Methods for lubricating pitch bearing of wind turbine

Abstract

To provide methods (100, 200) for lubricating a pitch bearing (72) connecting a blade (22) and a hub (20) of a wind turbine (10).SOLUTION: The method (100) comprises: determining (110) that the pitch bearing (72) is to be lubricated; determining (120) a top limit of a thrust gradient, the thrust gradient being a derivative of thrust with respect to time; determining (130) a pitch rate for lubricating the pitch bearing (72) based at least partially on the top limit of the thrust gradient; and pitching (140) the wind turbine blade (22) at the determined pitch rate for lubricating the pitch bearing (72) such that the top limit of the thrust gradient is not exceeded.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to a method for lubricating pitch bearings of a wind turbine, and further to a wind turbine controller and a wind turbine configured to perform such a method. [Background technology]

[0002] Modern wind turbines are commonly used to provide electricity to the power grid. This type of wind turbine typically includes a tower and a rotor located on the tower. The rotor usually consists of a hub and a number of blades, which rotate under the influence of the wind on the blades. This rotation generates a torque, which is usually transferred via the rotor shaft to a generator, either directly ("direct drive" or "gearless") or using a gearbox. In this way, the generator produces electricity, which can be supplied to the power grid.

[0003] The wind turbine hub may be rotatably coupled to the front of the nacelle. The wind turbine hub is coupled to a rotor shaft, which is rotatably mounted to the nacelle using one or more rotor shaft bearings located in a frame within the nacelle. The nacelle is a housing located on top of the wind turbine tower that can house and protect the gearbox and generator (if not located outside the nacelle) and, depending on the wind turbine, other components such as power converters and auxiliary systems.

[0004] Wind turbine blades are typically coupled to the hub by pitch bearings. Pitch bearings typically include an inner ring and an outer ring, between which a plurality of rolling or roller elements are typically arranged. The wind turbine blades may be attached to one of the inner or outer ring, and the hub may be attached to the other of the inner or outer ring. The attachment may be performed, for example, by nuts and bolts.

[0005] When the pitch bearing ring is rotated relative to the other bearing ring, for example by an electric or hydraulic pitch actuator, the blades may rotate about their longitudinal axis relative to the hub. For example, blades mounted on a bearing inner ring rotate relative to the hub. Rotating wind turbine blades in this manner is commonly referred to as "pitching" the blades. In (electro)mechanical pitch systems, a pinion may mesh with a ring gear that is typically provided on the bearing ring to which the blades are joined, to rotate the wind turbine blades. In hydraulic pitch systems, a hydraulic piston moves the blades.

[0006] Pitch bearings are generally subjected to high loads and vibrations. For this reason, if the lubricant is not evenly distributed in the pitch bearing, it can lead to wear and damage to the bearing rings and / or the roller elements between them. Grease lubricants are commonly used. The lubricant must be well distributed and reach all relevant parts and elements of the bearing. Pitching of the blades may be necessary to promote an even distribution of the lubricant, especially if the pitch angle of the blades has not changed for a long period of time or if the pitch angle changes many times by small amounts, e.g. less than 1° (degree).

[0007] Below nominal wind speeds, e.g., 11-13 m / s, the power generated is below nominal (i.e., rated, maximum) power and the pitch angle is generally maintained at or near a preset position, typically at or near 0°. When operating below nominal wind speeds, or with insufficient motion to distribute lubricant, the pitch angle may be constant but vary by small amounts over time, so the blades may need to be pitched to specifically distribute the lubricant. For example, a wind turbine's main controller may be configured to indicate that the blades are to be pitched X° every Y hours.

[0008] Above the nominal wind speed, the power generated is nominal (usually until the cut-out wind speed is reached) and the blade pitch angle is typically varied to control the loads acting on the blade and the wind turbine. As the blades are pitched frequently or continuously, the lubricant that lubricates the pitch bearings tends to be well distributed in this area. In some cases, the blades are pitched to specifically redistribute the lubricant in this area if it is determined that the lubricant is not adequately distributed.

[0009] The speed at which the pitching occurs is important: pitching too quickly to lubricate the pitch bearings can result in too high loads on the blades, but pitching relatively slowly to lubricate the pitch bearings can result in a significant reduction in power generation.

[0010] The present disclosure aims to optimize the balance between the loads on the blades and the energy production during pitching to lubricate the pitch bearings. Summary of the Invention

[0011] In one aspect of the disclosure, a method for lubricating a pitch bearing connecting a blade and a hub of a wind turbine is provided. The method includes determining that lubricating a pitch bearing is to be carried out. The method further includes determining a top limit for a load gradient, the load gradient being a derivative of the load with respect to time. The method further includes determining a pitch rate for lubricating the pitch bearing based at least partially on the top limit for the load gradient. The method further includes pitching the wind turbine blade at the determined pitch rate for lubricating the pitch bearing such that the top limit for the load gradient is not exceeded.

