Claims:We claim:

1. A method of controlling a wind turbine (10) in relation to an FRT-Event, wherein during said FRT-Event oscillations in a drive-train of the wind turbine (10) are caused, and wherein, during said FRT-Event, the wind turbine (10) provides decreased power in relation to the power provided by the wind turbine (10) before the FRT-Event, and wherein after the end of the FRT-Event the power provided by said wind turbine (10) is increased again by means of a power return procedure (24), characterised in that the power return procedure (24) is initiated dependent on the phase (23) of the drive-train oscillations.

2. The method according to claim 1, characterized in that the wind turbine (10), which is connected to an electric grid, is operated in an FRT-mode with decreased power during the FRT-Event, preferably after a grid fault, a voltage drop for example, has occurred in the electric grid, whereby, after the grid fault, the voltage drop for example, in the electric grid has been terminated, which is the end of the FRT-Event, the power of the wind turbine (10) is increased again, in particular to the power base level before the grid fault, the voltage drop for example, in the electric grid.

3. The method according to claim 1 or 2, characterized in that the power return procedure (24) is initiated dependent on the phase of the rotational speed of drive-train and/or dependent on the phase (27) of the generator speed.

4. The method according to anyone of claims 1 to 3, characterized in that the power return procedure (24) is initiated with a time delay (30) to the end of the FRT-Event.

5. The method according to anyone of claims 1 to 4, characterized in that the power return procedure (24) is initiated during an increasing portion (28) of the drive-train oscillations, in particular during an increasing portion of the rotational speed of the drive-train and/or during an increasing portion (28) of the generator speed.

6. The method according to anyone of claims 1 to 5, characterized in that the phase (23) of the drive-train oscillations, in particular the phase of the rotational speed of drive-train and/or the phase (27) of the generator speed, is, preferably constantly, estimated and/or monitored.

7. The method according to anyone of claims 1 to 6, characterized in that the power return procedure (24) is initiated, after the drive train oscillations, in particular the rotational speed of drive train and/or the generator speed, has gone through a minimum (29).

8. The method according to claim 7, characterized in that the power return procedure (24) is initiated at a defined point of time, said point of time lying behind said minimum (29).

9. The method according to claim 7, characterized in that the power return procedure (24) is initiated during a defined time period (31), said time period (31) lying behind said minimum (29).

10. The method according to anyone of claims 1 to 9, characterized in that the power return procedure (24) is performed by use of a power-return-path (26).

11. A computer program product, which enables a data processing unit, once the computer program product is executed on the data processing unit, and is preferably stored in a storage device of the data processing unit, to perform a method of controlling a wind turbine (10) in relation to an FRT-Event according to anyone of claims 1 to 10.

12. A control device (20) of a wind turbine (10), said control device (20) being provided for controlling the wind turbine (10) in relation to an FRT-Event, said control device (20) comprising: a data processing unit, said data processing unit comprising means for performing the method according to anyone of claims 1 to 10, or said control device (20) comprising a computer program product according to claim 11, which is at least temporarily executed in or by the data processing unit.

13. The control device (20) according to claim 12, characterized in that the control device (20) further comprises a device (21) for detecting parameters of an FRT-Event or for receiving data of parameters of an FRT-Event as input data, and/or a device (22) for estimating and/or monitoring and/or determining the phase(23) of the drive train oscillations, in particular the phase of the rotational speed of drive-train and/or the phase (27) of the generator speed, or for receiving data thereof as input data.

14. A wind turbine (10), said wind turbine (10) comprising a tower (12), a nacelle (11) being mounted on top of the tower (12), said nacelle (11) incorporating a drive train of the wind turbine (10), a rotor (14) being mounted to the nacelle (11), said rotor (14) comprising a number of rotor blades (16, 17, 18), characterized in that the wind turbine (10) comprises a control device (20) being provided for controlling the wind turbine (10) in relation to an FRT-event, said control device (20) being provided in accordance with anyone of claims 12 or 13, or that the wind turbine (10) comprises means for performing the method according to anyone of claims 1 to 10.
, Description:FIELD OF THE INVENTION

The present invention generally relates to a wind turbine. More particularly, the present invention relates to a method of controlling a wind turbine due to an FRT-Event, in particular to a method concerning a phase dependent power return of a wind turbine during FRT events.

