Yaw control using average motor speed reference as feedback

Abstract

This invention relates to a method for controlling the yaw of a wind turbine system by controlling multiple yaw drive actuators. Based on a requested motor speed reference as an input signal and an average motor speed reference as a feedback signal, the method determines a desired motor torque reference as the output signal for the multiple yaw drive actuators. The multiple yaw drive actuators rotate the nacelle or a structure comprising multiple nacelles, thereby providing a uniform load distribution to the multiple yaw drive actuators.

Description

Technical Field

[0001] The present invention relates to a method for controlling the yaw of a wind turbine system, and more particularly to a method for controlling a plurality of yaw drive actuators. Background Technology

[0002] The yaw system's task is to determine the nacelle's orientation relative to the wind. Most of the time, the yaw system is inactive or stopped. It only activates when the nacelle's orientation needs to be changed (usually due to a change in wind direction) to align the nacelle with the wind.

[0003] In normal operating mode, the deviation between the nacelle and the wind direction (yaw angle) should be as small as possible to avoid power generation losses and reduce load. However, at the same time, the yaw system should not be too sensitive to avoid continuous small yaw movements that would reduce the lifespan of mechanical components.

[0004] In modern wind turbine systems, the yaw system uses multiple yaw actuators to determine the nacelle's orientation relative to the wind. In such a system, multiple yaw actuators engage with a single yaw ring. Deviation from any of the co-operating yaw actuators will result in uneven wear of that actuator. Therefore, it is important to operate the yaw actuators in a manner that ensures the most uniform operating conditions possible among the multiple yaw actuators.

[0005] Therefore, an improved method for controlling yaw systems would be advantageous, and in particular, a more efficient and / or reliable method for controlling multiple yaw drive actuators would be advantageous. Summary of the Invention

[0006] One object of the present invention is to provide uniform load distribution for multiple yaw drive actuators when the yaw system is active.

[0007] Another objective of this invention is to improve the control method for wind turbines, particularly to improve the control method for increasing the service life of components in the yaw system.

[0008] Therefore, the above-mentioned and several other objectives are achieved in the first aspect of the invention by providing a method for controlling the yaw of a wind turbine system, the wind turbine system including a nacelle, a tower, a turbine controller, and a yaw system.

[0009] The yaw system can be operated to rotate the nacelle relative to the tower.

[0010] The yaw system includes a motor controller and multiple yaw drive actuators.

[0011] The motor controller receives:

[0012] - The requested motor speed reference is used as the input signal, and

[0013] -The average motor speed reference is used as a feedback signal, and

[0014] The motor controller provides:

[0015] - The required motor torque reference is used as the output signal, which is determined based on the input signal and the feedback signal, to rotate the nacelle using the plurality of yaw drive actuators.

[0016] By referencing the required motor torque for the plurality of yaw drive actuators to an average motor speed reference as a feedback signal, a control scheme with a virtual master drive constructed based on the average motor speed is provided. In this way, the load is distributed with respect to the virtual master drive, which is consistent with the drive operating at the average speed. As a result, the invention is particularly advantageous, but not exclusively advantageous, for achieving a uniform load distribution for multiple yaw drive actuators. Thus, each yaw drive actuator delivers substantially the same torque, performs uniform action to avoid imbalance, and avoids overloading of individual yaw drive actuators, thereby increasing the lifespan of the yaw system and increasing the wind turbine's production capacity by reducing downtime when the wind turbine is not generating electricity.

[0017] The yaw system includes multiple yaw drive actuators, each comprising a motor and a pinion gear connected to the yaw drive actuator. The yaw system also includes a yaw ring, to which multiple actuators are connected. The yaw ring is located on the tower to allow nacelle rotation. Furthermore, the yaw drive actuators include a variable frequency drive (VFD). In this document, the VFD is generally considered part of the yaw drive actuator, even though the VFD may be configured separately from other parts of the motor.

[0018] Yaw or rotation is understood, as is commonly understood in this field, as the rotation of the cabin.

[0019] The motors are preferably electric drive motors (which are typically asynchronous induction motors), but can also be permanent magnet motors. Each motor is powered by a separate variable frequency drive (VFD), enabling individual motor control. The VFD is connected to the motor controller in the yaw system and receives the required motor torque reference from the motor controller. Alternatively, these motors can be hydraulic drive motors.

