HYDROSTATIC YAW BEARING

APPLICANT

(a) Name Suzlon Energy Limited

(b) Nationality Indian

(c) Address Shrimali Society, Near Shri Krishna

Complex, Navrangpura, Ahmedabad 380 009, Gujarat, India

PREAMBLE TO THE DESCRIPTION:

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION:

The present invention relates to a yaw system for a wind turbine, to a wind turbine and to a method for adjusting a yaw angle of a nacelle of a wind turbine.

BACKGROUND

For an efficient power generation, a rotor of a wind turbine needs to be oriented towards the wind, such that the wind direction is perpendicular to a rotor plane defined by circulating blades of the rotor. A nacelle supports the rotor. Rotating the nacelle with respect to the ground that supports the wind turbine modifies the orientation of the rotor.

DE4413688 A1 describes a wind turbine with one or more rotors mounted on a tower, wherein a lower end of the tower is not fixed to a foundation but rotatably supported by means of a drive mechanism including a plain bearing. In this way, the wind turbine including the rotors and the tower may be rotated for orienting the rotors towards the wind. The drive mechanism of this wind turbine needs to support the huge weight of the entire tower and is thus complex and expensive.

Commonly, wind turbines comprise a yaw system rotatably connecting the nacelle with the top of the tower, wherein the tower is fixed to the foundation. The yaw system comprises a drive mechanism for rotating the nacelle and for holding the nacelle at a desired yaw angle. The drive mechanism needs to be powerful enough for rotating the nacelle, and for holding it, when no rotation is intended, and is thus cumbersome. The yaw-system drive mechanism normally comprises or consists of a driving unit, e.g. an electrical motor with a gear transmission, to provide a rotational force and a braking unit to provide a required brake capacity to hold the nacelle in position against external forces. The braking unit can be integrated into the motor as a mechanical brake acting through the same gear transmission and/or it can be an active brake acting on a rim of the tower.

OBJECT OF INVENTION An object of the present invention is to provide an optimized yaw system. SUMMARY OF THE INVENTION

Such a yaw system is adapted for supporting a nacelle of the wind turbine on a tower of the wind turbine, particularly on the top of the tower, such that the nacelle is rotatable with respect to the tower. The yaw system comprises a yaw bearing having a rim, at least one sliding pad slidably connected with the rim, and at least one pre-tensioning device. The at least one sliding pad and the rim are pressed against each other with a pre-tensioning force that is at least partially exerted by the pre- tensioning device. Therein, a hydrostatic system is provided, being operatively coupled with the at least one sliding pad, and being configured such as to be operable for continuously exerting a lifting force on the sliding pad, the lifting force partially (not entirely) releasing or counteracting the pre-tensioning force. In particular, the lifting force may act opposite to the pre-tensioning force and have a smaller absolute value than the pre-tensioning force. An idea is to utilize a bearing friction to hold the nacelle in position against external forces. In this case an active drive can be used to act against the external forces as well as the bearing friction. This would normally require an increased active drive force. For this reason the hydrostatic system is provided which may be operated to adjust the bearing friction in order to meet the different requirements of driving (low friction) and holding (high friction).

The yaw system allows to counteract the pre-tensioning force partially, i.e., to decrease the force urging the sliding pad against the rim, in particular when the sliding pad is to be moved with respect to the rim. The sliding pad and the rim may act as a plain bearing, in particular as a plain bearing being partially unloaded (by means of the hydrostatic system). By counteracting the pre-tensioning force, the friction between sliding pad and the rim is reduced so that a less powerful drive mechanism is sufficient to drive the sliding pad along the rim. Alternatively or additionally, the drive mechanism may be operated with less power.

The hydrostatic system is selectively operable. The hydrostatic system has a holding mode in which the lifting force is not exerted on the sliding pad, and a releasing mode in which the lifting force is exerted on the sliding pad. Alternatively, a (small) first lifting force is exerted in the holding mode and a (larger) second lifting force is exerted in the releasing mode. In the holding mode the friction is higher than in the releasing mode, e.g. by a factor of 2, 3 or 10. The hydrostatic system may have a plurality of releasing modes with different lifting forces. A range of lifting forces may be given for the releasing mode. Alternatively or in addition, the lifting force may be adjustable. By means of the hydrostatic system, the friction between sliding pad and the rim may be adjustable, in particular in a stepless manner.

