EP4713580A1

EP4713580A1 - Tension control of rotor support cables in a wind turbine

Info

Publication number: EP4713580A1
Application number: EP24729960.5A
Authority: EP
Inventors: Thomas S. Bjertrup Nielsen; Kim Hylling SØRENSEN; Torben Juul Larsen; Eik Herbsleb; Mikkel Kiilerich ØSTERLUND
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2023-05-16
Filing date: 2024-05-15
Publication date: 2026-03-25
Also published as: CN121464275A; WO2024235412A1

Abstract

Wind turbines (10) and methods of controlling wind turbines (10) including cable supported rotors (22). The cable supported rotor (22) includes a plurality of blades (26) and a plurality of cable assemblies (32) that support the blades (26). A plurality of tension mechanisms (44) controls the amount of tension in each of the cable assemblies (32). At start up, the tension is increased until a tension setpoint is reached, and tension levels are balanced across the plurality of cable assemblies (32). Tension may be readjusted periodically, and may be actively controlled during operation of the wind turbine (10) based on functions of tension versus azimuth, wind speed, rotor or generator speed, and blade pitch angle.

Description

TENSION CONTROL OF ROTOR SUPPORT CABLES IN A WIND TURBINE

Technical Field

The invention relates generally to wind turbines, and more particularly to systems and methods for setting and controlling the tension of rotor support cables.

Wind turbines produce electrical energy using a renewable resource. Generally, a wind turbine converts kinetic wind energy into mechanical energy and subsequently converts the mechanical energy into electrical energy. A common type of wind turbine is the upwind horizontal-axis wind turbine. This type of wind turbine typically includes a tower, a nacelle located at the apex of the tower, and a rotor having a central hub and one or more blades (e.g., three blades) mounted to and that extend radially from the hub. The rotor is supported by the nacelle and positioned at the front of the nacelle so that the rotor faces into the wind upwind of its supporting tower. The rotor is operationally coupled to a generator that is housed inside the nacelle and configured to convert mechanical energy received from the rotor into electrical energy.

Wind turbine manufacturers continually strive to increase power production from wind turbines. In this regard, the design of the wind turbine often plays a significant role in the power output generated from the wind. For example, energy obtained from the wind is generally proportional to the sweep area of the blades. Because longer blades trace a larger area with their blade tips than shorter blades, the swept area of the rotor can be increased by using longer blades. Thus, all else being equal, more energy can be extracted from a given amount of wind by a single rotor wind turbine having longer blades than one with shorter blades. However, the continued increase in the length of the blades has certain practical limits and poses significant design challenges for wind turbine manufacturers.

For example, the increased blade weight and root diameter associated with longer blades pose design challenges that can limit maximum blade length. One specific limiting factor is the need to support an increasingly heavier blade at its rotor attachment point. Increased loading at the root magnifies fatigue at this location due to rotation and yawing of the rotor during operation of the wind turbine. Increasing blade length, root diameter, and weight also makes transportation of blades to wind turbine sites more difficult.

One design solution that enables increased blade length is to support the blades with cables. The two main sources of blade loading in wind turbines are wind and gravity, with wind generally producing flapwise moments at the blade root, and gravity generally producing edgewise moments at the blade root. Cable-supported rotors include a system of cables that couples the blades to each other in a manner that reduces these loads. Thus, cable-supported rotors can generally use longer blades than rotors lacking cable supports for a given set of root size and blade weight design parameters. For example, a cable-supported rotor having a given bolt circle diameter at the hub-to-blade connection can typically have longer blades than a rotor having the same bolt circle diameter that is not cable-supported. By enabling increased sweep areas, cable-supported rotors may allow wind turbines to produce more energy than would otherwise be possible.

At the time wind turbines including cable-supported rotors are commissioned, technicians typically adjust the support cables so that they have a predetermined amount of tension. However, the amount of tension can change over time such that maintenance to adjust the tension should be performed on a regular basis. Thus, wind turbines including cable- supported rotors may be subject to more frequent maintenance windows during which they must be taken out of service. In addition, deviations from optimal cable tension can reduce the advantages of cable-supported rotors.

Accordingly, there is a need for improved systems and methods for controlling tension in cable-supported rotors.

Summary

In an aspect of the invention, a method of operating a wind turbine is disclosed. The wind turbine includes a plurality of operational modes and a rotor. The rotor has a plurality of blades, a plurality of cable assemblies that support the blades, and a plurality of tension mechanisms each configured to adjust a tension of a respective cable assembly. The method includes receiving an indication that the tension of one or more of the cable assemblies needs adjustment, determining an operational mode in which the wind turbine is operating, and controlling one or more tension mechanisms to adjust the tension of the one or more cable assemblies according to a tensioning process associated with the operational mode in which the wind turbine is operating.

In an embodiment of the method, the indication that the tension of the one or more cable assemblies needs adjustment may include one or more of the tension being outside an acceptable tension range, an operational parameter of the wind turbine exceeding an operational limit, and a position of the tension mechanism being outside an acceptable position range.

In another embodiment of the method, the operational parameter may include one or more of a period of time, a temperature, a product of the period of time and the temperature, an amount of energy produced by the wind turbine, and a number of rotations of the rotor since a previous tension adjustment or a maintenance window.

In another embodiment of the method, the operational mode in which the wind turbine is operating may be a power production mode, and the method may further include measuring one or more operational parameters including at least one of a wind speed, a rotor speed, a generator speed, an amount of power being produced, and a blade pitch angle. The method may further include, for each cable assembly of the plurality of cable assemblies, measuring the tension in the cable assembly and storing data indicative of the tension of the cable assembly and the one or more operational parameters.

In another embodiment of the method, the method may further include, for each of the one or more cable assemblies, defining a mapping function based on the stored data indicative of the tension of the cable assembly and the one or more operational parameters, using the mapping function to generate an expected tension in the cable assembly based on current operational parameters of the wind turbine, and adjusting the tension provided by the tension mechanism of the cable assembly to match the expected tension.

In another embodiment of the method, the mapping function may include a function of tension versus rotor speed or generator speed, and using the mapping function to generate the expected tension may include providing at least one of the rotor speed or the generator speed to the mapping function while the wind turbine is operating at a partial load.

In another embodiment of the method, the mapping function may include a function of tension versus the blade pitch angle, and using the mapping function to generate the expected tension may include providing the blade pitch angle to the mapping function while the wind turbine is operating at a full load.

In another embodiment of the method, adjusting the tension provided by the respective tension mechanism to match the expected tension may include determining a difference in tension between the expected tension and the tension being provided by the tension mechanism, generating an error signal based on the difference in tension, generating one or more of a proportional term, an integral term, and a derivative term of the error signal, summing the one or more of the proportional term, the integral term, and the derivative term to generate a control signal, and using the control signal to adjust the tension provided by the tension mechanism.

In another embodiment of the method, the operational mode in which the wind turbine is operating may be an idle mode, and the method may further include, measuring the tension of each cable assembly of the plurality of cable assemblies over a period of time that includes at least one full rotation of the rotor, determining a mean tension in each cable assembly over the period of time, and adjusting the tension provided by the one or more tension mechanisms so that the mean tension of each cable assembly is within a predetermined amount of the mean tension of the other cable assemblies.

In another embodiment of the method, the operational mode in which the wind turbine is operating may be the idle mode, each tension mechanism of the plurality of tension mechanisms may include a hydraulic actuator, and the method may further include increasing a pressure in a chamber of the hydraulic actuator of each tension mechanism until a pressure setpoint is reached thereby causing each tension mechanism to increase the tension in its respective cable assembly, fluidically coupling the chamber of the hydraulic actuator of each tension mechanism to the chamber of the hydraulic actuator of each of the other tension mechanisms, rotating the rotor by a predetermined amount of rotation, comparing the pressure in the chamber of the hydraulic actuator of at least one tension mechanism to the pressure in the chamber of the hydraulic actuator of at least one other tension mechanism, and in response to the pressure in the chamber of the hydraulic actuator of the at least one tension mechanism varying from the pressure in the chamber of the hydraulic actuator of the at least one other tension mechanism by more than a predetermined amount of pressure, releasing the pressure from the chamber of the hydraulic actuator of each tension mechanism.

