Wind turbine
Patent Information
- Application Number
- ES2023725951T
- Authority / Receiving Office
- ES · ES
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2023-05-12
- Publication Date
- 2026-09-14
- Estimated Expiration
- 2043-05-12
Smart Images
Figure 00000013_0000 
Figure 00000014_0000 
Figure 00000015_0000
Abstract
Description
Wind turbine Technical field The present invention relates generally to wind turbines and, more particularly, to a wind turbine having a rotor comprising at least three wind turbine blades and a plurality of blade connecting members, each of which is connected between a pair of wind turbine blades. Background There is a continued desire to generate higher levels of energy from onshore and offshore wind farms. One way to achieve this is by equipping modern wind turbines with larger blades. Providing larger blades increases the rotor's swept area, allowing the turbine to capture more energy from the wind. However, wind turbine blades experience various loads and stresses in use, and increasing the length of a blade increases the magnitude of the loads that the blade, hub, and blade bearing must withstand. For example, flank loads resulting from wind pressure on the blade, and edge loads resulting from the blade's weight, are both greater for larger blades. Wind turbine blades comprise a blade root configured to connect the blade to the turbine hub. To help withstand the increased loads in use, longer blades typically have a larger blade root diameter. However, this can make blade manufacturing and transportation more difficult. In some cases, a support frame or support cables may be included on the rotor to transfer some of the blade loads to the hub and / or other blades. As such, some of the blade loads can be diverted away from the blade root, thus reducing the structural requirements of the blade root. However, the loads experienced by a wind turbine blade vary during turbine operation. For example, loads can vary due to changing wind conditions and / or the changing effect of gravity on the blade when it is in different positions around the rotor, among other factors. As such, the wind turbine, and in particular any support components on the rotor, must be configured to account for these varying blade loads to ensure safe and reliable operation. US patent 2008 / 124216 A1 and WO2014 / 187933 describe wind turbine generators with supports between each blade and the hub. It is in this context that the present invention has been developed. Compendium In a first aspect of the present invention, a wind turbine according to claim 1 is provided, comprising a tower, a nacelle mounted on the tower, and a rotor mounted on the nacelle. The rotor comprises a hub and at least three wind turbine blades. Each blade extends from a root to a tip. Each blade further comprises a connection point located between the root and the tip. The wind turbine further comprises a plurality of blade connecting members, each connecting member connecting between the corresponding connection points of a pair of wind turbine blades. The wind turbine further comprises a tensioning system for adjusting the tension on each blade connecting member. The tensioning system comprises a plurality of linear actuators, each linear actuator coupled between the hub and a respective blade connecting member. Each linear actuator is configured to adjust the tension on the blade connecting member.In preferred examples, the connection point between the root and the tip is located away from both the blade root and the blade tip. For instance, in some examples, the connection point may be located at least 20% of the total blade length away from the root. Similarly, in some examples, the connection point may be located at least 20% of the total blade length away from the blade tip. In preferred examples, the connection point may be located at least 30% of the total blade length away from both the blade root and the blade tip. Separating the connection point from the blade root ensures that a substantial proportion of the blade loads can be transferred to the blade connecting member through the connection point, bypassing the blade root.Furthermore, separating the connection point away from the blade tip reduces the effect of the increased aerodynamic drag caused by the connection point, and also reduces the noise resulting from the connection point and the blade connecting member in use. Each linear actuator can be connected directly to the hub and / or directly to a blade connecting member. Alternatively, each linear actuator can be connected indirectly to the hub via another component and / or indirectly to a blade connecting member via another component. As such, "coupled between the hub and a respective blade connecting member" encompasses both direct and indirect connections to the hub and the blade connecting member. Therefore, it will be appreciated that, in some examples, the linear actuator may be one of a plurality of components coupled together between the hub and a blade connecting member. In the present context, 'blade connecting member' should be interpreted broadly to include examples such as flexible connecting members and rigid connecting members. As such, in some examples, the blade connecting member may comprise a cable. Preferably, the blade connecting member may therefore be a blade connecting cable, and the rotor may be a cable-stayed rotor. For example, in the present context, a 'cable' may be a braided or laid rope of metal wires (such as steel wires, for example), polymer fibers (such as polyethylene, polypropylene, nylon, polyester, aramid fibers, for example), inorganic fibers (such as carbon fibers, for example), or hybrid ropes of such materials.In some other examples, a blade connecting member may comprise a composite member, such as a pultrusion, or, alternatively, a blade connecting member may comprise a metal rod, to name a few possible examples. The tensioning system may be referred to as a pretensioning system in some examples, because it is preferably configured to pretension the wind turbine blades through the connecting members. Consequently, the tensioning system can be configured to apply a prestressing force in one direction to counteract the gravitational and / or wind load forces experienced by the blade during operation. Applying a prestressing force can help pull the rotor blades further upwind, away from the tower, thereby helping to reduce the risk of the blades striking the tower in high wind conditions. Each linear actuator preferably has an adjustable length to facilitate tension adjustment on the blade connecting member. It should be noted that the length of a linear actuator refers to the distance between the points where other wind turbine components, such as the hub and the blade connecting member, can be connected to the linear actuator. Linear actuators can have a stroke length of at least 1 m, preferably at least 2 m, and more preferably at least 3 m. The stroke length is the maximum length adjustment provided by the linear actuator. The maximum stroke length varies depending on the actual design and can be, for example, a maximum of 6 m, preferably a maximum of 5 m. In other words, the stroke length is the difference between the maximum (i.e., extended) length of the linear actuator and its minimum (i.e., retracted) length.Again, the maximum and minimum lengths of the linear actuator refer to the maximum and minimum distances between the connection points where