WO2025008033A1

WO2025008033A1 - Improvements relating to commissioning of wind turbines

Info

Publication number: WO2025008033A1
Application number: PCT/DK2024/050147
Authority: WO
Inventors: Peter Sigfred MORTENSEN
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2023-07-06
Filing date: 2024-06-18
Publication date: 2025-01-09
Anticipated expiration: 2026-01-06

Abstract

A method of commissioning a wind turbine, wherein the wind turbine comprises a tower extending along a tower axis (X), a tower top and at least one connection flange, the wind turbine having a tower damping system actuatable to control oscillatory movement of the tower, in use. The method comprises: controlling the tower damping system during a tower-settling phase of operation to cause the tower top to move in a plurality of directions (F1-F4) about at least a part of the tower axis to release residual stress in the at least one connection flange of the tower of the wind turbine. A benefit of the invention is the release of stresses built in during the manufacturing of tower sections of the wind turbine can be released rapidly allowing a more efficient commissioning process to be achieved.

Description

IMPROVEMENTS RELATING TO COMMISSIONING OF WIND TURBINES

Technical Field

This disclosure relates to systems, apparatus and methods for controlling side-side and fore-aft oscillatory movement of a wind turbine during wind turbine installation. Aspects of the invention relate to a wind turbine and a method for controlling the wind turbine, together with a controller for the wind turbine and a computer program product.

Background

Wind turbines are complex structures that require regular maintenance and inspection to ensure their safe and efficient operation. One critical aspect of wind turbine maintenance is the inspection and maintenance of bolted connections which are used to join various components of the wind turbine together. For example, typically bolted connections are used at the flanged interface between a foundation of a wind turbine and the lowermost tower section in order to provide a secure connection of the wind turbine to the foundation. Flanged interfaces are also present between mutually adjoining tower sections that make up a wind turbine tower.

Flanged interfaces in general are highly stressed during use and so the establishment of a bolted connection at the flanged interface is a crucial part of the installation process to achieve a long operating life of the wind turbine. The bolts in the bolted connection are typically pre-tensioned as part of the installation process which improves the robustness of the flanged interface. However, after installation, residual stress in the flanged connection may release over time which reduces the pretension in the bolts. This effect may compromise the fatigue life of the bolts, which is undesirable. Although bolt pretension can be measured and corrected, this may only take place after a considerable period of wind turbine operation. Furthermore, this is a time consuming and physically demanding task.

It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. Summary of the Invention

According to a first aspect of the invention, there is provided a method of commissioning a wind turbine, wherein the wind turbine comprises a tower extending along a tower axis (X), a tower top and at least one connection flange, the wind turbine having a tower damping system actuatable to control oscillatory movement of the tower, in use, the method comprising: controlling the tower damping system during a tower-settling phase of operation to cause the tower top to move in a plurality of directions (F1-F4) about at least a part of the tower axis so as to release residual stress in the at least one connection flange of the tower of the wind turbine. In this respect, the term 'commissioning’ may be considered to be the process of setting up a wind turbine for operation. During commissioning, the wind turbine may be run so as to produce power and to export that power to a power grid. Power export may be below typical nominal levels, however, due to the control functions implemented in the wind turbine systems during such commissioning process.

A benefit of the invention is that the release of stresses built in during the manufacturing and assembly of tower sections of the wind turbine can be released rapidly during application of control algorithms that are conventionally used to control damping of oscillatory motion of the wind turbine. Hence any concerns related to later release of such stresses and implications on flange bolt pre-tension, buckling or fatigue strength of weldings can be eliminated in a short and controlled manner.

In one example, controlling the tower damping system causes the tower top to move in a plurality of directions completely encircling the tower axis. Thus, the damping system is adapted to cause the tower to oscillate or vibrate in specifically selected directions in order to release residual stress in a controlled way. By controlling the damping system so that the tower oscillates in many different directions about the tower axis, stress relief can be achieved in a uniform way. Notably, the damping system is used here in a way that is not conventional. As is known, the damping system is used conventionally to reduce oscillatory motion of the wind turbine tower, whereas in the examples of the invention the damping system is used to increase, enhance or augment oscillatory tower motion to achieve a stress relieving function. The effect may be enhanced by controlling the damping system such that the induced oscillations of the tower have a frequency near to or substantially at the eigenfrequency of the tower. In another example, controlling the tower damping system causes the tower top to follow a generally circular path (F5) about the tower axis. This achieves uniforms applications of loads about the tower axis but in a different manner than described previously.

