Wind turbine rotor blade pitch control for tower fatigue reduction

A wind turbine, comprising:

• a tower;
• a nacelle rotatably mounted to the tower;
• a rotor supported by the nacelle and having pitch-adjustable rotor blades;
• and
• a controller, configured to:
• receive sensor data, from a flap loading sensor of each of the pitch-adjustable rotor blades, indicative of flap loading on each of the respective pitch-adjustable rotor blades, and obtain, based on the received sensor data, a flap loading vector in a rotor coordinate frame of the wind turbine;
• apply an m-blade coordinate transformation to the flap loading vector to obtain first and second mutually orthogonal components in a fixed coordinate frame of the wind turbine;
• determine first and second 3P components in the fixed coordinate frame based on the obtained first and second components, the first and second 3P components being indicative of 3P frequency content of the tower of the wind turbine;
• apply a control action to the first and second 3P components to obtain respective first and second 3P control components for mitigating the 3P frequency content of the tower;
• apply an inverse m-blade coordinate transformation to the first and second 3P control components to obtain pitch reference offset values for the respective pitch-adjustable rotor blades in the rotor coordinate frame;
• and
• transmit a control signal to adjust pitch of the pitch-adjustable rotor blades based on the obtained pitch reference offset values,
• wherein applying the control action comprises determining an activation gain that activates the control action when a ratio between a 3P frequency and a natural frequency of the tower is within a prescribed range, and wherein the activation gain is a constant value when the ratio between the 3P frequency and the natural frequency is within a prescribed sub-range of the prescribed range and the activation gain changes from the constant value to zero from an extreme value of the prescribed sub-range to an extreme value of the prescribed range.

A method for adjusting rotor blade pitch of a wind turbine, the method comprising:

• receiving sensor data, from a flap loading sensor of each of three rotor blades of the wind turbine, indicative of flap loading on each of the respective rotor blades, and obtaining, based on the received sensor data, a flap loading vector in a rotor coordinate frame of the wind turbine;
• applying an m-blade coordinate transformation to the flap loading vector to obtain first and second mutually orthogonal components in a fixed coordinate frame of the wind turbine;
• determining first and second 3P components in the fixed coordinate frame based on the obtained first and second components, the first and second 3P components being indicative of 3P frequency content of a tower of the wind turbine;
• applying a control action to the first and second 3P components to obtain respective first and second 3P control components for mitigating the 3P frequency content of the tower;
• applying an inverse m-blade coordinate transformation to the first and second 3P control components to obtain pitch reference offset values for the respective rotor blades in the rotor coordinate frame;
• and
• transmitting a control signal to adjust pitch of the rotor blades based on the obtained pitch reference offset values, and
• wherein applying the control action comprises determining an activation gain that activates the control action when a ratio between a 3P frequency and a natural frequency of the tower is within a prescribed range, and wherein the activation gain is a constant value when the ratio between the 3P frequency and the natural frequency is within a prescribed sub-range of the prescribed range and the activation gain changes from the constant value to zero from an extreme value of the prescribed sub-range to an extreme value of the prescribed range.