DESC:FIELD OF THE INVENTION

The present invention generally relates to a wind turbine. More particularly, the present invention relates to a method of damping oscillations in the drive train of the wind turbine, which are caused by an FRT event.

The present invention relates to a method of generating an FRT-power-return-path for damping oscillations in a drive train of a wind turbine caused by an FRT-event, as well as to a method of damping such oscillations. 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 particu-lar 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.

BACKGROUND

With the rise in generation of renewable energies, such as the generation of electric energy based on wind energy, it has 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 sys-tems, 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 pow-er. This time period can be described as a “Failure Ride Through – FRT”. FRT describes the requirement that the generation system, the wind turbine for exam-ple, continues to operate during short periods of low grid voltage and is not dis-connected from the grid.

If the grid fault is not eliminated during the FRT-period, the generation system, the wind turbine for example, will get disconnected from the power supply grid.

However, if the grid fault is successfully eliminated during the FRT-period, 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 tur-bine for example, is increased again, 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.

A known method concerning the technical field of FRT is described in WO 2011/009958 A2. This known solution relates to a method for operating a wind turbine connected to a power grid for generating electric energy when a change in the grid voltage occurs, and to a wind turbine for carrying out the method. Ac-cording to the method, when a deviation of a grid voltage from a certain regular grid voltage range occurs, the current residual grid voltage is measured, the current wind speed is measured, depending on the value of the residual grid voltage a certain time period is defined starting with the detection of the change in grid voltage, and the wind turbine is operated within the defined time period, depend-ing on the value of the residual grid voltage, in a certain operating mode that devi-ates from regular operation with respect to the effective power, and the wind tur-bine is operated again in the regular operating mode after the grid voltage has normalized again within the defined time period, or is shut off at the end of the time period if the deviation of the grid voltage persists during the defined time period. According to the invention, voltage ranges are defined for grid voltage values that are not part of the regular grid voltage range, wherein each defined voltage range is associated with a plurality of voltage values and at least one first factor, which is different for each voltage range, for controlling the wind turbine so as to implement the operating mode deviating from the regular operating mode and, depending on the measured wind speed, one of the first factors for controlling the wind turbine is used to implement the deviating operating mode.

If an FRT, caused by a grid fault, occurs, the rapid power change strongly excites oscillation of the drive train 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 the exceedance of the maximum coupling-torque, are either design driving or may lead to coupling damages. In any case such peaks, if they occur, require expensive countermeasures.

For reducing such peaks, one may consider to perform suitable pitch-offset-procedures or to use flat-power-return-paths. However, such measures may lead to other problems such as power fluctuations after the power return or the danger of a speed exceedance.

On the other hand, an efficient damping of the drive train oscillations by use of drive train dampers is crucial for a smooth FRT run. Present solutions with drive train dampers can have the following restrictions and disadvantages: Usually the drive train dampers are disabled below a certain voltage drop. In general, drive train dampers are designed for a normal operation only and not for heavy oscilla-tions which may occur during FRT. Moreover, drive train dampers need tuning time for the oscillation. However, sudden oscillations, which occur during FRT, need a quick as possible reaction.

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

OBJECT OF THE INVENTION

Starting from the aforementioned prior art, it is the object of the present invention to provide a solution in the field of wind turbines, which is simple in construction, and by means of which negative oscillations of the drive train, which occur during an FRT in particular during an LVRT, can be reduced or compensated.

According to the present invention, the object is solved by the method of generat-ing an FRT-power-return-path being provided for damping oscillations in a drive train of a wind turbine, said drive train oscillations being caused during an FRT-event of the wind turbine, which is the first aspect of the invention;, by the method of damping oscillations in the drive train of a wind turbine, said oscillations being caused during an FRT-event, which is the second aspect of the present invention, by the computer program product which enables a data processing unit , which is the third aspect of the invention, by the control device of a wind turbine , which is the fourth aspect of the present invention, and by the wind turbine, which is the fifth aspect of the invention.

Further features and details of the invention become apparent from the 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 ap-ply with respect to their disclosure in their entirety also to the other aspects ac-cording 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 modulating the FRT-power-return-path, in particular by superimpos-ing a power-counter wave to an existing FRT-power-return-path. 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 pre-ferred embodiment the present invention is applied in connection with an LVRT.

