Patent Publication Number: US-2023151796-A1

Title: Controlling the yaw to reduce motor speed

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for controlling the yaw of a wind turbine system, more particular for controlling the plurality of yaw drive actuators to avoid the motors of the yaw drive actuators to over-speed. 
     BACKGROUND OF THE INVENTION 
     The yaw system has the task of orienting the nacelle into relation to the wind. Most of the time the yaw system is inactive or parked. Only when the orientation of the rotor and the nacelle need to be changed, usually due to changes in the wind direction, the yaw system is active to turn the nacelle into the wind. 
     In a normal operation mode, the deviation between the nacelle and the wind direction, the yaw angle, is supposed to be as small as possible to avoid power production loss and to reduce loads. However, at the same time the yaw system must not respond to sensitively, to avoid continuous small yaw movements, which would reduce the life of the mechanical components. 
     In modern wind turbine systems a plurality of yaw drive actuators are used in the yaw system to orient the nacelle in relation to the wind. 
     However, if all yaw drive actuators are applying the same torque to all the motors this can lead to some motors overspeeding, if the motor is not engaged when the yaw system is activated. 
     Hence, an improved method for controlling the yaw system would be advantageous, and in particular a more efficient and/or reliable method to control a plurality of yaw drive actuators would be advantageous. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide, when the yaw system is active, a method to avoid or at least reduce overspeeding of a motor in a yaw drive actuator. 
     It is also an object of the invention to improve control methods of wind turbines, particularly to controlling methods, which improve lifetime of components of the yaw system. 
     Thus, the above described objects and several other objects are intended to be obtained in a first aspect of the invention by providing a method for controlling the yaw of a wind turbine system, the wind turbine system comprises a nacelle, a tower, a turbine controller and a yaw system, 
     the yaw system is operable to yaw the nacelle with respect to the tower, 
     the yaw system comprises a motor controller and a plurality of yaw drive actuators, 
     wherein, the motor controller receives an actual motor speed reference of each yaw drive actuator, and if the actual motor speed reference of a yaw drive actuator is higher than a specific motor speed reference, then an output signal to reduce the actual motor speed reference is applied to the yaw drive actuators with an actual motor speed reference higher than the specific motor speed reference. 
     To avoid too high or prolonged over-speed of a motor in a yaw drive actuator, the motor controller sends an output signal to the yaw drive actuator to reduce the actual motor speed when over-speed is detected. The advantage of reducing over-speed is that the load of the motor is reduced, and thereby lowering the risk of overloading the motor and/or the motor controller, with a possible failure that might close down and stop. Further, by avoiding overloading a motor and/or motor controller, the lifetime of the motor and/or motor controller may be increased. Yet a further advantage relates to avoidance of peak loads on the yaw gear system (pinion and yaw ring) resulting from a speeding motor, Motor over-speed is generally set a given speed above which prolonged operation cannot be sustained. Motor over-speed may e.g. be set as for motor speeds exceeding the specified design limit of the motor. However motor over-speed may also be set as a function of the design limit, such as at a given speed above or below the design limit. 
     The yaw system comprises a plurality of yaw drive actuators, each comprising a motor and a pinion connecting the yaw drive actuator. The yaw system further comprises a yaw ring to which the plurality of actuators are connected. The yaw ring it located on the tower to allow the nacelle to rotate. Further, the yaw drive actuators comprises a variable frequency drive. In this document, the variable frequency drives are generally considered part of the yaw drive actuators even though the variable frequency drives can be located separated from the other parts of the motor. 
     Yawing or rotating is understood as is common in the art, as rotation of the nacelle. 
     The motors are preferable electrical drive motors which typically will be asynchronous induction motors, but also can be permanent magnet motors and are each powered by a separate variable frequency drive, enabling individual motor control. The variable frequency drives are connected to the motor controller in the yaw system and receives an output signal from the motor controller. The output signal is a required motor torque reference from the motor controller, but in case a motor is overspeeding, the output signal is reduced to a reduced motor torque reference. Alternatively, the motors can be hydraulic drive motors. 
     A tower can be any support structure or construction on which one or more nacelles can be mounted and be rotatable relative to the tower. The tower can comprise support arms, with nacelles mounted on each support arm; therefore, the wording “to rotate the nacelle with respect to the tower” also covers when a nacelle placed on a support arm is rotated. Further, an embodiment is possible, wherein the support arms are rotatable relative to the tower, so that the nacelles all are rotated simultaneous relative to the tower, when the support arms are rotated relative to the tower; therefore, the wording “to rotate the nacelle with respect to the tower” also covers this situation. 