the linear actuator can be connected to other components of the wind turbine. In preferred examples, the stroke length is configured or set according to the anticipated variations during wind turbine operation. For example, factors influencing the stroke length configuration include anticipated changes in operating conditions, anticipated changes in the preload of the blade connecting members (e.g., during maintenance operations), and anticipated creep of the material of the components coupled between the blade and the hub. A linear actuator with a stroke length that accounts for these variations is selected. Furthermore, the adjustable length of the linear actuators can facilitate improved wind turbine assembly and maintenance. For example, the actuators can be fully extended, i.e., to their maximum stroke length, to loosen the blade connecting members when the wind turbine is not in operation. Therefore, in preferred examples, the linear actuators can be configured with a maximum stroke length that facilitates the loosening, i.e., unloading, of the blade connecting members. In examples where the blade connecting members are flexible, such as ropes or cables, for example, loosening the blade connecting members by extending the linear actuators can particularly advantageously facilitate simplified wind turbine assembly and maintenance.In some examples, the ease of extending the actuators and thus loosening the blade connecting members can mean that maintenance operations can be performed on the blades without disconnecting the blade connecting members from the blades. In some examples, the linear actuators can be electric linear actuators. For example, the linear actuators may comprise an electric motor. Electric actuators can facilitate the use of standard electrical components, which can reduce the cost of the tensioning system. Furthermore, electric actuators have a reduced risk of fluid leakage because the fluid is only included in small quantities, for example, for lubricating moving parts. Additionally, in some examples, a wind turbine may include an electric pitch mechanism to adjust the pitch of a wind turbine blade.In such an example, the use of electric actuators in a tensioning system can be particularly advantageous, because the existing electrical infrastructure in the wind turbine can be reused for the electric linear actuator. The wind turbine may further comprise a plurality of strain sensors, each strain sensor configured to measure the strain in a respective blade connecting member and / or a respective linear actuator. Such a strain sensor may be in contact with, or coupled to, the blade connecting member, or, in other instances, the strain sensor may be used to determine / estimate the strain in the blade connecting member without being located on or in the blade connecting member. In preferred examples, linear actuators can be either hydraulic or pneumatic. Advantageously, hydraulic or pneumatic linear actuators facilitate tension measurement in a blade connecting member by measuring the fluid pressure in the linear actuator. As such, additional sensors for measuring tension in the blade connecting member may not be required and / or may be included only as a precaution. Each linear actuator may comprise a piston, and the tensioning system may further comprise a plurality of stiffness response systems, each comprising a high-pressure fluid source and a low-pressure fluid source. Each high-pressure fluid source is preferably selectively and fluidly coupled to a first side of the piston in a linear actuator, and each low-pressure fluid source is preferably selectively and fluidly coupled to a second side of the piston in the linear actuator to facilitate control of the stiffness response of the linear actuator. As such, the stiffness response of the linear actuator can be controlled or set by varying the pressure on each side of the piston, for example, by varying the pressure in each of the high- and low-pressure fluid sources. As described above, linear actuators can be either hydraulic or pneumatic. Therefore, unless otherwise stated, any reference herein to a "fluid" includes liquids and / or gases. For example, for a pneumatic linear actuator, the high- and low-pressure fluid sources may be high- and low-pressure gas sources. For a hydraulic linear actuator, the high- and low-pressure fluid sources may be high- and low-pressure liquid sources. The stiffness response of a linear actuator can also be referred to as the stiffness of the linear actuator. For example, the linear actuator can function as a hydraulic or pneumatic spring. As such, the linear actuator can extend or retract in response to the tension in the blade connecting member during use. Therefore, the stiffness response of a linear actuator can be synonymous with a spring constant. Each linear actuator is preferably fluidly coupled to its respective high-pressure and low-pressure fluid sources via a hydraulic or pneumatic manifold. Such a manifold is preferably made of substantially rigid piping. With reference in particular to a high-pressure manifold that fluidly couples a linear actuator to a high-pressure fluid source, the use of substantially rigid piping helps reduce the risk of fluid leakage when the wind turbine is in operation. In some examples, the high-pressure fluid source may be a high-pressure accumulator. Also, in some examples, the low-pressure fluid source may be a low-pressure accumulator. The pressure in the high- and low-pressure accumulators can determine the stiffness response of the linear actuator. In some examples, one or both of the accumulators may be charged by gas pressure.For example, each accumulator can be a piston accumulator, a bladder accumulator, or a diaphragm / membrane accumulator. Each of these accumulator types comprises a separating element between a gas volume and a fluid volume at high or low pressure. Varying the gas pressure on one side of the separating element facilitates control of the fluid pressure on the other side. In some other examples, one or both accumulators can be additionally or alternatively loaded by a mechanical spring. For example, a mechanical spring can apply a force to one side of the separating element, with the high- or low-pressure fluid acting against the separating element from the other side. As such, the high- or low-pressure fluid pressure in the accumulator can be determined by the mechanical spring. In some examples, high-pressure fluid sources can be selectively and smoothly coupled to each other. Alternatively, low-pressure fluid sources can also be selectively and smoothly coupled to each other. High-pressure fluid sources are preferably selectively and smoothly coupled to each other via one or more regulating valves. Low-pressure fluid sources are preferably selectively and smoothly coupled to each other via one or more regulating valves. During an initial phase, the regulating valves can be opened to smoothly couple the high-pressure fluid sources, thereby equalizing the pressure in each of them.Furthermore, the regulating valves can be opened to smoothly couple the low-pressure fluid sources, thereby equalizing the pressure in each of the low-pressure fluid sources before the wind turbine operates. After pressure equalization in each of the high-pressure fluid sources, and / or pressure equalization in each of the low-pressure fluid sources, the regulating valves can be closed to isolate each high- or low-pressure fluid source from the