BACKGROUND

The present invention relates to a method of controlling a wind turbine in relation to an FRT-Event according to the preamble of independent claim 1. Furthermore, the present invention is directed to a computer program product, to a control device of a wind turbine and to a wind turbine itself.

The present invention is directed to the technical field of wind turbines, in particular to wind turbines of the horizontal type, which means that the wind turbines comprise a horizontal rotor axis and a rotor being directed against the wind. Such wind turbines generally comprise a nacelle incorporating a drive train. In general, the drive train comprises at least a generator device. The nacelle is mounted to a tower. A rotor with a number of rotor blades, particularly with three rotor blades, is connected to the drive train via a hub, to which the rotor blades are mounted. The rotor rotates around its rotational axis. According to one type of wind turbines the rotor blades are adjustably mounted to the hub. This is realized by means of respective pitch drives, said pitch drives being part of a pitch system. The pitch system, which is generally known in the prior art, participates in the control of the rotor speed to given set points. In general, the rotor speed is controlled by the load and by the pitch angle. By means of the pitch drives, the rotor blades may be moved about rotor blade axes into different pitch positions.

With the rise in generation of renewable energies, such as the generation of electric energy based on wind energy, it became current practice that such generation systems of renewable energy, wind turbines for example, feed the produced electric energy into an electric power supply grid. For the electric power supply, the grid stability and the security of supply are very important requirements.

Due to grid faults, caused by external conditions such as lightning strokes, short-circuits and the like, temporary voltage dips and jumps may occur in the power supply grid.

At the beginning, it was common practice in such cases that the generation systems, the wind turbines for example, were disconnected from the power supply grid, until the grid fault was eliminated.

However, due to this practice it can come to massive continuing malfunctions in the power supply grid, up to extensive black-outs. Therefore, the technical and legal standards have been developed in recent years. Nowadays, the generation systems, the wind turbines for example, have to remain connected to the power supply grid in case of a grid fault. Based on this practice, the power supply grid is stabilized and protected. In this case, during a grid fault, the generation system, the wind turbine for example, rides through this time period with decreased power. This time period can be described as a “Failure Ride Through – FRT”-Event. FRT describes the requirement that the generation system, the wind turbine for example, continues to operate during short periods of instable grid voltage and is not disconnect from the grid.

With regard to wind turbines, during a so called LVRT-Event (Low Voltage Ride Through) for example, it may come to a short-term drop in the mains voltage. During this voltage-drop, the power of the wind turbine is controlled in dependency of the currently existing mains voltage. As soon as the regular mains voltage returns, the power is returned to its original value.

If the grid fault is not eliminated during the FRT-Event, the generation system, the wind turbine for example, will get disconnected from the power supply grid. The maximum mandated duration of FRT event to stay connected is defined by grid requirements. For longer time the generation system, the wind turbine for example, can be disconnected from the power supply grid.

However, if the grid voltage recovers within appropriate time, defined by grid requirements, the electric power of the generation system, the wind turbine for example, is increased again, preferably up to the value which was existent before the grid fault occurred. This can be described as the “power return”.

During the power return phase, the power of the generation system, the wind turbine for example, is increased again, usually by means of and according to a characteristic curve, preferably to the base level which existed before the grid fault occurred. This kind of characteristic curve can be described as the “FRT-power-return-path”. Generally, the FRT-power-return-path has a linear curve shape but can also have other shapes.

If an FRT-Event, caused by a grid fault, occurs, the rapid power change strongly excites oscillation of the drive train of the wind turbine, which possibly lead to overspeed and/or high extreme loads. Due to the FRT it comes to oscillations in the drive train, which already appear at the beginning of the FRT. In particular, peaks in speed, torque and/or power can occur. Such peaks, which may cause too high loads for the drive train components, could be design driving or lead to damages. In any case such peaks, if they occur, require expensive countermeasures.

With regard to wind turbines and with regard to an LVRT-Event, during such an LVRT-Event, drive train oscillations having high amplitudes are excited, which may lead to an exceedance of a critical rotational speed and therefore to a shutdown of the wind turbine. This, however has to be avoided.

Therefore, FRTs are challenging events for the wind turbine, especially for the drive train.

US 10,112,533 B2 for example discloses a system and a method for reducing wind turbine oscillations caused by grid faults. According to this known solution, if a grid fault occurs, a controller of the wind turbine determines an operation catch point for a wind turbine component, and once this operation catch point is reached, the controller applies a torque demand to wind turbine component.