     According to an embodiment, the method comprises that the motor controller, as a feedback signal, receives a mean motor speed reference. The mean motor speed reference can be determined by the turbine controller or by the motor controller itself. If the motor controller determines the mean motor speed reference this is done in a separate computing block as to the computing block handling the feedback control. 
     The mean motor speed reference is used to determine whether a motor is overspeeding. 
     According to an embodiment, the method comprises that the specific motor speed reference is the mean motor speed reference where to a threshold value is added. Adding a threshold value to the mean motor speed reference to determine whether a motor is running faster than the mean motor speed reference where to a threshold value is added is used to make an early detection of whether a motor might be moving towards overspeeding. All motors should preferable rotate with the same speed, therefore, if a motor moves more than a threshold value faster than the mean motor speed reference, it is a sign the motor is moving towards overspeeding and therefore measures is taken to reduce the speed of the motor. 
     According to an embodiment, the method comprises that the specific motor speed reference is a maximum motor speed reference. 
     The maximum motor speed reference is a maximum value that the motors should not exceed, even though it may be possible for a motor to run a little faster than the maximum motor speed. This is to avoid overloading the motor and to avoid the motor closing down. Therefore, if the motor speed is higher than the maximum motor speed measures is taken to reduce the speed of the motor. The maximum motor speed reference will typically be stored as a parameter in computer memory entered when setting up the system. 
     According to an embodiment, the method comprises that the motor controller receives from the turbine controller
         a requested motor speed reference, as an input signal, and the motor controller provides   a required motor torque reference, as an output signal, for the plurality of yaw drive actuators to rotate the nacelle, determined according to the requested motor speed reference and the mean motor speed reference.       

     By basing the required motor torque reference for the plurality of yaw drive actuators on a mean motor speed reference as feedback signal, a control scheme with a virtual master drive is provided, the virtual master drive being constructed based on the mean motor speed. The virtual master drive is that all yaw drive actuators receives the same required motor torque references, acting together, as if there were only one yaw drive actuator. Therefore, the visual master drive is controlling all the yaw drive actuators that are running normally, not running in over-speed or in any other special mode. In this way, loads are shared in relation to the virtual master drive that is in accordance with a drive operating with the mean speed. As a result, the invention is particularly, but not exclusively, advantageous for obtaining an even load distribution for a plurality of yaw drive actuators. Hereby each yaw drive actuator substantially delivers the same torque, performs an even action avoiding imbalances, and avoids a single yaw drive actuator to be overloaded and thereby improve lifetime of the yaw system as well as production capabilities of the wind turbine due to reduced down time, where the wind turbine is not producing power. 
     However, when one motor is overspeeding, for instance because the pinion is not engaged with the yaw ring, the motor overspeeding need special treatment, and then is not part of the virtual drive, where all yaw drive actuators receives the same signal. 
     According to an embodiment, the method comprises that the output signal to reduce the actual motor speed reference, applied to the yaw drive actuator with an actual motor speed reference higher than the specific motor speed reference, is a reduced motor torque reference. 
     The motor controller sends the required motor torque reference to all the yaw drive actuators. However, if it is determined that a motor is overspeeding or at least running faster than the specific motor speed reference, then the required motor torque reference, sent to the specific yaw drive actuator with the overspeeding motor, is a reduced motor torque reference. Hereby, the speed of the overspeeding motor will be reduced, reducing the risk of the motor shutting down. 
     According to an embodiment, the method comprises that the reduced motor torque reference is the required motor torque reference reduced by a factor or a percentage. 
     Different strategies can be applied to reduce the motor speed by applying a reduced motor torque reference. The reduced motor torque reference can be a percentage subtracted from the required motor torque reference, or the required motor torque reference can be divided by a factor. 
     According to an embodiment, the method comprises that the reduced motor torque reference is reduced proportionally from the required motor torque reference relative to the detected speed. 
     The reduced motor torque reference can be a proportional reduction of the required motor torque reference calculated based in the mean motor speed reference from the motors running normally, for instance if the motor is running 30% to fast, the reduced motor torque reference is set to be 30% lower. 
     According to an embodiment, the method comprises that if an output signal to reduce the actual motor speed reference is applied to a yaw drive actuator, the remaining yaw drive actuators receives an output signal to increase the actual motor speed reference. 
     If a motor is overspeeding, it is doing so because the yaw drive actuator is not engaged with the yaw ring and therefore not participating in the actual yawing putting a higher load on the motors of the other yaw drive actuators. It can therefore be advantageous to increase the speed of the motors of the yaw drive actuators running normally, by increasing the required motor torque reference for these motors to increase the motor speed for these motors so they can compensate for the motors, which is not engaged with the yaw ring. 