The rim may have an interface for connecting the rim with the tower or the nacelle. Correspondingly, the sliding pad may have an interface for connecting the sliding pad with the other one of the tower or the nacelle. The rim may be ring-shaped. The rim may have the shape of a disc. The sliding pad comprises a sliding surface that is in contact with a sliding surface of the rim. The sliding surfaces are in surface contact with each other when the hydrostatic system exerts the lifting force on the sliding pad and when the hydrostatic system does not exert the lifting force on the sliding pad. The lifting force only partially counteracts the pre-tensioning force such that the sliding pad does not lift off from the rim. When the sliding pad would lift off from the rim, the friction would be reduced to a large extent and the nacelle could be unintentionally rotated by the wind, in particular when the drive mechanism is not powerful enough to withstand the unintended rotation. The hydrostatic system is adapted to exert a lifting force that is smaller than a force that would lift off the sliding pad. By this, a contact pressure of the contacting sliding surfaces of the sliding pad and the rim is reduced in the releasing mode compared to the holding mode. A holding torque of the sliding pad is controllable by means of the hydrostatic system. The sliding surface of the sliding pad may be small compared to the sliding surface of the rim.

According to an embodiment, at least one channel or chamber is formed in the sliding surface of the sliding pad. The hydrostatic system may be adapted to apply a pressure in the channel or chamber so as to exert the lifting force. The hydrostatic system may be in fluid connection with the sliding pad. By providing the channel or chamber, the lifting force may be efficiently applied. The cannel or chamber of the sliding pad may be sealed against the rim, in particular by means of a sealing element arranged in or at the sliding surface of the sliding pad. By sealing the channel or chamber, a loss of lubricant may be avoided. Therefore, no recovery or collection of leaking lubricant has to be provided. Only little amounts of lubricant have to be retained. Moreover, the pump may be activated for pumping lubricant until a predetermined target pressure is reached (that may result in the lifting force). The pump may then be operated at a lower power or even be deactivated as the sealed sliding pad holds the pressure of the lubricant. Due to the sealing, the yaw angle may be adjusted (in particular over large distances and/or several times) and the predetermined pressure maintained without supplying additional lubricant. The system can be operated in a way that pressure is built up and released from one centralized device like a hydraulic piston. In this case it can be operated with nearly no loss of lubricant (e.g. grease). The system may also be equipped with several pressure release points near to the friction pads. The pressure release points are operable to release pressure of the lubricant. Pressure release points allow a faster pressure drop. The lubricant that is lost from the system through the multiple pressure release points can be collected and re-used e.g. to lubricate the gear of the yaw bearing. For this purpose, a lubricant collector may be provided. The hydrostatic system can comprise a pump or any other device adapted to build up pressure in the system, in particular between the at least one sliding pad and the rim. In this manner, the lifting force can be generated. The device, in particular the pump may be adapted and arranged for pumping a lubricant, e.g. water or a grease, in particular between the at least one sliding pad and the rim. Pumping the lubricant between rim and sliding pad generates the lifting force. The pump of the hydrostatic system may be in fluid connection with the channel or chamber. Advantageously, the pump of the hydrostatic system is adapted for pumping the lubricant in the channel or chamber of the sliding pad, in particular with a predetermined pressure. The predetermined pressure may correspond to the lifting force.

The hydrostatic system may be adapted to exert and maintain a lifting force with an absolute value substantially smaller than the absolute value of the pre-tensioning force. The hydrostatic system may be adapted to exert and maintain a lifting force with an absolute value greater than 10 % and/or smaller than 90 % of the absolute value of the pre-tensioning force, in particular greater than 30 % and/or smaller than 70 % of the absolute value of the pre-tensioning force. These forces lead to improved ratios of static friction to dynamic friction. When the lifting force exceeds 70 % of the pre-tensioning force, lubricant may start to leak out and the sliding surface of the sliding pad may at least partially lift off from the sliding surface of the rim. The hydrostatic system may be adapted to exert and a lifting force that is approximately constant over a certain time frame, e.g. the time used for adjusting a yaw angle. The pre-tensioning force is partially exerted on the sliding pad by a weight of the nacelle and/or the yaw system or parts of the yaw system. The contribution of the pre-tensioning device to the pre-tensioning force may be larger than the contribution of the weight of the nacelle and/or the yaw system or parts of the yaw system. The pre-tensioning device may serve to balance an unevenness of the rim.

