WO2023088432A1

WO2023088432A1 - Controller for a wind turbine

Info

Publication number: WO2023088432A1
Application number: PCT/CN2022/132899
Authority: WO
Inventors: Fabio Caponetti; Jacob Deleuran GRUNNET; Ebbe Nielsen
Original assignee: Shanghai Electric Wind Power Group Co., Ltd
Priority date: 2021-11-19
Filing date: 2022-11-18
Publication date: 2023-05-25
Also published as: DK202170577A1; DK181381B1

Abstract

A controller(101) for controlling a wind turbine(100) configured to:provide a first shutdown-mode in which the controller(101) is configured to provide one or more first shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades(108a,108b,108c) to slow the rotor(103) and (b) a change in a torque applied to the rotor(103) by the generator(104) to slow the rotor(103);and provide a second shutdown-mode,different to the first shutdown-mode,in which the controller(101) is configured to slow the rotor(103) at a faster rate than the first shutdown-mode at least over a predetermined range of rotational speeds that corresponds to a resonant frequency of one or more of the rotor(103),the tower(102) or the blades(108a,108b,108c);and wherein the controller(101) is configured to provide the second shutdown-mode based on receipt of rapid-wind-direction-change-information.

Description

CONTROLLER FOR A WIND TURBINE

TECHNICAL FIELD

This invention relates to a controller for a wind turbine. It also relates to a controller configured to implement a control action for reducing rotor speed, such as to zero. The invention also relates to a wind turbine including such a controller; a method for implementing a control action for reducing rotor speed; and a computer program product and computer program code for implementing the method. It also relates to a controller for a wind turbine configured to detect rapid changes in wind direction.

BACKGROUND

A wind turbine typically comprises a tower and a rotor mounted to the tower. The rotor comprises a hub and a plurality of blades configured to extend from the hub. The rotor typically comprises three blades, although other numbers of blades are possible. Each blade is operably coupled to the hub by a blade bearing which allows for rotation of the blades relative to the hub, such that the pitch of the blades is adjustable. The rotor is coupled to a generator, and may be coupled to the generator via a gearbox. The generator is configured to convert the rotational energy of the rotor to electrical energy. The generator and optional gearbox are housed within a nacelle. A main bearing supports the rotor and allows for rotation of the rotor relative to the nacelle and generator. In some examples, the wind turbine may include a brake to slow and stop the rotation of the rotor.

A wind turbine may, during operation, experience rapid changes in wind direction and/or rapid changes in wind speed. These rapid changes can cause extreme loading on the main bearing and other parts of the wind turbine and may introduce undesirable vibration. In order to manage the loading and vibration experienced during rapid changes in wind direction and/or speed, the controller of the wind turbine may provide a control action to mitigate the issue or shut down the wind turbine. It is a challenge to detect a rapid change in wind direction and/or wind speed event and provide an effective control action in a timely manner.

SUMMARY

According to a first aspect of the invention we provide a controller for controlling a wind turbine having a tower and a rotor comprising a plurality of blades and wherein the rotor is coupled to a generator, wherein a pitch of the blades is controllable and a torque applied to the rotor by the generator is controllable, wherein the controller is configured to:

provide a first shutdown-mode in which the controller is configured to provide one or more first shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades to slow the rotor and (b) a change in a torque applied to the rotor by the generator to slow the rotor; and

provide a second shutdown-mode, different to the first shutdown-mode, in which the controller is configured to provide one or more second shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades to slow the rotor and (b) a change in a torque applied to the rotor by the generator to slow the rotor;

wherein the one or more second shutdown control signals are configured to slow the rotor at a faster rate than the one or more first shutdown control signals at least over a predetermined range of rotational speeds, wherein said predetermined range of rotational speeds is defined such that it includes at least one rotational speed that corresponds to a resonant frequency of one or more of the rotor, the tower or the blades; and wherein the controller is configured to:

receive a shutdown-request comprising a request to reduce the rotational speed of the rotor to at least a predetermined minimum-rotor-speed, wherein provision of the second shutdown-mode rather than the first shutdown-mode is based on receipt of rapid-wind-direction-change-information indicative of the occurrence of a rapid change in wind direction above a threshold level.

In one or more embodiments, the controller may be configured to, in response to receipt of the shutdown-request and absent the rapid-wind-direction-change-information being indicative of the occurrence of a rapid change in wind direction above a threshold level, provide the first shutdown-mode.

Thus, one or more components of the wind turbine, such as the tower and the blades, may have a respective resonant frequency that can be excited by one or more particular rotational speeds, or equivalently rotational frequencies, of the rotor. By resonant frequency we mean to include vibrational modes of the tower or blades or rotor that occur at particular frequencies. At different rotational speeds or frequencies, different modes of vibration can be excited in said one or more components of the wind turbine. When there has been a rapid change in wind direction, damping of the vibration caused by at these resonant frequencies by the wind flow can be reduced and therefore providing a second shutdown-mode can be advantageous to mitigate against the effect of the rotor moving through undamped (including less damped) resonant frequencies during its shutdown, which in the second shutdown-mode will be transitioned at a faster rate.

In one or more embodiments, said predetermined range of rotational speeds may include rotational speeds that correspond to one or more of the following resonant frequencies:

(a) a first flapwise collective excitation frequency;

(b) an excitation frequency associated with a fore-aft oscillation of the tower;

(c) an excitation frequency associated with a side-to-side oscillation of the tower;

(d) a blade flap frequency of one or more of the plurality of blades;

(e) a collective flap frequency of all of the plurality of the blades;

(f) a collective edge frequency of all of the plurality of the blades;

(g) a forward and backwards whirling flap frequency;

(h) a forward and backwards whirling edge frequency;

(i) a tower torsional excitation frequency; and

(j) a blade torsional excitation frequency.

In one or more examples, the said predetermined range of rotational speeds includes rotational speeds that correspond to a combination of said flap frequencies.

In one or more embodiments, said predetermined range of rotational speeds may include rotational speeds that at 1P and at least one of 2P, 3P, 4P, 6P and 9P, wherein P designates the rotational speed of the rotor, correspond to one or more of:

(a) a first flapwise collective excitation frequency;

(c) a blade flap frequency of one or more of the plurality of blades;

(d) a collective flap frequency of all of the plurality of the blades; and

(e) a forward and backwards whirling flap frequency.

In one or more examples, the flap frequencies are prone, in one or more examples, to provide damaging oscillations when undamped by wind flow and therefore defining the predetermined range of rotational speeds based on those frequencies, the controller can mitigate against the effect by passing through those frequencies when slowing the rotor by slowing more quickly during said predetermined range of rotational speeds.

In one or more embodiments, said predetermined range of rotational speeds may extend between a lower rotational speed and an upper rotational speed and wherein one or both of:

said upper rotational speed is defined by a rotational speed that corresponds to a resonant frequency of one or more of the rotor or the tower or the blades plus a first threshold amount; and

said lower rotational speed is defined by a rotational speed that corresponds to a resonant frequency of one or more of the rotor or the tower or the blades minus a second threshold amount.

In one or more embodiments, said one or more second shutdown control signals may provide for application of a greater generator torque to slow the rotor than the one or more first shutdown control signals, at least at rotational speeds corresponding to the predetermined range of rotational speeds.

In one or more examples, said application of the greater generator torque is provided by said one or more second shutdown control signals being configured to (a) increase the torque applied by the generator by causing an increase in the voltage across one or more coils of the generator and (b) increase the power output of the generator.

In one or more embodiments, the controller may be configured to enforce a generator torque limit which defines the maximum torque that the one or more first shutdown control signals cause the generator to apply to the rotor during the first shutdown-mode, wherein the controller is configured to provide said one or more second shutdown control signals to cause the generator to apply a torque greater than said generator torque limit during the second shutdown-mode thereby exceeding the generator torque limit.

In one or more embodiments, the controller may be configured to provide said one or more second shutdown control signals such that they cause a change the pitch of the plurality of blades towards a feathered blade orientation during the second shutdown-mode when the torque applied by the generator is greater than said generator torque limit.

In one or more embodiments, said controller may be configured to receive blade pitch limit information which defines a temporary limit on the blade pitch at a point in time, wherein said one or more second shutdown control signals are configured to change the pitch of the plurality of blades towards a feathered blade orientation without exceeding said temporary limit on the blade pitch, wherein said blade pitch limit information is determined by the controller to mitigate against a negative thrust force being exerted on the wind turbine, where a negative thrust force acts in a direction to urge the rotor in the direction in which it is pointing.

In one or more embodiments, the controller may be configured to receive rotational speed information indicative of the rotational speed of the rotor and, in the second shutdown-mode, said second shutdown control signals are configured to:

when the rotational speed of the rotor is greater than a low-rotor-speed threshold, cause the blades to pitch towards a feathered orientation at a first pitch rate at least within a threshold of a maximum pitch rate of the blades; and

when the rotational speed of the rotor is less than the low-rotor-speed threshold, cause the blades to pitch towards the feathered orientation at a second pitch rate less than said first pitch rate.

In one or more embodiments, said second pitch rate may comprise a constant pitch rate.

In one or more embodiments, said controller may be configured to receive blade pitch limit information which defines a temporary limit on the blade pitch at a point in time, and wherein said second pitch rate comprises a constant pitch rate at least at times when the change in blade pitch at the second pitch rate is unaffected by said temporary limit defined by said blade pitch limit information.

In one or more embodiments, the controller may be configured to, during provision of said second shutdown control signals, determine whether the blade pitch is at a predetermined pitch-angle and, when said predetermined pitch-angle is reached, increase the blade pitch rate to cause the blades to pitch towards the feathered orientation at a rate greater than the second pitch rate.

In one or more embodiments, the controller may be configured to receive acceleration information indicative of the acceleration experienced by one or both of the tower and a nacelle of the wind turbine, and wherein during provision of the second shutdown-mode;

the controller is configured to provide said one or more second shutdown control signals such that they cause a change in the pitch of the plurality of blades towards a feathered blade orientation;

wherein a pitch rate towards the feathered blade orientation is increased based on the acceleration information being indicative of vibrations above a first threshold vibration level; and

wherein the pitch rate towards the feathered blade orientation is decreased based on the acceleration information being indicative of vibration below a second threshold vibration level, lower than the first threshold vibration level.

In one or more examples, the increase in pitch rate is an increase to within a threshold of a maximum pitch rate of the blades the maximum pitch rate defining the maximum rate the blade pitch can change. In one or more examples, the decrease in pitch rate is a decrease to predetermined pitch rate.

In one or more embodiments, said predetermined minimum-rotor-speed may comprise less than 0.3 radians/second.

In one or more embodiments, said predetermined minimum-rotor-speed may comprise less than 25%of a rated rotational speed of the wind turbine, wherein the rated rotational speed comprises a predetermined value.

In one or more embodiments, the controller may be configured such that:

the one or more first shutdown control signals provided during the first shutdown-mode are configured to provide a generator-disconnect procedure in which the torque applied to the rotor by the generator is reduced to within a torque threshold of zero torque before the generator is disconnected from a grid connected power converter of the wind turbine; and

the one or more second shutdown control signals provided during the second shutdown-mode are configured to cause the generator to disconnect from the grid connected power converter while the torque applied by the generator is greater than the torque threshold.

This may be advantageous because the generator-disconnect procedure can take time and while it may minimize stress on the generator, it does not allow for the wind turbine to be stopped quickly. Accordingly, allowing for generator disconnect at above zero-torque levels, the second shutdown mode can complete more quickly.

In one or more examples, said rapid change in wind direction above a threshold level may comprise a change in wind direction greater than 30 degrees that occurs within a time of up to 30 seconds.

According to a second aspect of the invention, we provide a wind turbine including the controller of the first aspect.

According to a third aspect of the invention, we provide a method of controlling a wind turbine having a tower and a rotor comprising a plurality of blades and wherein the rotor is coupled to a generator, wherein a pitch of the blades is controllable and a torque applied to the rotor by the generator is controllable, the method comprising:

receiving a shutdown-request comprising a request to reduce the rotational speed of the rotor to at least a predetermined minimum-rotor-speed;

receiving rapid-wind-direction-change-information indicative of the occurrence of a rapid change in wind direction above a threshold level;

providing a second shutdown-mode rather than a first shutdown-mode based on receipt of rapid-wind-direction-change-information indicative of the occurrence of a rapid change in wind direction above a threshold level when the shutdown-request is received;

wherein

the first shutdown-mode comprises providing one or more first shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades to slow the rotor and (b) a change in a torque applied to the rotor by the generator to slow the rotor; and

the second shutdown-mode is different to the first shutdown-mode, and comprises providing one or more second shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades to slow the rotor and (b) a change in a torque applied to the rotor by the generator to slow the rotor;

wherein the one or more second shutdown control signals are configured to slow the rotor at a faster rate than the one or more first shutdown control signals at least over a predetermined range of rotational speeds, wherein said predetermined range of rotational speeds is defined such that it includes at least one rotational speed that corresponds to a resonant frequency of one or more of the rotor, the tower or the blades.

According to a fourth aspect of the invention, we provide a computer program product comprising computer program code, the computer program code configured to, when executed by a processor having memory, provide the method of the third aspect.

