DK201470481A1 - Improvements relating to wind turbine operation - Google Patents

Abstract

A wind turbine system comprising a tower, an electrical generator operatively coupled to a rotor having a plurality of blades, and a control system configured to control a rotor blade pitch adjustment means and a generator torque control means. The control system includes an active damping means configured to apply damping control inputs to one or both of the blade pitch adjustment means and the generator torque control means whilst the rotational frequency of the rotor coincides with the natural frequency of the tower.

Description

Improvements Relating to Wind Turbine Operation

Technical field

The present invention relates to a wind turbine system and also to a method of operating a wind turbine system to reduce structural loading on structural components thereof.

Background to the invention

Wind turbines generally take the form of tall slender structures comprising a nacelle mounted on top of a tower, the nacelle carrying the rotor of the wind turbine. As with all tall slender structures, the tower has a tendency to sway in use. The tower will oscillate in accordance with its natural frequency or ‘eigenfrequency’ that is determined largely by features of the tower such as its height, diameter, material of fabrication, internal structure, torsional stiffness, nacelle mass and so on.

The oscillations of the tower generate significant stresses through the tower that, over time, can affect its structural integrity. So, it is an important design consideration to avoid exciting the tower with forces coinciding with its natural frequency or ‘eigenfrequency’.

One significant source of excitation for a wind turbine is the dynamic load generated due to the motion of the rotor mounted to the nacelle of the wind turbine. Wind turbines are sometimes designed to operate most efficiently when the rotor spins at a single predetermined rotational speed or angular frequency or, more commonly, within a predetermined rotational speed range. In the art this is referred to as the 1P frequency. In comparatively small wind turbines, the 1P frequency tends to be significantly lower than the eigenfrequency of the tower. However, wind turbines are being designed ever larger due to the commercial and political incentives to increase energy production from renewable resources and so, as the overall size of wind turbines increase, the tower eigenfrequency tends to reduce towards the rotor 1P frequency. This can result in tower damage since it can lead to the tower being excited at or near to its eigenfrequency.

One approach is to optimise the strength of the tower so that it is better able to withstand the stresses exerted on it by the rotor. Often this involves adding strengthen structures to the tower, thereby increasing its mass, or by increasing the tower diameter. In effect, engineering the tower in this way changes the eigenfrequency of the tower so that it does not coincide with, and is spaced from, the 1P frequency of the rotor and generation equipment. However, the downside of this approach is the tower tends to be heavier, more complex, more difficult to install and more costly.

It is against this background that the invention has been devised.

Summary of the invention

In a first aspect, the invention provides a wind turbine system comprising a tower, an electrical generator operatively coupled to a rotor having a plurality of blades, and a control system configured to control a rotor blade pitch adjustment means and a generator torque control means, wherein the control system includes an active damping means configured to apply damping control inputs to one or both of the blade pitch adjustment means and the generator torque control means whilst the rotational frequency of the rotor coincides with the natural frequency of the tower.

The invention extends to, and therefore embraces, a method of controlling the operation of a wind turbine system including a tower, a rotor having a plurality of blades, a control system, an electrical generator coupled to the rotor, rotor blade pitch adjustment means and a generator torque control means, the method comprising: determining the natural frequency of the tower; determining the rotational frequency of the rotor; determining that the rotation frequency of the rotor coincides with the natural frequency of the tower and, in response, applying damping control inputs to one or both of the rotor blade pitch adjustment means and the generator torque control means.

Beneficially, the invention enables a less restrictive design process to be applied to wind turbines since there is less importance on designing the tower in such a way that its natural frequency is spaced by a certain margin from the rated speed of the wind turbine. This reduces the design and manufacture costs for wind turbines which improves their cost effectiveness and, ultimately, is a factor that applies a downward pressure on energy production cost.

The active damping means may apply the damping control inputs in circumstances where the rotational frequency of the rotor is within a predetermined range of the natural frequency of the tower. The predetermined range may be defined as within 20% of the tower natural frequency and, more preferably, within 10% of the tower natural frequency.

The natural frequency of the tower may coincide with a rated operational speed of the wind turbine. Thus, the system is designed for long term operation at the rated speed of the wind turbine.

The control system may be configured to apply a modulated torque demand signal to control the generator in order to damp oscillatory movement of the tower in a direction transverse to the rotor axis.

Further, the control system may be configured to apply a modulated collective pitch adjustment signal to the rotor blade pitch adjustment means to control the pitch of the blade collectively in order to damp oscillatory movement of the tower in a direction substantially in line with the rotor axis.

