Patent Publication Number: US-2022228556-A1

Title: Method of shutting down a wind turbine

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method of shutting down a wind turbine. 
     BACKGROUND OF THE INVENTION 
     Conventionally, in the event of an emergency, such as a high yaw error, a wind turbine is shut down by pitching out the wind turbine blades against the wind so as to immediately slow the rotor. 
     Such a method can result in large asymmetric loads being experienced by the blades. 
     Accordingly, there is a need for a method of shutting down a wind turbine that avoids or mitigates such loads. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides a method of shutting down a wind turbine, the wind turbine comprising a rotor with a plurality of blades; and a generator system coupled to the rotor. The method comprises: operating the generator system to generate electrical power and apply a load torque to the rotor; controlling the electrical power generated with a power reference signal; determining that a shutdown of the wind turbine is required; in response to the determination that a shutdown of the wind turbine is required, changing the power reference signal so as to increase the electrical power generated thereby slowing the rotor; determining that a speed of the rotor has reduced below a threshold; and in response to the determination that the speed of the rotor has decreased below the threshold, changing a pitch of the blades to further slow the rotor. 
     Controlling the electrical power generated indirectly controls the load torque applied to the wind turbine because there is a relationship between the generated electrical power and the load torque applied by the generator. The threshold may be a predetermined threshold. 
     The generator system may comprise a generator and an electrical converter; the electrical power and load torque being produced by the generator; the electrical converter converting a frequency of the electrical power to generate a grid signal which is input to a grid; and the power reference signal controlling the electrical converter which in turn controls the electrical power and load torque produced by the generator. 
     The pitch of the blades may be kept substantially constant until the speed of the rotor has reduced below the threshold. 
     The pitch of the blades may be kept substantially constant as the power reference signal changes in response to the determination that the shutdown of the wind turbine is required. 
     The pitch of the blades may be kept substantially constant from the time of determination that the shutdown of the wind turbine is required until the speed of the rotor has reduced below the threshold. 
     The term “substantially constant” as used above may mean that the pitch changes by no more than a small amount, for example 2°, 5° or 10°. 
     The power reference signal may be changed by an amount which is determined, at least in part, in accordance with a load torque limit so that the load torque applied to the rotor does not exceed the load torque limit. 
     The power reference signal may be changed after determining that the speed of the rotor has reduced below the threshold, so as to decrease the electrical power generated. For instance the power reference signal may be reduced to cause the electrical power to reduce to zero. 
     Determining that a shutdown is required may comprise identifying an error condition. The error condition may be a yaw error condition. The threshold may be determined based on the error condition. In other words, the threshold may be set based on what is causing the shutdown to take place. 
     The threshold may be a percentage of an initial speed, for instance a speed of the rotor at the time of changing the power reference signal. Alternatively, the threshold may be an absolute speed. 
     Changing the pitch of the blades may slow the rotor to a stop. Alternatively, changing the pitch of the blades may slow the rotor to an idling speed. 
     The changing of the pitch of the blades to further slow the rotor may change the pitch by more than 30°, more than 50° or more than 70°. 
     A further aspect of the invention provides a wind turbine comprising: a rotor with a plurality of blades; a generator system coupled to the rotor; and a control system, wherein the control system is configured to: operate the generator system to generate electrical power and apply a load torque to the rotor; control the electrical power generated and the load torque with a power reference signal; determine that a shutdown of the wind turbine is required; in response to the determination that a shutdown of the wind turbine is required, change the power reference signal so as to increase the electrical power generated and the load torque, the increased load torque slowing the rotor; determine that a speed of the rotor has reduced below a threshold; and in response to the determination that the speed of the rotor has decreased below the threshold, change a pitch of the blades to further slow the rotor. 
     The control system may be further configured to: after the power reference signal has been changed, determine that a maximum time has been exceeded (for example the maximum time being measured relative to the time that the power reference signal has been changed, or relative to the time of determination that a shutdown of the wind turbine is required); and in response to the determination that the maximum time has been exceeded, change a pitch of the blades to slow the rotor. 