According to a first further aspect of the invention we provide a controller for controlling a wind turbine having a rotor comprising two or more blades, wherein a pitch of the blades is controllable, and wherein the controller is configured to: receive wind speed information indicative of wind speed at the wind turbine; receive rotational speed information indicative of a rotational speed of the rotor; determine a minimum thrust coefficient, C _t-min, comprising the thrust coefficient that provides a predetermined minimum thrust force on the wind turbine, the minimum thrust coefficient comprising a function of a wind speed based on the received wind speed information, air density based on received air density information, a predetermined value indicative of the swept area of the rotor of the wind turbine and said predetermined minimum thrust force; and determine a maximum pitch angle, β _max, based on the minimum thrust coefficient, C _t-min, the wind speed information and the rotational speed information, and wherein said controller is configured to provide one or more control signals to control the blade pitch of the two or more blades of the wind turbine to a controlled blade pitch and wherein the controller is configured to ensure the controlled blade pitch does not exceed said maximum pitch angle. Thus, in one or more examples, the thrust limit defines a minimum thrust force on the wind turbine the controller seeks to maintain by control of the blade pitch using the maximum pitch angle. The predetermined minimum thrust force may be a predetermined positive thrust force. The predetermined minimum thrust force may comprise a negative thrust force within a predetermined negative-thrust-threshold of zero thrust. The magnitude of the thrust force may be defined relative to a maximum expected thrust force under normal operating conditions. For example, the predetermined minimum thrust force may be between 10%and -10%of an expected maximum thrust force expected to be applied to the wind turbine by the wind during normal operation. In other embodiments, the predetermined minimum thrust force may be between 5%and -5%, 3%and –3%or 1%and -1%of an expected maximum thrust force expected to be applied to the wind turbine by the wind during normal operation. In one or more embodiments, the controller being configured to determine the minimum thrust coefficient comprises the controller being configured to determine the minimum thrust coefficient, C _t-min, based on the equation:

wherein V comprises the wind speed based on the received wind speed information, ρcomprises the air density based on the received air density information, A comprises the predetermined value indicative of the swept area and F _thrust-limit comprises the predetermined minimum thrust force. However, in general the minimum thrust coefficient may comprise a function of the predetermined minimum thrust force divided by the wind speed squared. In one or more embodiments, the controller is configured to determine the maximum pitch angle angle β _max by reference to a predetermined look-up table that provides values of maximum pitch angle angle β _max for respective values of rotational speed, wind speed and minimum thrust coefficients C _t-min. In one or more embodiments, the controller is configured to determine the maximum pitch angle and provide said one or more control signals without exceeding said maximum pitch angle in response to an occurrence of a rapid change in wind direction above a threshold level. Thus, in one or more examples, one or more thresholds define the threshold level and thereby define when a change in wind direction is considered rapid. Further, the loading on the wind turbine during such events may define when the threshold level has been exceeded and thereby determine when the control action is provided. In one or more examples, the controller is configured to provide said one or more control signals without exceeding said maximum pitch angle for at least a predetermined time period after said occurrence of a rapid change in wind direction above a threshold level, and, after said predetermined time period has expired, the controller may be configured to provide said one or more control signals without limiting them to the maximum pitch angle. In one or more embodiments, said controller is configured to determine said predetermined minimum thrust force dynamically as a function of a thrust force experienced by the wind turbine with a recent time period. In one or more embodiments, said controller is configured to determine said predetermined minimum thrust force dynamically as a function of the thrust force experienced by the wind turbine in a recent time period and prior to an occurrence of a rapid change in wind direction above a threshold level in said recent time period. In one or more embodiments, said function of the thrust force comprises a proper fraction of the thrust force experienced by the wind turbine in the recent time period between -0.1 and 0.75. In one or more embodiments, said controller is configured to determine the current thrust force, F _t, based on the equation:

wherein v comprises the wind speed from the wind speed information, β comprises a current blade pitch, ρ comprises air density based on the air density information, Ω comprises the rotational speed of the rotor from the rotational speed information, and A comprises the predetermined value indicative of the swept area of the rotor of the wind turbine; and in response to receipt of an indication of the occurrence of a rapid change in wind direction above the threshold level, determine said predetermined minimum thrust force as a proper fraction of said current thrust force, F _t, determined in said recent time period. Thus, in one or more examples, the current thrust force at the time of the occurrence of a rapid change in wind direction may be used. In other examples, the current thrust force up to 5, 10, 15 or 20 seconds prior to the occurrence of a rapid change in wind direction may be used. The controller may be configured to buffer recent calculations of said current thrust force in a buffer. In one or more embodiments, in said determination of the minimum thrust coefficient, C _t-min, the wind speed based on the received wind speed information comprises an average wind speed over a recent time period. In one or more embodiments, the controller is configured to determine said maximum pitch angle, β _max, based on an average wind speed within a second recent time period based on the wind speed information. In one or more examples, said average wind speed is determined by low-pass filtering said wind speed information. In one or more examples, the second recent time period may be different to the recent time period used in the determination of the minimum thrust force. In one or more embodiments, the controller is configured to determine said maximum pitch angle, β _max, based on an average rotational speed of the rotor within a third recent time period based on the rotational speed information. In one or more examples, said average rotational speed is determined by low-pass filtering said rotational speed information. In one or more embodiments, the controller is configured such that if the controlled blade pitch is greater than the determined maximum pitch angle, the controller is configured to limit the controlled blade pitch to the determined maximum blade pitch angle. According to a second further aspect of the invention we provide a wind turbine including the controller of the first further aspect wherein the pitch of the blades of the wind turbine is limited by the maximum pitch angle. According to a third further aspect of the invention we provide method of controlling a wind turbine having a rotor comprising two or more blades, wherein a pitch of the blades is controllable, and wherein the method comprises: receiving wind speed information indicative of wind speed at the wind turbine; receiving rotational speed information indicative of a rotational speed of the rotor; determining a minimum thrust coefficient, C _t-min, comprising the thrust coefficient that provides a predetermined minimum thrust force on the wind turbine, the minimum thrust coefficient comprising a function of a wind speed based on the received wind speed information, air density based on air density information, a predetermined value indicative of the swept area of the rotor of the wind turbine and said predetermined minimum thrust force; determining a maximum pitch angle β _max based on the minimum thrust coefficient C _t-min, the wind speed information and the rotational speed information; and providing one or more control signals to control the blade pitch of the two or more blades of the wind turbine without exceeding said maximum pitch angle. According to a fourth further aspect of the invention we provide a computer program or computer program product comprising computer program code, the computer program code configure to, when executed by a processor having memory, provide the method of the third further aspect.

According to a first aspect of the invention we provide a controller for providing cyclic individual pitch control of a plurality of blades of a wind turbine, wherein the controller is configured to: receive at least a first pitch angle control signal and a second pitch angle control signal for control of the pitch of the blades during cyclic individual pitch control, wherein the first pitch angle control signal and the second pitch angle control signal define the change in blade pitch of the plurality of blades over a rotation of the rotor; receive wind direction information, the wind direction information indicative of the wind direction incident on said wind turbine relative to a direction in which the wind turbine is facing; determine an output first pitch angle control signal based on the first pitch angle control signal and determine an output second pitch angle control signal based on the second pitch angle control signal; and if the wind direction information is indicative of a wind direction incident at a first side of the wind turbine, provide one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine based on said output first pitch angle control signal and the output second pitch angle control signal, and with a predetermined phase shift; if the wind direction information is indicative of a wind direction incident at a second side of the wind turbine opposite the first side, provide one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine based on said output first pitch angle control signal and the output second pitch angle control signal without the predetermined phase shift. Thus, in one or more examples, a pitch sequence that is provided with a rotation of the rotor by the one or more control signals for cyclic IPC when the wind is one side will be out of phase with the pitch sequence that is provided with a rotation of the rotor by the one or more control signals for cyclic IPC when the wind is the other side, which has been found to be advantageous. In one or more examples said wind direction information is derived from one or more sensors. The one or more sensors may comprise acceleration sensors. In one or more examples, said wind direction information is from a wind direction sensor. In one or more examples, the wind turbine has two blades and the first pitch angle control signal is for controlling a first of the two blades and the second pitch angle control signal is for controlling a second of the two blades. In other examples, the wind turbine has three blades and three respective pitch angle control signals are provided. In one or more embodiments, the first pitch angle control signal comprises a D-component of a direct-quadrature-transform for control of the pitch of the blades during cyclic individual pitch control; and the second pitch angle control signal comprises a Q-component of a direct-quadrature-transform for control of the pitch of the blades during cyclic individual pitch control. Thus, in one or more examples, the wind turbine has three blades and the D-component and Q-component thereby comprise two pitch angle control signals for controlling the pitch of three blades. In one or more embodiments, the controller is configured to: if the wind direction information is indicative of a wind direction incident at a first side of the wind turbine, determine the output first pitch angle control signal by application of the predetermined phase shift to the first pitch angle control signal and to determine the output second pitch angle control signal by application of the predetermined phase shift to the second pitch angle control signal such that said one or more control signals for implementation of cyclic individual pitch control of the blades are provided with the predetermined phase shift; and if the wind direction information is indicative of a wind direction incident at the second side of the wind turbine, determine the output first pitch angle control signal without application of the predetermined phase shift to the first pitch angle control signal and determine the output second pitch angle control signal without application of the predetermined phase shift to the second pitch angle control signal such that said one or more control signals for implementation of cyclic individual pitch control of the blades are provided without the predetermined phase shift. In one or more embodiments, said provision of the one or more control signals for implementation of cyclic individual pitch control includes application of a Coleman Transform to said output first pitch angle control signal and said output second pitch angle control signal, and wherein controller is configured to apply the predetermined phase shift to a phase offset input of the Coleman Transform, wherein the controller is configured to provide said one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine based on a Coleman transform of said output first pitch angle control signal and said output second pitch angle control signal, and said phase offset input and information indicative of a current azimuth angle of the rotor in order to provide the one or more control signals for implementation of cyclic individual pitch control with the predetermined phase shift. Thus, in one or more examples, the one or more control signals for implementation of cyclic individual pitch control without the predetermined phase shift is provided by the controller by not applying the predetermined phase shift to the phase offset input of the Coleman transform. In one or more embodiments, said predetermined phase shift comprises a phase shift between 120 and 240 degrees. In one or more embodiments, said predetermined phase shift comprises a 180 degree phase shift. Thus, the phase shift may apply relative to first pitch angle control signal and the second pitch angle control signal. In one or more embodiments, said predetermined phase shift comprises a function of the wind direction information. In one or more embodiments, the controller is configured to: receive rapid-change-of-wind-direction information which is indicative of the occurrence of a rapid change in wind direction event above a threshold level; and wherein said provision of the one or more control signals for implementation of the cyclic individual pitch control provided by the controller is conditional on and in response to said rapid-change-of-wind-direction information indicating that there is a rapid change in wind direction event. In one or more examples, the occurrence of a rapid change in wind direction event is based on a change in wind direction greater than 30 degrees that occurs in less than 30 seconds. In one or more embodiments, said controller is configured to provide for application of a Coleman Transform to said output first pitch angle control signal and said output second pitch angle control signal to provide said one or more control signals for cyclic individual pitch control of the blades of the wind turbine. In one or more embodiments, the controller is configured to: receive a collective pitch reference angle indicative of the current pitch angle of the blades of the wind turbine; provide said one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine only if said collective pitch reference angle is indicative of a pitch at which the blades are not stalled. In one or more embodiments, the controller is configured to: receive a collective pitch reference angle indicative of the current pitch angle of the blades of the wind turbine; receive a minimum collective pitch angle indicative of a blade pitch angle at which it is determined that blade stall occurs; determine whether or not the collective pitch reference angle is greater than the minimum collective pitch angle; provide said one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine only if said collective pitch reference angle is greater than the minimum collective pitch angle. In one or more examples, the collective pitch reference angle comprises an angle provided to the controller indicative of the pitch of the blades. In some examples, the pitch of the blades may be measured by a blade pitch sensor. If any one of the blades is at a different pitch angle to the others, the collective pitch reference angle may comprise an average of the pitch angle of the blades. In one or more examples, said minimum collective pitch angle comprises a pitch angle at which it is determined the wind turbine will extract power from the wind at or above an upper-power-output-level, which may be the determined optimum blade pitch. In one or more embodiments, the controller is configured to: receive a collective pitch reference angle indicative of the current pitch angle of the blades of the wind turbine; receive a minimum collective pitch angle comprising a blade pitch angle at which it is determined that blade stall occurs; calculate a maximum-IPC-pitch-angle, IPC _max, based on a difference between the collective pitch reference angle, β _coll, and the minimum collective pitch angle β _opt, wherein: IPC _max=f (β _coll-β _opt) , that is a function of β _coll-β _opt; and wherein the maximum-IPC-pitch-angle is indicative of a maximum pitch angle deviation from the collective pitch reference angle that avoids stall of the blades; and wherein said provision of the one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine is further based on the maximum-IPC-pitch-angle. In one or more examples, said provision of the one or more control signals for implementation of cyclic individual pitch control is further based on said maximum-IPC-pitch-angle being greater than zero. In one or more embodiments, the controller is configured to: compare the first pitch angle control signal to the maximum-IPC-pitch-angle; compare the second pitch angle control signal to the maximum-IPC-pitch-angle; wherein if the first pitch angle control signal is greater than the maximum-IPC-pitch-angle then the output first pitch angle control signal is based on said maximum-IPC-pitch-angle; and if the first pitch angle control signal is less than the maximum-IPC-pitch-angle then the output first pitch angle control signal is based on said first pitch angle control signal; and wherein if the second pitch angle control signal is greater than the maximum-IPC-pitch-angle then the output second pitch angle control signal is based on said maximum-IPC-pitch-angle; and if the second pitch angle control signal is less than the maximum-IPC-pitch-angle then the output second pitch angle control signal is based on said second pitch angle control signal. In one or more embodiments, said determination of the output first pitch angle control signal and the output second first pitch control signal is based on an average wind direction over a recent time period derived from the wind direction information. In one or more examples, the controller is configured to determine said average wind direction by way of application of a low-pass filter to the wind direction information. In one or more embodiments, the controller is configured to provide the predetermined phase shift, referred to as the first predetermined phase shift, and a second predetermined phase shift, wherein the controller is configured such that if the wind direction information is indicative of a wind direction incident at a first side of the wind turbine, then the controller is configured to provide the one or more control signals for implementation of cyclic IPC based on said output first pitch angle control signal and the output second pitch angle control signal, and with the first predetermined phase shift; and if the wind direction information is indicative of a wind direction incident at the second side of the wind turbine, then the controller provides the one or more control signals for implementation of cyclic IPC based on said output first pitch angle control signal and the output second pitch angle control signal without the first predetermined phase shift and with the second predetermined phase shift. According to a second still further aspect of the disclosure, we provide a wind turbine including the controller of the first still further aspect. According to a third still further aspect of the disclosure, we provide a method for providing cyclic individual pitch control of blades of a wind turbine, the method comprising: receiving at least a first pitch angle control signal and a second pitch angle control signal for control of the pitch of the blades during cyclic individual pitch control, wherein the first pitch angle control signal and the second pitch angle control signal define the change in blade pitch of the plurality of blades over a rotation of the rotor; receiving wind direction information, the wind direction information indicative of the wind direction incident on said wind turbine relative to a direction in which the wind turbine is facing; determining an output first pitch angle control signal based on the first pitch angle control signal and determining an output second pitch angle control signal based on the second pitch angle control signal; and if the wind direction information is indicative of a wind direction incident at a first side of the wind turbine, providing one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine based on said output first pitch angle control signal and the output second pitch angle control signal, and with a predetermined phase shift; if the wind direction information is indicative of a wind direction incident at a second side of the wind turbine opposite the first side, providing one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine based on said output first pitch angle control signal and the output second pitch angle control signal without the predetermined phase shift. According to a fourth still further aspect of the disclosure, we provide computer program product comprising computer program code, the computer program code configure to, when executed by a processor having memory, provide the method of the third still further aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

There now follows, by way of example only, a detailed description of embodiments of the invention with reference to the following figures, in which:

Figure 1 shows a side view of an example wind turbine and controller;

Figure 2 shows a front view of the example wind turbine and controller of Figure 1;

Figure 3 shows an example controller, which may be provided in combination with one or more sensors;

Figure 4 shows an example flow chart illustrating an overview of a control scheme for use in detecting changes in wind direction above a threshold level and acting on said detection;