In another embodiment of the method, the chamber of the hydraulic actuator of each tension mechanism may be fluidically decoupled from the chamber of the hydraulic actuator of each of the other tension mechanisms while the pressure is being increased, the chamber of the hydraulic actuator of each tension mechanism may be fluidically coupled to the chamber of the hydraulic actuator of each of the other tension mechanisms after the pressure setpoint is reached, and the pressure in the chamber of the hydraulic actuator of the at least one tension mechanism may be compared to the pressure in the chamber of the hydraulic actuator of the at least one other tension mechanism after the rotor has rotated by the predetermined amount of rotation.

In another embodiment of the method, the method may further include pitching each blade of the rotor to a pitch angle that produces a minimum amount of tension in each of the cable assemblies prior to increasing the pressure in the chamber of the hydraulic actuator of each of the tension mechanisms.

In another embodiment of the method, the operational mode in which the wind turbine is operating may be the idle mode, each of the tension mechanisms may include a hydraulic actuator having a chamber, and the method may further include controlling a pitch angle of each of the blades to maintain an amount of tension in each cable assembly that is within a predetermined amount of the amount of tension in the other cable assemblies while the rotor is rotating, fluidically coupling the chamber of the hydraulic actuator of each tension mechanism to the chamber of the hydraulic actuator of each of the other tension mechanisms, increasing a pressure in the chamber of the hydraulic actuator of each tension mechanism until a pressure setpoint is reached thereby causing each tension mechanism to increase the tension in its respective cable assembly, measuring a measured tension versus rotor azimuth in each cable assembly while the rotor is rotating and the pressure is increasing, comparing the measured tension to an expected tension in each cable assembly, and if the measured tension deviates from the expected tension by more than an allowable amount, releasing the pressure in the chamber of the hydraulic actuator of each tension mechanism.

In another embodiment of the method, the operational mode in which the wind turbine is operating may be the idle mode, and the method may further include pitching each blade of the rotor to a pitch angle that produces a minimum amount of pitch-induced tension in each of the cable assemblies. In this embodiment, for each cable assembly of the plurality of cable assemblies, the method may further include, increasing the tension provided by the respective tension mechanism until a tension setpoint is reached in the cable assembly while the tension mechanisms of other cable assemblies are providing a minimum amount of tension, measuring a measured tension versus a rotor azimuth in the cable assembly while the rotor is rotating, and defining a function of tension versus rotor azimuth for the cable assembly based on the measured tension versus the rotor azimuth.

In another embodiment of the method, each of the tension mechanisms may include a hydraulic actuator having a chamber, the minimum amount of tension may be provided by each tension mechanism while the chamber of the hydraulic actuator of the tension mechanism is unpressurized, and increasing the tension provided by the tension mechanism until the tension setpoint is reached in the cable assembly may include increasing a pressure in the chamber of the hydraulic actuator of the tension mechanism until a pressure setpoint is reached.

In another aspect of the invention, a wind turbine is disclosed. The wind turbine includes the rotor having the plurality of blades, the plurality of cable assemblies that support the plurality of blades, and the plurality of tension mechanisms each configured to adjust a tension of a respective cable assembly and a controller operatively coupled to each of the tension mechanisms and including one or more processors and a memory storing program code that, when executed by the one or more processors, causes the controller to perform the above described method and/or any of the embodiments thereof.

Brief Description of the Drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention.

Fig. 1 is a diagrammatic front view of an exemplary wind turbine including a cable-supported rotor with a cable system.

Fig. 2 is a diagrammatic front view of the wind turbine of Fig. 1 showing additional details of the cable system.

Fig. 3 is a schematic view of an exemplary cable tension system for controlling tension in the cable system of Figs. 1 and 2 that includes a plurality of tension mechanisms.

Fig. 4 is a diagrammatic front view of the wind turbine with an alternative embodiment of the cable system.

Fig. 5 is a diagrammatic view of a portion of an exemplary cable tension system of the wind turbine of Fig. 4 including a tension mechanism. Fig. 6 is a schematic view of an exemplary tension mechanism that may be used in the cable tension systems of Figs. 3 and 5.

Fig. 7 is a schematic view of another exemplary tension mechanism that may be used in the cable tension systems of Figs. 3 and 5.

Fig. 8 is a schematic view of an exemplary cable tension system that may be used in the wind turbines of Figs. 1-5.

Figs. 9-11 are flowcharts of processes that may be used to operate the wind turbines of Figs. 1-5.

Fig. 12 is a schematic view of an exemplary control system that may be used to implement the processes of Figs. 9-11 .

Fig. 13 is a graphical view of a plot illustrating an exemplary relationship between cable tension and wind speed for the wind turbines of Figs. 1-5.

Fig. 14 is a graphical view of a plot illustrating an exemplary relationship between cable tension and generator speed for the wind turbines of Figs. 1-5.

Fig. 15 is a graphical view of a plot illustrating an exemplary relationship between cable tension and blade pitch angle for the wind turbines of Figs. 1-5.

Fig. 16 is a flowchart of another process that may be used to operate the wind turbines of Figs. 1-5.

Fig. 17 is a schematic view of a controller that may be used in the control system of Fig. 12.

Detailed Description

Embodiments of the invention are directed to cable systems for cable-supported rotors that include an active cable tension system which ensures proper cable tension in all situations during the lifetime of the wind turbine. One aspect of the cable tension system is to perform a cable pre-tension cycle that is used during start up. If needed, the cable tension system may also actively adjust tension while the wind turbine is producing power. The cable system includes a plurality of cable assemblies and a cable tension system configured to control tension in each of the cable assemblies. In one embodiment, each of the cable assemblies is tensioned by applying tension to a respective center cable. Each center cable may be connected to a structure mounted on the hub of the rotor by a respective tension mechanism. In another embodiment, each of the cable assemblies is tensioned by applying tension to a respective tip cable of the cable assembly. In this embodiment, the tip cable may be connected to an anchor point mounted in or on one of the blades. In each case, the tension mechanisms may be configured to balance tension between each cable assembly of the cable system. The tension mechanisms may also provide stiffness and damping characteristics that optimize functionality of the cable system.

When necessary for servicing (e.g., for service of a blade pitch system) or during installation, the tension provided by the tension mechanisms can be relieved to remove any tension placed on the pitch system or other blade components by the cable assemblies. If permanent elongation of the cables occurs (sometimes referred to as “creep”), the cable tension system may be used to retighten the cable system so that it maintains load alleviation according to design specifications. The cable tension system may be configured to apply an initial tension force in the cables before operation and to sustain a generally linear relationship between cable tension and displacement, i.e. , a spring-like characteristic.

Figs. 1 and 2 depict an exemplary wind turbine 10 including a tower 12 and an energy generating unit 14 disposed at the apex of the tower 12. The tower 12 is coupled to a foundation 16 at a lower end thereof. The foundation 16 may be a relatively large mass formed from concrete, steel, etc., that transfers forces acting on the wind turbine 10 into the ground. In an alternative embodiment, the foundation 16 may include a pile or other structure at an offshore location to which the wind turbine 10 is attached. The tower 12 is configured to support the weight of the energy generating unit 14 and elevate the energy generating unit 14 to a height above ground level or sea level at which faster moving air currents of lower turbulence are typically found.

The energy generating unit 14 includes a nacelle 20, a cable-supported rotor 22, and an electrical system, including a generator and a converter (not shown). The rotor 22 includes a central hub 24 and a plurality of wind turbine blades 26 (e.g., three blades) that are operatively coupled to the hub 24. Each blade 26 may extend from a root end to a tip end, with the root end being coupled to the hub 24 through a pitching system (Fig. 12). The pitching system selectively rotates (or “pitches”) each blade 26 about a pitch axis thereof to adjust their angle of attack with respect to the wind. The energy generating unit 14 is coupled to the tower 12 by a yaw system (Fig. 12) that rotates the energy generating unit 14 relative to the tower 12, e.g., for upwind control.

The rotational position of the rotor may be referred to as the azimuth of the rotor 22. By way of example, the position of the rotor 22 shown in Figs. 1 and 2 depicts the tip of one blade 26 at the apex of its path around the rotor’s axis of rotation. If this rotor position is considered as an azimuth of 0 degrees, then, for the depicted three-blade rotor 22, the rotor azimuth when the tip of the next blade 26 reaches its apex would be about 120 degrees, and the rotor azimuth when the tip of the next blade 26 after that reaches its apex would be about 240 degrees.