In a different context, US 9,567,975 B2 describes a solution of reducing oscillations in a wind turbine by means of changing the rotational speed, wherein the wind turbine is operated with a variable rotational speed between a minimum rotational speed and a maximum rotational speed.

OBJECT OF THE INVENTION

It is the object of the present invention to provide a solution in the field of wind turbines, which is simple in performance, and by means of which negative oscillations of the drive train, which occur during an FRT-Event, in particular during an LVRT, can be reduced or compensated.

According to the present invention, the object is solved by the method with the features according to independent claim 1, which is the first aspect of the invention, by the computer program product with the features according to independent claim 11, which is the second aspect of the invention, by the control device with the features according to independent claim 12, which is the third aspect of the present invention, and by the wind turbine with the features according to independent claim 14, which is the fourth aspect of the invention. Further features and details of the invention become apparent from the dependent claims, from the description as well as from the drawings. Therein, features and details which are described in connection with one aspect according to the invention apply with respect to their disclosure in their entirety also to the other aspects according to the invention, so that any statements made with respect to one aspect of the invention also apply to their full extent to the other aspects of the invention, and vice versa.

The underlying concept of the present invention is that oscillations of the drive train, which occur based on a grid fault during FRT, are reduced or compensated by means of a phase dependent power return after FRT-Events. Such an FRT (Failure Ride Through) comprises different scenarios, for example a so called LVRT (Low Voltage Ride Through) or a HVRT (High Voltage Ride Through). The present application is applicable for any kind of FRT. According to a preferred embodiment the present invention is applied in connection with an LVRT.

During a temporal grid fault, which usually occurs for a short time period, the wind turbine rides through this time period with decreased power. According to the present invention, this time period is named as the “Failure Ride Through – FRT”-Event. FRT describes the requirement that the wind turbine continues to operate during this short time period of low grid voltage, is not disconnect from the grid and recovers the power within a certain time after the FRT, the LVRT for example. If the grid fault is not eliminated during the FRT-Event, the wind turbine will get disconnected from the power supply grid. However, if the grid fault is successfully eliminated during the FRT-Event, the electric power of the wind turbine is increased again, preferably up to the value which was existent before the grid fault occurred. According to the present invention this is named as the “power-return”. During the power-return-phase, the electric power of the wind turbine is increased again. This is preferably achieved by means of and according to a characteristic curve, preferably up to the base level which existed before the grid fault occurred. According to the present invention, this characteristic curve is named as the “power-return-path”.

The present invention is based on the following principles:

During a drop in the voltage during an FRT-Event, an LVRT-Event for example, the power of the wind turbine is controlled dependent on the main voltage. This short-term loss in voltage leads to a sudden drop in the power and therefore to a sudden decrease of the torque at the drive train. Dependent on the depths of this voltage-drop, drive train oscillations with high amplitudes may occur. If the voltage returns, according to legal requirements, the power must return to the original value as soon as possible. However, this power return also effects a sudden torque variation.

Now, the inventors of the present invention have found out in an unforeseeable way, that this impulse of sudden torque variation during power return may either counteract the drive train oscillations, or it may additionally amplify those drive train oscillations, dependent on the point of time of the power return.

The inventors have found out, that, if the power return is initiated during an increasing or rising edge of the drive train oscillation, in particular of the rotational speed of the drive train, the rapid change of the torque counteracts on the drive train oscillations which results in a reduction of the drive train oscillations. However, if the power return is initiated during the decreasing or falling edge of the drive train oscillation, the rapid change of the torque negatively affects the drive train oscillations, which get amplified in such a case.

Therefore, it is the general principle of the present invention to provide the right timing of the power return at an favourable point of time.

SUMMARY OF THE INVENTION

The present invention according to its four aspects is directed to the technical field of wind turbines, in particular to the technical field of horizontal wind turbines. Such wind turbines are generally known in the prior art.

