Abstract:
A method is provided for controlling a DC link voltage of a power converter connecting an electric generator of a wind turbine with a power grid. The method includes determining the rotational speed of a rotor of the electric generator. The method further includes determining a strength of harmonic components of the rotational speed of the rotor, wherein the harmonic components have frequency components being different from a fundamental rotational frequency of the electric generator. The method also includes determining a damping reference signal as a function of the determined strength of the harmonic components, and controlling the DC link voltage of the power converter in response to the determined damping reference signal. A corresponding control system is also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of European Patent Office application No. 11154346.8 EP filed Feb. 14, 2011. All of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The present invention generally relates to the technical field of operating wind turbines. In particular, the present invention generally relates to a method and to a system for controlling a DC link voltage of a power converter connecting an electric generator of a wind turbine with a power grid. The present invention further related to a computer program for carrying out the described method. 
     ART BACKGROUND 
     Wind power generation has received a major impetus due to ever increasing demand for energy, depleting fossil fuel reserves and environmental benefits in particular with respect to the emission of greenhouse gases. 
     A wind turbine comprises a drive train, which includes all mechanical components, which are in a functional view connected upstream with respect to an electric generator being used for converting mechanical energy/power captured from wind into electric energy/power being supposed to be provided to a power grid. Important drive train components are e.g. rotor blade, a hub, a main shaft and, if applicable, a gearbox. 
     When operating a wind turbine regularly mechanical oscillations within the drive train occur. Such oscillations may cause an out-of-round operation of the electric generator which has a negative impact not only on the lifetime of the wind turbine but also on the electric quality of the energy/power being provided by the wind turbine to a power grid. Such mechanical oscillations may be so called “torque oscillations” and/or “speed oscillations”. A pure torque oscillation is an oscillation, wherein in the absence of any internal movement of at least one component of the drive train a value for the torque within the at least one component varies over time. A pure speed oscillation is any oscillation, wherein in the absence of torque variations the speed of at least a part of a component of the drive train varies over time. The speed may be in particular a rotational speed, however also a translational or a combination of a translational and a rotational speed is possible. Typically, the variations over time occur periodically. 
     In particular in a variable speed wind turbine, mechanical oscillations of drive train and in particular of the rotor blades at resonance frequencies can be excited by grid voltage dips and/or by wind speed changes. Mechanical oscillations resulting from such perturbations can cause a trip of the wind turbine and sometimes even structure damages of components (in particular rotor blades) of the drive train. Therefore, it is necessary to damp mechanical drive train oscillations in particular in variable speed wind turbines. 
     A known basic principle for damping such mechanical oscillations is to control the power output of the wind turbine being fed to the power grid at drive train resonance frequencies so that mechanical oscillations at this frequency can be reduced. In this context wind turbine controllers have been proposed, which comprise appropriate electronic hardware and/or software filters being comprised within the controllers. However, when applying this kind of active damping control by means of a dedicated wind turbine controller, power oscillations at the mechanical resonance frequency of components of the drive train are sent to the power grid, which is not preferred in both a steady state operational condition and intermediate operational condition comprising a ride through a grid fault. This has the effect that it is difficult for a wind turbine operator to meet power quality requirements given by the operator of a power grid, which receives electric power from the wind turbine. 
     There may be a need for an effective method which allows for reducing mechanical oscillations of a drive train of a wind turbine. 
     SUMMARY OF INVENTION 
     This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims. 
     According to a first aspect of the invention there is provided a method for controlling a DC link voltage of a power converter connecting an electric generator of a wind turbine with a power grid. The provided method comprises (a) determining the rotational speed of a rotor of the electric generator, (b) determining a strength of harmonic components of the rotational speed of the rotor, wherein the harmonic components have frequency components being different from a fundamental rotational frequency of the electric generator, (c) determining a damping reference signal as a function of the determined strength of the harmonic components, and (d) controlling the DC link voltage of the power converter in response to the determined damping reference signal. 
     The described control method is based on the idea that by controlling the DC link voltage of a power converter, mechanical oscillations of a wind turbine and in particular mechanical oscillations of a drive train of the wind turbine can be effectively reduced. 
