Wind turbine vibration control system

Abstract

A pitch controlled wind turbine has a tower, a nacelle mounted on the tower, a hub mounted on the nacelle, and blades. The wind turbine includes blade connecting members, each extending between neighbouring blades, and pre-tension members, each connected to one of the blade connecting members and to the hub via a tensioning device, the tensioning device provides radial movement of the pre-tension member due to extension / retraction of the tensioning device, each pre-tension member provides pre-tension in ta respective blade connecting member. A cable vibration control system is coupled to one or more of the tensioning devices, and to one or more sensors for detecting a vibration of, or resultant noise from, one or more of the blades, blade connecting members and pre-tension members. The control system is configured to control the tensioning devices to extend or retract so as to control vibrations and noise generated by the wind turbine.

Description

[0001] WIND TURBINE VIBRATION CONTROL SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a pitch controlled wind turbine having a cable vibration control system.

[0004] BACKGROUND OF THE INVENTION

[0005] Vibrations or oscillations of wind turbine components naturally occur. Vortex-induced vibrations (VIV) are a known phenomenon in wind turbines, characterised by the oscillations induced in wind turbine components due to the shedding of vortices in the wake of such components.

[0006] These vibrations not only pose structural integrity concerns but also contribute to noise pollution, impacting both the environment and the surrounding communities. Further detrimental effects from VIV may include increased maintenance costs and reduced operational lifespan.

[0007] The effects of VIV are particularly prevalent in pitch controlled wind turbines having blade load sharing connecting members, which include various additional components intended to support larger blades. These additional components may be susceptible to unwanted oscillations, which if not mitigated could result in a reduction in structural integrity of the wind turbine, and an increase in noise pollution.

[0008] As a result, systems aiming to mitigate against the unwanted effects of VIV have become important to ensure the reliable and continuous operation of wind turbines in all climates.

[0009] SUMMARY OF THE INVENTION

[0010] A first aspect of the invention provides a pitch controlled wind turbine comprising a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three wind turbine blades, wherein each wind turbine blade extends between a root end connected to the hub via a pitch mechanism, and a tip end, the wind turbine further comprising: at least three blade connecting members, each blade connecting member extending from a connection point on one wind turbine blade towards a connection point on a neighbouring wind turbine blade, where the connection point on a given wind turbine blade is arranged at a distance from the root end and at a distance from the tip end of the wind turbine blade; at least three pre-tension members, each pre-tension member being connected to one of the blade connecting members and to the hub via a tensioning device, the tensioning device provides radial movement of a radially inward end of the pre-tension member with respect to an axis of rotation of the hub due to extension or retraction of the tensioning device, each pre-tension member thereby providing pre-tension in the blade connecting member to which it is connected; and a cable vibration control system coupled to one or more of the tensioning devices, and to one or more sensors for outputting one or more signals influenced by a vibration of, or resultant noise from, one or more of the wind turbine blades, the blade connecting members and the pre-tension members; wherein the cable vibration control system is configured to control the one or more tensioning devices to extend or retract so as to control vibrations and related noise generated by the wind turbine.

[0011] Advantageously, a cable vibration control system can control noise generated by the wind turbine, thereby avoiding the wind turbine from being excessively loud, which can have a negative impact on the environment. Moreover, controlling the noise generated by the wind turbine can assist in compliance with regulatory requirements. Noise may be either a general (levelized) noise or specific frequencies potentially associated with standing waves in blade connecting members, pre-tension members or blades. Standing waves may be particularly relevant when the rotor is stationary for example during installation, decommissioning or service, or during idling of the rotor.

[0012] The one or more sensors are configured to output a signal influenced by a vibration or resultant noise from one or more of the wind turbine blades, the blade connecting members and the pre-tension members. It will be understood that in some arrangements, the one or more sensors directly measure vibration or associated noise, e.g. one or more sensors are for detecting a vibration of, or resultant noise from, one or more of the wind turbine blades, the blade connecting members and the pre-tension members.

[0013] In alternative arrangements, one or more sensors may infer the presence of vibrations or associated noise indirectly. For example, it is known that ice accumulation on wind turbine components causes vibration. In this case, ice presence and thus component vibration may be inferred via one or more sensors that records power generation of the wind turbine or that generates the wind turbine power curve (e.g. a reduction in power output of the wind turbine may suggest ice presence). In another example, a tension in a blade connection member and / or a pre-tension member may indicate the presence of vibrations or associated noise. In this case, the one or more sensors may include a cable tension sensor. Tension in a blade connection member and / or a pre-tension member may be inferred via one or more sensors that records a pressure in the tensioning device or that measures an extension position of the tensioning device. In this way, the one or more sensors may include a pressure sensor or a position sensor, respectively.

[0014] It will be understood that noise in a wind turbine may be generated by oscillations or vibrations of one or more blade connecting members and / or pre-tension members and / or wind turbine blades. Utilising the cable vibration control system to control the tensioning devices can assist in controlling the size and / or frequency of such oscillations, thereby allowing effective control over noise emitted therefrom. Moreover, controlling the tensioning devices to reduce noise generated by the wind turbine utilises pre-existing equipment to alleviate the negative impacts of cable vibrations, allowing improved control over cable vibrations and associated noise without a significant increase in complexity of the wind turbine design.

[0015] Implementing the control system allows the process of controlling cable vibrations to be substantially automated, as the control system can alter the tensioning devices in response to an input from a sensor.

[0016] The cable vibration control system may be configured to determine that vibration and / or associated noise generated by the wind turbine are at or above a predetermined level based on a signal from the one or more sensors, and may be configured to control one or more of the tensioning devices to extend or retract such that vibrations and / or associated noise are below the predetermined level.

[0017] Advantageously, the cable vibration control system is operable only when cable vibrations and / or associated noise is above a predetermined level (e.g. a level inputted by a user or from simulation or from feedback). The predetermined level may be any value, e.g. based on regulatory requirements or based on a maximum noise level that is acceptable to the public. The energy requirements and effectiveness of the control system can be reduced, as the system is only operable when cable vibration or noise is expected to exceed or exceeds this level. The predetermined level may be based on individual frequencies (i.e. frequencies of cable vibrations being at / below a threshold) or based on integrated sound power level or levelized noise being at / below a threshold. The integrated sound power level may also be assessed over a specific frequency band or specified independently for multiple bands.

[0018] The cable vibration control system may be configured to control one or more of the tensioning devices to extend or retract to alter a tension in a respective pre-tension member until the cable vibration control system determines that vibrations or noise generated by the wind turbine is at or below the predetermined level.

[0019] Advantageously, the cable vibration control system operates until cable vibrations and / or associated noise generated by the wind turbine has been appropriately reduced. In some arrangements, the control system operates with a feedback loop, in which the control system can determine the vibration or noise level is too high, alter a tension in a pre-tension member, determine whether the vibration or noise is still too high and either alter the tension again, or maintain tension in the pre-tension member. This has been found to improve the efficiency of the control system.

[0020] The cable vibration control system may be configured to control one or more of the tensioning devices to extend or retract in a substantially stepwise manner so as to alter the tension in a respective pre-tension member in a substantially stepwise manner.

