Stopping a wind turbine rotor based on a stored pitch angle signal

Abstract

The invention relates to controlling a wind turbine during a stopping process in response to a rotor stop signal. The rotor stopping process comprises the following steps: generating a stored pitch angle signal by storing the pitch angle signal for at least a portion of a full rotation of the rotor before receiving the rotor stop signal, and determining at least one periodic component of the stored signal. Controlling a rotor blade of the wind turbine towards a feathered position using a pitch control signal comprising the at least one periodic component.

Description

Technical Field

[0001] The present invention relates to controlling a wind turbine based on a stored pitch angle signal and, in particular, to controlling a wind turbine to stop a rotor in response to a rotor stop signal. Background Art

[0002] In a pitch controlled wind turbine, the pitch of the wind turbine blades is adjusted in response to wind conditions in order to obtain a desired energy production. As the wind turbine rotor rotates during operation, the load on each wind turbine blade varies, for example due to wind shear, tower passage, turbulence, etc. This may cause an asymmetric load distribution between the wind turbine blades. This is undesirable as it may result in high loads on the rotor, drive train, etc. In order to avoid this, an independent pitch control strategy is sometimes used. According to the independent pitch control strategy, the pitch angles of the individual blades are adjusted in order to take into account the conditions of the azimuthal position of each blade.

[0003] When a stop command or shutdown command for a pitch-controlled wind turbine is generated and a safety operation is initiated, for example due to an emergency, the wind turbine blades are generally moved towards the feathered position. However, for wind turbines in which the wind turbine blades are pitched independently, the pitch angles of the wind turbine blades are not identical when receiving the stop command or shutdown command. Instead, the pitch angle of each wind turbine blade has been adjusted in a manner that takes into account the prevailing conditions at the exact position of the blade in question. In this case, if all wind turbine blades are moved towards the feathered position as quickly as possible when receiving the stop command or shutdown command, the mutual difference in pitch angles will be maintained as the wind turbine blades continue to rotate in the rotor plane while moving towards the feathered position. As a result, the positive effect of the azimuth-dependent pitching is lost. In addition, the turbine safety system that performs the shutdown can operate with reduced sensing and drive capabilities, further complicating the application of independent pitching during the stop process.

[0004] A solution to this problem is disclosed in EP2290232 A1 (MHI) wherein, when a wind turbine parking command is received, the pitch angles of the wind turbine blades are first adapted and once adapted, the blades are moved towards a feathered position.

[0005] Furthermore, WO14173417A1 (Vestas) discloses a solution to ensure pitch alignment as a first step of blade feathering. In this disclosure, the timing of alignment is addressed.

[0006] WO 2015 / 003710 A1 (Vestas) discloses a solution in which, after receiving a shutdown command, the independent pitch is maintained to a certain extent. Here, the safety control system takes over the pitching of the blades and maintains the independent pitch in a manner that simulates the independent pitching with a constant pitch rate, thereby also obtaining a certain degree of asymmetric load relief during the feathering process.

[0007] WO 2006 / 007838 A1 (Vestas) also discloses a solution in which independent pitching continues during the feathering process.

[0008] It is in this context that the present invention is designed. Summary of the invention

[0009] It is an object of embodiments of the present invention to improve the handling of asymmetric loads acting on components of a wind turbine during a shutdown process.

[0010] Accordingly, in a first aspect, a method of controlling a wind turbine in response to a rotor stop signal is provided, the wind turbine comprising a rotor with one or more pitch-adjustable rotor blades; wherein the method comprises the following steps:

[0011] generating a stored pitch angle signal by storing the pitch angle signal for each of the pitch-adjustable rotor blades for at least a portion of a full rotation of the rotor prior to receiving the rotor stop signal;

[0012] determining at least one periodic component of the stored signal before or upon receiving the rotor stop signal;

[0013] Upon receiving the rotor stop signal, controlling the pitch adjustable rotor blades toward a feathered position using the pitch control signal including the at least one periodic component.

[0014] In this way, during the stopping process of the pitch-adjustable rotor blades moving towards the feathered position, the independent pitch change activated before receiving the rotor stop signal is maintained at least to a certain extent. Although the prior art also provides solutions for alleviating asymmetric loads on the rotor or tower during the stopping process, the advantage of the present invention is that it integrates the load relief pitch action used before stopping the rotor and applies this integrated load relief pitch action during the stopping process. The load relief pitch action used before stopping the rotor is obtained by determining at least one periodic component of the stored signal. The application of the integrated load relief pitch action is accomplished by including the pitch control signal containing the at least one periodic component in the applied pitch signal of the pitch-adjustable rotor blade. In this way, an approximation of the optimal trajectory can be obtained without increasing the complexity, for example having blade load sensors as part of the stopping (safety) system.