     By controlling the DC link voltage in the described manner harmonic power, which is generated by the electric generator in response to unwanted mechanical oscillations of and/or within a drive train of the wind turbine, is not forwarded to the power grid via the power converter. Instead, this generated harmonic power is sent to the DC link of the power converter and in particular to a DC link capacitor of the power converter. Thereby, a time dependent variation of the DC link voltage arises which has a retroaction to the electric generator. This retroaction causes a mechanical damping capability of the electric generator, which is capable of effectively reducing the mechanical oscillations of and/or within the drive train, which mechanical oscillations represent(ed) the source of the unwanted generation of harmonic power. 
     The described damping reference signal may by for instance a damping power reference signal or a damping torque reference signal. In this respect the damping power reference signal represents a reference or nominal value for the actual damping power of the electric generator. Correspondingly, the damping torque reference signal represents a reference or nominal value for the actual damping toque of the electric generator. 
     The described determining of the rotational speed may be realized by means of a measurement procedure relying on the output of an appropriate rotational speed sensor, by means of an estimation procedure, wherein at least one input value is used in order to estimate the actual rotational speed at least approximately. 
     In this document the term “electric generator” is used for any electromechanical device, which is capable of converting mechanic energy in electric energy. For generating a magnetic field, which in accordance with basic physical principles is necessary for the energy conversion, the electric generator may have current driven coils and/or permanent magnets. 
     The term “power converter” may denote any electric power device, which is used for an adaptation of a variable voltage and frequency of the electric generator to a nominally fixed voltage and frequency of a power grid. A power converter includes a generator bridge, which in normal operation operates as an active rectifier in order to supply power to a DC link. The generator bridge can have any suitable topology with a series of semiconductor power switching devices fully controlled and regulated using a pulse width modulation (PWM) strategy. A power converter typically also comprises a network bridge that converts electrical energy between dc link and power grid. Since a power converter always accomplishes a frequency conversion, such power converters are often also denominated as frequency converters. 
     The term “DC link voltage” denotes the voltage being present at the above described DC link of the power converter. 
     The term “fundamental rotational frequency” may particularly denote the rotational frequency of the rotor of the electric generator, which is associated with a desired operation of the wind turbine in the absence of any mechanical oscillations within the drive train of the wind turbine. Of course, if the electric generator is used in a variable speed wind turbine, which is a preferred application field for the control method described in this document, the fundamental rotational frequency will not be constant over time. 
     The drive train includes any mechanical components of a wind turbine, which are in a functional view connected upstream with respect to the electric generator. Important drive train components are e.g. rotor blade, a hub, a main shaft and, if applicable, a gearbox. 
     As has already been described above, the harmonic power components are sent to the DC link of the power converter and not to the power grid. This may provide the advantage that grid power harmonics can be reduced effectively. 
     The described method is in particular suitable for wind turbines, which depending on the actual wind condition are operated with variable rotational speed of the rotor. In this context the “rotor” may denote the rotor of the wind turbine, which comprises the rotor blades being mounted to the hub, and/or the rotor of the electric generator. The wind turbine may be either a so called direct drive wind turbine, wherein the main shaft of the wind turbine rotor directly drives the electric generator, or a wind turbine comprising a gearbox, which is connected between the main shaft and the electric generator. 
     Descriptive speaking, the described control method may be able to damp mechanical drive train and in particular mechanical blade oscillations in a variable speed wind turbine without feeding harmonic power to grid. In order to damp the mechanical oscillation of the drive train/blade, harmonic power at resonance frequencies of drive train/blade is produced by the electrical generator. Instead of sending the produced harmonic power to the power grid, the power converter sends this harmonic power to a DC link capacitor of the DC link of the power converter which will control the respective DC link voltage varying at certain range. 
     According to an embodiment of the invention determining the strength of harmonic components of the rotational speed of the rotor comprises (a) receiving a signal being indicative for the actual rotational speed of the rotor and (b) filtering the signal with a filter, which is capable of suppressing at least partially frequency components corresponding to the fundamental rotational frequency. This may provide the advantage that the strength of the harmonic components of the rotational speed of the rotor can be determined easily and in an effective manner. 
