Method and arrangement for damping of tower-oscillations

1. A method for damping tower-oscillations, comprising:

transforming a rotation into an electrical power by a generator located on top of a tower;

transforming the electrical power from AC to DC and back to AC by a converter; and

controlling a delivered electrical power by the converter using a power-reference-signal,

wherein a variable offset-signal is added to the power-reference-signal before the controlling is performed, and

wherein the variable power-offset-signal is based on a mean value of the power-reference-signal and a side-to-side-oscillation of the tower,

wherein the variable power-offset-signal is calculated by a multiplication of a first signal and a second signal,

wherein the first signal represents the mean value of the power-reference-signal, and

wherein the second signal represents the side-to-side-oscillation of the tower.

2. The method as claimed in claim 1 , wherein the variable power-offset-signal is saturated without clipping before the variable power-offset signal is added to the power-reference-signal.

3. The method as claimed in claim 2 , further comprising a soft-saturation which includes:

calculating a tower-oscillation-signal by a multiplication of the first signal with the second signal,

choosing a gain-factor to be multiplied with the tower-oscillation-signal,

reducing the gain-factor dynamically, if a peak-value of the tower-oscillation-signal reaches a predetermined saturation-limit, and

wherein the soft saturation is done to keep the variable power-offset-signal under the predetermined saturation-limit.

4. The method as claimed in claim 1 , wherein the mean value of the power-reference-signal is calculated using a chosen time interval.

5. The method as claimed in claim 4 , wherein the chosen time interval is a few seconds.

6. The method as claimed in claim 1 , wherein the side-to-side-oscillation of the tower is determined by measuring a vibration signal of the tower, by removing an interference signal from the vibration-signal, and by adjusting an amplitude of the interference-free signal.

7. The method as claimed in claim 1 , wherein tower-oscillations of a wind turbine are damped.

8. The method as claimed in claim 7 , wherein the wind turbine is damped using a same frequency as the tower oscillations with a phase shift of 180°.

9. The method as claimed in claim 7 , wherein a G-sensor is used to measure tower oscillations or tower vibrations.

10. The method as claimed in claim 8 , wherein the G-sensor is placed on top of the tower of the wind turbine or the G-sensor is placed within a nacelle of the wind-turbine.

11. An arrangement for the damping of tower-oscillations, comprising:

a generator, which transforms a rotation into electrical power;

a converter, which is connected to the generator and transform a variable-frequency AC power into a static-frequency AC power to match a frequency of a connected grid;

a power controller, which is connected to the converter; and

a summation-unit located between the power controller and the converter, the summation-unit adds a variable power-offset-signal to a power-reference signal before the power-reference signal is used to control the converter,

wherein the variable power-offset-signal is based on a mean value of the power-reference-signal and a side-to-side-oscillation of the tower,

wherein the power controller is connected with a first multiplier, and

wherein the first multiplier calculates the variable power-offset-signal by multiplying the mean value of the power-reference-signal with the side-to-side-oscillation of the tower.

12. The arrangement as claimed in claim 11 ,

wherein the power controller is connected to the first multiplier via a filtering unit, and

wherein the filtering unit calculates the mean value of the power-reference-signal using a predetermined time-interval.

13. The arrangement as claimed in claim 12 , wherein the predetermined time-interval is a few seconds.

14. The arrangement as claimed in claim 11 , further comprising:

an accelerometer, used to measure a vibration signal of the tower,

a unit for filtering and phase adjustment, connected with the accelerometer to remove an interfering signal from the measured vibration-signal,

a damp-gain-unit, connected with the unit for filtering and phase adjustment, the damp-unit gain adjusts an amplitude of the interference-free vibration-signal which represents the side-to-side-oscillation of the tower, and

wherein the damp-gain-unit is connected with the first multiplier.

15. The arrangement as claimed in claim 14 , wherein a G-sensor is used as the accelerometer.

16. The arrangement as claimed in claim 15 , wherein the G-sensor is placed on top of the tower of the wind turbine or the G-sensor is place within a nacelle of the wind turbine.

17. The arrangement as claimed in claim 11 , wherein a unit for saturation is located between the first multiplier and the summation-unit, the unit for saturation saturates the variable power-offset-signal without clipping before the variable power-offset-signal is used to control the converter.

18. The arrangement as claimed in claim 11 , wherein the tower is the tower of the wind-turbine or of the nacelle.