[0015] In a further aspect, the invention relates to a wind turbine comprising a control system arranged to perform the method according to the first aspect, and to a computer program product comprising software code adapted to control the wind turbine when executed on a data processing system, the computer program product being adapted to perform the method of the first aspect.

[0016] The computer program product may be provided in a computer-readable storage medium or may be downloaded from a communication network. When loaded onto a data processing system, the computer program product includes instructions to cause the data processing system, for example in the form of a controller, to execute the instructions.

[0017] In general, a controller may be a unit or collection of functional units including one or more processors, one or more input / output interfaces, and a memory capable of storing instructions executable by the processors.

[0018] In general, the various aspects of the invention may be combined and coupled in any possible way within the scope of the invention.These and other aspects, features and / or advantages of the invention will become apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0020] Figure 1 An example of a wind turbine is shown schematically;

[0021] Figure 2 schematically illustrates a controller unit implemented to control a wind turbine in response to a rotor stop signal;

[0022] Figure 3Aspects involving determining a periodic component of a stored signal are shown;

[0023] Figure 4 shows a pitch signal obtained using an embodiment of the present invention;

[0024] Figure 5 An embodiment is shown in which the safety controller is implemented as a stop controller arranged to pitch the blades individually at a plurality of preset substantially constant pitch rates. DETAILED DESCRIPTION

[0025] Figure 1 An example of a wind turbine 1 is shown in a schematic diagram. The wind turbine 1 includes a tower 2, a nacelle 3 placed at the apex of the tower, and a rotor 4 operably coupled to a generator housed in the nacelle 3. In addition to the generator, the nacelle houses various components required to convert wind energy into electrical energy and various components required to operate, control and optimize the performance of the wind turbine 1. The rotor 4 of the wind turbine includes a central hub 5 and a plurality of blades 6 protruding outward from the central hub 5. In the embodiment shown, the rotor 4 includes three blades 6, but the number thereof may vary. In addition, the wind turbine includes a control system. The control system may be placed in the nacelle or distributed at multiple locations within (or outside) the turbine and is communicatively connected. The rotor blades are pitch-adjustable, represented here by a pitch angle θ. The rotor blades may be adjusted according to a collective pitch setting, wherein each blade is arranged to the same pitch value. In addition, the rotor blades may be adjustable according to an independent pitch setting, wherein each blade may provide an independent pitch set point. The rotor blades move in the rotor plane, where the angular position of each blade in the rotor plane is represented by the azimuth angle express.

[0026] The rotor blades 6 may be mechanically connected to the generator via a gearbox. In direct drive systems and other systems, the gearbox may not be present. The electrical energy generated by the generator is injected into the grid via a converter. The generator and converter may be based on a full-scale converter (FSC) architecture or a double-fed induction generator (DFIG) architecture, but other types may also be used.

[0027] The control system comprises a plurality of elements, including at least one main controller having a processor and a memory, so that the processor can perform computing tasks based on instructions stored in the memory. In general, the wind turbine controller ensures that the wind turbine generates the desired power output level during operation. This is achieved by adjusting the pitch angle and / or the power extraction of the converter. To this end, the control system comprises a pitch system, which includes a pitch controller using a pitch reference, and a power system, which includes a power controller using a power reference. The power controller controls various electronic components of the generator converter system in order to transmit the desired power, hereinafter controlling the torque of the generator, which is required to extract the desired power from the wind through the rotor. The wind turbine rotor includes rotor blades that can be pitched by a pitch mechanism. The control system or elements of the control system can be placed in an electric field controller (not shown) so that the turbine can be operated based on instructions provided externally.

[0028] Figure 2 A controller unit 20 is schematically shown. An embodiment of the present invention relates to controlling a wind turbine in response to a rotor stop signal. The controller unit is implemented with a signal input 21 arranged to receive such a rotor stop signal. The stopping process is obtained by controlling the pitch-adjustable rotor blades towards a feathered position based on a pitch signal 28 to be sent to an actuator of the pitch-adjustable rotor blades. The pitch signal 28 uses a pitch control signal 22 obtained by using an embodiment of the present invention. It should be noted that the term stopping the rotor should be interpreted broadly to include reducing the rotor speed to a lower speed, including a lower control speed and an idling speed, as well as reducing the rotor speed to a complete stop. The rotor stop signal may be a shutdown signal. The rotor speed is reduced (stopped) by adjusting the rotor blades, i.e. the pitch angle θ of the rotor blades towards the feathered position. It should be noted here that the feathered position with respect to the wind turbine blades refers to a preset angle selected in the range of 85° to 120° depending on the mechanical design of the pitch system. The specific feathering position of the wind turbine is a design choice.