Figure 5 shows an example functional block diagram illustrating an embodiment of the functionality provided by the controller according to a first example embodiment for a control action for cyclic individual blade pitch control;

Figure 6 shows an example functional block diagram illustrating a second embodiment of the functionality provided by the controller according to a second example embodiment for a control action for cyclic individual blade pitch control;

Figure 7 shows a flowchart illustrating an example method of providing a control action for cyclic individual blade control;

Figure 8 shows a schematic diagram illustrating the concept of positive and negative thrust force on a wind turbine;

Figure 9 shows an example functional block diagram illustrating an embodiment of the functionality of provided by the controller according to a second example embodiment for a control action for negative thrust mitigation;

Figure 10 shows a flowchart illustrating an example method of providing a control action for negative thrust mitigation;

Figure 11 shows an example graph illustrating the effect of the control action for negative thrust mitigation on the blade pitch the controller can instruct the blades to adopt;

Figure 12 shows an example graph of side-to-side position derived from acceleration information measured by an acceleration sensor configured to measure the side-to-side acceleration the tower is subjected to relative to a long-term average over time;

Figure 13 shows a graph of resonant frequencies;

Figure 14 shows an example controller for slowing the rotor according to two or more different shutdown-modes; and

Figure 15 shows a flowchart illustrating an example method for controlling a wind turbine during shutdown;

Figure 16 shows an example computer readable medium.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Example figures 1 and 2 show a side view and a front view of an example wind turbine 100 and a controller 101 for the wind turbine. The wind turbine 100 comprises a tower 102 and a rotor 103 which is operably coupled to a generator 104 mounted within a nacelle 105. The controller 101 is shown schematically within a base of the tower 102 but, in other examples, it could be mounted elsewhere. The rotor 103 may be coupled to the generator 104 via a gearbox 106, which is also mounted within the nacelle 104. A main bearing (not visible in Figure 1) supports the rotor 103 and allows for its rotation. The rotor 103 comprises a hub 107 and three

blades

108A, 108B and 108C (shown collectively as 108 in Figure 1) that extend from the hub 107. While this example wind turbine 100 has three blades, other numbers of blades are possible, such as two or more blades. Each blade is operably coupled to the hub 107 by a blade bearing which allows for rotation of the blades relative to the hub, such that the pitch, that is rotation around a longitudinal axis, of each of the blades is adjustable. The generator 104, and optional gearbox 105, is controllable and may, during operation, be controlled to efficiently extract energy from the wind. The generator 104 and optional gearbox 105 are also controllable such that a torque can be applied to the rotor 103, which can be used to control its rotational speed.

In one or more examples, the pitch of

blades

108A, 108B, 108C may be controlled collectively or individually. Accordingly, each

blade

108A, 108B, 108C may be rotatably mounted to the hub 107 and be coupled with an actuator to control the pitch of the blade. Thus, the controller 101 may be configured to provide one or more control signals to control the actuator and thereby change the pitch of one or more of the blades of the rotor. In one or more examples, the controller may be configured to change the pitch of

blades

108A, 108B, 108C collectively. Thus, the pitch of each of the blades is changed to the same pitch. In other examples the controller may be configured to change the pitch of

blades

108A, 108B, 108C individually, such that at least one of the blades is at a different pitch relative to the others.

The controller is also shown separate from the wind turbine 100 that it controls in example Figure 3. The controller 101 is operably coupled to receive information at inputs 301 to 304, such as from one or more sensors associated with and optionally mounted on the wind turbine 100. The controller 101 is configured to transmit one or more control signals to components of the wind turbine at one or

more outputs

305, 306. The information the controller receives at its one or more inputs 301 to 304 may vary depending on the functionality it is configured to provide. However, examples of a plurality of sensors or other processing modules that may be configured to provide the controller 101 with information at the inputs are described.

The one or more sensors may include one or more of a wind speed sensor 307, a wind direction sensor 308 and a rotational speed sensor 309. The wind speed sensor 307 is configured to measure the current wind speed experienced by the wind turbine. The wind speed sensor is typically mounted on the wind turbine 100 to the nacelle 105 behind the rotor 103 but could be separate therefrom in the wind field that will be incident on the wind turbine. In other examples a LIDAR based sensor may be used. It will be appreciated that the wind speed sensor may comprise one or more sensors from which wind speed may be derived, whether that be by direct measurement of wind speed or inference from one or more other measured variables. For example, the wind speed sensor could be embodied as a wind speed estimator which determines or estimates wind speed based on rotor speed, produced power and pitch angle. Alternatively, a tip speed sensor, such as a pilot tube, may be used which is then used to calculate the tip-speed ratio and the wind speed may be derived therefrom. Thus, the controller 101 may be configured to receive wind speed information from the wind speed sensor, in whatever form it may take, at input 301.

The wind direction sensor 308 may comprise a wind vane mounted to the nacelle 105. In other examples, the wind direction sensor may comprise a LIDAR based sensor. Thus, the controller 101 may be configured to receive wind direction information from the wind direction sensor, in whatever form it may take, at input 302. It will be appreciated that the wind direction sensor 308 may be mounted on the wind turbine or may be separate therefrom and located in the wind field that will be incident on the wind turbine. It will be appreciated that the wind direction sensor may comprise one or more sensors from which wind direction may be derived, whether that be by direct measurement of wind direction or inference from one or more other measured variables.

The rotational speed sensor 309 is configured to measure the rotational speed of the rotor 103. Again, it will be appreciated that the rotational speed sensor may comprise one or more sensors from which rotational speed may be derived, whether that be by direct measurement of rotational speed or inference from one or more other measured variables. For example, a generator speed sensor may be used, which is then used to calculate the rotor speed with predetermined knowledge of the gearing ratio between the rotor and the generator. Alternatively, an azimuth sensor may detect blade position, which is then used to calculate the rotor speed. Alternatively, a centripetal force sensor may be used which is then used to calculate the rotor speed. Alternatively, a tip speed sensor, such as a pilot tube, may be used which is then used to calculate the tip-speed ratio and the rotational speed may be determined therefrom. Alternatively, a vibration sensor, such an accelerometer, may estimate rotor speed based on observed vibration modes in the tower or blades. Alternatively a blade load sensor may determine the load on a blade and with predetermined information regarding the relationship between load and rotor speed in terms of centripetal force or vibration, the rotor speed may be derived. In other examples, GPS data from a GPS receiver can be used to determine tower movement and derive rotor speed. Thus, the controller 101 may be configured to receive rotational speed information from the rotational speed sensor, in whatever form it may take, at input 303.

While the above description provides an example in which the information is received from sensors, it will be appreciated that the same information may be received via signal processing modules, which may be configured to process the information before receipt by the controller 101. Thus, the information may be one or more of filtered, digitally sampled, de-noised or averaged before it arrives at inputs 301 to 304.

The controller 101 may receive information from other sensors or other control processes at input 304. Thus, box 310 represents one or more other sensors or other signal processing modules or other controllers from which information required by the controller 101 can be received.

The controller 101 may be configured to control the generator 104 and, in particular, the torque applied to the rotor 103 by providing one or more control signals, such as at the first output 305. The controller 101 may be configured to control the pitch of each of the plurality of blades 108A-C by providing one or more control signals, such as at the second output 306. It will be appreciated that the controller 101 may be configured to provide one or more other control signals, such as to apply a braking force by way of a rotor brake or to effect changes to the gearing in the gearbox 106. The control signals may be provided at separate outputs as shown in example Figure 3 or at a single output wherein the control signals are addressed to the components they control.

The wind turbine 100 may experience rapid changes in wind direction and/or speed wherein these rapid changes induce undesirable loading on components of the wind turbine, such as the main bearing. The examples that follow relate to the operation of the controller 101 and, in particular, to a control action that may be provided by the controller in response to detection of changes in wind direction that may exceed a predetermined threshold and, optionally, to the detection of changes in wind direction and wind speed that may exceed predetermined thresholds. The predetermined threshold (s) may therefore define what is considered a rapid or “extreme” change in wind direction and/or wind speed event, which may be determined to cause undesirable damage, vibration or fatigue to the wind turbine. The detection of such above-threshold changes may be used by the controller 101 to trigger the control action to manage the operation of the wind turbine 100 during such events or in response to such events. However, the control action may be beneficial outside of the occurrence of a rapid or “extreme” change in wind direction and/or wind speed event. The control action may include the issue of one or more control signals at

outputs

305, 306 for controlling one or both of generator torque and blade pitch.

What qualifies as a rapid change in wind direction and/or wind speed event to cause the controller 101 to issue the control action may differ between wind turbines. However, as an example, a change in wind direction greater than 30 degrees that occurs in less than 30 seconds may be consider a rapid change in wind direction and trigger the control action. Thus, the operation of the controller 101 may be calibrated to identify when such a change occurs. In other examples, the operation of the controller 101 may be calibrated to identify an extreme change in wind direction and/or speed as defined in the IEC standard 61400.

Example figure 4 shows a flowchart illustrating a method the controller 101 may follow to mitigate against the effects of rapid changes in wind direction.

Block 401 represents the start of the method. Block 402 illustrates the provision of a control algorithm that detects the occurrence of at least a change in wind direction above a threshold level. The occurrence of a change in wind direction above a threshold level may be termed a “rapid change of direction” event or “ECD” event (standing for Extreme Change in Direction) , as will be known to those skilled in the art. Exactly how the controller 101 is configured to provide the functionality of block 402 is not the focus here, but in general it may be considered to involve comparison of information indicative of a change in wind direction to a threshold and if the threshold is exceeded, the controller deems a rapid change in wind direction event to have occurred.

If a rapid change of direction event or ECD event is detected, the method proceeds to block 403 which is termed “safe mode” in which a control action is taken to mitigate against the effects of the “rapid change of direction” event or ECD event. The functionality of the controller 101 described in the embodiments below may be provided as part of block 403.

If a rapid change of direction event or ECD event is not detected at block 402, the method proceeds to block 404 in which the controller 101 determines if there is a high yaw error. A yaw error may be determined by calculation of the difference between the direction the wind turbine is pointing and the wind direction. The direction in which the wind turbine is pointing may be defined, for example, by the position or orientation of the nacelle. Thus, if the rotor is facing directly into the wind the yaw error may be zero degrees. If the wind direction is incident from the right, the yaw error may be +90 degrees. If the wind direction is incident from the left, the yaw error may be -90 degrees. In very general terms, a high yaw error may be determined by comparing the yaw error to a high-yaw-error-threshold. If a high yaw error is detected, the method may also proceed to block 403. If a high yaw error is not detected in this example the method ends at 405 and restarts at block 401.

In some examples, the “safe mode” of block 403 also includes checking various conditions to determine if the wind turbine is to be shut down (i.e., the rotor speed reduced, such as to a stop) . If the various “shut down” conditions are met, the method may proceed to block 406 in which the turbine is shutdown. The method arrives at block 407 once the turbine is shutdown. Block 407 shows the step of controlling the yaw of the turbine such that it points in the current wind direction. Block 408 illustrates the restarting of the wind turbine. The method then proceeds to step 401. It will be appreciated that the method illustrated in Figure 4 is focussed on detecting rapid change of direction events or ECD events and providing a control action, however, various other control actions may be provided in parallel.

There will now be described a plurality of control actions. The controller 101 may be configured to provide one or more, two or more or three of said plurality of control actions. In some examples, two or more of said plurality of control action may be provided in parallel, i.e., simultaneously, with another control action for the wind turbine.

Control Action for Cyclic Individual Blade Control

In the present example, the pitch of

blades

108A, 108B, 108C may be controlled individually. Accordingly, each

blade

108A, 108B, 108C may be rotatably mounted to the hub 107 and be coupled with an actuator to control the pitch of the blade. Thus, the controller 101 may be configured to provide one or more control signals to control the actuator (s) and thereby change the pitch of one or more of the blades of the rotor.

It will be known to those skilled in the art of wind turbine control that, in one or more examples, it may be desirable to control the pitch angles of the blades such that a pitch angle of one of the blades 108 differs from the pitch angle of the other blade or blades. Thus, a controller may be configured to provide individual pitch control (IPC) by control of the pitch angles of the blades such that the pitch angle of at least one of the blades differs from the pitch angle of the other blades at least at some time during a full rotation of the rotor 103.

In one or more examples, the controller may be configured to provide cyclic individual pitch control, i.e., cyclic IPC, which comprises a type of individual pitch control. In providing cyclic IPC, the controller is configured to control the instantaneous pitch angle of each of the blades as a function of the instantaneous rotation angle of a respective blade relative to a notional fixed reference angle. The function is typically the same or at least substantially the same for each blade. Thus, the controller will cause each blade to pitch through substantially the same pitch angle sequence during a full rotation of the rotor depending on the rotation angle of the respective blade relative to the same notional fixed reference angle.

The functionality provided by the controller 101 in terms of providing for cyclic IPC will now be described with reference to the example functional block diagram of figure 5. It will be appreciated that the example functional block diagram illustrates the function (s) provided by the controller to implement cyclic IPC and the controller 101 may be configured to provide other control actions, which can include control actions provided simultaneously. The functions shown here may be provided by a programmable logic controller. Alternatively, the filters and/or amplifiers may comprise discrete signal processing components. In a further embodiment, the controller may provide a software based implementation of the functions shown. A combination of the above implementations is also within the scope of the disclosure.

In one or more examples, the controller is configured to receive a collective pitch reference angle, β _coll, at input 501. Thus, with reference to the controller 101 shown in figure 3, the collective pitch reference angle may be received at the input 304. The collective blade pitch reference angle represents the current pitch angle of the blades of the wind turbine. In this example, the collective pitch reference angle comprises an angle provided to the controller 101.

The pitch of the blades may be measured by one or more blade pitch sensors to provide the collective blade pitch reference angle to the controller 101. In other examples, it will be appreciated that the controller 101, as part of a different control action, may be actively controlling blade pitch by the issue of one or more control signals to the blade pitch actuator (s) and the collective blade pitch reference angle may be based on those control signals, i.e. the blade pitch the controller is instructing the blades to adopt. If any one of the blades is at a different pitch angle to the others, the collective pitch reference angle may comprise an average of the pitch angle of the blades. The collective blade pitch reference angle may comprise the current, instantaneous pitch angle of the blades, or, in other embodiments, a recent average thereof.

The pitch angle of the blades may be measured as the rotation angle between a fixed part and a rotatable part of the blade bearing. It will be appreciated that a 0 degree pitch angle reference may be freely selected, but in one or more embodiments, the zero pitch angle reference may be defined as the pitch angle for which optimal power extraction can be achieved. Thus, in the examples herein a high positive blade pitch angle (such as around +90 degrees) may be indicative of a blade in a feathered orientation and thus the angle of attack may be oriented approximately 90 degrees to direction of rotation. A zero degree blade pitch angle is such that the angle of attack is closer to the direction of rotation and the blade may be in a fine orientation. The pitch angle may take positive or negative values and may range, for example, from -90 to 120 degrees, from -30 –100 degrees, from -5 to 90 degrees or any other angular range.