     The described filter may be realized by means of a hardware (electronic) filter, by means of a software filter or by means of a so called hybrid filter which relies on a combination between software and hardware filtering. 
     The described filter may be a single filter or a filter system comprising different single filters. In particular, the described filter may comprise a high pass filter or a band pass filter. 
     According to a further embodiment of the invention determining the damping reference signal comprises (a) receiving the filtered signal from the filter and (b) limiting the strength the received filtered signal. This may provide the advantage that it will be ensured that the strength of the damping reference signal will be kept within technically meaningful limits and an unwanted oversteer of the DC link voltage control can be avoided. 
     The range into which the damping reference signal will be restricted can be defined by two limiting values, wherein the absolute value of the two limiting values is the same and the sign of the two limiting values is different. 
     In this respect it is mentioned that the step of determining the damping reference signal may optionally further comprise an amplification of the filtered signal with a predefined amplification gain. An appropriate amplifier may be connected preferably in between the filter and a limiting unit, which is configured to perform the described limiting procedure. 
     According to a further embodiment of the invention controlling the DC link voltage of the power converter is carried out in further response to the actual DC link voltage. This may provide the advantage a closed loop control is realized, which may allow for an accurate and precise control of the DC link voltage. 
     According to a further embodiment of the invention controlling the DC link voltage of the power converter is carried out in further response to a reference signal for the DC link voltage. This may provide the advantage that the accuracy of the DC link voltage can be even further improved. 
     According to a further embodiment of the invention the method further comprises filtering a measurement signal being indicative for the actual DC link voltage with a further filter, which is capable of suppressing at least partially frequency components corresponding to mechanical resonance frequencies of a drive train of the wind turbine. Thereby, the frequency components after the filter may be related to the fundamental rotational frequency and/or to harmonics of this fundamental rotational frequency. This may mean that the filter can suppress resonance frequencies/harmonics in the measured dc link voltage. Therefore, compared to the signal before the filter the signal after the filter contains weaker harmonics or less harmonics caused by mechanic resonance frequencies of the wind turbine drive train. 
     The further filter may be a band stop filter so that a DC link controller, which is responsible for controlling the DC link voltage, has no response to harmonic voltages being present at the DC link of the power converter. Thereby, the frequency response of the further filter may be preferably designed such that predominately only signal portions having a frequency being in the regime of the fundamental frequency and not within higher frequency regimes can pass the further filter. This may provide the advantage that bandwidth limitations of the DC link controller are removed. As a consequence, the described method will be also applicable for applications wherein the fundamental frequency is relatively high (e.g. higher than 20 Hz). 
     According to a further embodiment of the invention controlling the DC link voltage comprises controlling an AC-to-DC conversion being carried out in a generator bridge of the power converter. Thereby, the generator bridge may be controlled by a so caller generator control portion of the power converter controller. 
     According to a further embodiment of the invention a reference signal for the amount of power to be generated by the wind turbine is used for controlling a DC-to-AC conversion being carried out in a network bridge. 
     In other words, the described control method further takes into account a general power reference for the wind turbine. This may provide the advantage that the described control method may contribute to maintain the amount of power being generated by the wind turbine at a predetermined level, which may be given for instance by a superordinate wind farm controller controlling the general operation of a plurality of wind turbines of a wind farm. 
     According to a further embodiment of the invention controlling the DC link voltage comprises controlling a DC-to-AC conversion being carried out in a network bridge of the power converter. Thereby, the network bridge may be controlled by a so caller grid control portion of the power converter controller. 
     According to a further embodiment of the invention a reference signal for the amount of power to be generated by the wind turbine is used for controlling a AC-to-DC conversion being carried out in a generator bridge. This means that the described control method also takes into account a general power reference for the wind turbine, which may be given for instance by a superordinate wind farm controller controlling the general operation of a plurality of wind turbines of a wind farm. This may provide the advantage that the described control method may contribute to maintain the amount of power being generated by the wind turbine at a predetermined level. 
