque-speed curve of said generator to the shifted torque-speed curve, wherein said means for determining a shifted torque-speed curve is adapted to determine, in the under excited range, a shifted torque-speed curve assigning a higher speed to the same torque as compared to the actual torque-speed curve, wherein said means for determining a shifted torque-speed curve is further adapted to determine, in the overexcited range, a shifted torque-speed curve assigning a lower speed to the same torque as compared to the actual torque-speed curve.

2. The wind turbine according to claim 1 wherein said means for shifting the actual torque-speed curve comprise means for varying the rotor current of said generator rotor.

3. The wind turbine according to claim 2 wherein said means for shifting the actual torque-speed curve comprise a converter connected to said generator rotor.

4. The wind turbine according to claim 2 wherein said means for shifting the actual torque-speed curve comprise means for varying the resistance of said generator rotor.

5. The wind turbine according to claim 1 further comprising:

means for determining an actual generator power factor;

means for determining a shifted generator power factor in response to the determined actual grid frequency if the determined actual grid frequency is higher than a predetermined upper frequency limit or lower than a predetermined lower frequency limit, wherein the shifted generator power factor is more in the inductive range than the actual power factor; and

means for shifting the actual generator power factor to the shifted generator power factor.

6. The wind turbine according to claim 5 wherein said means for shifting the actual generator power factor to the shifted generator power factor comprises a converter.

7. A wind farm comprising:

at least two wind turbines, each of said at least two wind turbines comprising:

an asynchronous generator having a rotor and a stator;

means for determining an actual torque-speed curve of said generator;

means for determining an actual grid frequency;

means for determining a shifted torque-speed curve in response to the determined actual grid frequency if the determined actual grid frequency is higher than a predetermined upper frequency limit or lower than a predetermined lower frequency limit, wherein said means for determining a shifted torque-speed curve is further adapted to determine the shifted torque-speed curve such that an increase in the rotor voltage of said generator rotor is counteracted when shifting the actual torque-speed curve to the shifted torque-speed curve; and

means for shifting the actual torque-speed curve of said generator to the shifted torque-speed curve, wherein said means for determining actual torque-speed curves of said generators, said means for determining an actual grid frequency, and said means for determining shifted torque-speed curves in response to the determined actual grid frequency are integrated in a centralized wind farm management system, and wherein said means for determining a shifted torque-speed curve is connected to each of said means for shifting the actual torque-speed curve of said generator, respectively.