Patent Description:
Wind turbine control technology is used for optimisation of power and minimisation of loads. Many different loads act on a wind turbine, such as aerodynamic, gravity, centrifugal and inertial loads. Changes in the loads experienced by a wind turbine may be caused by wind conditions in the vicinity of the wind turbine, e.g. wind shear or turbulence, or may be caused by changing operation of the wind turbine, e.g. grid loss.

Wind turbine control approaches use a combination of collective and cyclic control, for instance controlling collective and individual pitch of a number of blades of a wind turbine, or controlling torque or power output of a generator of the wind turbine. Cyclic or individual pitch is used to control a number of factors, such as lateral (side-side) tower damping, asymmetric rotor load control, and tower torsional dampening.

As more advanced control approaches are implemented to control an ever-greater number of factors, there is a risk that some of the control approaches or features may in fact counteract one other. That is, implementation of one control approach directed to one control objective may degrade the performance or effectiveness of another control approach directed to another control objective that is being implemented at the same time.

<CIT> discloses a diagnostic system for use in a wind turbine yaw system. In particular, errors in the yaw system are detected based on measured tower side-side oscillations. <CIT> discloses a method for wind turbine tower damping. A blade reference signal is determined based on a tower damping controller output added to the output of a full or partial load controller. The blade pitch reference signal is then compared to a saturation pitch reference signal, and the maximum value is used as a maximum pitch reference signal to control the wind turbine. <CIT> discloses a method for controlling wind turbine rotor blades during an extreme wind event. <CIT> discloses a method for controlling a wind turbine in accordance with a controllable parameter only when a determined variation of loading on the wind turbine is below an alert threshold.

The inventor of the present invention has realised that different individual or cyclic pitch control approaches or algorithms can have a detrimental effect on the effectiveness of one another's performance. In particular, instead of reducing structural loading, control actions such as cyclic pitch control may in fact have the opposite effect. Clearly, this is undesirable and could negatively impact on the lifespan of certain structural components of a wind turbine. It has therefore been understood that there is a need to ensure that particular control actions can continue to address the particular factors that they are aimed at addressing without having a negative effect on other factors that also need to be addressed.

According to an aspect of the present invention there is provided a method of controlling operation of a wind turbine having a tower. The method comprises determining an overall control output according to a wind turbine control strategy and based on monitored operation of the wind turbine. The overall control output includes rotor load control for reducing asymmetric loading on a rotor of the wind turbine and includes lateral oscillation control for dampening, or countering, lateral oscillation of the tower. The method comprises using the determined overall control output to control operation of the wind turbine. The method further comprises receiving lateral oscillation sensor data indicative of a level of lateral (side-to-side) oscillation of the tower, determining a rated lateral oscillation control output in dependence on the received lateral oscillation data, and receiving an indication of a yaw error of the wind turbine. A lateral oscillation control output included in the overall control output is determined to be de-rated from the rated lateral oscillation control output when the indicated yaw error of the wind turbine is above a predetermined lower threshold level in order to prioritise the rotor load control over the lateral oscillation control.

The control strategy can include any suitable or desired objectives for controlling operation of the wind turbine, such as optimising or limiting power output of the turbine. The monitored operation can include measured and/or estimated values of various operating parameters of, or associated with, the wind turbine, e.g. rotor speed, power output, loading on various wind turbine components, wind conditions, etc. Yaw error refers to a misalignment of a rotor plane of the wind turbine relative to a wind direction in the vicinity of the turbine, in particular a misalignment in a yaw direction (which constitutes rotation of a nacelle of the wind turbine about an axis defined by the wind turbine tower), but can also include misalignment in a tilt direction of the turbine. The rated lateral oscillation control output may be regarded as an optimal control output for dampening or countering the lateral oscillations, and de-rating from the rated value may be regarded as reducing the effectiveness of the control output in dampening or countering the lateral oscillations.

The lateral oscillation control output may be determined to be the rated lateral oscillation control output when the indicated yaw error of the wind turbine is below the predetermined lower threshold level.