In another aspect, the examples of the invention provide a method of commissioning a wind turbine, wherein the wind turbine comprises a tower extending along a tower axis (X), a tower top having a nacelle, and at least one connection flange, the method comprising controlling the wind turbine during a tower-settling phase of operation, including the steps of: starting the wind turbine and controlling a yaw system of the wind turbine so that the nacelle faces in a first one of a predetermined plurality of directions relative to the tower axis, performing an emergency stop of the wind turbine whilst the nacelle faces in the first one of the predetermined plurality of directions; and repeating the steps of starting the wind turbine, controlling the yaw system, and performing an emergency stop of the wind turbine for each one of the predetermined plurality of directions, thereby to increase the loading applied to the tower in each of the predetermined plurality of directions, so as to release residual stress in the at least one connection flange of the tower of the wind turbine.

It should be noted that the second aspect of the invention may achieve the same or similar technical advantages over the known art as the first aspect of the invention.

The method may include stopping operation of the wind turbine following the tower-settling phase of operation, and re-tensioning of bolts in one or more of the at least one flanged connection of the tower.

The examples of the invention also embrace i) a controller for a wind turbine control system comprising a processor and a memory module, wherein the memory module comprises a set of program code instructions which when executed by the processor implements a method as defined in the aspects set out above, ii) a computer program product downloadable from a communication network and/or stored on a machine readable medium comprising program code instructions for implementing a method according to the aspects set out above, and iii) a wind turbine comprising a tower, and a controller as set out above.

Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all examples and/or features of any example can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

Brief Description of the Drawings

The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a front view of a typical horizontal-axis wind turbine in which examples of the invention may be incorporated;

Figure 2 is a schematic view of a wind turbine which exemplifies oscillatory movement experienced by a tower of the wind turbine during use;

Figure 3 is a schematic systems view of the wind turbine in Figure 1 ;

Figure 4 is a more detailed schematic systems view of a monitoring and control system of the wind turbine system of Figure 4;

Figure 5 is a flow chart representing an example method for controlling the wind turbine;

Figures 6 and 7 are pictorial representations of principles applied by the examples of the invention;

Figure 8 is a flow chart representing another example method for controlling the wind turbine. Detailed Description

A specific embodiment of the invention will now be described in which numerous features will be discussed in detail in order to provide a thorough understanding of the inventive concept as defined in the claims. However, it will be apparent to the skilled person that the invention may be put into effect without the specific details and that in some instances, well known methods, techniques and structures have not been described in detail in order not to obscure the invention unnecessarily. Moreover, it would be apparent to the skilled reader that structural, logical, and electrical changes may be made without departing from the scope of the invention as defined in the appended claims.

In general terms, the examples of the invention described here provide systems and corresponding methods for improving the way in which a wind tower installation is carried out so as to make the ‘settling’ process of a newly installed flanged connection of a wind turbine tower more predictable. Since the settling process is more predictable, action can be taken early to address outcomes from the settling process, such as re-tensioning bolted connections, which improves the long-term reliability of the flanged connection.

Figure 1 shows a wind turbine, generally designated as 10, comprising a tower 12. The tower 12 supports a nacelle 14 to which a rotor 16 is mounted. The rotor 16 is operatively coupled to a generator (not shown) housed inside the nacelle 14. The rotor 16 is configured to rotate about a central axis to drive the generator to generate electrical energy. In addition to the generator, the nacelle 14 houses miscellaneous components required for converting wind energy into electrical energy, along with various other components needed to operate, control, and optimise the performance of the wind turbine 10.

The rotor 16 comprises a plurality of rotor blades 18 extending radially from a central hub 20. The blades 18 rotate in a rotor plane. In this example, the rotor 16 comprises three rotor blades 18, although it will be apparent to those skilled in the art that other configurations are possible. The rotor blades 18 are pitch-adjustable. That is to say, the pitch of the rotor blades 18 can be adjusted, about their respective longitudinal axis 19, in accordance with a collective pitch setting, where each rotor blade 18 is set to the same pitch value relating to the collective pitch setting and/ or in accordance with individual pitch settings, where each rotor blade 18 may be set to its own pitch value corresponding to its individual pitch setting. The nacelle 14 is able to yaw relative to the tower 12 under the control of a yaw system (not shown) to ensure that the rotor 16 and the blades 18 face into the flow of wind during operation in order to optimise energy capture.

The tower 12 can experience oscillations/vibrations along its length during the operation of the wind turbine 10, particularly due to the vibrational coupling between the rotor 16 and the tower 12, which can be a source of self-excitation. Oscillations of the tower 12 might also arise as a result of external forces. Self-excitation is typically caused by asymmetries or mass imbalances in the rotor 16. For example, asymmetries in the rotor 16 may come about due to geometric errors in or misalignment of the rotor blades 18, giving rise to aerodynamic asymmetries, which vary with the rotational speed of the rotor 16.