[0012] According to this aspect, an upper limit of the load gradient is determined, and a pitch speed is determined based at least in part on the determined upper limit. In this aspect, pitching to lubricate the pitch bearing, e.g., to distribute lubricant along the interface between the inner and outer rings of the pitch bearing, can be performed as fast as possible without loading the wind turbine blades higher than acceptable loads. Unnecessary losses of power generation are avoided, and fatigue loads are maintained at acceptable levels.

[0013] In a further aspect of the disclosure, a wind turbine controller is provided comprising a processor and a memory, the memory comprising instructions that, when executed by the processor, cause the processor to determine that lubrication of a pitch bearing is required, instructions to determine an upper load gradient limit, the load gradient being a derivative of thrust with respect to time, and instructions to determine a pitch speed for lubricating a pitch bearing connecting a blade and a hub of a wind turbine based on the upper load gradient limit, such that the upper load gradient limit is not exceeded during pitching.

[0014] In a further aspect of the disclosure, there is provided a wind turbine including a tower, a nacelle disposed on the top of the tower, and a rotor coupled to the nacelle, the rotor including a hub and a number of blades coupled to the hub via pitch bearings, the wind turbine further including the wind turbine controller of the previous aspect.

[0015] In a further aspect of the present disclosure, a method is provided. The method includes the steps of: determining that a pitch bearing connecting a blade and a hub of a wind turbine requires lubrication; determining a prevailing wind speed; determining a variation of thrust with respect to pitch angle based on the prevailing wind speed; determining a pitch rate for lubricating the pitch bearing based on the variation of thrust with respect to pitch angle and a maximum limit for a thrust gradient, the thrust gradient being a derivative of thrust with respect to time; and pitching the wind turbine blade at the determined pitch rate for lubricating the pitch bearing such that the maximum limit of thrust gradient is not exceeded. rate for lubricating the pitch bearing such that the maximum limit for the thrust gradient is not exceeded).

[0016] Aerodynamic thrust on the rotor of a wind turbine is a load that represents the fatigue load of the wind turbine. According to this aspect, the thrust gradient is kept below an acceptable limit in order to optimize the annual energy yield while maintaining the fatigue load at an acceptable level. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a perspective view of an example wind turbine. [Diagram 2] FIG. 2 is a simplified internal view of an example nacelle of the wind turbine of FIG. 1. [Diagram 3] 1 is a flow chart illustrating an example of a method for lubricating a pitch bearing that connects the blades and the hub of a wind turbine. [Figure 4] 1 is an example of a simulation of thrust as a function of wind speed. [Diagram 5] 1 shows an example of a simulation of thrust gradient as a function of wind speed. [Figure 6] 6 is a flowchart showing another example of a method for lubricating a pitch bearing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Reference will now be made in detail to the embodiments of the present disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation only, and not by way of limitation. Indeed, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure. For example, features illustrated or described as part of one embodiment can be used with another embodiment to yield still further embodiments. Thus, it is intended that the present disclosure cover such modifications and variations as come within the scope of the appended claims and their equivalents.

[0019] FIG. 1 is a perspective view of an example of a wind turbine 10. In this example, the wind turbine 10 is a horizontal axis wind turbine. Alternatively, the wind turbine 10 may be a vertical axis wind turbine. In the example, the wind turbine 10 includes a tower 15 extending from a support system 14 on a ground 12, a nacelle 16 attached to the tower 15, and a rotor 18 coupled to the nacelle 16. The rotor 18 includes a rotatable hub 20 and at least one rotor blade 22 coupled to the hub 20 and extending outwardly from the hub 20. In this example, the rotor 18 has three rotor blades 22. In alternative embodiments, the rotor 18 includes more or less than three rotor blades 22. The tower 15 may be fabricated from tubular steel to define a cavity (not shown in FIG. 1) between the support system 14 and the nacelle 16. In alternative embodiments, the tower 15 may be any suitable type of tower having any suitable height. According to an alternative, the tower may be a hybrid tower made of concrete and steel tubular sections. It may also be a partial or complete lattice tower.

[0020] The rotor blades 22 are spaced relative to the hub 20 to rotate the rotor 18 and convert kinetic energy from the wind into usable mechanical energy, and therefore electrical energy. The rotor blades 22 are mated to the hub 20 by coupling blade root regions 24 to the hub 20 at a number of load transfer regions 26. The load transfer regions 26 may include hub load transfer regions and blade load transfer regions (both not shown in FIG. 1 ). Loads induced in the rotor blades 22 are transferred to the hub 20 via the load transfer regions 26.

[0021] In examples, rotor blades 22 may have lengths ranging from about 15 m (meters) to about 90 m or more. Rotor blades 22 may have any suitable length that enables wind turbine 10 to function as described herein. For example, non-limiting examples of blade lengths include lengths of 20 m or less, 37 m, 48.7 m, 50.2 m, 52.2 m, or greater than 91 m. When wind strikes rotor blades 22 from wind direction 28, rotor 18 rotates about rotor axis 30. As rotor blades 22 rotate and experience centrifugal forces, rotor blades 22 also experience various forces and moments. As such, rotor blades 22 may deflect and / or rotate from a neutral, i.e., unbiased, position to a biased position.