[0021] Advantageously, a substantially stepwise change in tension in a pre-tension member can improve the speed at which the control system can have an impact on cable vibration (e.g. compared to a more gradual, continuous change), thereby improving the effectiveness of the cable vibration control system. This is particularly advantageous when cable vibrations are of the standing wave type, where a certain step change may be advantageous and / or required to eliminate the standing wave, e.g. a step change of 3-10% of tension in blade connecting member may be preferable.

[0022] The term “substantially stepwise” refers to the limitations of equipment to achieve a completely instantaneous change in state of the tensioning device. It should be understood that the change in state of the tensioning device is intended to be as instantaneous as is permitted by the equipment. The cable vibration control system may comprise a memory unit, and may be configured to identify one or more conditions at which vibrations or associated noise is at or above the predetermined level and store the one or more conditions in the memory unit.

[0023] Advantageously, the cable vibration control system has improved capability to learn from experience, improving the effectiveness of the system.

[0024] The cable vibration control system may be configured to determine that one or more conditions stored in the memory unit is occurring based on a signal from the one or more sensors, and may be configured to control one or more of the tensioning devices to extend or retract to alter a tension in a respective pre-tension member in response thereto.

[0025] Advantageously, the cable vibration control system can effectively predict that cable vibrations or the associated noise generated by the wind turbine is likely to exceed a predetermined level based on previous events (i.e. based on the stored conditions), and in response thereto, pre-emptively alter a tension in a pre-tension member to avoid the cable vibration or associated noise ever exceeding the predetermined level. In this way, the wind turbine is less likely to generate cable vibrations or associated noise that exceeds an acceptable level.

[0026] At least one of the one or more conditions may be a wind speed and / or a wind direction, and the one or more sensors may include a wind speed sensor and / or a wind direction sensor.

[0027] Advantageously, wind speed proximal to the wind turbine has been found to have an impact on the frequency of oscillation of one or more of the wind turbine blades, the blade connecting members and the pre-tension members, and by extension, the noise generated by the wind turbine. As such, utilising the wind speed sensor and / or wind direction sensor to predict the likelihood that cable vibrations or associated noise exceeding an acceptable level will occur was found to be a direct / controllable means of pre-emptively adjusting tension in the pre-tension members so as to avoid excessive cable vibration or associated noise generation in response to a change in wind speed or direction. The cable vibration control system may be configured to control one or more of the tensioning devices to extend or retract such that a tension in a respective pre-tension member is adjusted to a fraction of a pre-adjustment tension.

[0028] The fraction may be within + / - 50% of the pre-adjustment tension, i.e. between 50% - 150% of the pre-adjustment tension.

[0029] Advantageously, the alteration in tension is generally suitable for reducing cable vibration and / or associated noise as desired, without altering the tension of a pretension member beyond the given fraction, which may lead to the pre-tension members and blade connecting members net working effectively.

[0030] In some arrangements, the cable vibration control system can supplement this alteration in tension with other changes in the wind turbine (e.g. if altering tension by between + / - 50% is not sufficient to reduce vibrations and / or noise as desired). For example, the alteration in tension may be performed in conjunction with an alteration in rotational speed of the wind turbine, an alteration in pitch angle of one or more blades and / or yawing of the nacelle. Such an arrangement prevents an alteration of tension outside the between + / - 50% range (i.e. maintaining the effectiveness of the pretension members and / or blade connecting members), while still reducing undesired vibrations and noise generation.

[0031] The cable vibration control system may be configured to control one or more of the tensioning devices to extend or retract such that one or more of the wind turbine blades, the blade connecting members and the pre-tension members oscillate at a frequency outside a predetermined range.

[0032] Advantageously, controlling a frequency of oscillation or vibration of one or more of the tensioning devices to extend or retract is an effective means of controlling cable vibration and associated noise generation from the wind turbine.

[0033] The predetermined range may be between 5% greater than and 5% less than a natural frequency (eigen frequency) of a respective wind turbine blade, blade connecting member or pre-tension member. Advantageously, it has been found that altering the frequency of oscillation to be 5% greater than or less than a natural frequency of the wind turbine blade, blade connecting member or pre-tension member has been found to greatly reduce the noise generated by the wind turbine.

[0034] It will be understood that the term “natural frequency” refers to the frequency at which a component oscillates when not subject to a continuous external force (e.g. wind). Put another way, the term “natural frequency” refers to the frequency at which a component oscillates when disturbed without being subject to a driving or a damping force. An example of such a driving force is periodic gravitational loading during rotor rotation.

[0035] The cable vibration control system may comprise a memory unit, and the cable vibration control system may be configured to store the natural frequency of one or more of the wind turbine blades, blade connecting members or pre-tension members in the memory unit.

[0036] Advantageously, the natural frequency of a component can be recalled by the control unit such that the control unit can quickly identify the change in frequency required to greatly reduce cable vibration and associated noise generation.

[0037] The natural frequency of a component may change as rotor loading changes, e.g. due to changes in wind speed, rotational speed of the blades, blade pitch etc. In this case, the cable vibration control system may be configured to store a range of natural frequencies of one or more of the wind turbine blades, blade connecting members or pre-tension members in the memory unit (e.g. based on different configurations of the wind turbine).

[0038] The cable vibration control system may be coupled to the pitch mechanism and may be configured to control the pitch mechanism to alter a pitch of a respective wind turbine blade so as to control vibration and associated noise generated by the wind turbine.

[0039] Advantageously, the cable vibration control system utilises a pre-existing mechanism to further assist in the control of noise generation. Utilising a change in pitch alongside controlling tension in the pre-tension members (and thus oscillations in the wind turbine blades) has been found to further improve control of cable vibration and associated noise generated by the wind turbine. Moreover, the pitch of wind turbine blades can be adjusted to allow the change in tension in the pre-tension members to have maximum impact on the blades.

[0040] Similarly, the cable vibration control system can alter the blade pitch or a generator torque to modify the rotational speed and thereby change the flow velocities and therefore the excitation frequencies of standing waves in the blade connecting members and the pre-tension members. The cable vibration control system could also activate a wind turbine yaw system to change the flow velocities and therefore the excitation frequencies of standing waves in the blade connecting members 6 and the pre-tension members 8.

[0041] Each tensioning device may comprise an actuator having a first portion coupled to the hub and a second portion movable with respect to the first portion and coupled to the respective pre-tension member, wherein the tensioning device is configured to extend and retract by movement of the second portion with respect to the first portion.

[0042] Advantageously, the actuator provides a simple to control means of changing tension and oscillation frequency in at least some of the wind turbine blades, the blade connecting members and the pre-tension members.

[0043] The actuator may be a linear actuator, such as a hydraulic actuator, an electrical actuator, or a mechanical actuator. The actuator may have a cylinder and a rod moveable into and out of the cylinder. The first portion of the actuator may include the cylinder, and the second portion of the actuator may include the rod.

[0044] The actuator may be a rotary actuator. The actuator may comprise a motor and a tension element, wherein a radially distal end of the tension element is connected to the radially inward end the pre-tension member, and a radially inward end of the tension element is wound around a drum connected to a rotatable shaft of the motor. The first portion of the actuator may include the motor, and the second portion of the actuator may include the tension element.

[0045] The one or more sensors may comprise one or more of: an acoustic sensor, a microphone, a position sensor, a load sensor, a tension sensor, an accelerometer, a pressure sensor, a wind speed sensor, a wind direction sensor, and a wind turbine power generation sensor. The blade connecting members may cause the wind turbine blades to mutually support each other, in the sense that loads on the wind turbine blades, in particular edgewise loads and to some degree flapwise loads, are ‘shared’ among the wind turbine blades.

