Abstract:
A method for damping torsional oscillations in a power production plant includes a drive train, a rotor fitted to a rotor-side end of the drive train, and a generator driven via the drive train and fitted to a generator-side end of the drive train. By using at least one angular position of the drive train at the generator-side end thereof, a torsional moment acting on the drive train is determined and used to control the generator.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2010/007497, filed on Dec. 9, 2010, which claims the benefit of priority to Serial No. DE 10 2009 059 669.0, filed on Dec. 19, 2009 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The disclosure relates to a method for damping torsional vibrations, in particular in the drive train of energy generation installations, by means of closed-loop generator control and a device for implementing the method. 
     Drive trains comprising components such as gears, clutches and connecting elements (shafts), for example, are important parts of various electrical energy generation installations, such as, for example, wind energy installations, water power installations, etc. 
     The drive train performs the task of producing a mechanical connection between an input drive (for example a rotor of a wind energy installation) and an output drive (for example a corresponding generator), via which energy is transmitted by a rotary movement. Drive train components, such as gears, serve the purpose of converting the speed and the torque present at the input drive to values which correspond to the working range of the generator. If required, clutches are used for isolation between the input drive and the output drive and shafts produce the mechanical connection between the components involved. Other components, such as mechanical brakes or the like, can also be integrated in the drive train. 
     Since the components involved cannot be manufactured with any desired stiffness, but have a finite stiffness, they can be excited to produce natural vibrations. Such excitation can take place, for example, as a result of an input power which is not constant (in the case of wind energy installations, for example, as a result of gusts of wind or wind turbulences), as a result of external faults or as a result of self-motions of other installation components. Vibrations of a different origin can also result in vibrations in the drive train, in the case of a wind energy installation, for example, vibrations of the tower or vibrations as a result of meshing of a gear. 
     Vibrations have a disadvantageous effect on the life of the components involved, in particular the gear. Continuous pulsating loads increase the wear on the affected component parts and result in shorter replacement intervals, which has financial and technical implications for the installation and grid operator and reduces the installation&#39;s profits. In particular from the point of view of the predicted increase in the prevalence of wind energy installations in the offshore sector in the foreseeable future, this aspect will play an ever greater role since the replacement of damaged components will be even more difficult there. The aim is therefore to reduce these vibrations in order to extend the life of the components. 
     Although the present disclosure in the context of this application is described primarily with respect to wind energy installations, it is in no way restricted to wind energy installations or devices for energy generation, but can be used in principle in all devices in which torsional vibrations of shafts, spindles or the like, in particular also of shafts and spindles with interposed gears, can occur. 
     In order to avoid vibrations, it is known to configure the generator of an energy generation installation such that it can be adjusted by means of external actuating signals. In this case, the generator may be a double-fed asynchronous generator, for example, which is connected directly to the power supply system on the stator side and is supplied on the armature side via a DC voltage intermediate circuit, as a result of which voltages and currents of different frequencies and amplitudes can be impressed on the armature. Synchronous generators which are connected to the power supply system via converters with a DC intermediate circuit and are correspondingly adjustable are also used in the prior art. By virtue of the mentioned actuation possibilities, it is possible for the generator to be provided with a torque which is matched to the damage-causing vibrations, as a result of which it reduces these vibrations and correspondingly damps the torque present in the drive train. 
     Thus, DE 10 2007 019 907 B4 has disclosed a method which forms a control deviation on the basis of the generator speed via a vibrational delay element, from which a correction torque is determined for open-loop generator control. 
     A similar approach is pursued in US 2008/0067815, in accordance with which a signal is generated from changes in the generator speed, and this signal is used to realize damping via the generator actuating torque. 
     EP 1 507 331 A2 describes a method in which a generator actuating torque is generated from the generator speed specifically for synchronous generators. 
     Disadvantages with all of these solutions are considered to be the fact that the damage-causing vibrations of the torque are only determined with restricted accuracy and accordingly also the quality of the compensation operations is limited. In particular, the determination of the vibrations from the speed is subject to considerable inaccuracies. 
     By virtue of filtering the speed signal, this problem can be partially eliminated, but this increases the complexity in respect of damping and can result in further undesirable side effects, such as phase shifts of the signals. 
     U.S. Pat. No. 7,173,399 B2 has proposed, in this regard, measuring the torque via torque sensors and generating a corresponding actuating torque via load-commutated converters and current intermediate circuits. Instead of deriving an actuating torque from the speed, therefore, the torque itself is used directly, as a result of which the accuracy of the damping is improved. However, in this case the use of at least one further sensor is necessary in order to detect the torque to be damped. Therefore, the damping cannot be achieved on its own with the hardware already existing in an installation. 