According to a preferred embodiment, the wind turbine comprises a nacelle which incorporates a drive train. The drive train transmits the rotor speed to the generator where it is converted into electric energy. In particular, the drive train is the mechanical energy transmitting line starting from the hub of the rotor right up to the generator. The wind turbine comprises a rotor being connected to said drive train, said rotor preferably comprising a hub and a number of rotor blade, preferably three rotor blades, said rotor blades being mounted to said hub. The rotor is pivotally mounted around a rotational axis to the drive train. In order to transform the rotational energy of the rotor into electric energy, the drive train, to which the rotor of the wind turbine is mounted, comprises a number of different components. One of these components is a generator device. The generator device generates electric energy. For this purpose, the generator device preferably comprises a stator component and a rotor component, said rotor component being coupled to the generator shaft. The nacelle is mounted at the top of a tower of said wind turbine. At its lower base end, the tower is anchored to the ground by means of a foundation.

In particular, the rotor blades are adjustably mounted on the hub. This is realized by means of a pitch drive, said pitch drive being part of a pitch drive unit, which in turn is part of a pitch system. The pitch system, which is generally known in the prior art, participates in the control of the rotor speed to given set points. In general, the rotor speed is controlled by the load and by the pitch angle. By means of the pitch-drives, the rotor blades may be moved about rotor blade axes into different pitch positions, said rotor blade axes extending in an axial direction of the rotor blade. A rotor blade having an adjustable pitch can be particularly understood in such a manner, that the angle of attack of the rotor blade, which may be defined as the pitch angle, can be adjusted or is provided in an adjustable manner, during a pitch-offset for example.

The present invention will now be explained in detail and with reference to the different aspects of the present invention.

According to the first aspect of the present invention, the object is solved by the method comprising the features of independent claim 1.

This method is a method of controlling a wind turbine in relation to an FRT-Event. That means that drive train oscillations arise during the FRT-Event. In particular, this method is a method concerning a phase dependent power return of a wind turbine during an FRT event, preferably at an end thereof.

In particular, the FRT-Event is initiated by a grid fault that temporarily occurs in the power supply grid. During said FRT-Event, oscillations in the drive train of the wind turbine are caused. In particular the oscillations of the drive train can be the oscillations with eigenfrequency of the drive train.

During said FRT-Event, the wind turbine provides decreased power in relation to the power provided by the wind turbine before the FRT-Event. After the end of the FRT-Event, the power provided by said wind turbine is increased again by means of a power return procedure.

According to the present invention the method is characterized in that the power return procedure is initiated dependent on the phase of the drive train oscillations. that means that the starting point of the power return depends on the phase of the drive train oscillations.

In particular, the phase of an oscillation describes a specific point of time relative to a characteristic point of the oscillation, for example a maximum or a minimum.

With regard to the general configuration and to the general functionality of the method, full reference is also made to the general description of the invention as described further above.

With the method according to the present invention, any drive train oscillations which may occur during an FRT-Event, can successfully get damped or at least significantly reduced.

According to a preferred embodiment, the wind turbine, which is connected to an electric grid, is operated in an FRT-mode with decreased power during the FRT-Event, preferably after a grid fault, a voltage drop for example, has occurred in the electric grid, whereby, after the grid fault, the voltage drop for example, in the electric grid has been terminated, which is the end of the FRT-Event, the power of the wind turbine is increased again, in particular to the power base level before the grid fault, the voltage drop for example, in the electric grid,

In general, the present invention is directed to a solution of initiating or starting a power return procedure dependent on the phase of the drive train oscillations. With this regard, the present invention is open to different configurations. In the following, some preferred embodiments are described in more detail.

According to preferred embodiments, the power return procedure is initiated dependent on the phase of the rotational speed of the drive train and/or dependent on the phase of the generator speed.

According to a preferred embodiment, the power return procedure is initiated at the rising course of the drive train oscillations, at an increasing portion thereof for example, in particular of the rotational speed of drive train and/or of the generator speed.

As explained further above, the starting point of the power return procedure depends on the phase of the drive train oscillations. When the power return procedure starts, if the course of the drive-train oscillations increases, the drive train oscillations can get reduced. However, if the course of the drive train oscillations is dropping, the drive train oscillations can get enhanced. Therefore, it is preferably provided that the power return procedure only starts, when the drive train oscillations, in particular of the rotational speed of drive train and/or of the generator speed, shows an increasing course or a rising edge.

Therefore, an optimum timing for the begin of the power return procedure during an increasing course of the drive train oscillations leads to a significant reduction of the drive train oscillations. At the same time, the maximum values of the torque can get reduced as well.