[0022] Additionally, the pitch angles of the rotor blades 22, e.g., angles determining the orientation of the rotor blades 22 relative to the wind direction, may be varied by the pitch system 32 to control the load and power generated by the wind turbine 10 by adjusting the angular position of at least one rotor blade 22 relative to the wind vector. A pitch axis 34 of the rotor blades 22 is shown. During operation of the wind turbine 10, the pitch system 32 may vary the pitch angles of the rotor blades 22, among other things, to reduce the angle of attack of (some of) the rotor blades, thereby facilitating a reduction in rotational speed and / or facilitating a stall of the rotor 18.

[0023] In this embodiment, the blade pitch of each rotor blade 22 is individually controlled by the wind turbine controller 36 or the pitch control system 80. Alternatively, the blade pitch of all rotor blades 22 may be simultaneously controlled by these control systems.

[0024] Furthermore, in this embodiment, as the wind direction 28 changes, the yaw direction of the nacelle 16 can be rotated about a yaw axis 38 to position the rotor blades 22 relative to the wind direction 28 .

[0025] In this example, the wind turbine controller 36 is shown as being centrally located in the nacelle 16, but the wind turbine controller 36 may be a distributed control system located throughout the wind turbine 10, on the support system 14, in the wind farm, and / or at a remote control center. The wind turbine controller 36 may include one or more processors configured to perform one or more steps of the methods described herein. Additionally, many of the other components described herein include one or more processors. The wind turbine controller 36 may also include memory, e.g., one or more memory devices. As used herein, memory may include memory elements including, but not limited to, computer-readable media (e.g., random access memory (RAM)), computer-readable non-volatile media (e.g., flash memory), floppy disks, compact disks-read only memory (CD-ROM), magneto-optical disks (MOD), digital versatile disks (DVD), and / or other suitable memory elements.

[0026] FIG. 2 is an enlarged cross-sectional view of a portion of the wind turbine 10. In this example, the wind turbine 10 includes a nacelle 16 and a rotor 18 rotatably coupled to the nacelle 16. More specifically, a hub 20 of the rotor 18 is rotatably coupled to a generator 42 disposed within the nacelle 16 by a main shaft 44, a gearbox 46, a high-speed shaft 48, and a coupling 50. In the example, the main shaft 44 is disposed at least partially coaxially with a longitudinal axis (not shown) of the nacelle 16. Rotation of the main shaft 44 drives a gearbox 46, which drives the high-speed shaft 48 by converting the relatively slow rotational motion of the rotor 18 and main shaft 44 into a relatively fast rotational motion of the high-speed shaft 48. The latter is connected, with the aid of the coupling 50, to a generator 42 for generating electrical energy. Further, a transformer 90 and / or suitable electronics, switches and / or inverters may be arranged in the nacelle 16 to convert the electrical energy generated by the generator 42 having a voltage of, for example, 400V to 1000V into electrical energy having a medium voltage (for example, 10 to 35 kV). An offshore wind turbine may have a generator voltage of, for example, 650V to 3500V, and the transformer voltage may be, for example, 30 kV to 70 kV. Said electrical energy is conducted from the nacelle 16 to the tower 15 via a power cable.

[0027] In some examples, the wind turbine 10 may include one or more shaft sensors 51. The shaft sensors may be configured to monitor at least one of a torque load acting on the main shaft 44 and / or the high speed shaft 48 and the rotational speed of the shafts 44, 48. In some examples, the wind turbine 10 may include one or more generator sensors 53. The generator sensors may be configured to monitor at least one of a rotational speed of the generator 42 and a generator torque. The shaft sensors 51 and / or the generator sensors 53 may include, for example, one or more torque sensors (e.g., strain gauges or pressure sensors), optical sensors, accelerometers, magnetic sensors, speed sensors, and micro inertial measurement units (MIMUs).

[0028] The gearbox 46, the generator 42 and the transformer 90 may be supported by a main support structural frame of the nacelle 16, optionally embodied as a main frame 52. The gearbox 46 may include a gearbox housing coupled to the main frame 52 by one or more torque arms 103. In an embodiment, the nacelle 16 also includes a main forward support bearing 60 and a main aft support bearing 62. Additionally, the generator 42 may be mounted to the main frame 52 by a decoupling support means 54, particularly to prevent vibrations of the generator 42 from being introduced into the main frame 52 and thereby causing a source of noise emissions.

[0029] Optionally, the main frame 52 is configured to carry the weight of the rotor 18 and nacelle 16 components, as well as the overall loads caused by wind and rotational loads, and to introduce these loads into the tower 15 of the wind turbine 10. The rotor shaft 44, generator 42, gearbox 46, high speed shaft 48, coupling 50, and supports 52, and any associated fastening, supporting, and / or fastening devices, including, but not limited to, forward and aft support bearings 60 and 62, may be referred to as a drive train 64.

[0030] In some embodiments, the wind turbine may be a direct drive wind turbine without a gearbox 46. The generator 42 operates at the same rotational speed as the rotor 18 of the direct drive wind turbine and therefore typically has a much larger diameter than the generator used in a wind turbine with a gearbox 46 to provide the same amount of power as a wind turbine with a gearbox 46.