The lateral oscillation control output may be de-rated monotonically for increasing indicated yaw error of the wind turbine above the predetermined lower threshold level.

The monotonic de-rating may be a linear de-rating from the rated lateral oscillation control output.

The lateral oscillation control output may be de-rated to substantially zero lateral oscillation control when the indicated yaw error of the wind turbine is above a predetermined upper threshold level greater than the predetermined lower threshold level.

In some embodiments, the method comprises determining a lateral oscillation activity level, based on the received indication of yaw error, indicative of a degree to which the rated lateral oscillation control output is to be de-rated, wherein the lateral oscillation control output is determined as a proportion of the rated lateral oscillation control output using the determined lateral oscillation activity level.

The lateral oscillation control output may be included in the overall control output by at least one of: gain scheduling the rated lateral oscillation control output; and, reducing an amplitude of the rated lateral oscillation control output.

The indication of yaw error of the wind turbine may include data indicative of wind conditions in the vicinity of the wind turbine.

The data indicative of wind conditions may include data indicative of a wind direction in the vicinity of the wind turbine relative to a rotor plane of the wind turbine. Optionally, this data is sensor data.

The data indicative of wind conditions may include data indicative of at least one of: positive wind shear; negative wind shear; and, wind veer.

The indication of yaw error of the wind turbine may include data indicative of an asymmetric loading experienced by one or more components of the wind turbine, optionally an asymmetric moment experienced by the one or more components, further optionally a yaw moment experienced by the one or more components.

The received indication of yaw error may include data received from external to the wind turbine and indicative of at least one of: operation of one or more further wind turbines adjacent to the wind turbine; and, meteorological conditions in the vicinity of the wind turbine.

The indication of yaw error of the wind turbine may include an indication of a level of rotor load control in the overall control output. Alternatively, such an indication of (implemented or required) rotor load control level can be separate from the indication of yaw error, but de-rating of the rated lateral oscillation control output can additionally or alternatively be based on the indication of rotor load control and, in particular, de-rating of the rated lateral oscillation control output may be performed/determined when it is indicated that a level of rotor load control is above a predetermined lower threshold level.

In some embodiments, the lateral oscillation control output comprises a pitch modulation value for each of a plurality of respective blades of the wind turbine, and wherein the overall control output comprises an individual pitch reference value, including the respective pitch modulation value, for each of the blades.

In some embodiments, the lateral oscillation control output comprises a generator power offset value, and wherein the overall control output comprises a generator power reference value, including the generator power offset value, for a generator of the wind turbine.

According to another aspect of the present invention there is provided a non-transitory, computer-readable storage medium storing instructions thereon that when executed by a processor causes the processor to perform the method described above.

According to another aspect of the present invention there is provided a controller for controlling operation of a wind turbine having a tower. The controller is configured to determine an overall control output according to a wind turbine control strategy and based on monitored operation of the wind turbine. The overall control output includes rotor load control for reducing asymmetric loading on a rotor of the wind turbine and includes lateral oscillation control for dampening lateral oscillation of the tower. The controller is configured to use the determined overall control output to control operation of the wind turbine. The controller is further configured to receive lateral oscillation sensor data indicative of a level of lateral oscillation of the tower, determine a rated lateral oscillation control output in dependence on the received lateral oscillation data, and receive an indication of a yaw error of the wind turbine. A lateral oscillation control output included in the overall control output is less than the rated lateral oscillation control output when the indicated yaw error of the wind turbine is above a predetermined lower threshold level in order to prioritise the rotor load control over the lateral oscillation control.

According to another aspect of the present invention there is provided a wind turbine comprising a controller as described above.

One or more examples of the invention will now be described, by way of example only, with reference to the accompanying figures, in which:.

