A wind farm controlling a wind turbine with reduced tower oscillations

Abstract

A method for controlling at least one wind turbine (103, 203) having a wind turbine tower (102, 202) and connected to a public power grid (125), the method comprising: dynamically filtering the active power of the public power grid (109, 209) according to a damping signal (111, 211) provided for counteracting the oscillations of the wind turbine tower (102, 202); and controlling the wind turbine (103, 203) according to the filtered active power of the public power grid (119).

Description

Technical Field

[0001] The present invention relates to a method and an arrangement for controlling at least one wind turbine having a wind turbine tower and connected to a public power grid. The present invention further relates to a wind turbine having said arrangement. Background Art

[0002] A wind turbine may include a wind turbine tower and a nacelle mounted on top of the wind turbine tower, the nacelle accommodating a main rotor to which a plurality of rotor blades are connected. The main rotor is mechanically coupled to a generator to generate electrical energy when the rotating shaft rotates.

[0003] During operation of the wind turbine, tower oscillations may occur, including fore-and-aft oscillations of the wind turbine tower. Conventionally, a method has been proposed to attenuate tower oscillations by modulating the active power production by means of the tower characteristic frequency of the tower oscillations. However, it has been observed that when the wind turbine is controlled and limited by a wind farm-level active power controller, the wind turbine will introduce an error in the active power production caused by tower damping. Thereby, control instability may be caused, or the stability of the public power grid may be impaired. Since the tower characteristic frequencies (e.g., 0.21 Hz to 0.25 Hz) are within the bandwidth of the wind farm-level active power feedback controller, the wind farm-level controller will attempt to cancel the tower damping, thereby causing more fatigue loads on the wind turbine and possibly pushing the tower damping controller into its operating range.

[0004] Therefore, there may be a need for a method and an arrangement for controlling at least one wind turbine such that errors in active power production can be reduced while effectively attenuating tower oscillations (therefore, the attenuation of tower oscillations is effective). In addition, there may be a need for a control method in which grid stability can be improved while attenuating wind turbine tower oscillations. Summary of the Invention

[0005] This need can be met by the subject matter of the independent claims. The dependent claims describe advantageous embodiments of the present invention.

[0006] According to an embodiment of the present invention, there is provided a method for controlling at least one wind turbine having a wind turbine tower and connected to a public power grid, the method comprising: dynamically filtering the public power grid active power according to a damping signal provided for canceling the oscillations of the wind turbine tower; and controlling the wind turbine according to the filtered public power grid active power.

[0007] The method can be implemented partially or fully by a wind turbine controller and / or partially or fully by a wind farm controller. Embodiments of the invention operate at the wind turbine level, and other embodiments of the invention operate at the wind farm level. The active power of the public power grid can be obtained, for example, by measuring the active power provided by the wind turbine or the entire wind farm to the public power grid. Thus, measuring devices can be utilized, for example, installed at or near the point of common coupling or installed between the wind farm and the public power grid.

[0008] Controlling the wind turbine can include supplying corresponding control signals to the converter of the wind turbine. The converter can include a plurality of controllable switches, such as IGBTs, where the switch pairs of each phase can be provided in series. The converter can include an AC-DC section, a DC link, and a DC-AC section. The converter can be connected between the generator of the wind turbine and the output terminals of the wind turbine or the wind turbine transformer.

[0009] The damping signal is designed to counteract the oscillations of the wind turbine tower, particularly the fore-aft oscillations of the wind turbine tower. The damping signal can define a supplementary active power signal that is added, for example, to a preliminary power reference at the wind turbine level. The damping signal can counteract one or more characteristic oscillations of the wind turbine tower. Thus, for a single wind turbine, the damping signal can include one or more frequency components or frequency ranges and have corresponding or associated amplitude information. If the method is implemented at the wind farm level, the damping signal can include a combination of individual wind turbine damping signals. The damping signal can include frequency components within one or more frequency ranges.

[0010] The filtered active power of the public power grid may reduce the amplitude of the frequency components included in the damping signal. When filtering the active power of the public power grid according to the damping signal, the damping signal is not regarded as an error that the controller needs to counteract in further control methods. Thus, the output of the wind turbine can better conform to the externally provided active power reference. To control the wind turbine, one or more controllers can be provided at the wind farm level and / or the wind turbine level.