others. During wind turbine operation, each linear actuator and its associated pair of high- and low-pressure fluid sources can act as an independent stiffness response system for a blade connecting member. In some examples, each linear actuator can be coupled to a respective blade connecting member via a separate tension member. It should be appreciated that the term "tension member" should be interpreted broadly to include examples such as flexible and rigid tension members. As such, in some examples, a tension member may comprise a composite member, such as a pultrusion, or alternatively, it may comprise a metal rod, to name a few possible examples. However, in preferred examples, the tension member may comprise a cable.For example, in the present context, a 'cable' may be a braided or stranded rope of metal wires (such as steel wires, for example), polymer fibers (such as polyethylene, polypropylene, nylon, polyester, aramid fibers, for example), inorganic fibers (such as carbon fibers, for example), or hybrid ropes of such materials. Most preferably, the tension member comprises a polymer rope, such as a braided polymer rope. For example, a tension member may comprise ultra-high molecular weight polyethylene fibers, such as Dyneema® DM20-based polymer rope. It will be noted that the examples comprising a tension member coupled between the linear actuator and the blade connecting member are examples in which the linear actuator is indirectly connected to the blade connecting member via another component. The tension member in such an example is, therefore, one of a plurality of components, including the linear actuator, that are coupled between the hub and the blade connecting member. In some other examples, the wind turbine may further or alternatively comprise a tension member coupled between the linear actuator and the hub. As such, the linear actuator may be indirectly connected to the hub. However, in preferred examples, the linear actuator may be positioned as close as possible to the hub, i.e., as far inward as possible, to reduce the rotational inertia of the rotor. A separate tension member coupled between the linear actuator and the blade connecting member can simplify wind turbine assembly. In particular, where the tension member is a flexible member, such as a rope or cable, connecting the linear actuator between the hub and the blade connecting member, assembly can be easier. Furthermore, coupled between one or more tension members between the linear actuator and the blade connecting member, and / or between the linear actuator and the hub, can facilitate the use of a smaller linear actuator and / or shorter blade connecting member(s), thereby saving costs and weight. The total distance between the hub and a blade connecting member can be referred to as the tensioning system length. The maximum tensioning system length can be defined by the maximum extended length of the actuator and the length of any tensioning member through which the actuator connects between the hub and the blade connecting member. The tension on the blade connecting member can be adjusted by adjusting the tensioning system length, for example, by varying the length of the linear actuator. The stroke length of each linear actuator, i.e., the maximum length adjustment facilitated by adjusting the length of the linear actuator, can be at least 10%, preferably at least 15%, and more preferably at least 20% of the maximum length of the tensioning system. For example, the maximum length of the tensioning system can be 15 m, where the stroke length of the linear actuator can be 3 m, in some preferred examples. In some examples, each linear actuator is preferably rotatably coupled to the hub, so that the linear actuator is free to rotate relative to the hub. As such, the linear actuator preferably self-orients according to the tension in the tension member and thus takes into account, for example, variations in the orientation of the tension member due to, for example, wind gusts, pitching, maintenance, and / or creep.As such, in preferred examples, the extension shaft of the linear actuator can be coaxial with the resulting tensile force on the respective tension member. Advantageously, such a configuration reduces the risk of moment loads on the linear actuator, which could cause wear, damage, or misalignment problems. Furthermore, this configuration advantageously reduces fatigue on the tension member. In preferred examples, each linear actuator can be rotatably coupled to the hub with three degrees of rotational freedom. This configuration advantageously ensures that the tension force does not cause any moment force or torsional load on the linear actuator during wind turbine operation, when blade loads and their magnitude may vary or shift, for example, during operation (e.g., due to wind gusts, pitching, and / or creep) or during installation or overhaul. In preferred examples, the hub may include an upwind hub extension. That is, the hub may include an extension component or arrangement that extends in an upwind direction. Linear actuators may be coupled to the upwind hub extension. Advantageously, such a configuration can cause the tension in the blade connecting members to pull the wind turbine blades upwind, thereby reducing the risk of the blade striking the tower in high wind conditions. Furthermore, such a configuration can effectively increase the stiffness of the wind turbine blade in the span direction. In particular, coupling the linear actuator between the blade connecting member and the upwind hub extension increases the stiffness of the inner portion of the blade, between the connection point and the blade root. In preferred examples, the linear actuators (or other tensioning system components, such as tension members, through which the linear actuators are coupled to the hub) are preferably coupled to the hub at coupling points evenly distributed around the rotor's axis of rotation. As such, the tensioning forces on the respective linear actuators can substantially cancel each other out. The linear actuators can be coupled to the hub via a suspension bracket. For example, the suspension bracket can define the coupling points. The suspension bracket can help ensure that the loads transferred from the linear actuators are resolved in the same plane, thereby minimizing the loads transferred to the hub. In some examples, the wind turbine may include tiltable wind turbine blades. For example, each wind turbine blade may be rotatably connected to the hub via a pitch mechanism. As such, in some examples, the rotor may be referred to as a "cable-stayed tilt rotor." The bypass mechanisms are preferably hydraulic bypass mechanisms. In some examples, the high-pressure fluid source in a stiffness response system, for example, a high-pressure accumulator, can be selectively and fluidly coupled to a hydraulic bypass mechanism. Alternatively, in some examples, the low-pressure fluid source in a stiffness response system, for example, a low-pressure accumulator, can also be selectively and fluidly coupled to a hydraulic bypass mechanism. Such a configuration can facilitate the simple and efficient supply and withdrawal of hydraulic fluid from the accumulators when required, for example, during maintenance operations, during initial wind turbine assembly, and / or as a safety backup system. In another aspect of the present invention, a method of operating a wind turbine, according to claim 7, is provided. The wind