<FIG> shows a wind turbine <NUM> in which an example of the invention may be incorporated. The wind turbine <NUM> comprises a tower <NUM> supporting a nacelle <NUM> to which a rotor <NUM> is mounted. The rotor <NUM> comprises a plurality of wind turbine blades <NUM> that extend radially from a hub <NUM>. In this example, the rotor <NUM> comprises three blades <NUM> and a single rotor <NUM>, although other configurations including any suitable number of blades and rotors are possible.

The wind turbine <NUM> includes a number of different sensors for measuring various features of the operation of the turbine <NUM>, and of the conditions in the vicinity of the turbine <NUM>. Shown within each blade <NUM> is an optional blade load sensor <NUM> (in other examples there may be multiple blade load sensors allowing blade loads to be represented by more than a single variable). The sensing element may be a fibre optic strain gauge, a resistive strain gauge, or any other appropriate detector. A rotor wind speed and/or direction detector <NUM> is also shown - again, this measurement may be performed in several ways as the skilled person will appreciate, one being through a wind vane and an anemometer, and another through LIDAR, as the skilled person will appreciate from the literature of wind turbine design and control. A rotational speed sensor <NUM> is also shown - this may be, for example, in the form of a rotary encoder on a generator shaft of the turbine <NUM>; however, the rotor speed may be determined in any suitable manner. An accelerometer <NUM> for measuring lateral, or side-to-side, oscillations or vibrations of the tower <NUM> is also included at a suitable location. Further sensors for measuring data indicative of misalignment of the turbine in one or both of the yaw and tilt directions, and/or for measuring asymmetric (tilt/yaw) loading moments of components of the wind turbine <NUM>, may also be included.

<FIG> shows a wind turbine control system <NUM> in accordance with an example of the invention which may be implemented in the wind turbine <NUM> of <FIG>. Here, the control system <NUM> includes an actuator system <NUM> that is controlled by a control unit or (overall) controller <NUM>. In this particular example, the actuator system <NUM> may be, or may comprise, a pitch system for controlling pitch of one or more of the wind turbine blades <NUM> which may include a hydraulic actuator <NUM> arranged to adjust blade pitch in a known manner. The actual position of the actuator <NUM> is controllable by an actuator position control unit <NUM> which provides a positioning command signal to the hydraulic actuator <NUM>. The controller <NUM> and actuator system <NUM> may be replicated for each of the blades <NUM> of the wind turbine <NUM> so that the position of each blade <NUM> may be controlled independently.

The pitch system of the wind turbine <NUM> is just one example of a wind turbine system that may be controlled. The controller <NUM> may also be used to control other wind turbine systems and/or components. For instance, the actuator system <NUM> may be an electric or hydraulic yaw drive for the nacelle <NUM> of the wind turbine <NUM> to provide rotational position control of the nacelle <NUM> with respect to the tower <NUM>. Another example would be a converter control system where the actuator system <NUM> may be a power converter of the generation system of the wind turbine <NUM> that converts AC power delivered by the generator to a variable-frequency AC power output via a DC link in a process known as 'full power conversion', i.e. changing the synchronous speed of the generator independently from the voltage and frequency of the grid.

In one example method of the invention, the pitch of individual blades <NUM> of the wind turbine <NUM> may be controlled according to a control strategy to maximise energy production and minimise loads based on the monitored operation of the wind turbine. In particular, the individual blade pitch may be controlled to alleviate fatigue loading on the turbine tower <NUM> caused by lateral (or side-to-side) oscillations of the tower <NUM>. That is, the pitch is controlled to create a lateral or sideways force to counteract, and therefore dampen, the tower lateral oscillations. The individual blade pitch may also be controlled to reduce loading on one or more components of the turbine <NUM> caused by misalignment of a plane of the rotor <NUM> and blades <NUM> of the turbine <NUM> relative to wind direction in a tilt and/or yaw direction, or by excessive tilt and/or yaw moments.