[0020] The tower can be any support structure or construction on which one or more nacelles can be mounted and rotated relative to the tower. The tower may include support arms on each support arm, so the phrase "rotate the nacelles relative to the tower" also covers the case where the nacelles placed on the support arms are rotated. Furthermore, it is possible in embodiments where the support arms are rotatable relative to the tower, so that when the support arms rotate relative to the tower, the nacelles all rotate relative to the tower simultaneously; therefore, the phrase "rotate the nacelles relative to the tower" also covers this situation.

[0021] According to one embodiment, the method includes: when the yaw system is operating in normal operating mode, the motor controller calculates a desired motor torque reference, and the motor controller sends the desired motor torque reference to all of the plurality of yaw drive actuators.

[0022] When the yaw system is activated and turns the nacelle to a new position relative to the wind, the yaw system operates in normal operating mode. In normal operating mode, the motor controller calculates the required motor torque reference and sends it to all yaw drive actuators. This is advantageous for achieving even load distribution to multiple yaw drive actuators and also simplifies control because only one required motor torque reference is calculated. Therefore, under normal operating conditions, the yaw drive actuators operate as a single unit.

[0023] To calculate the required motor torque reference, the motor controller receives a requested motor speed reference as an input signal from the turbine controller, and an average motor speed reference is determined as a feedback signal. The average motor speed reference can be determined by the turbine controller or the motor controller itself. If the motor controller determines the average motor speed reference, this is done in a separate calculation block outside the calculation block handling feedback control. The requested motor speed reference is, for example, a request from the turbine controller to yaw the nacelle at a certain speed.

[0024] According to one embodiment, the method includes: each yaw drive actuator including a motor, and an average motor speed reference being calculated as the average of the motor speeds of all motors.

[0025] According to one embodiment, the method includes: each yaw drive actuator includes a motor, and an average motor speed reference is calculated as the average of the motor speeds of a selected subgroup of motors.

[0026] According to one embodiment, the method includes: in the calculation of an average motor speed reference, a selected motor subgroup excludes motors with motor speeds above a high-speed threshold speed and / or motor speeds below a low-speed threshold speed.

[0027] If some motors are not running or are running at significantly different speeds than other motors, it is advantageous to exclude those motors from the calculation of the average motor speed reference. This typically occurs when initiating yaw if the actuator must rotate a greater distance than other actuators to engage the pinion with the yaw ring gear. This can happen, for example, if there are broken teeth on the yaw ring.

[0028] According to one embodiment, the method includes: a required motor torque reference is limited by a torque limiter to not exceed a maximum torque.

[0029] The maximum torque is determined by the motor's maximum capacity and therefore remains constant. Consequently, the motor cannot deliver torque exceeding its maximum torque. It is advantageous to include the maximum torque in the calculations of the required motor torque reference in order to manage the system in a controlled manner.

[0030] According to one embodiment, the method includes: limiting the required motor torque reference to avoid a motor speed exceeding the maximum motor speed.

[0031] In one embodiment, the maximum motor speed should not be exceeded, even if the motor may be operated slightly faster than the maximum motor speed, because doing so could overload the motor and potentially damage it, which is undesirable. It is advantageous to include the maximum motor speed in the calculation of the required motor speed torque reference, so that the process can be controlled and the anticipated consequences are included in the control.

[0032] According to one embodiment, the method includes: the motor controller further receiving a maximum power usage reference as input, and wherein a finite motor speed reference is limited to not exceeding the power available according to the maximum power usage reference.

[0033] When available power is limited, it is advantageous to control its use in order to distribute power to the motor in a controlled manner. To do this, it is advantageous to prevent the use of power exceeding the available power in order to control the process and include the expected consequences in the control.

[0034] According to one embodiment, the method includes: the change in motor speed does not exceed the maximum rate of change of speed.

[0035] Especially during startup, it is advantageous to avoid high instantaneous peak torque caused by attempting to accelerate the motor beyond the system's capacity to handle effectively and in a controlled manner. Therefore, speed increase / decrease is implemented to limit acceleration to the maximum rate of speed change. Limiting acceleration reduces the risk of damaging or overloading the motor.

[0036] According to one embodiment, the method includes: a motor controller including a dynamic speed limiter,

[0037] - The dynamic speed limiter calculates a finite motor speed reference based on the requested motor speed reference and the maximum power usage reference, such that:

[0038] - A limited motor speed reference should not exceed the maximum motor speed, and

[0039] - The change in motor speed does not exceed the maximum rate of change of speed, and

[0040] - Power usage should not exceed the maximum power usage reference.