[0011] According to an embodiment of the invention, controlling the wind turbine includes: supplying a first active power difference between an externally defined active power reference and the filtered active power of the public power grid to an active power controller; and outputting a preliminary power reference from the active power controller to at least one wind turbine.

[0012] When the method is implemented at the wind turbine level, the externally defined active power reference may refer to the desired active power output of an individual or single wind turbine. When the method is implemented at the wind farm level, the externally defined active power reference may refer to the desired wind farm active power output of the entire wind farm.

[0013] The active power controller derives a preliminary power reference based on the supplied first active power difference. The preliminary power reference is not ultimately used to control the wind turbine, but at least a damping signal of the corresponding wind turbine is added to the preliminary power reference. Thereby, effective attenuation of the tower oscillation of the corresponding wind turbine is achieved while conforming to the externally defined active power reference in an improved manner.

[0014] In this embodiment, the active power controller has been provided with an input difference signal from which the frequency components of the damping signal are reduced or even removed.

[0015] According to another embodiment of the present invention, controlling a wind turbine includes: supplying a second active power difference between an externally defined active power reference and the utility grid active power to an active power controller; outputting a control signal by the active power controller, and subtracting the filtered utility grid active power from the control signal to derive a preliminary power reference supplied to at least one wind turbine.

[0016] As can be conventionally known, the second active power difference is derived as the difference between the externally defined active power reference and the utility grid active power (i.e., the unfiltered signal). According to this embodiment, consideration of the damping signal is implemented at the output stage of the active power controller. Thereby, cancellation of the desired damping action is also avoided.

[0017] According to an embodiment of the present invention, the active power controller is configured to derive a preliminary control signal such as to reduce the active power difference. The active power controller may include, for example, a PID or PI controller. The active power controller may have been tuned to derive a corresponding output such that the input difference becomes smaller and smaller. Thereby, conventionally known controllers can be utilized.

[0018] According to an embodiment of the present invention, dynamically filtering the utility grid active power according to the damping signal includes using a band-stop filter that reduces the amplitude of the frequency components of the damping signal in the utility grid active power to between 0% and 10%, wherein the damping signal particularly includes frequency components in the range between 0.1 Hz and 1 Hz, especially between 0.2 Hz and 0.3 Hz.

[0019] A band-stop filter can be configured to filter out or at least reduce the frequency components of a corresponding input signal in one or more frequency ranges. The one or more frequency ranges can be determined based on a damping signal to cover those frequencies included in the damping signal. The ranges can define the corresponding bandwidths of the filter. The filter can include several filter components, each filter component for each frequency range. Filter out the corresponding frequency ranges in the active power of the utility grid. Frequency components in the range between 0.1 and 1 Hz are typical for tower oscillations. Thus, effectively filter out the damping signal in the filtered active power of the utility grid that cancels the corresponding tower oscillations. Thus, with respect to a better compliance of the actual active power output of the wind turbine with an externally defined active power reference, the method can still be improved.

[0020] According to an embodiment of the invention, the method further comprises generating, by a damping controller, a damping signal based on a tower oscillation indication signal to at least cancel the fore-and-aft oscillations of the wind turbine tower, wherein the tower oscillation indication signal is particularly measured by an accelerometer.

[0021] The tower oscillation indication signal can indicate the frequency and amplitude of one or more tower oscillations. The damping signal can be derived to be phase-shifted from the tower oscillation indication signal in order to cancel the tower oscillations. For example, when the tower moves fore-and-aft, during the time span when the tower moves backward, the load on the generator can be reduced (particularly in terms of reducing the active power output). When the wind turbine tower moves forward or is in the front position of the fore-and-aft oscillation, it can be desirable to increase the load (involving increasing the active power output of the wind turbine). In this canceling manner, the damping signal can involve modulating the active power output of the wind turbine. The accelerometer can be arranged at the nacelle or on the wind turbine tower. Thus, the implementation of the method is simplified, including using conventionally available equipment. Also, the damping controller can include a PID or PI controller.

[0022] According to an embodiment of the invention, the method further comprises: adding a corresponding damping signal to a preliminary power reference at each wind turbine to obtain a wind turbine active power reference signal; and controlling the wind turbine using the wind turbine active power reference signal.