     In the mentioned methods, there is the additional problem that vibrations are impressed on the torque present at the generator as a result of the compensation processes themselves, and these vibrations generally result in a correspondingly non-uniform electrical output power. 
     There is therefore a need for solutions which make it possible to continuously determine a torque, in particular in a drive train, on the basis of variables which are already determined, generally by means of existing hardware, and back-control (closed-loop) by means of adjusting a generator torque. In addition, there is a requirement for solutions for smoothing vibration-related fluctuations in the output power of such generators. 
     SUMMARY 
     Against this background, the present disclosure provides a method for damping torsional vibrations in an energy generation installation and a device, in particular a closed-loop control device of an energy generation installation, having the features of the disclosure. Advantageous developments are the subject matter of the dependent claims and the description below. 
     On the basis of the proposed measures, the present disclosure provides solutions for at least the three abovementioned problems: 
     First, the torque to be damped or compensated for is determined more precisely and fed directly to the controller, as a result of which greater accuracy is achieved in comparison with speed-based damping methods. 
     The determination of the torque can also take place without any additional sensors, purely by means of the sensor equipment already existing in the installation and with the additional use of computation models. 
     The fluctuations in the generator output power arising in this method can ultimately be smoothed by a damping device before the electrical energy generated is fed to the power supply system. 
     In energy generation installations, the speed on the input drive and/or output drive side is generally known. On the input drive side, for example in the case of the rotor of a variable-speed wind energy installation, this speed is recorded by speed sensors and transmitted to the installation controller, which controls, by means of open-loop or closed-loop control, the operation of the installation on the basis of this value. On the generator side, the speed can be derived from electrical variables in the generator if it is not likewise determined via a speed sensor. In some installations, it is conventional to determine the speed only on the generator side and to feed this speed to the open-loop or closed-loop installation control. 
     In the context of this application, “rotor” should be understood to mean that part of an energy generation installation which is subjected to and driven by, for example, water or wind. The mobile part of the generator, on the other hand, is denoted by the term “armature”. 
     In both cases, advantageously the torque can be determined from the variables known in the installation, wherein the accuracy of the determined torque is greater if the rotor speed is also measured in addition to the generator speed. A determination merely via the generator speed, on the other hand, enables a particularly quick and simple implementation without an additional sensor in corresponding installations. Both variants are based on the principle of determining the rotation angle between the input drive side and the output drive side continuously or time-discretely and from this determine the torque which is present in the drive train via a corresponding computation model. The advantageously used computation model is based on the equations of motion of a two-mass torsional vibration system. The rotation angle can in this case be obtained from the speeds on the rotor and generator side. 
     If only the speed on the generator side is known, a conclusion can be drawn on the rotor-side angular position by means of this speed and the torque present at the generator and therefore the rotation angle can likewise be determined. This can advantageously likewise take place by means of a computation model which is based on the equations of motion of a two-mass rotational vibration system. 
     Undesirable high-frequency or low-frequency vibrations which cannot be damped or are not intended to be damped and should therefore not be used as an input for the damping controller can be suppressed by filters. The installation torque is supplied to the damping controller by the superordinate installation controller, and said damping controller determines from this and from the determined torque the actuating torque for the generator, which can then be adjusted via the corresponding power electronics. In this way, an actuating torque can be generated with a high degree of accuracy without the use of additional sensor technology being necessary. 
     Expediently, in the case of closed-loop control of the generator according to the disclosure, one or more types of limitation are monitored. For example, it is possible to monitor the event of a maximum generator power P=M·ω not being exceeded by the controller intervention. In addition, monitoring by means of which a power variation over time is limited (dP/dt limitation) is also conceivable. Likewise expedient is a limitation of the generator torque M and/or of the generator torque variation dM/dt. 
     Furthermore, the disclosure solves the problem of the fluctuating output power in a particularly advantageous manner by virtue of a damping device being connected upstream of the feed point to the power supply system. In this regard, an intermediate circuit capacitor of the DC voltage intermediate circuit and/or further electrical or mechanical energy stores can be used which can be connected in parallel with the intermediate circuit capacitor or other components of the drive train. In this case, the intermediate circuit voltage can be variable or kept constant. 