     According to a further aspect of the invention there is provided a control system for controlling a DC link voltage of a power converter connecting an electric generator of a wind turbine with a power grid. The provided control system comprises (a) a first determining unit for determining the rotational speed of a rotor of the electric generator, (b) a second determining unit for determining a strength of harmonic components of the rotational speed of the rotor, wherein the harmonic components have frequency components being different from a fundamental rotational frequency of the electric generator, (c) a third determining unit for determining a damping reference signal as a function of the determined strength of the harmonic components, and (d) a control unit for controlling the DC link voltage of the power converter in response to the determined damping reference signal. 
     Also the described control system is based on the idea that by controlling the DC link voltage of a power converter, mechanical oscillations of a wind turbine and in particular mechanical oscillations of a drive train of the wind turbine can be effectively reduced. 
     It is mentioned that depending on the specific application the described units of the control system may be physically different units or may be realized within at least one combined unit. 
     According to an embodiment of the invention the control system is realized by a power converter controller for controlling the operation of the power converter. Thereby, the power converter controller comprises (a) a first portion for controlling the operation of a generator bridge carrying out a AC-to-DC conversion within the power converter and (b) a second portion for controlling the operation of a network bridge carrying out a DC-to-AC conversion within the power converter. 
     Since the described generator bridge is assigned to the electric generator side or the wind turbine side of the power converter, the first portion is often denominated a generator controller. Accordingly, since the described network bridge is assigned to the power grid side of the power converter, the second portion is often denominated a (power) grid controller. 
     According to a further aspect of the invention there is provided a computer program for controlling a DC link voltage of a power converter connecting an electric generator of a wind turbine with a power grid. The computer program, when being executed by a data processor, is adapted for carrying out the method as set forth in any one of the preceding claims. 
     As used herein, reference to a computer program is intended to be equivalent to a reference to a program element and/or to a computer readable medium containing instructions for controlling a computer system to coordinate the performance of the above described method. 
     The computer program may be implemented as computer readable instruction code in any suitable programming language, such as, for example, JAVA, C++, and may be stored on a computer-readable medium (removable disk, volatile or non-volatile memory, embedded memory/processor, etc.). The instruction code is operable to program a computer or any other programmable device to carry out the intended functions. The computer program may be available from a network, such as the World Wide Web, from which it may be downloaded. 
     The invention may be realized by means of a computer program respectively software. However, the invention may also be realized by means of one or more specific circuits respectively hardware. Furthermore, the invention may also be realized in a hybrid form, i.e. in a combination of software modules and hardware modules. 
     It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to method type claims whereas other embodiments have been described with reference to apparatus type claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters, in particular between features of the method type claims and features of the apparatus type claims is considered as to be disclosed with this document. 
     The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a drive train damping controller, which provides a damping power reference signal being used by different control systems representing embodiments of the invention described in this document. 
         FIG. 2  shows a control system for a drive train damping procedure, wherein in accordance with a first embodiment of the invention a DC link voltage control is carried out by a generator control portion of a power converter controller. 
         FIG. 3  shows a control system for a drive train damping procedure, wherein in accordance with a second embodiment of the invention a DC link voltage control is carried out by a generator control portion of a power converter controller. 
         FIG. 4  shows a control system for a drive train damping procedure, wherein in accordance with a third embodiment of the invention a DC link voltage control is carried out by a grid control portion of a power converter controller. 
         FIG. 5  shows a diagram depicting an intermediate dip in the amount of power being transferred from a wind turbine to a power grid, which in accordance with a prior art damping procedure causes (a) a short rise in the voltage of the DC link of a power converter and (b) successive oscillations of the amount of transferred. 
         FIG. 6  shows a diagram depicting an intermediate dip in the amount of power being transferred from a wind turbine to a power grid, which in accordance with a damping procedure described in this document causes (a) a short rise in the voltage of the DC link of a power converter followed by successive oscillations of the voltage of the DC link and (b) a comparatively smooth behavior of the amount of transferred power after the end of the intermediate dip. 
         FIG. 7  shows a drive train damping controller, which provides a damping torque reference signal being used by different control systems representing embodiments of the invention described in this document. 
         FIG. 8  shows a control system comprising the drive train damping controller of  FIG. 7 , wherein a DC link voltage control is carried out by a generator control portion of a power converter controller. 