Still further, the control system may be configured to apply a modulated pitch adjustment signal associated with each of the plurality of blades of the wind turbine system to control the pitch of each of the blades individually in order to damp oscillatory movement of the tower in a direction transverse to the rotor axis.

In running the comparison between the rotor speed and the tower natural frequency, the control means may be configured to compare the rotational frequency of the rotor to a predetermined value of the natural frequency of the tower. Alternatively, the control means may be configured to compare the rotational frequency of the rotor to a value of the natural frequency of the tower that is determined during operation of the wind turbine system.

Brief description of the drawings

In order for the invention to be more fully understood, it will now be described by way of example only with reference to the following drawings, in which:

Figure 1 is a schematic view of a wind turbine system;

Figure 2 is a process diagram illustrating a strategy for damping oscillations of the wind turbine; and

Figure 3 is a graph illustrating the frequency response of the tower to rotor frequency excitation, and the effects of active damping according to the strategy in Figure 2.

Description of embodiments of the invention

Figure 1 shows a wind turbine system 2 represented schematically including features that are significant for this discussion. Therefore, it will be appreciated that many features that are common to wind turbines are not shown here for brevity, for example the nacelle, tower, control network, power distribution network and so on. However, the skilled person would understand that these features are implicit.

The wind turbine system 2 includes a rotor 4 having a set of blades 5 which drives a gearbox 6 by way of an input shaft 8. Note that although a gearbox 6 is shown here, it is also known for wind turbines to have a direct-drive architecture in which a gearbox is omitted. The gearbox 6 has an output shaft 10 which drives a generator 12 for generating electrical power. Three phase electrical power generation is usual in utility scale wind turbine systems, but this is not essential for the purpose of this discussion.

The generator 12 is connected to a frequency converter 14 by a suitable three-phase electrical connector such as a cable or bus 16. The frequency converter 14 is of conventional architecture and, as is known, converts the output frequency of the generator 12 to a voltage level and frequency that is suitable for supplying to the grid 18. Various frequency converter architectures are known in the art and the particular type selected is not central to the invention and so will not be described here in detail.

Although fixed-speed wind turbines are appropriate for wind turbines having a comparatively low power output, for example of below 1MW, in this embodiment the wind turbine system 2 is able to operate at variable speed so as to be more efficient at capturing energy from the wind at a wind range of wind speeds.

As is known, variable-speed wind turbines typically operate under two main control strategies: below-rated power and above-rated power. As is known, the term ‘rated power’ is used here in its accepted sense to mean the power output at which the wind turbine system is rated or certified to produce under continuous operation. Similarly, the use of the term ‘rated wind speed’ should be understood to mean the lowest wind speed at which the rated power of a wind turbine is produced.

Below rated power occurs at wind speeds between the cut-in speed and rated wind speed which, typically, is between 10and 17m/s. In this operating region, the wind turbine system 2 is operable to control the rotor speed so as to maximise the energy captured from the wind.

This is achieved by controlling the rotor speed so that the tip speed ratio is at an optimum value, namely between 6 and 7. To control the rotor speed, the wind turbine system is provided the facility to control the generator torque to track a commanded power reference, as will be described.

Above-rated power occurs when the wind speed has increased to, or has exceeded, the rated wind speed. In this operating condition, the objective of the wind turbine system 2 is to maintain a constant output power. This is achieved by controlling the generator torque or power to be substantially constant, but varying the pitch angle of the blades which adjusts the resulting drag and lift forces in the rotor plane. This will slow down the turbine’s rotational speed or the torque transferred to the rotor shaft so that the rotational speed or the torque is kept constant below a set threshold.

Referring again to Figure 1, in order to achieve the below-rated power and above-rated power control objectives, the wind turbine system 2 is equipped with a control system 20. The control system 20 includes a speed controller 22 which is operable to control the frequency converter 14 to influence the torque exerted on the rotor 4 by the generator 12, and also to control the pitch of the blades 5 through a blade pitch adjustment system comprising a pitch control module 24 and a pitch actuation module 26.

It should be noted at this point that the architecture of a wind turbine speed control system that acts through power electronics such as a frequency converter to control generator torque and also acts through a pitch control system to control the pitch angle of the blades is generally known in the art, so a detailed description of the electronic architecture will not be given here.