     By virtue of this method, it is possible to avoid a situation in which undesirable vibrations are impressed on the electricity system or on a connected load. In addition, the proposed solution can be used for positively influencing further installation variables which are coupled directly or indirectly to the torque, such as, for example, for damping lateral tower vibrations in a wind energy installation and/or for reducing loads in the rotor blades. 
     Further advantages and configurations of the disclosure result from the description and the attached drawing. 
     It goes without saying that the features mentioned above and yet to be explained below can be used not only in the respectively given combination, but also in other combinations or on their own, without departing from the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is illustrated schematically using exemplary embodiments in the drawings and will be described in detail below with reference to the drawings. 
         FIG. 1  shows an energy generation installation in accordance with a particularly preferred embodiment of the disclosure. 
         FIG. 2  shows an energy generation installation in accordance with a further preferred embodiment of the disclosure. 
         FIG. 3  shows graphs illustrating the method according to the disclosure in accordance with particularly preferred embodiments of the disclosure in comparison with the prior art. 
         FIG. 4  shows a graph illustrating the method according to the disclosure in accordance with a particularly preferred embodiment of the disclosure in comparison with the prior art. 
         FIG. 5  shows an energy supply installation in accordance with a particularly preferred embodiment of the disclosure having means for smoothing a generator output power. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows the basic design  100  of a device for implementing an associated method in accordance with a particularly preferred embodiment of the disclosure, in which case an angular position on the rotor and generator side is determined. 
     The following are illustrated: a rotor  1  and a generator  6  of a wind energy installation, which are connected to one another mechanically by a drive train  2 - 5  consisting of a rotor output drive shaft  2 , a gear  3 , a clutch  4  and a generator input drive shaft  5 . The rotor is fitted to a rotor-side end  1 ′ of the drive train  2 - 5 , and the generator is fitted to a generator-side end  6 ′. Two position encoders  7 ,  8  on the rotor side  1 ′ and the generator side  6 ′ determine the angular positions f R  (rotor  1 ) and f G  (generator  6 ) which are used in a model  9  of the drive train  2 - 5 . The model  9  can in particular simulate the stiffness of the drive train and can be based on an estimation or a measurement. 
     By way of example, the torque M T  is determined from a stiffness characteristic or a known stiffness c of the system in the model  9 , omitting damping terms using the equation M T =c·Δφ from the measured angular difference Δφ, wherein this torque M T , together with a setpoint installation torque M A  originating from a superordinate closed-loop installation control  10 , represents an input for a damping controller  11 , in which the actuating signal for the generator  6  M GS  is generated. In this case, for example, the difference M A −M T  is given to a controller, for example a P controller, and this intensified control deviation is in turn added to M A  (controller with feedforward control). 
       FIG. 2  shows the basic design  200  of a device for implementing an associated method in accordance with a further preferred embodiment of the disclosure, wherein in this case, however, only the angular position at the generator  6  is known or is determined. Identical or functionally identical elements to those in  FIG. 1  are denoted by identical reference symbols and will not be explained again for reasons of clarity. 
     In the context of this embodiment, the generator torque M GA , in addition to the generator angular position f G , is input to a correspondingly matched installation model  12 , in which the rotor angle f R  and, with the aid of this, then the torque M T  can be determined internally. The installation model  12  can have a two-mass vibration system model of the drive train. The acceleration of the generator (derivative of the generator speed) and the present generator torque M GA  are used to draw a conclusion on the acceleration of the rotor on the basis of the model via a known stiffness and known mass ratios. By virtue of double integration, the angles at both ends and therefore the angle difference Δφ are produced. In this case too, the determination of the torque M T  is therefore based on two angular position values, namely a generator angular position f G  and a rotor angular position f R , wherein, however, the rotor angular position f R  is determined from the generator torque M GA  in the installation model  12 . The torque M T  is used, together with the setpoint installation torque M A  originating from the superordinate closed-loop installation control  10 , as an input to the damping controller  11 , in which, in turn, the actuating signal for the generator M GS  is generated. 
     Subfigures  3 A,  3 B,  3 C and  3 D in  FIG. 3  show graphs  310 - 340  illustrating the method according to the disclosure in comparison with the prior art. In graphs  310  ( FIG. 3A ),  320  ( FIG. 3B ),  330  ( FIG. 3C) and 340  ( FIG. 3D ), in each case a torque M in the gear (graph  310 ), a torque M determined from the angular positions (graph  320 ), a speed difference Δn (graph  330 ) and a generator speed n (graph  340 ) are plotted on the y axes  312 ,  322 ,  332  and  342  over a time t of 10 s on the x axes  311 ,  321 ,  331  and  341 . 