     The control system may be further configured to: monitor the speed of the rotor after the power reference signal has been changed to identify an abnormal behaviour (for instance the speed is increasing rather than decreasing); and in response to an identification of abnormal behaviour, change the pitch of the blades to slow the rotor. 
     A further aspect of the invention provides a computer-readable storage medium having computer-readable program code embodied therewith, the computer-readable program code executable by one or more computer processors to perform an operation for a wind turbine, the wind turbine comprising a rotor with a plurality of blades; and a generator system coupled to the rotor, wherein the operation comprises: operating the generator system to generate electrical power and apply a load torque to a rotor; controlling the electrical power generated and the load torque with a power reference signal; determining that a shutdown of the wind turbine is required; in response to the determination that a shutdown of the wind turbine is required, changing the power reference signal so as to increase the electrical power generated and the load torque, the increased load torque slowing the rotor; determining that a speed of the rotor has reduced below a threshold; and in response to the determination that the speed of the rotor has decreased below the threshold, changing a pitch of the blades to further slow the rotor. 
     A further aspect of the invention provides a method of shutting down a wind turbine, the wind turbine comprising a rotor with a plurality of blades; and a generator system coupled to the rotor, the method comprising: operating the generator system to generate electrical power and apply a load torque to the rotor; controlling the electrical power generated with a power reference signal; determining that a shutdown of the wind turbine is required; in response to the determination that a shutdown of the wind turbine is required, changing the power reference signal so as to increase the electrical power generated; and after the power reference signal has been changed, determining that a maximum time has been exceeded; and in response to the determination that the maximum time has been exceeded, changing a pitch of the blades to slow the rotor. 
     A further aspect of the invention provides a method of shutting down a wind turbine, the wind turbine comprising a rotor with a plurality of blades; and a generator system coupled to the rotor, the method comprising: operating the generator system to generate electrical power and apply a load torque to the rotor; controlling the electrical power generated with a power reference signal; determining that a shutdown of the wind turbine is required; in response to the determination that a shutdown of the wind turbine is required, changing the power reference signal so as to increase the electrical power generated; monitoring the speed of the rotor after the power reference signal has been changed to identify an abnormal behaviour; and in response to an identification of abnormal behaviour, changing the pitch of the blades to slow the rotor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of a wind turbine; 
         FIG. 2  is a schematic representation of a control system of the wind turbine; and 
         FIG. 3  illustrates various turbine parameters during a shutdown. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
       FIG. 1  illustrates, in a schematic perspective view, an example of a wind turbine  1 . The wind turbine  1  includes a tower  2 , a nacelle  3  at the apex of the tower, and a rotor  4  operatively coupled to a generator housed inside the nacelle  3 . In addition to the generator, the nacelle houses miscellaneous components required for converting wind energy into electrical energy and various components needed to operate, control, and optimize the performance of the wind turbine  1 . The rotor  4  includes a central hub  5  and a plurality of blades  6  that project outwardly from the central hub  5 . In the illustrated embodiment, the rotor  4  includes three blades  6 , but the number may vary, for example the rotor may have two or four blades. Moreover, the wind turbine comprises a control system. The control system may be placed inside the nacelle or distributed at a number of locations inside the turbine and communicatively connected. 
     The wind turbine  1  may be included among a collection of other wind turbines belonging to a wind power plant, also referred to as a wind farm or wind park, that serve as a power generating plant connected by transmission lines with a power grid. The power grid generally consists of a network of power stations, transmission circuits, and substations coupled by a network of transmission lines that transmit the power to loads in the form of end users and other customers of electrical utilities. 