During a temporal grid fault, which occurs for a short time period, the wind tur-bine rides through this time period with decreased power. According to the pre-sent invention, this time period is named as the “Failure Ride Through – FRT”. 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 re-covers the power within a certain time after the FRT, the LVRT for example.

If the grid fault is not eliminated during the FRT-period, the wind turbine will get disconnected from the power supply grid.

However, if the grid fault is successfully eliminated during the FRT-period, 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 inven-tion this is named as the “FRT-power-return”.

During the FRT-power-return-phase, the electric power of the wind turbine is in-creased again. This is 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 “FRT-power-return-path”.

The present invention is based on the technical principle that any drive train oscil-lations which occur, based on a grid fault, a voltage-drop for example, during an FRT-event, an LVRT-event for example, in particular during the FRT-power-return, an LVRT-power-return for example, can now be reduced by modulating the FRT-power-return-path. According to the present invention, the “FRT-event” starts with the begin of the FRT due to the occurrence of a temporal grid fault, and it ends when the FRT-power-return has been finished. Therefore, the FRT itself as well as the FRT-power-return are parts of the FRT-event. During the FRT, the wind turbine is run with decreased power. According to the present invention this time-period of decreased power is also mentioned as the “FRT-mode”. Due to the occurrence of a grid fault, the drive train of the wind turbine receives a strike at the beginning of the FRT-event, said strike causing the negative oscillations in the drive train that have to be reduced or compensated by means of the present invention.

SUMMARY OF THE INVENTION

The present invention according to its five 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 me-chanical 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 pivot-ally 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 ro-tor 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 founda-tion.

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 gen-eral, 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 differ-ent pitch positions, said rotor blade axes extending in an axial direction of the ro-tor 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 of generating an FRT-power-return-path, an LVRT-power-return-path for example, being provided for damping oscillations in a drive train of a wind tur-bine, said drive train oscillations being caused during an FRT-event, an LVRT-event for example, of the wind turbine, said method comprising the following steps:
• Selecting or providing a first FRT-power-return-path ;
• Dependent on the drive train oscillations, modulating the first FRT-power-return-path in its curve shape into a second FRT-power-return-path, said second FRT-power-return-path having a curve shape that deviates from the curve shape of the first FRT-power-return-path.

According to the present invention the FRT-power-return-path represents a suita-ble FRT-power-return-strategy as well.

That means that the drive train oscillations arise during the FRT-event, whereby the FRT-event is initiated by a grid fault that temporarily occurs in the power sup-ply grid. In particular the oscillations of the drive train can be the oscillations with eigenfrequency of the drive train.

In a first step, a first FRT-power-return-path is selected or provided. For example, the first FRT-power-return-path can be selected from a set of pre-existing first FRT-power-return-paths. The first FRT-power-return-path is preferably an initial return-path which, in particular, is a predetermined FRT-power-return-path.

Dependent on the drive train oscillations, which occur during the FRT-event based on the grid fault, caused by a voltage drop for example, the first FRT-power-return-path gets modulated in its curve shape into a second FRT-power-return-path, said second FRT-power-return-path having a curve shape that deviates from the curve shape of the first FRT-power-return-path. The second FRT-power-return-path is preferably a resulting return-path.

According to the present invention, the first FRT-power-return-path is modulated in its curve shape into the second FRT-power-return-path. This particularly means that the first FRT-power-return-path gets changed or varied in its curve shape and that, as a result of this change or variation, the second FRT-power-return-path is formed. Preferred embodiments, how this can be achieved are described in more detail further below.

According to a preferred embodiment, the first FRT-power-return-path has a linear curve shape and the modulated second FRT-power-return-path has a curve shape which deviates from the linear curve shape of the first FRT-power-return-path.

The modulation of the first FRT-power-return-path into the second FRT-power-return-path can be performed in different ways. According to a preferred embodi-ment, the second FRT-power-return-path is modulated from the first FRT-power-return-path by superimposing or inducing or applying a counter-acting power wave, in particular a sinusoidal counter-acting power wave, to the first FRT-power-return-path. In this case, the counter-acting power wave counter-acts to the oscillations in the drive train caused by the FRT-event.