A specific example implementing this approach in the controller or control system <NUM> is shown schematically in <FIG>. Several functional elements are shown: a main controller <NUM> determines and generates control actions or outputs 42a, 42b according to a specified control strategy and based on a monitored - e.g. measured and/or estimated - operation of the wind turbine relative to the control strategy. The control actions 42a, 42b from the main controller <NUM> include actions to provide rotor load control for reducing asymmetric loading on the rotor <NUM> of the wind turbine <NUM>. In particular, the control output 42a includes collective and individual pitching control, and the control output 42b controls wind turbine power output by, e.g. generator torque control.

Also included are functional elements for determining and generating control actions or modifications to provide lateral (side-to-side) oscillation control for dampening lateral oscillation of the tower <NUM>. In particular, control elements 44a, 44b are provided to determine (rated) control actions for dampening lateral tower oscillation based on acquired signals indicative of side-to-side acceleration of the tower <NUM>. Different elements 46a, 46b are provided to determine an activity level for the lateral tower oscillation control, based on various acquired information. The activity level is combined with the (rated) control actions from control elements 44a, 44b to provide lateral oscillation control outputs 48a, 48b to be combined with the control actions 42a, 42b from the main controller <NUM> to determine overall control outputs 50a, 50b to be used in controlling operation of the wind turbine <NUM>. This is discussed in greater detail below.

The various functional elements or units of the controller <NUM> may be provided by suitable software running on any suitable computing substrate using conventional or customer processors and memory. These various functional elements may use a common computing substrate (for example, they may run on a single server) or separate substrates, or one or each may themselves be distributed between multiple computing devices.

In the example illustrated in <FIG>, the overall control output 50a, 50b includes both: individual and collective pitch references 50a for each of the plurality of blades <NUM>; and, a power set point or reference 50b for the wind turbine generator, e.g. torque or speed. However, it may be the case that only one of collective pitch or generator torque is controlled (by means of an appropriate actuator) in accordance with the appropriate reference point(s) at any given time. For instance, it is commonly the case that collective pitch may be controlled during rated operation of the wind turbine <NUM> ('full load'), whereas generator torque may be controlled during below-rated operation of the turbine <NUM> ('partial load'). Note that individual pitch is commonly controlled during both full and partial loading of the wind turbine <NUM>. In the following, operation is mainly described in the case where the individual pitch SSTD control system 44a is active, but it will be understood that the power SSTD control system 44b may additionally, or alternatively, be used.

The main controller <NUM> generates a blade pitch control output 42a that includes both collective and individual pitch control outputs. As mentioned, the individual pitch control outputs 42a include rotor load control for reducing asymmetric loading experienced by the turbine <NUM>. This is combined with individual pitch modulation 48a to generate overall individual pitch references 50a that also include tower lateral oscillation control.

The respective individual pitch control algorithms for rotor load control and tower lateral oscillation control can, in certain situations, degrade each other's performance when control outputs from each algorithm are both included in the overall individual pitch references. This can mean that asymmetric loading is not reduced and/or lateral tower oscillations are not dampened, as intended by the respective control algorithms.

It is noted that the degradation in performance of the load-reducing cyclic pitch algorithms is particularly apparent in conditions of large yaw error - i.e. significant misalignment between a rotor plane of the turbine <NUM> defined by the rotor <NUM> and blades <NUM>, and wind direction, or excessive moments experienced by wind turbine components in the tilt and/or yaw direction. A large yaw error may mean that the asymmetric rotor load controller needs to be active to reduce asymmetric loads caused by the yaw error. However, in such a scenario it has been realised that operation of the SSTD controller to dampen lateral tower oscillations can degrade the performance of the asymmetric rotor load controller.

The present disclosure addresses this issue by recognising that in such conditions the SSTD controller can be de-rated to ease degradation in performance of the asymmetric rotor load controller, but without significant negative effects on its own objectives. In particular, it is recognised that in such conditions asymmetric rotor load control can be prioritised over tower lateral oscillation control because tower lateral oscillation control is used for reducing fatigue loads which are not so sensitive to (relatively rare) de-rating of its individual pitch contribution compared to those of rotor load control. That is, in conditions of significant tilt/yaw misalignment and/or moments performance of tower lateral oscillation control is de-rated from its determined rated or optimal performance.