       FIG. 2  schematically illustrates an embodiment of a control system  20  together with elements of the wind turbine. The rotor blades  6  are mechanically connected to an electrical generator  22  via a gearbox  23  with an input shaft and an output shaft  21 . In direct drive systems, and other systems, the gearbox  23  may not be present. The electrical power generated by the generator  22  is injected into a power grid  24  via an electrical converter  25 . The electrical generator  22  and the converter  25  may be based on a full scale converter (FSC) architecture or a doubly fed induction generator (DFIG) architecture, but other types may be used. 
     The control system  20  comprises a number of elements, including at least one main controller  200  with a processor and a memory, so that the processor is capable of executing computing tasks based on instructions stored in the memory. In general, the wind turbine controller ensures that in operation the wind turbine generates a requested power output level. This is obtained by adjusting the pitch angle and/or the power extraction of the converter  25 . To this end, the control system comprises a pitch system including a pitch controller  27  using a pitch reference signal  28 , and a power system including a power controller  29  using a power reference signal  26 . The power controller  29  controls the various electric components of the generator converter system in order to deliver the requested power, which in turn controls the torque of the generator  22  that is needed to extract the requested power by the rotor from the wind. 
     The torque of the generator  22  (referred to below as the generator torque) is applied to the output shaft  21  of the gearbox  23 . 
     The rotor blades  6  can be pitched by a pitch control mechanism. The rotor may comprise a common pitch system which adjusts all pitch angles on all rotor blades at the same time, as well as in addition an individual pitch system which is capable of individual pitching the rotor blades. The control system  20 , or elements of the control system  20 , may be placed in a power plant controller (not shown) so that the turbine may be operated based on externally provided instructions. In embodiments of the present invention the pitch is controlled based on a common pitch reference onto which the individual pitch reference is superimposed. In embodiments, the slowing of the rotor may be obtained by changing the common pitch reference. Also keeping the pitch of the blades constant may amount to keeping the common pitch reference constant 
     To ensure that the power reference signal  26  does not damage the power controller  29  or the converter  25 , the size of the power reference signal  26  is limited so that it cannot exceed a predetermined maximum value, such as a maximum voltage value. 
     The rotor  4  experiences aerodynamic torque and load torque that act in opposite directions. The aerodynamic torque is the result of wind acting on the rotor blades  6 . Changing the pitch of the blades changes the aerodynamic torque, by varying the angle of attack and thus changing the aerodynamic lift. The load torque acts on the rotor against the aerodynamic torque. The load torque has a component which originates from the generator torque applied to the output shaft  21  of the gearbox, and a component caused by friction in the gearbox  23  and bearings. 
     The difference between the aerodynamic torque and the load torque is the net torque applied to the rotor. If the net torque is zero, i.e. the aerodynamic and load torques are equal, then the speed of the rotor will not change. If the aerodynamic torque is greater than the load torque, the rotor will speed up. If the load torque is greater than the aerodynamic torque, the rotor will slow down. 
     As explained above, the generator torque produces most of the load torque applied to the rotor. Hence, requesting more power from the generator  22  results in a greater load torque being experienced by the rotor  4 . The relationship between torque and power is: P=ω×T, where P is power generated, T is the load torque and ω is the rotational speed of the rotor. 
     During normal operation of the wind turbine, the speed of the rotor is controlled using the pitch angle of the blades and/or the load torque, which are controlled by the pitch reference signal  28  and the power reference signal  26  respectively. 
     In a low wind speed operating region, the pitch reference signal  28  may be held constant, and the power reference signal  26  and rotor speed increased as the wind speed increases. In a high wind operation region, the power reference signal  26  and rotor speed may be held constant (at a rated power and a rated speed respectively), and the pitch reference signal  28  adjusted to respond to the wind speed changing. 