[0029] The stored pitch angle signal 23 is generated by storing a pitch angle signal for each of the pitch adjustable rotor blades for at least a portion of a full rotor rotation while the turbine is operating prior to receiving the rotor stop signal, ie during a period prior to receiving the rotor stop signal.

[0030] Before or upon receipt of the rotor stop signal, at least one periodic component 24 of the stored signal is determined. The calculation unit may determine the periodic component continuously during normal operation or may determine it upon receipt of the rotor stop signal 21 .

[0031] Then, upon receipt of the rotor stop signal 21 , a pitch control signal 25 for each of the rotor blades comprising the at least one periodic component is determined and used to move the pitch angle towards a feathered position to reduce the rotor speed.

[0032] In general, a signal component is understood to be periodic with a period T if the following conditions are met:

[0033] x(A)=x(A+nT), where n is an integer, that is, n∈[0,1,2,...].

[0034] The constant signal x(A)=k is understood to be periodic since it is independent of the parameter A, so for all n, x(A)=x(A+nT)=k.

[0035] In an embodiment, the at least one periodic component is determined based on a Fourier transform or an equivalent frequency estimation algorithm. By Fourier transform is also understood a discrete Fourier transform and an implementation of a discrete Fourier transform including a Goertzel algorithm for evaluating independent terms of the discrete Fourier transform. Expressions based on the Fourier transform also include the application of an inverse transform.

[0036] Advantageously, the stored pitch signal is the rotor azimuth angle, ie the angle φ of the blade in the rotor plane (see Figure 1 ), a pitch angle signal as a function of . In particular, it may be advantageous to store the pitch signal as pitch angle signals at equidistant azimuth angles, as this allows the use of a discrete Fourier transform. When using azimuth as the sampled variable, frequency domain filtering using a discrete Fourier transform (or equivalent) is accomplished by replacing time with azimuth. This is advantageous because the signal synthesis is independent of the rotor rate, while still being able to correctly synthesize the nP component.

[0037] The pitch signal may be stored in any suitable manner, including as a stored pitch angle reference or a stored pitch angle position.

[0038] It may be advantageous not to apply too many rotations when determining the periodic component. In this way, smearing of the signal can be avoided and only the periodic content in the period shortly before the rotor stop signal is obtained.

[0039] In an embodiment, the pitch signal is stored in such a way that the fraction of a full rotor revolution is between 0.5 and 1.5 revolutions. However, it may be advantageous to derive the 1P signal component using at least one revolution. In an embodiment, the fraction of a full rotor revolution is substantially one full rotor revolution.

[0040] Storing the pitch angle signal of each of the pitch-adjustable rotor blades to generate the stored pitch angle signal may be a continuous storage of the pitch angle signal for at least the last part of a full rotation of the rotor, for example by applying a circular buffer. The continuous storage may be continuous in the sense that a (substantially) complete series of samples of the part of the full rotation of the rotor under investigation is stored.

[0041] An example embodiment is explained below. This example embodiment is based on a stored pitch signal as a function of the rotor azimuth angle. Although as mentioned above it is advantageous to base the stored pitch signal on the rotor azimuth angle, it may also be based on an alternative variable, for example on time.

[0042] The pitch position is based on the rotor azimuth angle of a predetermined sequence of N equally spaced azimuth angles. In summary:

[0043]

[0044] Furthermore, the pitch angle is defined as a function of the azimuth angle:

[0045]

[0046] That is, the stored pitch angle signal is represented as a series of N pitch angle samples for a given rotor azimuth position.

[0047] The at least one periodic component in the stored signal is periodic in azimuth. In this embodiment, the pitch angle sequence is subjected to discrete Fourier transform to obtain the periodic component:

[0048]

[0049] Therefore, the nP component of the pitch angle is given by the element θ ±n±pN describe.