The yaw system may further comprise a control unit adapted for receiving at least one input value and being configured for increasing the lifting force if the input value is above a predetermined threshold and/or decreasing the lifting force if the at least one input value is below the same or another predetermined threshold. Alternatively, the control unit may be configured for decreasing the lifting force if the input value is above a predetermined threshold and/or increasing the lifting force if the at least one input value is below the same or another predetermined threshold.

The yaw system may further comprise at least one drive mechanism, in particular at least one drive motor for moving the at least one sliding pad along the rim, particularly along the sliding surface of the rim. The yaw system is adapted to set the hydrostatic system in the releasing mode before driving the drive mechanism. The control unit may further be adapted for determining a torque of the drive motor as input value. The torque can determined by measuring a drive speed and/or a current or power consumed by the at least one drive motor. The drive motor could be a hydraulic motor or an electric motor. The rim may be formed as a gear rim being engaged by a pinion driven by the drive motor. The hydrostatic system may be configured to maintain the lifting force as long as an adjustment of the yaw angle is effected by means of the drive mechanism. The sliding surfaces of the sliding pad and the rim are in contact while moving the sliding pad along the rim. In the Yaw system, the drive mechanism may be of a simpler construction or of smaller scale compared to conventional yaw systems.

According to an embodiment, the yaw system comprises a sensor. For example, the sensor is adapted for determining a friction force of the at least one sliding pad along the rim. The sensor can be configured to provide the determined friction force as input value to the control unit.

The yaw system may comprise a plurality of sliding pads, e.g. more than ten sliding pads. About half of or at least half of the plurality of sliding pads may be operatively coupled with the hydrostatic system, in particular for providing an efficient application of the hydrostatic system.

The pre-tensioning device may comprise an elastic element. The elastic element can compensate tolerances of the rim and/or the sliding pad. By means of an elastic element the pre-tensioning force can be maintained in a in a pre-defined range. The elastic element may comprise an elastic spring, e.g., a leaf spring, a plate spring or a coil spring, in particular a plurality of springs. Alternatively or additionally to an elastic element, the hydrostatic system or another hydrostatic system may be used as the pre-tensioning device or be a part of the pre-tensioning device. In this manner, the setup of the yaw system can be simplified.

The object is also solved by a wind turbine having a tower and a nacelle rotatably coupled to an upper end of the tower by means of a yaw system, the yaw system comprising a rim and at least one sliding pad slidably coupled to the rim, wherein the at least one sliding pad and the rim are pressed against each other by a pre-tensioning force, the yaw system further comprising a hydrostatic system operatively coupled with the at least one sliding pad and adapted for exerting and maintaining a lifting force on the sliding pad partially counteracting the pre-tensioning force. The yaw system of the wind turbine may particularly be the yaw system according to any aspect or embodiment described herein.

The object is also solved by a method for adjusting a yaw angle of a nacelle of a wind turbine, the method comprising the following steps: providing a yaw system being adapted for rotatably supporting the nacelle on a tower of the wind turbine, the yaw system comprising a rim and at least one sliding pad slidably coupled to the rim, wherein the at least one sliding pad and the rim are being pressed against each other by a pre-tensioning force, the yaw system further comprising a hydrostatic system operatively coupled with the at least one sliding pad; exerting (and maintaining) a lifting force on the sliding pad partially counteracting the pre-tensioning force by means of the hydrostatic system; and

while exerting the lifting force, rotating the nacelle around a yaw axis with respect to the tower of the wind turbine for adjusting the yaw angle. The provided yaw system may particularly be the yaw system according to any aspect or embodiment described herein. By the above-described yaw system, wind turbine and method a hydrostatic yaw bearing may be provided, which can facilitate rotatable motion of the nacelle with respect to the wind turbine tower. Further, a hydrostatic yaw bearing may be provided, which can adjust the bearing friction in order to meet the different requirements of low friction while driving and high friction while holding.

Further, a hydrostatic yaw bearing may be provided, which can facilitate adjusting a yaw angle of a nacelle of a wind turbine in order to make the entire operation smooth and more effective.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention are shown in the figures, where

Figure 1 shows a schematic view of a wind turbine with a tower, a nacelle and a rotor;

Figure 2 shows a schematic side view of a various components located within the nacelle of the turbine according to Figure 1 , including a drive train and a yaw system;

Figure 3 shows a schematic cross-sectional view of the yaw system according to Figure 2;

Figure 4 shows an enlarged schematic view of the yaw system according to Figures 2 and 3;

Figures 5A to 5C show schematic cross-sectional views of various embodiments of sliding pads;

Figures 6A and show schematic views of the sliding pads according 6B to Figures 5A and 5B, respectively;

Figures 7A to 7C show a sliding pad of the yaw system according to

Figures 2 to 4 with a lubricant applied at different pressures;

Figure 8 shows an applied lifting force and a resulting frictional force associated to a movement of the sliding pad along a rim of the yaw system according to Figures 2 to 4 in a diagram versus the time; and

Figure 9 shows a method for adjusting a yaw angle of the nacelle of the wind turbine according to Figure 1.