     In an emergency situation, the wind turbine may be conventionally shut down by pitching out the blades. However, pitching out the blades results in a large variation in the distribution of load experienced across the surface of the blade. This is because different portions along the length of the blade experience different lift coefficient profiles as the blade completes a revolution. When the angle of attack is changed during a pitching action, the lift coefficient profile for each portion of the blade changes differently, meaning that across the whole of the surface of the blade, the change in lift coefficient experienced is not uniform. Therefore, a large variation in the distribution of load across the surface of the blade may be experienced. This is undesirable because it may cause damage or fatigue to various parts of the wind turbine such as bearings etc. 
     The invention avoids or mitigates these problems by determining that a shutdown of the wind turbine is required: then in response to the determination that a shutdown of the wind turbine is required, changing the power reference signal  26  so as to increase the electrical power generated thereby slowing the rotor; determining that a speed of the rotor has reduced below a speed threshold; and in response to the determination that the speed of the rotor has decreased below the threshold, changing a pitch of the blades (by increasing the pitch reference signal  28 ) to further slow the rotor. 
     The increase in the power reference signal decreases the rotor speed using the principles described above, i.e. increasing the generated power which in turn increases the load torque applied to the rotor. The aerodynamic torque does not change because the wind speed does not vary much, and the blade pitch does not change. Therefore, the net torque causes the rotor to slow down. 
     A wind direction sensor  30  (such as a wind vane) may be used to determine that a shutdown of the wind turbine is required due to a high yaw error. The wind direction sensor  30  determines the angle of the wind relative to the nacelle. If the magnitude of the angle increases above a threshold angle, then it is determined that there is a high yaw error which triggers a shutdown. 
     Other possible reasons for determining that a shutdown of the wind turbine is required include: extreme wind events; failure of a sensor, actuator or component; or loss of connection to the grid. 
       FIG. 3  illustrates this technique, showing an example of system parameters when the invention is employed. 
     Normal operation of the wind turbine takes place until time t 0 . During this normal operation period before time t 0 , the electrical power generated is controlled as described above using the power reference signal  26  and/or the pitch reference signal  28 . 
     At time t 0 , a condition changes. For example, this condition may be that the wind direction changes so that the wind is no longer aligned with the rotor, and instead the wind is coming into the turbine sideways. Thus, at time t 0  the yaw error signal begins to increase as shown at  40 . 
     As the yaw error increases, the power reference signal  26 , power, pitch reference signal  28  and load torque all decrease as shown at  41 ,  42 ,  43  and  44  respectively. The generator speed dips and then recovers as shown at  45 . 
     At time t 1 , a determination is made by the controller  200  that a shutdown of the wind turbine is required, since the yaw error signal has crossed a threshold  50 . In response to the determination that a shutdown of the wind turbine is required, the power reference signal  26  is increased (as shown at  51 ) so as to increase the electrical power generated (as shown at  52 ). The wind speed does not vary much, and the pitch of the blades does not change (as shown at  54  by the constant pitch reference signal), so the aerodynamic torque does not change. The increased power requirement causes the load torque to increase (as shown at  55 ). Since the aerodynamic torque has not changed, there is now a net torque being applied which causes the rotor to slow (as shown at  53 ). 
     The power reference signal  26  is increased until it reaches a power offset level  60  at time t 2 . The power offset level  60  may be determined in a variety of ways. 
     In one example the power reference signal  26  may be changed by a fixed and predetermined amount (for instance 1 MW). 
     In another example the power reference signal  26  may be changed by a fixed percentage (for instance 50%, 100% or 200%). 
     In other examples, the power reference signal  26  may be changed by an amount which is determined on the basis of a variety of parameters such as load torque, blade pitch and power. 
     Optionally the power reference signal  26  may be changed by an amount which is determined, at least in part, in accordance with a load torque limit so that the load torque applied to the rotor does not exceed the load torque limit as the speed gets low (torque=power/speed). So for example if increasing the power reference signal  26  by 1 MW would cause the load torque to exceed the load torque limit, then the power reference signal  26  is increased by a smaller amount which keeps the load torque below the load torque limit. 