Tower movement/oscillation generates bending moments along the tower 12, both at the flanges between adjacent tower sections and also at the base of the tower 12 where the distance from the free mass, i.e. the nacelle 14, is greatest. Aspects of tower oscillation are illustrated schematically in Figure 2. In this figure, the wind turbine 10 is illustrated by a beam structure 60, which is fixed at its lower end at a ground plane 62 and provided with a mass (i.e. nacelle 14) at its free end. When the top of the beam structure 60 oscillates in a direction X, that is, in a side-to-side direction with respect to the hub 20, varies between two maxima defined by the maximum deflection of the tower structure 60 during the oscillation. The position that is representative of the displacement of the nacelle 14 in the direction X may indicate the position of the nacelle’s 14 centre-of-mass, the position of a sensor housed within the nacelle 14, or the position of other fix-points representing the movement of the nacelle 14 in the direction X.

The tower 12 is considered to have at least two degrees of freedom, or vibrational modes, being able to bend laterally and longitudinally. Lateral bending is bending in a side-side direction, parallel to the plane of the rotor 16, as is indicated as direction X on Figure 2. However, longitudinal oscillation is bending in a fore-aft direction, perpendicular to the plane of the rotor 16, as is indicated by direction Y on Figure 2.

Bending in two degrees of freedom, in the fore-aft and side-side directions, can be quantified as movement or loads experienced by the tower 12, and mapped for one or more horizontal planes through the tower and positioned along the length of the tower 12. Averaged vibrational movement and/ or loads along the length of the tower 12 can also be mapped. Vibration of the tower 12 in any direction can be expressed as the product of excitation in the fore-aft and the side-side directions, and so is considered to have a fore- aft component and a side-side component.

Returning to Figure 1 , further technical context of the invention is illustrated. The tower 12 is connected to a foundation 22 which is embedded in the ground. Foundation structures are well known in the art. As is also known, foundation structures may be provided in onshore as well as offshore wind turbine installations. In an offshore context, foundation structures may include a component known as a transition piece.

The fixing between the tower 12 and the foundation 22 is achieved by a flanged connection or ‘coupling’ 25. A portion of a flanged connection 25 between the tower 12 and the foundation 22 can be seen in the inset panel in Figure 1.

The flanged connection 25 comprises a tower flange 26 and base flange 28 which are adjacent one another so as to fit together in a butted interface. In this example, the tower flange 26 is part of the tower, and the base flange 28 is part of the foundation 22. The tower flange 26 and base flange 28 define respective through-bores that are aligned so as to provide a bolt hole 30 that extends through both flanges 26,28. The bolt hole 30 accommodates a bolted connection 32.

The bolted connection 32 includes an anchor bolt 34 and a nut 36. The bolt 34 in this example is a stud bolt, having a shank 38. The shank 38 has a threaded section 39 onto which the nut 36 is screwed in a conventional manner to bear against a washer 37. In other examples the bolted connection 25 may include a headed bolt, rather than a stud bolt.

It will be noted at this point that the bolted connection 32 shown in Figure 1 illustrates a single bolt, although in practice a bolted connection would comprise a circular array of bolts arranged circumferentially around the base flange 28. It should be noted, for the avoidance of doubt, that the term ‘bolted connection’ should not be considered limited to the specific bolted connection as shown in the illustrated example. It should also be noted that the flanged connection shown in Figure 1 is only one example of such a connection, and that flanged connections may also be provided between adjacent tower sections in wind turbine towers that are made from several tower sections, as is known generally in the art. Such bolted connections as described here typically are pretensioned or preloaded to increase the effectiveness of the bolted connection and its resistance to cyclical loading. Although the abutting surfaces of flanged connections can be assumed to be flat, there may be small variations from idealised flatness. These variations are more significant for large tower diameters for example at or above 7 metres in diameter meaning that small gaps may exist between the abutting surfaces, which is undesirable. Applying pretension to the bolted connections about the connection flange reduces these gaps and improves the robustness of the flanged connection.