turbine comprises a tower, a nacelle mounted on the tower, and a rotor mounted on the nacelle. The rotor comprises a hub and at least three wind turbine blades. Each wind turbine blade extends from a root to a tip. Each blade further comprises a connection point located between the root and the tip. The wind turbine further comprises a plurality of blade connecting members. Each blade connecting member connects between the corresponding connection points of a pair of wind turbine blades. The wind turbine further comprises a tensioning system comprising a plurality of linear actuators. Each linear actuator is coupled between the hub and the respective blade connecting member. The method comprises adjusting each linear actuator depending on the tension in the respective blade connecting member to adjust, therefore, the tension in the blade connecting member. In preferred examples, each linear actuator has an adjustable length. Therefore, adjusting each linear actuator preferably involves adjusting the length of the respective linear actuator to thereby adjust the tension on the blade connecting member. Advantageously, this simplifies the operation of adjusting the tension on a wind turbine blade connecting member. In some examples, the method may involve establishing a target tension on the blade connecting member and, depending on the tension in the blade connecting member in use, either a) extending the linear actuator or b) retracting the linear actuator to maintain the target tension on the blade connecting member. In preferred examples, maintaining a target tension, or maintaining a constant tension, may involve keeping the tension within a predetermined range. For example, the tension may be maintained within a range of 3–10% of the target tension in some examples. If the tensioning system were rigid, high wind loads could cause increased tension in a blade connecting member. However, in such a high-wind situation, the linear actuators of the tensioning system described above can extend—that is, extend the overall length of the tensioning system—to maintain constant tension in the blade connecting member. Furthermore, with a rigid tensioning system set to an appropriate average tension, in low-wind conditions, the tension in the blade connecting member can decrease substantially, causing the connecting member to loosen.However, in such low wind conditions, the linear actuators of the tensioning system described above can retract, shortening the overall length of the tensioning system, thereby maintaining constant tension on the blade connecting member and ensuring that some of the blade loads are still transferred to the blade connecting member. As such, the linear actuators can respond to external factors, such as wind and / or gravity, to maintain constant tension on the blade connecting member. Furthermore, a tensioning system and its use, as described herein, can compensate for other factors, such as temperature effects and material creep, which can cause variations in the length of the blade connecting members in use. Furthermore, adjusting a linear actuator, for example by extending or retracting it, to adjust the tension in the blade connecting member also adjusts the stiffness of the tensioning system and the associated blade connecting member. By providing adjustable stiffness in the blade connecting members and the tensioning system, the tensioning system and its use, as described herein, ensure that the variable blade loads experienced during wind turbine operation are distributed between the blades and the blade connecting members. In preferred examples, the variable blade loads can be distributed equally between the blades and the blade connecting members as a result of the adjustable stiffness of the blade connecting members and the tensioning system. In some examples, linear actuators can be hydraulic or pneumatic. In use, linear actuators can extend and retract as a result of pressure, without any active electronic control input. As such, the extension and retraction of linear actuators can be described as passive, since the actuator's extension and retraction can occur without an active electrical control input. In use, the linear actuator can therefore be a "dumb" system, such as a spring, that only responds passively to external stimuli, such as a pressure change, causing the linear actuator to extend and retract. In preferred examples, the tensioning system can be configured to allow the linear actuators to extend / retract passively by + / -0.3 m in use, preferably + / -0.45 m in use, and more preferably + / -0.6 m in use.In preferred examples, the tension in the blade connecting member can be measured or calculated by measuring the pressure in the hydraulic or pneumatic linear actuator. For example, during wind turbine operation in high wind conditions or other situations where the blade experiences a high load, the tensioning system, and in particular the linear actuators of the tensioning system, can act as a slave component, following the loads on the blade connecting member and extending the actuator to reduce the tension on the blade connecting member. The linear actuators can extend passively, that is, without an active electrical control input, when the blade experiences a high load. Furthermore, during wind turbine operation under low load conditions, the linear actuators can act as a master component and retract to increase the tension on the blade connecting member. This ensures that the blade connecting member remains loaded and under tension, so that at least some blade loads are still transferred to the blade connecting member.Linear actuators can be passively retracted, i.e., without an active electrical control input, under low load conditions. In some examples, each linear actuator may comprise a piston, and the tensioning system may comprise a plurality of stiffness response systems. Each stiffness response system may comprise a high-pressure fluid source and a low-pressure fluid source. Each high-pressure fluid source may be selectively and fluidly coupled to a first side of the piston in a linear actuator, and each low-pressure fluid source may be selectively and fluidly coupled to a second side of the piston in the linear actuator. The method may further comprise controlling the stiffness response of the linear actuator by controlling the high-pressure and low-pressure fluids. Controlling the stiffness of the linear actuator also results in controlling the combined stiffness of the tensioning system and the blade connecting member.Controlling this combined stiffness facilitates control of the load distribution between the blade and the blade connecting cable. For example, increasing the combined stiffness of the blade connecting cable and the tensioning system results in a greater proportion of the blade loads being transferred to the blade connecting cable. Consequently, this allows control over how much load is transferred to the hub through the blade root and inner portion of the blade, and how much load bypasses the blade root and inner portion and is instead transferred to the hub through the blade connecting member. In some examples, the high-pressure fluid source may be a high-pressure accumulator, and the low-pressure fluid source may be a low-pressure accumulator. Therefore, controlling the stiffness response of the linear actuator may involve controlling the pressure in the high-pressure accumulator and / or controlling the pressure in the low-pressure