     According to an embodiment, the method comprises that the mean motor speed reference is calculated as the average of the actual motor speed reference of all motors. 
     According to an embodiment, the method comprises that each yaw drive actuator comprises a motor, and the mean motor speed reference is calculated as the average of the actual motor speed reference of a selected subgroup of motors. 
     According to an embodiment, the method comprises that the selected subgroup of motors do not include, in the calculation of the mean motor speed reference, the motors with an actual motor speed reference higher than a high-speed threshold speed and/or the motors with an actual motor speed reference lower than a low-speed threshold speed. 
     It is advantageous to be able to exclude some motors from the calculation of the mean motor speed reference in case some motors are not operating or are operating with speeds that differs considerable from the other motors. This can happen typically when starting up the yawing, if an actuator has to rotate a larger distance than the other actuators for the pinion to engage with the gear of the yaw ring. This can for instance happen, if there is a broken tooth in the yaw ring. Therefore, the overspeeding motors are excluded from the calculation of the mean motor speed reference, and thereby excluded from effecting the calculation of the required motor torque reference for the normally running motors. 
     According to an embodiment, the method comprises that the wind turbine system comprises a plurality of nacelles and the yaw system is arranged to rotate one or more of the plurality of nacelles. Thus, the method of the invention can also be applied to a multi-rotor wind turbine. 
     In a multi-rotor turbine, the nacelles may be mounted on support arms or other support structures allowing more nacelles mounted on the same wind turbine system. The method of the invention can be used to the plurality of nacelles individually, so that a single nacelle placed on a support arm can be rotated while the other nacelles are not being rotated. The method can also be used to rotate all the nacelles by rotating the entire structure, on which the plurality of nacelles are mounted, so that the plurality of nacelles are rotated simultaneous. The plurality of nacelles are then rotated relative to the tower and therefore each individual nacelle is also rotated relative to the tower. 
     A second aspect of the invention relates to a control system for controlling the yaw of a wind turbine, where the control system is arranged to perform the steps according to the method of the first aspect. 
     A third aspect of the invention relates to a wind turbine, where the wind turbine further comprises a control system for controlling the yaw of the wind turbine system according to the second aspect. 
     A 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. 
     The different parts of the motor controller, the dynamical speed limiter, the speed control the torque limiter etc. can be implemented in separate computer programs or as different functions in the same computer program running on the same or on separate microprocessors. Likewise, the motor controller and the turbine controller can be implemented in different software programs running on separate computers or microprocessors, or be implemented in the same software programs running on the same computer or microprocessor or in any combination hereof. 
     In general, the various aspects and embodiments of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which 
         FIG.  1    illustrates a wind turbine, 
         FIG.  2    illustrates wind turbines configured as multi-rotor wind turbines, 
         FIG.  3    illustrates the yaw system, 
         FIG.  4    illustrates the yaw control, 
         FIG.  5    illustrates the operation envelope, 
         FIG.  6    illustrates the motor controller for each drive. 
         FIG.  7    illustrates the difference running with over-speed protection and without over-speed protection. 
     
    
    
     The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG.  1    shows a wind turbine  100  (WTG) comprising a tower  101  and a rotor  102  with at least one rotor blade  103 . Typically, three blades are used, but a different number of blades can also be used. The blades  103  are connected with the hub  105 , which is arranged to rotate with the blades. The rotor is connected to a nacelle  104 , which is mounted on top of the tower  101  and being adapted to drive a generator situated inside the nacelle via a drive train. The rotor  102  is rotatable by action of the wind. The wind induced rotational energy of the rotor blades  103  is transferred via a shaft to the generator. Thus, the wind turbine  100  is capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades and, subsequently, into electric power by means of the generator. The generator is connected with a power converter. 
       FIG.  2    shows alternative wind turbines  100  configured as multi-rotor wind turbines. Multi-rotor wind turbines comprises a plurality of nacelles  104 . Here an example of 4 nacelles is shown, but in general two or more nacelles may be used in a multi-rotor turbine. The nacelles  104  can be supported, as illustrated in the upper drawing, via a tower  101  and support arms  106  extending outwardly from the tower  101  so that the nacelles are placed away from the tower and on opposite sides of the tower. In multi-rotor wind turbines, the yaw system can be placed at the tower for collective rotation of an arm structure, and/or as individual yaw systems for each nacelle. Another example of a multi-rotor structure is illustrated in the lower drawing, here the nacelles  104  are supported by angled towers  101  extending from a foundation  130 , e.g. a ground or floating foundation, so that two or more nacelles  104  are sufficiently separated from each other at a given height. Embodiments of the present invention may be used with multi-rotor wind turbines or single-rotor wind turbines. 