The control element 44a - or side-to-side tower dampening (SSTD) control system 44a - receives data <NUM> indicative of lateral (side-to-side) oscillations experienced by the wind turbine tower <NUM>. In the described example this data is in the form of sensor data received from the accelerometer measuring lateral acceleration of the tower <NUM>.

Based on the received tower acceleration data, the control element 44a then determines a control output that is needed to dampen the lateral oscillations of the tower <NUM>. In particular, this determination may be considered as an optimal control output in terms of effectiveness of dampening the lateral oscillations being experienced by the tower <NUM>. That is, the control element 44a determines a control output for delivering rated performance of the tower lateral oscillation controller of the turbine <NUM>.

The rated control output from the control element 44a is then to be combined with the output from the control element 46a. The control element 46a - or SSTD activity level unit <NUM> - receives an indication of yaw error of the wind turbine <NUM>. This indication can take different forms.

In the described example, the indication includes sensor data or signals <NUM> indicative of a level of tilt and/or yaw misalignment of the turbine <NUM>. For instance, the sensor data <NUM> may be indicative of wind conditions in the vicinity of the wind turbine <NUM>. Specifically, the sensor data <NUM> may indicate a level of yaw misalignment or error of the turbine <NUM>, i.e. the error or difference between wind direction and the axis of the rotor <NUM>. Alternatively, yaw misalignment can be determined based on wind direction relative to a plane of the turbine <NUM> defined by the swept area of the rotor <NUM> and blades <NUM> of the turbine, where ideal yaw alignment is when wind direction is perpendicular to the rotor plane. Tilt misalignment may be determined in an equivalent manner. In this case, the received data <NUM> may include measures of relative wind direction, and/or absolute wind direction with a yaw position of the nacelle <NUM>.

The sensor data <NUM> may optionally include data indicative of extreme coherent wind gusts with direction changes, and/or of other extreme wind conditions that give rise to extreme rotor loads, such as extreme positive or negative wind shear, or wind veer. Sensor data indicative of such wind conditions may be acquired from one or more LIDAR sensors of the wind turbine <NUM>, for instance.

The indication of yaw error of the wind turbine <NUM> may optionally include sensor data or signals indicative of an asymmetric (yaw) loading experienced by one or more components of the wind turbine <NUM>. For instance, this could include a measured and/or estimated (significant) asymmetric moment experienced by one or more turbine components. In particular, these could be compensated for cyclic pitch control amplitudes that are being implemented by the controller <NUM>. Alternatively, or in addition, this could include measured and/or estimated (significant) asymmetric moments combined with rotational frequency rotor load control amplitudes. Also, asymmetric loading data may include data indicative of extreme blade flap moments from the blade sensor <NUM>.

Data indicative of wind conditions, and/or turbine component loading caused by wind conditions, may optionally be obtained from external to the wind turbine <NUM>. For instance, data may be received by the controller <NUM> - and, in particular, the control element 46a - that adjacent turbines in a wind park of the wind turbine <NUM> have experienced, or are experiencing any of the above-mentioned wind conditions or loading. Also, external data received by the controller <NUM> may include central server meteorology information indicative of wind conditions in the vicinity of the wind turbine <NUM>.

It will be understood that any suitable combination of the above examples, or any other suitable data, may be used to provide the indication of yaw (and/or tilt) error of the wind turbine <NUM>.

Based on the received indication of yaw error, the control element 46a determines an activity level for the tower lateral oscillation controller. The activity level is an indication of the level of tower lateral oscillation control that is to be utilised relative to its optimal or rated performance level. In general, the greater the level of yaw error of the wind turbine, the lower the determined activity level. This is to ensure that the tower lateral oscillation control does not degrade the performance of the rotor load controller, while still ensuring that the objectives of the tower lateral oscillation control, i.e. dampening lateral oscillations or vibrations of the tower <NUM>, are being addressed.