Further, in one or more examples, the controller 101 is configured to receive a minimum collective pitch angle, β _opt, at input 502. The minimum collective pitch angle is indicative of a blade pitch angle at which it is determined that blade stall occurs. Blade stall is a familiar concept to those in the art and it is appreciated that the angle at which it is determined a particular blade stalls can be calculated in several ways. For example, the minimum collective pitch angle may be the collective pitch angle at which the gradient of the power coefficient curve of the wind turbine efficiency is negative. In yet another example, the minimum collective pitch angle may be the collective pitch angle at which more than x%of blade sections of the blade stall, where x can be defined as an acceptable limit. In general however, for the purpose of this controller, the minimum collective pitch angle is a minimum pitch angle for the purpose of providing cyclic IPC. Thus, with reference to the controller 101 shown in figure 3, the minimum collective pitch angle may be received at input 304. The minimum collective pitch angle is, in one or more examples, equivalent to a power optimal blade pitch reference angle. The minimum collective pitch angle (or optimal blade pitch reference angle) may be determined by the controller 101 or a further controller. How to calculate the minimum collective pitch angle will be known to those skilled in the art. However, in one or more examples, the minimum collective pitch angle may be determined by a process of the controller and may be based on a model that can be used to predict blade stall angles. In one or more examples, the minimum collective pitch angle may include a margin for error and therefore the minimum collective pitch angle may comprise the pitch angle at which the chance of stall is at a predetermined level less than 100%.

The power optimal pitch angle comprises a blade pitch angle at which it is determined the wind turbine 100 will extract power from the wind at or above a upper-power-output-level. In some examples, this may be an optimal power level or the upper-power-output-level may be a function of the optimal power level, such as to define the upper-power-output-level as a level within a threshold of the optimal power level. The power optimal pitch angle may be received from another process provided by the controller that determines the power optimal pitch angle. The power optimal pitch angle may comprise a function of the effective wind speed, which comprises the component of wind speed incident perpendicular to the plane of the rotor 103. Thus, effective wind speed, v _eff, may be determined by v _eff=v _measured cos γ, wherein v _measured comprises the measured wind speed, such as from a wind speed sensor or more generally from wind speed information provided to the controller, and γ comprises an angle indicative of the wind direction measured relative to the direction in which the rotor 103 is currently pointing, often called yaw error in the art. In one or more examples:

wherein and C _p (β, v _eff, Ω) comprises a thrust coefficient, which comprises a function of β which comprises blade pitch angle, Ω which comprises the rotational speed of the rotor and, as mentioned above, v _eff which comprises the effective wind speed.

Further, in one or more examples, the controller 101 is configured to receive wind direction information, such as from a wind direction sensor at input 503. The controller 101 may be configured to determine a moving average of the wind direction over a recent time period, such as over the last three seconds, from the wind direction information received at input 503. In other examples, the recent time period may comprise up to or at least one, two, three, four, five, six, seven, eight, nine or ten seconds. In one or more examples, rather than a moving average, the controller may be configured to filter the wind direction information using a low pass filter 504, which may, in effect, provide a recent average wind direction. A time constant of the low pass filter may be configured to define the recent time period. In one or more examples, the time constant, tau, of the low pass filter is set to between 1 to 4 seconds, such as 3 seconds.

The controller 101 may be configured to generate one or more control signals configured to provide for cyclic IPC. In the present example, the controller 101 is configured to modify or modulate one or more input control signals configured to provide for cyclic IPC. Thus, one or more input cyclic IPC control signals may be determined by a different process, such as a different process provided by the controller 101 or a different controller 101, to control the blade pitch of each of the blades 108A-C to provide cyclic IPC and may be received by the controller 101. In some examples, the input cyclic IPC control signals are predetermined and recalled from a memory to define a pitch angle sequence.

In the present example, the one or more input cyclic IPC control signals comprise a first pitch angle control signal and a second pitch angle control signal. The first pitch angle control signal and the second pitch angle control signal comprise periodic signals that define the change in blade pitch of the plurality of blades over a rotation of the rotor. In some examples, there may be a separate pitch angle control signal for each blade. Thus, in one or more examples, for a wind turbine that has two blades, the first pitch angle control signal may be for controlling a first of the two blades and the second pitch angle control signal may be for controlling a second of the two blades. In the present example, the wind turbine has three blades and in one or more examples, three respective pitch angle control signals may be provided, one for each blade.

However, in this example, the first pitch angle control signal comprises a D-component of a direct-quadrature-transform. The second pitch angle control signal comprises a Q-component of the direct-quadrature-transform. The controller may be configured to receive the first pitch angle control signal at input 505. The controller may be configured to receive the second pitch angle control signal at input 506. The first pitch angle control signal may be understood as a change in blade pitch to introduce a yaw moment in the rotor axis. The second pitch angle control signal may be understood as a change in blade pitch to introduce a tilt moment in the rotor axis. However, it will be appreciated that depending on how the direct-quadrature-transform is configured, the first and second pitch angle control signal could be understood differently.

As will be known to those skilled in the art, the direct-quadrature-transform or DQ-transform is a common method for defining blade pitch control of the blades 108 of the rotor 107 based on rotation angle. Thus, the D and Q components of the DQ-transform allow for two control signals to provide for control of the pitch of three blades during cyclic IPC. Further, as will also be known to those skilled in the art, a Coleman transform may be used to receive said D-component and Q-component and transform the values into a plurality of control signals, one for controlling the pitch of each of the blades 108A-C with reference to a current azimuth angle of the rotor.

In general terms and as will be described in more detail below, the controller 101 may be configured to provide the one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine based on the first pitch angle control signal, the second pitch angle control signal and (a) said collective pitch reference angle and the minimum collective pitch angle and/or (b) wind direction information. The output of the controller 101, that is the one or more control signals for implementation of cyclic individual pitch control as modulated by the controller 101, may comprise the D-component signal and Q-component signal, provided at

outputs

507 and 508, or the Coleman transform thereof, provided at

outputs

510, 511, 512.

It has been found that how cyclic IPC is implemented can have a significant effect, especially in changeable wind fields. Thus, in one or more examples, the controller may be configured to apply the cyclic IPC control described herein based on received rapid-change-of-wind-direction information. As mentioned above, rapid-change-of-wind-direction information is indicative of the occurrence of a rapid change in wind direction event above a threshold level. Thus, the control action provided by the controller 101 in the present example may be conditional on said rapid-change-of-wind-direction information indicating that there is a rapid change in wind direction event. Further, the controller may be configured to provide the control action in response to, that is within a predetermined amount of time, of it occurring. However, in the present example, the controller is configured to apply the cyclic IPC control described herein irrespective of said rapid-change-of-wind-direction information.

The controller 101 is configured to receive the wind direction information at the input 503. The controller may be configured to determine an average over a recent time period of the wind direction information by way of optional low pass filter 504. The controller 101 is configured to modify the implementation of cyclic individual pitch control based on the direction from which the wind is incident on the wind turbine relative to the direction the rotor is facing.

In particular, the controller, represented by the action at block 515, is configured to determine if the wind is incident at a first side of the wind turbine or a second side of the wind turbine, opposite the first side. In the present example, the wind direction information is indicative of a yaw error. A yaw error may be determined by calculation of the difference between the direction the wind turbine is pointing, i.e., nacelle position, and the wind direction. Thus, if the rotor is facing directly into the wind the yaw error may be zero degrees. If the wind direction is incident from the right, the yaw error may be +90 degrees. If the wind direction is incident from the left, the yaw error may be -90 degrees. It will be appreciated that the sign of the yaw error may be different, such that a negative number is indicative of a wind direction incident from the right rather than the left in other examples. The block 515 represents the controller being configured to determine the sign of the yaw error and, as such, the side of the wind turbine from which the wind is incident.

Based on the wind being incident on the first side or the second side, the controller is configured to determine an output first pitch angle control signal based on the first pitch angle control signal at block 516 and an output second pitch control signal based on the second pitch control signal at block 517.

In particular, if the wind direction is determined to be incident at the first side of the wind turbine (e.g., the left side) , the controller is configured to determine the output first pitch angle control signal and an output second first pitch control signal by application of, at

blocks

516, 517, a predetermined phase shift to the first pitch angle control signal and the second pitch angle control signal respectively.

Further, if the wind direction is determined to be incident at the second side of the wind turbine (e.g., the right side) , the controller is configured to determine the output first pitch angle control signal and the output second first pitch control signal without application of the predetermined phase shift to the first pitch angle control signal and the second pitch angle control signal. Thus, the first pitch angle control signal may be unmodified in phase and provided as the output first pitch angle control signal. Likewise, the second pitch angle control signal may be unmodified in phase and provided as the output second pitch angle control signal.

The

blocks

516 and 517 are thereby configured to modify, selectively, the phase of the first pitch angle control signal and the second pitch angle control signal to form the output first pitch angle control signal and an output second first pitch control signal. By modifying the phase of the cyclic IPC that will be applied, the inventors have found that the forces applied to the main bearing may be reduced in some examples. It will be appreciated that application of the phase shift when the wind direction is from the first side is provided with the intention of reducing a moment on the main bearing during cyclic IPC. Likewise it will be appreciated that non-application of the phase shift when the wind direction is from the second side is done with the intention of reducing or not increasing a moment on the main bearing during cyclic IPC.

Accordingly, the controller 101 is configured to provide one or more control signals, i.e. those at 507 and 508 or at 510-512, for implementation of cyclic individual pitch control of the blades of the wind turbine based on said output first pitch angle control signal and said output second pitch angle control signal.

In the present embodiment, the sign determined at block 515 is expressed as either +1 or -1 and multiplied by the first pitch angle control signal and the second pitch angle control signal at

blocks

516, 517. Thus, if the sign at block 515 is positive then the first pitch angle control signal is multiplied by +1 and is left unchanged to comprise the output first pitch angle control signal. Likewise, the second pitch angle control signal is multiplied by +1 and is left unchanged to comprise the output second pitch angle control signal.

If the sign at block 515 is negative, then the first pitch angle control signal is multiplied by -1 and is, in effect, phase shifted by 180 degrees to comprise the output first pitch angle control signal. Likewise, the second pitch angle control signal is multiplied by -1 and is, in effect, phase shifted by 180 degrees to comprise the output second pitch angle control signal. Thus, in the present example, the predetermined phase shift is 180 degrees. Accordingly, the blade pitch change that occurs over a rotation of the rotor will be 180 degrees out of phase when the wind direction is at the first side of the wind turbine relative to the second side of the wind turbine 100.

In the present examples, a 180 degree phase shift is applied to the cyclic IPC control signals but in other examples, the predetermined phase shift comprises a phase shift between 120 degrees and 240 degrees. In one or more examples, the predetermined phase shift comprises a function of the wind direction information. As an example, the function may be formulated such that the amount of yaw moment depends on the magnitude of the wind direction change.

In one or more examples, said wind direction information is derived from one or more sensors, which may comprise a wind direction sensor, such as a wind vane or LIDAR based sensor. In other examples, the one or more sensors may comprise acceleration sensors. Accordingly, the controller may be configured to receive acceleration information indicative of the acceleration subjected to the tower and/or nacelle. The example graph of Figure 12 shows two

traces

1201 and 1202 of acceleration information derived from an acceleration sensor configured to measure the side-to-side acceleration the tower is subjected to relative to a long-term average. It will be appreciated by those skilled in the art that a long-term average of the acceleration information provides a reference point because it can be assumed that over the long term the tower will be subject to acceleration in all directions and an average of this acceleration information will be indicative of a neutral tower acceleration. In other examples the controller may be configured to use a reference point determined in a different way, such as from position sensors determining a neutral position of the tower. Trace 1201 shows the acceleration information relative to the average acceleration information when the wind direction is from the first side. Trace 1202 shows the acceleration information relative to the average acceleration information when the wind direction is from the second side. It can be readily appreciated that there is a strong correlation between the sign of the acceleration information relative to the average acceleration information. It has been found that there are several effective methods that may be used to derive the wind direction from the acceleration information. Thus, in summary and in one or more examples, we provide a controller configured to determine a side of the wind turbine upon which the wind is incident based on acceleration information indicative of the side-to-side acceleration experienced by the wind turbine. In a first example, the controller may be configured to low pass filter the acceleration information at a frequency lower than the resonant frequency of the tower (e.g., less than 0.5 of the resonant frequency of the tower) and integrate the acceleration information over time and then further integrate the result over time to obtain a displacement. The direction of the displacement has been found to be indicative of the wind direction as the force of the wind against the wind turbine displaces it in the direction of the wind flow. In other examples, rather than a low-pass filter, a notch filter tuned to the resonant frequency of the tower may be used. In a further example, the controller may be configured to provide the tower side-to-side acceleration information to a cumulative sum block, such that the input to the block is long-term de-trended to provide the reference point. The cumulative sum block may be configured to provide the CUSUM (or cumulative sum control chart) sequential analysis technique. Such a cumulative sum block may be used to determine if the current acceleration information is positive or negative and thereby from which side of the wind turbine the wind is incident, while being robust to noise.

In the present examples, the controller is configured to provide for application of a Coleman Transform by block 518 to said output first pitch angle control signal 507 and said output second pitch angle control signal 508 to provide said cyclic individual pitch control of the blades of the wind turbine, which are shown at 510-512. The Coleman Transform block is configured to receive a zero-point pitch at input 513 about which the cyclic IPC pitch changes are made. This value may be understood as the DC component input to the Coleman transform block. In this example, a zero value is provided at input 513. The Coleman Transform block is configured to receive a phase offset input at input 514. The value provided at input 531 controls the collective pitch angle contribution for the Coleman transform, which happens to be set to zero for this embodiment. The phase offset input at input 514 determines the phase offset applied by the Coleman transform when transforming the D and Q component signals into the three control signals at outputs 510-512 for control of each respective blade. In the present example, the phase offset input is not controlled because the phase shift has been achieved by applying it to the D-component and the Q-component. However, in an alternative embodiment described later, the phase shift applied by block 515 may be applied to the phase offset input 514.

The Coleman Transform block 518 provided by the controller is further configured to receive the present azimuth angle of the rotor at input 528 based on azimuth angle information received from one or more sensors and/or derived from other information. It will be familiar to those skilled in the art that the Coleman transform provides its transformation of the D-component and Q-component into the one or more control signals at outputs 510-512 based on the phase offset input at 514 and the current azimuth angle received at 528.

