System and method for reducing rotor loads in a wind turbine upon detection of blade-pitch failure and loss of counter-torque

1. A method for braking a wind turbine including a brake and at least one rotor blade coupled to a rotor, the method comprising the steps of:

detecting a loss of counter torque on the rotor;

controlling a pitch angle of the at least one rotor blade in response to the detected loss of counter torque on the rotor;

detecting a blade-pitch failure in the at least one rotor blade;

determining a reduced generator overspeed threshold value in response to the blade-pitch failure;

determining whether a generator speed exceeds the reduced generator overspeed threshold value; and

applying the brake to slow the rotor in response to the loss of counter torque on the rotor and the blade-pitch failure when the generator speed exceeds the reduced generator overspeed threshold value.

2. A method according to claim 1 , wherein the brake is applied when a rotor speed is above a reduced rotor speed threshold value.

3. A method according to claim 1 , wherein the wind turbine is coupled to a utility power grid, and wherein the loss of counter-torque comprises a grid anomaly in the utility power grid.

4. A method according to claim 3 , wherein the grid anomaly in the utility power grid comprises sensing at least one of an undervoltage condition, an overvoltage condition, and an out-of-frequency condition.

5. A method according to claim 1 , wherein the brake comprises a hydraulic parking brake, an electromechanical brake, a centrifugal brake, an Eddie-current brake (retarder), or a magnetic brake.

6. A method for braking a wind turbine including at least two rotor blades coupled to a rotor, the method comprising the steps of:

detecting a loss of counter torque on the rotor;

controlling a pitch angle of the at least two rotor blades in response to the detected loss of counter torque on the rotor;

detecting a blade-pitch failure in at least one of the rotor blades; and

varying a rate of change of pitch angle of all functioning rotor blades in response to the loss of counter torque on the rotor and the blade-pitch failure in at least one of the rotor blades.

7. A method according to claim 6 , wherein varying the rate of change of pitch angle of all functioning rotor blades comprises changing the pitch angle at a first rate and thereafter changing the pitch angle at a second rate that is different from the first rate.

8. A method according to claim 6 , wherein varying the rate of change of pitch angle of all functioning rotor blades comprises varying the rate of change of the pitch angle based on at least one of a design parameter of a component of the wind turbine and a measured parameter of a component of the wind turbine.

9. A method according to claim 6 , wherein the wind turbine also includes a brake, and wherein the method further comprises the step of applying the brake to slow the rotor.

10. A method according to claim 9 , wherein the brake comprises a hydraulic parking brake, an electromechanical brake, a centrifugal brake, an Eddie-current brake (retarder), or a magnetic brake.

11. A method according to claim 6 , wherein the wind turbine is coupled to a utility power grid, and wherein the loss of counter-torque comprises a grid anomaly in the utility power grid.

12. A wind turbine system configured to be coupled to a power grid, comprising:

a rotor comprising at least one rotor blade;

a brake coupled to the rotor;

a blade pitch actuator; and

a processor coupled to the blade pitch actuator, wherein the processor is configured to detect a loss of counter torque on the rotor and a blade-pitch failure in the at least one rotor blade, and wherein the processor determines a decreased generator overspeed threshold value in response to the blade-pitch failure, and whether a generator speed exceeds the decreased generator overspeed threshold value, and

wherein the processor applies the brake to slow the rotor in response to the loss of counter torque on the rotor and the blade-pitch failure when the generator speed exceeds the decreased generator overspeed threshold value.

13. A wind turbine system according to claim 12 , wherein the brake is applied when a rotor speed is above a reduced rotor speed threshold value.

14. A wind turbine system according to claim 12 , wherein the wind turbine comprises at least two rotor blades, and wherein the processor varies a rate of change of pitch angle of all functioning blades in response to the detected blade-pitch failure.

15. A wind turbine system according to claim 14 , wherein the processor is configured to vary the rate of change of pitch angle of all functioning blades at a first rate and thereafter change the pitch angle at a second rate that is different than the first rate.

16. A wind turbine system according to claim 12 , wherein the brake comprises a hydraulic parking brake, an electromechanical brake, a centrifugal brake, an Eddie-current brake (retarder), or a magnetic brake.

17. A wind turbine system configured to be coupled to a power grid, comprising:

a rotor comprising at least two rotor blades;

a blade pitch actuator; and

a processor coupled to the blade pitch actuator, wherein the processor is configured to detect a loss of counter torque on the rotor and a blade-pitch failure in one of the at least two rotor blades,

wherein the processor varies a rate of change of pitch angle of all functioning blades in response to the loss of counter torque on the rotor and the blade-pitch failure in one of the at least two rotor blades.

18. A wind turbine system according to claim 17 , wherein the processor is configured to vary the rate of change of pitch angle of all functioning blades at a first rate and thereafter change the pitch angle at a second rate that is different than the first rate.

19. A wind turbine system according to claim 17 , further comprising a brake coupled to the rotor.

20. A wind turbine system according to claim 19 , wherein the brake is applied when a generator speed is above a reduced generator overspeed threshold value.

21. A wind turbine system according to claim 19 , wherein the brake is applied when a rotor speed is above a reduced rotor speed threshold value.

22. A wind turbine system according to claim 19 , wherein the brake comprises a hydraulic parking brake, an electromechanical brake, a centrifugal brake, an Eddie-current brake (retarder), or a magnetic brake.