The generator is operatively coupled to the hub 24, e.g., by a drive train that typically includes a gear arrangement that interconnects the rotor 22 and the generator. The rotational speed of the generator is connected to the rotational speed of the rotor 22 by the gear ratio. Thus, the speed of the generator can be determined from the speed of the rotor, and vice-versa. The generator and a substantial portion of the drive train may be positioned inside the nacelle 20. The generator converts mechanical energy received from the rotor 22 into electrical energy, which is typically supplied to the grid. In addition to the generator, the nacelle 20 may also house other miscellaneous components required for converting wind energy into electrical energy and that are needed to operate, control, and optimize the performance of the wind turbine 10. The blades 26 are configured to interact with the wind in a manner that generates lift. This lift causes the rotor 22 to rotate to generally define a sweep area of the blades 26. The energy generating unit 14 thus generates power from the wind that passes through the sweep area of the rotor 22.

The blades 26 are supported by a cable system 30 that carries as least some of the static and dynamic loads. The cable system 30 transfers these loads between the blades 26 so that the blades 26 mutually support each other. Opposing loads may cancel each other, and some loads may be transferred from the blades 26 to the hub 24. Edgewise loads and flapwise loads are thereby shared among the blades 26 and between the blades 26 and the hub 24 via the cable system 30.

The exemplary cable system 30 of Fig. 1 includes three cable assemblies 32 (one for each pair of blades 26), with each cable assembly 32 being connected to the rotor 22 at three locations. One of these connection points is to the hub 24, and each of the remaining two connection points is to a respective one of two adjacent blades 26. That is, each cable assembly 32 is coupled to and between a respective pair of adjacent blades 26 and to the hub 24. As shown, this forms a Y-shaped cable configuration between the adjacent blades 26 and hub 24. Although not shown in detail, the connection between the cables of each assembly 32 and the rotor 22 may be by way of cable end fittings commonly used in the industry. For example, a thimble with an integrated bearing may define one or more cable ends in each assembly 32. The cable end fittings may cooperate with other fittings or receptacles on the hub 24 and/or on the blades 26 to mechanically attach the assembly 32 so that it carries a portion of the loads on the rotor 22.

Each of the blades 26 may include a cable-to-blade connection point to which the cable assemblies 32 are coupled. The connection points on the blades 26 may be arranged at a distance midway between the root end and the tip end, e.g., at a distance between 10% and 60% of the length of the blade 26 from the root end. In cases where the blades 26 are assembled from seperate sections (e.g., an inboard section including the root end and an outboard section including the tip end), the inboard section and the outboard section may be connected at a split position. In this case, the connection points of the blades 26 may be provided at the split positions, e.g., as part of a coupling mechanism that joins the inboard and outboard sections of the blade 26. The connection point itself may be positioned outside of the blade at the split position. For example, a connection point may extend outwardly from the blade 26 and be available for connection to the cable system 30. Blade connection points may be preferably located on or proximate to the pitch axis of the blade 26 to reduce interaction between blade pitch and tension in the cable assemblies 32.

In the depicted embodiment, each cable assembly 32 includes two tip cables 34 that are connected to one another and to the blades 26, and a center cable 36 that is connected to the two tip cables 34 and the hub 24. Thus, each cable assembly 32 includes three separate cables corresponding to each portion of the Y-shaped cable assembly 32. Each of the tip cables 34 and center cable 36 are coupled together at an intersection point 38. Each intersection point 38 may include a connector 40 to which one end of each cable of the respective cable assembly 32 is operatively coupled. One or more of the ends of the tip cables 34 and center cable 36 may include a thimble configured to connect to connection points on the hub 24, blade 26, and/or connector 40, respectively. In an alternative embodiment, the tip cables 34 may be manufactured as one unit, with each end of the unit being operatively coupled to a blade connection point on a respective blade 26. In another embodiment, the tip cables 34 and center cable 36 may be manufactured as one unit (e.g., a Y- or T shaped cable with three ends), with each leg of the unit forming one of the tip cables 34 and the center cable 36, respectively.

By way of example, one or more of the cables of the cable system 30 may comprise a polymer material as a load bearing component. The polymer material may be, for example, an ultra-high molecular weight polyethylene, e.g., of the kind being manufactured under the tradename ‘Dyneema’. Ultra-high molecular weight polyethylene fibers may be particularly advantageous due to a combination of a high strength/weight ratio and good fatigue properties. As an alternative, the polymer material may be based on polyester, polyamid, nylon, polypropylene, aramid, etc. As another alternative, the polymer material may be a composite material, e.g., a liquid crystal polymer, such as polybenzoxazole (PBO). However, the cable system 30 is not limited to a polymer material as various types of steel cabling may also be utilized alone or in combination with polymer materials. Moreover, other tension members may be used as “cables” instead of flexible members, such as steel rods and other rigid or semi-rigid materials.

Each cable assembly 32 may be taut in its attachment between the hub 24 and the blades 26. Tensioning the cable assemblies 32 may be achieved using a cable tension system. The cable tension system may be located in the hub 24, and may include one or more tension mechanisms configured to pull on one or more of the center cables 36 following installation of the cable assemblies 32. Pulling on the center cable 36 in a direction toward the hub 24 places tip cables 34 and center cables 36 in tension so that the cable assemblies 32 transmit loads imposed on the blades 26.

Fig. 3 presents a cross sectional view of the hub 24 illustrating an exemplary cable tension system 42 including a plurality of (e.g., three) tension mechanisms 44. Each tension mechanism 44 includes a distal end 46 and a proximal end 48. The distal end 46 of each tension mechanism 44 is operatively coupled to a proximal end 50 of a respective center cable 36, e.g., by a thimble and pin arrangement. The proximal end 48 of each tension mechanism 44 is operatively coupled to an anchor point 52 located within or proximate to the hub 24, e.g., by a ball joint or other type of bearing 55 that allows the tension mechanism 44 to pivot with respect to the anchor point 52. Each tension mechanism 44 is configured to apply a selectable amount of force to the center cable 36, thereby enabling individual adjustment of the tension in each cable assembly 32. The anchor point 52 may “float” in the sense that it is allowed to move within certain limits. This movement may facilitate load sharing and balancing among the center cables 36 during operation of the wind turbine 10. In an alternative embodiment, the anchor point 52 may be fixed to the hub 24 so that when unequal forces are applied to the anchor point 52 by the tension mechanisms 44, at least some of these forces are transferred to the hub 24.

The tension mechanisms 44 may form part of the hub 24 and be oriented radially about the rotational axis thereof. In accordance with one embodiment, the anchor point 52 may be operatively coupled to the hub 24 by an extended hub structure (e.g., a tripod or beam - not shown) that extends outward from the hub 24 along the rotor’s axis of rotation. The extended hub structure may locate the anchor point 52 so that it is offset axially in a windward direction from the rotational plane of the rotor 22. This axial offset may enable the cable assemblies 32 to provide support to the blades 26 that counteracts forces (e.g., flapwise forces) applied to the blades 26 by the wind. Each tension mechanism 44 may include one or more linear actuators, such as a hydraulic, electrical, pneumatic, or mechanical actuator, configured to apply a selective amount of tension to the center cable 36.

Fig. 4 depicts an exemplary cable system 30 in accordance with an alternative embodiment that omits the center cable 36 from the cable assemblies 32. In this embodiment, each cable assembly 32 includes a single tip cable 34 operatively coupled to each of two adjacent blades 26. In each cable assembly 32, one end of the tip cable 34 is operatively coupled to one of two adjacent blades 26, and the other end is operatively coupled to the other of the two adjacent blades 26. One end of the tip cable 34 may include a thimble configured to connect to a connection point on the blade 26, and the other end may include a thimble configured to connect to a connection point on a tension mechanism 44 housed within or otherwise attached to the adjacent blade 26.