The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.

DESCRIPTION OF THE INVENTION

Reference is made to Figure 1 to Figure 9 which discloses details pertaining to various embodiment of the present invention. However, the details, modalities and illustrations mentioned herein shall not to be construed as limitation for the present invention.

Figure 1 shows a wind turbine (2) for generating electrical energy by rotation of a rotor (22) by means of a wind. The rotor (22) is mounted at a nacelle (21 ) arranged at an upper end (200) of a tower (20) of the wind turbine (2). The tower (20) extends between its upper end (200) and a foundation (201) at the ground. The tower (20) is elongate and has a longitudinal axis. The rotor (22) comprises several, in the present case three blades (221) mounted on a hub (220). The rotor (22) is rotatable around a rotor axis x with respect to the nacelle (21 ). The rotor blades (221 ) can be revolved within a rotor plane.

For an efficient extraction of wind energy, the rotor (22) is oriented towards the wind. In particular, the rotor plane is oriented perpendicular to the direction of the incoming wind. For this purpose, the nacelle (21) together with the rotor (22) is rotatable around a yaw axis z with respect to the tower (20). The yaw axis z corresponds to the longitudinal axis of the tower (20). The yaw axis z is substantially perpendicular to the rotor axis x. For effecting a rotation of the nacelle (21 ) around the yaw axis z, the wind turbine comprises a yaw system, that will be described below. Figure 2 shows the hub (220) of the rotor (22) being connected to a drive train (23) mounted on a main frame (210) of the nacelle (21 ), and the yaw system (1 ). The hub (220) is coupled to a gearbox (230) of the drive train (23) via a low speed shaft. A high speed shaft leaves the output side of the gearbox (230) and is coupled to a generator (231) for converting mechanical energy to electrical energy. The main frame (210) may also be referred to as machine carrier.

By means of the yaw system (1 ), the nacelle is rotatable around the yaw axis z, e.g. by a predetermined yaw angle φ. The yaw system (1 ) allows a controlled orientation of the rotor (22) relative to the wind and/or the tower (20) by means of a plurality of drive motors (10). The drive motors (10) are fixed to motor supports (211 ) of the main frame (210). The motor supports (211 ) may be attached to and/or formed in one piece with the main frame (210). Each drive motor (10) drives a pinion (11 ). The pinions

(11 ) mesh with a toothing of a rim (12) of the yaw system (1 ). The rim

(12) is formed as a gear rim. The tooting of the rim (12) is provided at an outer shell surface of the rim (12). Alternatively, the toothing may be provided on an inner surface of the rim (12), or provided at another component than the rim (12). The drive motors (10) are fixedly connected with the nacelle (21 ). The rim (12) is fixed to the tower (20). Alternatively, the rim (12) may be fixed to the nacelle (21 ) and the drive motors (10) may be fixed to the tower (20). The main frame (210) of the nacelle (21) is rotatably supported on the rim (12) by means of a plurality of sliding pads of the yaw system (1 ), as will be explained in more detail below. An activation of one or more of the drive motors (10) effects a rotation of the nacelle (21) around the yaw axis z with respect to the rim (12) (and to the tower (20)).

Figures 3 and 4 show further details of the yaw system (1 ) in a cross- sectional view. In particular, the yaw system (1 ) comprises a plurality of sliding pads (13), one of which is shown in Figures 3 and 4. The sliding pad (13) is supported on the rim (12). The sliding pad (13) may slide along a sliding surface (120) of the rim (12). The sliding surface (120) may have the shape of a ring. In a view along the yaw axis z, the sliding pad (13) is arranged above the rim (12). A pre-tensioning device (14) urges the sliding pad (13) against the rim (12). The pre-tensioning device (14) may be formed as, or comprise a resilient spring. On its end opposite the sliding pad (13), the pre- tensioning device (14) exerts a force against a clamp (17). The clamp (17) (partially) encompasses the rim (12). An opposing pad (16) is provided on an opposite side of the rim (12) with respect to the sliding pad (13). The sliding pad (13) is mounted on the clamp (17) so as to be movable along the yaw axis z. By means of the pre-tensioning device (14), the opposing pad (16) and the clamp (17), the sliding pad (13) is urged against the sliding surface (120) of the rim (12). In an alternative embodiment, the opposing pad (16) may be formed as the sliding pad (13) and/or another pre-tensioning device (14) may be provided for the opposing pad (16).