[0023] The preliminary power reference may not be designed to attenuate tower oscillations. In contrast, the corresponding damping signal is designed to cancel tower oscillations. Thus, adding the corresponding damping signal to the preliminary power reference results in the wind turbine active power reference signal effectively causing attenuation of the tower oscillations and compliance with the desired or externally defined active power reference.

[0024] According to an embodiment of the invention, for other control purposes such as torque ripple reduction, other supplementary control signals can be added.

[0025] According to an embodiment of the invention, the method further comprises: filtering the filtered grid active power using a transition filter triggered by a detected transient in the grid active power of the public power grid, and the output of the transition filter is derived as a weighted sum of the filtered grid active power and the (unfiltered) grid active power.

[0026] The transition filter can be provided to reduce the fluctuations that may occur after large transients in the measured grid active power. Thus, the filtered grid active power and the (unfiltered) grid active power are added in a weighted manner to obtain a composite signal, and the composite signal can effectively reduce the fluctuations caused by large changes in the measured grid active power. The transient can be defined according to a specific application. For example, a transient can be defined when the grid active power of the public power grid changes by more than a threshold within a specific time span. The threshold and the length of the time span can be adjusted according to a specific application. Thus, the method can still be improved.

[0027] According to an embodiment of the invention, the sum of the weighting factors is 1 and changes with the time elapsed since the occurrence of the transient. Thus, for each transient, the transition filter is triggered again, and the weighted sum is calculated at the beginning of the occurrence of the transient. The longer the time after the occurrence of the transient, the higher the weight of the filtered grid active power. Thus, smooth, reliable and safe control of the wind turbine can be achieved.

[0028] According to an embodiment of the invention, the weighting factor of the grid active power of the public power grid is the highest at the time of the occurrence of the transient (or immediately or shortly after the occurrence of the transient), and then decreases to zero over time, particularly according to an exponential function. Thus, an effective implementation scheme is achieved. In addition, since the output of the transition filter gets closer and closer to the filtered grid active power used when no transient is detected in the grid active power of the public power grid, a smooth transition to the situation where there is no transient is achieved.

[0029] According to an embodiment of the invention, a transient is detected if the grid active power of the public power grid changes by more than an active power threshold, particularly between 5% and 20% of the rated power, during a time span, particularly between 0.1 second and 10 seconds. According to other embodiments, the threshold can be set to different values.

[0030] According to another embodiment of the invention, the method is implemented at a wind farm level, wherein the method is adapted to control a plurality of wind turbines of a wind farm, wherein the externally defined active power reference is an externally defined wind farm active power reference; wherein the damping signal is a (sum) signal composed of individual damping signals of all the wind turbines, each damping signal being provided to counteract the oscillations of the respective wind turbine tower, and wherein the preliminary active power reference defines a preliminary wind farm active power reference.

[0031] Herein, the damping signal can be the sum of the individual damping signals of all the wind turbines. This sum of the individual damping signals can include a plurality of frequency components that can be in one or more frequency ranges. The corresponding filtering may involve reducing the amplitude of the frequencies in this one or more frequency ranges.

[0032] However, according to another embodiment, the method can be implemented individually for each wind turbine in the wind farm. In this case, the externally defined active power reference can have the meaning of an externally defined wind turbine active power reference. The damping signal can be the damping signal of the wind turbine under consideration. The preliminary control signal can define a preliminary wind turbine active power reference. The grid active power can relate to the actual active power output of the wind turbine under consideration.

[0033] According to this embodiment, the method can further include dividing the preliminary wind farm active power reference into a plurality of preliminary power references for all the wind turbines.

[0034] Thus, each wind turbine can receive a corresponding preliminary power reference that does not interfere with the damping controller, which can be included in and operate within the wind turbine to attenuate tower oscillations.

[0035] It should be understood that the features disclosed, described, explained, or applied to the method of controlling at least one wind turbine having a wind turbine tower, either individually or in any combination, are also applicable, either individually or in any combination, to an arrangement for controlling at least one wind turbine having a wind turbine tower according to an embodiment of the invention, and vice versa.

[0036] According to an embodiment of the invention, there is provided an arrangement for controlling at least one wind turbine having a wind turbine tower and connected to a public power grid, the arrangement comprising: a band-stop filter adapted to dynamically filter the grid active power according to a damping signal provided to counteract the oscillations of the wind turbine tower; and a controller adapted to control the wind turbine according to the filtered grid active power.