The process of applying pretension to bolted connections is a known technique in this regard. It is also known that the pretension or preload induced in such bolted connections may reduce or ‘settle’ over time. This can be due to the release of residual stress that exists in the material of the tower, which is usually steel, following the manufacturing process. Conventionally, a post-installation protocol is observed which involves the wind turbine being allowed to operate for a defined period, which may be between 3 to 12 months, during which time it is subject to a variety of wind conditions and loading conditions. The varied loading conditions have the effect of releasing residual stresses in the tower structure near to the flanged connection which may cause a degree of detensioning of the bolted connections. After this defined period, the bolts in the flanged connection can be inspected and re-tensioned if necessary. However, this is a considerable maintenance task, the period for which the wind turbine needs to be allowed to operate during this ‘post-installation protocol’ is significant which places delays on possible maintenance activities and a degree of uncertainty over the integrity of the flanged connection.

It is an object of the invention to address the technical challenge of carrying out the postinstallation process described above in a more efficient way that is done at present as part of the wind turbine commissioning process. Wind turbine commissioning can be a lengthy task, which can delay the hand over process of the wind turbine to the control of the operator/owner. Therefore, any measure that can reduce the commissioning time is desirable.

In one aspect, an aim is to intentionally increase the loading applied to the wind turbine tower during a ‘settling phase’ of operation during commissioning so that loading is applied to the flanged connections of the wind turbine tower through a range of directions in order to accelerate the release of residual stress in the tower near or at the flanged connections. It is envisaged that this technical effect may be achieved by operating a tower damping system of the wind turbine to induce oscillatory movement at the top of the wind turbine tower so that the tower top moves in a plurality of directions about the tower axis in order to achieve the required loading profile at one or more flanged connections of the tower. The plurality of directions may involve the tower damping system being controlled so that the tower top oscillates to and fro with components of fore-aft and side-side motion. The effect of this will be to cause the tower top to oscillate in a plurality of selected radial directions about the tower axis which preferably covers a spread of angles that encircle the tower axis so that the reduction of residual stress of the flanged connections is suitably uniform about the circumference thereof.

An exemplary control means of the wind turbine to achieve this technical functionality will now be discussed with reference to Figure 3 and 4.

In Figure 3, the wind turbine 10 comprises control means 50 that is operable to monitor the operation of the wind turbine 10 and to issue commands thereto to achieve a set of control objectives. The control means 50 is shown in Figure 3 as a simplified, schematic overview of a plurality of control units and modules, and also in Figure 4, as a more detailed example of how specific units and modules may be arranged in order to facilitate data exchange between them.

The wind turbine 10 also includes a gearbox 52 and a power generation system 54 including a generator 56 and a power converter system 58. The gearbox 52 gears up the rotational speed of the rotor 16 and drives the generator 56, which in turn feeds generated power to the power converter system 58. Usually such a system will be based on three- phase electrical power, although this is not essential. Other wind turbine designs are known, such as “gearless” types, also known as “direct drive”, as well as “belt drive” transmission types.

The generator 56 and the power converter system 58 may, as an example, be based on a full-scale converter (FSC) architecture or a doubly-fed induction generator (DFIG) architecture, although other architectures would be known to the skilled person.

In the illustrated embodiment, the power output of the power converter system 58 is transmitted to a load 60, which may be an electrical grid. The skilled person would be aware that different power conversion and transmission options exist and that the system schematic of Figure 3 is merely to provide an example of a system architecture that would be suitable for implementation of the functionality discussed here.

Returning to the control means 50 of Figures 3 and 4, the control means 50 comprises a processor 62 configured to execute instructions that are stored in and read from a memory module 64 and/or an external data store that forms part of an external network 66. Measurement data may also be stored in the memory module 64 and recalled in order to execute processes according to the instructions being carried out by the processor 62.

Instructions and data may also be received from external controllers or sensors that form part of the external network 66, and recorded data and/or alerts may be issued over the external network 66 to be stored/displayed at an external source for analysis and remote monitoring.

In addition, the processor 62 is in communication with a plurality of sensors 68 that are disposed within the wind turbine 10. For example, as shown in Figure 4, the plurality of sensors 68 may comprise a tower accelerometer 72, a rotor speed sensor 74, a blade pitch angle sensor 76, a nacelle yaw angle sensor 78 and a rotor position sensor 79.

The control means 50 of the wind turbine 10 also includes at least one control unit 70. Five control units are shown in the configuration shown in Figure 4. These are a blade pitch angle control unit 82, a nacelle yaw angle control unit 84, a speed control unit 86 a fore- aft tower damping (FATD) control unit 85 and a side-side tower damping (SSTD) control unit 87. The blade pitch angle control unit 82 and the nacelle yaw angle control unit 84 are arranged to alter the pitch angle of the rotor blades 18 and the yaw angle of the nacelle 14, respectively, and the speed control unit 86 functions to control the rotational speed of the rotor 16 through converter control and pitch control.