We also disclose a further embodiment in which the block 515 is configured to provide for output of two different predetermined phase shifts; that is the predetermined phase shift mentioned above (which may be referred to here as the first predetermined phase shift) and a second predetermined phase shift. Thus, the controller is configured such that if the wind direction information is indicative of a wind direction incident at a first side of the wind turbine, then it provides the one or more control signals for implementation of cyclic IPC based on said output first pitch angle control signal and the output second pitch angle control signal, and with the first predetermined phase shift. Further, if the wind direction information is indicative of a wind direction incident at the second side of the wind turbine, then the controller provides the one or more control signals for implementation of cyclic IPC based on said output first pitch angle control signal and the output second pitch angle control signal without the first predetermined phase shift but with the second predetermined phase shift. Accordingly, in this example, the predetermined phase shift may be +90 degrees and the second predetermined phase shift may be -90 degrees. Thus, the cyclic IPC implemented by such an embodiment will be 180 degrees out of phase depending on which side of the wind turbine the wind direction is incident, similar to the first embodiment. It will be appreciated that other values for the first predetermined phase shift and the second predetermined phase shift may be used.

Thus, in one or more examples, the controller 101 is configured to provide for conditional application of a phase shift to the cyclic IPC control signals based on wind direction. The magnitude of the blade pitch changes implemented as part of the present cyclic IPC control action will now be described. In some examples, only the phase control described above is provided and, in other examples, only the magnitude control described below is provided and, in other examples, both are provided.

The controller 101 receives the collective pitch reference angle indicative of the current pitch angle of the blades of the wind turbine at 501 and is referred to herein as β _coll. The controller 101 receives the minimum collective pitch angle indicative of a blade pitch angle at which it is determined that blade stall occurs (determined by a different process) at 502 and is referred to herein as β _opt. It will be appreciated that the minimum collective pitch angle comprises a prediction of when blade stall occurs and therefore may include a margin for error and thereby represents a blade pitch angle approaching stall or, put another way, when stall is likely to occur plus a margin pitch angle.

The controller is configured to calculate a maximum-IPC-pitch-angle, IPC _max, based on a difference between the collective pitch reference angle, β _coll, and the minimum collective pitch angle β _opt, wherein:

IPC _max= (β _coll-β _opt) .

It will be appreciated that in other examples a different function may be used and therefore IPC _max may be expressed more generally as a function of (β _coll-β _opt) . For example, IPC _max=f (β _coll-β _opt) , such as IPC _max= (β _coll-β _opt) . k where k is a proper fraction to scale the difference between β _coll and β _opt or IPC _max= (β _coll-β _opt) ±j where j is an adjustment parameter.

Block 520 comprises a difference block that receives β _coll at a non-inverting input and β _opt at an inverting input, wherein its output comprises β _coll-β _opt.

A maximum determination block 521 is configured to output the maximum of its two inputs. The block 521 receives IPC _max from block 520 at a first input and a predetermined zero value at its second input shown at block 522. The block 521 thereby determines the greater of zero and IPC _max and provides it at output 523. Thus, the combination of

blocks

520, 521 and 523, in effect, is to determine if the collective pitch reference angle, β _coll, is greater than the minimum collective pitch angle β _opt, and control the amplitude of the pitch changes as part of the cyclic IPC control action only if said collective pitch reference angle is greater than the minimum collective pitch angle. In providing this control, the controller at block 521 may output IPC _max , such that provision of cyclic IPC is based on IPC _max as well as the first pitch angle control signal and the second pitch angle control signal. Otherwise, if the collective pitch reference angle, β _coll, is less than the minimum collective pitch angle β _opt, the controller, at block 521, outputs zero and thereby inhibits the provision of cyclic IPC in terms of provision of the one or more control signals at

outputs

507, 508 or at outputs 510-512.

Put another way, the collective pitch reference angle being less than the minimum collective pitch angle may be indicative of the blades being at a pitch at which the blades are stalled or approaching stall given that the minimum collective pitch angle may include a margin for error. Thus, the controller, is configured to provide said one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine only if said collective pitch reference angle is indicative of a pitch at which the blades 108 are not stalled. Further, the difference between the collective pitch reference angle, β _coll, and the minimum collective pitch angle β _opt can be understood as the angle through which the blades can pitch from their current pitch while avoiding stall. It is therefore termed the maximum-IPC-pitch-angle because it is indicative of a maximum pitch angle deviation from the collective pitch reference angle that avoids stall of the blades 108.

A first minimum determination block 524 is configured to output the minimum of its two inputs. The first minimum determination block 524 is configured to receive the first pitch angle control signal 505 at a first input and the calculated IPC _max at a second input from the output 523. The output of the first minimum determination block 524 is the smaller of the first pitch angle control signal 505 and the maximum-IPC-pitch-angle. Accordingly, this block 524 effectively ensures that the first pitch angle control signal 505 does not instruct cyclic IPC with an amplitude (i.e., magnitude of blade pitch) that exceeds the maximum-IPC-pitch-angle.

Similarly, a second minimum determination block 525 is configured to output the minimum of its two inputs. The second minimum determination block 525 is configured to receive the second pitch angle control signal 506 at a first input and the calculated IPC _max at a second input from the output 523. The output of the second minimum determination block 525 is the smaller of the second pitch angle control signal 506 and the maximum-IPC-pitch-angle. Accordingly, this block 525 effectively ensures that the second pitch angle control signal 506 does not instruct cyclic IPC with an amplitude (i.e., magnitude of blade pitch) that exceeds the maximum-IPC-pitch-angle.

Thus, to summarize, the controller 101 is configured to:

compare each of the first pitch angle control signal and the second pitch angle control signal to a maximum-IPC-pitch-angle, which represents a maximum angle through which the blades can pitch before stall (which may include a margin for error) ;

wherein if the first pitch angle control signal 505 is greater than the maximum-IPC-pitch-angle IPC _max then value at output 526 that goes on to form the output first pitch angle control signal is said maximum-IPC-pitch-angle; and if the first pitch angle control signal is less than the maximum-IPC-pitch-angle then then value at output 526 that goes on to form the output first pitch angle control signal is said first pitch angle control signal; and

wherein if the second pitch angle control signal is greater than the maximum-IPC-pitch-angle then the value at output 527 that goes on to form the output second pitch angle control signal is based on said maximum-IPC-pitch-angle; and if the second pitch angle control signal is less than the maximum-IPC-pitch-angle then the value at output 527 that goes on to form the output second pitch angle control signal is based on said second pitch angle control signal. This feature may form an aspect of the invention based on the maximum-IPC-pitch-angle being received by the controller or calculated from the collective pitch reference and minimum pitch angle as described above.

With reference to figures 1 and 2, the present disclosure shows a wind turbine 100 including the controller 101 as described herein wherein the controller is configured to provide for cyclic IPC in accordance with the control action described herein.

In one or more examples and with reference to figure 3, we provide a controller 101 for a wind turbine 100 in combination with one or more acceleration sensors 207, 208 the controller configured to detect rapid changes in wind direction based on acceleration information from said one or more acceleration sensors 207, 208.

Figure 6 shows a second example embodiment in the form of a functional block diagram similar to Figure 5. The same reference numerals have been used for like parts/functions. As explained above, the present control action is configured to provide two main actions. Firstly, to control the phase shift applied in the determination of control signals to cause cyclic IPC based on wind direction. Secondly, to determine a maximum-IPC-pitch-angle to control the magnitude of the pitch changes provided as part of cyclic IPC. In the controller described in the following embodiment, the control of the phase shift is applied at a different point.

Thus, block 515, similar to the previous embodiment is configured to determine if the wind is incident at a first side of the wind turbine or a second side of the wind turbine, opposite the first side and thereby whether or not the predetermined phase shift is to be applied. However, rather than applying the predetermined phase shift by acting on the D-component at output 526 and the Q-component at output 527 by way of

blocks

516 and 517, the phase shift is applied to the phase offset input of the Coleman transform at input 514, Accordingly, with reference to Figure 6, the Coleman transform block 518 provided by the controller is configured to transform the D-component and Q-component received at 507 and 508 to the one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine based on the phase offset input received from at input 550 and the present azimuth angle of the rotor at input 528.

Thus, the controller is configured to receive a predetermined reference phase offset at input 514. By way of a sum block 551 the controller is configured to add or, more generally, selectively apply the predetermined phase shift determined at block 515 to the phase offset input of the Coleman transform as shown at 550. Accordingly, the phase of the cyclic IPC applied to the blades is similarly shifted to provide for improved cyclic IPC in one or more examples. Thus, in both embodiments, the selective change in phase of the cyclic IPC has been found to reduce main bearing stresses. In this example, the block 515 may selectively apply the predetermined phase shift directly to the sum block 551 without block 529.

In a further embodiment, the block 515 is configured to provide for output of two different predetermined phase shifts; that is the predetermined phase shift mentioned above (which again may be referred to here as the first predetermined phase shift) and a second predetermined phase shift. Similar to the alternative embodiment to Figure 5, a similar approach can be provided for the embodiment of figure 6, wherein the first predetermined phase shift and the second predetermined phase shift are applied/added at sum block 551. Thus, in this example, the block 515 is configured to output a plus (+1) or negative (-1) sign based on the side of the wind turbine the wind is incident. The sign then controls the sign of the phase shift added by the sum block 551 by the block 529. Thus, block 529 may be set to provide a 90 degree phase shift, which becomes a +90 degree phase shift (the first predetermined phase shift) when the block 515 determines that the wind direction is from the first side and a -90 degree phase shift (the second predetermined phase shift) when the block 515 determines that the wind direction is from the second side. It will be appreciated that other first predetermined and second predetermined phase shift values may be applied by the controller. In addition, the first predetermined and second predetermined phase shifts may be of different magnitudes.

In terms of the magnitude of the pitch changes, the first and second minimum determination blocks 524, 525 provide the input to the Coleman transform block 518 and the phase offset application blocks 516 and 517 are absent. Otherwise, the functions provided by the controller are the same as the embodiment described with reference to Figure 5.

Figure 7 shows an example method for providing cyclic individual pitch control of a plurality of blades of a wind turbine. The method comprises:

receiving 701, 702 at least a first pitch angle control signal and a second pitch angle control signal for control of the pitch of the blades during cyclic individual pitch control, wherein the first pitch angle control signal and the second pitch angle control signal define the change in blade pitch of the plurality of blades over a rotation of the rotor;

receiving 703 wind direction information, the wind direction information indicative of the wind direction incident on said wind turbine relative to a direction in which the wind turbine is facing;

determining 704 an output first pitch angle control signal based on the first pitch angle control signal and determining an output second pitch angle control signal based on the second pitch angle control signal; and

if the wind direction information is indicative of a wind direction incident at a first side of the wind turbine, providing 705 one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine based on said output first pitch angle control signal and the output second pitch angle control signal, and with a predetermined phase shift;

if the wind direction information is indicative of a wind direction incident at a second side of the wind turbine opposite the first side, providing 705 one or more control signals for implementation of cyclic individual pitch control of the blades of the wind turbine based on said output first pitch angle control signal and the output second pitch angle control signal without the predetermined phase shift.

It will be appreciated that the controller 101 of Figure 3 may be configured to provide the Control Action for Cyclic IPC described in this section and as such may only require the input of the collective pitch reference angle, the minimum blade pitch reference angle, the at least first and second blade pitch control signals and the wind direction information.

Control Action for Negative Thrust Control

Control of the wind turbine in the event of negative thrust can be extremely important in reducing wear and undesirable loads on the main bearing of the wind turbine 100. When the wind turbine faces directly into the wind such that the direction of flow of the wind field is incident perpendicular to a plane in which the rotor 103 rotates, the wind turbine is loaded by a thrust force, designated F _t. With reference to example figure 8, the wind turbine designated 100A has the thrust force 801 acting in a positive direction. Although shown in an exaggerated manner for clarity, the wind turbine 100A is loaded and will bend in the direction of the thrust force from its upright position, shown by line 802. It will be appreciated that even when the wind turbine is not directly facing into the wind, but the wind arrives from the front of the rotor, a component of the force from the incident wind will create a positive thrust force.

The thrust force, F _t, as will be familiar to those skilled in the art, is expressed by the following equation:

wherein A comprises the swept area of the rotor 103, ρ comprises air density, v comprises wind speed incident on the wind turbine 101 and Ct (Ω, β, v) comprises the thrust coefficient.

The thrust coefficient is a function of Ω comprising the rotational speed of the rotor 103, βcomprising the pitch angle of the blades 108A-C, and wind speed, v.

In certain circumstances caused by one or more of changes in wind direction, wind speed and blade pitch angle, a negative thrust which acts in direction 803 may be generated. Thus, the thrust force, F _t, is negative. When the thrust force is negative, the wind turbine is pulled forward (or pushed forward by wind behind the rotor) , shown as 100B, and this has been found potentially to cause a high loading on the main bearing.

In the event of a large change in wind direction, such as a change of 90 degrees, the wind turbine 100A is no longer loaded by the thrust force in the direction 801. Thus, the tension in the tower and other parts of the wind turbine may cause the wind turbine to spring forward as the tower resiles, which may cause undesirable vibration and oscillatory bending motion of the blades and tower. Further, because the wind direction may now be at 90 degrees to the direction the rotor is facing, the wind flow may not act to damp the oscillatory bending motion of the blades. Alternatively or in addition, the wind field following a change in wind direction may act on the rotor to provide a negative thrust force.

Thus, in one or more examples, it can be important to provide a control action that mitigates against the generation of a negative thrust force. Further, it may be particularly important to provide said control action if negative thrust is detected or determined likely to occur, such as in response to the occurrence of a rapid change in wind direction above a threshold level.

Figure 9 shows an example functional block diagram of the controller 101 according to an example embodiment. It will be appreciated that the example functional block diagram illustrates the function (s) provided by the controller 101 to mitigate against negative thrust and the controller may be configured to provide other control actions, which can include control actions provided simultaneously. The functions shown here may be provided by a programmable logic controller. Alternatively, the filters and/or amplifiers may comprise discrete signal processing components. In a further embodiment, the controller may provide a software based implementation of the functions shown. A combination of the above implementations is also within the scope of the disclosure.

In general, the control action to mitigate against negative thrust of the present embodiment is configured to define a maximum blade pitch angle which is not to be exceeded. Thus, the controller 101 may be configured to provide one or more other control actions that control blade pitch, but if those one or more other control actions determine a blade pitch greater than (or, more generally, that exceeds) the determined maximum blade pitch angle defined by the present control action, the controller may be configured to one or more of (a) limit the blade pitch to the determined maximum blade pitch angle or (b) disregard the blade pitch determined by the one or more other control actions and thereby withhold control signals that would otherwise cause the blade to adopt such a pitch angle.

The example control action will now be described with reference to Figure 8, which shows the functionality for this embodiment provided by the controller 101. Firstly, the controller 101 is configured to receive a plurality of inputs.

The controller 101 may be configured to receive rotational speed information from the rotational speed sensor 309 at input 303. It will be appreciated that two different functional blocks receive the rotational speed information and therefore they are both labelled as inputs 303.