[0046] The connection points on the wind turbine blades may be arranged at a distance from the root end which is between 10% and 60% of the length of the wind turbine blades from the root end to the tip end, preferably radially inboard of 50% of the length of the respective wind turbine blade from the root end, and more preferably radially inboard of 45% of the length of the respective wind turbine blade from the root end.

[0047] The connection points on the wind turbine blades may be arranged at a position where a thickness-to-chord ratio of the wind turbine blade is between 20% and 50%.

[0048] The wind turbine blades may each comprise an inboard blade part comprising the root end and an outboard blade part comprising the tip end. The inboard blade part and the outboard blade part may be connected to each other at a split position. The inboard blade part may be joined to the outboard blade part by a connection joint. The connection joint may comprise a connector.

[0049] The connector may be a metallic component, preferably a cast component or a machined component. The connector may be a composite component. The connector may be a co-cured or co-bonded component. Providing such a connector may improve ease of manufacture of the connector and provide a lightweight, high strength connector.

[0050] The connector may be coupled to transfer load between a spar cap portion of the inboard blade part and a spar cap portion of the outboard blade part. The connector may be arranged to transfer load from the blade connecting members into the spar cap portion of the inboard blade part. This may improve the load transfer efficiency of the turbine blade as the spar cap portions may be designed to withstand higher loads relative to the blade shell. The connector may comprise connection point for a blade connecting member.

[0051] Each wind turbine blade may comprise a leading edge, a leading edge extension, and a blade shell, wherein the leading edge extension extends forward of the leading edge, and the connection point of the respective wind turbine blade is located forward of the leading edge on the leading edge extension, and each wind turbine blade further comprises a respective fairing extending over at least the leading edge extension.

[0052] The leading edge extension may be integrally formed with the connector. The fairing may cover the connection joint. The fairing may be secured to the connector. The connector may extend outside the profile of the blade shell at the connection joint. This may improve the load transfer across the connection joint but may require a larger fairing.

[0053] Each wind turbine blade may be coupled to two of the blade connecting members. Each of the two blade connecting members may extend from respective connection points of one of the blades. The connection points of the one of the wind turbine blades may be adjacent each other on the same leading edge extension. Each blade connecting member may be independently moveable at the respective first and second connection points to which it attaches. The connection point(s) may comprise a bearing structure.

[0054] The wind turbine may be an upwind wind turbine.

[0055] A second aspect of the invention provides a method of controlling vibration, or resultant noise from, one or more components of a pitch controlled wind turbine, the pitch controlled wind turbine comprising: a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three wind turbine blades, wherein each wind turbine blade extends between a root end connected to the hub via a pitch mechanism, and a tip end; at least three blade connecting members, each blade connecting member extending from a connection point on one wind turbine blade towards a connection point on a neighbouring wind turbine blade, where the connection point on a given wind turbine blade is arranged at a distance from the root end and at a distance from the tip end of the wind turbine blade; at least three pre-tension members, each pre-tension member being connected to one of the blade connecting members and to the hub via a tensioning device, the tensioning device provides radial movement of a radially inward end of the pre-tension member with respect to an axis of rotation of the hub due to extension or retraction of the tensioning device, each pretension member thereby providing pre-tension in the blade connecting member to which it is connected; and a cable vibration control system coupled to one or more of the tensioning devices, and to one or more sensors for outputting one or more signals influenced by a vibration of, or resultant noise from, one or more of the wind turbine blades, the blade connecting members and the pre-tension members; the method comprising controlling the one or more tensioning devices to extend or retract so as to control vibrations and related noise generated by the wind turbine.

[0056] The method may comprise determining that vibration and / or associated noise generated by the wind turbine are at or above a predetermined level based on the one or more signals from the one or more sensors, and controlling one or more of the tensioning devices to extend or retract such that vibrations and / or associated noise are below the predetermined level.

[0057] The method may comprise controlling one or more of the tensioning devices to extend or retract to alter a tension in a respective pre-tension member until the vibrations or noise generated by the wind turbine is at or below the predetermined level.

[0058] The method may comprise controlling one or more of the tensioning devices to extend or retract in a substantially stepwise manner so as to alter the tension in a respective pre-tension member in a substantially stepwise manner.

[0059] The method may comprise identifying one or more conditions at which vibrations or associated noise is at or above the predetermined level and storing the one or more conditions in a memory unit.

[0060] The method may comprise determining that one or more conditions stored in the memory unit is occurring based on a signal from the one or more sensors, and controlling one or more of the tensioning devices to extend or retract to alter a tension in a respective pre-tension member in response thereto.

[0061] At least one of the one or more conditions may be a wind speed and / or a wind direction, and the one or more sensors may include a wind speed sensor and / or a wind direction sensor.

[0062] The method may comprise controlling one or more of the tensioning devices to extend or retract such that a tension in a respective pre-tension member is adjusted to within + / - 50% of the pre-adjustment tension. The method may comprise controlling one or more of the tensioning devices to extend or retract such that one or more of the wind turbine blades, the blade connecting members and the pre-tension members oscillate at a frequency outside a predetermined range.

[0063] The predetermined range may be between 5% greater than and 5% less than a natural frequency of a respective wind turbine blade, blade connecting member or pre-tension member.

[0064] The method may comprise storing the natural frequency of one or more of the wind turbine blades, blade connecting members or pre-tension members in a memory unit.

[0065] The method may comprise controlling the pitch mechanism to alter a pitch of a respective wind turbine blade so as to control vibration and associated noise generated by the wind turbine.

[0066] Each tensioning device may comprise an actuator having a first portion coupled to the hub and a second portion movable with respect to the first portion and coupled to the respective pre-tension member, wherein the tensioning device is configured to extend and retract by movement of the second portion with respect to the first portion.

[0067] The one or more sensors may comprise one or more of: an acoustic sensor, a microphone, a position sensor, a load sensor, a tension sensor, an accelerometer, a pressure sensor, a wind speed sensor, a wind direction sensor, and a wind turbine power generation sensor.

[0068] Any of the method steps outlined herein may be performed by the cable vibration control system described in the first aspect.

[0069] BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0071] Figure 1 shows a front view of a wind turbine according to a first example;

[0072] Figure 2 shows a side view of the wind turbine; Figure 3 shows a partially side view of components adjacent the hub of a wind turbine;

[0073] Figure 4 shows a schematic of a cable vibration control system according to an example;

[0074] Figure 5A shows a schematic of a blade connecting member or pre-tension member according to a first example;

[0075] Figure 5B shows a schematic of a blade connecting member or pre-tension member according to a second example;

[0076] Figure 6 shows a wind turbine blade.

[0077] DETAILED DESCRIPTION OF EMBODIMENT(S)

[0078] In this specification, terms such as leading edge, trailing edge, pressure surface, suction surface, thickness, and chord are used. While these terms are well known and understood to a person skilled in the art, definitions are given below for the avoidance of doubt.

[0079] The term leading edge is used to refer to an edge of the blade which will be at the front of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

[0080] The term trailing edge is used to refer to an edge of a wind turbine blade which will be at the back of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

[0081] The chord of a blade is the straight line distance from the leading edge to the trailing edge in a given cross section perpendicular to the blade spanwise direction. The term chordwise is used to refer to a direction from the leading edge to the trailing edge, or vice versa.