The function of the FATD and SSTD control units 85, 87 is discussed in more detail below. In the embodiment shown, the blade pitch angle control unit 82 and the FATD and SSTD control units 85, 87 are separate control units. However, the skilled reader will appreciate that the respective functionalities of these separate control units 82, 85, 87 could be delivered from a single control unit or two control units.

A network 88 forms a central connection between each of the modules (according to a suitable protocol), allowing the relevant commands and data to be exchanged between each of the modules accordingly. However, it will be appreciated that suitable cabling may be provided to interconnect the units. It will also be appreciated that the wind turbine 10 could include more control units 70, and that Figure 4 is provided only to illustrate an example of a system architecture in which the invention may be implemented.

A principal function of the control means 50 is to control power generation of the wind turbine 10 so that it optimises power production under current ambient wind conditions and in accordance with demanded power generation by a transmission grid operator. However, in addition to its main power control tasks, the control means 50 may be operable to perform a suite of other functions. In the examples of the invention, one of these functions is to operate the wind turbine during commissioning so that it is ready for handover to the end-user of the wind turbine or wind farm of which the wind turbine forms a part. As part of this, the control means 70 may be configured to control the FATD control unit 85 and the SSTD control unit 87 to influence the movement of the tower, in particular the oscillatory motion of the tower top, to increase the loading applied to the one or more connection flanges of the tower with the aim to release the residual stress within the tower in the region of the connection flanges at an accelerated rate. In this manner, the bolts at the connection flanges settle at a more predictable and accelerated rate which means follow up maintenance can be planned more accurately, and commissioning can be completed more quickly.

At this point, it should be appreciated that the functionality of active tower damping systems, namely the FATD control unit 85 and the SSTD control unit 87 are known generally in the art as being measures that are implemented in wind turbines to reduce oscillatory movements of the tower. As such systems would be well-known to a skilled person, a full discussion will not be provided here so as to avoid obscuring the invention. However, at a high level, it can be noted that side-side oscillatory movement of the tower can be influenced principally by altering the individual pitch of each blade 18 of the wind turbine 10 i.e. ‘cyclical blade pitch control’ or by altering the power generation of the turbine by adjusting the generator torque which is sometimes known as ‘generator speed control’ or ‘generator torque control’. Either way, the function is to use the electrical characteristics of the generator to control the torque on the main rotor shaft which generates a force on the nacelle in a sideways direction. The application of the sideways force thereby may be used, conventionally, to counteract sideways movement of the tower. Cyclical blade pitch control and generator torque control in an active damping sense are managed by the SSTD control unit 87 in this example. In some embodiments, the SSTD control unit 87 may receive the control signal and may control the blade-pitch angle control unit 82 and/or the speed control unit 86 to perform the relevant alteration or actuation. Other control units may be used as would be understood by the skilled person.

In contrast to the adjustment of loading in the side-side direction, loading is altered in the fore-aft direction by adjusting the collective pitch of the blades 18. This may be achieved by the operation of the FATD control unit 85 which is configured to adjust the collective pitch of the blades 18 in order to carry out active damping in the fore-aft direction in response to a suitable control signal. The FATD control unit 85 may carry out this function through communication with the blade-pitch angle control unit 82. In some examples, however, the collective pitch may be controlled directly by the FATD control unit 85. Collectively, the FATD control unit 85 and the SSTD control unit 87 can be considered to constitute an active tower damping system for the wind turbine 10.

In order to perform the functionality discussed above, the control means 50 operates according to a control protocol. A general protocol is shown by a method 100 illustrated in Figure 5.

The method starts at step 102 at which point a tower settling phase of operation is initiated. It should be noted that the tower settling phase of operation may be a typical operational condition of the wind turbine in which it is connected to a power distribution network and generating power. However, as is discussed below the control of the wind turbine during the initial settling phase is adjusted such that the tower motion is enhanced or augmented in a controlled way so as to achieve material stress relieve at flanged connections of the tower.

Once the tower settling phase of operation is initiated, the method determines at step 104 a target load profile that should be met to enable residual stress relief in the steel at the one or more tower flange connections.

The load profile may be determined in various ways. One example is that the load profile may be stored in memory module 64 ready to be recalled by the processor 62 of the control means 50. The load profile may therefore take the form of an array of force values or bending moments, as is appropriate, that need to be applied to individual flanged connections of the wind turbine tower. Such force values or bending moments may be translatable directly into displacements/velocity/acceleration components that should be achieved at the tower top in order to achieve the desired force values.