Fig. 5 depicts a detailed view of a section of a blade 26 illustrating a portion of an exemplary cable tension system 42 in accordance with the cable system 30 of Fig. 4. The depicted portion of the cable tension system 42 includes a tension mechanism 44 having a distal end 46 operatively coupled to a proximal end 53 of a respective tip cable 34a, e.g., by a thimble and pin arrangement. The proximal end 48 of the tension mechanism 44 may be operatively coupled to an anchor point 52 located within or proximate to the blade 26, e.g., by a ball joint or other type of bearing 55 that allows the tension mechanism 44 to pivot with respect to the anchor point 52. The anchor point 52 may be rigidly attached to the blade 26, or allowed to float to a certain extent, e.g., through a compliant coupling to the blade 26. The tension mechanism 44 may be one of a plurality of mechanisms configured to apply a selectable amount of force to each tip cable 34, thereby enabling individual adjustment of the tension in each cable assembly 32. The tip cable 34b connecting the depicted blade 26 the other adjacent blade 26 may be operatively coupled to the anchor point 52, e.g., by a thimble/pin arrangement, so that the forces applied to the blade 26 by the tip cables 34a, 34b act on the blade 26 through the anchor point 52.

Fig. 6 illustrates an exemplary tension mechanism 44 of the cable tension system 42. The tension mechanism 44 may be in communication with a tension system controller 54, and includes a hydraulic actuator 56 and an actuator valve 58 (e.g., a proportional valve) that couples the hydraulic actuator 56 to a source of pressurized hydraulic fluid 60. Typically, the cable tension system 42 includes multiple hydraulic actuators 56 and actuator valves 58, e.g., one for each cable assembly 32 of rotor 22. The tension system controller 54 may be configured to specifically control the tension applied to one or more of the cable assemblies 32 of the wind turbine 10, or may be provided by an application running on another controller, such as a controller that also controls the wind turbine 10, blade pitch system, or power unit yaw system.

The hydraulic actuator 56 may include a piston 62 located within a cylinder 64 that is terminated on one end by a cylinder cap 66 and on the other end by a cylinder head 68. The piston 62 is coupled to a piston rod 70 and divides the interior of the cylinder 64 into a front chamber 72 (also known as a piston rod chamber) through which the piston rod 70 passes, and a rear chamber 74 (also known as a bottom chamber) that is terminated by the cylinder cap 66.

The piston rod 70 may pass through a sealed opening in the cylinder head 68 and includes a distal end operatively coupled to a cable (e.g., a tip or center cable 34, 36) of cable assembly 32 by a coupling 76, such as a thimble and a pin arrangement. Movement (e.g., retraction) of the piston rod 70 may apply a tensile force to the cable assembly 32. The cylinder cap 66 may be operatively coupled to the anchor point 52 so that movement of the piston 62 causes a linear displacement of the end of the center cable 36 relative to the hub 24 or the end of the tip cable 34 relative to the blade 26, as the case may be.

In response to receiving a signal from the tension system controller 54, the actuator valve 58 may selectively fluidically couple an output port of the hydraulic fluid source 60 to one of the front chamber 72 and rear chamber 74 of hydraulic actuator 56, and selectively fluidically couple a return port of the fluid source to the other of the front chamber 72 and rear chamber 74 of hydraulic actuator 56. The tension system controller 54 may thereby control the flow of fluid between the hydraulic fluid source 60 and the hydraulic actuator 56 via actuation of the actuator valve 58. The hydraulic fluid source 60 may include one or more pumps, valves, accumulators, etc. configured to provide pressurized fluid. The hydraulic fluid source 60 may be dedicated to operation of a single hydraulic actuator 56, or may provide hydraulic fluid to multiple hydraulic actuators 56 of cable tension system 42.

The cable tension system 42 may further include one or more of a front chamber pressure sensor 78, a rear chamber pressure sensor 80, and a force sensor 82. The front chamber pressure sensor 78 may be configured to sense the pressure of the fluid in, or that is being provided to, the front chamber 72 of hydraulic actuator 56. The rear chamber pressure sensor 80 may be configured to sense the pressure in, or that is being provided to, the rear chamber 74 of hydraulic actuator 56. Each pressure sensor 78, 80 may output a respective pressure signal 84, 86 indicative of the pressure sensed by the pressure sensor 78, 80. For example, each pressure signal 84, 86 may have one or more characteristics (e.g., a voltage, current, impedance, frequency, phase, etc.) that provide information to the tension system controller 54 indicative of the sensed pressure.

When present, the force sensor 82 may be configured to provide a force signal 88 to the tension system controller 54 indicative of an amount of tension being provided to the cable assembly 32 in a similar manner as described above with respect to the pressure sensors 78, 80. By way of example, the force sensor 82 may be configured to measure the strain in the piston rod 70 using a suitable strain sensor, such as one or more strain gauges or one or more optical fibers, that generates signals indicative of strain.

The tension system controller 54 may also use the pressure data received from the pressure sensors 78, 80 to determine the actuator force being applied to the cable assembly 32 by the hydraulic actuator 56. The actuator force may be determined, for example, using the following equation: where FA is the actuator force applied by the piston rod 70, PFC is the pressure in the front chamber 72, PRC is the pressure in the rear chamber 74, AFF is the effective area of the piston 62 facing the front chamber 72, and ARF is the effective area of the piston 62 facing the rear chamber 74. As can be seen from Equation 1 , a positive value of actuator force FA would indicate the piston rod 70 is pushing on the cable assembly 32, while a negative value of actuator force F_P indicates the piston rod 70 is pulling on the cable assembly 32. The effective area of the piston 62 facing the rear chamber 74 is typically larger than the effective area of the piston 62 facing the front chamber 72 due to the presence of the piston rod 70. If the force sensor 82 is present, the tensile force on the tip or center cable may be determined without pressure measurements, or the force can be calculated based on pressure measurements merely to check operation of the force sensor 82.

Fig. 7 illustrates an exemplary tension mechanism 44 of cable tension system 42 in accordance with an alternative embodiment thereof. The depicted tension mechanism 44 includes a mechanical actuator 90 comprising a motor 92 (e.g., an electric motor) operatively coupled to a screw 94 that provides an actuator force F_A to the cable assembly 32. As with Fig. 4 above, the cable tension system 42 is depicted with a single mechanical actuator 90 for simplicity. However, it should be understood that the cable tension system 42 may include a separate mechanical actuator 90 for each cable assembly 32 of rotor 22. The screw 94 may include a cylindrical shaft 96 having a helical ridge 98 and a threaded collar 100 including a hole having a helical grove configured to mesh with the helical ridge 98 of cylindrical shaft 96. The screw 94 may be configured so that when the motor 92 rotates the cylindrical shaft 96, the collar 100 is urged longitudinally along an axis of the cylindrical shaft 96 in a direction dependent on the direction of rotation.

The collar 100 may be operatively coupled to the coupling 76 by a connecting rod 102 so that the actuator force F_A generated by the screw 94 is operatively coupled to the cable assembly 32, i.e., so that movement of the collar 100 alters the tension on the tip cable 34 or center cable 36, as the case may be. The cable tension system 42 may also include the force sensor 82 that provides the force signal 88 to the tension system controller 54 indicative of the actuator force F_A being applied to the cable assembly 32 by the mechanical actuator 90.

The above tension mechanisms are exemplary only, and it should be understood that embodiments of the cable tension system 42 may be based on hydraulic, electric, pneumatic, or any other suitable type of tension mechanism 44. Moreover, the various components comprising the cable tension system 42 may be located in or proximate to the hub 24, in or near the blades 26, or in any other suitable location from which they can provide tension to the cable assemblies 32.

During normal operation of the wind turbine 10, the cable tension system 42 may maintain each of the tension mechanisms 44 in a fixed condition, referred to as “passive mode”, e.g., by closing actuator valve 58 or keeping motor 92 stationary. While in passive mode, cable system 30 provides load support to the blades 26 based on the existing tension provided by the cable assemblies 32. The tension may be set during an initial pre-tension procedure that is completed during commissioning of the wind turbine 10. This procedure may only need to be repeated occasionally, e.g., during scheduled cable re-tension or when necessary to address warnings or alarms indicating the cable system 30 needs re-tensioning. The cable system 30 may also be designed to enable slacking of the cables when required, e.g., during a wind turbine maintenance window.