<FIG> shows an illustrative plot of how the determined activity level may vary according to the indication of yaw error which, in the described example, is provided by the sensor signals <NUM>. The SSTD activity level of the described example may be regarded as a scaling factor for the rated lateral oscillation control output determined by the control element 44a. As such, the SSTD activity level can vary on a scale between zero and one.

Three illustrative examples <NUM>, <NUM>, <NUM> of how the activity level may vary with varying sensor signals <NUM> are shown. In the described example, an increasing magnitude of the sensor signals <NUM> is indicative of an increasing yaw misalignment or error; however, this can also mean increasing tilt misalignment or increasing asymmetric moments, for instance.

In each of the three examples <NUM>, <NUM>, <NUM>, the activity level is equal to one for sensor signal levels below a trigger lower level <NUM>. That is, in the described example, for yaw misalignment errors less than a lower threshold level the activity level is one such that, effectively, no scaling is applied to the rated lateral oscillation control output that is used to modulate the overall control output. Expressed differently, up to a certain level of yaw misalignment the tower lateral oscillation (SSTD) controller is fully operational. This ensures that SSTD operation is not shut off or de-rated in conditions where operation of the SSTD controller does not have a significant impact on operation of the rotor load controller. In a non-limiting example, the lower trigger level <NUM> may correspond to a rotor yaw misalignment of around <NUM> degrees; however, any suitable value may be used.

In each of the examples <NUM>, <NUM>, <NUM> the activity level decreases monotonically from one at the trigger lower level <NUM> to zero at a trigger higher level <NUM> of the sensor signals <NUM>. In a first of the examples <NUM>, the activity level decreases linearly from the trigger lower level <NUM> to the trigger higher level <NUM>. In each of the other two examples <NUM>, <NUM>, the activity level decreases according to a quadratic curve, one <NUM> with downward curvature, and the other <NUM> with upward curvature.

In each of the three examples <NUM>, <NUM>, <NUM>, the activity level is equal to zero for sensor signal levels above the trigger higher level <NUM>. That is, in the described example, for yaw misalignment errors greater than an upper threshold level the activity level is zero such that no lateral oscillation control output is used to modulate the overall control output, i.e. tower lateral oscillation control is not present in the overall individual pitch control outputs 50a. That is, when rotor yaw misalignment, for instance, is particularly severe then the lateral oscillation controller does not contribute to controlling individual blade pitch.

The lower and upper trigger levels <NUM>, <NUM> are tuning parameters that may be set in accordance with desired operation of the wind turbine <NUM>. In some examples, only the lower trigger level may be present, and the activity level may asymptotically approach zero for increasing sensor signals <NUM>.

The relationship between activity level and the sensor signals <NUM> may be continuous, and optionally smooth, to ensure predictable, consistent, and smooth operation of the tower lateral oscillation controller and of the wind turbine <NUM>.

It will be understood that any suitable relationship between the SSTD activity level and data indicative of yaw error (in this case, sensor signals <NUM>) may be defined or used.

Referring back to <FIG>, the activity level scaling factor determined in the SSTD activity level module 46a is applied to the rated control output determined by the SSTD control system 44a in the processing element 70a, thus de-rating the rated control output (when the activity level is less than one). he controller <NUM> includes a lateral (side-to-side) pitch modulation element <NUM> that receives this de-rated control output and determines individual pitch modulation 48a to be included in the overall individual pitch control outputs 50a so as to include a certain amount of tower lateral oscillation dampening in the overall individual pitch references 50a used to control operation of the wind turbine <NUM>. A lower activity level can mean that lower amplitude modulation - i.e. less tower lateral oscillation control - is included in the overall control outputs 50a. Alternatively, or in addition, the lower activity level can mean applying gain scheduling to the amplitude modulation of the rated control output.

It will be understood that the above-described functionality for the control elements used to determine individual pitch outputs for tower lateral oscillation control are applicable in an equivalent manner to the control elements 44b, 46b for determining an adjusted power offset, for providing tower lateral oscillation control, to determine the overall power set point used to control turbine generator operation.