The speed controller 22 receives a plurality of control inputs, but two input parameters are shown specifically here: a rotor speed input parameter 30 which is provided by a suitable rotor speed sensing means, and a demanded power input parameter 32 or 'power reference' which is provided by a higher level controller (not shown) of the wind turbine system 2 either directly to the speed controller 22 or through a data distribution network based on a suitable protocol, such as ethernet.

The speed controller 22 is operable to control the generator torque by outputting a demanded torque signal TDEm to the frequency converter 14, more specifically a control module operatively linked to the converter 14, during below-rated power operating condition in order to minimise the error between the rotor speed input parameter 30 and the speed reference 32 and, thus, to bring the generated power to match the power reference 32. Similarly, at operating conditions above-rated power, the speed controller 22 is operable to hold the generator torque constant but to provide a control input to the pitch control module 24 to modulate, collectively, the pitch angles of all three blades 5 of the rotor 4. The pitch control module 24 receives the control input from the speed controller, shown here as Pcoll_dem and converts this into a pitch angle adjustment value for each of the blade 5. The pitch angle adjustment values are shown here as Padjj Padj_3 and PAdj_3 that represent values for a three bladed rotor. These control inputs are fed to the pitch actuation module 26 which controls the pitch actuating motors for the respective blades 5.

As will be appreciated from the above discussion, the wind turbine system 2 is provided with a facility to control the rotor speed during a wide range of wind speed in order to optimise the power generation of the system. However, in addition to this speed control facility, the wind turbine system also is provided with a facility to control the way in which the tower oscillates in use. To this end the control system 20 includes a damping controller 40 which cooperates with the speed controller 22, as will be explained, in order to apply forces into the rotor 4 via the generator 14 and the blade pitch adjustment system in order to counter the oscillation of the tower. Advantageously, the damping controller 40 is operable when the rotor speed coincides with the natural frequency of the tower. The term 'coincides' used in this context means that the rotor speed is equal to the natural frequency or 'eigenfrequency' of the tower, or within a predetermined range of the natural frequency, as will be explained in more detail later.

This is a particular benefit for relatively large wind turbines whose rotor frequency at rated power (‘rated rotor frequency/speed’) tends to be lower, which ordinarily would mean a risk that the rated speed would be nearly the same as the natural frequency of a tower having an optimized construction. Note here that the terms ‘speed’ and ‘frequency’ in relation to the rotor may be used interchangeably as referring to revolutions per minute of the rotor. Up to now, known strategies for dealing with this problem are to design the tower in such a way that the natural frequency of the tower is shifted along the frequency spectrum so that it is moved away from the rated speed of the wind turbine. This may be achieved by adding material to the tower, thereby increasing its strength and reducing its natural frequency, or through other known design approaches. However, this is not an optimum solution since this increases the cost of the tower, its complexity and its weight. This may be a particular issue for offshore installation where there is a strong incentive to keep the installed mass as low as possible and to keep the size of the foundation as small as possible.

The invention provides a solution to this issue by utilising an active damping system which acts through the pitch control facility, the generator torque control facility, or through a combination of both of these facilities to provide forces on the tower that oppose the cyclical excitation of the rotor during circumstances where the rotor speed is at or within a predetermined range of the natural frequency of the tower.

Figures 1 and 2 illustrate an example of how this control strategy may be embodied. As can be seen in Figure 1, the damping system or ‘controller’ 40 includes three main control modules, which are a lateral motion torque damping module 42, a lateral motion pitch damping module 44, and an axial motion pitch damping module 46. The damping controller 40 also includes a supervisory module 48 which controls the activation of each of the aforementioned modules 42, 44, 46.

At this point it should be noted that although the modules 42,44,46,48 have been described as being separate, this is not intended to confer a particular physical structure on the modules. For example, the modules may be separate firmware units or they may be individual functional software units implemented on a common processing platform. Also it should be noted that although damping modules 42-46 may be operated simultaneously, it may also be appropriate for them to be operated separately. For instance, the lateral motion torque damping module 42 tends to be more effective when operating at partial load conditions, that is to say at below rated power, whilst the lateral motion pitch damping module 44 and the axial motion pitch damping module 46 tend to be more effective when operating at full load conditions, that is to at or above rated power.

The lateral motion torque damping module 42 receives as an input signal a lateral acceleration parameter 49 which may be sourced from a suitable acceleration sensor or sensors installed on the wind turbine, for example on the tower or nacelle. In response to the input signal, the lateral motion torque damping module 42 is operable to output a torque offset signal TOFfset which serves to modulate the output of the speed controller 22 at summing junction 52. The modulated signal is coupled to the frequency converter 14, via a generated torque signal TGen, which controls the generator 12 accordingly.