The rim (12) and the sliding pads (13) together form a yaw bearing. The nacelle (21 ) is supported by the yaw bearing. In the example of Figure 3, the main frame (210) of the nacelle (21 ) is fixed to the clamp (17).

The weight force of the nacelle (21 ) and the rotor (22), and the force exerted by the pre-tensioning device (14) together create a pre- tensioning force Fp pressing the sliding pad (13) against the rim (12). For example, the weight may be about 120 tonnes, while the pre-pensioning element (14) may exert a force corresponding to about 400 tonnes. Due to the pre-tensioning force F _P , a friction between the sliding pad (13) and the rim (12) is increased. An increased friction is desired, e.g., when no rotation of the nacelle (21 ) around the yaw axis z is intended, e.g. , when the rotor (22) already faces the wind.

Before and while rotating the nacelle (21 ) around the yaw axis z, a lifting force F _L is applied to the sliding pad (13), acting in opposite direction as the pre-tensioning force F _P .

For applying the lifting force F _L , the yaw system (1 ) comprises a hydrostatic system (15). The hydrostatic system (15) is a hydraulic system. The hydrostatic system (15) comprises a pump (150) for pumping a lubricant such as a grease, e.g. with a flow rate of about 7 ccm/min. The pump (150) is in fluid connection with the sliding pad (13), in the example via a tube (152). The pump (150) presses lubricant between the sliding pad (13) and the rim (12).

The pressurized lubricant between the sliding pad (13) and the rim (12) exerts the lifting force F _L on the sliding pad (13). For example, the pump (150) may be adapted to pump lubricant between the sliding pad (13) and the rim (12) at a pressure of up to 350 bar, in particular of between 50 and 100 bar, more particularly of between 80 and 90 bar. Depending on the pre-tensioning force, these pressures may lead to an optimal lifting force FL. The pump (150) is controlled by a control unit (18). The pump (150) may have a pressure sensor and provide pressure readings of the pressure sensor to the control unit (18). The control unit (18) may further be operatively connected to one or more sensors (180). The one or more sensors (180) may comprise a pressure sensor, a force sensor, a position sensor and/or a path sensor.

The hydrostatic system (15) is adapted to exert and maintain the lifting force F _L with an absolute value smaller than the pre-tensioning force F _P so that the pre-tensioning force F _P is partially counteracted. For this purpose, the control unit (18) controls the pump (150) to apply lubricant to the sliding pad at a predetermined pressure.

In this manner, the sliding pad (13) remains in contact with the rim (12), both when the lifting force F _L is exerted and when it is not exerted on the sliding pad (13). Therefore, essentially no lubricant is lost during operation of the yaw system (1 ).

With particular reference to Figure 4 more details of the sliding pad (13) and the pre-tensioning device (14) are now described. The sliding pad (13) has a sliding surface (130). The sliding surface (130) of the sliding pad (13) is in surface contact with a sliding surface (120) of the rim (12). The sliding surfaces (120), (130) of the rim (12) and the sliding pad (13) together form a plain bearing.

The sliding pad (13) further has a chamber (131 ) formed as a recess in its sliding surface (130). The sliding surface (130) of the sliding pad (13) is ring-shaped. The chamber (131) is sealed with respect to the environment by the contact of the sliding surfaces (120), (130) of the rim (12) and the sliding pad (13). The chamber (131 ) is in fluid connection with the pump (150) (by means of the tube (152)), through which lubricant may be pumped into the chamber (131 ) as indicated by arrows in Figure 3.

According to the example shown in Figure 4, the pre-tensioning device (14) comprises a screw (141 ) for creating the pre-tensioning force F _P . The screw (141 ) urges the sliding pad (13) against the rim (12). During assembly, the screw (142) is adjusted such as to exert the pre-tensioning force Fp. The tube (152) is partially or at least partially formed by and/or inside the screw (141 ). The pre-tensioning device (14) comprises elastic elements. The elastic elements are arranged between the screw (141 ) and the sliding pad (13). The force exerted by the screw (141 ) is transmitted by the elastic elements. According to Figure 4, the pre-tensioning device (14) comprises a plurality of springs as elastic elements, namely a pack of plate springs (140). Such springs are particularly suitable to exert the desired forces and to balance an unevenness of the rim (12). Between the screw (141 ) and the elastic elements (the plate springs (140)), a sensor (180) is arranged. The sensor (180) shown in Figure 4 is a force sensor. The sensor (180) measures the sum of the absolute values of the pre-tensioning force F _P and the lifting force F _L . The sensor (180) is operatively connected with the control unit (18).