Preferably, the FRT-power-return-path also includes a “fade in” and a “fade out” of the overlaid counter-acting power wave, which is a sinus-curve for example. That means that the counter-acting power wave is multiplied by a “fade-in/out” factor. The phase of the wave is strongly dependent from the time of the start of the FRT, the time of a power drop for example.

According to a preferred embodiment, the counter-acting power wave is generated by combining, in particular by multiplying, an, in particular undiluted-acting, power wave with a fading function.

According to a preferred embodiment, the counter-acting power wave is generated based on the oscillations of the drive train during the FRT-event, in particular at the beginning of the FRT-event or at the beginning of the FRT-power-return.

In this case it is preferably provided that characteristic parameters of the oscilla-tions of the drive train caused during the FRT-event, are determined and that the counter-acting power wave is generated based on these characteristic parameters of the oscillations of the drive train.

It is a general principle that the oscillations of the drive train during the FRT-event, caused by a grid fault at the beginning of the FRT-event, are deterministic. This particularly means that the curve shape of the oscillations can be clearly de-scribed with regard to their characteristic parameters. Therefore, according to this preferred embodiment, the characteristic parameters of the counter-acting power wave can be derived from the characteristic parameters of the oscillations of the drive train and this, be described as well.

According to this preferred embodiment, characteristic parameters of the oscilla-tions of the drive train are determined. For example, such oscillations can be measured or evaluated or detected or calculated or determined by use of a combi-nation thereof.

Characteristic parameter of the oscillations are particularly such properties or fac-tors of the oscillations which are capable of describing or defining the oscillations. In particular, a characterizing parameter of the oscillation of the drive train can be the oscillation frequency. Another characterizing parameter can be the oscillation phase(s). Both parameters are known during the FRT-event, or they can be derived at the time of the beginning of the FRT-power-return. Another characteristic parameter of the oscillation of the drive train is the amplitude thereof. The ampli-tude is dependent from different factors, from the wind speed, the kind of the used FRT-power-return-path, the depth of the voltage drop for example. A simple estimate of the amplitude is sufficient for the purpose of the present invention.

According to the aforementioned preferred embodiment it is preferably provided, that at least one characteristic parameter of the oscillations of the drive train is determined and that the counter acting-power wave is generated having at least one characteristic parameter which corresponds to or complies with the respective characteristic parameter of the drive train oscillations, in particular at the beginning of the FRT-event or at the beginning of the FRT-power-return.

By way of example, different embodiments thereof are now described in more detail.

According to a preferred embodiment, the frequency of the oscillations of the drive train is determined and that the counter-acting power wave is generated hav-ing a frequency which corresponds to or complies with the frequency of the drive train oscillations, in particular at the beginning of the FRT-event or at the begin-ning of the FRT-power-return. In this case the frequency of the oscillations of the drive train can be its natural or self-frequency.

According to a different preferred embodiment, or in addition to the aforemen-tioned embodiment, the phase of the oscillations of the drive train is determined and that the counter-acting power wave is generated having a phase which corre-sponds to or complies with the phase of the drive train oscillations, in particular at the beginning of the FRT-event or at the beginning of the FRT-power-return.

According to yet another preferred embodiment, or in addition to one or both of the aforementioned embodiments, the amplitude of the counter-acting power wave is determined or estimated by means of a simulation process and/or by means that the amplitude of the counter-acting power wave is individually adjusted and/or by means that the amplitude of the counter-acting power wave is determined or estimated based on the windspeed and/or by means that the amplitude of the counter-acting power wave is determined or estimated in accordance with the first FRT-power-return-path and/or by means that the amplitude of the counter-acting power wave is determined or estimated in accordance with the depth of the first FRT-power-return-path.

According to a preferred embodiment, the frequency and the phase of the counter-acting power wave are determined, preferably just at the beginning of the FRT-event which is the FRT starting time. The amplitude of the counter-acting power wave can be approximated. Since the oscillations of the drive train, in particular the oscillations of the generator speed, have all roughly the same phase and ampli-tude the same modulations of the first FRT-power-return-path can be used which only depend on the FRT start time.