The lateral motion pitch damping module 44 and the axial motion pitch damping module 46 in this embodiment are operable during above-rated power operating conditions and complement the functionality of the speed controller 22 to damp a different oscillatory motion of the tower. Both of these damping modules 44, 46 operate together via the pitch control module 24 to control the pitch adjustment commands for the blades 5, as will now be explained.

The axial motion damping module 46 functions to damp the oscillations of the tower in a direction in line with the rotor axis; that is to say the ‘fore-aft’ motion of the tower. In order to do this, the module 46 receives as an input signal an axial acceleration parameter 54 from an acceleration sensor that is configured to measure the axial acceleration of a suitable point or points on the wind turbine. Typically such a sensor will be mounded toward the top of the tower, and possibly in the nacelle, in order to maximise the accelerations to which it is subjected.

In more detail, the axial motion damping module 46 calculates the collective pitch change that is required to cause the rotor to apply a force to the nacelle that is counter to the fore-aft motion. Thus, the module 46 outputs a collective pitch offset that modulates the collective pitch demand PColl signal that is output by the speed controller 22 at summing junction 60. The modulated collective pitch demand PColl_dem is then provided to the pitch control module 24.

Conversely, the lateral motion pitch damping module 44 functions to damp the oscillations of the tower in a direction that is transverse to the rotor axis; that is to say the ‘side-to-side’ motion of the tower. To do this, it receives as input signal the lateral acceleration parameter 49 from the same acceleration sensor that provides data to the lateral motion torque damping module 42 and then calculates the pitch adjustments needed to each of the blades individually to result in the rotor applying a sideways force that is counter to the sideways motion of the tower. The module 44 outputs three separate pitch offset values as separate command signals that are shown here as Poffsetj, Poffset 2 and POFfsetj- These command signals are then fed into the pitch control module 24.

As will be now appreciated from the above discussion, the pitch control module 24 receives the modulated collective pitch demand Pcoll_dem from the speed controller 22 and also receives the three pitch offset values Poffsetj, Poffsetj and Poffsetj from the lateral motion damping module 44. The pitch control module 24 combines the aforementioned signals and outputs three separate signals to the pitch actuation module 26, in order to adjust the pitch angles of each of the blades. The pitch adjustment angle values PAdj 1, Padjj, and Padjjj as mentioned above, therefore are modulated by the pitch control module 24 so as to factor in the offset values Poffsetj, Poffsetj and Poffsetj from the lateral motion pitch adjustment module 44. The pitch actuation module 26 therefore controls the blades 5 of the wind turbine system 2 in accordance with the adjustments determined by the damping controller 40. Optionally, rather than three offset signals sent to the pitch control module 24 from the damping module 44, the damping module 44 may alternatively output a single damping demand signal, which may then be converted into three offset signals by the pitch control module 24.

The above discussion has explained the functionality of the damping controller 40 to modulate the torque demand to the generator and also the pitch of the blades. In the invention, the damping controller 40 is operable to damping the oscillation of the tower during circumstances where the rotor speed is at or within a predetermined range of the natural frequency of the tower. With this in mind, Figure 2 illustrates a control strategy or process 100 that is implemented by the control system 20 in accordance with an embodiment of the invention.

The control strategy begins at step 102 which corresponds to the wind turbine system 2 being powered up, at which point the control system 20 initiates a speed control strategy, at step 102, in order to transition the wind turbine through the cut-in speed towards a speed point at which electricity may be generated. Such a speed control strategy has been explained above and would be understood by the skilled person.

At step 104 the control strategy moves into an initial condition monitoring loop. One option here is to monitor the rotor speed and to compare this with predetermined speed range data, as is illustrated here as being provided by a suitable database 105. Note that the rotor frequency may be determined by various techniques, for example from direct sensing of the rotor or by converting the speed of the generator into rotor speed.

In checking the rotor speed, the objective for the control strategy 100 is to monitor for when the rotor speed is equal to or within a predetermined range of the natural frequency of the tower. In doing so, the current rotor speed may be compared to speed range data that has been calculated in an offline process to take into account the natural frequency of the tower such that only an absolute speed range value is compared with the current rotor speed. The term ‘coincides’ is used here as meaning that the rotor speed is within a range of about 20% (+/- 20%) of the tower natural frequency, although it is currently envisaged that the rotor speed should be within 10% (+/- 10%) of the tower natural frequency, and preferably within 5% (+/- 5%) of the tower natural frequency. So, the process is operable to damp the motion of the tower whilst the rotor frequency coincides with the tower natural frequency, that is to say during circumstances where the rotor frequency coincides with the tower natural frequency. In some embodiments, it is envisaged that the process is operable to damp the tower motion only in circumstances where the rotor frequency coincides with the tower natural frequency.