[0082] A pressure surface (or windward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which, when the blade is in use, has a higher pressure than a suction surface of the blade.

[0083] A suction surface (or leeward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which will have a lower pressure acting upon it than that of a pressure surface, when the blade is in use. The thickness of a wind turbine blade is measured perpendicularly to the chord of the blade and is the greatest distance between the pressure surface and the suction surface in a given cross section perpendicular to the blade spanwise direction.

[0084] The term spanwise is used to refer to a direction from a root end of a wind turbine blade to a tip end of the blade, or vice versa. When a wind turbine blade is mounted on a wind turbine hub, the spanwise and radial directions will be substantially the same.

[0085] The term outboard refers to a radial direction from the hub of the blade towards the tip end of the blade. The term inboard refers to a radial direction from the tip end towards the hub.

[0086] A view which is perpendicular to both of the spanwise and chordwise directions is known as a planform view. This view looks along the thickness dimension of the blade.

[0087] Figures 1 and 2 show a pitch controlled wind turbine 1 according to a first example. Figure 1 is a front view of the wind turbine 1 , and Figure 2 is a side view of the wind turbine 1 . The wind turbine 1 includes a tower 2 and a nacelle 3 mounted on the tower 2. A hub 4 is mounted rotatably on the nacelle 3, and carries three wind turbine blades 5 projecting outwardly from the nacelle 3. While the example wind turbine 1 shown in Figures 1 and 2 has three blades 5, it will be appreciated that other numbers of blades 5 are possible. When wind blows against the wind turbine 1 , the wind turbine blades 5 generate a lift force which causes a generator (not shown) within the nacelle 3 to generate electrical energy.

[0088] It will be appreciated that the wind turbine 1 depicted may be any suitable type of wind turbine 1. The wind turbine 1 shown is an upwind wind turbine, although it will be appreciated the wind turbine 1 may be a downwind wind turbine. The wind turbine 1 may be an onshore wind turbine such that the foundation is embedded in the ground, or the wind turbine 1 may be an offshore installation in which case the foundation would be provided by a suitable marine platform.

[0089] Blade connecting members 6 interconnect neighbouring wind turbine blades 5 between connection points 7a, 7b on the wind turbine blades 5. In the illustrated arrangement, three blade connecting members 6 are provided (i.e. one per blade 5). The connecting members 6 are cables, e.g. metallic (e.g. steel) or polymer (for example comprising ultra-high molecularweight polyethylene - LIHMWPE) cables. In some examples, each wind turbine blade 5 is coupled to two blade connecting members 6. Each two blade connecting members 6 extend from respective connection points 7a, 7b on one of the blades 5. Each blade connecting member 6 may be independently moveable at the respective first and second connection points 7a, 7b to which it attaches. The connection point(s) 7a, 7b may comprise a bearing structure.

[0090] A pre-tension member 8 extends from one of each of the blade connecting members 6 and towards a common point 25 (see Figure 3) arranged at or adjacent the hub 4. In the example shown in Figures 1 and 2, the pre-tension members 8 extend to the hub 4. The pre-tension members 8 are configured to provide pre-tension in the blade connecting members 6. The pre-tension members 8 are typically cables, e.g. metallic or polymer cables. In the illustrated arrangement, three pre-tension members 8 are provided (i.e. one per blade 5).

[0091] The pre-tension members 8 are coupled to the hub 4 by respective tensioning devices 9. Each tensioning device 9 provides radial movement of a radially inward end of the pre-tension member 8 with respect to an axis of rotation 10 (see Figure 3) of the hub due to extension or retraction of the tensioning device 9. Each pre-tension member 8 thereby provides pre-tension in the blade connecting member 6 to which it is connected. In the figures, three tensioning devices 9 are provided (i.e. one per pretension member 8).

[0092] The wind turbine blades 5 have a root end 11 proximal to the hub 4, adapted to be connected to the hub 4 via a pitch mechanism 50 (see Figure 3), and a tip end 12 distal from the hub 4. The blades 5 include a leading edge 13 and a trailing edge 14 that extend between the respective root end 11 and tip end 12. Each of the blades 5 include a suction side 15 and a pressure side 16 (see Figure 5). A thickness dimension of the blade 5 extends between the suction side 15 and the pressure side 16.

[0093] The connection points 7a, 7b are provided between the root end 11 and the tip end 12 of a respective blade 5 (i.e. at a distance from the root end 11 and at a distance from the tip end 12). The connection points 7a, 7b may be between 10% and 60% of the length of the wind turbine blade 5 from the root end 11 to the tip end 12 in the radial direction but are preferably radially inboard of 50% of the length of the wind turbine blade 5 from the root end 11 to the tip end 12, and more preferably radially inboard of 45% of the length of the wind turbine blade 5 from the root end 11 to the tip end 12, e.g. around 30-40%. It will be appreciated that the connection points 7a, 7b may be adjacent each other. Alternatively, the connection points 7a, 7b may be spaced from one another.

[0094] The wind turbine 1 includes a cable vibration control system 17 illustrated schematically in Figures 3 and 4. The cable vibration control system 17 is configured to detect a vibration of, or resultant noise from, one or more of the wind turbine blades 5, the blade connecting members 6, and the pre-tension members 8, and in response thereto, control various components of the wind turbine 1 to control vibrations and related noise generated by the wind turbine 1 .

[0095] The cable vibration control system 17 is coupled to one or more sensors 19 for outputting one or more signals influenced by a vibration of, or resulting noise from, one or more of the wind turbine blades 5, blade connecting members 6, and the pre-tension members 8.

[0096] The one or more sensors 19 are configured to output a signal influenced by a vibration or resultant noise from one or more of the wind turbine blades 5, the blade connecting 6 members and the pre-tension members 8. It will be understood that in some arrangements, the one or more sensors directly measure component vibration or associated noise, e.g. one or more sensors 19 are for detecting a vibration of, or resultant noise from, one or more of the wind turbine blades 5, the blade connecting members 6 and the pre-tension members 8.

[0097] In alternative arrangements, one or more sensors 19 may infer the presence of vibrations or associated noise indirectly. For example, it is known that ice accumulation on wind turbine components causes vibration thereof. In this case, ice presence and thus component vibration may be inferred via one or more sensors 19 that records power generation of the wind turbine 1 or that generates a wind turbine 1 power curve (e.g. a reduction in power output of the wind turbine may suggest ice presence due to reduced aerodynamic performance).

[0098] In another example, a tension in a blade connection member 6 and / or a pre-tension member 8 may indicate the presence of vibrations or associated noise. In this case, the one or more sensors 19 may include a cable tension sensor. Tension in a blade connection member 6 and / or a pre-tension member 8 may be inferred via one or more sensors 19 that records a pressure in the tensioning device 9 or that measures an extension position of the tensioning device 9. In this way, the one or more sensors 19 may include a pressure sensor or a position sensor, respectively.

[0099] The one or more sensors 19 may include an acoustic sensor, a microphone, a position sensor, a load sensor, a tension sensor, an accelerometer, a pressure sensor, a wind speed sensor, a wind direction sensor, a power generation sensor, or any suitable sensor that can provide the cable vibration control system 17 with information regarding the presence of vibrations in wind turbine components and / or associated noise.

