| CPC F03D 7/0224 (2013.01) [F03D 7/0296 (2013.01); F05B 2270/334 (2013.01); F05B 2270/807 (2013.01)] | 20 Claims |

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1. A controller for a wind turbine having three rotor blades, the controller being for adjusting pitch of the three rotor blades, the controller being configured to:
receive an acceleration signal from an acceleration sensor located in a tower or a nacelle of the wind turbine, the acceleration signal being indicative of side-to-side motion of the tower or nacelle of the wind turbine;
determine a first component in a fixed coordinate frame of the wind turbine based on the received acceleration signal, the first component being indicative of a whirling mode of the wind turbine caused by edgewise vibrations of one or more of the rotor blades;
generate, based on the determined first component, a second component in the fixed coordinate frame that is orthogonal to the first component, the second component is generated by application of a second-order generalised integrator to the first component;
apply a control action to the first and second components to obtain respective first and second control components for mitigating the whirling mode;
apply an inverse m-blade coordinate transformation to the first and second control components to obtain pitch reference offset values for the respective rotor blades in a rotor coordinate frame of the wind turbine; and,
transmit a control signal to adjust pitch of the rotor blades based on the obtained pitch reference offset values.
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