According to a preferred embodiment of the invention the drive train oscillation is a deterministic, particularly sinusoidal, oscillation with fixed frequency and phase determined by the beginning of the FRT-event. Therefore, the FRT-power-return after the FRT shall not be linear but modulated with a counter-acting wave or sig-nal, which is preferably sinusoidal, to damp the drive train oscillation.

While the first aspect of the invention provides a method how such a suitable FRT-power-return-path is generated, the following second aspect of the invention describes a method, how, by use of such a modulated FRT-power-return-path, the oscillations in the drive train, which occur during an FRT-event can actually get damped.

According to the second aspect of the present invention the object is solved by a method of damping oscillations in the drive train of a wind turbine, said oscilla-tions being caused during an FRT-event, , whereby the wind turbine which is connected to an electric grid is operated in an FRT-mode with decreased power after a grind 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, 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, and whereby the power increase is performed according to an FRT-power-return-path , that has been generated with a method according the first as-pect of the present invention.

With regard to the configuration, to the performance and to the function of the method according to the second aspect, full reference is also made to the general description of the invention further above, and to the description of the other as-pects of the invention as disclosed further above and further below.

According to the second aspect of the invention, the method is performed in such a way, that the wind turbine, which is connected to an electric grid, is operated in an FRT mode with decreased power after a grid fault, a voltage drop for example, has occurred in the electric grid, whereby, after the grid fault in the electric grid has been terminated, the power of the wind turbine is increased again, in particular to the power base level before the grid fault in the electric grid. According to the invention the power increase is performed according to an FRT-power-return-path that has been generated with a method according to the first aspect of the inven-tion as described in detail further above.

That means that for damping the oscillations of the drive train that have been caused during the FRT-event, a modulated FRT-power-return-path is used. Ac-cording to a preferred embodiment, the FRT-power return after the FRT shall not be linear but modulated with a counter-acting power wave to damp the drive train oscillations.

The actual damping of the oscillations, by using the specifications provided by the modulated FRT-power-return-path, can be performed by means of suitable ad-justments of the wind turbine, by means of the orientation of the rotor blades per-forming a pitch-offset, by means of the activation of suitable damper devices, or the like.

According to the third aspect of the invention, the object is solved by a computer program product , which enables a data processing unit, once the computer pro-gram 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 generating an FRT-power-return-path being provided for damping oscillations in a drive train of a wind turbine caused by an FRT-event according to the first aspect of the present invention, or to perform a method of damping oscillations in the drive train of a wind turbine , said oscillations being caused during an FRT-event, according to the second aspect of the present invention.

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 descrip-tion 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 generating an FRT-power-return-path being provided for damping oscillations in a drive train of a wind turbine caused by an FRT-event according to the first aspect of the in-vention, or to perform a method of damping oscillations in the drive train of a wind turbine, said oscillations being caused during an FRT-event, according to the second aspect of the invention.

According to the fourth aspect of the invention, the object is solved by a control device of a wind turbine, said control device being provided for controlling the wind turbine during an FRT-event, said control device comprising: a data pro-cessing unit, a first component being provided for selecting or providing a first FRT-power-return-path, and a second component being provided for modulating the first FRT-power-return-path in its curve shape into a second FRT-power-return-path dependent on the drive train oscillations caused during the FRT-event, said second FRT-power-return-path having a curve shape that deviates from the curve shape of the first FRT-power-return-path.

With regard to the configuration, to the performance and to the function of the control device 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 and further below.

According to the fourth aspect, the invention is directed to a control device of the wind turbine. This control device is provided for controlling the wind turbine dur-ing 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.

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

Furthermore, the control device comprises a first component being provided for selecting or providing a first FRT-power-return-path, as well as a second compo-nent being provided for modulating the first FRT-power-return-path in its curve shape into a second FRT-power-return-path dependent on the drive train oscilla-tions caused during the FRT-event, said second FRT-power-return-path having a curve shape that deviates from the curve shape of the first FRT-power-return-path.

