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

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.

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.

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.