         FIG. 9  shows a control system comprising the drive train damping controller of  FIG. 7 , wherein a DC link voltage control is carried out by a grid control portion of a power converter controller. 
     
    
    
     DETAILED DESCRIPTION 
     The illustration in the drawing is schematically. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit. 
       FIG. 1  shows a drive train damping controller  110 , which provides a damping power reference signal PDTD being used by different control systems representing embodiments of the invention described in this document. 
     The drive train damping controller  110  comprises a high pass filter  112 , which receives as an input signal cog the actual rotational speed of the rotor of an electric generator (not depicted) of a wind turbine (also not depicted). The actual rotational speed cog may be measured for instance by means of an encoder and/or any type of speed/position observer and/or estimator. 
     According to the embodiment described here the high pass filter  112  is realized by a low pass filer  112   a , a bypass line  112   b  and a subtracting unit  112   c . Further, the output signal of the high pass filter  112  is fed to an amplifier  114 , which has a predetermined gain. In  FIG. 1  this gain is denominated as “damping gain” (DampGain). 
     As can be seen from  FIG. 1 , the output signal provided by the amplifier  114  is supplied to a limiting unit  116 , which limits the output signal PDTD of the whole drive train damping controller  110  within a range extending from −Pdtd_Lim to +Pdtd_Lim. Limiting the damping power reference signal PDTD within a predefined range provides the advantage that an unwanted oversteer by the control systems, which are described further below, can be avoided. According to the embodiment described here the limit values −Pdtd_Lim and +Pdtd_Lim for the damping power reference signal PDTD depend on the nominal power of the respective wind turbine. 
     It is mentioned that the high pass filter in  FIG. 1  can be replaced by other type of filter or filter systems such as a band pass filter or a combination of several high pass filters. 
       FIG. 2  shows a control system  200  of a power converter controller (not depicted) for a drive train damping procedure. The control system  200  comprises two portions, a first portion  202 , which is also denominated a generator control portion or in short a generator controller, and a second portion  204 , which is also denominated a grid control portion or in short a grid controller. According to the embodiment described here, a DC link voltage control is carried out by the generator controller  202  of the power converter controller. 
     The control system  200  comprises the drive train damping controller  110 , which is illustrated in  FIG. 1  and which has been described above in detail. The damping power reference signal PDTD, which is provided by the drive train damping controller  110  in response to the actual measured and/or observed rotational speed cog of the rotor (not depicted) of the electric generator (also not depicted), is supplied to a calculation unit  222  for calculating a reference signal VACREF for the harmonic voltage. This calculation is carried out in consideration of a signal VDC being indicative for the actually measured DC link voltage. 
     As can be seen from  FIG. 2 , the reference signal VACREF for the harmonic voltage being provided by the calculation unit  222  is fed to an adding unit  224 , which adds the signal VACREF to a signal VDCREF representing a reference signal for the fundamental voltage of the DC link of the power converter. The output signal from the adding unit  224  is fed, together with the above described (measured) DC link voltage VDC, to a DC link controller  250 . In response to the output signal of the adding unit  224  and to the signal VDC, the DC link controller  250  determines a reference signal PGenRef for the power to be generated by the electric generator. 
     Further, the signal PGenRef is fed to a calculation unit  262  for calculating a generator current reference signal Idqrefm representing the reference signal for the amount of current to be generated by the electric generator. Based on this signal Idqrefm and on a signal Idqm representing the measured current generated actually by the electric generator, a generator current controller  264  determines a reference voltage signal Vdqm for a Pulse Width Modulation (PWM) of an AC-to-DC conversion being carried out in a generator bridge (not depicted) of the power converter. 
     It is mentioned that the power reference signal PGenRef with could also be replaced by a torque reference signal TGenRef. Further details are given below with reference to the description of  FIGS. 7, 8 and 9 . 
     It is further mentioned that the indexes “dq” are indicative for the so called direct quadrature or in short d-q rotating reference frame, which is associated with a common mathematical transformation being often used to simplify the analysis of three-phase electric circuits. In the case of balanced three-phase electric circuits, application of the d-q transformation reduces the three AC quantities to two DC quantities. As a consequence, simplified calculations can be carried out on these imaginary DC quantities before performing the inverse transform to recover the actual three-phase AC results. 