[0037] The described arrangement structure may be partially or fully included in a wind farm controller and / or a wind turbine controller. The arrangement structure may include hardware components and software components.

[0038] According to an embodiment of the present invention, there is also provided a wind turbine, which includes a wind turbine tower and an arrangement structure according to the foregoing embodiment.

[0039] The above-defined aspects and other aspects of the present invention are apparent from and will be explained with reference to the examples of the embodiments described hereinafter. The present invention will be described in more detail hereinafter with reference to the examples of the embodiments, but the present invention is not limited thereto. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Embodiments of the present invention will now be described with reference to the drawings. The present invention is not limited to the embodiments illustrated or described.

[0041] Figure 1 Schematically illustrated is a wind farm including an arrangement structure for controlling at least one wind turbine having a wind turbine tower according to an embodiment of the present invention;

[0042] Figure 2 Schematically illustrated is a wind turbine according to an embodiment of the present invention;

[0043] Figure 3 、 Figure 4 and Figure 5 Illustrated are graphs showing the performance of control methods according to the present invention and the prior art. DETAILED DESCRIPTION

[0044] The illustrations in the drawings are in schematic form. It should be noted that in Figure 1 and Figure 2 elements that are similar or identical in structure and / or function are provided with the same reference numerals or reference numerals that differ only in the first digit. The description of elements not described in one embodiment may be taken from the description of this element with respect to another embodiment.

[0045] Schematically illustrated in Figure 1 is a wind farm 100 including a plurality of wind turbines 103 and a wind farm-level controller 105 as an implementation of an arrangement structure for controlling at least one wind turbine having a wind turbine tower according to an embodiment of the present invention. The arrangement structure 105 includes a band-stop filter 107, which is adapted to dynamically filter the active power 109 of the public power grid according to a damping signal 111 provided for canceling the oscillations of the respective wind turbine towers of all the wind turbines 103. The damping signal 111 is the sum of the individual wind turbine damping signals 110 of the individual wind turbines.

[0046] Arrangement structure 105 further includes a control device, in particular including an active power controller 113 and an active power scheduler 115, which represent an implementation of controller 117. Controller 117 is adapted to control wind turbine 103 according to the filtered utility grid active power 119 output by band-stop filter 107 based on the measured grid active power 109 and damping signal 111. Wind farm 100 further includes a wind turbine transformer 121 and a grid measurement device 123 for measuring the grid active power 109. During normal operation, wind turbine 103 supplies electrical energy to the utility grid 125 via the wind farm transformer 121.

[0047] In the embodiment illustrated as Figure 1 arrangement structure 105 further includes a transition filter 127, which can be triggered by a detected transient of the utility grid active power. Therefore, the transition filter also receives the utility grid active power 109 and detects whether a transient has occurred. The transient may involve a large change in the utility grid active power 109 within a specific time window. Transition filter 109 further receives the filtered utility grid active power 119 output by band-stop filter 107 as an input. Transition filter 127 derives a weighted sum of the filtered utility grid active power 119 and the (unfiltered) utility grid active power 109 and outputs the weighted sum 129 (e.g., as the controller grid active power), thereby supplying it to difference element 131. Thus, difference element 131 calculates a first active power difference 133 between an externally defined active power reference 135 and the weighted sum 129 output by transition filter 127.

[0048] The first active power difference 133 is supplied to the active power controller 113, and the active power controller 113 outputs a preliminary power reference 136 based on it, here a preliminary wind farm power reference. This preliminary wind farm power reference 136 is supplied to the active power scheduler 115, and the active power scheduler 115 divides the wind farm active power reference 136 into a plurality of preliminary power references 137 for all wind turbines.

[0049] As Figure 1 illustrated, the wind farm level controller 105 requests the tower damping frequency and amplitude (e.g., as damping signal 111) from the turbine. The resulting frequency and bandwidth are calculated online in the wind farm level controller 105 and used in the dynamic band-stop filter 107 to remove the resulting tower damping frequency from the measured active power fed back to the wind farm level active power feedback controller 113.