The controller 101 may be configured to receive wind speed information from the wind speed sensor 307 at input 301. It will be appreciated that two different functional blocks receive the wind speed information and therefore they are both labelled as inputs 301.

The controller 101 may be configured to receive air density information, which may be predetermined air density information, at an input 901. Thus, input 901 may be an example of the information from one or more other sources received at the input 304. The air density information may be indicative of the current density of the air in the wind field incident on the wind turbine. The air density information may be calculated by a different process executed by the controller 101 or another controller. In one or more examples, the air density information may be determined based on information from an atmospheric pressure sensor (not shown) and/or a temperature sensor (not shown) . In one or more examples, the controller may be configured to recall a predetermined air density from a data store (not shown) , which may be a fixed value or dynamically updated value. The air density may be estimated and provided to the controller. The air density may be estimated based on one or more of temperature and atmospheric pressure.

The controller 101 may be configured to receive blade pitch angle information, comprising the current pitch angle of the blades 108A-C, at input 902. Thus, again, input 901 may be an example of the information from one or more other sources received at the input 304. In one or more examples, the pitch of

blades

108A, 108B, 108C, that is rotation around a longitudinal axis of each blade, may be controlled. Accordingly, each

blade

108A, 108B, 108C is rotatably mounted to the hub 107 and is coupled with an actuator to control the pitch of the blade. Thus, the controller 101 may be configured to provide one or more control signals to control the actuator and thereby change the pitch of one or more of the blades 108A-C of the rotor 103. One or more blade pitch sensors may be provided to measure the current pitch of the blades 108 to provide the blade pitch angle information to the controller. In other examples, it will be appreciated that the controller 101, as part of a different control action, may be actively controlling blade pitch by the issue of one or more control signals to the blade pitch actuator (s) and the blade pitch information may be based on those control signals.

In one or more examples, the controller 101 may optionally receive an indication of an occurrence of a rapid change in wind direction above a threshold level event at an input 903. Thus, in one or more examples, the control action to determine the maximum blade pitch angle may be performed in response to an occurrence of a rapid change in wind direction above a threshold level event, but in other examples the control action to determine the maximum blade pitch angle may be performed at other times. A rapid change in wind direction may be defined as a wind direction change of 30 degrees or more that occurs over a time period of less than 30 seconds. Other threshold levels and ways of determining such the occurrence of the rapid change event may be used.

The controller 101 may be configured to determine a thrust limit wherein the thrust limit defines a minimum thrust force on the wind turbine 100 the controller seeks to maintain by control of the blade pitch. In the example that follows the controller seeks to maintain a minimum positive thrust force. As mentioned above, a positive thrust force is in direction 801 directed towards a front of the rotor. The thrust limit is determined by the functional block 904. However, in other examples, a small negative thrust force may be permitted.

In one or more other examples, the thrust limit may comprise a predetermined value and therefore determination of the thrust limit by block 904 may not be required. Accordingly, in such other examples, the block 904 may not be present and, instead, the predetermined thrust limit may be provided at input 905.

The predetermined thrust limit may comprise zero. Thus, the controller may be configured to prevent negative thrust by providing a control action comprising control of the blade pitch to avoid the thrust force falling below zero (or attempting to do so) and therefore subjecting the wind turbine 100 to a negative thrust force. In other examples, the predetermined thrust limit may be greater than zero. A greater than zero thrust limit may be advantageous because providing a control action to ensure that a (e.g., small) positive thrust force is present may act to suppress or damp any undesirable vibration or oscillatory bending of the blades 108 or tower 102. In one or more examples where the thrust limit is defined in Newtons, a beneficial thrust limit may be between 200 kN and 600 kN. Alternatively, the trust limit may be expressed in terms of a rated thrust: in such examples, a beneficial thrust limit may be between 25%and 100%. The rated thrust may comprise a predetermined thrust force which may be determined to be an upper limit force the wind turbine should be subjected to in normal use.

In one or more other examples, the predetermined minimum thrust force may comprise a negative thrust force within a predetermined negative-thrust-threshold of zero, which may be deemed to be acceptable. Thus, the controller may be configured to allow for a small negative thrust force.

It will be appreciated that wind turbines are subjected to constantly fluctuating environmental conditions and the controller 101 or other controllers may provide for changes in blade pitch to achieve other goals and therefore the control action described herein to prevent negative thrust can be understood as a control action that attempts to prevent negative thrust, but given said changing environmental conditions it may not always be achieved in practice. However, it has been found that the control action described in this section may provide for a reduced chance of the wind turbine being subjected to negative thrust, which is advantageous.

In the present example, the thrust limit is determined by the controller 101. Block 906 shows the controller 101 configured to determine a current thrust force based on the equation:

wherein the wind speed information received at input 301 provides term v, the blade pitch angle information received at input 902 provides term β, the air density information received at input 901 provides term ρ, the rotational speed information received at input 303 provides term Ω, a predetermined value indicative of the swept area of the rotor 103 provides term A.

The block 906 and thereby the controller 101, in one or more examples, is configured to provide an output of the current thrust for use determining the thrust limit at output 907, wherein the thrust limit defines a minimum positive thrust force on the wind turbine the controller seeks to maintain by control of the blade pitch. Thus, the controller may be configured to determine a thrust limit based on the current thrust experienced by the wind turbine or the thrust force experienced within a recent time period. This may be advantageous because the thrust limit is then dynamic and based on the current conditions and can therefore be of a magnitude to damp any resultant vibration caused by a change in the thrust force or, potentially, a negative thrust force. The recent time period may comprise up to 100 seconds prior to the current time or between 1 and 100 seconds prior to the current time or between 10 and 20 seconds prior to the current time. Thus, in one or more examples, the current thrust force at the time of the occurrence of a rapid change in wind direction may be used. In other examples, the current thrust force up to 5, 10, 15 or 20 seconds prior to the occurrence of a rapid change in wind direction may be used. The controller may be configured to buffer recent calculations of said current thrust force in a buffer, such that the controller can react with an appropriate current thrust value when a rapid change in wind direction is detected.

In the present examples, the thrust limit provided at input 905 comprises a proper fraction of the current thrust determined at 907. An amplifier or attenuator 908 is shown that determines a proper fraction of the current (positive) thrust. The amplifier 908 may therefore have a gain K (representing the aforementioned proper fraction) , wherein K is between -0.1 and 0.75 and preferably between 0.1 and 0.3. Thus, the controller may be configured to determine a thrust limit based on a proper fraction of the current thrust experienced by the wind turbine, whether by use of the amplifier 908 (which could equally be termed an attenuator) or by a software-defined calculation. It will further be appreciated that when K is between 0 and -0.1 a small negative thrust force is permitted, which may be acceptable in one or more examples.

Block 904 additionally includes a latch 910 which receives the indication of an occurrence of a rapid change in wind direction above a threshold level event. The latch 910 may be configured to latch, that is retain, the current thrust value at output 907 based on the occurrence of the rapid change in wind direction above a threshold level event. Thus, in one or more examples, the controller 101 may be configured to determine a thrust limit based on a proper fraction of the thrust force experienced by the wind turbine 100 at the time of detection of rapid change in wind direction above a threshold level or within a predetermined time thereof, such as within a predetermined time before the rapid change in wind direction above a threshold level event.

The thrust limit, whether predetermined or calculated in block 904, is provided to a thrust-coefficient-determination block 911. The block 911 is configured to receive the air density at input 901 and the wind speed information at input 912. In one or more examples, the wind speed information provided to the block 911 comprises average wind speed information over a predetermined recent time period. Accordingly, one example implementation may comprise the provision of a low-pass filter 913 configured to receive the wind speed information from input 301. The low-pass filter 913 is configured to filter the wind speed information to determine filtered wind speed information, which is indicative of said average wind speed information over a predetermined recent time period. The time constant of the low pass filter may be set to five seconds or between 1 and 10 seconds or between 3 and 7 seconds.

The thrust-coefficient-determination block 911 calculates a minimum thrust coefficient C _t-min (Ω, β， v) that will provide the thrust limit force determined and received at input 905 using the equation:

wherein V comprises a current or average wind speed based on the wind speed information received at input 301, ρ comprises the air density information received at input 901, A comprises the predetermined value indicative of the swept area of the rotor 103 and F _thrust-limit comprises the thrust limit received at 905. More generally the minimum thrust coefficient C _t-min is determined as a function of the thrust limit divided by a square of the current or average wind speed. Accordingly:

The minimum thrust coefficient C _t-min (Ω, β, v) calculated by block 911 is provided to maximum pitch angle determination block 914. The block 914 is configured to receive wind speed information from input 301 or a wind speed based on said wind speed information, such as the average wind speed information from low-pass filter 913. The block 914 is also configured to receive rotational speed information from the input 303. In one or more examples, the rotational speed information provided to the block 914 comprises average rotational speed information over a predetermined recent time period. Accordingly, one example implementation may comprise the provision of a low-pass filter 915 configured to receive the rotational speed information from input 301. The low-pass filter 915 is configured to filter the rotational speed information to determine filtered rotational speed information, which is indicative of said average rotational speed information over a predetermined recent time period. The time constant of the low pass filter 915 may be set to five seconds or between 1 and 10 seconds or between 3 and 7 seconds. In some examples, the time constant of the low pass filter 915 may be set up to 100 seconds.

The maximum pitch angle determination block 914 is configured to determine a maximum pitch angle β _max based on the minimum thrust coefficient C _t-min, the wind speed information (which may comprise the current wind speed or a moving average) and the rotational speed information (which may comprise the current rotational speed of the rotor or a moving average) .

It will be appreciated that β _max can be determined by the following equation:

In one or more examples, the controller is configured to determine β _max with reference to a look-up table that provides β _max for a plurality of rotational speeds and wind speeds and minimum thrust coefficients C _t-min. The look-up table may be predetermined and stored in a memory accessible to the controller. In other examples, the maximum pitch angle β _max is determined by an optimization problem the controller is configured to solve by minimizing or maximizing an objective function for β _max, as will be familiar to those skilled in the art.

The output of the controller 101 and, in particular, the output of the present negative thrust mitigation control action is the maximum pitch angle at output 916.

Thus, in general, the controller 101 may be configured to:

receive wind speed information from a wind speed sensor indicative of the wind speed;

receive rotational speed information from a rotor speed sensor indicative of the rotational speed of the rotor;

determine a minimum thrust coefficient, C _t-min, comprising the thrust coefficient that provides a predetermined minimum positive thrust force on the wind turbine, wherein

wherein V comprises a wind speed based on the received wind speed information, ρ comprises air density based on air density information, A comprises a predetermined value indicative of the swept area of the rotor 103 of the wind turbine and F _thrust-limit comprises the predetermined minimum positive thrust force; and

determine a maximum pitch angle β _max based on the minimum thrust coefficient C _t-min, the wind speed information and the rotational speed information, wherein said controller is configured to provide control signals to control the blade pitch of one or more blades of the wind turbine without exceeding said maximum pitch angle.

Example figure 10 shows a flow chart illustrating a method controlling the wind turbine 100. The method comprises:

receiving 1001 wind speed information from a wind speed sensor;

receiving 1002 rotational speed information from a rotor speed sensor configured to measure a rotational speed of the rotor;

determining 1003 a minimum thrust coefficient, C _t-min, comprising the thrust coefficient that provides a predetermined minimum positive thrust force on the wind turbine, the minimum thrust coefficient comprising a function of a wind speed based on the received wind speed information, air density based on air density information, a predetermined value indicative of the swept area of the rotor of the wind turbine and said predetermined minimum positive thrust force;

determining 1004 a maximum pitch angle β _max based on the minimum thrust coefficient C _t-min, the wind speed information and the rotational speed information; and

providing 1005 one or more control signals to control the blade pitch of the two or more blades of the wind turbine without exceeding said maximum pitch angle.

Example figure 11 shows a graph of blade pitch angle on axis 1101 versus time on axis 1102 to provide an example of the action of the controller 101. It will be appreciated that the controller or other process may be controlling the blade pitch, which will be termed the controlled blade pitch. The controller in this example is ensuring that the controlled blade pitch does not exceed the determined maximum pitch angle.

The dashed line shows the determination of the maximum pitch angle. Thus, at time 1103, an occurrence of a rapid change of wind direction above a threshold level event may have been determined by any appropriate method. It is appreciated that for the purpose of this negative thrust mitigation control action it is immaterial how this is detected, but provision of the control action during or in response to the event is advantageous. Accordingly, the maximum pitch angle is established. At times 1104, that is between time 1103 and time 1105, the controller is configured to ensure the controlled blade pitch does not exceed said maximum pitch angle defined by the dashed line. If the controlled blade pitch is greater than the determined maximum pitch angle, the controller is configured to limit the controlled blade pitch to the determined maximum blade pitch angle. Thus, the controlled pitch angle can be set to any pitch angle in region 1106 but if the controller determines that the controlled pitch angle is in region 1107, the controller may limit the pitch angle to the follow the dashed line at times 1104.

In the present example, the controller is configured to provide the control action to mitigate against negative thrust for a predetermined duration after the detection of the rapid change in wind direction event. The predetermined duration is shown here as the difference between time 1105 and time 1104.

Control Action for Shutdown of the Wind Turbine

In some instances it may be necessary to shutdown the wind turbine 100. As shown at step 406 in Figure 4, shutdown may be provided in response to an occurrence of a rapid change in a wind direction above a threshold. Shutdown involves slowing the rotor to a minimum-rotor-speed, which typically comprises bringing the rotor to a stop or close to a stop. The shutdown of a wind turbine may be defined by a control action executed by the controller 101. However, it has been found that a default shutdown control action may not be appropriate in all circumstances or, put another way, may have limitations in some circumstances.

After the wind direction has changed rapidly, the wind turbine 100 will be operating at high yaw error. In very general terms, a high yaw error occurs when the difference between the direction the wind turbine is pointing, i.e., the nacelle orientation, and the current wind direction is high, such as above 30 or 40 degrees. A rapid change in wind direction such that there is a high yaw error has been found to induce high levels of vibration in the wind turbine by vibration or oscillation of components in various modes, as will be known to those in the art. In particular, vibrations at that occur at 1P and 3P frequencies in the fixed frame and the 2P frequency in the rotating frame are excited (where P designates the rotational speed of the wind turbine) . Furthermore, the aerodynamic damping of these vibrations is low when at high yaw error because the wind flow may not be acting against the directions of vibration. As a result, operating the wind turbine where the rotational frequencies of the rotor coincide with structural, resonant frequencies (including vibration modes) of the tower and/or blades has a tendency to cause high level of vibrations, which may be much higher than during normal operation. Thus, in summary, rapid changes in wind direction may cause high levels of excitation in the tower and/or blades at a time when the aerodynamic damping is low.

