WIND TURBINE TOWER SYSTEM FOR SECOND NATURAL FREQUENCY MODIFICATION

Abstract

A method for second natural frequency wind turbine tower modification is provided. The method includes the steps of: a) determining the second natural frequency of the wind turbine; b) calculating the anti-node of the second natural frequency to determine the point of the wind turbine tower that suffers the greatest displacement during the second natural vibration mode; c) determining an height of the tower corresponding with the anti-node calculated in b); d) calculating a mass to be placed at the height of the tower to modify the second natural frequency considering that a heavier mass leads to a lower second natural frequency; e) placing the mass calculated in step d) at the height determined in step c).

Claims

1. A method for second natural frequency wind turbine tower modification wherein it comprises the steps of: a) determining the second natural frequency of the wind turbine; b) calculating the anti-node of the second natural frequency to determine the point of the wind turbine tower that suffers the greatest displacement during the second natural vibration mode; c) determining a height of the tower corresponding with the anti-node calculated in b); d) calculating a mass to be placed at said height of the tower to modify the second natural frequency considering that a heavier mass leads to a lower second natural frequency; and e) placing the mass calculated in step d) at the height determined in step c).

2. The method for second natural frequency wind turbine tower modification according to claim 1 wherein the step d) is calculated considering how much is going to be modified the second natural frequency of the wind turbine tower, the rigidity of the tower, the mass of the tower and the top head mass comprising the mass of the rotor and of the nacelle.)

3. The method for second natural frequency wind turbine tower modification according to claim 1 wherein the step b) is calculated analytically or with numerical simulation.

4. The method for second natural frequency wind turbine tower modification according to claim 1 the step d) is calculated with numerical simulation or according to Rayleigh method.

5. The method for second natural frequency wind turbine modification according to claim 1 wherein it further comprises a sub-step of placing a receptacle at the height determined in step c) and a sub-step of filling the receptacle with a mass as calculated in step d).

6. The method for second natural frequency wind turbine modification according to claim 5 wherein the receptacle is a sandbox and the mass which the receptacle is filled is sand.

7. The method for second natural frequency wind turbine modification according to claim 1 wherein step e) is done by welding the mass in the interior of the tower).

8. The method for second natural frequency wing turbine modification according to claim 1 wherein the wind turbine tower is manufactured in sections and step d) is performed placing the mass in the section of the tower corresponding with the specific location of the height of the tower determined in step c).

9. A wind turbine tower wherein it comprises a mass placed at a height of the tower corresponding to the anti-node of the second natural frequency said mass causing a modification of a previous wind turbine second natural frequency.

Description

BRIEF DESCRIPTION

[0027] Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

[0028] FIG. 1. shows a wind turbine tower with the system of the invention; and

[0029] FIG. 2. shows a zoom view of the section of the tower in which the mass is added.

DETAILED DESCRIPTION

[0030] A method for second natural frequency wind turbine tower modification to avoid the collisions between the second vibration modes with possible exciting frequencies of the wind turbine is proposed.

[0031] The method comprises the following steps:

a) determining the second natural frequency of the wind turbine;
b) calculating the anti-node of the second natural frequency to determine the point of the wind turbine tower (1) that remains invariant during the second natural vibration mode; [0032] c) determining a height (H) of the tower (1) corresponding with the anti-node calculated in b); [0033] d) calculating a mass (2) to be placed at said height (H) of the tower to modify the second natural frequency considering that a heavier mass (2) leads to a lower second natural frequency; [0034] e) placing the mass (2) calculated in d) at the height (H) determined in c).

[0035] The step of determining the second natural frequency of the wind turbine considers the frequencies of the wind turbine with all its components. It is to be considered the whole wind turbine with the rotor and nacelle and not only the frequencies of the tower (1). This is essential since the wind turbine will comprise all its components when completely installed and the possible collisions with the second natural frequency will appear in that situation.

[0036] The anti-node is a point of the tower (1) which is placed between two invariant nodes, that is to say, between two points that do not displace. The anti-node to be found in step b) is to be found approximately at a 0.6*H (around the 60% of the height of the tower). The exact location can be obtained by the deflection of the second vibration modes. The anti-node can be determined analytically or with numerical simulation.

[0037] When the anti-node has been calculated, the next step is to determine the height (H) of the tower (1) in which the anti-node has been found. Said height (H) corresponds to the distance between the anti-node and the base of the tower (1).

[0038] Step d) of calculating a mass (2) to be placed is done considering how much is going to be modified the second natural frequency of the wind turbine tower (1), the rigidity of the tower (1), the mass of the tower (1) and the top head mass comprising the mass of the rotor and of the nacelle.

[0039] As explained above, the rotor and nacelle have to be considered in the method because both elements are present when the wind turbine is working and thus, they are present when the possible collisions between vibration modes appear.

[0040] The step d) of calculating the mass (2) to be placed in the anti-node is done with numerical simulation or according to Rayleigh method. The determination of the mass (2) can be done with iterative calculations, for example, it can be simulated a 1 ton mass (2) placed in the anti-node and the natural frequencies of the wind turbine are calculated.

[0041] Then, considering that placing a higher mass (2) leads to a decrease in the second natural frequency, the simulation can be repeated with a higher or a lower mass (2) according to the modification to be made. When a lower second natural frequency is to be achieved, the mass has to be increased.

[0042] In an exemplary embodiment of the invention the method further comprises a sub-step of placing a receptacle (3) at the height (H) determined in step c) and a sub-step of filling the receptacle (3) with a mass (2) as calculated in step d).

[0043] In FIG. 1 it can be appreciated a wind turbine tower (1) section with a receptacle (3), placed at the height (H) corresponding to the anti-node. Said figure represents an embodiment of the invention in which the receptacle (3) is a sandbox and the mass (2) with which the receptacle (3) is filled is sand. It can be appreciated also a sand pump (4) with which the sand is pumped to the receptacle (3).

[0044] In another embodiment of the invention step e) of placing the mass (2) at the height corresponding with the anti-node is done by welding the mass (2) in the interior of the tower (1).

[0045] When the method is going to be performed in a wind turbine tower that is manufactured in sections (S), step d) is performed placing the mass (2) in the section (S) of the tower (1) corresponding with the specific location of the height (H) of the tower determined in step c). In FIG. 2 it can be appreciated a zoom view of the section of the wind turbine tower in which the receptacle (3) is placed.

[0046] A further aspect relates to provide a wind turbine tower (1) that comprises a mass (2) placed at a height (H) of the tower corresponding to the anti-node of the second natural frequency. The mass (2) comprised by the tower being a mass (2) causing a desired modification of a previous wind turbine second natural frequency.

[0047] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0048] For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.