<FIG> summarises the steps of a method <NUM> performed by the controller <NUM> to determine and output the overall control outputs 50a for controlling operation, in particular individual pitch, of the wind turbine <NUM>. At step <NUM>, the controller <NUM> - in particular, the SSTD control system 44a - receives data <NUM> indicative of the lateral or side-to-side oscillations being experienced by the tower <NUM>. In the described example, this data is in the form of measured lateral acceleration from the accelerometer <NUM>.

At step <NUM>, the controller <NUM> - in particular, the SSTD control system 44a - determines a rated lateral oscillation control output in dependence on the received lateral oscillation data <NUM>. In the described example, this is in the form of individual blade pitch, but in different examples this can alternatively, or additionally, be in the form of generator torque (power) offset. This rated lateral oscillation control output would be in some sense an optimum determined control output for dampening the tower oscillations, for instance in the absence of other control considerations related to controlling rotor loads (cyclic loads).

At step <NUM>, the controller <NUM> - in particular, the SSTD activity level unit 46a - receives an indication of a yaw (and/or tilt) error of the wind turbine <NUM>. That, is, the indication may be indicative of rotor/asymmetric loads that are, or may be, experienced by the wind turbine <NUM> such that a certain level of rotor load control is included in the control output by an asymmetric rotor load controller (which is included in the main controller <NUM> in the described example). The yaw (and/or tilt) error may be indicative of misalignment of the rotor <NUM> relative to wind direction in the yaw (and/or tilt) direction. The yaw error may additionally, or alternatively, be indicative of significant moments experienced by the wind turbine. In the described example, the indication of yaw error of the wind turbine <NUM> is in the form of sensor signals <NUM> indicative of measured yaw(/tilt) misalignment and/or moments. This can include measurements of wind conditions, such as wind direction, positive wind shear, negative wind shear, and/or wind veer. Estimated values of these variables/parameters may additionally, or alternatively, be used. The received indication of yaw error may additionally or alternatively include data received from external to the wind turbine <NUM>. For instance, this can include information on the operation of one or more further wind turbines adjacent to the wind turbine <NUM> in a wind park, and/or meteorological conditions in the vicinity of the wind turbine <NUM> from a central server.

At step <NUM>, the controller <NUM> determines a lateral oscillation control output 48a to be included in the overall control output 50a. In particular, the lateral oscillation control output 48a is determined to be de-rated from the rated lateral oscillation control output (determined in step <NUM>) when the indicated yaw error <NUM> of the wind turbine <NUM> is greater than a prescribed threshold level. In the described example, this threshold level corresponds to the sensor signal trigger lower level <NUM>. In the described example, the SSTD activity level unit 46a determines an activity level <NUM>, <NUM>, <NUM> based on the sensor signals <NUM>, and this activity level is used to scale the rated lateral oscillation control output in the unit/module 70a. If the scaling is less than one, then the lateral oscillation control output to be used to control the turbine <NUM> is de-rated from the rated lateral oscillation control output. In the described example, the lateral oscillation control output is in the form of individual pitch modulation values 48a as determined in the SSTD pitch modulation unit <NUM> using the output from the scaling/de-rating unit 70a.

At step <NUM>, the controller <NUM> - in particular, the main controller <NUM> - determines and outputs a control output 42a for controlling the wind turbine <NUM> according to a wind turbine control strategy and based on monitored operation of the wind turbine <NUM>, in a known manner. This control output from the main controller <NUM> also includes asymmetric rotor load control for reducing asymmetric loading on the wind turbine <NUM>.

At step <NUM>, the control output 42a from the main controller <NUM> is combined with the individual and cyclic pitch modulation 48a to determine the overall control output 50a. For instance, the control output 42a may be amplitude modulated using the control output 48a to determine the overall control output 50a. That is, the overall control output 50a includes a de-rated level of tower lateral oscillation control in a case in which rated - or at least a greater level - of tower lateral oscillation control would negatively affect the effectiveness of the rotor load control also included in the overall control output 50a. At step <NUM>, the wind turbine <NUM> is controlled using the determined overall control output 50a.