According to a preferred embodiment, the phase of the drive train oscillations, in particular of the rotational speed of drive train and/or of the generator speed, is, preferably automatically, estimated and/or monitored and/or determined. In particular an automatic live detection of the phase or of at least specific points of time, such as minima or maxima, is realised. Therefore, it is known at any point of time, when the power return procedure should be initiated and whether it is suitable for initiating the return of power procedure at a given time.

According to a preferred embodiment, the power return procedure is initiated, after the drive train oscillations, in particular of the rotational speed of drive train and/or of the generator speed, have/has gone through a minimum. Once, such a minimum, or more than one minimum, has/have been found or detected or evaluated, the power return procedure can get initiated in time relation to this minimum or these minima. Nevertheless, the present invention can be realised, according to a different embodiment, by initiating the power return procedure, after the phase of the drive train oscillations, in particular of the rotational speed of drive train and/or of the generator speed, has gone through other characteristic points in time of the oscillation, for example a maximum, as well. In the latter case, the method is performed in an analogous manner.

According to a preferred embodiment, the power return procedure is initiated with a time delay to the end of the FRT-Event. Usually, the end of an FRT-Event does not coincide in time with a suitable phase of the drive train oscillation for starting the power return procedure. In such a case, the starting point of the power return procedure gets delayed until a suitable phase has been reached. Nevertheless, if both events fall chronologically together, for example if the end of an FRT-Event coincides with a minimum of the phase curve of the drive train oscillations, the power return procedure may get initiated without a time delay.

According to a first preferred embodiment, the power return procedure is initiated at a defined point of time, said point of time lying timely behind, that is after said minimum.

According to a second different preferred embodiment, the power return procedure is initiated during a defined time period or time slot, said time period lying timely behind said minimum. This time period or time slot is preferably defined in relation to the said minimum. In this case the time period begins at a defined first point of time after a minimum and ends at a defined second point of time after said minimum, said second point of time being, in comparison to said minimum, later than the first point of time.

A suitable point of time according to said first embodiment as well as a suitable period of time according to said second embodiment can get determined based on the actual conditions, under which the method is performed and, therefore, can change.

According to a preferred embodiment, the power return procedure is performed by use of a power-return-path, which per se is known in the prior art. For example, such a power-return-path may have a linear course or curve shape. Nevertheless power -return-paths having a course or shape deviating from a linear course or shape can get used as well.

According to the second aspect of the invention, the object is solved by a computer program product comprising the features of independent claim 11.

With regard to the configuration, to the performance and to the function of the computer program product according to the third aspect, full reference is also made to the general description of the invention further above, and to the description of the other aspects of the invention as disclosed further above and further below.

The computer program product enables a data processing unit, once the computer program product is executed on the data processing unit, and is preferably stored in a storage device of the data processing unit, to perform a method of controlling a wind turbine in relation to an FRT-Event according to the first aspect of the invention.

According to the third aspect of the invention, the object is solved by a control device of a wind turbine, said control device comprising the features of independent claim 12.

With regard to the configuration, to the performance and to the function of the control device according to the third aspect, full reference is also made to the general description of the invention further above, and to the description of the other aspects of the invention as disclosed further above and further below.

According to the third aspect, the invention is directed to a control device of the wind turbine. This control device is provided for controlling the wind turbine in relation to an FRT-Event. For example, the control device can be an individual control device being used for this purpose only. Nevertheless, the control device can be part of the central control system of the wind turbine as well. Or the control device can be implemented in another component, in another control device for example such as in the control device of the converter.

In order to be capable of solving the object of the present invention the control device comprises a number of different components. In particular, the control device comprises a data processing unit, or according to a different embodiment the control device is provided as a data processing unit.

According to a preferred embodiment, the data processing unit comprising means for performing the method according to the first aspect of the invention. According to a different embodiment, the control device comprises a computer program product according to the second aspect of the invention, which is at least temporarily executed in or by the data processing unit.

Preferably, the control device further comprises a device for detecting parameters of an FRT-Event or for receiving data of parameters of an FRT-Event as input data. Alternatively, or in addition, the control device preferably comprises a device for estimating and/or monitoring and/or determining the phase of the drive train oscillations, in particular of the rotational speed of drive train and/or of the generator speed, or for receiving data thereof as input data. In particular the latter device is preferably provided for estimating and/or monitoring and/or determining minima and/or maxima of the drive train oscillations, in particular of the rotational speed of drive train and/or of the generator speed. According to a very preferred embodiment, this device is provided for estimating and/or monitoring and/or determining minima of the phase curve.