Figures 5A and 6A show the sliding pad (13) of the yaw system (1 ) in a cross-sectional view (Figure 5A) and in a view from below (Figure 6A). The sliding pad (13) has a generally flat shape. The sliding pad (13) comprises a circular disc and a hollow-cylindrical part projecting from the edge of the disc. The hollow-cylindrical part and the disc define the cylindrical chamber (131 ) having an opening. The opening is delimited by the sliding surface (130) of the sliding pad (13). The sliding surface (130) forms the end face of the hollow-cylindrical part opposite the disc. Within the disc, an inlet (133) is formed for connection with the pump (150). For example, the sliding pad (13) has an outer diameter of 10 cm and an inner diameter of 9 cm. Figures 5B and 6B show an alternative embodiment of a sliding pad (13') for use with the yaw system (1 ) described above. The sliding pad (13') corresponds to the sliding pad (13) according to Figures 5A and 6A, with the difference that within the sliding surface (130), a groove (134) is formed. The groove (134) has a circular form. A seal (135) in the form of an o-ring is arranged in the groove (134). The seal seals the sliding pad (13') against the rim (12) for sealing the chamber (131).

Figure 5C shows another alternative embodiment of a sliding pad (13") for use with the yaw system (1 ) described above. The sliding pad (13") corresponds to the sliding pad (13) according to Figures 5A and 6A, with the difference that the wall-thickness of the hollow-cylindrical part increases towards the sliding surface (130). The sliding pad (13") forms an undercut within the chamber (131). A pressure in the chamber (131 ) will urge the sliding surface (130) against the rim (12) for a tight sealing of the chamber (131).

The yaw system (1 ) according to Figures 2 to 4 comprises a plurality of sliding pads (13), (13'), (13") according to Figures 5A, 5B and/or 5C, for example more than 10 sliding pads, in particular 80 sliding pads.

With particular reference to Figures 7A to 7C, the operation of the hydrostatic system (15) will be described. Figure 7A shows the sliding pad (13) pressed against the rim (12). The hydrostatic system (15) provides lubricant (3) at a low pressure or without an overpressure with respect to the environment. In this configuration, the friction between the sliding pad (13) and the rim (12) is high. A high friction is desired when no rotation of the nacelle (21 ) is intended. The hydrostatic system (15) comprises a holding mode. When no rotation of the nacelle (21 ) is intended, the control unit (18) controls the hydrostatic system (15) to assume the holding mode and provide no or only little pressure to the sliding pad (13). A corresponding coefficient of friction may have a value of about or up to 0.5.

Figure 7B shows the sliding pad (13), wherein the hydrostatic system (15) provides lubricant (3) at a predetermined pressure to the chamber (131 ) of the sliding pad (13). The predetermined pressure leads to a lifting force F _L approximately 0.5 to 0.7 times the pre-tensioning force F _P . The pre-tensioning force F _P is reduced to 50 % to 30 % (compared to the holding mode). In this manner, the friction between the sliding pad (13) and the rim (12) is strongly reduced and only little power is needed to drive the yaw system (1 ) compared to the holding mode. At the same time the pressure is low enough so that the sliding pad (13) does not lift off from the rim (12). The hydrostatic system (15) comprises a releasing mode, in which it provides the predetermined pressure (higher than in the holding mode) to the sliding pad (13). When a rotation of the nacelle (21 ) is intended, the control unit (18) controls the hydrostatic system (15) to assume the releasing mode. Figure 7C shows an undesired configuration, wherein the predetermined pressure is exceeded so that the sliding pad (13) is lifted off from the rim (12). In this configuration, the friction is determined by the lubricant (3) and reduced to a very small value when compared to the releasing mode. A corresponding coefficient of friction may have a value of about or down to 0.05. In such a situation, very powerful drive motors would be necessary in order to withstand an unintended rotation by the wind. Moreover, the chamber (131 ) would no longer be sealed by the rim (12) and lubricant (3) could escape from the chamber (131 ). Escaped lubricant (3) would have to be collected by additional means.