[0100] The cable vibration control system 17 is coupled to one or more of the tensioning devices 9. In Figures 3 and 4, only one tensioning device 9 is indicated, but it will be appreciated that the cable vibration control system 17 may be coupled to a plurality (e.g. all) of the tensioning devices 9 in some arrangements. The cable vibration control system 17 is configured to control one or more of the tensioning devices 9 to extend or retract so as to control vibrations and related noise generated by the wind turbine 1.

[0101] Utilising the cable vibration control system 17 to control the tensioning devices 9 can assist in controlling the size and / or frequency of vibrations or oscillations in one or more of the blade connecting members 6, pre-tension members 8, and / or wind turbine blades 5, thereby allowing effective control over vibrations and resultant noise emitted therefrom. Controlling the tensioning devices 9 to reduce noise generated by the wind turbine 1 utilises pre-existing equipment to alleviate the negative impacts of vibrations (e.g. vibrations in the cables of the blade connecting members 6 and / or pre-tension members 8), allowing improved control over cable vibrations and associated noise without a significant increase in complexity of the wind turbine design.

[0102] In Figure 3, the hub 4 of the wind turbine is generally shown. The hub 4 may include a hub member 57 extending from the hub substantially along a direction defined by a rotational axis of the hub 10. The pre-tension members 8 may be connected to the hub member 57. As can be seen, the tensioning devices 9 are connected at a radial inner end thereof to or towards the common region or point 25. The common point 25 may provide a pivotal connection to each of the tensioning devices 18. In Figure 3, the common point 25 is on a projection 37 extending forward of the hub 4 in an upwind direction of the wind turbine 1. The projection 37 may be a framework or structure rigidly coupled to the hub 4. A spinner 29 may be coupled to the hub to cover a radially inner portion of each tensioning device 9. The spinner 29 may shroud the common point 25. In this way, the spinner 29 can be seen as an important component in providing protection to the radially inner portion of each tensioning device 9.

[0103] The tensioning device 9 may include an actuator 51 having a first portion 53 coupled to the hub 4 and a second portion 54 movable with respect to the first portion 53 and coupled to the respective pre-tension member 8. The tensioning device 9 extends and retracts by movement of the second portion 54 with respect to the first portion 53. Extension and retraction of the tensioning device 9 changes the tension in pre-tension member 8. In such an arrangement, the cable vibration control system 17 may be coupled to the actuator 51 so as to control movement of the second portion 54 relative to the first portion 53 and thus alter a tension and control a vibration in a respective pre-tension member 8.

[0104] The actuator 51 may be a linear actuator having a cylinder and a rod moveable into and out of the cylinder. The actuator 51 may be hydraulic or electro-mechanical, for example. The cylinder may be the first portion 53 and the rod may be the second portion 54. In such an arrangement, the sensor 19 may include a pressure sensor coupled to the actuator 51 to determine the presence and / or extent of vibrations in a pre-tension member 8.

[0105] The actuator 51 may be a rotary actuator. Although not shown, the actuator 51 may include a motor and a tension element, wherein a radially distal end of the tension element is connected to the radially inward end the pre-tension member 8, and a radially inward end of the tension element is wound around a drum connected to a rotatable shaft of the motor. The first portion 53 of the actuator 51 may include the motor, and the second portion 54 of the actuator 51 may include the tension element.

[0106] It will be appreciated that the tensioning device 9 may be in any suitable form to alter a tension in at least some of the wind turbine blades 5, blade connecting members 6 and the pre-tension members 6.

[0107] The cable vibration control system 17 may be configured to determine that cable vibration and / or associated noise generated by the wind turbine are at or above a predetermined level based on a signal from the one or more sensors 19. The cable vibration control system 17 may be configured to control one or more of the tensioning devices 9 to extend or retract such that cable vibrations and / or associated noise are below the predetermined level.

[0108] The predetermined level may be any value, e.g. based on regulatory requirements or based on a maximum noise level that is acceptable to the public. The predetermined level may be based on individual frequencies (i.e. frequencies of vibrations of the blade connection members 6 or pre-tension members 8 being at / below a threshold) or based on an integrated sound power level or levelized noise being at / below a threshold. The integrated sound power level or levelized noise may also be assessed over a specific frequency band or specified independently for multiple bands.

[0109] The cable vibration control system 17 may include a memory unit 21. The memory unit 21 may be configured to store relevant data and information, either inputted by a user or obtained by the control system 17 (e.g. based on data from the sensor 19). In some arrangements, the predetermined level is inputted by a user into the memory unit 21 such that the cable vibration control system 17 can compare data from one or more sensors 19 against the stored predetermined level. The memory unit 21 may be in any suitable form, e.g. a dual inline memory module (DIMM).

[0110] The cable vibration control system 17 may be configured to convey a signal to a respective tensioning device 9 (e.g. in response to a signal from a sensor 19 that vibration and / or noise are at or above the predetermined level). The cable vibration control system 17 may be configured to control one or more of the tensioning devices 9 to extend and / or retract so as to alter a tension in a respective pre-tension member 8. The cable vibration control system 17 may control the tensioning device 9 to alter the tension until the control system 17 determines that vibrations or noise generated by the wind turbine 1 is at or below the predetermined level. Alternatively or additionally, the cable vibration control system 17 may control the tensioning device 9 to alter tension in a pre-tension member 8 for a predetermined amount of time (i.e. after which it is expected that the vibration or noise generation will have ceased).

[0111] It should be understood that signals between the cable vibration control system 17 and other components (e.g. sensors 19, tensioning devices 9) are conveyed via any suitable means, e.g. electrical signals via electrical wiring, wireless communication, mechanical signals, optical signals. For example, the cable vibration control system 17 may include an electrical circuit for receiving signals from the sensors 19 and for sending electrical power to operate the actuator 51. Alternatively, where the actuator 51 is hydraulically or pneumatically powered, the control system 17 may include a pneumatic or hydraulic circuit for controlling the actuator 51 to alter tension in the pretension members 8.

[0112] Altering the tension in a respective pre-tension member 8 will alter the frequency of vibrations occurring in the member 8, as well as in the blade connection members 6 and the blades 5. In this way, an alteration in tension in the pre-tension member 8 can impact the extent of vibrations and noise generated by vibrations in the blade connection members 6, pre-tension members 8 and blades 5.

[0113] In some arrangements, the cable vibration control system 17 is configured to operate in a feedback loop. In such an arrangement, the control system 17 is configured to determine whether the vibration or noise level is too high (e.g. based on a signal from a sensor 19 and comparing said signal against the predetermined level stored in the memory 21). In response to determining that the vibration or noise level is above the predetermined level, the cable vibration control system 17 may be configured to instruct one or more tensioning devices 9 to alter a tension in one or more pre-tension members 8. The cable vibration control system 17 may then check whether the vibration or noise level is still too high (i.e. by comparing updated signals from the sensors 19 against the predetermined level in the memory unit 21), and either control the tensioning device 9 to alter the tension again (i.e. if the level is still above the predetermined level), or maintain the tension in the pre-tension member 8 (i.e. if the level is below the predetermined level).