For example, the different components of the control device can be provided as electronic devices or as software components or as combinations of electronic devices and software components.

According to the fourth aspect of the invention, the object is solved by a wind turbine comprising the features of independent claim 14.

With regard to the configuration, to the performance and to the function of the wind turbine according to the fourth aspect, full reference is also made to the general description of the invention further above, and to the description of the other aspects of the invention as disclosed further above.

According to the fourth aspect, the wind turbine comprises a tower, a nacelle being mounted on top of the tower, said nacelle incorporating a drive train of the wind turbine, a rotor being mounted to the nacelle, said rotor comprising a number of rotor blades. According to the invention, the wind turbine further comprises a control device being provided for controlling the wind turbine in relation to an FRT-Event. According to a preferred embodiment, the control device is provided in accordance with the third aspect of the invention. According to a different preferred embodiment, the wind turbine comprises means for performing the method according to the first aspect of the invention.

The present invention according to its four aspects provides a solution of a delayed power return after the end of an FRT-Event, in particular after a voltage return.

The invention will now be explained in more detail with respect to exemplary embodiments with reference to the enclosed drawings, wherein the foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures:

BRIEF DESCRIPTION OF THE INVENTION

Figure 1 shows a schematic view of a wind turbine incorporating the principles of the present invention; and
Figure 2 depicts a schematic representation of a control device according to the present invention, by means of which the method according to the present invention is performed.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 depicts a wind turbine (10) of the horizontal type with a tower (12) and a nacelle (11), which incorporates the principles of the present invention. Nacelle (11) is rotatable mounted to the top of tower (12). At its lower base end, the tower (12) is anchored to the ground by means of a foundation (13). Nacelle (11) incorporates a drive train (not shown), said drive train being mounted inside nacelle (11) and being connected to a rotor (14). Rotor (14) comprises three rotor blades (16), (17), (18) which are mounted to a hub (15). Hub (15) of rotor (14) is connected to a drive shaft of the drive train. The rotor blades (16), (17), (18) are adjustably mounted on the hub (15). This is realized by means of pitch drives (not shown), said pitch drives being part of a pitch system. The pitch system controls the rotor speed to given set points. By means of the pitch drives, the rotor blades (16), (17), (18) may be moved about rotor blade axes such that the pitch of the rotor blades (16), (17), (18) can be changed. This means that the pitch-angles of the rotor blades (16), (17), (18) can be changed such that the orientation of the rotor blades (16), (17), (18) can be varied.

The rotor (14) is rotatable connected to the drive train via its rotational axis. The drive train transmits the rotor speed to a generator device, where it is converted into electric energy. In order to transform the rotational energy of the rotor (14) into electric energy, the drive train particularly comprises a gear device.

The electric energy which is produced by the wind turbine (10) is fed into the electric power supply grid. For the electric power supply, the grid stability and the security of supply are very important requirements. Due to grid faults, caused by external conditions such as lightning strokes, short-circuits and the like, temporal voltage dips may occur in the power supply grid. During such grid faults, the wind turbine (10) must stay connected to the power supply grid. In case of a grid fault, the wind turbine rides through the FRT – Failure Ride Through - with decreased power. FRT describes the requirement that the wind turbine (10) continues to operate during short periods of low grid voltage and is not disconnect from the grid.

If the grid fault has been successfully eliminated during the FRT-period, the power of the wind turbine (10) is increased again, preferably up to the value which was existent before the grid fault occurred. During the FRT power return phase, the power of the wind turbine (10) is increased by means of and according to a characteristic curve, which is the FRT-power-return-path.

Preferably the wind turbine (10) is controlled by means a control device (20) during such an FRT-Event. The control device (20) can be realised as an individual device or as a component or a feature of a different device. For example, the control device (20) can be implemented in the control device of the converter.

Figure 2 schematically shows such a control device (20) being responsible for controlling the wind turbine (10) during an FRT-Event. The control device (20) comprises a device (21) for detecting parameters of an FRT-Event, said device (21) being provided for detecting a voltage-drop (25) in the present embodiment. This is particularly shown in the upper part of Figure 2. Furthermore, the control device (20) comprises a device (22) for estimating and/or monitoring and/or determining the phase (23) of the drive train oscillations, the phase (27) of the generator speed in the present embodiment. This is particularly shown in the lower part of Figure 2.