     The grid control portion  204  shown in  FIG. 2  comprises a calculation unit  272  for calculating a power grid current reference signal Idqrefg in response to a reference signal PWTCRef for the total power to be generated by the respective wind turbine. The reference signal PWTCRef may be received for instance from a wind turbine controller (not depicted), which in known manner controls the operation of the whole wind turbine. 
     The signal Idqrefg, which is indicative for the amount of current being supposed to be received by the power grid (not depicted), is supplied to a grid current controller  274 . As can be seen from  FIG. 2 , in response to the signal Idqrefg and further in response to a signal Idqg representing the measured current, which is actually received by the power grid, the grid current controller  274  determines a reference voltage signal Vdqg, which is a reference voltage signal for the Pulse Width Modulation (PWM) of a DC-to-AC conversion being carried out in a network bridge of the power converter. 
     Descriptive speaking, in the described control system  200  the change of DC link voltage reference VDCREF corresponding to the damping power PDTD is calculated. The DC link voltage reference VDCREF with both DC component and harmonics is sent to the DC link controller  250 . The output of the DC link controller  250  is the generator power reference PGenRef. The generator power is controlled to follow this reference by the generator current controller  264  which is the inner loop of the generator controller. 
     The described control system  200  is in particular suitable when resonance frequency is relatively low due to the limitation of bandwidth of the DC link controller  250 . 
       FIG. 3  shows a control system  300  for a drive train damping procedure, wherein in accordance with the control system  200  the DC link voltage control is also carried out by the generator control portion  202  of the respective power converter controller. Compared to the control system  200  the control system  300  is more suitable when the mechanical resonance frequency of the comparatively high (e.g. more than 20 Hz). 
     As can be easily seen from a comparison of  FIG. 2  and  FIG. 3 , there are significant similarities between the two control systems  200  and  300 . For the sake of conciseness of this document, in the following focus is made to the differences between the two control systems  200  and  300 . 
     The output signal PDTD of the drive train damping controller  110  is added by an adding unit  352  to the output PDCRef of the DC link controller  250  directly. The signal PDCRef can be understood as a reference signal for the fundamental power to be generated by the electric generator. 
     The measured feedback signal VDC of the DC link voltage is filtered by a band stop filter  340  so that the DC link controller  250  has no response to harmonic voltages of the DC link at resonance frequencies. 
       FIG. 4  shows a control system  400  for a drive train damping procedure, wherein by contrast to the embodiments described above, the DC link voltage control is carried out by a grid control portion  204  of the respective power converter controller. 
     Again, there are many similarities between the control system  400  and the two control systems  200  and  300  described above. Therefore, in the following it will be focused on the differences between the two control systems  300  and  400 . 
     As can be seen from  FIG. 4 , the reference signal PWTCRef for the power to be generated by the wind turbine is sent from a non depicted wind turbine controller to an adding unit  454  of the generator controller  202 . The adding unit  454  adds (a) the damping power reference signal PDTD to (b) the reference signal PWTCRef, the result of which is the reference signal PGenRef for the power to be generated by the electric generator. 
     As can be further seen from  FIG. 4 , in the grid controller  204  the DC link controller  250  determines, in response to (a) the reference signal VDCREF for the fundamental voltage of the DC link of the power converter and to (b) the output signal of the band stop filter  340 , a reference signal PGridRef for the amount of power to be received by the power grid. 
     It is again mentioned that in some applications it might be advantageous to replace the band stop filter  340  by low-pass filter. 
       FIG. 5  shows a diagram  580  depicting the time dependency  582  of the amount of power being transferred from the electric generator of a wind turbine to the power grid, when a conventional control procedure for controlling the operation of a power converter connecting the electric generator and the power grid is employed. 
     As can be seen from  FIG. 5 , in case an intermediate dip  584  in the power curve  582  occurs (for instance due to an unwanted voltage dip in the power grid), a short rise  594  in the time dependency of the DC link voltage  590  appears. As can be further seen from  FIG. 5 , after the end of the power dip  584 , comparatively strong power oscillations  586  at the mechanical resonance frequency of the drive train of the wind turbine in the power curve  582  occur. These oscillations  586  are only slowly damped in time. 