Furthermore, the control device preferably comprises a third component which is provided to operate the wind turbine, which is connected to an electric grid, in an FRT mode after grid fault, a voltage drop for example, has occurred in the electric grid, and, after the grid fault in the electric grid has been terminated, to increase the power of the wind turbine again, in particular to the power base level before the grid fault in the electric grid, and to perform the power increase in accordance with the second FRT-power-return-path.

According to yet another preferred embodiment, the control device comprises a fourth component which is provided for determining characteristic parameters of the oscillations of the drive train and for generating a counter-acting power wave based on these characteristic parameters of the oscillations of the drive train. Pref-erably, the fourth component can be provided to actuate different parts of the wind turbine in order to realize the specifications provided by the modulated FRT-power-return-path, or according to a different embodiment, the fourth component may comprise an interface for transferring data to respective actuator means.

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

According to yet another preferred embodiment, the control device comprises a computer program product according to the third aspect of the invention, which is at least temporarily executed in or by the data processing unit of the control de-vice.

Preferably the control device comprises means for performing the methods ac-cording to the first aspect and the second aspect of the invention.

According to the fifth aspect of the invention, the object is solved by a wind tur-bine comprising 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 na-celle, said rotor comprising a number of rotor blades, and a control device being provided for controlling the wind turbine during an FRT-event, characterized in that the control device is provided in accordance with the fourth aspect of the present invention.
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With regard to the configuration, to the performance and to the function of the wind turbine according to the fifth aspect, full reference is also made to the gen-eral description of the invention further above, and to the description of the other aspects of the invention as disclosed further above.

According to the fifth 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, and a control device being provided for controlling the wind turbine during an FRT-event. According to the invention, the control device is provided in accordance with the fourth aspect of the present invention.

The present invention according to its five aspects has a number of effects and advantages, which are particularly as follows:

- Reduction of loads and therefore cost reduction in the drive train;
- Steeper FRT-power-return-paths are possible;
- More adjustable power return settings are possible to fulfil present, future and more
rigid grid requirements can be realized easily;
- Grid agreeable FRT-power-return is possible due to less oscillations during and
after the end of the FRT-event.

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;

Figure 2 depicts a schematic representation of a control device ac-cording to the present invention;

Figure 3 depicts simulated FRT events for three different slopes of a linear power return path; and

Figures 4a to 4c show different diagrams how an FRT-power-return-path gets modulated in accordance with the present invention.

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) incor-porates 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 con-nected to a drive shaft of the drive train. The rotor blades (16), (17), (18) are ad-justably mounted on the hub (15). This is realized by means of pitch drives (not sown), 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 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 must stay connected to the power supply grid. In case of a grid fault, the wind turbine rides through the FRT with decreased power. FRT describes the re-quirement 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 pow-er 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.

Figure 2 schematically shows a control device (20), which controls the FRT as well as the FRT-power-return. The control device (20) can be an individual control device, or the control device (20) can be part of the central control system of the wind turbine (10).

The control device (20) comprises a number of different components. First of all, the control device (20) comprises a data processing unit (21).Furthermore, the control device (20) comprises a first component (22) being provided for selecting or providing a first FRT-power-return-path, as well as a second component (23) being provided for modulating the first FRT-power-return-path in its curve shape into a second FRT-power-return-path dependent on the drive train oscillations caused during the FRT-event, said second FRT-power-return-path having a curve shape that deviates from the curve shape of the first FRT-power-return-path. Ad-ditionally, the control device (20) comprises a third component (24) which is pro-vided to operate the wind turbine, which is connected to an electric grid, in an FRT mode after grid fault, a voltage drop for example, has occurred in the electric grid, and, after the grid fault in the electric grid has been terminated, to increase the power of the wind turbine again, in particular to the power base level before the grid fault in the electric grid, and to perform the power increase in accordance with the second FRT-power-return-path. Moreover, the control device (20) com-prises a fourth component (25) which is provided for determining characteristic parameters of the oscillations of the drive train and for generating a counter-acting power wave based on these characteristic parameters of the oscillations of the drive train. Preferably, the fourth component (25) can be provided to actuate dif-ferent parts of the wind turbine in order to realize the specifications provided by the modulated FRT-power-return-path.