The yaw system (1 ) comprises means for controlling the lifting force F _L in order to avoid the sliding pad (13) to lift off from the rim (12), e.g. one more of the following means. The pump (150) and or the control system (18) may have a pressure sensor providing a pressure reading as input value to the control system (18). The control system (18) may control the pump (150) to such as to adjust the pressure to equal the predetermined pressure. Alternatively or additionally, a sensor (180) may provide a force reading as input value to the control system (18). Further alternatively or in addition, a position, path and/or speed sensor may provide a position, path or speed reading of the gliding pad (13) with respect to the rim (12) as input value to the control system (18). By detecting that a change of position or speed is above a predetermined threshold, the control system (18) may conclude that the friction is too low and reduce the lifting force F _L . By detecting that a change of position or speed is below a predetermined threshold, the control system (18) may conclude that the friction is too high and increase the lifting force F _L . According to a further option, a torque and/or a power consumption (in particular determined by measuring a drawn current) of a drive motor (10) may be determined and provided as input value to the control unit (18). The torque and/or a power consumption is determined while adjusting the yaw angle. The torque may be calculated by the control unit (18) on the basis of the power consumption. The torque may be used by the control unit (18) to determine a frictional force F _F between the sliding pad (13) and the rim (12). For example, three phase asynchronous electric motors can be used as drive motors (10). In such motors, the rotational speed and the torque are directly related. This relation may be used by the yaw system (1 ) as described above for protecting the drive motors (10).

The control unit (18) may comprise a control loop using one or more input values for adjusting an optimal (and/or predetermined) friction between the rim and the sliding pad. By detecting that the frictional force F _F is above a predetermined threshold, the control system (18) may increase the lifting force F _L . By detecting that the frictional force F _F is below a predetermined threshold, the control system (18) may decrease the lifting force F _L .

The yaw system (1 ) may further comprise a sensor for determining the speed and/or direction of the wind, e.g. a wind vane mounted on the top of the nacelle (21 ). In response of the speed and/or direction of the wind, the control system (18) may control the hydrostatic system (15) to assume the holding mode or the releasing mode, and/or to drive the drive motors for adjusting the yaw angle φ.

Turning now to Figure 8, a result of at test operation of the yaw system (1 ) is described. Therein, the applied lifting force F _L and the resulting frictional force F _F are shown while the sliding pad (13) was constantly moved back and forth along the rim (12), travelling a distance d. The upper curve shows the force measured by the sensor (180) being arranged as shown in Figure 4, i.e., the upper curve corresponds to the sum of the absolute values |F _P | + |F _L | of the pre-tensioning force F _P and the lifting force F _L . The pre-tensioning force F _P has a constant value of about 52 kN. The lifting force F _L was varied between 0 kN (what may correspond to the holding mode) and about 19 kN by corresponding application of a pressure by means of the pump (150). After about 250 seconds, the lifting force F _L was maintained at a constant value of about 19 kN.

The hydraulic force (the lifting force F _L ) is added while the sliding pad (13) is driven on the rim (12). The frictional force F _F is the force required to drive the sliding pad on the rim. The driven distance in cm is denoted as d. It can be seen that the frictional force F _F is approx. 10 kN if |F _P | + |F _L | is 52 kN (no additional hydraulic force) and is reduced to approximately 3 kN at |F _P | + |F _L | = 69 kN (about 17kN of added hydraulic force). Application of a lifting force F _L of about 17 kN may correspond to the releasing mode. Alternatively, application of a lifting force F _L of 15 kN +/- 2 kN may correspond to the releasing mode.

After approximately 80 seconds the pressure is released, which results in an increase of the frictional force F _F to the initial value of about 10 kN. After about 125 s the driving direction is reversed which results in a frictional force F _F of about -10 kN. While the pressure is rising, the frictional force F _F is reducing accordingly. Pressure is released again at 150 s which results in a high frictional force F _F of -10 kN. As pressure is rising again, the frictional force F _F is reducing and reaches to approximately -3 kN at 250 s. After 250 s the hydraulic pressure (and thus the lifting force F _L ) has reached a maximum. At this high pressure grease starts leaking out. At the same time the frictional force F _F reduces further to approximately 1 kN. As it is advantageous to avoid leaking grease, the lifting force F _L may be adjusted such as to remain below a predefined value, e.g. below 19, 18 or 17 kN in the releasing mode.