Once the FRT-Event has ended, the device (21) detects a return of the voltage to its original value before the FRT-Event. Now, the control device (20) starts a power return procedure (24), by means of which the power of the wind turbine (10) returns to its values before the FRT-Event. This power return procedure (24) is performed by means of a suitable power-return-path (26). This is particularly shown in the middle part of Figure 2. As evident from the middle part of Figure 2, the power-return-path (26) may have a different course, three of them being shown in the Figure, a first one being represented by a solid line, the others being represented by a dotted and a dashed line respectively.

According to the embodiment shown in Figure 2, the power return procedure (24) is initiated dependent on the phase (23) of the drive train oscillations, which here is the phase (27) of the generator speed in the present embodiment.

According to the embodiment shown in Figure 2, in particular in the lower part thereof, the power return procedure (24) is not initiated immediately after the end of voltage-drop (25) and, therefore after the end of the FRT-Event, but with a time delay (30). The time delay (30) is chosen or determined in such a way, that the power return-procedure (24) is initiated during an increasing portion (28) or a rising edge of the generator speed. Thus, the drive train oscillations can get reduced. An optimum timing for the begin of the power return procedure (24) during an increasing portion (28) of the generator speed leads to a significant reduction of the drive train oscillations.

Preferably the phase (23) of the drive train oscillations, in particular the phase (27) of the generator speed, is constantly and/or automatically estimated and/or monitored and/or determined.

According to the shown embodiment, the power return procedure (24) is initiated, after the drive train oscillations, in particular the generator speed, has gone through a minimum at tmin (29). Once, such a minimum (29), or more than one minimum, has/have been found or detected or evaluated, the power return procedure (24) can get initiated in time relation to this minimum (29) or these minima.

As can be seen from figure 2, the power return procedure (24) is initiated with a time delay (30) to the end of the FRT-Event, which is the end of voltage-drop (25). Preferably, the power return procedure (24) gets initiated during a defined time period (31), said time period (31) lying timely behind said minimum (29). This time period (31) or time slot is preferably defined in relation to the said minimum (29). The time period (31) begins at a defined first point of time (?t1) after the minimum (29) and ends at a defined second point of time (?t2) after said minimum (29).

The drive train oscillation depends on the starting point of the power return. The inventors of the present invention have found out that it is critical when the drive train oscillations, the generator speed for example, is dropping. This enhances oscillations. However, it is favourable, when the drive train oscillations, the generator speed for example, is increasing. This reduces oscillations. According to the present invention it is preferably provided that the – fast – power return starts only, when the drive train oscillation, the generator speed for example, is on the rising edge. This, for example is represented by the grey shaded areas shown in the lower part of Figure 2.

The present invention provides a phase dependent power return after an FRT-Event with a delayed power return after the voltage comeback. This goal is achieved, among other aspects, by a method of controlling a wind turbine (10) in relation to an FRT-Event, wherein during said FRT-Event oscillations in a drive-train of the wind turbine (10) are caused, and wherein, during said FRT-Event, the wind turbine (10) provides decreased power in relation to the power provided by the wind turbine (10) before the FRT-Event, and wherein after the end of the FRT-Event the power provided by said wind turbine (10) is increased again by means of a power return procedure (24). According to the present invention, the power return procedure (24) is initiated dependent on the phase (23) of the drive-train oscillations, in particular during an increasing portion (28) of drive train oscillations. Thus, drive train oscillations occurring due to the FRT-Event can get reduced.

LIST OF REFERENCE NUMERALS

10 Wind turbine
11 Nacelle
12 Tower
13 Foundation
14 Rotor
15 Hub
16 Rotor blade
17 Rotor blade
18 Rotor blade

20 Control device
21 Device for detecting parameters of the FRT-Event
22 Device for estimating and/or monitoring and/or determining the phase of the drive train oscillations
23 Phase of drive train oscillations
24 Power return procedure
25 Voltage drop
26 Power return path
27 Phase of the generator speed
28 Rising edge of the generator speed
29 Minimum
30 Time delay
31 Time period

The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles described herein can be applied to other embodiments without departing from the spirit or scope of the invention. Thus, it is to be understood that the description and drawings presented herein represent a presently preferred embodiment of the invention and are therefore representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art and that the scope of the present invention is accordingly not limited.