accumulator. In some examples, high-pressure fluid sources can be selectively and fluidly coupled to each other. Additionally, low-pressure fluid sources can be selectively and fluidly coupled to each other. The method may further comprise equalizing the pressure in the high-pressure fluid sources by fluidly coupling them, rotating the rotor through at least one complete rotation, and then fluidly decoupling the high-pressure fluid sources from each other. The method may further comprise equalizing the pressure in the low-pressure fluid sources by fluidly coupling them, rotating the rotor through at least one complete rotation, and then fluidly decoupling the low-pressure fluid sources from each other.In preferred examples, equalizing the pressure at high-pressure fluid sources and equalizing the pressure at low-pressure fluid sources can result in the stress in the blade connecting member being adjusted to the target stress of the blade connecting member. Brief description of the drawings Examples of the present invention will now be described, by way of non-limiting example, with reference to the accompanying figures, in which: Figure 1 is a schematic perspective view of a wind turbine comprising a plurality of blades, a plurality of blade connecting members connected between pairs of blades, and a tensioning system for adjusting the tension on the blade connecting members; Figure 2 is a schematic perspective view of the tensioning system and a hub of the wind turbine; and Figure 3 is a fluid diagram describing an example of the tensioning system comprising hydraulic or pneumatic actuators. Detailed description Figure 1 shows a wind turbine 10 comprising a tower 12 and a nacelle 14 mounted on the tower 12. A rotor 16 is mounted on the nacelle 14, and the rotor 16 comprises a hub 18 and a plurality of wind turbine blades 20. In this example, the wind turbine 10 comprises three wind turbine blades 20. The blades 20 can be tiltable wind turbine blades 20, i.e., the pitch of the blades 20 can be adjustable. As such, the wind turbine blades 20 can be connected to the hub 18 via respective pitch mechanisms (not shown), by means of which the blades 20 can be rotated relative to the hub 18. Consequently, in preferred examples, the pitch of the wind turbine blades 20 can be controlled depending on the relative speed of the incident wind to ensure that the blades 20 are oriented at an advantageous angle of attack to capture wind energy. Each wind turbine blade 20 extends in a span-along direction between a blade root 22 and a blade tip 24. The blade root 22 is preferably configured for connection to the hub 18, either directly (as shown in Figure 1) or via another wind turbine component, such as a blade root extension (not shown). Each blade 20 comprises a connection point 26 located between the root 22 and the tip 24. The connection points 26 of each blade 20 are configured to facilitate the connection of a blade connecting member 28 to the wind turbine blade 20. As shown in Figure 1, the wind turbine 10 comprises a plurality of blade connecting members 28, and each blade connecting member 28 is connected between the corresponding connection points 26 of a pair of wind turbine blades 20. The blade connecting members 28 can be flexible connecting members, such as cables, or substantially rigid connecting members, such as rods or pultrusions. In the example shown in Figure 1, the blade connecting members 28 comprise cables, and the rotor 16 can therefore be referred to as a "cable-stayed rotor." The blade connecting members 28 are configured to transfer some of the loads experienced by the blade 20 in use to another wind turbine blade 20 and / or to the hub 18. In particular, the blade connecting members 28 reduce the loads experienced by an inner portion 30 of the blade 20 and, therefore, also reduce the amount of blade loads transferred from the blade 20 to the hub 18 through the blade root 22.This is because some of the blade loads are transferred to the blade connecting member 28 at the connection point 26, and these loads are then transferred to another blade 20 or to the hub 18 via a separate route, avoiding the blade root 22. As described in the background, increasing the length of a wind turbine blade 20 typically requires an increase in the blade root diameter 22 to provide greater strength in the inner portion 30 of the blade 20 to accommodate the higher loads experienced by longer blades in use. However, the wind turbine 10 described herein includes blade connecting members 28 that facilitate the use of longer wind turbine blades 20 without necessarily requiring an increase in the blade root dimensions for structural purposes.As such, the use of blade connecting members 28 at least reduces the degree to which the blade root dimensions can be increased for longer blades 20 (compared to the blades of a wind turbine that does not include blade connecting members), and, in preferred examples, the use of blade connecting members 28 can advantageously facilitate an increase in blade length without increasing the blade root dimensions. As noted above, this is because some of the blade loads are unloaded, i.e., transferred, to the blade connecting member 28 at the connection point 26. As such, the swept area of the rotor 16 can be increased, meaning that more energy can be captured from the wind, without increasing the blade root diameter 22. Referring again to Figure 1, the wind turbine 10 also includes a tensioning system 32 for adjusting the tension on each blade connecting member 28. The tensioning system 32 comprises a plurality of linear actuators 34, each coupled between the hub 18 and its respective blade connecting member 28. The linear actuators 34 are configured to adjust the tension on their associated blade connecting member 28. For example, the linear actuator 34 can be adjusted depending on the tension on its associated blade connecting member 28, as will be described in more detail below. With further reference to Figure 2, which shows a schematic view of the hub 18 and the tensioning system 32, in some examples, each linear actuator 34 can be coupled to a respective blade connecting member 28 via a separate tension member 36. The tension member 36 can be rigid or, alternatively, flexible. The inclusion of a tension member 36 can facilitate the use of a smaller, i.e., shorter, linear actuator 34, because the tension member 36 can be configured with a length to span the required distance between the linear actuator 34 and the blade connecting member 28. Furthermore, coupling the linear actuator 34 to the blade connecting member 28 via a tension member 36 means that the mass of the linear actuator 34 is kept substantially inside, i.e., close to the rotor axis, thereby reducing the rotational inertia of the rotor 16. Each linear actuator 34 preferably has an adjustable length to facilitate tension adjustment on the blade connecting member 28. Therefore, in some instances, adjusting a linear actuator 34 to adjust the tension on a blade connecting member 28 may involve adjusting the length of the respective linear actuator 34. It will be appreciated that the length of a linear actuator 34 refers to the distance between the points where the actuator 34 connects to other components of the wind turbine 10. A linear actuator 34 can be retracted, i.e., shortened, to increase the tension on the blade connecting member 28 to which the actuator 34 is coupled. It follows that the linear actuator 34 can