It may also necessary to account for limitations in how quickly control actions can be implemented. For example, there are limits on how much torque the generator can apply before manufacturer defined thresholds are exceeded. Further, there are limits on how fast the blade pitch actuators can change the pitch of the blades. Further, there may be other limitations on the rate of change in blade pitch because other control actions may enforce limits to mitigate against other undesirable effects. For example, the present shutdown control action may be provided in combination with a control action for negative thrust control and therefore the change in blade pitch may be restricted if it is determined that the change would cause an undesirable negative thrust force on the wind turbine.

Figure 13 shows an example graph of rotor frequency vs. rotor speed. A first, second and third dashed

line

1301, 1302, 1303 is shown illustrating the 1P, 2P and 3P relationship respectively where P designates the rotor speed. Further horizontal lines are shown at different frequencies that correspond to resonant frequencies or modes of vibration of the tower and blades. In particular, line 1304 designates an edgewise backward whirl vibration mode. Line 1305 designates a first flapwise collective vibration mode. Line 1306 designates a first fore-aft or side-to-side tower vibration mode. Line 1307 designates a flapwise cyclic vibration mode.

Figure 13 also shows where these vibration modes coincide with the 1P, 2P and 3P lines 1301-1303. Circle 1308 shows where the 3P line coincides with the line 1305 indicating the rotor speed which will cause first flapwise collective undesirable vibration. Circle 1309 shows where the 2P line coincides with the line 1306 indicating the rotor speed which will cause undesirable tower vibration. Circle 1310 shows where the 2P line coincides with the line 1307 indicating the rotor speed which will cause undesirable first flapwise cyclic vibration. Circle 1311 shows where the 3P line coincides with the line 1304 indicating the rotor speed which will cause undesirable edgewise backward whirl vibration.

It has been identified that there is a range of rotational speeds designated by box 1312 that contains a large number of these coincidences. It is appreciated that there are typically other vibrational modes not shown for clarity. Box 1313 shows a lower range of rotor speeds to which the shutdown control action may be aiming to bring the rotor.

Example Figure 14 illustrated a functional block diagram illustrating how the controller 101 is configured to operate. The controller comprises a shutdown-mode-decision block 1400. The block 1400 is configured to receive a shutdown-request at a first input 1401 comprising a request to reduce the rotational speed of the rotor to at least a predetermined minimum-rotor-speed, such as speed 1320 in figure 13. In one or more examples, that predetermined minimum-rotor-speed is zero or substantially zero. In other examples a small residual rotor speed may be acceptable. In these examples, the predetermined minimum-rotor-speed comprises less than 0.3 radians/second. However, the predetermined minimum-rotor-speed may be expressed differently. Thus, the predetermined minimum-rotor-speed may comprise less than 25%, 20%, 15%or 10%of a rated rotational speed of the wind turbine, wherein the rated rotational speed comprises a predetermined value. The rated rotational speed of a wind turbine is a common parameter associated with a wind turbine and comprises a rotational speed at which the wind turbine provides its full rated power output.

The block 1400 is also configured to receive rapid-wind-direction-change-information at a second input 1402. The rapid-wind-direction-change-information informs the controller and the block 1400 when there is an occurrence of a rapid change in wind direction above a threshold level. In one or more examples, said rapid change in wind direction above a threshold level may comprise a change in wind direction greater than 30 degrees that occurs within a time of up to 30 seconds. However, what is determined to be a rapid change in wind direction upon which the controller should act may vary between wind turbines. However, it will be appreciated that the rapid-wind-direction-change-information informs the controller when there is a high yaw error and therefore low aerodynamic damping and that there is a chance for the vibrational modes of Figure 13 to occur, undamped, at the rotor speeds shown.

The block 1400 may also be configured to receive the current rotor speed at a third input 1403. One or more of the control actions provided by the controller may be based on or require feedback of the current rotor speed or a recent average thereof.

As mentioned above, a pitch of the blades 108 is controllable and a torque applied to the rotor 103 by the generator 105 is controllable and the controller may be configured to control these parameters to effect shutdown of the wind turbine.

The controller is configured to provide a first shutdown-mode, perhaps considered a default shutdown mode, represented by block 1404. In provision of the first shutdown-mode, the controller is configured to provide one or more first shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades to slow the rotor and (b) a change in a torque applied to the rotor by the generator to slow the rotor. The first shutdown-mode may comprise a steady and constant reduction in rotor speed subject to any external constraints.

The controller is configured to provide a second shutdown-mode represented by block 1405. The second shutdown-mode is different to the first shutdown-mode. The second shutdown-mode may be different to the first shutdown-mode in terms of one or more of the speed at which shutdown is provided, the rate at which the rotor is slowed or the strategy with which the generator torque and blade pitch is changed to slow the rotor and achieve the shutdown. In provision of the second shutdown-mode, the controller 101 is configured to provide one or more second shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades to slow the rotor and (b) a change in a torque applied to the rotor by the generator to slow the rotor. Typically, both the generator torque and blade pitch are changed to achieve the shutdown in both modes.

The second shutdown mode may be provided to shutdown the wind turbine more quickly, which can be desirable when the wind turbine is subjected to a rapid change in wind direction.

Accordingly, the one or more second shutdown control signals of the second shutdown-mode are configured to slow the rotor at a faster rate than the one or more first shutdown control signals at least over a predetermined range of rotational speeds. The predetermined range of rotational speeds is defined such that it includes at least one rotational speed that corresponds to a resonant frequency of one or more of the rotor, the tower or the blades such as illustrated in Figure 13 for example. Thus, the predetermined range of rotational speeds may, in one or more examples, correspond to the rotational speeds of box 1312. The controller may be configured to receive rotational speed information indicative of the rotational speed of the rotor so that the control to slow the rotor over the predetermined range of rotational speeds can be provided. In other examples, the rotational speed may be inferred with knowledge of the expected effect of one or more second shutdown control signals or from other measurements.

The shutdown-mode-decision block 1400 is therefore configured to receive the shutdown-request at the first input 1401 and the rapid-wind-direction-change-information at the second input 1402 and determine whether to provide the first shutdown-mode or the second shutdown-mode. In the event of a rapid change in wind direction it has been found that it can be advantageous to shutdown the wind turbine 100 more quickly and, in particular, to shutdown the wind turbine more quickly when the rotor speed is in the predetermined range of rotational speeds. Thus, the shutdown-mode-decision block 1400 is therefore configured to provide the second shutdown-mode rather than the first shutdown-mode based on receipt of rapid-wind-direction-change-information indicative of the occurrence of a rapid change in wind direction above a threshold level.

It will be appreciated that the second shutdown mode may be activated at times other than at times of a rapid change in wind direction above a threshold level, such as in the event of an emergency. However, for the purpose of this embodiment, the selection of the second shutdown mode rather than the “default” first shutdown mode is based on there being or having recently been a rapid change in wind direction above a threshold level.

Thus, the controller 100 as shown by block 1400 is configured to select the default first shutdown mode when the shutdown-request is received and there is no indication of an occurrence of a rapid change in wind direction above a threshold level in the rapid-wind-direction-change-information.

Example figure 13, as described above, shows different modes of vibration that can be excited in the tower or blades of the wind turbine. However, as will be known to those skilled in the art, there are numerous other modes of vibration or oscillation.

Thus, the predetermined range of rotational speeds may include rotational speeds that correspond to one or more of the following resonant frequencies:

(a) a first flapwise collective excitation frequency;

(d) a blade flap frequency of one or more of the plurality of blades;

(e) a collective flap frequency of all of the plurality of the blades;

(f) a collective edge frequency of all of the plurality of the blades;

(g) a forward and backwards whirling flap frequency;

(h) a forward and backwards whirling edge frequency;

(i) a tower torsional excitation frequency; and

(j) a blade torsional excitation frequency.

Further, the predetermined range of rotational speeds may comprise a contiguous range of speeds that covers the one or more, two or more or three or more resonant frequencies listed above. In other embodiments, the predetermined range of rotational speeds may comprise a discontinuous range of rotational speeds that is focussed on two or more resonant frequencies.

It has been found that the frequencies that correspond to the flap modes of oscillation may be more problematic following a rapid change in wind direction. The flap frequencies are prone, in one or more examples, to provide damaging oscillations when undamped by wind flow and therefore by defining the predetermined range of rotational speeds based on those frequencies, the controller can mitigate against the effect by passing through those frequencies when slowing the rotor by slowing the rotor more quickly during said predetermined range of rotational speeds. According, in one or more examples, the said predetermined range of rotational speeds includes rotational speeds that correspond to a combination of one, two, three or more of said flap frequencies. Thus, the frequencies listed at (a) , (d) , (e) and (g) may be included in said predetermined range of rotational speeds.

Further, in one or more examples, said predetermined range of rotational speeds includes rotational speeds that at which the 1P and at least one of 2P and 3P, wherein P designates the rotational speed of the rotor, correspond to one or more of:

(a) a first flapwise collective excitation frequency;

(c) a blade flap frequency of one or more of the plurality of blades;

(d) a collective flap frequency of all of the plurality of the blades; and

(e) a forward and backwards whirling flap frequency.

Thus, it will be appreciated that certain modes of vibration are excited at multiples of the rotor speed and therefore identifying 2P, 3P and in some examples, 4P, 6P and 9P rotational speeds that correspond to the frequency of problematic modes of vibration can be advantageous in defining the predetermined range.

In one or more examples, the predetermined range of rotational speeds is tightly focussed on the rotational speeds and multiples thereof that correspond to the vibration modes of the tower and/or blades. Thus, the predetermined range of rotational speeds may extend between a lower rotational speed 1321 and an upper rotational speed 1322. The upper rotational speed 1322 is defined by a rotational speed that corresponds to a vibrational mode of one or more of the rotor or the tower or the blades plus a first threshold amount. Thus, in figure 13 it can be seen that the upper threshold amount is at a slightly greater value than the circle 1308 which corresponds to a 3P vibrational mode. Further, said lower rotational speed 1321 may be defined by a rotational speed that corresponds to a resonant frequency or vibrational mode of one or more of the rotor or the tower or the blades minus a second threshold amount. Thus, the difference between the circle 1310 and the lower rotational speed 1321 may correspond to the second threshold amount. Thus, in this example, the second threshold amount is greater than the first threshold amount. However, in other examples different first and second threshold amounts may be used.

We now consider further differences between the first shutdown-mode provided by the controller as represented by block 1404 and the second shutdown-mode provided by the controller as represented by block 1405.

In this and other examples, the one or more second shutdown control signals provide for application of a greater generator torque to slow the rotor than the one or more first shutdown control signals, at least at rotational speeds corresponding to the predetermined range of rotational speeds. Thus, the slowing of the rotor more quickly during the predetermined range of rotational speeds may be achieved, at least in part, by the selective application of a greater generator torque.

It will be appreciated that the controller can provide the effect of increased generator torque in any suitable manner. However, in a first example, the application of the greater generator torque is provided by the second shutdown control signals being configured to cause an increase in the voltage across one or more coils of the generator, which will, in turn, increase the current through the coils of the generator, with the effect of increase torque applied to slow the rotor. In other examples, the second shutdown control signals may be configured to request an increase in the power output of the generator, which can have the effect of increasing the torque on the rotor to slow the rotor.

In one or more examples, the torque applied at any one time during the second (and first) shutdown-mode may vary. Thus, the greater generator torque applied during the second shutdown-mode may be a greater average torque over a range of rotational speeds or an average over a predetermined time. Thus, the second shutdown-mode may provide a greater mean average torque over a time period between 10 seconds and 60 seconds, which may correspond to the time it takes the second shutdown-mode to complete or only part thereof. In other examples, the greater generator torque applied during the second shutdown-mode may be a greater peak torque.

It is common for the operation of wind turbines to be governed by various operational limits. The operational limits are typically manufacturer set and are present to prevent or limit the amount of stress or torque on components to levels that are considered acceptable, such that the components may function for their intended lifetime. Other operational limits may be imposed because if exceeded, the temperature, currents, voltages or forces on one or more components may reach levels deemed undesirable for the effective function of the turbine over its lifetime. One such operational limit may comprise a generator torque limit. The generator torque limit may be provided by the controller and defines an upper limit to the torque the generator 104 should apply to the rotor 103. It will be appreciated that the generator may be physically capable of applying a greater torque, but such application may be deemed to place undue stress or loading on components such that it is undesirable during normal operation. Rapid changes of wind direction above a threshold level may be rare events but the stress they have the potential to cause may be significant. Accordingly, in some examples, it may be determined or probabilistically understood that exceeding an operational limit, despite the stress that would be caused, may be less, on average, than the stress that could be caused by the effects of a rapid changes of wind direction above a threshold level.

Thus, in one or more examples, the controller 101 is configured to enforce a generator torque limit which defines the maximum torque that the one or more first shutdown control signals cause the generator to apply to the rotor during the first shutdown-mode. It will be appreciated that the generator torque limit may also apply to one or more or all control processes outside the second shutdown-mode.

However, the controller may be configured to provide said one or more second shutdown control signals to cause the generator to apply a torque greater than said generator torque limit during the second shutdown-mode thereby exceeding the generator torque limit. In some examples, exceeding the generator torque limit may only be caused to occur when the rotor has a rotational speed within the predetermined range of rotational speeds.

It will be appreciated that the slowing of the rotor can also be achieved through adjustment of the blade pitch towards a feathered orientation. In one or more examples, the blade pitch may be adjusted in combination with changes in the generator torque. Alternatively, in this example, the controller is configured to, while providing the second shutdown-mode of block 1405, provide said one or more second shutdown control signals such that they cause a change the pitch of the plurality of blades towards a feathered blade orientation when the torque applied by the generator is greater than said generator torque limit. In some examples, the controller may define a second generator torque limit comprising a greater generator torque than the generator torque limit that is acceptable in limited circumstances such as during the second shutdown-mode.

Further, in one or more examples, the adjustment of the blade pitch towards a feathered orientation may be provided in response to the generator torque reaching the second generator torque limit. Thus, prior to the second generator torque limit (or generator torque limit) being reached, the controller may not change the blade pitch as part of the second shutdown-mode.

While an objective of the second shutdown-mode is to reduce the rotational speed of the rotor quickly, it may not always be possible to do this at the fastest rate the various blade actuators and generator will allow. A wind turbine will be subjected to various forces during shutdown and these need to be managed by one or more other control processes outside the second shutdown-mode described here. However, in summary, for the purpose of the present control process, these one or more other control processes have the effect of defining limits on blade pitch angle or rate of change in blade pitch angle at particular points in time during execution of the second (and first) shutdown-mode.

Thus, the controller 101 may be configured to receive blade pitch limit information which defines a temporary limit on the blade pitch at a point in time. This blade pitch limit (including rate changes) may be explicit information provided to the presently described control action. In other examples, the blade pitch limit information may be inferred based on feedback on current blade pitch not matching expected blade pitch because said one or more other control processes have intervened to limit the blade pitch. As discussed herein, one example of said blade pitch limit information may be determined by the controller to mitigate against a (e.g., large) negative thrust force being exerted on the wind turbine.