A diagrammatic representation of this can be considered in Figure 6, although it should be noted that the force values represented here would be contained in a suitable data structure.

In Figure 6, a base tower section 90 is shown in solid lines, whilst a further tower section 92 above it is shown in dotted lines, for clarity. The flanged connection between the base tower section 90 and the adjoining tower section 92 is shown at 94. It should be noted that the dimensions of the tower sections shown in Figure 5 are not to scale.

As will be appreciated in Figure 6, there are four pairs of force vectors, labelled F1 to F4, that lay in a horizontal plane P1 above the flanged connection 94. Note that the plane P1 may be coincident with the flanged connection 94.

The force vector pairs F1-F4 extend in a plurality of different directions about the axis of the tower, labelled X. Here, the force vector pairs F1-F4 are distributed equally about the tower axis X to provide 360-degree coverage. As such, force vector pair +F1 and -F1 extends substantially in the fore-aft direction of the tower top, whereas force vector pair +F3 and -F3 extend substantially in the side-side direction of the tower top. Force vector pairs +/-F2 and +/- F4 are in intermediate positions. In total, therefore there are eight force vectors that extend about the tower axis X are angular increments of 45 degrees. It should be noted that this is just an example and that there may be more numerous or fewer force vectors than shown in Figure 6. Each force vector represents a minimum target load or force that is to be applied to the tower at the horizontal plane P1 and therefore also at the associated flanged connection 94. Note that the target load profile may be stored in the memory module 64 as part of the wind turbine commissioning process. The download of the target load profile may be carried out at the wind turbine or may be achieved by way of an over-the-air download over a suitable communications network, as the skilled person would appreciate.

The control means 50 is configured to interpret the data contained in the target load profile in order to operate the wind turbine to release residual stress in the tower. Thus, once the load profile has been determined at step 104, the control means starts operation of the wind turbine at step 106. In order to apply the increased loading as dictated by the desired loading profile, the control means calculates control signal components at step 108 based on the target load profile.

At step 108, the control means 50 uses the determined target load profile which are used to operate the tower damping system, in particular the FATD control unit 85 and the SSTD control unit 87, as described above, in order to control the oscillations of the tower top thereby to achieve the increased loading required by the desired load profile that has been determined at step 104.

The control signal components determined at step 108 are configured to increase the oscillational movement of the tower 12 such that the side-side and the fore-aft components of the vibrational movement meet or exceed that determined load profile.

In Figure 6, it will be appreciated that the load profile is depicted by eight force vectors that are spaced at equal angular increments about the tower axis. The control means 50 may therefore be configured to operate the tower damping system so that the tower top, i.e. the nacelle of the wind turbine, oscillates along the direction of each one of the vector pairs F1 to F4 in turn. For example, the control means 50 may be configured to control the FATD control unit 85 to cause the tower top substantially only in the fore-aft direction in line with vector pair +/-F1 for a predetermined time period. The predetermined time period may be any time period that is suitable to release residual stress in the tower, for example 1 hour, or 5 hours, or 10 hours.

Once that predetermined time period has finished, the control means 50 may then select another vector pair e.g. +/-F2 and operate the tower damping system to cause the tower top to oscillate in the direction of that vector pair. Since the vector pair +/-F2 is in between vector pair +/-F1 which is in the fore-aft direction and vector pair +/-F3 which is in the sideside direction, the control means 50 is configured to generate appropriate control signals for each of the FATD control unit 85 and the SSTD control unit 87 so that the two control unit operate in synchrony to cause the tower top to oscillate in the desired intermediate direction.

Similar considerations apply for the vector pair +/- F4. Once the control means 50 has calculated suitable control signal components, at step 108, those control signal components are then applied to the relevant control units 85,87 of the tower damping system, at step 110.

The FATD control unit 85 and the SSTD control unit 87 may be operated for a suitable time period and a suitable manner in order to achieve the desired loading on the connection flanges of the tower 12. As shown in Figure 5, this may be achieved my operating the FATD control unit 85 and the SSTD control unit 87, in accordance with the determined control signals calculated at step 108, for a predetermined period of time that is calculated to result in sufficient release of residual stress in the tower, as indicated at step 112.

As an alternative, step 114 indicates a feedback approach in which the loading applied to the connection flanges of the tower 12 may be suitably monitored. In this way, the tower damping system may be operated until the required loading indicated by the loading profile is achieved. The loading applied to the tower 12 may be determined in various ways. One way in which this may be achieved is to equip the connection flanges with suitable load cells which provide a loading indication to the control means. The load cells may form part of the plurality of sensor 68 as indicated in Figures 3 and 4.