[0050] The pitch angle, i.e. the pitch control signal, can be synthesized into the frequency component θ k Apply the inverse transform and find the given azimuth The pitch angle is obtained as:

[0051] Mathematically, the pitch angle of the pitch control signal is expressed as:

[0052]

[0053] In general, the pitch angle of the pitch control signal can be determined based on a selected number of periodic components, such as by a finite number of selected periodic components. These components can be selected based on knowledge of the components that are generally most prominent in the pitch angle signal. In one embodiment, the selected number includes at least zero-order and first-order periodic components. An example of using zero-order and first-order periodic components is provided below.

[0054] In this example of implementation, the pitch control signal is determined based on the 0P (zero order) and 1P (first order) components of the pitch signal stored for the preceding rotor full revolution. The 0P component helps to take into account any (quasi) static offset that may be applied. The 1P component may take into account any wind field-based cyclic pitch action that may exist.

[0055] In this case, for each azimuth step k, the Fourier coefficient Θ is calculated 0 and θ 1 :

[0056]

[0057]

[0058] After shutdown, the pitch sequence in It is synthesized for the nth pitch sample after shutdown and can be derived as:

[0059]

[0060] This results in:

[0061]

[0062] in, and denote the real and imaginary parts of the complex number x, respectively.

[0063] In a specific implementation, this formula means that the coefficients Θ0 and Θ1 are updated at each azimuth step until shutdown, at which point the updating is interrupted. During shutdown, the coefficients Θ0 and Θ1 are updated at each azimuth step. 0 and θ 1 and the cosine and sine terms, with time step n, calculate the resultant pitch as described above If needed, trigonometric terms can be precomputed and resolved using a simple table lookup.

[0064] An important advantage of embodiments of the present invention is that the pitch control signal can be based on a stored pitch signal and applied during shutdown without further input. In embodiments, this can be used to generate the pitch control signal without regard to sensor input. For example, in embodiments, the pitch control signal can be generated without regard to blade load sensor signals or other load sensor signals. This is advantageous because the system component used to handle shutdown can be a system component that operates in a safety domain, and typically load sensors are not placed in a safety domain.

[0065] As described above, the pitch-adjustable rotor blades are controlled toward the feathered position using the pitch control signal 22 including the at least one periodic component. The actual pitch signal 28 sent to or used by the pitch actuator need not be identical to the pitch control signal 22, and in many cases will not be. The actual pitch angle or pitch trajectory used to stop a full rotation of the rotor may include an additional signal that is combined with the pitch control signal to move the pitch-adjustable rotor blades toward the feathered position. This combination is Figure 2 2 is shown by a controller element 27 which represents a further controller element for determining a further pitch signal for stopping the rotor.

[0066] In this way it can be ensured that any asymmetric load on the rotor is relieved during the stopping process, but further control of the stopping process is also contemplated. Such further control may include control of the average pitch rate.

[0067] Figure 3 Further elements of embodiments of the invention are shown.

[0068] Figure 3 A shows a schematically stored pitch angle signal, θ represents equidistant azimuth angles from 0 to 2π. The figure schematically shows the pitch change applied in the pitch change before stopping the rotor.

[0069] Figure 3 B shows the Figure 3 The pitch angle signal of A is discretely transformed with respect to the azimuth angle to determine the periodic component of the stored signal. The resulting spectrum is as follows: Figure 3 C. It can be seen that the average value (0P) and the first harmonic (1P) are the main components. Figure 3 In C, only the 0P component and the 1P component are selected. This corresponds to frequency domain filtering.

[0070] exist Figure 3 D, the pitch control signal including the at least one periodic component is obtained by Figure 3 The signal at C is obtained by applying an inverse discrete Fourier transform. This yields the pitch as a function of azimuth as shown in the figure.

[0071] Figure 4 Shown based on Figure 3 B (solid line) and Figure 3 C (marked line). It can be seen that the integrated signal based only on the 0P and 1P components approximates the ideal signal, but without the higher frequency components.

[0072] The control system that handles controlling the pitch-adjustable rotor blades toward the feathered position may be based on a general-purpose or standard industrial computing device, but it may advantageously be based on a safety computing device instead; that is, on a safety controller. Although the safety controller in an embodiment may be a safety-certified controller, it need not be safety-certified, but rather a controller based on rigorously tested software and hardware that meets the same or similar requirements as required for certification. This type of controller may also be referred to as a safety-certifiable controller. Often, the safety-related parts of the control system may be built according to the principles given in recognized functional safety standards. Such standards include, for example, ISO 13849, IEC 61508, and IEC 62061. In addition to meeting specific, quantitative requirements for reliability and diagnostic coverage, systems built according to these standards are assumed to be immune to systematic failures, such as software defects. The control system can therefore be divided into a normal control domain that uses standard computing devices to perform computing tasks and a safety control domain that uses safety-related computing devices to perform computing tasks.