Fig. 8 depicts an exemplary cable tension system 42 including a plurality of hydraulic actuators 56, e.g., one for each cable assembly 32 of cable system 30. The front and rear chambers 72, 74 of each hydraulic actuator 56 may be operatively coupled to a respective accumulator 104 by an actuator valve 106. The front chamber 72 of each hydraulic actuator 56 may be operatively coupled to the front chamber 72 of each of the other hydraulic actuators 56 by a respective front chamber balance valve 107. Similarly, the rear chamber 74 of each hydraulic actuator 56 may be operatively coupled to the rear chamber 72 of each of the other hydraulic actuators 56 by a rear chamber balance valve 108. Each accumulator 104 may be operatively coupled to a hydraulic power unit 114 of the wind turbine 10 by a charge valve 109.

Each of the valves 106-109 may be in communication with the tension system controller 54 (Fig. 6). The tension system controller 54 may be configured to cause the valves 106-108 to selectively fluidically couple the upper and lower chambers 72, 74 to one or more of the accumulator 104, hydraulic power unit 114, and/or each other in order to control tension in the cable assemblies 32 during commissioning, maintenance windows, and/or operation of the wind turbine 10. The tension system controller 54 may also be configured to selectively fluidically couple one or more of the accumulators 104 to the hydraulic power unit 114. The one or more of the charge valves 109 may be opened during maintenance, for example, while the corresponding actuator valve 106 is closed to selectively charge the accumulator 104 with pressurized hydraulic fluid from the hydraulic power unit 114, or to release pressure into the hydraulic power unit 114.

Fig. 9 depicts a flow chart illustrating an exemplary pre-tensioning process 120 for setting tension levels in the cable system 30. In block 122, the process 120 may determine if one or more preconditions are met. Exemplary preconditions may include, for example, that the wind turbine 10 is in an idle mode (e.g., a state in which there is little or no rotation of the rotor 22), that the blades 26 are pitched at idle mode angles (e.g., feathered), and that there are no active alarms. If the preconditions are not met (“NO” branch of decision block 124), the process 120 may proceed to block 126, adjust one or more operational parameters and/or issue an alarm, and return to block 122. If the preconditions are met (“YES” branch of decision block 124), the process 120 may proceed to block 128.

In block 128, the process 120 may set the blade pitch to a value that minimizes tension in the cable system 30. A typical blade pitch angle that produces the minimum amount of tension may be in the range of 40-50 degrees, depending on the location of the cable attachment points on the blade 26 and the geometry of the blade 26 and blade pitch system. Setting blade pitch to an angle that results in the minimum amount of tension in the cable system 30 may minimize any differences between the pressure in the hydraulic actuators 56 of cable tension system 42.

In block 130, the process 120 may increase the pressure in one or more of the front and rear chambers 72, 74 (e.g., the front chamber 72) of each hydraulic actuator 56. The pressure may be increased by activating one or more of the actuator valve 106 and charge valve 109 of cable tension system 42. For example, the process 120 may cause each of the actuator valves 106 to fluidically couple the front chamber 72 of each hydraulic actuator 56 to its respective accumulator 104 for a period of time that causes the pressure inside the front chamber 72 to increase a predetermined amount with respect to the pressure of the rear chamber 74. This increase in relative pressure in the front chamber 74 may cause the tensile force exerted on the cable assembly 32 by the hydraulic actuator 56 to increase.

In block 132, the process 120 may determine if a pressure setpoint has been reached, e.g., based on readings taken from the front and/or rear chamber pressure sensors 78, 80. If the pressure setpoint has not been reached (“NO” branch of decision block 132), the process 120 may return to block 130 and continue to increase the pressure in the one or more cylinder chambers 72, 74. If the pressure setpoint has been reached (“YES” branch of decision block 132), the process 120 may stop increasing the pressure (e.g., by closing the actuator valves 106 and/or charge valves 109) and proceed to block 134.

In block 134, the process 120 may open one or more (e.g., all) of the balance valves 107,108, thereby fluidically coupling the front chambers 72 of the hydraulic actuators 56 to each other and/or fluidically coupling the back chambers 74 of the hydraulic actuators 56 to each other. The process 120 may then proceed to block 136 and rotate the rotor 22 by a predetermined amount, e.g., 3600 degrees (10 rotations). As the rotor 22 rotates, the forces applied to the cable assemblies 32 by the blades 26 may vary with azimuth. For example, when the azimuth of a three-bladed rotor 22 is such that one blade 26 of rotor 22 is in a vertical position with its tip at the highest position, the downward bending of the two lower blades 26 under the force of gravity may tend to decrease the distance between their tips. This decrease in distance between the tips may, in turn, tend to reduce the amount of tension in the cable assembly 32 connecting those two blades 26.

In contrast, when the one blade 26 of rotor 22 is in a vertical position with its tip at the lowest position, the downward bending of the two upper blades 26 under the force of gravity may tend to increase the distance between their tips. This increase in distance between the tips may tend to increase the amount of tension in the cable assembly 32 connecting those two blades 26. Thus, the tension in each of the cable assemblies 32 may be expected to vary between a minimum amount of tension and a maximum amount of tension as the rotor 22 rotates about its axis. When the balance valves 107, 108 are open, this variation in tension may result in changes in the force applied to the pistons 62 of the hydraulic actuators 56 that causes pressure to be transmitted between hydraulic actuators 56 by the action of the pistons 62 on the hydraulic fluid as the rotor 22 rotates.

In block 138, the process 120 may determine if the pressure in the front and/or rear chambers 72, 74 has reached an equilibrium pressure. By way of example, the equilibrium pressure may be considered as being reached if the pressure in each of the front and/or in each of the rear chambers of the hydraulic actuators 56 is within a predetermined amount (e.g., within 10 percent) of the pressure in the other front or rear chambers. If the chamber pressures in question have not reached equilibrium (“NO” branch of decision block 138), the process 120 may proceed to block 140, release the pressure in the cylinders 64 of each hydraulic actuator 56, and return to block 122 to repeat the pre-tensioning process. If the chamber pressures in question have reached equilibrium (“YES” branch of decision block 138), the process 120 may proceed to block 142, close the balance valves 109, and terminate.

The pre-tensioning process 120 may be run at wind turbine commissioning, and prior to restart after certain blade or hub services, e.g., services that require de-tensioning of the cable system 30. After wind turbine commissioning or replacement of the cables of cable system 30, the pre-tensioning process 120 may be run several times at predetermined intervals to ensure correct blade load support. The number and timing of the repeated pre-tensioning processes 120 may depend on the amount of initial cable creep during the first operating weeks. After this initial break-in period, the pre-tensioning process 120 may only need to be performed if cable tension falls below a minimum value (e.g., as determined from pressure, force, or strain measurements) or after a service that requires re-tensioning of the cable system 30.

Fig. 10 depicts a flow chart illustrating another pre-tensioning process 150 for setting tension levels in the cable system 30. In block 152, the process 150 adjusts the pitch angle of each blade 26 so that the tension in the cable assemblies 32 is equalized. The tension levels may be considered as being equalized if, for example, the tension at each tension mechanism 44 is within a predetermined percentage of the tension at the other tension mechanisms, e.g., if there is less than a 10 percent difference in tension between the tension mechanisms 44. In any case, the pitch angles that produce equal tension in each cable assembly 32 may be different for each blade 26 due to the varying effects of gravity on the blades 26 at different azimuth positions of the rotor 22.

Once the tension is equalized across the cable assemblies 32, the process 150 may proceed to block 154, and open the balance valves 107, 108. The process 150 may then proceed to block 156, and begin increasing the pressure in one or both chambers 72, 74 (e.g., the front chambers 72) of the hydraulic actuators 56. The pressure may be increased, for example, by opening the actuator valves 106 and/or charge valves 109 so that pressurized hydraulic fluid flows into the chambers in question.

In block 158, the process 150 monitors the tension in the cable assemblies 32, e.g., by determining the tension at each tension mechanism 44. The amount of tension in each cable assembly 32 may be compared to an expected amount of tension based on the pitch of the blades 26 connected to the cable assembly 32 and the azimuth of the rotor 22 as the pressure increases and the rotor 22 rotates. This comparison may include, for example, comparing the determined tension to a value in a lookup table. The lookup table may include tension values that are associated with various combinations of pressure, pitch, azimuth, wind speed, generator or rotor speed, and blade pitch angle. These tension values may be determined, for example, based on empirical data gathered from the wind turbine being monitored, or from other similar wind turbines. If the tension values determined for the cable assemblies 32 being pre-tensioned vary from those expected by more than an allowable amount (e.g., more than 10 percent), the cable tension may be considered as being out of tolerance.