[0114] The cable vibration control system 17 may be configured to control one or more tensioning devices 9 to extend or retract in a substantially stepwise manner so as to alter a tension in a respective pre-tension member 8 in a substantially stepwise manner. Put another way, the cable vibration control system 17 may control one or more of the tensioning devices 9 to extend or retract so as to cause a step change (i.e. an increase or reduction) of the tension in the pre-tension member 8.

[0115] The term “substantially stepwise” refers to the limitations of equipment to achieve a completely instantaneous change in tension applied by the tensioning device 9. It should be understood that the change in tension applied by the tensioning device 9 is intended to be as instantaneous as is permitted by the equipment (e.g. by the control system 17 and the actuator 51).

[0116] The substantially stepwise extension or retraction of a given tensioning device 9 can improve the speed at which the control system 17 can have an impact on vibration of the blade connecting members 6, pre-tension members 8 and / or wind turbine blades 5 (e.g. compared to a more gradual, continuous change), thereby improving the effectiveness of the cable vibration control system 17. This is particularly advantageous when cable vibrations are of the standing wave type, where a certain step change may be advantageous and / or required to eliminate the standing wave, e.g. a step change of 3-10% of tension in blade connecting member may be preferable.

[0117] In an example arrangement, the cable vibration control system 17 is configured to control one or more of the tensioning devices 9 to extend or retract such that a tension in a respective pre-tension member 8 is adjusted to a fraction of a pre-adjustment tension. The term “pre-adjustment tension” is intended to refer to the tension in a respective pre-tension member 8 prior to the cable vibration control system 17 intervening. In some arrangements, the cable vibration control system 17 is configured to adjust the tension in a respective pre-tension member 8 to within + / - 50% of the preadjustment tension. Such a fraction may avoid damage to the pre-tension members 8 or blade connection members 6, while also assisting the pre-tension members 8 and blade connection members 6 in working effectively.

[0118] The cable vibration control system 17 may be configured to supplement the alteration in tension with other changes in the wind turbine (e.g. if altering tension to within + / - 50% is not sufficient to reduce vibrations and / or noise as desired). For example, the alteration in tension may be performed in conjunction with an alteration in rotational speed of the wind turbine 1. Such an arrangement avoids the need to alter the tension beyond the + / - 50% range (i.e. maintaining the effectiveness of the pre-tension members 8 and / or blade connecting members 6), while still reducing undesired vibrations and noise generation. Alternatively, or additionally, alteration in tension may be performed in conjunction with pivoting one or more blades and / or yawing of the nacelle.

[0119] The cable vibration control system 17 may be configured to control one or more of the tensioning devices 9 to extend or retract such that one or more of the wind turbine blades 5, the blade connecting members 6 and the pre-tension members 8 oscillate or vibrate at a frequency outside a predetermined range. The predetermined range may be a range within which the vibration and associated noise of wind turbine 1 components is undesirable (e.g. based on regulatory requirements of noise generation, or based on structural requirements of the various components). The predetermined range may be stored in the memory unit 21 (e.g. inputted by a user) such that the cable vibration control system 17 can compare measured frequency values (e.g. from the sensors 19) against the stored predetermined range.

[0120] The predetermined range may be between 5% greater than and 5% less than a natural frequency of a respective wind turbine blade 5, blade connecting member 6 or pretension member 8. It will be understood that the term “natural frequency” refers to the frequency at which a component oscillates when not subject to a continuous external force (e.g. wind). The cable vibration control system 17 may control the change in tension in a substantially stepwise manner.

[0121] It has advantageously been found that altering the frequency to be outside such a range greatly reduces the noise generated by the respective wind turbine 1 component. This arrangement is particularly advantageous when a respective wind turbine 1 component is experiencing vortex induced vibration (VIV), e.g. where the VIV causes vibration at a frequency that is the same as the natural frequency of the respective component. This can result in excessive vibrations and resulting noise generation. In this way, altering the tension in a respective component such that the frequency of vibration is outside the predetermined range is a simple means of preventing or reducing such vibration and noise generation. In some arrangements, the predetermined range may be different.

[0122] It will be understood that the predetermined range may be between at least 5% greater than and at most 5% less than the natural frequency of a wind turbine blade 5. In some cases, a greater increase or reduction in frequency may be implemented by the cable vibration control system 17.

[0123] The cable vibration control system 17 may be configured to store the natural frequency of one or more wind turbine blades 5, blade connecting members 6 or pre-tension members 8 in the memory unit 21 (e.g. the natural frequencies may be user-inputted, or a series of look-up tables may be stored in the memory unit 21). In such an arrangement, the cable vibration control system 17 can quickly identify the change in frequency required to reduce vibration and associated noise in response to identifying that excessive vibration and / or noise generation is occurring (e.g. via a signal from the one or more sensors 19).

[0124] In some arrangements, it may not be possible to store the natural frequency of various wind turbine 1 components in the memory unit 21 , for example, the natural frequency of the blade connection members 6, blades 5 and / or pre-tension members 8 may change based on environmental factors, for instance if ice has accumulated on a surface thereof. Additionally, or alternatively, it may be preferred for the cable vibration control system 17 to control various components to avoid excessive cable vibration or noise generation without requiring a user to input the natural frequencies of various components. To address these issues, the cable vibration control system 17 may be configured to learn from experience by recording conditions when excessive vibration or noise occurs. The cable vibration control system 17 may be configured to identify one or more conditions at which vibrations or associated noise is at or above the predetermined level and store the one or more conditions in the memory unit 21.

[0125] In an example arrangement, the one or more sensors 19 may include a first sensor arrangement configured to identify the presence of vibrations or noise (e.g. an accelerometer or motion sensor coupled to a surface of a respective component to identify vibrations, and / or a microphone positioned near a respective component to identify noise generation). The one or more sensors 19 may include a second sensor arrangement configured to identify conditions (e.g. ambient conditions) proximal the wind turbine 1 , for example, weather conditions. In this case, the second sensor arrangement may include one or more of a wind speed sensor, a wind direction sensor, a temperature sensor, a position sensor (e.g. on the surface of a wind turbine component to detect the presence of ice thereon), or the like. As discussed above, the cable vibration control system 17 is configured to determine whether vibration or noise generation is at or above the predetermined level (e.g. by comparing the signal from the first sensor arrangement of the one or more sensors 19 with the predetermined level stored in the memory unit 21). If the cable vibration control system 17 determines the vibration or noise generation is at or above the predetermined level, the cable vibration control system 17 may store the conditions detected by the second sensor arrangement when the vibrations or noise generation exceeds the predetermined level in the memory unit 21. The cable vibration control system 17 may be configured to determine that one or more conditions stored in the memory unit 21 is occurring based on a signal from the sensors 19 (i.e. from the second sensor arrangement). In response thereto, the cable vibration control system 17 is configured to control one or more of the tensioning devices 9 to extend or retract to alter a tension in a respective pre-tension member 8. The cable vibration control system 17 is configured to effectively predict that vibrations or associated noise generated by the wind turbine 1 is likely to exceed the predetermined level based on previous events (e.g. based on stored conditions at which the predetermined level had previously been exceeded). In response to this prediction, the cable vibration control system 17 can pre-emptively alter the tension in a pre-tension member 8 to avoid the vibration or associated noise ever exceeding the predetermined level. The wind turbine 1 is thus less likely to generate such vibrations or noise that exceeds an acceptable level. In some arrangements, the conditions may be inputted into the memory unit 21 by a user based on previous experience (e.g. with other pitch controlled wind turbines 1).