As shown in Figure 8, the lifting force F _L exerted by the pump (150) is maintained at a value larger than 0 kN, in particular larger than 10 kN over a given time span, e.g. several seconds or minutes, in particular the time span it takes to adjust the yaw system (1 ). In this time span, the lifting force F _L counteracts the pre-tensioning force F _P . Therein, the absolute value |F _L | of the lifting force F _L is smaller than the absolute value |F _P | of the pre-tensioning force F _P so that the pre-tensioning force F _P is only partially, not fully counteracted. The at least one sliding pad (13) and the rim (12) are then pressed against each other by a reduced tensioning force that is smaller than the pre-tensioning force F _P .

Individual strokes of the pump (150) cause a wave-like line of the force |Fp| + |F _L | measured by the sensor (180). Approximately 30 strokes of the pump (150) are required to build up the full pressure resulting in a lifting force F _L of about 17 kN in the releasing mode. The resulting frictional force F _F between the sliding pad (13) and the rim

(12) decreased with increasing lifting force F _L and increased with decreasing lifting force F _L . With the covered range of the lifting force F _L , the frictional force F _F was varied between absolute values of about 13 kN and about 3 kN.

With reference to Figure 9, a method for adjusting the yaw angle φ of the nacelle (21 ) of the wind turbine (2) according to Figure 1 will be described.

At step S100, the yaw system (1 ) being adapted for rotatably supporting the nacelle (21) on the tower (20) of the wind turbine (2) is provided, the yaw system (1 ) comprising the rim (12) and the at least one sliding pad

(13) slidably coupled to the rim (12), wherein the at least one sliding pad (13) and the rim (12) are being pressed against each other by a pre- tensioning force F _P , the yaw system (1 ) further comprising the hydrostatic system (15) operatively coupled with the at least one sliding pad (13).

At step S101 , a lifting force F _L is exerted on the sliding pad (121 ) partially counteracting the pre-tensioning force F _P by means of the hydrostatic system (15). In other words, the hydrostatic system (15) is controlled to assume the releasing mode. Preferably, the releasing mode is assumed just before rotating the yaw system (1 ). At step S102, the nacelle (21 ) is rotated around the yaw axis z with respect to the tower (20) of the wind turbine (2) while exerting the lifting force FL (while in the releasing mode) for adjusting the yaw angle φ. By assuming the releasing mode, only little power of the drive motors (10) is necessary to affect the rotation of the nacelle (21 ).

While rotating the nacelle (21 ), the dynamic friction can be measured and provided to the control unit (18) as input value. The control unit (18) may control the pressure of the lubricant between the sliding pad (13) and the rim (12) such as to maintain the dynamic friction at an approximately constant level.

At step S103, the lifting force F _L is partially or completely released (by decreasing the pressure provided by the pump (150)). In other words, the hydrostatic system (15) is controlled to assume the holding mode. In the holding mode, only little power of the drive motors or an optional additional break is necessary for securely holding the nacelle (21 ) at its current yaw angle φ. Preferably, the holding mode is assumed right after the yaw angle φ has been adjusted.

When another rotation of the nacelle (21 ) around the yaw axis x is desired, the process may return to step S101 . Since a reduced power of the drive motors (10) is needed both in the holding mode and in the releasing mode, the drive capacity may be reduced compared to a conventional yaw system, allowing a simplified yaw system (1 ) with reduced production cost. Furthermore, no lubricant collector such as a lubricant gutter is needed, further simplifying the yaw system (1 ).

List of Reference Numbers

1 yaw system

10 drive motor

1 1 pinion

12 rim

120 sliding surface

13 sliding pad

30 sliding surface

131 chamber

132 seal ring

133 inlet

134 grove

135 seal

14 pre-tensioning device

140 spring

141 screw

15 hydrostatic system

150 pump

152 tube

16 opposing pad

17 clamp

18 control unit 1 80 sensor 2 wind turbine

20 tower

200 upper end

201 foundation

21 nacelle

21 0 main frame

21 1 motor support 22 rotor

220 hub

221 blade

23 drive train

230 gearbox

231 generator

3 lubricant

FP pre-tensioning force F _L lifting force

F _F frictional force d distance X rotor axis

z yaw axis φ yaw angle

The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles described herein can be applied to other embodiments without departing from the spirit or scope of the invention. Thus, it is to be understood that the description and drawings presented herein represent a presently preferred embodiment of the invention and are therefore representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art and that the scope of the present invention is accordingly not limited.