[0050] If there are large transients in the measured grid active power, the band-stop filter 107 may introduce fluctuations into the filtered grid active power 119. Therefore, a transition filter 127 is introduced to combine the filtered grid active power and the measured grid active power (signals 109 and 119) with different weights wA and wB such that the weighted sum 129 (also referred to as the controller grid active power) is given by:

[0051] Controller grid active power = wA * Filtered grid active power (signal 119) + wB * Grid active power (signal 109)

[0052] where

[0053] wA + wB = 1

[0054] When a large change in the measured grid active power 9 is detected, wB = 1, and wA = 0, and wB will tend to 0 through a mathematical function such as: wB = exp(−λ*t)

[0055] Therefore, the weighted sum 29 (controller grid active power) is given by:

[0056] Controller grid active power = (1 - exp(−λ*t)) * Filtered grid active power + exp(−λ*t) * Grid active power

[0057] In this context, t will increase over time (since a transient occurs) and λ is chosen such that the minimum fluctuations at the transient response occur and the settling time of the filter is within a reasonable time.

[0058] Figure 2 Schematically illustrates a wind turbine 203 according to another embodiment, in which an arrangement structure 205 is included in the wind turbine 203. The arrangement structure 205 receives the measured utility grid active power 209. In addition, the controller 205 receives a damping signal 211 from a damping controller 239 also included in the wind turbine 203. The arrangement structure 205 is similar to Figure 1 the arrangement structure 105 illustrated in Figure 1 The band-stop filter derives the filtered active utility grid power from the utility grid active power 209 and the damping signal 211. Then the filtered active utility grid power is subtracted from the utility grid active power and supplied to an active power controller (similar to

[0059] The damping signal 211 is added to the preliminary power reference 237. The output of the adder element 241 is the active power reference signal 243 of the wind turbine. This signal is supplied to the converter 245, which is connected to the generator 247, which is connected to the main rotating shaft 249, to which a plurality of rotor blades 251 are attached. Thus, the active power reference signal 243 is adapted to effectively attenuate the tower oscillations of the tower 202 of the wind turbine 203 and to comply with the external defined active power reference 209 that defines the desired active power of the active power output of the wind turbine.

[0060] The arrangement 205 receives the externally defined wind turbine active power reference (embodiment of the externally defined active power reference) 235. The damping controller 239 receives the oscillation indication signal 246 from an accelerometer mounted in the nacelle 210.

[0061] Figure 3 and Figure 4 respectively indicate the measurement results according to the prior art and according to the present invention or an embodiment of the present invention, wherein the abscissa 3 indicates the number of samples or time, and the ordinate 4 indicates the power output (in megawatts).

[0062] Figure 3 shows the wind turbine active power reference 35 derived from a conventional wind farm controller of a conventional control method and the turbine active power output 38 output by the corresponding wind turbine. As can be seen from Figure 3 it can be seen that the wind turbine active power reference 35 includes an oscillatory behavior having the same frequency as the active power output 38 but with a phase shift. Thus, the two signals cancel each other out.

[0063] In the Figure 4 embodiment according to the present invention, the wind turbine active power reference, in particular the preliminary active power reference 37, and the actual power output 38 of the wind turbine are also shown. It can be seen that the preliminary active power reference 37 does not include Figure 3 the oscillatory behavior as shown in the conventional curve illustrated in

[0064] Figure 5 A graph is illustrated having an abscissa 3 indicating the number of samples or time and an ordinate 4 indicating power. The curve 60 indicates the wind turbine active power reference, and the curve 62 indicates the turbine active power. These data have been obtained after the reset of the transition filter 27. It can be seen that the turbine active power reference 60 decays rapidly to a small value. Thus, fluctuations are effectively avoided or even reduced.

[0065] Embodiments of the present invention may include the following features or achieve the following advantages:

[0066] - Use a wind farm-level controller using the tower damping frequency and amplitude from the turbine to avoid canceling the tower damping, which will allow the tower damping controller to operate within its full dynamic range to cancel tower oscillations, which will cause less fatigue on the turbine and extend the life of the turbine;

[0067] - The wind farm-level active power controller will not experience additional errors caused by tower damping on turbines that cannot be controlled at the wind farm level, so a wind farm-level active power controller with a narrower operating range can be designed, which will allow less active power overshoot, which will improve compliance with grid codes and protection of transmission equipment such as high-voltage transformers.