[0041] The dynamic speed limiter is a function block in the motor controller that calculates a limited motor speed reference taking into account different parameters to avoid exceeding the maximum motor speed and maximum power usage, and also to prevent high acceleration of the motor.

[0042] If there is no risk of exceeding the limit parameters, the finite motor speed reference is the same as the requested motor speed reference.

[0043] According to one embodiment, the method includes: a motor controller including a speed control unit,

[0044] The speed control unit includes a PID controller.

[0045] - This PID controller calculates the motor torque reference based on a finite motor speed reference and an average motor speed reference, and

[0046] The calculated motor torque reference was used to determine the required motor torque reference.

[0047] The speed control unit is a functional block in a motor controller that includes a PID controller. A PID (Proportional-Integral-Derivative) controller is a control loop mechanism that uses feedback and is widely used in industrial control systems requiring continuous modulation control. The PID controller continuously calculates the error value between the desired setpoint (in this case, a finite motor speed reference) and the measured process variable (in this case, an average motor speed reference), and corrects for it based on the proportional, integral, and derivative terms (denoted as P, I, and D, respectively), hence the name. In a PID controller, D can be zero; in this case, the PID controller is a PI controller.

[0048] According to one embodiment, the method includes: the speed control unit further includes a torque limiter,

[0049] - The torque limiter determines the required motor torque reference based on a calculated motor torque reference, such that:

[0050] - The required motor torque reference should not exceed the maximum torque.

[0051] The torque limiter is a sub-function block within the speed control unit. The torque limiter function block is a programmed function block that ensures the motor torque reference does not exceed the maximum torque. The speed control unit and torque limiter are implemented as part of the motor controller, and this implementation can be done in many different ways, either programmed into the software that is part of the motor controller or in separate software routines used by the motor controller.

[0052] According to one embodiment, the method includes: a wind turbine system comprising multiple nacelles, and a yaw system arranged to rotate one or more of the multiple nacelles. Therefore, the method of the present invention can also be applied to multi-rotor wind turbines.

[0053] In multi-rotor turbines, nacelles can be mounted on support arms or on other support structures that allow for the installation of more nacelles on the same wind turbine system. The method of this invention can be used individually for multiple nacelles, such that a single nacelle placed on a support arm can rotate while other nacelles remain stationary. The method can also be used to rotate all nacelles by rotating the entire structure on which multiple nacelles are mounted, causing multiple nacelles to rotate simultaneously. The multiple nacelles then rotate relative to the tower, thus each individual nacelle also rotates relative to the tower.

[0054] The second aspect of the invention relates to a control system for controlling the yaw of a wind turbine, wherein the control system is arranged to perform the steps of the method according to the first aspect.

[0055] A third aspect of the invention relates to a wind turbine, wherein the wind turbine further includes a control system according to the second aspect for controlling the yaw of the wind turbine system.

[0056] The fourth aspect of the invention relates to a computer program product comprising software code adapted to control a wind turbine when executed on a data processing system, the computer program product being adapted to perform the method of the first aspect.

[0057] Different parts of a motor controller, dynamic speed limiter, speed controller, torque limiter, etc., can be implemented in separate computer programs, or as different functional blocks within the same computer program running on the same or different microprocessors. Similarly, motor controllers and turbine controllers can be implemented in different software programs running on separate computers or microprocessors, in the same software program running on the same computer or microprocessor, or in any combination thereof.

[0058] Generally speaking, aspects and embodiments of the present invention can be combined and coupled in any possible manner within the scope of the invention. These and other aspects, features, and / or advantages of the invention will become apparent and will be elucidated with reference to the embodiments described below. Attached Figure Description

[0059] Embodiments of the present invention will be described by way of example only with reference to the accompanying drawings, wherein:

[0060] Figure 1 The illustration shows a wind turbine.

[0061] Figure 2 The illustration shows a wind turbine configured as a multi-rotor wind turbine.

[0062] Figure 3 The diagram illustrates the yaw system.

[0063] Figure 4 The diagram illustrates the yaw controller.

[0064] Figure 5 The diagram illustrates the running envelope.

[0065] Figure 6 The diagram illustrates the motor controller for each drive.

