Reduction of edgewise vibrations using blade load signal

1. A rotor control system for actuating pitch of pitch adjustable rotor blades of a wind turbine, the rotor control system comprising:

a pitch actuating unit for determining a pitch modification signal to be applied to a pitch actuator for actuating the pitch of the pitch adjustable rotor blades; the pitch actuation unit being arranged to:

receive an edgewise load signal for each of the pitch adjustable rotor blades, the edgewise load signal being measured in a rotating reference frame;

apply an m-blade coordinate transformation to the edgewise load signal to transform the edgewise load signal to a whirling reference frame along a first and a second reference direction;

select a signal component of either the first or the second reference direction to obtain a selected signal component;

filter the selected signal component at either a backward whirling frequency or a forward whirling frequency to obtain a filtered signal component;

apply an inverse m-blade coordinate transformation to the filtered signal component to obtain the pitch modification signal, wherein only a whirling mode of either backward or forward whirling is targeted in obtaining the pitch modification signal; and

apply the pitch modification signal to the pitch actuator.

2. The rotor control system according to claim 1 , wherein the selected signal component is filtered to couple an output of the m-blade coordinate transformation to a selected backward whirling component of an edgewise vibration or a selected forward whirling component so that the filtered signal component reflects a magnitude of the selected component of the backward or forward whirling in the whirling reference frame of the edgewise vibration.

3. The rotor control system of claim 1 wherein the filtering of the selected signal comprises applying a bandpass filter including the backward whirling frequency or the forward whirling frequency.

4. The rotor control system of claim 1 wherein the filtering of the selected signal includes an inverse gain to the selected signal.

5. The rotor control system of claim 1 , wherein the inverse m-blade coordinate transformation taking as input a first signal and a second signal; and wherein the second signal is determined by further filtering the filtered signal with a further signal filter with a quadrature phase shift filter phase response.

6. The rotor control system according to claim 5 , wherein the further signal filter is a leaky integrator.

7. The rotor control system of claim 5 , wherein the further signal filter is selectable for either a positive filter phase response or a negative filter phase response.

8. The rotor control system of claim 1 , wherein the m-blade coordinate transformation is based on a Coleman transformation.

9. The rotor control system of claim 1 further comprising:

determine a collective pitch reference for the pitch adjustable rotor blades, the collective pitch reference being determined based on a rotor speed,

apply a resulting pitch modification signal to the pitch adjustable rotor blades, the resulting pitch modification signal being applied to the pitch adjustable rotor blades individually, and for each individual blade being based on a combined signal of the collective pitch reference and the individual pitch modification signals.

10. The rotor control system of claim 1 further comprising an adjustment gain with a gain scheduling term, the gain scheduling term being dependent upon an operational point of the wind turbine.

11. The rotor control system of claim 1 wherein the inverse m-blade coordinate transformation further includes a phase shift to adjust a phase of the filtered signal component.

12. The rotor control system of claim 1 wherein the pitch modification signal is filtered with a notch filter around either an edgewise vibration frequency forward whirling frequency or around an edgewise vibration frequency backward whirling frequency.

13. The rotor control system of claim 1 further comprising an activation element, the activation element being dependent upon an operational point of the wind turbine.

14. A method of actuating pitch of pitch adjustable rotor blades of a wind turbine, the wind turbine comprising a pitch actuator for actuating the pitch of the pitch adjustable rotor blades, the method comprising:

receiving an edgewise load signal for each of the pitch adjustable rotor blades, the edgewise load signal being measured in a rotating reference frame;

applying an m-blade coordinate transformation to the edgewise load signal to transform the edgewise load signal to a whirling reference frame along a first and a second reference direction;

selecting a signal component of either the first or the second reference direction to obtain a selected signal component;

filtering the selected signal component at either a backward whirling frequency or a forward whirling frequency to obtain a filtered signal component;

applying an inverse m-blade coordinate transformation to the filtered signal component to obtain a pitch modification signal, wherein only a whirling mode of either backward or forward whirling is targeted in obtaining the pitch modification signal; and

applying the pitch modification signal to the pitch actuator.

15. A wind turbine, comprising:

a tower;

a nacelle disposed on the tower;

a rotor extending from the nacelle and having a plurality of pitch adjustable rotor blades disposed thereon; and

a pitch actuating unit to determine a pitch modification signal to be applied to a pitch actuator for actuating the pitch of the pitch adjustable rotor blades; the pitch actuation unit being arranged to:

receive an edgewise load signal for each of the pitch adjustable rotor blades, the edgewise load signal being measured in a rotating reference frame;

apply an m-blade coordinate transformation to the edgewise load signal to transform the edgewise load signal to a whirling reference frame along a first and a second reference direction;

select a signal component of either the first or the second reference direction to obtain a selected signal component;

filter the selected signal component at either a backward whirling frequency or a forward whirling frequency to obtain a filtered signal component;

apply an inverse m-blade coordinate transformation to the filtered signal component to obtain the pitch modification signal, wherein only a whirling mode of either backward or forward whirling is targeted in obtaining the pitch modification signal; and

apply the pitch modification signal to the pitch actuator.

16. A computer program product comprising a non-transitory computer readable medium storing software code adapted to control a wind turbine when executed on a data processing system, the data processing system being adapted to perform, upon execution of the software code, an operation of actuating pitch of pitch adjustable rotor blades of a wind turbine, the wind turbine comprising a pitch actuator for actuating the pitch of the pitch adjustable rotor blades, the operation comprising:

receive an edgewise load signal for each of the pitch adjustable rotor blades, the edgewise load signal being measured in a rotating reference frame;

apply an m-blade coordinate transformation to the edgewise load signal to transform the edgewise load signal to a whirling reference frame along a first and a second reference direction;

select a signal component of either the first or the second reference direction to obtain a selected signal component;

filter the selected signal component at either a backward whirling frequency or a forward whirling frequency to obtain a filtered signal component;

apply an inverse m-blade coordinate transformation to the filtered signal component to obtain a pitch modification signal, wherein only a whirling mode of either backward or forward whirling is targeted in obtaining the pitch modification signal; and

apply the pitch modification signal to the pitch actuator.