If an FRT, caused by a grid fault, occurs, the rapid power drop strongly excites oscillation of the drive train which possibly lead to overspeed and 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 power, torque and speed can occur. Such peaks, which may cause the exceedance of the maximum coupling-torque, are either design driving or may lead to coupling damages. In any case such peaks, if they occur, require expensive counter measures. This is shown in connection with Figure 3.

Figure 3 depicts a three phase (3ph) 0%-FRT for 500ms. 0%-FRT means that the FRT is performed after a grid fault, by the occurence of which the voltage has been dropped down to 0%.

Figure 3 depicts simulated FRT events for three different slopes of a linear power return path, i.e. without application of the counter-acting power wave.

The upper part of Figure 3 shows the curve shape of the power for three different simulated paths (a), (b) and (c). From the mid part of Figure 3 which shows the curve shape for these three paths with regard to the torque, one can derive that a slow power return is better. The lower part of Figure 3 which shows the curve shape for these three paths with regard to the generator speed, depicts, that for the speed a quick power return is better.
Therefore, one has to find a compromise to satisfy both, torque and speed. How-ever, optimizing the slope of the linear FRT-power-return-path is limited. There-fore, decreasing the oscillations of the drive train is suitable since they are benefi-cial for both, torque and speed.

How such a reduction of the oscillations can be achieved will now be explained in connection with Figures 4a to 4c.

For reducing drive train oscillations which are caused during an FRT-event, it is provided to superimpose a conventional linear FRT-power-return-path (30) with a predetermined counter-acting power wave (31), said counter-acting power wave (31) counter-acting the drive train oscillations. Figure 4a depicts a diagram show-ing such a conventional linear FRT-power-return-path. The counter-acting power wave (31), which is depicted in Figure 4b, can be determined since the drive train oscillations are deterministic sinusoidal oscillations with fixed frequency and phase determined by the beginning of the FRT-event. The counter-acting power wave (31) therefore is a sinusoidal curve. As evident from Figures 4b1 and 4b2, the counter-acting power wave (31) is generated by multiplying an undiluted-acting power wave (31a) with a fading function (31b). Preferably, the FRT-power-return-path also includes a “fade in” and a “fade out” of the overlaid counter-acting power wave (31). That means that the counter-acting power wave is multi-plied by a “fade-in/out” factor. Therefore, the resulting FRT-power-return-path (32) after the FRT, as depicted in Figure 4c, has a non-linear curve shape since it has been modulated with a counter-acting signal to damp the drive train oscilla-tion. The phase of the wave is strongly dependent from the time of the start of the FRT, the time of a power drop for example.

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 Data processing unit
22 First component
23 Second component
24 Third component
25 Fourth component

30 Linear FRT-power-return-path (first path)
31 Counter-acting power wave
31a Undiluted-acting power wave
31b Fading function
32 Resulting FRT-power-return-path (second path)

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 ge-neric 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.
,CLAIMS:We claim:

1. A method of generating an FRT-power-return-path being provided for damp-ing oscillations in a drive train of a wind turbine, said drive train oscillations being caused during an FRT-event of the wind turbine, characterized by the following steps:
• Selecting or providing a first FRT-power-return-path (30);
• Dependent on the drive train oscillations, modulating the first FRT-power-return-path (30) in its curve shape into a second FRT-power-return-path (32), said second FRT-power-return-path (32) having a curve shape that deviates from the curve shape of the first FRT-power-return-path (30).

2. The method according to claim 1, characterized in that the first FRT-power-return-path (30) has a linear curve shape and that the modulated second FRT-power-return-path (32) has a curve shape which deviates from the linear curve shape of the first FRT-power-return-path (30).

3. The method according to claim 1 or 2, characterized in that the second FRT-power-return-path (32) is modulated by superimposing or inducing or applying a counter-acting power wave (31), in particular a sinusoidal counter-acting power wave, to the first FRT-power-return-path (30), said counter-acting power wave (31) counter-acting to the oscillations in the drive train caused by the FRT-event.