       FIG.  3    shows an embodiment of a yaw system in accordance with the present invention. In the illustrated example, the yaw system comprises a plurality of yaw drive actuators  301 . In other configurations, more or less yaw drive actuators may be used. Each yaw drive actuator  301  comprises a motor  302 , in this embodiment an electrical drive motor, and a pinion  304 . Additionally a gearing may be included. The pinion  304  is connecting the yaw drive actuator  301  and the yaw ring  305 . Further, the yaw drive actuator  301  comprises a variable frequency drive (VFD)  306 . 
     The motors  302  may be of the asynchronous induction motor type, each being powered by a separate variable frequency drive  306 , and enabling individual motor control. The frequency drives  306  are seen in  FIG.  3    to be clustered in a cabinet in the centre and being connected to the motor controller  307 , however the frequency drives  306  can be placed in other locations as well. 
     The motor  302  comprises an encoder, which is a position meter, detecting the position of the motor, and from the changes in the position, the motor speed can be derived. The encoder is used to detect the speed of the motor  302  and return the speed to the frequency drive  306 . 
     The encoder may be used for every motor  302  to detect the position and speed of the motor  302  and to ensure great load sharing, while avoiding overloading any of the motors  302 . 
     The motor controller  307  outputs the required motor torque reference  403  to the variable frequency drives  306 , and the motor controller  307  receives information about the motor speed either through communication with the encoder, the individual variable frequency drives  306  or through communication with the turbine controller  308 , which calculates the mean motor speed reference  402 . Further, the motor controller  307  receives signals from the main turbine controller  308  about when to yaw and in which direction based on input from the wind direction device  309 . 
     The turbine controller  308  may control the yaw system  300 , and the turbine controller  308  activates the motor controller  307  when yawing is needed. 
       FIG.  4    is a schematic illustration of an embodiment of the yaw control scheme. The yaw control comprises a centralized control structure, where a single motor controller  307  is operating all the yaw drive actuators  301  based on a requested motor speed reference  401  and a mean motor speed reference  402  of all the motors  302 , or of a selected subgroup of the motors  302 . 
     The selected subgroup of motors  302  might not include the fastest and/or the slowest motor  302  for increased robustness or the selected subgroup of motors  302  might not include motors  302  running faster than a high-speed threshold speed and/or slower than a low speed threshold speed. 
     Under normal operation the same torque, the required motor torque reference  403 , is requested for all yaw drive actuators  301  to ensure even load distribution. However, the required motor torque reference  403  for each individual yaw drive actuator may be distributed unevenly, if requested, for instance when passing a yaw segment split where a reduced torque may be desired or during self-check where each drive is tested individually. Also, if a motor  302  is overspeeding the required motor torque reference  403  can differ for the yaw drive actuator  301  with the overspeeding motor  302  relative to the other yaw drive actuators  302 . 
     The required motor torque reference  403  sent from the motor controller  307  to the yaw drive actuator is received by the variable frequency drives  306 . The variable frequency drives  306  then sends a motor signal  420  to the motor  302 , which is then applying the motor shaft torque  421  to the yaw system  300  and the pinion  304  to transfer the torque to the yaw ring  305 . 
     Based on the motor speed  422  from each motor  302 , the turbine controller  308  or the motor controller  307  calculates a mean motor speed reference  402  as a feedback signal. The motor controller may calculate it own feedback signal in a computing block separate from the control loop computing block. In addition, the motor speed for each motor  302  is sent as a feedback signal via the inner torque control loop  423  back to the variable frequency drive  306 . 
       FIG.  5    shows an example of an operation envelope  501  used for 4-quadrant control of the motors. The operation envelope  501  is a coordinate system where the x-axis is the mean motor speed reference  402  and the y-axis is the required motor torque reference  403  applied to the motors  302 . 
     The limited motor speed reference  407  ( FIG.  6   ) calculated by the motor controller  307  is not allowed to be higher than the maximum motor speed  506 . The maximum motor speed  506  is illustrated by the vertical curve part in the first quadrant  502 . 
     The required motor torque reference  403  calculated by the motor controller  307  is not allowed to be higher than the maximum torque  507 . The maximum torque  507  is illustrated by the horizontal curve part in the first quadrant  502 . 
     The curved part  508  of the curve in the first quadrant  502  is illustrating the relationship between speed and torque under consideration of the maximum power use reference  405 . When the motors are running with a high speed, the torque that can be applied is limited by the maximum power use reference  405 . 