An alternative way of achieving the load monitoring is for the loading to be estimated based on tower acceleration signals that are obtainable from the tower accelerometer 72. Since acceleration is related directly to the force applied to the tower, loading data can readily be determined from accelerometer data.

Note that a combination of operating the tower damping system for a determined period of time, as in step 112, and monitoring the applied loads, as in step 114, can be carried out.

Following the operation of the tower damping system to achieve the desired loading profile of the one or more connection flanges of the tower, the process ends, as shown at step 116.

The ending of the method 100 may trigger an inspection event in which the bolts within the one or more connection flanges of the tower 12 are inspected for tension. This can be achieved by an appropriate technique, for example by observing whether the nuts of the bolted connections have moved from a datum position, and/or through a non-destructive inspection technique as known in the art. One known system for determining the tension of bolted connections is available commercially as a ‘Bolt-Check’ product from R&D Test Systems A/S.

Any bolts that are observed to have de-tensioned, can be re-tensioned immediately or can be scheduled for re-tensioning at a subsequent maintenance event.

Various modifications may be made to the specific embodiments that have been discussed above with reference to the accompanying figures. Some variants have already been discussed but others would be apparent to the skilled person. Therefore, the scope of the invention should be determined from the appended claims rather than with reference to the specific examples discussed in this text.

In the above discussion, the loading profile as illustrated in Figure 6 has been explained as being implemented by the control means 50 such that the tower damping system causes the tower to oscillate in each of the directions of the force vectors F1 to F4 separately and in sequence. However, other options are possible. One possible option is illustrated in Figure 7, which is similar to Figure 6 but which depicts a different loading profile.

In Figure 7, it can be observed that the loading profile comprises a force vector F5. That force vector F5 is configured, when implemented by the control means 50 in the method 100 described above, to cause the tower top to follow a circular movement path above the tower axis X.

A further example of the invention is now described with respect to Figure 8 which discloses method 200. In common with method 100 illustrated in Figure 5, method 200 has the aim to increase the loading applied to the one or more connection flanges of the tower in a plurality of directions with the aim to release the residual stress within the tower in the region of the connection flanges at an accelerated rate. In this manner, the bolts at the connection flanges settle at a more predictable and accelerated rate which means follow up maintenance can be planned more accurately, and commissioning can be completed more quickly.

However, in contrast to method 100, method 200 achieves the release of residual stress in a different manner, and, more specially, makes use of the emergency stop function of the wind turbine 10 which can be performed when the nacelle 14 of the wind turbine 10 is facing in each one of a predetermined plurality of directions. In an emergency stop, typically the blades 18 of the rotor 16 are pitched aggressively to reduce aerodynamic lift, and the generator is operated to increase generator torque. In combination, these measures act to apply a torque brake to the rotor which slows the rotor quickly at which point a parking brake may be applied to the rotor. In performing this function, the tower will bend substantially in the direction the nacelle faces which applies a significant bending moment to the connection flanges of the tower. By performing the emergency stop function repeatedly in a plurality of different yaw directions of the nacelle, a predictable and consistent loading can be applied to the connection flanges of the tower extending above the yaw axis.

Method 200 starts at step 202 at which point a tower settling phase of operation is initiated, as discussed above. Once the tower settling phase of operation is initiated, the method determines at step 204 a target load profile that should be met to enable residual stress relief in the steel at the one or more tower flange connections.

In the context of the application of an emergency response procedure, the determined load profile may consist of a plurality of yaw headings that should be applied to the wind turbine and, optionally, the number of instances that the emergency stop procedure should be applied. As in the previous method, the load profile may be stored in memory module 64 ready to be recalled by the processor 62 of the control means 50 or may be downloaded from a remote location.

Once the target load profile has been determined, the method 200 moves to step 206 at which point the wind turbine is started, as long as there is sufficient wind, and suitable control signals to operate the emergency stop procedure are calculated at step 208. Such control signals may take into account wind speed, rotor speed, yaw heading, and so on, in order to carry out the emergency stop function at a point that would generate sufficient loading on the tower in a desired one of a plurality of yaw headings. Note that the plurality of yaw headings may be selected to map the loading about the tower axis X in a suitable way. As such, the predetermined plurality of yaw headings may be in suitable angular increments to give sufficient release of residual stress in the tower. The incremental angular intervals may be 10 degrees, or 20 degrees, or 45 degrees, by way of example. It should be noted that there is a balance to be struck between the time taken to perform emergency stops in the desired wind conditions in many different nacelle yaw headings and the magnitude of the residual stress release.