4. The method according to claim 3, characterized in that the counter-acting power wave (31) is generated based on the oscillations of the drive train during the FRT-event, in particular at the beginning of the FRT-event or at the beginning of the FRT-power-return.

5. The method according to claim 4, characterized in that characteristic parame-ters of the oscillations of the drive train are determined and that the counter-acting power wave (31) is generated based on these characteristic parameters of the os-cillations of the drive train.

6. The method according to claim 5, characterized in that the frequency of the oscillations of the drive train is determined and that the counter-acting power wave (31) is generated having a frequency which corresponds to or complies with the frequency of the drive train oscillations, in particular at the beginning of the FRT-event or at the beginning of the FRT-power-return.
7. The method according to claim 5 or 6, characterized in that the phase of the oscillations of the drive train is determined and that the counter-acting power wave (31) is generated having a phase which corresponds to or complies with the phase of the drive train oscillations, in particular at the beginning of the FRT-event or at the beginning of the FRT-power-return.

8. The method according to anyone of claims 5 to 7, characterized in that the amplitude of the counter-acting power wave (31) is determined or estimated by means of a simulation process and/or that the amplitude of the counter-acting power wave is individually adjusted and/or that the amplitude of the counter-acting power wave is determined or estimated based on the windspeed and/or that the amplitude of the counter-acting power wave is determined or estimated in ac-cordance with the first FRT-power-return-path (30) and/or that the amplitude of the counter-acting power wave (31) is determined or estimated in accordance with the depth of the first FRT-power-return-path (30).

9. A method of damping oscillations in the drive train of a wind turbine (10), said oscillations being caused during an FRT-event, whereby the wind turbine (10) which is connected to an electric grid is operated in an FRT-mode with decreased power after a grind 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, the power of the wind turbine is increased again, in particular to the power base level before the grid fault, the voltage drop for ex-ample, in the electric grid, and whereby the power increase is performed according to an FRT-power-return-path (32), that has been generated with a method ac-cording to anyone of claims 1 to 8.

10. A computer program product, which enables a data processing unit (21), once the computer program product is executed on the data processing unit (21), and is preferably stored in a storage device of the data processing unit (21), to perform a method of generating an FRT-power-return-path (32) being provided for damping oscillations in a drive train of a wind turbine (10) caused by an FRT-event accord-ing to anyone of claims 1 to 8, or to perform a method of damping oscillations in the drive train of a wind turbine (10), said oscillations being caused during an FRT-event, according to claim 9.

11. A control device (20) of a wind turbine (10), said control device (20) being provided for controlling the wind turbine (10) during an FRT-event, said control device (20) comprising: a data processing unit (21), a first component (22) being provided for selecting or providing a first FRT-power-return-path (30), and a sec-ond component (23) being provided for modulating the first FRT-power-return-path (30) in its curve shape into a second FRT-power-return-path (32) dependent on the drive train oscillations caused during the FRT-event, said second FRT-power-return-path (32) having a curve shape that deviates from the curve shape of the first FRT-power-return-path (30).

12. The control device (20) according to claim 11, characterized in that it com-prises a third component (24) which is provided to operate the wind turbine (10), which is connected to an electric grid, in an FRT-mode after grid fault, for exam-ple a voltage drop, has occurred in the electric grid, and, after the grid fault, the voltage drop for example, in the electric grid has been terminated, to increase the power of the wind turbine (10)again, in particular to the power base level before the grid fault, the voltage drop for example, in the electric grid, and to perform the power increase in accordance with the second FRT-power-return-path (32).

13. The control device (20) according to claim 11 or 12, characterized in that the control device (20) comprises a fourth component (25) which is provided for de-termining characteristic parameters of the oscillations of the drive train and for generating a counter-acting power wave (31) based on these characteristic pa-rameters of the oscillations of the drive train, and/or that the control device (20) comprises a computer program product according to claim 10, which is at least temporarily executed in or by the data processing unit (21).

14. The control device (20) according to anyone of claims 11 to 13, characterized in that it comprises means for performing the method according to anyone of claims 1 to 9.

15. 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), and a control device (20) being provided for controlling the wind turbine (10) during an FRT-event, characterized in that the control device (20) is provided in accordance with anyone of claims 11 to 14.