Many modifications may be made to the above-described example without departing from the scope of the appended claims.

In the above-described example, de-rating of the determined rated tower lateral oscillation control output is achieved by applying a scaling factor in the form of a determined activity level to the rated control output. However, it will be understood that in different examples de-rating may be achieved in any suitable manner, for instance by subtraction from the rated control output by an appropriate/determined amount. In some examples, the rated control output may not be determined (explicitly), and a de-rated control output may be determined directly.

In the above-described example, de-rating of the rated lateral oscillation control output is dependent on signals indicative of a yaw error of the wind turbine. The de-rating determination - e.g. the activity level determination - may also be dependent on additional factors. For instance, when the monitored tower lateral oscillations reach a predetermined level the controller may be configured to raise an alarm or alert to the large oscillations. De-rating of the tower lateral oscillation controller may therefore be made dependent on how close to this predetermined level of oscillation the tower is. For instance, the closer the tower is to the predetermined level the less the lateral tower level controller is de-rated, to ensure that the predetermined level is not exceeded, thus avoiding an alert being raised. As another example, the wind turbine may have certain thresholds of asymmetric loading moments (associated with one or more wind turbine components) that, if exceeded, cause an alert to be raised or even require shutdown of the turbine. De-rating of the tower lateral oscillation controller may therefore be made dependent on how close to these thresholds the measured and/or estimated asymmetric loading moments are. For instance, the closer to these threshold moments the measured and/or estimated values are, the more the tower lateral oscillation control may be de-rated to ensure that the tower lateral oscillation control does not degrade performance of the rotor loading controller trying to reduce these moments.

In the above-described example, de-rating of the rated lateral oscillation control output is dependent on signals indicative of a yaw error. In this example, the yaw error can be indicative that a certain level of asymmetric rotor load control is included in the overall control output; however, the yaw error need not necessarily be indicative of this. In different examples, de-rating of the rated lateral oscillation control output may additionally, or alternatively, be dependent on an indication of a level of asymmetric rotor load control that is implemented or required. In particular, lateral oscillation control included in the overall control output may be de-rated when the indication of the amount of asymmetric rotor load control required or included in the overall control output is above a threshold level. For instance the indication of the amount of rotor load control can be based on blade root load measurements or on a reading of a resulting gain factor that is used to scale a signal from the asymmetric rotor load controller superposed onto the individual pitch or power reference signal.

Claim 1:
A method (<NUM>) of controlling operation of a wind turbine (<NUM>) having a tower (<NUM>),
the method (<NUM>) comprising determining (<NUM>) an overall control output (50a, 50b) according to a wind turbine control strategy and based on monitored operation of the wind turbine (<NUM>), the overall control output (50a, 50b) including rotor load control for reducing asymmetric loading on a rotor (<NUM>) of the wind turbine (<NUM>) and including lateral oscillation control for dampening lateral oscillation of the tower, and the method (<NUM>) comprising using (<NUM>) the determined overall control output (50a, 50b) to control operation of the wind turbine (<NUM>),
the method (<NUM>) further comprising:
receiving (<NUM>) lateral oscillation sensor data (<NUM>) indicative of a level of lateral oscillation of the tower (<NUM>);
determining (<NUM>) a rated lateral oscillation control output in dependence on the received lateral oscillation data; and,
receiving (<NUM>) an indication (<NUM>) of a yaw error of the wind turbine (<NUM>), wherein the yaw error is a misalignment of a rotor plane of the wind turbine (<NUM>) relative to a wind direction in the vicinity of the wind turbine (<NUM>),
wherein a lateral oscillation control output (48a, 48b) included in the overall control output (50a, 50b) is determined (<NUM>) to be de-rated from the rated lateral oscillation control output when the indicated yaw error (<NUM>) of the wind turbine (<NUM>) is above a predetermined lower threshold level (<NUM>) in order to prioritise the rotor load control over the lateral oscillation control.