     The third quadrant  503  is equivalent to the first quadrant  502 , only rotating the motors  302  in the opposite direction. 
       FIG.  6    illustrates a graphical illustration of an embodiment of the motor controller  307 . 
     The objective for the motor controller  307  is to determine the required motor torque reference  403  needed to yield the requested motor speed reference  401 . 
     The control strategy comprises two feedback control loops in a cascaded structure where both torque and speed control is used. The two feedback loops comprises an inner loop and an outer loop. The inner loop is shown on  FIG.  4    as an inner torque control loop, where it is illustrated to be handled by the variable frequency drive  306 , which receives the required motor torque reference  403  from the motor controller  307 . While, the variable frequency drive  306  is not shown on  FIG.  6    is placed between the motor controller  307  and the motor  302  as illustrated in  FIG.  4   . 
     The inner torque control loop  423  is handled by the variable frequency drive  306 , such 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 . 
     The outer feedback loop is illustrated in  FIG.  6    as the speed control loop  612  returning the mean motor speed reference  402  to the speed control unit  408 . The speed control unit receives the limited motor speed reference  407  as input signal and the mean motor speed reference  402  as a feedback signal and sends the required motor torque reference  403  as output to the motors  302 . 
     The speed control unit consists of a PI controller  409 , an over-speed damping function  450  and a torque limiter  404 . 
     The PI-controller  409  is a proportional-integral controller used for speed control, since it yields unity DC-gain and great disturbance rejection. The speed control loop  612  refers the mean motor speed reference  402  back to the speed control unit  408 . The mean motor speed reference  402  is subtracted from the limited motor speed reference  407  to give an error signal  424  as input to the PI controller  409 , and the PI controller  409  provides the calculated motor torque reference  410  as output. The PI control can also be a PID control, but in the embodiment described, the derivative (D) part in the PID is zero. 
     The over-speed damping function  450  receives the calculated motor torque reference  410  as input from the PI-controller  409  and an actual motor speed reference  451  from each motor  302 . In case over-speed is detected for a motor  302 , the over-speed damping function  450  reduces the torque reference and sends a reduced motor torque reference as an output signal to the torque limiter  404  for the specific motor that is overspeeding. If a motor is not overspeeding, then the output signal is the calculated motor torque reference  410  received from the PI controller. 
     The torque limiter  404  limits the torque to the maximum torque  507  during operation in quadrant  1  and  3  in  FIG.  5   . The torque limiter  404  receives an output signal  452  from the over-speed damping function  450  and delivers the required motor torque reference  403  as an output signal. Preferable, the torque limiter  404  sends the same required motor speed reference  403  to all yaw drive actuators  301 , but it can, in case of a motor overspeeding or other special cases, send a different required motor speed reference  403  to the individual yaw drive actuators  301 . 
     The dynamical speed limiter  406  is used to limit the speed reference determining the limited motor speed reference  407 . The requested speed reference  401  is received from the turbine controller  308 , but may be reduced due to power limitation according to the maximum power use reference  405 . Further, the speed can be reduced in the speed saturation routine  615  to not exceed the maximum power speed and the speed ramp routine  616  ensures not to accelerate the motors to exceed a maximum speed change rate. 
     A feedback of the required motor torque reference  403  for the yaw drive actuators running normally is also used by the dynamic speed limiter  406  after going through a low-pass filter  618  and a direction saturation filter  617 , which ensures the feedback signal has a minimum numerical value and is not zero. 
       FIG.  7    is illustrating the advantage of using over-speed protection.  FIG.  7   a - b    illustrates the method using over-speed protection.  FIG.  7   a    illustrates motor speed using over-speed protection and  FIG.  7   b    illustrates the resulting motor torque. 
       FIG.  7   c - d    illustrates not using over-speed protection.  FIG.  7   c    illustrates the motor -  speed not using over-speed protection and  FIG.  7   d    illustrates the resulting motor torque. 
       FIGS.  7   a  and  7   c    illustrates the speed reference  701  the motor receives as input and the speed  702  of the motors, which is engaged with the yaw ring. While the speed  703  and  704  are from two motors not engaged with the yaw ring. These two motors are therefore increasing the speed rapidly.  FIG.  7   c    illustrates that the speed increases without over-speed protection while in  FIG.  7   a    illustrates that the over-speed protection cuts of the speed.  FIG.  7   d    illustrates that all motors applies the same torque  705  when no over-speed protection is used, while in  FIG.  7   b    it is illustrated that when over-speed protections is used the two motor not engaged with the yaw ring applies a reduced torque  707 ,  708 , while the motors engaged with the yaw ring applies the requested torque  706 . 
     Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.