[0031] The nacelle 16 may also include a yaw drive mechanism 56 that is used to rotate the nacelle 16, and thereby the rotor 18, about the yaw axis 38 to control the approach or approach of the rotor blades 22 relative to the wind direction 28.

[0032] To properly position the nacelle 16 relative to the wind direction 28, the nacelle 16 may also include at least one meteorological measurement system including a wind vane and anemometer. The meteorological measurement system 58 may provide information including the wind direction 28 and / or wind speed to the wind turbine controller 36.

[0033] In this embodiment, pitch system 32 is disposed, at least in part, within hub 20 as a pitch assembly 66. Pitch assembly 66 includes one or more pitch drive systems 68 and at least one sensor 70. Each pitch drive system 68 is coupled to a respective rotor blade 22 (shown in FIG. 1 ) for adjusting the pitch angle of rotor blade 22 along pitch axis 34. Only one of the three pitch drive systems 68 is shown in FIG. 2 .

[0034] In this embodiment, the pitch assembly 66 includes at least one pitch bearing 72 coupled to the hub 20 and each rotor blade 22 (shown in FIG. 1 ) to rotate each rotor blade 22 about the pitch axis 34. The pitch drive system 68 includes a pitch drive motor 74, a pitch drive gearbox 76, and a pitch drive pinion 78. The pitch drive motor 74 is coupled to the pitch drive gearbox 76 such that the pitch drive motor 74 provides a mechanical force to the pitch drive gearbox 76. The pitch drive gearbox 76 is coupled to the pitch drive pinion 78 such that the pitch drive pinion 78 is rotated by the pitch drive gearbox 76. The pitch bearing 72 is coupled to the pitch drive pinion 78 such that rotation of the pitch drive pinion 78 causes rotation of the pitch bearing 72.

[0035] The pitch drive system 68 is coupled to the wind turbine controller 36 for adjusting the pitch angle of the rotor blades 22 in response to one or more signals from the wind turbine controller 36. In this example, the pitch drive motor 74 is any suitable motor driven by an electrical power and / or hydraulic system that enables the pitch assembly 66 to function as described herein. Alternatively, the pitch assembly 66 may include any suitable structure, configuration, arrangement, and / or components, such as, but not limited to, hydraulic cylinders, springs, and / or servo mechanisms. In certain embodiments, the pitch drive motor 74 is driven by the rotational inertia of the hub 20 and / or energy extracted from a stored energy source (not shown) that provides energy to components of the wind turbine 10.

[0036] The pitch assembly 66 may also include one or more pitch control systems 80 for controlling the pitch drive systems 68 in accordance with control signals from the wind turbine controller 36 in case of certain preferred conditions and / or upon overspeed of the rotor 18. In an example embodiment, the pitch assembly 66 includes at least one pitch control system 80 communicatively coupled to each pitch drive system 68 for controlling the pitch drive systems 68 independently from the wind turbine controller 36. In an example embodiment, the pitch control system 80 is coupled to the pitch drive systems 68 and to the sensors 70. During normal operation of the wind turbine 10, the wind turbine controller 36 may control the pitch drive systems 68 to adjust the pitch angle of the rotor blades 22.

[0037] According to one embodiment, a power generator 84, including, for example, batteries and electrical capacitors, is disposed at or within the hub 20 and is coupled to the sensors 70, pitch control system 80, and pitch drive system 68 to provide a source of electrical power to these components. In this example, the power source 84 provides a continuous source of electrical power to the pitch assembly 66 during operation of the wind turbine 10. In an alternative embodiment, the power source 84 provides power to the pitch assembly 66 only during a power loss event of the wind turbine 10. A power loss event may include a loss or dip in the electrical grid, a malfunction of the electrical system of the wind turbine 10, and / or a failure of the wind turbine controller 36. During a power loss event, the generator 84 operates to provide electrical power to the pitch assembly 66 such that the pitch assembly 66 can operate during the power loss event.

[0038] In this example, pitch drive system 68, sensor 70, pitch control system 80, cables, and power supply 84 are each disposed within a cavity 86 defined by an inner surface 88 of hub 20. In alternative embodiments, these components may be disposed relative to and directly or indirectly coupled to an outer surface of hub 20.

[0039] As used herein, the term "processor" is not limited to integrated circuits referred to in the art as computers, but refers broadly to controllers, microcontrollers, microcomputers, programmable logic controllers (PLCs), application specific integrated circuits, and other programmable circuitry, and these terms are used interchangeably herein.

[0040] In one aspect of the disclosure, a method 100 is provided for lubricating a pitch bearing 72 connecting a blade 22 and a hub 20 of a wind turbine 10. The method 100 is shown generally in the flow chart of FIG. 3. The method includes, in block 110, determining that lubricating the pitch bearing is to be carried out. The method further includes, in block 120, determining a top limit for a load gradient, the load gradient being a derivative of the load with respect to time. In this specific example, the load gradient is a thrust gradient ie a variation of aerodynamic thrust with respect to time.