[0066] The accompanying drawings illustrate one mode of carrying out the invention and should not be construed as limiting other possible embodiments falling within the scope of the appended claims. Detailed Implementation

[0067] Figure 1 A wind turbine 100 (WTG) is shown, comprising a tower 101 and a rotor 102 with at least one rotor blade 103. Three blades are typically used, but a different number can also be used. The blades 103 are connected to a hub 105, which is arranged to rotate together with the blades. The rotor is connected to a nacelle 104, which is mounted on top of the tower 101 and adapted to drive a generator located within the nacelle via a drivetrain. The rotor 102 rotates under the influence of wind. The rotational energy of the rotor blades 103 caused by the wind is transferred to the generator via a shaft. Thus, the wind turbine 100 is able to convert the kinetic energy of the wind into mechanical energy by means of the rotor blades, and subsequently into electrical energy by means of the generator. The generator is connected to a power converter.

[0068] Figure 2An alternative wind turbine 100 configured as a multi-rotor wind turbine is shown. The multi-rotor wind turbine includes multiple nacelles 104. An example with four nacelles is shown here, but in general, two or more nacelles can be used in a multi-rotor turbine. As shown in the figure above, the nacelles 104 can be supported via a tower 101 and support arms 106 extending outward from the tower 101, thereby positioning the nacelles away from the tower and on opposite sides of the tower. Alternatively, as shown in the figure below, the nacelles 104 can be supported by an inclined tower 101 extending from a base 130 (e.g., the ground or a floating base), thereby allowing two or more nacelles 104 to be sufficiently separated from each other at a given height. Embodiments of the invention can be used with both multi-rotor and single-rotor wind turbines.

[0069] Figure 3 An embodiment of the yaw system according to the present invention is shown. In the illustrated example, the yaw system includes eighteen yaw drive actuators 301, ten of which are located in… Figure 3 As shown in the diagram. In other configurations, more or fewer yaw drive actuators can be used. Each yaw drive actuator 301 includes a motor 302 (an electric drive motor in this embodiment) and a pinion 304 connecting the yaw drive actuator 301 and the yaw ring 305. Furthermore, the yaw drive actuator 301 includes a variable frequency drive (VFD) 306.

[0070] Motor 302 can be an asynchronous induction motor, each powered by a separate variable frequency drive 306, thus enabling individual motor control. Figure 3 As can be seen, the frequency converter drive 306 is clustered in the central cabinet and connected to the motor controller 307, but the frequency converter drive 306 can also be placed in other locations.

[0071] Motor 302 includes an encoder for detecting the motor's position. This encoder is a position gauge, and the motor speed can be derived from changes in position. The encoder is used to detect the speed of motor 302 and return the speed to the frequency converter drive 306.

[0072] An encoder can be used for each motor 302 to detect the position and speed of the motor 302 and ensure load sharing while avoiding overloading any of the motors 302.

[0073] The motor controller 307 outputs the required motor torque reference 403 to the variable frequency drive 306. The motor controller 307 receives information about the motor speed through communication with the encoder, each variable frequency drive 306, or with the turbine controller 308. The average motor speed reference 402 can be calculated by the motor controller or the turbine controller. Furthermore, based on input from the wind direction device 309, the motor controller 307 receives signals from the turbine controller 308 regarding when and in which direction to yaw.

[0074] The turbine controller 308 can control the yaw system 300, and the turbine controller 308 activates the motor controller 307 when yaw is required.

[0075] Figure 4 This is a schematic diagram of an embodiment of a yaw control scheme. The yaw controller includes a centralized control structure in which a single motor controller 307 operates all yaw drive actuators 301 based on a requested motor speed reference 401 and an average motor speed reference 402 of all motors 302 or a selected subgroup of motors 302.

[0076] The selected motor 302 subgroup may exclude the fastest and / or slowest motor 302 to increase robustness, or the selected motor 302 subgroup may exclude motors 302 that operate faster than the high-speed threshold speed and / or slower than the low-speed threshold speed.

[0077] Under normal operation, the same torque, the required motor torque reference 403, is requested for all yaw drive actuators 301 to ensure uniform load distribution. However, the torque reference for each individual drive may be distributed unevenly if requested, for example, when passing through a divided yaw section, a reduction in torque may be desired, or each drive may be tested individually during self-test.

[0078] The required motor torque reference 403 sent from the motor controller 307 to the yaw drive actuator is received by the frequency converter drive 306. Then, the frequency converter drive 306 sends a motor signal 420 to the motor 302, and the motor 302 applies the motor shaft torque 421 to the yaw system 300 and the pinion 304 to transmit the torque to the yaw ring 305.