be extended to reduce the tension on the blade connecting member 28. In some examples, the operation of the wind turbine 10 may involve establishing a target tension on the blade connecting member and varying the length of the linear actuator 34 to achieve, or maintain, that target tension on the blade connecting member 28. For example, depending on the tension on the blade connecting member 28 in use, the linear actuator 34 may either extend or retract to maintain the target tension on the blade connecting member 28. In practice, adjusting the length of the linear actuator 34 changes the stiffness of the associated blade connecting member 28 (and, in some examples, also changes the stiffness of an associated tension member 36). Controlling the stiffness of the blade connecting member 28 relative to the stiffness of the inner portion 30 of the blade 20 determines the load distribution between the inner portion 30 of the blade 20 and the blade connecting member 28. For example, by increasing the stiffness of the blade connecting member 28 relative to the blade 20, a greater proportion of the blade loads can be transferred to the blade connecting member 28. This ensures that the variable loads experienced by the blade 20 in use are safely distributed between the blade root 22 and the blade connecting member 28.The stiffness of the blade connecting member 28 can be increased by retracting, i.e., shortening, the associated linear actuator 34, with the increased stiffness resulting in greater stress on the blade connecting member 28, as described above, because more load is transferred to the blade connecting member 28 from the blade 20. The linear actuators 34 are preferably rotatably coupled to the hub 18. As such, in preferred examples, the linear actuators 34 can be free to rotate relative to the hub 18. This means that the linear actuators 34 can self-orient according to the tension in the tension member 36, which is particularly advantageous when the direction of the tension force varies in use. Therefore, the linear actuator 34 preferably maintains an orientation in which the extension axis of the actuator 34 is substantially coaxial with the tension force experienced by the linear actuator 34 during the operation of the wind turbine 10. Orienting the linear actuator 34 and the tension member 36 in this way advantageously reduces fatigue in the tension member 36, because the tension forces are consistently directed along the longitudinal axis of the tension member 36.This orientation also reduces the risk of moment loads acting on the linear actuator 34, which could cause wear, damage, or misalignment problems in the actuator 34. In some examples, hub 18 may comprise an upwind hub extension 38, i.e., a portion of hub 18 that extends in an upwind direction. Linear actuators 34 may be coupled, for example, rotatably, to the upwind hub extension 38. This configuration may help pull the wind turbine blades 10 upwind (via tension in the blade connecting member 28 and the tension member 36), reducing the risk of a blade 20 striking the tower 12 in high wind conditions. This may also increase the stiffness of the wind turbine blade 20 in a span-along direction; in particular, this configuration may increase the stiffness of the inner portion 30 of the blade 20. As shown in Figure 2, in preferred examples, the linear actuators 34 can be either hydraulic or pneumatic actuators, i.e., actuators driven by liquid or gas pressure, respectively. It follows that the linear actuators 34 can therefore extend and retract as a result of fluid pressure, i.e., liquid or gas pressure. As will be described in more detail later, in preferred examples, the linear actuators 34 can extend and retract without any active electronic control input. For example, in use, the linear actuators 34 can extend and retract as a result of the tension force acting against the fluid pressure in the actuator 34. Referring now to Figure 3, this shows a fluid system diagram describing an example of the tensioning system 32 of the wind turbine 10. As shown in Figure 3, the tensioning system 32 may include a plurality of stiffness response systems 40, each of which is indicated schematically by the dashed perimeter lines in Figure 3. Each stiffness response system 40 is preferably configured to control the stiffness of a blade connecting member 28 by controlling the fluid pressure in the associated linear actuator 34. Accordingly, each stiffness response system 40 preferably includes a high-pressure fluid source 42 and a low-pressure fluid source 44. The high-pressure fluid source 42 in the associated stiffness response system 40 can be selectively and fluidly coupled to a first side of a piston 46 in the linear actuator 34. It follows that the low-pressure fluid source 44 in the associated stiffness response system 40 can be selectively and fluidly coupled to a second side of the piston 46. The position of piston 46 within linear actuator 34 determines the length of linear actuator 34. For example, moving piston 46 within linear actuator 34 can cause the actuator 34 to extend or retract. The movement of piston 46 in linear actuator 34 is resisted by high- and low-pressure fluid on either side of the piston 46. Therefore, the stiffness response of linear actuator 34 can be controlled by regulating the high-pressure and low-pressure fluid levels. As shown in Figure 3, and with brief reference again to Figure 2, in some examples, the high-pressure fluid source 42 may be a high-pressure accumulator, and the low-pressure fluid source 44 may be a low-pressure accumulator. The high- and low-pressure accumulators 44, 42 of each stiffness response system 40 may be located inside the hub 18 in some examples, or in other examples, the accumulators 44, 42 may be mounted on the hub 18 or on the inner portion 30 of the blade 20. The pressure in the accumulators 42, 44 can determine the stiffness response of the associated linear actuator 34. As such, the stiffness response of the linear actuator 34 can be controlled by controlling the pressure in the high-pressure accumulator 42 and / or the pressure in the low-pressure accumulator 44.In addition, accumulators 42, 44 can provide a damping effect when the linear actuator 34 extends or retracts as a result of the variable tension on the blade connecting member 28, in use. In some examples, the high-pressure fluid sources 42 and / or the low-pressure fluid sources 44 can be selectively and fluidly coupled to a hydraulic pitching mechanism (not shown) that is configured to adjust the pitch of a wind turbine blade 20. In some examples, this configuration can facilitate the simple addition and removal of fluid from the high- and low-pressure fluid sources 42, 44, for example, during an initial phase of operation of the wind turbine 10, following maintenance work, or to replace fluid lost in a leak, for example. Referring again to Figure 3, in some preferred examples, the high-pressure fluid sources 42 can be selectively and fluidly coupled to each other. When the high-pressure fluid sources 42 are fluidly coupled, the pressure in each of the high-pressure fluid sources 42 can be equalized, as will be described in more detail below. Furthermore, the low-pressure fluid sources 44 can be selectively and fluidly coupled to each other, and such a configuration means that the pressure in each of the low-pressure fluid sources 44 can also be equalized. Referring initially to the high-pressure fluid sources 42, the tensioning