[0073] In an advantageous embodiment, the rotor stop signal is generated by a safety controller. In this way, a high reliability of the safety controller for controlling the stop of the rotor can be ensured.

[0074] In an advantageous embodiment, the generation of the rotor stop signal is triggered by the safety controller when a fault signal occurs. The fault signal may originate from a sensor also located in the safety domain, but this is not necessary.

[0075] Figure 5 Embodiments are shown, wherein the safety controller is implemented as a stop controller arranged to individually pitch the blades at a plurality of preset substantially constant pitch rates, the pitch rates comprising a first pitch rate and a second pitch rate lower than the first pitch rate, and wherein the safety controller is further arranged to:

[0076] Obtaining a desired pitch angle 52 for the stopping process;

[0077] adding an envelope band 54 to the desired pitch angle by adding positive and negative values, thereby defining upper and lower band values; and

[0078] Among them, the safety controller is arranged as follows:

[0079] moving the pitch-adjustable rotor blade at a pitch rate selected from the plurality of preset pitch rates until the pitch angle signal is outside a pitch angle value defined by an envelope band,

[0080] If the pitch angle signal is greater than the upper band value, changing the selected pitch rate to a lower pitch rate among the plurality of preset pitch rates, and

[0081] If the pitch angle signal is less than the lower band value, the selected pitch rate is changed to a higher pitch rate among the plurality of preset pitch rates.

[0082] Figure 5 A schematic embodiment of the stopping process is shown, wherein at t 0 The rotor stop signal is received at t. The pitch angle signal stored during at least a portion of a full rotation of the rotor is generated by storing a pitch angle signal 51 for each of the pitch adjustable rotor blades in a period 50 prior to receiving the rotor stop signal. As described above, before or upon receiving the rotor stop signal, one or more periodic components of the stored signal are generated and used to control the rotor blades toward the feathered position. In the figure, at t 0 Three varying pitch trajectories 51 are shown in the preceding period. 0 In the following time periods 57, 58, only a single pitch trajectory and a single envelope band are shown. This is for illustrative reasons, since there may be more envelope bands, since a common envelope or an independent envelope for each blade may be envisaged.

[0083] During the stopping process, the desired pitch angle (desired pitch trajectory) of the stopping process is obtained. The desired pitch angle is defined by the superposition of the pitch ramp and the pitch control signal containing the at least one periodic component, resulting in the pitch control signal 52 being overlaid on the pitch ramp 53. Figure 5 In the example of , the pitch ramp 53 comprises a first phase 57 with a high average pitch rate and a second phase 58 with a lower average pitch rate.

[0084] The figure further shows the rotor speed, showing that the turbine is operating at rated speed until t 0 The rotor stop signal is received at , after which the rotor speed decreases towards zero. The rotor speed only represents a general characteristic of the rotor speed, not the exact rotor speed that matches the pitch angle shown.

[0085] The stop controller comprises a controller unit implemented to obtain a desired pitch angle 52 for the stop process and to add an envelope band 54 to the desired pitch angle by adding positive and negative values, thereby defining an upper band value and a lower band value. After defining the band, the stop controller is arranged as follows:

[0086] moving the pitch-adjustable rotor blade at a pitch rate 55, 56 selected from said plurality of preset pitch rates until the pitch angle signal is outside a pitch angle value defined by an envelope band,

[0087] If the pitch angle signal is greater than the upper band value, changing the selected pitch rate to a lower pitch rate among the plurality of preset pitch rates, and

[0088] If the pitch angle signal is less than the lower band value, the selected pitch rate is changed to a higher pitch rate among the plurality of preset pitch rates.

[0089] exist Figure 5 In the example of , the stop controller is arranged to pitch the blades at two preset pitch rates, a first high pitch rate 55 and a second low pitch rate 56. The pitch rates may be set according to a given design. The first pitch rate may be between 3 and 15° / s, while the second low pitch rate may be between 0 and 5° / s.

[0090] In t 0 Upon receiving the rotor stop signal at , the pitch blades are moved at a high pitch rate 55. When moving the blades towards feathering, the pitch angle signal is compared to the envelope band, and once it is detected that the pitch angle signal is above the upper band value, the selected pitch rate is changed to a low pitch rate 56. The comparison is maintained during the stopping process, and once it is detected that the pitch angle signal is less than the lower band value, the low pitch rate 56 is changed to a high pitch rate 55. This logic remains operational during the stopping process.