Alternatively, it is envisaged that the database 105 may include a data item for the tower natural frequency (TNF data item) and also data items which express the relative speed range as upper and lower limit thresholds based on the tower natural frequency (upper and lower limit data items). Other configurations are possible. The benefit of this approach is that the TNF data item and the upper and lower limit data items are configurable and so may be changed by uploading new values into the control system, either via a direct connection to the control system or via a remote uplink, which would be more convenient.

In addition to monitoring the rotor speed, this step may also be configured to monitor the tower oscillations directly and to check whether they exceed a predetermined threshold. Monitoring the tower oscillation may be a check that is mutually exclusive of the rotor speed check such that the process is triggered when either the rotor speed becomes close to the tower natural frequency OR that the tower oscillation exceeds a threshold. Alternatively, the initial conditions can be considered to have been met if both the rotor speed AND the tower oscillation are determined to be a problem.

If the comparison step 104 determines that the rotor speed (also known as Ί P’ in the art) is not within the predetermined speed range, as defined above, the process loops around the negative branch of decision step 106 until a positive determination is made. When this happens, the process moves to step 108 at which point the control system 20 activates the damping controller 40 in order to damp actively the oscillations of the tower through the operation of i) the lateral motion torque damping module 42 to modulate the generator torque demand, ii) the axial and lateral pitch damping modules 44,46 to modulate the blade-to-blade pitch, or iii) through a combination of these measures.

The result of this can be seen by the graph of Figure 3, in which the rotor speed defines the X-axis, and the frequency response of the tower defines the Y-axis. The tower natural frequency (TNF) is illustrated on the graph, and it will be seen that the undamped frequency response of the tower, as indicated by reference ‘A’, peaks at the tower natural frequency, as is to be expected. However, the predetermined speed range discussed above is indicated on Figure 3 by reference ‘B’ and it can be seen that the damped frequency response of the tower, as indicated by the dashed line marked ‘C’ is made less severe locally in the region of the tower natural frequency, as bounded by the upper and lower limits of the predetermined speed range.

Once the damping controller 40 has been activated at step 108, the process moves on to a monitoring loop as defined by steps 110 and 112 during which the rotor speed is monitored to determine whether it drops out of the predetermined speed range, by either reducing so that it moves under the lower limit (B1 on Figure 3), or by increasing so that it exceeds the upper limit (B2 on Figure 3).

If it is determined that the rotor speed is no longer within the predetermined speed range, then at step 114 the damping controller 40 is deactivated and the control moves back to step 102 whereby the wind turbine system 2 is controlled in accordance with a speed control strategy. A significant advantage of the control strategy of the invention is that it enables the wind turbine to be operated continuously at a 1P speed that coincides with the natural frequency of the tower by damping the tower oscillations during a restricted set of operating conditions, that is to say, when the 1P speed coincides with the tower natural frequency. By ‘coincides’ it is meant that the tower natural frequency is preferably within 10% of the rated operational speed of the wind turbine system by way of example.

This enables greater flexibility in designing tower structures of wind turbines since less focus needs to be placed on ensuring that the natural frequency of the tower has an operational margin away from the rated speed of the wind turbine.

The above discussion has assumed that the natural frequency of the tower is a predetermined value that is calculated in an 'offline' process either prior to installation based on tower modelling algorithms, or prior to operational certification during which the wind turbine system could be run in an experimental mode in order to identify its natural frequency. The calculated natural frequency value is then stored in a suitable accessible memory, e.g. database 105, of the system so that it can be accessed by the control system in implementing the control strategy. However, it is envisaged that the natural frequency value could also be determined in an 'online' process involving the evaluation of excitation parameters such as rotor speed and wind gusting conditions and also response parameters such as lateral and axial accelerations of the tower. By appropriate analysis of these parameters it is possible to determine the natural frequency of the tower. This online approach has the potential to provide a more accurate determination of the natural frequency of the tower than the offline approach and would also take into account factors that may change the natural frequency of the tower. For example, it is possible that components in the nacelle are changed for lighter or heavier alternatives, or components could be added or removed. Also possible is that the stiffness of the tower foundation changes over time. All of these things could have an appreciable effect on the natural frequency of the tower.