[0041] The method further comprises, at block 130, determining a pitch speed for lubricating pitch bearings 72 based at least in part on the upper load gradient value. The method further includes, at block 140, pitching the wind turbine blade 22 at the determined pitch speed for lubricating pitch bearings 72 such that the upper load gradient value is not exceeded.

[0042] In this way, the maximum value of the load variation over time is determined and used as an upper limit, and the pitch speed can be determined so that the upper limit is not exceeded. Thus, the distribution of lubricant in the pitch bearing due to the pitching of the wind turbine blades is performed as quickly as possible while controlling the load on the wind turbine blades, thereby reducing potential power losses.

[0043] Determining the upper limit for the load gradient comprises consulting a data structure, e.g., a look-up table, or receiving the upper limit from another controller, element, or system, etc. The upper limit value may be predefined, e.g., stored in a data structure, and determining the upper limit value comprises accessing the data structure. An appropriate value for the upper limit may be based, for example, on a dimension of a blade of the wind turbine or a position of the wind turbine.

[0044] In some examples, the top limit may be calculated. For example, the wind turbine controller may determine a first top limit, and then determine the actual top limit as a percentage of the first top limit. Fatigue, damage, and other parameters experienced by the wind turbine blades may affect the percentage that is applied. The load gradient top limit may change over the life of the wind turbine.

[0045] The order of steps as illustrated in this figure is not to be considered limiting. For example, step 110 of determining that lubricating pitch bearing 72 should be performed (i.e., is deemed beneficial or necessary) may be performed before the pitch speed determination in block 130, and in some examples may also be performed before the step of determining an upper limit for thrust gradient in block 120. The step of determining an upper limit in block 120 may be performed before the step in block 110 in other examples.

[0046] Determining that lubrication of the pitch bearings is necessary can be done in a number of ways. In some examples, determining that lubrication of the pitch bearings is necessary (or beneficial) can include determining that the pitch bearings 72 have not been lubricated for a (first) predefined period of time. In some of these examples, the method can include determining that the (first) predefined period of time has elapsed. For example, the frequency of pitching the blades 22 to lubricate the pitch bearing rings can be predefined. The controller 36 can be configured to periodically lubricate the pitch bearing rings in some examples. The memory of the wind turbine controller can include stored instructions indicating that the blades 22 of the wind turbine can be pitched X degrees every Y hours. In such a case, the time at which the pitch bearings 72 are lubricated can be predefined. Thus, if a certain predefined time has elapsed, a pitch speed at which to lubricate the pitch bearings 72 can be determined. That is, after Y hours, the blades can be pitched X degrees at the determined pitch speed. After another Y hours, the blades can be pitched X degrees again at the same pitch speed or a different pitch speed. Pitching X degrees can take, for example, tens of seconds to minutes.

[0047] Determining that pitch bearing lubrication is performed may include determining that the pitch bearings 72 have not been lubricated for a second predetermined period of time that is different from the first predetermined period of time. For example, the method may further include determining that the second predetermined period of time has elapsed. The wind turbine controller 36 may be configured to initiate pitch bearing lubrication after a first time period and after a second time period. For example, the wind turbine controller may be configured to pitch blade X° every Y hours and pitch the same blade W° every Z hours, where X is different from W and Y is different from Z.

[0048] Determining that lubrication of the pitch bearings is performed may consist of determining that the blades 22 have not been rotated a predefined amount for a predefined period of time. For example, determining that the blades 22 have not been rotated at all or by more than a threshold for a (first) predefined period of time. If the blades 22 should be pitched X° (e.g., pitched X° in the same direction) every Y time periods but this has not been achieved, a pitch speed for lubricating the pitch bearings 72 may be determined.

[0049] Additionally or alternatively, wind turbine 10 may include one or more sensors that may be used to obtain information related to lubrication of pitch bearings 72 and ascertain whether pitching is required for lubrication of the pitch bearings. For example, if it is detected that lubrication of pitch bearings 72 is required before a predefined period of time has elapsed, the pitch speed may be determined without waiting for the predefined period of time to elapse.

[0050] Determining that pitch bearing lubrication should be performed may include determining that the prevailing wind speed is less than a nominal wind speed. For wind speeds above the nominal wind speed, the blades 22 of the wind turbine 10 may be pitched frequently, and so redistribution of lubricant between the outer and inner bearing rings of the pitch bearing 72 may not be necessary. Thus, in some examples, the pitch speed 130 may be determined only if the prevailing wind speed is below the nominal (i.e., rated) wind speed. The wind turbine controller may be configured to have a first operating or control mode for wind speeds below the nominal wind speed in which the pitch speed 130 may be determined, and a second operating or control mode (different from the first mode) for wind speeds above the nominal wind speed in which the pitch speed 130 is not determined.