Thus, said one or more second shutdown control signals may be configured to change the pitch of the plurality of blades towards a feathered blade orientation without exceeding said temporary limit on the blade pitch.

When the rotational speed of the rotor is greater than a low-rotor-speed threshold, which may comprise the speed 1321 in Figure 13 which defines the lower rotational speed of the predetermined range, the controller may be configured to cause the blades to pitch towards a feathered orientation at a first pitch rate at least within a threshold of a maximum pitch rate of the blades. This may comprise pitching to feather at the fastest rate possible subject to any blade pitch limit information. In other examples, the threshold of a maximum pitch rate may be within 5%of the maximum pitch rate, i.e., between 95%and 100%of maximum pitch rate. The maximum pitch rate may be determined to be a maximum rate a blade pitch actuator is able to rotate the blades in the control of the blade pitch.

However, when the rotational speed of the rotor is less than the low-rotor-speed threshold, which could be speed 1321, the controller 101 may cause the blades to pitch towards the feathered orientation at a second pitch rate less than said first pitch rate. Thus, the rate of change of pitch may be slowed when the rotational speed of the rotor reaches the low-rotor-speed threshold.

The second pitch rate may comprise a constant pitch rate subject to any temporary limitations imposed by the blade pitch limit information.

Thus, in one or more examples, the controller may pitch at the first pitch rate during the predetermined range of rotational speeds and then, when the low-rotor-speed threshold is reached, change the process to pitch towards feather at the second rate. However, in a further example, while said second pitch rate is employed, the controller may be configured to determine when a predetermined pitch-angle is reached, and in response thereto increase the blade pitch rate to cause the blades to pitch towards the feathered orientation at a third rate, greater than the second pitch rate.

To exemplify the contrast between the default first shutdown-mode and the rapid-wind-direction-change second shutdown-mode described above, the first shutdown mode may be as follows. The one or more first shutdown control signals may provide for reducing rotor speed by control of the pitch the blades towards a feather orientation. The one or more first shutdown control signals may provide for application of a generator torque to slow the rotor. The one or more first control signals may provide for slowing of the rotor at one or more rates irrespective of the predetermined range of rotational speeds. For example, any change in the one or more first control signals is unrelated to the rotor speed being within the predetermined range of rotational speeds.

In the above examples, the controller may be configured to monitor rotor speed when providing the second shutdown-mode so that the relevant control of increased generator torque can be provided at least when the rotor speed is within the predetermined range of rotor speeds. However, we now disclose a different control scheme. As mentioned above in relation to Figure 13, the rationale for providing for quicker slowdown during the predetermined range of rotational speeds was to avoid excessive vibration at the resonant frequencies. Thus, in these alternate examples, the rotor speed being at a speed that is within the predetermined range of rotor speeds is determined based on received acceleration information that is indicative of the acceleration experienced by one or both of the tower and a nacelle of the wind turbine. Thus, when the received acceleration information is indicative of vibration levels higher than a threshold it may be inferred that the rotor speed is within said predetermined range of rotor speeds.

Thus, during provision of the second shutdown-mode, the controller is configured to provide said one or more second shutdown control signals such that a pitch rate towards the feathered blade orientation is increased based on the acceleration information being indicative of vibrations above a first threshold vibration level. The increase in pitch rate may be an increase to within a threshold of a maximum pitch rate of the blades or to the maximum pitch rate. The maximum pitch rate may be the greatest rate the blade pitch can change.

In one or more examples, the amount of generator torque may be increased based on the acceleration information being indicative of vibrations above the first threshold vibration level.

Further, in one or more examples, the controller may be configured such that the pitch rate towards the feathered blade orientation is decreased based on the acceleration information being indicative of vibration below a second threshold vibration level, lower than the first threshold vibration level. The decrease in pitch rate may be a decrease to predetermined pitch rate or to below such a predetermined pitch rate.

In one or more examples, the amount of generator torque may be decreased based on the acceleration information being indicative of vibrations below the second threshold vibration level.

The first threshold level and the second threshold level may be same or they may be different.

As part of shutdown, a controller may cause the generator to electrically disconnect from a power convertor that connects the wind turbine to the grid. By grid we refer to the power distribution grid to which wind turbines are typically coupled but it could equally be a local load. Thus, the controller may provide for control of one or more relays that provide the electrical coupling between the generator and the power convertor and/or grid. The first shutdown-mode and second shutdown-mode may differ in terms of how the disconnect is enacted.

Typically, the one or more first shutdown control signals are provided during the first shutdown-mode are configured to provide the generator-disconnect procedure such that the torque applied to the rotor by the generator is slowly reduced to within a torque threshold of zero torque (or to zero torque) over a non-zero torque-reduction time, before the generator is disconnected from the grid-connected power converter of the wind turbine. However, given that the second shutdown-mode is intended to provide for quicker shutdown, the slow reduction of the generator to a zero torque condition may not be viable.

Thus, in one or more examples, the second shutdown control signals provided during the second shutdown-mode may be configured to cause the generator to disconnect from the grid connected power converter while the torque applied by the generator is greater than the torque threshold. While this may not minimize stresses on the generator as much as in the first shutdown-mode, it may be deemed a compromise worth performing in the unlikely event of a rapid change in wind direction in order to reduce damage cause by the undamped vibrations.

We also disclose, with reference to figures 1 and 2, a wind turbine including the controller 101 that performs the above disclosed control action for shutdown.

Example figure 15 shows a flowchart for controlling a wind turbine during shutdown. That is a method for slowing the rotor to a stop or close to a stop.

The example method comprises receiving 1501 a shutdown-request comprising a request to reduce the rotational speed of the rotor to at least a predetermined minimum-rotor-speed;

receiving rapid-wind-direction-change-information 1502 indicative of the occurrence of a rapid change in wind direction above a threshold level; and

providing a second shutdown-mode 1504 rather than a first shutdown-mode 1503 based on receipt of rapid-wind-direction-change-information indicative of the occurrence of a rapid change in wind direction above a threshold level when the shutdown-request is received.

Figure 16 shows a computer program product 1600 comprising computer program code for implementing the method of figure 4 and/or the method of figure 7 and/or the method of figure 10 and/or the method of claim 15. The computer program product 1600 may comprise a USB mass storage device or other media for use in updating software or firmware of a controller 101 of a wind turbine 100.

Claims

A controller for controlling a wind turbine having a tower and a rotor comprising a plurality of blades and wherein the rotor is coupled to a generator, wherein a pitch of the blades is controllable and a torque applied to the rotor by the generator is controllable, wherein the controller is configured to:

provide a first shutdown-mode in which the controller is configured to provide one or more first shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades to slow the rotor and (b) a change in a torque applied to the rotor by the generator to slow the rotor; and

provide a second shutdown-mode, different to the first shutdown-mode, in which the controller is configured to provide one or more second shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades to slow the rotor and (b) a change in a torque applied to the rotor by the generator to slow the rotor;

wherein the one or more second shutdown control signals are configured to slow the rotor at a faster rate than the one or more first shutdown control signals at least over a predetermined range of rotational speeds, wherein said predetermined range of rotational speeds is defined such that it includes at least one rotational speed that corresponds to a resonant frequency of one or more of the rotor, the tower or the blades; and wherein the controller is configured to:

receive a shutdown-request comprising a request to reduce the rotational speed of the rotor to at least a predetermined minimum-rotor-speed, wherein provision of the second shutdown-mode rather than the first shutdown-mode is based on receipt of rapid-wind-direction-change-information indicative of the occurrence of a rapid change in wind direction above a threshold level.
The controller of claim 1, wherein the controller is configured to, in response to receipt of the shutdown-request and absent the rapid-wind-direction-change-information being indicative of the occurrence of a rapid change in wind direction above a threshold level, provide the first shutdown-mode.
The controller of claim 1 or claim 2, wherein said predetermined range of rotational speeds includes rotational speeds that correspond to one or more of the following resonant frequencies:

(a) a first flapwise collective excitation frequency;

(b) an excitation frequency associated with a fore-aft oscillation of the tower;

(c) an excitation frequency associated with a side-to-side oscillation of the tower;

(d) a blade flap frequency of one or more of the plurality of blades;

(e) a collective flap frequency of all of the plurality of the blades;

(f) a collective edge frequency of all of the plurality of the blades;

(g) a forward and backwards whirling flap frequency;

(h) a forward and backwards whirling edge frequency;

(i) a tower torsional excitation frequency; and

(j) a blade torsional excitation frequency.
The controller of claim 1 or claim 2, wherein said predetermined range of rotational speeds includes rotational speeds that at 1P and at least one of 2P, 3P, 4P, 6P and 9P, wherein P designates the rotational speed of the rotor, correspond to one or more of:

(a) a first flapwise collective excitation frequency;

(b) an excitation frequency associated with a fore-aft oscillation of the tower;

(c) a blade flap frequency of one or more of the plurality of blades;

(d) a collective flap frequency of all of the plurality of the blades; and

(e) a forward and backwards whirling flap frequency.
The controller of any preceding claim, wherein said predetermined range of rotational speeds extends between a lower rotational speed and an upper rotational speed and wherein one or both of:

said upper rotational speed is defined by a rotational speed that corresponds to a resonant frequency of one or more of the rotor or the tower or the blades plus a first threshold amount; and

said lower rotational speed is defined by a rotational speed that corresponds to a resonant frequency of one or more of the rotor or the tower or the blades minus a second threshold amount.
The controller of any preceding claim, wherein said one or more second shutdown control signals provide for application of a greater generator torque to slow the rotor than the one or more first shutdown control signals, at least at rotational speeds corresponding to the predetermined range of rotational speeds.
The controller of any preceding claim, wherein the controller is configured to enforce a generator torque limit which defines the maximum torque that the one or more first shutdown control signals cause the generator to apply to the rotor during the first shutdown-mode, wherein the controller is configured to provide said one or more second shutdown control signals to cause the generator to apply a torque greater than said generator torque limit during the second shutdown-mode thereby exceeding the generator torque limit.
The controller of claim 7, wherein the controller is configured to provide said one or more second shutdown control signals such that they cause a change the pitch of the plurality of blades towards a feathered blade orientation during the second shutdown-mode when the torque applied by the generator is greater than said generator torque limit.
The controller of claim 8, wherein said controller is configured to receive blade pitch limit information which defines a temporary limit on the blade pitch at a point in time, wherein said one or more second shutdown control signals are configured to change the pitch of the plurality of blades towards a feathered blade orientation without exceeding said temporary limit on the blade pitch, wherein said blade pitch limit information is determined by the controller to mitigate against a negative thrust force being exerted on the wind turbine, where a negative thrust force acts in a direction to urge the rotor in the direction in which it is pointing.
The controller of any preceding claim, wherein the controller is configured to receive rotational speed information indicative of the rotational speed of the rotor and, in the second shutdown-mode, said second shutdown control signals are configured to:

when the rotational speed of the rotor is greater than a low-rotor-speed threshold, cause the blades to pitch towards a feathered orientation at a first pitch rate at least within a threshold of a maximum pitch rate of the blades; and

when the rotational speed of the rotor is less than the low-rotor-speed threshold, cause the blades to pitch towards the feathered orientation at a second pitch rate less than said first pitch rate.
The controller of claim 10, wherein said second pitch rate comprises a constant pitch rate.
The controller of claim 10 or claim 11, wherein said controller is configured to receive blade pitch limit information which defines a temporary limit on the blade pitch at a point in time, and wherein said second pitch rate comprises a constant pitch rate at least at times when the change in blade pitch at the second pitch rate is unaffected by said temporary limit defined by said blade pitch limit information.
The controller of claim 10, wherein the controller is configured to, during provision of said second shutdown control signals, determine whether the blade pitch is at a predetermined pitch-angle and, when said predetermined pitch-angle is reached, increase the blade pitch rate to cause the blades to pitch towards the feathered orientation at a rate greater than the second pitch rate.
The controller of claims 1 to 7, wherein the controller is configured to receive acceleration information indicative of the acceleration experienced by one or both of the tower and a nacelle of the wind turbine, and wherein during provision of the second shutdown-mode;

the controller is configured to provide said one or more second shutdown control signals such that they cause a change in the pitch of the plurality of blades towards a feathered blade orientation;

wherein a pitch rate towards the feathered blade orientation is increased based on the acceleration information being indicative of vibrations above a first threshold vibration level; and

wherein the pitch rate towards the feathered blade orientation is decreased based on the acceleration information being indicative of vibration below a second threshold vibration level, lower than the first threshold vibration level.
The controller of any preceding claim, wherein said predetermined minimum-rotor-speed comprises less than 0.3 radians/second.
The controller of any preceding claim, wherein said predetermined minimum-rotor-speed comprises less than 25%of a rated rotational speed of the wind turbine, wherein the rated rotational speed comprises a predetermined value.
The controller of any preceding claim, wherein, the controller is configured such that:

the one or more first shutdown control signals provided during the first shutdown-mode are configured to provide a generator-disconnect procedure in which the torque applied to the rotor by the generator is reduced to within a torque threshold of zero torque before the generator is disconnected from a grid connected power converter of the wind turbine; and

the one or more second shutdown control signals provided during the second shutdown-mode are configured to cause the generator to disconnect from the grid connected power converter while the torque applied by the generator is greater than the torque threshold.
A wind turbine including the controller of any preceding claim.
A method of controlling a wind turbine having a tower and a rotor comprising a plurality of blades and wherein the rotor is coupled to a generator, wherein a pitch of the blades is controllable and a torque applied to the rotor by the generator is controllable, the method comprising:

receiving a shutdown-request comprising a request to reduce the rotational speed of the rotor to at least a predetermined minimum-rotor-speed;

receiving rapid-wind-direction-change-information indicative of the occurrence of a rapid change in wind direction above a threshold level;

providing a second shutdown-mode rather than a first shutdown-mode based on receipt of rapid-wind-direction-change-information indicative of the occurrence of a rapid change in wind direction above a threshold level when the shutdown-request is received;

wherein

the first shutdown-mode comprises providing one or more first shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades to slow the rotor and (b) a change in a torque applied to the rotor by the generator to slow the rotor; and

the second shutdown-mode is different to the first shutdown-mode, and comprises providing one or more second shutdown control signals to provide for one or both of (a) a change of blade pitch of one or more of the plurality of blades to slow the rotor and (b) a change in a torque applied to the rotor by the generator to slow the rotor;

wherein the one or more second shutdown control signals are configured to slow the rotor at a faster rate than the one or more first shutdown control signals at least over a predetermined range of rotational speeds, wherein said predetermined range of rotational speeds is defined such that it includes at least one rotational speed that corresponds to a resonant frequency of one or more of the rotor, the tower or the blades.
A computer program product comprising computer program code, the computer program code configured to, when executed by a processor having memory, provide the method of claim 19.