If the monitored cable tension does not stay within a predetermined tension tolerance level (“NO” branch of decision block 160), the process 150 may proceed to block 162 and abort the startup. Aborting startup may include, for example, releasing the tension provided by the tension mechanisms 44, e.g., by releasing the pressure in the hydraulic actuators 56. If the monitored cable tension does stay within the predetermined tolerance levels (“YES” branch of decision block 160), the process 150 may proceed to block 164 and determine if the pressure setpoint has been reached. If the pressure setpoint has not been reached (“NO” branch of decision block 164), the process 150 may proceed to block 166 and continue increasing the pressure. If the pressure setpoint has been reached (“YES” branch of decision block 164), the process 150 may proceed to block 168 and stop increasing the pressure, e.g., by closing one or more of the actuator valves 106 and charge valves 109.

The pre- tensioning process 150 described above may be performed to ensure that pressure differences between the hydraulic actuators 56 of cable tension system 42 are as low as possible during start-up, which may be desirable for a softer run-in of the cable system 30.

Fig. 11 depicts a flow chart illustrating an exemplary calibration process 170. In block 172, the process 170 determines if one or more preconditions are met. Exemplary preconditions may include, for example, closing the balance valves 107, 108 to fluidically decouple the cylinder chambers in different cable assemblies 32, checking chamber pressures to confirm they are at or close to zero (e.g., below 2000 Pascals), and checking that the wind speed is below a predetermined threshold (e.g., less than 5 m/s). If the preconditions are not met (“NO” branch of decision block 174), the process 170 may proceed to block 176, adjust one or more operational parameters and/or issue an alarm, and return to block 172. If the preconditions are met (“YES” branch of decision block 174), the process 170 may proceed to block 178.

In block 178, the process 170 may set the blade pitch to a pitch angle that produces a minimum amount of tension in the cable system 30 (e.g., idle pitch), and select an initial cable assembly 32 on which to gather calibration data. The process 170 may then proceed to block 180 and set the tension in the selected cable assembly 32. The tension may be set, for example, by causing the tension mechanism 44 in question to increase the tension applied to the cable assembly 32 (e.g., by pressurizing the cylinder 64 or activating the motor 92) until a predetermined tension setpoint is reached.

In response to the tension setpoint being reached, the process 170 may proceed to block 182 and determine a tension-azimuth relation for the selected cable assembly 32. This relationship may be determined, for example, by rotating the rotor 22 while monitoring both the azimuth of the rotor 22 and the tension at the tension mechanism 44. The resulting tension-azimuth relationship may be stored as values in a lookup table or otherwise used to define a function of tension versus azimuth for use during operation of the wind turbine 10.

Once the tension-azimuth relationship has been determined for the selected cable assembly 32, the process 170 may proceed to block 184, release the tension in the selected cable assembly 32, and proceed to block 186. Tension may be released in the selected cable assembly 32, for example, by releasing pressure from the cylinder 64 or activating the motor 92 to return the connecting rod 102 to its location prior to tensioning. If not all the cable assemblies 32 have been characterized (“NO” branch of decision block 186), the process 170 may proceed to block 188, select the next cable assembly 32, and return to block 180 to repeat the tension-azimuth relationship determination process. If all the cable assemblies 32 have been characterized (“YES” branch of decision block 186), the process 170 may proceed to block 190, update the calibration parameters of the cable tension system 42 with the tensionazimuth relationship data, and terminate.

Situations in which cable tension system 42 may be used to control or adjust tension in the cable system 30 include to compensate for permanent elongation (creep) of the cables in the cable system 30, to compensate for the thermal effects of seasonal climate variations (mean temperature) or for other changes in temperature, e.g., diurnal changes between daytime high temperatures and nighttime low temperatures. Actual short term mean temperature variations may be determined from direct measurements of air temperature or weather forecasts. The cable tension system 42 may also be used to compensate for mean temperatures found at different geographical site locations, such as artic or tropical locations, thereby avoiding the need to manufacture multiple cable assembly parts for different climates.

Other uses of the cable system 30 may include compensating for differences in cable tolerances. For example, daisy chaining hydraulic actuators 56 (as described above with respect to Fig. 9) or active control of tension during operation of the wind turbine 10 may be used to equalize the loads between blades and to compensate for differences in the length and amount of creep in the cables 34, 36 of cable system 30. Each of the above processes 120, 150, 170 may be performed independently of the cable configuration of the cable system 30. That is, cable configurations with and without a center cable 36 may use any of processes 120, 150, 170.

FIG. 12 illustrates an exemplary control system 200 that may be used to control the wind turbine 10. The control system 200 may include the cable tension system 42, a wind turbine controller 202 in communication with a wind sensor 204, a pitch system 206, a yaw system 208, a generator system 210, and a supervisory controller 212. The yaw system 208 may be used by the wind turbine controller 202 to control the direction in which the nacelle 20 is pointed, and may include one or more yaw controllers, drive systems, position sensors, etc. configured to implement a yaw command signal received from the wind turbine controller 202. The pitch system 206 may be configured to adjust the pitch of blades 26 collectively or independently in response to a pitch command signal received from the wind turbine controller 202, and may include pitch angle sensors 214 that provide signals indicative of the pitch angle of each blade 26. The generator system 210 may be operatively coupled to the rotor 22 and include the generator as well as any additional components necessary to control and monitor the wind turbine 10, such as rotor position sensor 216 and electrical power sensors 218, e.g., voltage and current sensors from which the power output of the generator can be determined. The supervisory controller 212 may be configured to implement a system-wide control strategy for a group of wind turbines 10 that manages the collective performance of the group.

Fig. 13 depicts a graph illustrating an amount of tension in the cable system 30 (e.g., as measured at a center cable 36) versus wind speed for three different levels of wind turbulence. Under normal power producing conditions (wind speeds above the cut-in wind speed and below the cut-out wind speed for the wind turbine 10), there is a low wind region 220 and a high wind region 222 in which the relationship between tension in the cable system 30 and wind speed is well defined and essentially independent of the level of turbulence. This relationship can be mapped for the wind turbine 10, for example, by defining a look-up table or curve-fitted polynomial based on empirical data collected during operation of the wind turbine 10.

The wind turbine controller 202 may estimate wind speed based on generator rpm, blade pitch angle, and the amount of electrical power being generated. Wind speed may also be determined directly based on signals from the wind sensor 204. Regardless of how wind speed is determined, the wind speed may be provided to the tension system controller 54 and used to actively control the amount of tension provided to the cable system 30 by the cable tension system 42. For example, one or more of the tension system controller 54 and wind turbine controller 202 may run an active control process that causes the cable tension system 42 to actively adjust the tension provided to the cable system 30 based on the wind speed. To this end, an expected tension may be determined based on a function of tension versus wind speed, and the tension of the tension mechanisms 44 adjusted to match the expected tension. The tension of the tension mechanisms 44 may be considered as matching the expected tension if the tensions are within a predetermined amount of each other, e.g., if there is less than a 1 percent, a 5 percent, or a 10 percent difference between the tension provided by the tension mechanisms 44 and the expected tension in the respective cable assemblies 32. The active control process may vary the tension slowly to avoid having an azimuthal impact on load variations. For example, the rate at which the tension is varied may be lower than the time it takes the rotor to make one revolution, e.g., for a rotor 22 rotating at 20 RPM, the active control process may have a cutoff frequency of 0.15 Hz.