[0051] The pitch speed may be determined in block 130 based on the prevailing wind conditions, specifically the prevailing wind speed. Determining the prevailing wind speed may include measuring or estimating the prevailing wind speed. The wind speed may be determined (immediately) after detecting that the pitch bearing lubrication is required. Thus, in some examples, it may be detected that the pitch bearing lubrication is required, and then, for example, the prevailing wind speed may be measured and an appropriate pitch speed may be determined. The wind speed may be determined in some examples based on the rotor speed, pitch angle, etc. In other examples, an anemometer or LIDAR may be used to determine the prevailing wind speed. Other wind conditions that may be considered include the level of turbulence, e.g., turbulence intensity. Further wind conditions that may be considered may include wind direction (variation), wind shear, and wind veer.

[0052] The load data as a function of pitch angle and as a function of wind speed may be known in advance. For example, the data may be predefined as shown in FIG. 4. In the following examples, aerodynamic thrust is used as an example load. In other examples, other loads, such as tower or blade bending loads, may be used.

[0053] Figure 4 shows an example of a simulation of thrust in kN (kilonewtons) as a function of wind speed in m / s (meters per second). In this figure, we can see the minimum, maximum and average thrust for a given wind speed. In some examples, it can be considered that the thrust gradient does not only depend on the wind speed, but also on turbulence for example. Turbulence can lead to different values ​​of thrust for the same average wind speed. For example, a specific wind profile with a specific average wind speed can be introduced. The instantaneous wind speed is not constant throughout the simulation, which leads to variations in thrust values.

[0054] In the example of Figure 4, the rotor 18 of the wind turbine 10 is assumed to face the wind at all wind speeds. In the simulation, variance can also be introduced in the incident wind direction, and the calculated thrust value takes into account the possibility that the incident wind direction may not be constant during the simulation. As can be seen from Figure 4, the thrust, i.e. the axial force exerted by the wind on the rotor of the wind turbine, is greatest when the wind turbine is operating near the rated (nominal) wind speed.

[0055] Such a simulation can be performed in advance, i.e. before the wind turbine is put into operation. The data may be stored in a memory of a wind turbine controller, e.g., the main wind turbine controller 36, for example, before the wind turbine is installed. In a further example, a remote operation center may store such data or a SCADA system may store such data. During operation, the wind turbine can access this information.

[0056] Figure 5 shows a simulated example of the change in thrust (kN / °) with respect to pitch angle as a function of wind speed (m / s). In particular, Figure 5 shows the pitch derivative of thrust for the example in Figure 4. Thus, Figure 5 shows the maximum, minimum and average change in thrust per degree of pitching at a given wind speed.

[0057] In absolute terms, the thrust gradient is generally smaller at lower wind speeds. Thus, the loads expected on the blades when pitching are lower at low wind speeds, and faster pitching is acceptable. At higher wind speeds, the thrust gradient is generally higher. Thus, the loads expected on the blades when pitching are higher at higher wind speeds, and slower pitching is acceptable to avoid excessive loads on the blades.

[0058] In some examples, the data of the change in thrust with respect to the pitch angle as a function of the wind speed may be known in advance. That is, the data as shown in FIG. 5 may be predefined, for example because a simulation may have been performed in advance. A known mathematical model may be used to perform the simulation. Instead of or in addition to the data shown in FIG. 4, the data in FIG. 5 may be stored in a memory of a wind turbine controller, for example the main wind turbine controller 36, before the installation of the wind turbine. As is conventional, such data may be stored in a SCADA system or at a remote location, for example a remote operating center.

[0059] During operation, the wind turbine can access this information. The predefined data (e.g., thrust as a function of wind speed or change in thrust with respect to pitch angle) can be stored in any suitable manner. In some examples, a look-up table can be used.

[0060] To determine, e.g., calculate or simulate, the thrust as a function of wind speed or the variation of thrust with respect to pitch angle, mathematical equations and models known in the art can be used to derive the aforementioned data.

[0061] Thus, if it is decided to redistribute the distribution of lubricant in the pitch bearing, the prevailing wind conditions may be determined, e.g., measured, and (predefined) data of the variation of thrust (or other load) versus pitch angle for those wind conditions may be used to determine the variation of thrust versus pitch angle for the prevailing wind speed. In some cases, the wind conditions may simply refer to wind speed, and in other cases, the wind conditions may be more advanced, including one or more of air density, wind direction, wind veer, wind shear, turbulence, temperature, etc.

[0062] The upper limit value of thrust gradient (eg, kN / s) can be divided by the determined change in thrust with respect to pitch angle (eg, kN / °) to obtain a value for pitch rate (eg, ° / s).

[0063] In some examples, a determined, e.g., measured, average of the variation of thrust relative to pitch for the prevailing wind speed may be used to determine the pitch speed in block 130.

[0064] The determination of the pitch rate can be done in several ways. In some examples, the determination of the pitch rate consists of using predefined data of thrust and the variation of thrust with respect to pitch angle. For example, a look-up table or any other suitable data structure may be consulted by the wind turbine controller 36 to determine the value of the variation of thrust with respect to pitch angle. In other examples, the data of thrust and the variation of thrust with respect to pitch angle may not be predefined and may be calculated or estimated each time such information is useful or necessary. For this purpose, suitable mathematical formulas may be used.