Once suitable control signals have been calculated, the control means 50 controls the wind turbine, at step 210, to perform an emergency stop in a first yaw direction from the plurality of predetermined yaw directions for the nacelle.

Once the first emergency stop has been completed successfully, the control means 50 restarts the wind turbine 10 at step 212 and operates the nacelle yaw angle control unit 84 (see Figure 4) to yaw the nacelle to a yaw heading consistent with the next one of the plurality of directions as indicated in the predetermined load profile. This process may be completed until an emergency stop has been performed in each of the plurality pf predetermined directions as dictated by the load profile.

Following the completion of emergency stops in each of the predetermined plurality of directions, the loads applied to the connection flanges are verified as being sufficient at step 214, at which point the method may be terminated at step 216.

In the context of the emergency stop procedure described above, it should be noted that the flange settling operation of method 200 could be interrupted by normal operation of the wind turbine. For example, when an emergency operation has been performed in one direction (see step 210), the wind turbine may return to normal operation until such time as the wind direction has changed sufficiently, as monitored by the yaw system. A suitable supervisory yaw heading monitoring function may be configured suitably for this task. Then the wind turbine re-enters tower flange settling and performs an emergency stop in the new direction before it returns to normal operation. This is repeated until the target load profile has been fulfilled.

Claims

1. A method (100) of commissioning a wind turbine (10), wherein the wind turbine comprises a tower (12) extending along a tower axis (X), a tower top and at least one connection flange (25), the wind turbine having a tower damping system (85,87) actuatable to control oscillatory movement of the tower, in use, the method comprising: controlling the tower damping system during a tower-settling phase of operation to cause the tower top to move in a plurality of directions (F1-F4) about at least a part of the tower axis so as to release residual stress in the at least one connection flange (25) of the tower of the wind turbine.

2. The method of Claim 1 , wherein controlling the tower damping system causes the tower top to move in a plurality of directions completely encircling the tower axis.

3. The method of Claim 2, wherein controlling the tower damping system causes the tower top to follow a generally circular path (F5) about the tower axis.

4. The method of any one of the preceding claims, wherein the step of controlling the tower damping system further comprises: determining a minimum load profile (104) for the at least one flange connection of the tower which represents the minimum load to be applied to the at least one flange connection in a plurality of directions extending about a tower axis, based on the required load profile, determine (108) a first control signal component in respect of a side-side component of oscillatory tower movement; based on the required load profile, determining (108) a second control signal component in respect of a fore-aft component of the oscillatory tower movement; and wherein the first and second control signal components are configured such that the tower damping system causes an increase in oscillatory tower movement in the horizontal plane such that the load applied to the at least one flanged connection exceeds the determined minimum load profile.

5. The method of Claim 4, wherein the control signal components are configured such that the load applied to the at least one flanged connection meets a target magnitude in exceedance of the determined load profile.

6. The method of Claim 4 or 5, wherein the determined load profile corresponds to a predetermined service load limit of the tower.

7. The method of any one of the preceding claims, wherein the tower-settling phase of operation is controlled remotely from the wind turbine.

8. A method (200) of commissioning a wind turbine (10), wherein the wind turbine comprises a tower (12) extending along a tower axis (X), a tower top having a nacelle (14), and at least one connection flange (25), the method comprising controlling the wind turbine during a tower-settling phase of operation, including the steps of: starting (206) the wind turbine and controlling a yaw system of the wind turbine so that the nacelle faces in a first one of a predetermined plurality of directions relative to the tower axis, performing (210) an emergency stop of the wind turbine whilst the nacelle faces in the first one of the predetermined plurality of directions; repeating (212) the steps of starting the wind turbine, controlling the yaw system, and performing an emergency stop of the wind turbine for each one of the predetermined plurality of directions, thereby to increase the loading applied to the tower in each of the predetermined plurality of directions, so as to release residual stress in the at least one connection flange of the tower of the wind turbine.

9. The method of any one of the preceding claims, further comprising: stopping operation of the wind turbine following the tower-settling phase of operation, and re-tensioning of bolts in one or more of the at least one flanged connection of the tower.

10. A controller (50) for a wind turbine control system comprising a processor (62) and a memory module (64), wherein the memory module comprises a set of program code instructions which when executed by the processor implements a method according to any one of Claims 1 to 8.

11. A wind turbine (10) comprising a tower (12), and a controller (50) according to Claim 10.

12. A computer program product downloadable from a communication network and/or stored on a machine readable medium comprising program code instructions for implementing a method according to any one of Claims 1 to 8.