Alternative embodiments may preform active tension control based at least in part on operational parameters such as generator/rotor speed and mean pitch angle. Fig. 14 depicts a graph of center cable tension versus generator speed (which is typically a fixed multiple of rotor speed), and Fig. 15 depicts a graph of center cable tension versus blade pitch angle. As can be seen from the graph of Fig. 14, the wind turbine 10 has an operational region 224 in which the relationship between generator speed and cable tension is well defined. This operational region 224 may, and correspond to the low-wind region 220 of Fig. 13 (e.g., wind speeds above cut-in wind speed and below rated wind speed) in which the speed of the rotor (and thus the generator) is strongly correlated with wind speed. When operating in this region, the wind turbine 10 may be considered as operating at a partial load. As can be further seen from the graph of Fig. 15, the wind turbine 10 has another operational region 226 in which the relationship between blade pitch angle and cable tension is well defined. This operational region 226 may correspond to the high-wind region 222 of Fig. 13 (e.g., wind speeds above rated wind speed and below cut-out wind speed) in which pitch angle is adjusted to control power output, and thus correlates strongly with wind speed. When operating in this region, the wind turbine 10 may be considered as operating at a full load. Thus, the active control process may adjust the tension provided to the cable system 30 based at least in part on the generator/rotor speed when the wind turbine 10 is operating below rated power level, and adjust the tension provided to the cable system 30 based at least in part on the mean pitch angle of the blades 26 when the wind turbine 10 is operating at rated power. The tension adjustment may process may include determining an expected tension based on a function of tension versus rotor speed or generator speed in the low wind region, and a function of tension versus blade pitch in the high wind region. The tension provided by the tension mechanisms 44 may then be controlled to match the expected tension.

Fig. 16 depicts a flow chart illustrating an exemplary operational process 230 that may be used to adjust the tension in the cable assemblies 32 of rotor 22. In block 232, the process 230 receives an indication that the tension of one or more cable assemblies 32 needs to be adjusted. This indication may include one or more of an indication the tension is outside an acceptable tension range, an operational parameter of the wind turbine 10 exceeds an operational limit, a position of the tension mechanism 44 is outside an acceptable position range, an indication related to a scheduled maintenance or commissioning activity, or any other suitable indication. Scheduled maintenance or commissioning activities may include, for example, an initial pre-tension procedure or configuring (e.g., training or programming) the cable system controller 54 to control tension in the cable assemblies 32.

In block 234, the process 230 determines the operational mode of the wind turbine 10. If the wind turbine is in an idle mode (“IDLE” branch of decision block 232), the process 230 may proceed to block 236 and measure the tension in one or more cable assemblies 32. The process 230 may measure the tension while the rotor 22 is rotating and over a period of time that includes at least one full rotation of the rotor 22. The process 230 may then determine a mean value of the tension over the period of time, e.g., by measuring the tension at multiple rotor azimuth positions and calculating an average value of the tension measurements.

Subsequent to or concurrently with measuring the tension in the one or more cable assemblies 32, the process 230 may proceed to block 238 and adjust the tension provided by one or more of the tension mechanisms 44. The tension may be adjusted, for example, in accordance with one of the processes 130, 150, 170 depicted in Figs. 9-11 and described in detail above. By way of example, the tension may be adjusted so that the tension in the one or more cable assemblies 32 being adjusted is within a predetermined amount (e.g., within 10 percent) of the tension in one or more other cable assemblies 32 of rotor 22. Adjusting the tension in this way may balance the mean tension in each cable assembly 32 with the mean tension in each of the other cable assemblies 32.

If the wind turbine 10 is in a power production mode (“POWER” branch of decision block 234), the process 230 may proceed to block 240 and determine one or more operational parameters of the wind turbine 10. Power production modes may include partial load operation, full load operation, or any other operating mode during which the wind turbine 10 produces power. The one or more operational parameters may be determined through direct measurement (e.g., using a sensor), or by calculation (e.g., based on other related operational parameters), and may include wind speed, rotor speed, generator speed, an amount of power being produced by the wind turbine 10, a blade pitch angle, a temperature (e.g., the ambient temperature), a period of time (e.g., the amount of time since a previous tension adjustment or maintenance window), a product of the period of time and the temperature (e.g., an integral of the temperature over the period of time multiplied by a constant), or a number of rotations of the rotor since a previous tension adjustment or a maintenance window, for example.

In block 242, the process 230 may determine if the tension in one or more of the cable assemblies 32 of rotor 22 needs to be adjusted. The process 230 may make this determination based on one or more of a tension being outside an acceptable tension range, an operational parameter of the wind turbine exceeding an operational limit, a position of the tension mechanism being outside an acceptable position range, or an input from a technician, e.g., an indication that the cable system controller 54 is in a training mode. If the tension does not need adjusting (“NO” branch of decision block 242), the process may proceed to block 244. If the tension does need adjusting (“YES” branch of decision block 242), the process 230 may proceed to block 246.

In block 244, the process 230 may measure the tension in one or more of the cable assemblies 32 while continuing to determine the one or more operational parameters of the wind turbine 10. In response to a predetermined amount of cable tension and operational parameter data being obtained (e.g., tension data over at least one revolution of the rotor 22 under each of a plurality of predetermined operating conditions), the process 230 may proceed to block 248 and define a mapping function. The mapping function may be defined by associating each measured cable tension value with one or more operational parameter values determined at the time the tension value was measured. The resulting mapping function may include a lookup table, neural network, or other algorithm that outputs a tension value for one or more cable assemblies 32 based on an input including at least one operational parameter. The mapping function may be configured to output an expected tension level for each of the one or more cable assemblies 32, and may be used by the cable system controller 54 or other system controller to control cable tension.

In block 246, the process 230 may determine the tension to apply to one or more cable assemblies 32. To this end, the process 230 may use a previously defined mapping function to generate an expected tension in one or more of the cable assemblies 32 based on at least one current operational parameter of the wind turbine 10. In an exemplary embodiment of the process 230, the mapping function may include a function of tension versus rotor speed or generator speed associated with operation below a rated wind speed, and a function of tension versus blade pitch angle associated with operation above the rated wind speed. In this embodiment, the process 230 may use the mapping function to generate the expected tension by providing the rotor or generator speed to the mapping function while the wind turbine 10 is operating at a partial load, and by providing the blade pitch angle to the mapping function while the wind turbine 10 is operating at a full load.

In block 250, the process 230 may use the expected tension to adjust the tension in one or more of the cable assemblies 32. One way the expected tension may be used in this manner is by comparing the expected tension to the measured tension in the cable assembly 32 being controlled. The difference in the expected and measured tension may be treated as an error signal, as non-zero values may indicate an undesirable difference between the tension levels. This error signal may be provided to a proportional-integral-derivative (PID) control system that generate one or more of a proportional term (e.g., K_P*error), an integral term (e.g., K^errof), and a derivative term (e.g., K_D ^xd/dt[e/TOr]) ofthe error signal. Each of the generated terms may then be weighted and the weighted values summed together to produce a control signal, e.g., a voltage, current, hydraulic pressure level, actuator position, etc. This control signal may then be provided to the appropriate tension adjustment mechanism 44 to adjust the tension in the cable assembly 32.

Fig. 17 illustrates an exemplary controller 260 that may be used to provide one or more components of embodiments of the invention, such as the tension system controller 54, the wind turbine controller 202, supervisory controller 212, or any other controller of wind turbine 10. The controller 260 may include a processor 262, memory 264, and an input/output (I/O) interface 266. The processor 262 may include one or more devices that perform operations on data based on internal logic or operational instructions that are stored in memory 264. Memory 264 may include a single memory device or a plurality of memory devices capable of storing data. Computer program code embodied as one or more computer software applications, such as an application 268 residing in memory 264, may have instructions executed by the processor 262. One or more data structures 270 may also reside in memory 264, and may be used by the processor 262 or application 268 to store or manipulate data. The I/O interface 266 may provide a machine interface that operatively couples the processor 262 to other devices and systems, such as various actuators, valves, and other components that control the wind turbine 10. The application 268 may thereby work cooperatively with the external devices and systems by communicating via the I/O interface 266 to provide the various features, functions, applications, processes, or modules comprising embodiments of the invention. While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in some detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the various features of the invention may be used alone or in any combination depending on the needs and preferences of the user.

Claims

1. A method of operating a wind turbine (10) including a plurality of operational modes and a rotor (22) that has a plurality of blades (26), a plurality of cable assemblies (32) that support the blades (26), and a plurality of tension mechanisms (44) each configured to adjust a tension of a respective cable assembly (44), the method comprising: receiving an indication that the tension of one or more of the cable assemblies (32) needs adjustment; determining an operational mode in which the wind turbine (10) is operating; and controlling one or more tension mechanisms (44) to adjust the tension of the one or more cable assemblies (32) according to a tensioning process associated with the operational mode in which the wind turbine (10) is operating.

