Tower vibration damper

1. A wind turbine tower comprising a tower vibration damper, the tower vibration damper comprising

a tuned mass damper comprising:

a pendulum structure suspended inside the wind turbine tower,

a chamber comprising an outer boundary, the chamber connecting a friction media to the pendulum structure, and

a suspension arrangement suspending the pendulum structure at least partially inside the chamber such that the pendulum structure is allowed to displace from a neutral position towards the outer boundary of the chamber, and

an impact damper comprising:

one or more impact damping units laterally positioned between the pendulum structure and the outer boundary of the chamber so as to provide an opening either between the pendulum structure and the one or more impact units or between the outer boundary of the chamber and the one or more impact units when the pendulum structure is in the neutral position,

wherein the impact damper is adapted to be deactivated in order to prevent collision of the outer boundary and the pendulum structure via the impact damper.

2. The wind turbine tower according to claim 1 , wherein the friction media is a damping liquid, and the outer boundary of the chamber has a bottom part extending between wall parts of the outer boundary, the chamber holding the damping liquid into which the pendulum structure is at least partly immersed.

3. The wind turbine tower of claim 1 , wherein the pendulum structure includes an upper rim and a lower rim, the one or more impact damping units being position between the upper rim and the lower rim.

4. The wind turbine tower according to claim 1 , wherein the impact damper is adapted to be deactivated by removing or relocating at least one of the impact damper units, changing the shape and/or dimensions of at least one of the impact damper units, or by rotating the pendulum structure relative to the outer boundary of the chamber.

5. The wind turbine tower according to claim 1 , wherein the one or more impact damping units are detachably secured to the pendulum structure.

6. The wind turbine tower according to claim 1 , wherein the one or more impact damping units are suspended from an upper rim of the pendulum structure.

7. The wind turbine tower according to claim 1 , wherein the number of impact damping units is larger than two.

8. The wind turbine tower according to claim 7 , wherein the impact damping units are evenly distributed around a centre axis of the pendulum structure and/or around a centre axis of the wind turbine tower.

9. The wind turbine tower according to claim 1 , wherein each impact damping unit comprises one or more resilient fenders each having a durometer hardness of at least Shore 50A.

10. The wind turbine tower according to claim 9 , wherein each impact damping unit comprises oppositely arranged first and second resilient fenders, wherein the first resilient fender faces the pendulum structure, and wherein the second resilient fender faces the outer boundary of the chamber.

11. The wind turbine tower according to claim 1 , wherein the outer boundary of the chamber is formed by a part of a wind turbine tower wall.

12. The wind turbine tower according to claim 1 , wherein the suspension arrangement comprises

a plurality of wires for suspending the pendulum structure, and

tuning means configured for adjusting the natural frequency of the suspended pendulum structure, the tuning means comprising, for each of said plurality of wires, a clamp secured to the wind turbine tower at one end and to the wire at the other end, wherein the securing of the clamp is configured such that the clamp is movable along a longitudinal direction of the wire.

13. A method of damping tower vibrations in a wind turbine tower, the method comprising the steps of

providing a tuned mass damper comprising:

a pendulum structure suspended inside the wind turbine tower,

a chamber comprising an outer boundary, the chamber connecting a friction media to the pendulum structure, and the outer boundary of the chamber has a bottom part extending between wall parts of the outer boundary, the chamber holding a damping liquid into which the pendulum structure is at least partly immersed, and

a suspension arrangement, for suspending the pendulum structure at least partially inside the chamber such that the pendulum structure is allowed to displace from a neutral position towards the outer boundary of the chamber,

during assembly, storage, transport and/or installation of the wind turbine tower, providing an impact damper comprising:

one or more impact damping units laterally positioned between the pendulum structure and the outer boundary of the chamber so as to create an opening either between the pendulum structure and the one or more impact units or between the outer boundary of the chamber and the one or more impact units when the pendulum is in the neutral position, wherein the impact damper is adapted to be deactivated in order to prevent collision of the outer boundary and the pendulum structure via the impact damper, and

activating the impact damper during assembly, storage, transport and/or installation of the wind turbine tower.

14. The method according to claim 13 , further comprising the step of dismantling the impact damper when the wind turbine tower is no longer during assembly, storage, transport and/or installation.