[0068] It should be noted that the term "comprising" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. Moreover, elements described in connection with different embodiments may be combined. It should also be noted that the reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

1. A method for controlling at least one wind turbine (103, 203) having a wind turbine tower (102, 202) and connected to a public power grid (125), the method comprising: dynamically filtering the active power of the public power grid (109, 209) according to a damping signal (111, 211) provided for counteracting the oscillations of the wind turbine tower (102, 202); and controlling the wind turbine (103, 203) according to the filtered active power of the public power grid (119).

2. The method according to claim 1, wherein, Controlling the wind turbine includes: supplying a first active power difference (133) between an externally defined active power reference and the filtered active power of the public power grid (119) to an active power controller (113); outputting a preliminary power reference to at least one wind turbine by the active power controller (113).

3. The method according to claim 1, wherein, Controlling the wind turbine includes: supplying a second active power difference between an externally defined active power reference and the active power of the public power grid (209) to an active power controller; outputting a control signal by the active power controller, and subtracting the filtered active power of the public power grid from the control signal to derive a preliminary power reference supplied to at least one wind turbine.

4. The method according to claim 2, wherein, The active power controller (113) is configured to derive a preliminary power reference so as to reduce the first active power difference (133).

5. The method according to any one of claims 1 to 3, wherein Dynamically filtering the active power of the public power grid according to the damping signal includes using a band-stop filter (107), which reduces the amplitude of the frequency components of the damping signal (111) in the active power of the public power grid (109) to between 0% and 10%, wherein the damping signal (111) includes frequency components in the range between 0.1 Hz and 1 Hz.

6. The method according to any one of claims 1 to 3, wherein Dynamically filtering the active power of the public power grid according to the damping signal includes using a band-stop filter (107), which reduces the amplitude of the frequency components of the damping signal (111) in the active power of the public power grid (109) to between 0% and 10%, wherein the damping signal (111) includes frequency components in the range between 0.2 Hz and 0.3 Hz.

7. The method according to any one of claims 1 to 3, further comprising: generating the damping signal (211) by a damping controller (239) based on a tower oscillation indication signal (246) to at least counteract the front-back oscillations of the wind turbine tower, wherein the tower oscillation indication signal (246) is measured by an accelerometer (248).

8. The method according to any one of claims 2 and 3, further comprising: adding (241) a corresponding damping signal (211) to the preliminary power reference at each wind turbine (203) to obtain a wind turbine active power reference signal (243); controlling the wind turbine (203) using the wind turbine active power reference signal (243).

9. The method according to any one of claims 1 to 3, further comprising: filtering the filtered grid active power (119) using a transition filter (127) triggered by a detected transient in the grid active power (109), the output (129) of the transition filter being derived as a weighted sum of the filtered grid active power (119) and the grid active power (109).

10. The method according to claim 9, wherein, The sum of the weighting factors is 1 and changes with the time elapsed since the transient occurred.

11. The method according to claim 10, wherein, The weighting factor of the grid active power (109) is highest at the time of the transient and then decreases to zero over time according to an exponential function.

12. The method according to claim 9, wherein A transient is detected if the grid active power changes by more than an active power threshold between 5% and 20% of the rated power during a time span between 0.1 seconds and 10 seconds.

13. The method according to any one of claims 2 and 3, Among them, The method is adapted to control a plurality of wind turbines (103) of a wind farm (1), wherein the externally defined active power reference is an externally defined wind farm active power reference (135); wherein the damping signal (111) is a signal composed of individual damping signals (110) of all wind turbines, each damping signal being provided to counteract the oscillation of the corresponding wind turbine tower, wherein the preliminary active power reference defines a preliminary wind farm active power reference.

14. The method according to claim 13, further comprising: dividing the preliminary wind farm active power reference into a plurality of preliminary power references for all wind turbines (103).

15. An arrangement (105, 205) for controlling at least one wind turbine having a wind turbine tower and connected to a public power grid, the arrangement comprising: a band-stop filter (107) adapted to dynamically filter the grid active power (109) according to a damping signal (111) provided to counteract the oscillation of the wind turbine tower; and a controller (113) adapted to control the wind turbine according to the filtered grid active power.

16. A wind turbine (203), comprising: a wind turbine tower (202); and the arrangement (205) according to claim 15.

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