2. The method of claim 1 , wherein the indication that the tension of the one or more cable assemblies (32) needs adjustment includes one or more of the tension being outside an acceptable tension range, an operational parameter of the wind turbine (10) exceeding an operational limit, and a position of the tension mechanism (44) being outside an acceptable position range.

3. The method of claim 2, wherein the operational parameter includes one or more of a period of time, a temperature, a product of the period of time and the temperature, an amount of energy produced by the wind turbine (10), and a number of rotations of the rotor (22) since a previous tension adjustment or a maintenance window.

4. The method of any of claims 1-3, wherein the operational mode in which the wind turbine (10) is operating is a power production mode, and further comprising: measuring one or more operational parameters including at least one of a wind speed, a rotor speed, a generator speed, an amount of power being produced, and a blade pitch angle; for each cable assembly (44) of the plurality of cable assemblies (32): measuring the tension in the cable assembly (44); and storing data indicative of the tension of the cable assembly (44) and the one or more operational parameters.

5. The method of claim 4, further comprising, for each of the one or more cable assemblies (32): defining a mapping function based on the stored data indicative of the tension of the cable assembly (44) and the one or more operational parameters; using the mapping function to generate an expected tension in the cable assembly (44) based on current operational parameters of the wind turbine (10); and adjusting the tension provided by the tension mechanism (44) of the cable assembly (44) to match the expected tension.

6. The method of claim 5, wherein: the mapping function includes a function of tension versus rotor speed or generator speed, and using the mapping function to generate the expected tension includes providing at least one of the rotor speed or the generator speed to the mapping function while the wind turbine (10) is operating at a partial load.

7. The method of claim 5 or 6, wherein: the mapping function includes a function of tension versus the blade pitch angle, and using the mapping function to generate the expected tension includes providing the blade pitch angle to the mapping function while the wind turbine (10) is operating at a full load.

8. The method of any of claims 5-7, wherein adjusting the tension provided by the respective tension mechanism (44) to match the expected tension comprises: determining a difference in tension between the expected tension and the tension being provided by the tension mechanism (44); generating an error signal based on the difference in tension; generating one or more of a proportional term, an integral term, and a derivative term of the error signal; summing the one or more of the proportional term, the integral term, and the derivative term to generate a control signal; and using the control signal to adjust the tension provided by the tension mechanism (44).

9. The method of any of claims 1-8, wherein the operational mode in which the wind turbine (10) is operating is an idle mode, and further comprising: measuring the tension of each cable assembly (32) of the plurality of cable assemblies (32) over a period of time that includes at least one full rotation of the rotor (22); determining a mean tension in each cable assembly (32) over the period of time; and adjusting the tension provided by the one or more tension mechanisms (44) so that the mean tension of each cable assembly (32) is within a predetermined amount of the mean tension of the other cable assemblies (32).

10. The method of any of claims 1-9, wherein the operational mode in which the wind turbine (10) is operating is an idle mode, each tension mechanism (44) of the plurality of tension mechanisms (44) includes a hydraulic actuator (56), and further comprising: increasing a pressure in a chamber (72, 74) of the hydraulic actuator (56) of each tension mechanism (44) until a pressure setpoint is reached, the increase in the pressure causing each tension mechanism (44) to increase the tension in its respective cable assembly (32); fluidically coupling the chamber (72, 74) of the hydraulic actuator (56) of each tension mechanism (44) to the chamber (72, 74) of the hydraulic actuator (56) of each of the other tension mechanisms (44); rotating the rotor (22) by a predetermined amount of rotation; comparing the pressure in the chamber (72, 74) of the hydraulic actuator (56) of at least one tension mechanism (44) to the pressure in the chamber (72, 74) of the hydraulic actuator (56) of at least one other tension mechanism (44); and in response to the pressure in the chamber (72, 74) of the hydraulic actuator (56) of the at least one tension mechanism (44) varying from the pressure in the chamber (72, 74) of the hydraulic actuator (56) of the at least one other tension mechanism (44) by more than a predetermined amount of pressure, releasing the pressure from the chamber (72, 74) of the hydraulic actuator (56) of each tension mechanism (44).

11 . The method of claim 10, wherein: the chamber (72, 74) of the hydraulic actuator (56) of each tension mechanism (44) is fluidically decoupled from the chamber (72, 74) of the hydraulic actuator (56) of each of the other tension mechanisms (44) while the pressure is being increased, the chamber (72, 74) of the hydraulic actuator (56) of each tension mechanism (44) is fluidically coupled to the chamber (72, 74) of the hydraulic actuator (56) of each of the other tension mechanisms (44) after the pressure setpoint is reached, and the pressure in the chamber (72, 74) of the hydraulic actuator (56) of the at least one tension mechanism (44) is compared to the pressure in the chamber (72, 74) of the hydraulic actuator (56) of the at least one other tension mechanism (44) after the rotor (22) has rotated by the predetermined amount of rotation.

12. The method of claim 10 or 11 , further comprising: pitching each blade (26) of the rotor (22) to a pitch angle that produces a minimum amount of tension in each of the cable assemblies (32) prior to increasing the pressure in the chamber (72, 74) of the hydraulic actuator (56) of each of the tension mechanisms (44).

13. The method of any of claims 1-12, wherein the operational mode in which the wind turbine (10) is operating is an idle mode, each of the tension mechanisms (44) includes a hydraulic actuator (56) having a chamber (72, 74), and further comprising: while the rotor (22) is rotating, controlling a pitch angle of each of the blades (26) to maintain an amount of tension in each cable assembly (32) that is within a predetermined amount of the amount of tension in the other cable assemblies (32); fluidically coupling the chamber (72, 74) of the hydraulic actuator (56) of each tension mechanism (44) to the chamber (72, 74) of the hydraulic actuator (56) of each of the other tension mechanisms (44); increasing a pressure in the chamber (72, 74) of the hydraulic actuator (56) of each tension mechanism (44) until a pressure setpoint is reached, the increase in the pressure causing each tension mechanism (44) to increase the tension in its respective cable assembly (32); while the rotor (22) is rotating and the pressure is increasing, measuring a measured tension versus rotor azimuth in each cable assembly (32); comparing the measured tension to an expected tension in each cable assembly (32); and if the measured tension deviates from the expected tension by more than an allowable amount, releasing the pressure in the chamber (72, 74) of the hydraulic actuator (56) of each tension mechanism (44).

14. The method of any of claims 1-13, wherein the operational mode in which the wind turbine (10) is operating is an idle mode, and further comprising: pitching each blade (26) of the rotor (22) to a pitch angle that produces a minimum amount of pitch-induced tension in each of the cable assemblies (32); and for each cable assembly (32) of the plurality of cable assemblies (32): while the tension mechanisms (44) of other cable assemblies (32) are providing a minimum amount of tension, increasing the tension provided by the respective tension mechanism (44) until a tension setpoint is reached in the cable assembly (32), while the rotor (22) is rotating, measuring a measured tension versus a rotor azimuth in the cable assembly (32), and defining a function of tension versus rotor azimuth for the cable assembly (32) based on the measured tension versus the rotor azimuth.

15. The method of claim 14, wherein: each of the tension mechanisms (44) includes a hydraulic actuator (56) having a chamber (72, 74), the minimum amount of tension is provided by each tension mechanism (44) while the chamber (72, 74) of the hydraulic actuator (56) of the tension mechanism (44) is unpressurized, and increasing the tension provided by the tension mechanism (44) until the tension setpoint is reached in the cable assembly (32) includes increasing a pressure in the chamber (72, 74) of the hydraulic actuator (56) of the tension mechanism (44) until a pressure setpoint is reached.

16. A wind turbine (10), comprising: a rotor (22) including a plurality of blades (26), a plurality of cable assemblies (32) that support the plurality of blades (26), and a plurality of tension mechanisms (44) each configured to adjust a tension of a respective cable assembly (32); and a controller (54, 202, 212, 260) operatively coupled to each of the tension mechanisms (44) and including one or more processors (262) and a memory (264) storing program code (268) that, when executed by the one or more processors (262), causes the controller (54, 202, 212, 260) to perform the method of any of claims 1-15.