System and method to manage torsional oscillation of a wind turbine tower

1. A wind turbine comprising:

a tower;

a nacelle;

a rotor including a plurality of blades;

an electrical generator operatively coupled to the rotor; and

a control system comprising:

a sensing system operable to output a torsional oscillation signal indicative of a torsional oscillation frequency of the nacelle;

a torsional damping module configured to:

monitor the torsional oscillation signal; and

determine one or more blade pitch command signals for damping a torsional oscillation of the tower; and

a filter module configured to:

receive the one or more blade pitch command signals as inputs:

filter the one or more blade pitch command signals to exclude frequency components greater than the torsional oscillation frequency, wherein the filter module defines a cut off frequency equal to a 4P frequency of the wind turbine; and

output a respective one or more modified blade pitch command signals.

2. The wind turbine of claim 1 , wherein determining the one or more blade pitch command signals includes:

multiplying a torsional moment reference signal with a modulation signal based on an azimuth angle of the rotor.

3. The wind turbine of claim 1 , wherein determining the one or more blade pitch command signals includes:

calculating a cyclic blade pitch input for each blade of the plurality of blades in order to generate a moment on the rotor that is out of phase with the torsional oscillation signal.

4. The wind turbine of claim 1 , wherein the control system is configured to control a pitch actuation system of the wind turbine based on the one or more modified blade pitch command signals.

5. The wind turbine of claim 1 ,

wherein the sensing system includes a sensor mounted in the nacelle or in a top of the tower of the wind turbine, and

wherein the sensing system is arranged to determine a yaw movement of the nacelle.

6. The wind turbine of claim 1 , wherein the torsional damping module is operable to damp the torsional oscillation of the tower when the torsional oscillation is detected to exceed a predetermined threshold.

7. The wind turbine of claim 1 , wherein the torsional damping module implements a gain such that the magnitude of the one or mode blade pitch command signals is proportional to the magnitude of the torsional oscillation of the tower.

8. The wind turbine of claim 7 , wherein the gain is dependent on an operating parameter of the wind turbine.

9. The wind turbine of claim 8 , wherein the operating parameter is at least one of: a wind speed, a rotor speed, a torsional oscillation frequency, a blade pitch angle, a generated power, or a commanded power reference.

10. The wind turbine of claim 1 , wherein the tower has a height exceeding 100 meters.

11. The wind turbine of claim 1 , wherein the tower is steadied by guy lines.

12. A method of controlling a wind turbine comprising a tower, a nacelle, a rotor including a plurality of blades, and an electrical generator operatively coupled to the rotor, wherein the method comprises:

monitoring a torsional oscillation frequency that indicates a yaw motion of the nacelle;

determining one or more blade pitch command signals for damping a torsional oscillation of the tower; and

filtering, by a filter, the one or more blade pitch command signals so as to generate one or more modified blade pitch command signals,

wherein, in filtering the one or more blade pitch command signals, those signals are filtered to exclude frequency components greater than the torsional oscillation frequency, wherein the filter defines a cut off frequency equal to a 4P frequency of the wind turbine.

13. The method of claim 12 , wherein determining the one or more blade pitch command signals includes:

multiplying a torsional moment reference signal with a modulation signal based on an azimuth angle of the rotor.

14. The method of claim 12 , wherein determining the one or more blade pitch command signals includes:

calculating a cyclic blade pitch input for each blade of the plurality of blades in order to generate a moment on the rotor that is out of phase with the torsional oscillation signal.

15. The method of claim 12 , further comprising:

controlling a pitch actuation system of the wind turbine based on the one or more modified blade pitch command signals.

16. A controller for a wind turbine system including a nacelle mounted on a tower, and a rotor having a plurality of blades, wherein the controller comprises:

a processor;

a memory module; and

an input/output interface,

wherein the memory module includes a set of program code instructions which when executed by the processor, performs an operation comprising:

monitoring a torsional oscillation frequency that indicates a yaw motion of the nacelle;

determining one or more blade pitch command signals for damping a torsional oscillation of the tower;

filtering, by a filter, the one or more blade pitch command signals so as to generate one or more modified blade pitch command signals,

wherein, in filtering the one or more blade pitch command signals, those signals are filtered to exclude frequency components greater than the torsional oscillation frequency, wherein the filter defines a cut off frequency equal to a 4P frequency of the wind turbine.

17. The controller of claim 16 , wherein determining the one or more blade pitch command signals includes:

multiplying a torsional moment reference signal with a modulation signal based on an azimuth angle of the rotor.

18. The controller of claim 16 , wherein determining the one or more blade pitch command signals includes:

calculating a cyclic blade pitch input for each blade of the plurality of blades in order to generate a moment on the rotor that is out of phase with the torsional oscillation signal.

19. The controller of claim 16 , wherein the input/output interface is coupled to a pitch actuation system of the wind turbine operable based on the one or more modified blade pitch command signals.