[0126] In an example arrangement, at least one of the conditions is a wind speed and / or wind direction and the one or more sensors 19 includes a wind speed sensor and / or a wind direction sensor. The wind speed sensor and / or wind direction sensor may be provided proximal to or on the wind turbine 1 to provide accurate information regarding the environmental conditions proximal to the wind turbine 1. In such an example, the cable vibration control system 17 may be configured to identify that VIV is likely to occur based on a wind speed and / or wind direction that previously caused excessive component vibration or noise generation. Allowing the cable vibration control system 17 to learn from previous incidents of excess vibration and noise generation can improve the effectiveness of the control system 17 and reduce the occurrence of future incidents of excess vibration and / or noise generation.

[0127] It should be noted that other conditions and sensor arrangements may be provided to assist the cable vibration control system 17 in identifying that excess vibration or noise generation is likely to occur. For example, the condition may include rotational speed of the wind turbine 1 may be utilised to indicate that VIV is occurring, and so the sensor 19 may include a rotational speed sensor connected for example to a rotor (not shown) of the wind turbine 1 . Blade orientation and / or pitch angle may also indicate that VIV is likely to occur, and so the sensor 19 may include a positional sensor coupled with the blade 5 and / or the pitch mechanism 50. Tension in the blade connecting members 6 and / or a difference in tension between the blade connecting members 6 may indicate that VIV is likely to occur. In this case, the sensor 19 may include one or more tension sensors on the surface of each blade connecting member 6. Any combination of the above conditions may be utilised by the cable vibration control system 17 to predict the likelihood of VIV occurring and / or the likelihood of excessive vibration and associated noise generation of wind turbine components 1.

[0128] The cable vibration control system 17 may include a built-in tolerance to avoid responding to an inaccurate signal from a sensor 19 regarding ambient conditions. For example, the cable vibration control system 17 may not respond to such a signal unless the same signal is repeated at least once, or continuously over a given time period. Such an arrangement avoids the control system 17 altering tension in the system based on faulty or anomalous signals.

[0129] As shown in Figures 3 and 4, the cable vibration control system 17 may be coupled to the pitch mechanism 50 in some arrangements. The cable vibration control system 17 may be configured to control the pitch mechanism 50 to alter a pitch of a respective wind turbine blade 5 or blades 5. Such an arrangement can assist in altering a frequency of vibration of the respective blade 5. Altering the pitch of a blade 5 can also alter the tension in the blade connecting members 6 and the pre-tension members 8, thereby facilitating control of the vibration and noise generation of such components. The cable vibration control system 17 may be configured to control the pitch mechanism 50 to alter the pitch of all of the blades 5 simultaneously in some arrangements, so as to avoid an imbalanced wind turbine 1.

[0130] Controlling the pitch is an effective way of using a pre-existing system (i.e. the pitch mechanism 50) to assist in controlling vibration and associated noise generation by wind turbine 1 components. As used herein, it will be understood that the term “pitch” refers an angle of the wind turbine blade with respect to the oncoming wind in the wind turbine 1. The cable vibration control system 17 may be configured to alter the pitch of a respective blade 5 in combination with any of the above configurations of controlling the tensioning device 9. For example, the system 17 may be configured to simultaneously alter the pitch angle and control the tensioning devices 9 to alter tension in the blade connecting members 6, pre-tension members 8 or blades 5 (e.g. as discussed in relation to the arrangements above). In some arrangements, the cable vibration control system 17 may be configured to alter the blade pitch or a generator (not shown) torque to modify the rotational speed of the wind turbine 1 and thereby change the flow velocities and therefore the excitation frequencies of standing waves in the blade connecting members 6 and the pre-tension members 8. The cable vibration control system 17 could also activate a turbine yaw system (not shown) to change the flow velocities and therefore the frequencies of standing waves in the blade connecting members 6 and the pre-tension members 8.

[0131] Referring to Figure 4, a schematic of the cable vibration control system 17 is represented. As has been discussed, the cable vibration control system 17 is coupled to the tensioning devices 9 and to the pitch mechanism 50. The cable vibration control system 17 includes the memory unit 21 for storing information (e.g. natural frequency of components, ambient conditions at which VIV is likely to occur, predetermined level of vibrations or noise generation that is deemed acceptable). The cable vibration control system 17 is coupled to one or more sensors 19.

[0132] One or more sensors 19 may be provided within or on one or more of the blade connecting members 6 or the pre-tension members 8 (e.g. embedded within or position on the cable), see for example, Figures 5A and 5B. In some examples one or more sensors 19 may be arranged away from the blade connecting members 6 and pretension members 8, such as embedded within or mounted on a part of the blade (e.g. see Figure 6) or on the nacelle, or external but proximal to the wind turbine 1 . The sensors 19 may supply the control system 17 with information regarding the vibration or noise generated by vibration of a respective blade connecting member 6, pre-tension member 8 or blade 5 and / or ambient conditions at or proximal to the surface of wind turbine 1 components. The cable vibration control system 17 may only be operational when the sensors 19 detect excessive vibration and / or noise generation and / or a condition at which excessive vibration and / or noise generation is expected to occur. In this way, the energy requirements of the control system 17 may be reduced as the system 17 may only be operational when required.

[0133] Figures 5A and 5B schematically indicate an example blade connecting member 6 or a pre-tension member 8 (i.e. a cable that could serve either function). A plurality of sensors 19 may be provided with the respective blade connecting member 6 or pretension member 8, e.g. as shown in Figure 5A, distributed along a length of a respective blade connecting member 6 or pre-tension member 8. A sensor 19 may alternatively be provided that extends along substantially the entire length of the respective blade connecting member 6 or pre-tension member 8, e.g. as shown in Figure 5B.

[0134] An example of a wind turbine blade 5 is shown in Figure 6. The blades 5 include a suction side 15 and a pressure side 16. A thickness dimension of the blade 5 extends between the suction side 15 and the pressure side 16. Each blade 5 may have a cross section which has a substantially circular profile near the root end 11. The blade 5 may transition from a circular profile to an aerofoil profile moving from the root end 11 of the blade 5 outboard. The blade 5 may comprise a “shoulder” 22 outboard of the root end 11 , which is the widest part of the blade where the blade 5 has its maximum chord. The blade 5 may have an aerofoil profile of progressively decreasing thickness in an outboard portion of the blade. The progressively decreasing thickness may extend from the shoulder 22 to the tip end 12.

[0135] Each of the blades 5 may be a split blade formed of an inboard blade portion 23 and an outboard blade portion 24 coupled together. Each blade portion 23, 24 has a blade shell 52 that defines a respective leading edge 13a, 13b, trailing edge 14a, 14b, suction side 15a, 15b, and pressure side 16a, 16b.

[0136] The inboard portion 23 and outboard portion 24 of each blade 5 may be connected at a connection joint indicated by connection line 40. The connection line 40 between the inboard and outboard blade portions 23, 24 may be a spanwise split, with the connection line 40 being chordwise. The inboard blade portion 23 extends from the blade root 11 to the connection line 40. The outboard blade portion 24 extends from the blade connection line 40 to the blade tip 12.

[0137] It will be appreciated that the blade 5 may have any number of blade portions 23, 24, with respective connection joints between them. Alternatively, the blades 5 may not be split blades and may instead extend continuously from the root end 11 to the tip end 12 without any connection joint. The split blade 5 may include a bolted connection in some examples.