[0079] Based on the motor speed 422 from each motor 302, the turbine controller 308 or the motor controller calculates an average motor speed reference 402 and sends it as a feedback signal to the motor controller 307. The motor controller can calculate its own feedback signal in a separate calculation block from the control loop calculation block. Furthermore, the motor speed of each motor 302 is sent back to the frequency converter drive 306 as a feedback signal via the internal torque control loop 423.

[0080] Figure 5An example of the operating envelope 501 for 4-quadrant control of a motor is shown. The operating envelope 501 is a coordinate system in which the X-axis is the average motor speed reference 402 and the Y-axis is the desired motor torque reference 403 applied to motor 302.

[0081] Finite motor speed reference 407 calculated by motor controller 307 Figure 6 The speed must not exceed the maximum motor speed 506. The maximum motor speed 506 is illustrated by the vertical curve portion in the first quadrant 502.

[0082] The required motor torque reference 403 calculated by the motor controller 307 must not exceed the maximum torque 507. The maximum torque 507 is illustrated by the horizontal curve portion in the first quadrant 502.

[0083] The curved portion 508 of the curve in the first quadrant 502 illustrates the relationship between speed and torque considering the maximum power usage reference 405. When the motor is running at high speeds, the applicable torque is limited by the maximum power usage reference 405.

[0084] The third quadrant 503 is equivalent to the first quadrant 502, except that the motor 302 is rotated in the opposite direction.

[0085] Figure 6 The illustration shows a graphical representation of an embodiment of the motor controller 307.

[0086] The goal of motor controller 307 is to determine the required motor torque reference 403 that generates the requested motor speed reference 401.

[0087] The control strategy comprises two feedback control loops in a cascaded structure, where both torque and speed control are used. These two feedback loops consist of an inner loop and an outer loop. The inner loop... Figure 4 The diagram shows an internal torque control loop, which is illustrated as being processed by a frequency converter drive 306, which receives the required motor torque reference 403 from the motor controller 307. Figure 6 The variable frequency drive 306, not shown in the diagram, is placed between the motor controller 307 and the motor 302, as follows: Figure 4 As shown in the image.

[0088] The internal torque control loop 423 is processed by the frequency converter drive 306 so that the output is a torque reference for obtaining the desired motor torque. Ideally, the motor shaft torque 421 is equal to the required motor torque 403.

[0089] External feedback loop in Figure 6The diagram shows a speed control loop 612, which returns an average motor speed reference 402 to a speed control unit 408. The speed control unit receives a finite motor speed reference 407 as an input signal and an average motor speed reference 402 as a feedback signal, and sends the desired motor torque reference 403 as an output to the motor 302.

[0090] The speed control unit consists of a PI controller 409 and a torque limiter 404.

[0091] The PI controller 409 is a proportional-integral controller for speed control because it produces a uniform DC gain and excellent noise suppression. The speed control loop 612 returns an average motor speed reference 402 to the speed control unit 408. Subtracting the average motor speed reference 402 from a finite motor speed reference 407 yields an error signal 424, which serves as the input to the PI controller 409, which provides a calculated motor torque reference 410 as its output. The PI controller could also be a PID controller, but in the described embodiment, the derivative (D) portion of the PID is zero.

[0092] Torque limiter 404 Figure 5 During operation in the first and third quadrants, the torque is limited to a maximum torque of 507. The torque limiter 404 receives a calculated motor torque reference 410 from the PI controller 409 as input and outputs the required motor torque reference 403 as an output signal.

[0093] Dynamic speed limiter 406 is used to limit the speed reference, thereby determining a limited motor speed reference 407. The requested speed reference 401 is received from turbine controller 308, but can be reduced due to power limitations based on maximum power usage reference 405. Furthermore, the speed can be reduced in speed saturation routine 615 to not exceed maximum power speed 506, and speed ramp-up routine 616 ensures that the motor is not accelerated beyond the maximum rate of speed change.

[0094] After passing through the low-pass filter 616 and the directional saturation filter 617 (which essentially ensures that the feedback signal has a minimum value, rather than zero), the dynamic speed limiter 406 also uses the feedback from the required motor torque reference 403.

[0095] Although the invention has been described in conjunction with specific embodiments, it should not be construed as being limited in any way to the examples presented. The scope of the invention is defined by the appended set of claims. In the context of the claims, the terms "comprising" or "including" do not exclude other possible elements or steps. Furthermore, references to words such as "a" or "an" should not be construed as excluding multiples. The use of reference numerals for elements shown in the figures in the claims should also not be construed as limiting the scope of the invention. Moreover, individual features mentioned in different claims may be advantageously combined, and mentioning these features in different claims does not preclude the possibility and advantage of combining features.