     Optionally the power reference signal  26  may be changed by a proportional-integral (PI) controller trying to reach a reference speed (for instance the speed threshold  70  mentioned below) using power with a load torque limit and relevant integrator wind up protection. 
     Alternatively the controller  200  may look at generator speed, tower oscillations, blade loads etc. and control the power reference signal  26  accordingly. 
     Between time t 2  and time t 3 , the power reference signal  26  is held constant (as shown at  61 ) so the power is also held constant (as shown at  62 ). Since the pitch reference signal  28  is also held constant during this phase (as shown at  63 ) the load torque starts to reduce slightly (as shown at  64 ). 
     At time t 3 , the controller  200  determines that the speed of the rotor has reduced below a speed threshold  70 . This determination may be made by monitoring the speed of the rotor or the generator with one or more sensors. 
     The determination that the speed of the rotor  4  has reduced below the speed threshold  70  triggers the pitch reference signal to be increased (as shown by ramp  71 ) so that the pitch of the blades is increased until it reaches 90°. This reduces the aerodynamic torque so as to further slow the rotor as shown at  72 . In this case the rotor is slowed to a complete stop. In other cases, the rotor may be slowed down to an idling speed. 
     The speed threshold  70  may be a percentage of an initial speed of the rotor at time t 1 , or it may be an absolute value. The threshold speed  70  may be predetermined or it may be determined by the controller  200  during the aforementioned shutdown process. 
     Note that the pitch reference signal  28  is illustrated as constant between time t 1  and t 3 . Thus, the pitch of the blades is kept substantially constant as the power reference signal  26  changes between time t 1  and time t 2 , and it is also kept substantially constant from the time of determination that the shutdown of the wind turbine is required (time t 1 ) until the speed of the rotor has reduced below the threshold (time t 3 ). 
     Note however that the individual pitch signals for each blade may vary slightly (for instance by a few degrees) during this time period between time t 1  and time t 3 , whereas the collective pitch angle may vary by a smaller amount. The change of pitch angle caused by the ramp  71  on the other hand is much greater (for instance it may be more than 40° or more than 60°). Thus, the term “substantially constant” as used above may mean that the pitch changes by no more than a small amount, for example 20, 5° or 10°. 
     At time t 3 , the power reference signal  26  and power are reduced (as shown at  72 ,  73  respectively) and the load torque reduces (as shown at  74 ). As the generator speed decreases, the generator is unable to generate as much power due to the P=ω×T relationship. 
     As the power reference signal  26  starts to increase at time t 1 , the speed should decrease fairly rapidly (as shown at  53 ). If the speed does not reduce as expected, then it may be desirable to change the pitch of the blades to shut down the rotor even if the speed has not reduced below the speed threshold  70 . This may be dealt with in a number of ways. 
     In a first example, the controller  200  may be configured to determine that a maximum time has been exceeded; and in response to the determination that the maximum time has been exceeded, change a pitch of the blades to slow the rotor. For example the maximum time may be measured relative to the time t 1  that the power reference signal has been increased, which in this case is also the time t 1  of determination that a shutdown of the wind turbine is required. 
     In a second example, the controller  200  may be configured to monitor the speed of the rotor after the power reference signal  26  has been increased at time t 1 , to identify an abnormal behaviour. For instance the speed may be increasing rather than decreasing. In response to an identification of such abnormal behaviour, the pitch of the blades may be changed immediately to slow the rotor, rather than waiting for the speed to drop below the speed threshold  70 . 
     The procedure described above has the advantage of limiting the mechanical forces acting on the wind turbine. After the power reference signal has been increased at t 1 , the absolute values of both the tilt moment and the yaw moment on the main bearing decrease significantly, reducing the mechanical loads on the wind turbine. 
     Other benefits include: reducing the negative thrust peak leading to lower tower loads and lower negative blade flap loads; and a reduction of the required maximum pitch rate (i.e. the rate of change of pitch angle caused by the ramp  71 ) leading to lower loads on the pitch control system. 
     Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.