Method and system for damping a wind turbine tower

1. A method for damping oscillations in a tower of a wind turbine, wherein the wind turbine includes a rotor with a plurality of rotor blades, and a wind turbine controller, the method comprising:

(a) determining a primary rotational frequency of the rotor (fp) that correlates to a tower resonance frequency (fr) that induces oscillations in the tower;

(b) defining an exclusion zone between a first rotational frequency of the rotor (f 1 ) that is less than the primary rotational frequency (fp) and a second rotational frequency of the rotor (f 2 ) that is greater than the primary rotational frequency (fp);

(c) at rotor frequencies below the exclusion zone, applying a first tower-damping force strategy that increases a tower frequency (ftw) towards or greater than the tower resonance frequency (fr); and

(d) at rotor frequencies above the exclusion zone, applying a second tower-damping force strategy that that decreases the tower frequency (ftw) towards or less the tower resonance frequency (fr).

2. The method according to claim 1 , wherein the first rotational frequency (f 1 ) and the second rotational frequency (f 2 ) are defined such that the exclusion zone is equal to or less than +/−3% of the primary rotational frequency (fp).

3. The method according to claim 1 , wherein the wind turbine comprises a generator with a generator rotor that produces rotational torque, wherein application of the first and second tower-damping strategies comprises controlling the generator with the wind turbine controller to vary the rotational torque from the generator rotor that is imparted to the tower.

4. The method according to claim 3 , wherein the tower oscillations being damped are primarily lateral oscillations generally perpendicular to an axis of the rotor.

5. The method according to claim 1 , wherein the wind turbine comprises a pitch control system, wherein application of the first and second tower-damping strategies comprises varying a pitch angle of the rotor blades with the pitch control system to change the rotational frequency of the rotor.

6. The method according to claim 5 , wherein the tower oscillations being damped are primarily longitudinal oscillations generally parallel to an axis of the rotor.

7. The method according to claim 1 , wherein the wind turbine comprises a pitch control system and a generator with a generator rotor that produces rotational torque, wherein application of the first and second tower-damping strategies comprises:

controlling the generator with the wind turbine controller to vary the rotational torque from the generator rotor that is imparted to the tower to dampen primarily lateral tower oscillations; and

varying a pitch angle of the rotor blades with the pitch control system to change the rotational frequency of the rotor to dampen primarily longitudinal tower oscillations.

8. A wind turbine, comprising:

a plurality of rotor blades on a rotatable rotor hub;

a pitch system configured to change a pitch angle of the rotor blades;

a generator with a generator rotor;

a controller, in operable communication with the pitch system and the controller, the controller configured to:

(a) determine a primary rotational frequency of the rotor (fp) that correlates to a tower resonance frequency (fr) that induces oscillations in the tower;

(b) define an exclusion zone between a first rotational frequency of the rotor (f 1 ) that is less than the primary rotational frequency (fp) and a second rotational frequency of the rotor (f 2 ) that is greater than the primary rotational frequency (fp);

(c) at rotor frequencies below the exclusion zone, apply a first tower-damping force strategy that increases a tower frequency (ftw) towards or greater than the tower resonance frequency (fr); and

(d) at rotor frequencies above the exclusion zone, apply a second tower-damping force strategy that decreases the tower frequency (ftw) towards or less the tower resonance frequency (fr).

9. The wind turbine according to claim 8 , wherein the first rotational frequency (f 1 ) and the second rotational frequency (f 2 ) are defined such that the exclusion zone is equal to or less than +/−3% of the primary rotational frequency (fp).

10. The wind turbine according to claim 8 , further comprising a generator with a generator rotor that produces rotational torque, wherein application of the first and second tower-damping strategies comprises controlling the generator with the wind turbine controller to vary the rotational torque from the generator rotor that is imparted to the tower to primarily damp lateral oscillations that are generally perpendicular to an axis of the rotor.

11. The wind turbine according to claim 8 , further comprising a pitch control system, wherein application of the first and second tower-damping strategies comprises varying a pitch angle of the rotor blades with the pitch control system to change the rotational frequency of the rotor to primarily damp longitudinal oscillations that are generally parallel to an axis of the rotor.

12. The wind turbine according to claim 8 further comprising:

a generator with a generator rotor that produces rotational torque, wherein application of the first and second tower-damping strategies comprises controlling the generator with the wind turbine controller to vary the rotational torque from the generator rotor that is imparted to the tower to primarily damp lateral oscillations that are generally perpendicular to an axis of the rotor; and

a pitch control system, wherein application of the first and second tower-damping strategies comprises varying a pitch angle of the rotor blades with the pitch control system to change the rotational frequency of the rotor to primarily damp longitudinal oscillations that are generally parallel to an axis of the rotor.