       FIG. 6  shows a corresponding diagram  680 , which shows the same situation of a power dip  684  in the time dependency  682  of the amount of power being actually transferred from the electric generator to the power grid via the power converter, when a damping procedure in accordance with a control system as described in this document is employed. 
     Reference numeral  690  denominates the time dependency of the DC link voltage of the power converter. Again, the power dip  684  causes a short rise  694  of the DC link voltage  690 . However, by contrast to the situation shown in  FIG. 5 , now there appear no oscillations in the power signal  680  after the end of the dip. Instead, damped oscillations  696  having again the frequency of the mechanical resonance of the drive train, appear in the time dependency  690  of the DC link voltage. 
       FIG. 7  shows a drive train damping controller  710 , which provides a damping torque reference signal TDTD being used by different control systems representing embodiments of the invention described in this document. The structure of the drive train damping controller  710  corresponds to the structure of the drive train damping controller  110  shown in  FIG. 1 . The only difference is that the output of the drive train damping controller  710  is the damping torque reference signal TDTD and not a damping power reference signal PDTD. Accordingly, a limiting unit  716  is used, which limits the output signal TDTD of the whole drive train damping controller  710  within a range extending from −Tdtd_Lim to +Tdtd_Lim. 
     It is mentioned that the high pass filter in  FIG. 7  can be replaced by other type of filter or filter systems such as a band pass filter or a combination of several high pass filters. 
       FIG. 8  shows a control system  800  comprising the drive train damping controller  710  shown in  FIG. 7 . The structure of the control system  800  corresponds to the structure of the control system  300  shown in  FIG. 3 . Also the control system  800  comprises two portions, a first portion  802 , which is also denominated a generator control portion or in short a generator controller, and a second portion  804 , which is also denominated a grid control portion or in short a grid controller. According to the embodiment described here, a DC link voltage control is carried out by the generator controller  802  of the power converter controller. 
     As can be seen from a comparison between the control system  800  and the control system  300  shown in  FIG. 3 , in the adding unit  352  there is added the damping torque reference signal TDTD provided by the drive train damping controller  710  and a reference signal TDCRef provided by the DC link controller  250 . The reference signal TDCRef is indicative for the fundamental torque which is supposed to be generated by the electric generator. Further, the output of the adding unit  352  is a reference signal TGenRef for the amount of torque to be generated by the electric generator. 
     Apart from these differences the structures and the operations of the control system  800  and the control system  300  shown in  FIG. 3  correspond to each other. Therefore, for the sake of conciseness at this point of the description reference can be made to the description above. 
       FIG. 9  shows a control system  900  comprising the drive train damping controller  710 , wherein a DC link voltage control is carried out by the grid control portion  804  of a power converter controller. The structure of the control system  900  corresponds to the structure of the control system  400  shown in  FIG. 4 . The only difference is that by means of the adding unit  454  there is added the damping torque reference signal TDTD provided by the drive train damping controller  710  and a reference signal TWTCRef for the torque to be generated by the electric generator. The reference signal TWTCRef may be provided in particular by a non depicted wind turbine controller. 
     The output of the adding unit  454  is the reference signal TGenRef, which has already been described above with reference to the control system  800 . The reference signal TGenRef is indicative for the amount of torque to be generated by the electric generator. 
     Apart from these differences the structures and the operations of the control system  900  and the control system  400  shown in  FIG. 4  correspond to each other. Therefore, for the sake of conciseness of this document at this point of the description reference can be made to the description above. 
     The above elucidated embodiments of the invention presented in this document may in particular provide the following advantages: 
     (A) Oscillation of torque and speed of the drive train, in particular of the blades, of the wind turbine can be damped actively. 
     (B) Power oscillations are not forwarded to the power grid. 
     (C) An implementation of the invention into actual available power converters can be realized by means of software. Therefore, there are no or only a negligible additional costs for realizing the invention in connection with available wind turbines. 
     It should be noted that the term “comprising” does not exclude other elements or steps and the use of articles “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.