[0065] In some examples, the determined pitch rate may be maximum relative to the upper limit previously determined for thrust gradient in block 120. That is, the highest possible pitch rate relative to the thrust limit may be determined. In this manner, pitching may be performed as fast as possible, but without exceeding the previously determined thrust limit.

[0066] In other examples, the determined pitch rate need not be maximum for a thrust gradient limit, for example, a pitch rate that is not the fastest possible pitch rate for a set thrust limit may be determined.

[0067] In some examples, the determined pitch speed may be above a lower pitch speed limit. The lower pitch speed limit may be predefined, i.e. known in advance, such that the determined pitch speed is sufficiently high. For example, predefined lower pitch speed limits may be provided for different wind speeds, e.g., different average wind speeds. The lower limit may be different for all of these wind speeds. The lower limit may be the same for at least some of the wind speeds in some examples. The one or more lower pitch speed limits may be stored in a memory of the wind turbine controller 36.

[0068] In other examples, the lower limit on the pitch rate may be implicit in the sense that the determined pitch rate may not fall below a certain value because a trade-off between maximizing power production and minimizing the load on the wind turbine during pitching may be achieved. Very low or excessively low pitch rates may be excluded.

[0069] The determined pitch speed may be maintained for the entire pitching operation to lubricate the pitch bearing. Pitching to lubricate the pitch bearing generally requires only a small amount of time, e.g., less than 5 minutes, or even less than 2 minutes, and maintaining the determined pitch speed may provide an effective and safe lubrication operation of the pitch bearing. The lubrication operation may include one lubrication stroke or one or more lubrication strokes. The lubrication stroke may include pitching in a first direction and then pitching back in the opposite direction.

[0070] In another example, the method can further include varying the pitch speed during lubrication of the pitch bearings. This option can be implemented depending, for example, on the duration of the pitching maneuver expected due to lubrication of the pitch bearings or if wind gusts or other significant changes in wind conditions are expected or experienced.

[0071] The method may be applied to all blades 22 of the wind turbine 10 simultaneously, for example, in some instances, all blades 22 may be pitched simultaneously to lubricate the corresponding pitch bearings. The duration of the lubrication operation and the determined pitch speed may also be the same for all blades of the wind turbine.

[0072] According to a further aspect of the disclosure, a wind turbine controller 36 is provided. The wind turbine controller includes a processor and a memory. The memory includes instructions that, when executed by the processor, cause the processor to: determine an upper load gradient limit, the load gradient being a derivative of thrust with respect to time; determine a pitch speed for lubricating a pitch bearing connecting the blades and a hub of the wind turbine based on the upper load gradient limit; and pitch the blades of the wind turbine at the determined pitch speed for lubricating the pitch bearing such that the upper load gradient limit is not exceeded.

[0073] The memory may generally be configured to store suitable computer readable instructions that, when executed by the processor, configure or trigger the wind turbine controller 36 to perform various steps disclosed herein, such as with respect to method 100. The memory may also be configured to store data, such as predefined data or data from measurements and / or calculations.

[0074] The wind turbine controller 36 may further include a communication module. The communication module may facilitate communication between the controller 36 and various components of the wind turbine 10. For example, the communication module may function as an interface to enable the turbine controller 36 to send control signals to the pitch drive system 66 for controlling the pitch angle of the rotor blades 22. The communication module may be configured to communicatively connect the controller 36 to other elements of the wind turbine 10. The connection may be implemented via a wired connection and / or via a wireless connection, e.g., by using any suitable wireless communication protocol known in the art. Additionally, the communication module may include a sensor interface, e.g., one or more analog-to-digital converters, to enable signals transmitted from one or more sensors (e.g., wind speed sensors) to be converted into signals that can be understood and processed by the processor.

[0075] Controller 36 may be configured to perform one or more of the steps and operations referred to in method 100 (also referred to in method 200 below).

[0076] According to a further aspect of the present disclosure, there is provided a wind turbine 10 comprising a tower 15, a nacelle 16 provided on top of the tower 15, and a rotor 18 coupled to the nacelle 16, the rotor comprising a hub 20 and a number of blades 22 coupled to the hub via pitch bearings, the wind turbine further comprising a wind turbine controller 36 of the previous aspect.

[0077] In some examples, the wind turbine is an offshore wind turbine, and the above method is particularly beneficial for offshore wind turbines because the power generation of the offshore wind turbine is never stopped to perform lubrication operations. Of course, the above method may also be performed by onshore wind turbines.

[0078] According to a further aspect of the present disclosure, there is provided a method 200. Method 200 is illustrated in the flow chart of Fig. 6. Details and descriptions of previous aspects, such as method 100, may be combined and applied to this aspect, and vice versa.