system 32 may include a plurality of regulating valves 48 through which the high-pressure fluid sources 42 can be selectively and smoothly coupled to and uncoupled from one another. The stiffness response systems 40 may further each include one or more one-way valves 50 to control the direction of fluid flow between the high-pressure fluid sources 42 of each stiffness response system 40. In an initial phase, before the wind turbine 10 is operated to generate electricity, the regulating valves 48 can be opened to smoothly couple the high-pressure fluid sources 42 to one another. Similarly, the regulating valves 52 can be opened to smoothly couple the low-pressure fluid sources 44 to one another. The rotor 16 can then be idled or spun.In examples involving tiltable wind turbine blades 20, this may involve turning the tiltable wind turbine blades 20 into a feathered position so that the wind does not drive the rotor 16. The coupling of low-pressure fluid sources or high-pressure fluid sources can also be performed during operation, if required. As the rotor 16 rotates, the loads experienced by the blades 20 vary throughout the cycle, for example, due to the varying weight force acting on the blades 20 when they are in different positions during the rotation of the rotor 16. It follows that the stress in the blade connecting members 28 also varies during the rotation of the rotor 16. Consequently, the linear actuators 34 and the stiffness response systems 40 of each actuator 34 will also experience varying stresses and pressures, respectively. When opened, the regulating valves 48 allow high-pressure fluid to flow between each of the stiffness response systems 40.The regulating valves 48 can be configured to open and close, respectively, to allow and block fluid communication between the high-pressure fluid sources 42 until the pressure in each of the high-pressure fluid sources 42 is substantially the same, i.e., equalized. It will be appreciated that the equalized pressure in the high-pressure fluid sources 42 is preferably an average pressure, i.e., a mean pressure, experienced by the high-pressure fluid sources 42 during the rotation of the rotor 16. After pressure equalization, the high-pressure fluid sources 42 are preferably decoupled, i.e., fluidly isolated from each other, so that each stiffness response system 40 acts as an individual system with an associated linear actuator 34 and blade connecting member 28 during wind turbine 10 operation. The linear actuators 34 are preferably configured with a stroke length, i.e., the maximum variable length, that accommodates stress variations and pressure variations above and below the equalized pressure during wind turbine 10 operation. It will be appreciated that the process described above for equalizing the pressure in the high-pressure fluid sources 42 is equally applicable for equalizing the pressure in the low-pressure fluid sources 44 using regulating valves 52. As such, for the sake of brevity, a description of the pressure equalization process for the low-pressure fluid sources 44 will not be repeated herein. In preferred examples, the pressures in the low-pressure fluid sources 44 are equalized at the same time, i.e., at the same initial phase of wind turbine operation 10, as when the pressures in the high-pressure fluid sources 42 are equalized. Fluid can be introduced into each stiffness response system 40 through a fluid inlet. Fluid can be extracted from each stiffness response system 40 through a fluid return. Each stiffness response system 40 preferably includes one or more controllable valves 54 between the respective linear actuator 34 and the fluid inlet and fluid return, as shown in Figure 3. The controllable valves 54 help ensure that fluid access to an external fluid supply can be controlled for each stiffness response system 40. As such, the stiffness response system 40 can be isolated from the external supply (fluid inlet / return) during normal wind turbine operation. This reduces the risk of fluid leakage, saves energy, and ensures that the linear actuator 34 and the associated stiffness response system 40 can act as a damper during wind turbine operation.This, in turn, helps to increase the longevity of the tensioning system 32 and, in particular, reduces fatigue of the linear actuator components, the tension member 36 and the blade connecting member 28. As shown in Figure 3, in some examples, each stiffness response system 40 may include one or more overpressure relief valves 56. The overpressure relief valves 56 facilitate smooth communication with the fluid return to reduce pressure at the high- or low-pressure sources 42, 44 if the pressure at these sources rises above a predetermined level. Therefore, the overpressure relief valves 56 help facilitate safe and reliable operation of the tensioning system 32. The wind turbine 10 described herein and the associated methods of operating the wind turbine 10 provide several advantages over prior art wind turbines. As previously described, the blade connecting members 28 relieve, i.e., transfer, some of the loads experienced by a blade 20 in use, thus avoiding the inner portion 30 and the blade root 22. This allows the use of larger, i.e., longer wind turbine blades 20 without requiring an increase in the dimensions or strength of the blade root 22, or with a smaller increase in the dimensions and strength of the blade root 22, compared to a wind turbine without blade connecting members 28.Therefore, the diameter of a rotor 16 can be increased without necessarily increasing the size of the rotor hub 18 or the interface associated with the blades 20, or with a smaller increase in the size of the rotor hub 18 and the interface associated with the blades 20. Furthermore, as described above, the tensioning system 32 and the wind turbine operating methods 10 allow for control of the blade load distribution between the inner portion 30 and the root 22 of the blade 20 and the blade connecting member 28. In particular, the linear actuators 34 are configured to adjust the stiffness and tension in their associated blade connecting members 28, meaning that the tension in a blade connecting member 28 can be maintained within a safe range. By facilitating the adjustment of the stiffness and tension of the blade connecting members 28, the linear actuators 34 of the tensioning system 32 help ensure that the variable loads experienced by the blade 20 in use can be evenly distributed between the inner portion 30 of the blade 20 and the associated blade connecting member 28.Furthermore, by facilitating the adjustment of the stiffness and tension of the blade connecting members 28, the tensioning system 32 advantageously helps to maintain a given tension in the blade connecting members 28, despite the variable factors during the operation of the wind turbine 10. For example, the stiffness and tension of the blade connecting members 28 can be adjusted using the tensioning system 32 to take into account manufacturing tolerances, material creep over time, and changes in material properties due to temperature variations, to name just a few factors. The description provided herein serves to illustrate a plurality of possible examples of the present invention. The features described in relation to any of the foregoing examples can be readily combined with any other feature described with reference to different examples without departing from the scope of the invention, as defined in the appended claims.