[0091] During the stopping process, the pitch ramp can be used to control the reduction of rotor speed and thrust problems, while the independent pitch angle settings according to embodiments of the present invention can be used to alleviate asymmetric loads on the rotor and tower vibrations during the stopping process.

[0092] Figure 5 Thus an embodiment is shown wherein the shutdown controller takes as input a shutdown pitch reference signal and determines a discrete pitch actuation signal such that a resulting pitch trajectory using a plurality of preset substantially constant pitch rates approximates the shutdown pitch reference signal.

[0093] Example embodiments of the present invention are described for illustrative purposes only, and not to limit the scope of the invention as defined in the appended claims.

Claims

1. A method for controlling a wind turbine in response to a rotor stop signal, the wind turbine comprising a rotor with one or more pitch-adjustable rotor blades; wherein: The method comprises the following steps: generating the stored pitch angle signal by obtaining, before receiving the rotor stop signal, for each of the pitch-adjustable rotor blades, a fractional storage pitch angle signal of a full rotor rotation of between 0.5 and 1.5 revolutions, such that smearing of the signal is avoided and only periodic content in a period shortly before the rotor stop signal is obtained; determining at least one periodic component of the stored signal before or upon receiving the rotor stop signal; Upon receiving the rotor stop signal, controlling the pitch adjustable rotor blades toward a feathered position using the pitch control signal including the at least one periodic component.

2. The method according to claim 1, wherein: The stored pitch signal is a pitch angle signal as a function of the rotor azimuth angle, which is the angle of the blades in the rotor plane.

3. A method according to any one of the preceding claims, wherein: The stored pitch signal is a stored pitch angle reference or a stored pitch angle position.

4. The method according to any one of claims 1 to 2, wherein: The at least one periodic component is determined based on a Fourier transform.

5. The method according to any one of claims 1 to 2, wherein: A selected number of periodic components is determined, and wherein the selected number includes at least zero-order and first-order periodic components.

6. The method according to any one of claims 1 to 2, wherein: A pitch control signal is generated without regard to sensor input.

7. The method according to any one of claims 1 to 2, wherein: The pitch actuation signal applied by the pitch actuator to move the pitch-adjustable rotor blade towards the feathered position is a combination of the pitch control signal and the further pitch signal.

8. The method according to any one of claims 1 to 2, wherein: Control of the pitch-adjustable rotor blades towards the feathered position is performed by a safety controller.

9. The method according to any one of claims 1 to 2, wherein: The rotor stop signal is generated by the safety controller.

10. The method according to any one of claims 1 to 2, wherein: When a fault signal occurs, the safety controller triggers the generation of a rotor stop signal.

11. The method according to claim 8, wherein: The safety controller is arranged to individually pitch the blades at a plurality of preset pitch rates, the plurality of preset pitch rates comprising a first pitch rate and a second pitch rate lower than the first pitch rate, and wherein the safety controller is further arranged to: Obtaining the desired pitch angle of the stopping process; adding envelope bands to the desired pitch angle by adding positive and negative values, thereby defining upper and lower band values; and Among them, the stop controller is arranged as follows: moving the pitch-adjustable rotor blade at a pitch rate selected from the plurality of preset pitch rates until the pitch angle signal is outside of a pitch angle value defined by an envelope band, If the pitch angle signal is greater than the upper band value, changing the selected pitch rate to a lower pitch rate among the plurality of preset pitch rates, and If the pitch angle signal is less than the lower band value, the selected pitch rate is changed to a higher pitch rate among the plurality of preset pitch rates.

12. Computer program products, including: A computer program product adapted as software code for controlling a wind turbine when executed on a data processing system, the computer program product being adapted to perform the method of any of the preceding claims.

13. A wind turbine comprising a control system, the wind turbine comprising a rotor with one or more pitch adjustable rotor blades, and the control system is arranged to control the wind turbine in response to a rotor stop signal, the control system comprising one or more controller units arranged to: generating the stored pitch angle signal by obtaining, before receiving the rotor stop signal, for each of the pitch-adjustable rotor blades, a fractional storage pitch angle signal of a full rotor rotation of between 0.5 and 1.5 revolutions, such that smearing of the signal is avoided and only periodic content in a period shortly before the rotor stop signal is obtained; determining at least one periodic component of the stored signal before or upon receiving the rotor stop signal; Upon receiving the rotor stop signal, controlling the pitch adjustable rotor blades toward a feathered position using the pitch control signal including the at least one periodic component.

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