       FIG. 3A  shows the time profile  313  of a torque which is actually present in the drive train, in the present example on the sun wheel of a three-stage gear mechanism with one planet stage and two spur stages, with this torque being influenced, as is clearly visible, by vibrations of approximately 2 Hz, corresponding to natural vibrations of the drive train. Further, lower-frequency vibrations, for example shading of the tower and natural vibrations of the tower, and high-frequency vibrations, as arise from meshing in the gear, are superimposed on the 2-Hz vibrations. 
     In contrast,  FIG. 3B  illustrates a torque  323  which is determined in accordance with a particularly preferred embodiment of the disclosure via a model from the rotation angle or from angular positions. The torque  323  demonstrates a virtually identical profile to the measured torque in the gear  313 , but without the high-frequency components of the gear vibrations which are virtually completely filtered out by the integral nature of the angular positions with respect to the speeds  333 . Since these high-frequency vibrations cannot be compensated for in any case, this signal is very useful for active damping control of the torsional vibrations. 
       FIG. 3C  shows a time profile showing a speed difference  333  which is used in accordance with the prior art to compensate for vibrations.  FIG. 3D  shows an actual generator speed  343 . 
     As can be seen from the combination of subfigures  3 A to  3 C, the torque  323  determined from the angular positions corresponds to the real torque in the gear much more precisely than the speed difference  333  used in accordance with the prior art, for example (see  FIG. 3C ). It therefore becomes clear that the torque  323  determined via the described method simulates the actually present torque  313  much better than the speed methods  333  and is therefore better suited as an input for control-engineering damping. The large discrepancies between the profile of the speed difference  333  of the rotor and the generator and the profile of the generator speed  343  with respect to the actual profile of the torque  313 , however, result in the expectation that closed-loop control based on these signals, as corresponds to the prior art, is not capable of achieving the same quality as the closed-loop control via the angular positions. 
       FIG. 4  illustrates, in the form of a graph  400 , a torque  403  without damping and a torque  404  after damping in accordance with a particularly preferred embodiment in the form of a torque M on the y axis  402  over a time of 10 s on the x axis  401 . As can be seen, a torque vibration is markedly reduced in comparison with the undamped state by means of the damping method in accordance with the preferred embodiment. 
       FIG. 5  shows the basic design  500  of a device for implementing an associated method in accordance with a particularly preferred embodiment of the disclosure, wherein possible, even alternative, embodiments with storage units  17 ,  18 ,  19  for smoothing the generator output power are illustrated. Identical or functionally identical elements to those in  FIGS. 1 and 2  are denoted by identical reference symbols and will not be explained again for reasons of clarity. 
     For damping purposes, firstly even one intermediate circuit capacitor  14  of the DC voltage intermediate circuit can be used which is arranged between the generator-side rectifier  13  and the power supply system-side inverter  15  before the output voltage is transformed to the system voltage via the system transformer  16 . The intermediate circuit capacitor  14  can in this case be designed for operation with a variable or with a constant DC voltage. 
     Alternatively or in addition to this, in accordance with preferred embodiments, further storage units  17 ,  18  and  19  with energy stores  21 ,  23  and  25  can be used. These storage units  17 ,  18  and  19  can in this case be used individually or else in any desired combination, in particular in the context of a cost-benefit analysis. 
     The storage unit  17  represents an embodiment of a damping system which is operated in parallel with the intermediate circuit. The damping system has a separate DC-to-DC converter  20  in order to provide the possibility of variable operating voltage of the DC store  21  and a variable power flow. 
     The storage unit  18  represents an embodiment of the damping system which is connected directly to the generator. In this case, coupling both to the stator and to the armature winding of the generator  6  is possible irrespective of the design of the main converter  13 ,  14  and  15 . The three-phase current generated by the generator  6  is converted into a DC voltage of variable level via an AC-to-DC converter  22 , for example a B6 rectifier. The storage of the damping energy is performed in a suitable DC store  32 . 
     The storage unit  19  represents an embodiment of the damping system with likewise direct electrical coupling to the stator-side or rotor-side generator terminals. In contrast to storage unit  18 , however, in this case an AC storage system  25  is provided which is fed via an AC-to-AC converter  24 . Such a storage system can represent, for example, a flywheel which is coupled to an AC motor and which advantageously provides a relatively high power density.