The skilled person would understand that variations could be made to the embodiments discussed above without departing from the inventive concept as defined by the claims.

Claims (19)

1. A wind turbine system comprising a tower, an electrical generator operatively coupled to a rotor having a plurality of blades, and a control system configured to control a rotor blade pitch adjustment means and a generator torque control means, wherein the control system includes an active damping means configured to apply damping control inputs to one or both of the blade pitch adjustment means and the generator torque control means whilst the rotational frequency of the rotor coincides with the natural frequency of the tower.

2. The wind turbine system of claim 1, wherein the active damping means applies the damping control inputs in circumstances where the rotational frequency of the rotor is within a predetermined range of the natural frequency of the tower.

3. The wind turbine system of claim 2, wherein the predetermined range is defined as within 10% of the tower natural frequency.

4. The wind turbine system of claims 1 to 3, wherein the natural frequency of the tower coincides with a rated operational speed of the wind turbine.

5. The wind turbine system of claims 1 to 4, wherein the control system is configured to apply a modulated torque demand signal to control the generator in order to damp oscillatory movement of the tower in a direction transverse to the rotor axis.

6. The wind turbine system of claims 1 to 5, wherein the control system is configured to apply a modulated collective pitch adjustment signal to the rotor blade pitch adjustment means to control the pitch of the blade collectively in order to damp oscillatory movement of the tower in a direction substantially in line with the rotor axis.

7. The wind turbine system of claims 1 to 6, wherein the control system is configured to apply a modulated pitch adjustment signal associated with each of the plurality of blades of the wind turbine system to control the pitch of each of the blades individually in order to damp oscillatory movement of the tower in a direction transverse to the rotor axis.

8. The wind turbine system of claims 1 to 7, wherein the control means is configured to compare the rotational frequency of the rotor to a predetermined value of the natural frequency of the tower.

9. The wind turbine system of claims 1 to 7, wherein the control means is configured to compare the rotational frequency of the rotor to a value of the natural frequency of the tower that is determined during operation of the wind turbine system.

10. A method of controlling the operation of a wind turbine system including a tower, a rotor having a plurality of blades, a control system, an electrical generator coupled to the rotor, rotor blade pitch adjustment means and a generator torque control means, the method comprising: determining the natural frequency of the tower; determining the rotational frequency of the rotor; determining that the rotation frequency of the rotor coincides with the natural frequency of the tower and, in response, applying damping control inputs to one or both of the rotor blade pitch adjustment means and the generator torque control means.

11. The method of claim 10, wherein the damping control inputs are applied in circumstances where the rotational speed of the rotor is within a predetermined range of the natural frequency of the tower.

12. The method of claim 11, wherein the predetermined range is defined as within 10% of the tower natural frequency

13. The method of claims 10 to 12, wherein the natural frequency of the tower coincides with a rated operational speed of the wind turbine.

14. The method of claims 10 to 13, wherein applying damping control inputs includes applying a modulated torque demand signal to control the generator in order to damp oscillatory movement of the tower in a direction transverse to the rotor axis.

15. The method of claims 10 to 14, wherein applying damping control inputs includes apply a modulated collective pitch adjustment signal to the rotor blade pitch adjustment means to control the pitch of the blade collectively in order to damp oscillatory movement of the tower in a direction substantially in line with the rotor axis.

16. The method of claims 10 to 15, wherein applying damping control inputs includes applying a modulated pitch adjustment signal associated with each of the plurality of blades to control the pitch of each of the blades individually in order to damp oscillatory movement of the tower in a direction transverse to the rotor axis.

17. The method of claims 10 to 16, wherein, in determining that the rotation frequency of the rotor coincides with the natural frequency of the tower, the rotational frequency is compared with a predetermined value of the natural frequency of the tower.

18. The method of claims 10 to 16, wherein, in determining that the rotation frequency of the rotor coincides with the natural frequency of the tower, the rotational frequency is compared with a value of the natural frequency of the tower that is determined during operation of the wind turbine system.

19. A method of controlling the operation of a wind turbine system including a tower, an electrical generator operatively coupled to a rotor having a plurality of blades, and a control system configured to control a rotor blade pitch adjustment means and a generator torque control means, wherein the control system includes an active damping means, wherein the method comprises applying damping control inputs to one or both of the blade pitch adjustment means and the generator torque control means whilst the rotational frequency of the rotor coincides with the natural frequency of the tower.