Claims
1. A wind turbine (10) comprising: a tower (12); a nacelle (14) mounted on the tower; a rotor (16) mounted on the nacelle, the rotor comprising a hub (18) and at least three wind turbine blades (20), each blade extending between a root (22) and a tip (24), and each blade further comprising a connection point (26) located between the root and the tip; a plurality of blade connecting members (28), each blade connecting member connecting between the corresponding connection points of a pair of wind turbine blades; and a tensioning system (32) for adjusting the tension on each blade connecting member, the tensioning system comprising a plurality of linear actuators (34), each linear actuator being coupled between the hub and a respective blade connecting member, and each linear actuator being configured to adjust the tension on the blade connecting member,wherein the linear actuators (34) are hydraulic or pneumatic actuators, and each linear actuator (34) comprises a piston (46), wherein the tensioning system (32) further comprises a plurality of stiffness response systems (40), each comprising a high-pressure fluid source (42) and a low-pressure fluid source (44), each high-pressure fluid source selectively and fluidly coupled to a first side of the piston in a linear actuator, and each low-pressure fluid source selectively and fluidly coupled to a second side of the piston in the linear actuator to facilitate control of the stiffness response of the linear actuator.
2. The wind turbine (10) of claim 1, wherein each linear actuator (34) has an adjustable length to facilitate adjustment of the tension in the blade connecting member (28).
3. The wind turbine (10) of claim 1,wherein the high-pressure fluid source (42) is a high-pressure accumulator and the low-pressure fluid source (44) is a low-pressure accumulator, the pressure in the high- and low-pressure accumulators determining the stiffness response of the linear actuator (34).
4. The wind turbine (10) of claim 1 or claim 3, wherein the high-pressure fluid sources (42) are selectively fluidly coupled to each other, and / or wherein the low-pressure fluid sources (44) are selectively fluidly coupled to each other.
5. The wind turbine (10) according to any preceding claim, wherein each linear actuator (34) is coupled to a respective blade connecting member (28) via a separate tension member (36).
6. The wind turbine (10) according to claim 5, wherein each linear actuator (34) is rotatably coupled to the hub (18),so that the linear actuator is free to rotate relative to the hub to self-orient according to the tension in the tension member (36).
7. A method of operating a wind turbine (10), the wind turbine comprising: a tower (12); a nacelle (14) mounted on the tower; a rotor (16) mounted on the nacelle, the rotor comprising a hub (18) and at least three wind turbine blades (20), each blade extending between a root (22) and a tip (24), and each blade further comprising a connection point (26) situated between the root and the tip; a plurality of blade connecting members (28), each blade connecting member being connected between the corresponding connection points of a pair of wind turbine blades; and a tensioning system (32) comprising a plurality of linear actuators (34),each linear actuator that is coupled between the hub and the respective blade connecting member; the method comprising: adjusting each linear actuator in relation to the tension of the respective blade connecting member to thereby adjust the tension in the blade connecting member; wherein each linear actuator (34) has an adjustable length, and wherein the adjustment of each linear actuator comprises adjusting the length of the respective linear actuator to thereby adjust the tension in the blade connecting member (28); wherein the linear actuators (34) are hydraulic or pneumatic actuators, and wherein in use, the linear actuators extend and retract as a result of pressure, without any active electronic control input; wherein each linear actuator (34) comprises a piston (46), wherein the tensioning system (32) comprises a plurality of stiffness response systems (40),Each comprising a high-pressure fluid source (42) and a low-pressure fluid source (44), each high-pressure fluid source being selectively and fluidly coupled to a first side of the piston in a linear actuator and each low-pressure fluid source being selectively and fluidly coupled to a second side of the piston in the linear actuator, the method further comprising controlling the stiffness response of the linear actuator by controlling the high-pressure fluid and the low-pressure fluid.
8. The method of claim 7, further comprising establishing a target tension in the blade connecting member and, depending on the tension in the blade connecting member (28) in use, either a) extending the linear actuator (34), or b) retracting the linear actuator, to maintain the target tension in the blade connecting member.
9. The method of claim 7 or 8,wherein the high-pressure fluid source (42) is a high-pressure accumulator and the low-pressure fluid source (44) is a low-pressure accumulator, and wherein controlling the stiffness response of the linear actuator (34) comprises controlling the pressure in the high-pressure accumulator and / or the pressure in the low-pressure accumulator.
10. The method of claim 7 or claim 9, wherein the high-pressure fluid sources (42) are selectively fluidly coupled to each other, and wherein the low-pressure fluid sources (44) are selectively fluidly coupled to each other, the method further comprising: equalizing the pressure in the high-pressure fluid sources by fluidly coupling the high-pressure fluid sources to each other, rotating the rotor (16) through at least one complete rotation, and subsequently,to smoothly decouple the high-pressure fluid sources from each other; and to equalize the pressure in the low-pressure fluid sources by smoothly coupling the low-pressure fluid sources to each other, rotating the rotor through at least one complete rotation and then smoothly decoupling the low-pressure fluid sources from each other.