Claims

1. A method for controlling yaw of a wind turbine system, the wind turbine system comprising a nacelle (104), a tower (101), a turbine controller (308) and a yaw system (300), the yaw system (300) is operable to rotate the nacelle (104) relative to the tower (101), the yaw system (300) comprises a motor controller (307) and a plurality of yaw drive actuators (301), the method comprising: receiving, by the motor controller (307), a requested motor speed reference (401) as an input signal, receiving, by the motor controller (307), an average motor speed reference (402) as a feedback signal, wherein the average motor speed reference (402) is calculated as an average of the sum of motor speeds (422) of a selected subset of motors (302) in the wind turbine system divided by the number of motors in the selected subset of motors (302), and wherein, in the calculation of the average motor speed reference (402), the selected subset of motors (302) does not include motors (302) with a motor speed above a high speed threshold speed and / or motors (302) with a motor speed below a low speed threshold speed, and providing, by the motor controller (307), a required motor torque reference (403) as an output signal, the required motor torque reference (403) being determined from the input signal and the feedback signal, for rotating the nacelle (104) by the plurality of yaw drive actuators (301).

2. The method for controlling yaw of a wind turbine system according to claim 1, wherein, when the yaw system (300) is operated in a normal operation mode, the motor controller (307) calculates one required motor torque reference (403), and the motor controller (307) sends the one required motor torque reference (403) to all of the plurality of yaw drive actuators (301).

3. The method for controlling yaw of a wind turbine system according to claim 1 or 2, wherein, the average motor speed reference (402) is calculated as an average of the sum of motor speeds (422) of all motors (302) in the wind turbine system divided by the number of all motors.

4. The method for controlling yaw of a wind turbine system according to claim 1 or 2, wherein, the required motor torque reference (403) is limited by a torque limiter (404) to not exceed a maximum torque.

5. The method for controlling yaw of a wind turbine system according to claim 1 or 2, wherein, the required motor torque reference (403) is limited to avoid a motor speed (422) above a maximum motor speed.

6. The method for controlling yaw of a wind turbine system according to claim 1 or 2, wherein, the motor controller (307) further receives as input a maximum power usage reference (405), and wherein a limited motor speed reference (407) is limited to not require more power than available according to the maximum power usage reference (405).

7. The method for controlling yaw of a wind turbine system according to claim 1 or 2, wherein, a change in motor speed (422) does not exceed a maximum speed change rate.

8. The method for controlling yaw of a wind turbine system according to claim 1 or 2, wherein, the motor controller (307) comprises a dynamic speed limiter (406), - the dynamic speed limiter (406) calculates the limited motor speed reference (407) based on the requested motor speed reference (401) and a maximum power usage reference (405) such that: - the limited motor speed reference (407) is not higher than the maximum motor speed, and - the limited motor speed reference (407) is not lower than the low speed threshold speed. - the change in motor speed (422) does not exceed a maximum speed change rate, and - the power usage does not exceed the maximum power usage reference (405).

9. The method for controlling yaw of a wind turbine system according to claim 8, wherein, The motor controller (307) comprises a speed control unit (408), The speed control unit comprises a PID controller (409), - the PID controller (409) calculates a calculated motor torque reference (410) based on the limited motor speed reference (407) and the average motor speed reference (402), and - the calculated motor torque reference (410) is used for determining the required motor torque reference (403).

10. The method for controlling yaw of a wind turbine system according to claim 8, wherein, The speed control unit (408) further comprises a torque limiter (404), - the torque limiter (404) determines the required motor torque reference (403) based on the calculated motor torque reference (410) such that - the required motor torque reference (403) does not exceed a maximum torque.

11. The method for controlling yaw of a wind turbine system according to claim 1 or 2, wherein, The wind turbine system comprises a plurality of nacelles (104), and the yaw system (300) is arranged to rotate one or more of the plurality of nacelles (104).

12. A control system for controlling yaw of a wind turbine system, wherein, The control system is arranged to perform the steps of the method according to any of claims 1-11.

13. A wind turbine system comprising a control system for controlling yaw of a wind turbine system according to claim 12.

14. A computer program product comprising software code adapted to control a wind turbine system when executed on a data processing system, the computer program product being adapted to perform the method of any of claims 1-11.

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