[0138] In Figure 6, a sensor 19 is provided within the inboard portion 23 of the blade 5. It should be understood that sensors 19 may be provided at any area of the blade 5, or in multiple areas of the blade. In some arrangements, no sensor 19 is provided with the blade.

[0139] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1. A pitch controlled wind turbine (1) comprising a tower (2), a nacelle (3) mounted on the tower (2), a hub (4) mounted rotatably on the nacelle (3), and at least three wind turbine blades (5), wherein each wind turbine blade (5) extends between a root end (11) connected to the hub (4) via a pitch mechanism (50), and a tip end (12), the wind turbine (1) further comprising: at least three blade connecting members (6), each blade connecting member (6) extending from a connection point (7a, 7b) on one wind turbine blade (5) towards a connection point (7a, 7b) on a neighbouring wind turbine blade (5), where the connection point (7a, 7b) on a given wind turbine blade (5) is arranged at a distance from the root end (11) and at a distance from the tip end (12) of the wind turbine blade (5); at least three pre-tension members (8), each pre-tension member (8) being connected to one of the blade connecting members (6) and to the hub (4) via a tensioning device (9), wherein the tensioning device (9) provides radial movement of a radially inward end of the pre-tension member (8) with respect to an axis of rotation (10) of the hub (4) due to extension or retraction of the tensioning device (9), each pre-tension member(8) thereby providing pre-tension in the blade connecting member (6) to which it is connected; and a cable vibration control system (17) coupled to one or more of the tensioning devices(9), and to one or more sensors (19) for outputting one or more signals influenced by a vibration of, or resultant noise from, one or more of the wind turbine blades (5), the blade connecting members (6) and the pre-tension members (8); wherein the cable vibration control system (17) is configured to control the one or more tensioning devices (9) to extend or retract so as to control vibrations and related noise generated by the wind turbine (1).

2. The pitch controlled wind turbine (1) according to claim 1 , wherein the cable vibration control system (17) is configured to determine that vibration and / or associated noise generated by the wind turbine (1) are at or above a predetermined level based on the one or more signals from the one or more sensors (19), and is configured to control one or more of the tensioning devices (9) to extend or retract such that vibrations and / or associated noise are below the predetermined level.

3. The pitch controlled wind turbine (1) according to claim 2, wherein the cable vibration control system (17) is configured to control one or more of the tensioning devices (9) to extend or retract to alter a tension in a respective pre-tension member (8) until the cable vibration control system (17) determines that vibrations or noise generated by the wind turbine (1) is at or below the predetermined level.

4. The pitch controlled wind turbine (1) according to claim 2 or 3, wherein the cable vibration control system (17) is configured to control one or more of the tensioning devices (9) to extend or retract in a substantially stepwise manner so as to alter the tension in a respective pre-tension member (8) in a substantially stepwise manner.

5. The pitch controlled wind turbine (1) according to any of claims 2 to 4, wherein the cable vibration control system (17) comprises a memory unit (21), and is configured to identify one or more conditions at which vibrations or associated noise is at or above the predetermined level and store the one or more conditions in the memory unit (21).

6. The pitch controlled wind turbine (1) according to claim 5, wherein the cable vibration control system (17) is configured to determine that one or more conditions stored in the memory unit (21) is occurring based on a signal from the one or more sensors (19), and is configured to control one or more of the tensioning devices (9) to extend or retract to alter a tension in a respective pre-tension (8) member in response thereto.

7. The pitch controlled wind turbine (1) according to claim 5 or claim 6, wherein at least one of the one or more conditions is a wind speed and / or a wind direction, and the one or more sensors (19) includes a wind speed sensor and / or a wind direction sensor.

8. The pitch controlled wind turbine (1) according to any preceding claim, wherein the cable vibration control system (17) is configured to control one or more of the tensioning devices (9) to extend or retract such that a tension in a respective pre-tension member (8) is adjusted to within + / - 50% of a pre-adjustment tension.

9. The pitch controlled wind turbine (1) according to any preceding claim, wherein the cable vibration control system (17) is configured to control one or more of the tensioning devices (9) to extend or retract such that one or more of the wind turbine blades (5), the blade connecting members (6) and the pre-tension members (8) oscillate at a frequency outside a predetermined range.

10. The pitch controlled wind turbine (1) according to claim 9, wherein the predetermined range is between 5% greater than and 5% less than a natural frequency of a respective wind turbine blade (5), blade connecting member (6) or pre-tension member (8).

11. The pitch controlled wind turbine (1) according to claim 10, wherein the cable vibration control system (17) comprises a memory unit (21), and wherein the cable vibration control system (17) is configured to store the natural frequency of one or more of the wind turbine blades (5), blade connecting members (6) or pre-tension members (8) in the memory unit (21).

12. The pitch controlled wind turbine (1) according to any preceding claim, wherein the cable vibration control system (17) is coupled to the pitch mechanism (50) and is configured to control the pitch mechanism (50) to alter a pitch of a respective wind turbine blade (5) so as to control vibration and associated noise generated by the wind turbine (1).

13. The pitch controlled wind turbine (1) according to any preceding claim, wherein the one or more sensors (19) comprise one or more of: an acoustic sensor, a microphone, a position sensor, a load sensor, a tension sensor, an accelerometer, a pressure sensor, a wind speed sensor, a wind direction sensor, and a wind turbine power generation sensor.

14. A method of controlling vibration, or resultant noise from, one or more components of a pitch controlled wind turbine (1), the pitch controlled wind turbine (1) comprising: a tower (2), a nacelle (3) mounted on the tower (2), a hub (4) mounted rotatably on the nacelle (3), and at least three wind turbine blades (5), wherein each wind turbine blade (5) extends between a root end (11) connected to the hub (4) via a pitch mechanism (50), and a tip end (12); at least three blade connecting members (6), each blade connecting member (6) extending from a connection point (7a, 7b) on one wind turbine blade (5) towards a connection point (7a, 7b) on a neighbouring wind turbine blade (5), where the connection point (7a, 7b) on a given wind turbine blade (5) is arranged at a distance from the root end (11) and at a distance from the tip end (12) of the wind turbine blade (5);at least three pre-tension members (8), each pre-tension member (8) being connected to one of the blade connecting members (6) and to the hub (4) via a tensioning device (9), wherein the tensioning device (9) provides radial movement of a radially inward end of the pre-tension member (8) with respect to an axis of rotation (10) of the hub (4) due to extension or retraction of the tensioning device (9), each pretension member (8) thereby providing pre-tension in the blade connecting member (6) to which it is connected; and a cable vibration control system (17) coupled to one or more of the tensioning devices (9), and to one or more sensors (19) for outputting one or more signals influenced by a vibration of, or resultant noise from, one or more of the wind turbine blades (5), the blade connecting members (6) and the pre-tension members (8); the method comprising controlling the one or more tensioning devices (9) to extend or retract so as to control vibrations and related noise generated by the wind turbine (1).

15. The method according to claim 14, further comprising determining that vibration and / or associated noise generated by the wind turbine (1) are at or above a predetermined level based on the one or more signals from the one or more sensors (19), and controlling one or more of the tensioning devices (9) to extend or retract such that vibrations and / or associated noise are below the predetermined level.

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