[0079] The method includes the steps of determining, in block 210, that a pitch bearing 72 connecting a blade 22 and a hub 20 of the wind turbine 10 requires lubrication; determining, in block 220, a prevailing wind speed; determining, in block 230, a variation in thrust with respect to pitch angle based on the prevailing wind speed; determining, in block 240, a pitch speed for lubricating the pitch bearing 72 based on the variation in thrust with respect to pitch angle and a maximum limit of thrust gradient, where the thrust gradient is the derivative of thrust with respect to time; and pitching, in block 250, the wind turbine blade 22 at the determined pitch speed for lubricating the pitch bearing such that the maximum limit of thrust gradient is not exceeded.

[0080] Determining that pitch bearings 72 need to be lubricated may include determining that pitch bearings 72 have not been lubricated for a predetermined period of time.

[0081] The pitch speed may be held constant until lubrication of the pitch bearings is terminated. The pitch speed determination may consist of dividing the maximum limit of thrust gradient by the change in thrust with respect to pitch angle. In some instances, additional safety or load factors may be considered.

[0082] The thrust data and the change in thrust with respect to pitch angle as a function of wind speed data may be predefined as described herein.

[0083] This specification uses examples to disclose the teachings, including preferred embodiments, and to enable one of ordinary skill in the art to practice the teachings, including making and using any device or system, and performing the incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those of ordinary skill in the art. Such other examples are intended to be included in the claims if they have structural elements that do not differ from the language of the claims, or if they include equivalent structural elements that do not differ substantially from the language of the claims. Aspects from the various embodiments described, and other known equivalents to each such aspect, may be mixed and matched by those of ordinary skill in the art to construct additional embodiments and techniques according to the principles of this application. When reference signs related to the drawings are placed in parentheses in the claims, they are merely intended to improve the clarity of the claims, and should not be construed as limiting the scope of the claims. [Explanation of symbols]

[0084] 10 wind turbine 12: ground 14: support system 15: tower 16: nacelle 18: rotor 20: hub 22: rotor blade 24: blade root area 26: load transfer area 28: wind direction 30: rotor shaft 32: pitch system 34: pitch axis 36: wind turbine controller 38: yaw axis 42: generator 44: main shaft 46: gearbox 48: high speed shaft 50: coupling 51: shaft sensor 52: main frame 53: generator sensor 54: decoupling support means 56: yaw drive mechanism 58: meteorological measurement system 60: main forward support bearing 62: main aft support bearing 64: drive train 66: pitch assembly 68: pitch drive system 70: sensor 72: pitch bearing 74: pitch drive motor 76: pitch drive gearbox 78: pitch drive pinion 80: pitch control system 84: Power supply / generator 86: Cavity 88: Inner surface 90: Transformer 103: Torque arm

Claims

1. A method (100) for lubricating a pitch bearing (72) connecting a wind turbine blade (22) and a hub (20) of a wind turbine (10), comprising: determining (110) that lubrication of the pitch bearing (72) is to be performed; determining (120) an upper limit for a load gradient, the load gradient being the derivative of the load with respect to time; determining (130) a pitch speed for lubricating the pitch bearings (72) based at least in part on an upper load gradient limit; and pitching (140) the wind turbine blade (22) at the determined pitch speed to lubricate the pitch bearing (72) such that an upper load gradient limit is not exceeded.

2. The method of claim 1 , wherein an upper limit of the load gradient is predefined.

3. The method of claim 1 , wherein the pitch speed is determined based on prevailing wind conditions.

4. The method of claim 3 , wherein the prevailing wind conditions include a prevailing wind speed.

5. The method of claim 4 , wherein the prevailing wind conditions further include one or more of turbulence intensity, wind direction, wind shear, and / or wind bear.

6. 6. The method of claim 5, wherein determining the pitch speed comprises determining a variation of load versus pitch angle for the determined prevailing wind conditions.

7. 7. The method of claim 6, wherein determining the pitch speed further comprises dividing an upper limit of the load gradient by the determined variation of load versus pitch angle for the prevailing wind speed.

8. 8. The method of claim 7, wherein data for load as a function of pitch angle and load as a function of wind speed are known.

9. The method of claim 1 , wherein the load is an aerodynamic thrust force of a rotor of a wind turbine.

10. 2. The method of claim 1, further comprising the step of maintaining the determined pitch speed constant during a complete pitching motion to lubricate the pitch bearings (72).

11. 2. The method of claim 1, wherein determining (110) that lubrication of the pitch bearings (72) should be performed comprises determining that a predefined period of time has elapsed or determining that the blades (22) have not rotated a predefined amount during a predefined period of time.

12. 2. The method according to claim 1, wherein the pitch bearing (72) is lubricated in less than 5 minutes, in particular in less than 2 minutes.

13. A wind turbine controller (36) including a processor and a memory, the memory including instructions that, when executed by the processor, cause the processor to perform a method according to any of the preceding claims.

14. 14. A wind turbine (10) comprising a tower (15), a nacelle (16) provided on top of the tower (15), and a rotor (18) connected to the nacelle (16), the rotor (18) including a hub (20) and a number of blades (22) connected to the hub (20) via pitch bearings (72), the wind turbine (10) further comprising a wind turbine controller (36) according to claim 13.

15. The wind turbine (10) of claim 14, wherein the wind turbine (10) is an offshore wind turbine.

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