Abstract:
A rotational speed controlling device for a wind turbine is provided. The controlling device includes a first input node for receiving a first signal, indicative of an actual rotational speed of the rotor, a second input node for receiving a second signal, indicative of a reference rotational speed for the rotor, a detection unit for detecting a defective operational state in which the wind turbine is impeded to export electrical power to an electricity network, a control unit for providing a reference signal for a characteristic operational parameter of the wind turbine based on the first signal and the second signal, and a pulse generator adapted to send a kick pulse to an integral control element of the control unit in response to the detection of the defective operational state. The characteristic operational parameter is indicative of the rotational speed of the rotor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of European Patent Office application No. 08021302.8 EP filed Dec. 8, 2008, which is incorporated by reference herein in its entirety. 
     FIELD OF INVENTION 
     The present invention relates to the field of wind turbines for generating electric power. In particular, the present invention relates to a device and to a method for controlling the rotational speed of a rotor of a wind turbine in an operational situation, in which the wind turbine is impeded to export electrical power to an electricity network being connected to the wind turbine. Further, the present invention relates to a wind turbine comprising the above mentioned rotational speed controlling device and to a computer program for controlling the above mentioned rotational speed controlling method. 
     ART BACKGROUND 
     Wind energy is increasingly recognized as a viable option for complementing and even replacing other types of energy sources such as for instance fossil fuels. However, the control of the operation of a wind turbine is often very sensitive because a wind turbine is typically exposed to ever-changing ambient conditions which may cause extreme mechanical loads for instance to the rotor and to the mast of the wind turbine. 
     Further, the operation of a wind turbine strongly depends on the availability and the electrical power absorption capability of the electricity network the wind turbine is connected to. Specifically, when an electricity network fault happens near a wind turbine, the voltage of the electricity network will be typically very low. This makes it impossible for the generator of the wind turbine to export the produced electrical power to the electricity network. The result is an increasing of the rotational speed of the rotor of the wind turbine with the risk of a dangerous over speed situation. 
     In order to avoid an over speed situation it is known to control the rotational speed of a speed controller of the wind turbine for instance by setting the pitch angle of the blades of the rotor to an angular position being different from the optimal blade pitch angle when the wind turbine is operated in an error-free operational state. Thereby, it is essential that the speed controller is able to react quickly on the electricity network fault. Further, an emergency stop of the wind turbines operation can be necessary due to the fact that without voltage on the electricity network, a blade pitch angle control system can only be operated in the emergency mode, where it is changing the blade pitch angle with a constant speed. However, pitching with a constant speed causes large loads on the tower and yaw system of the wind turbine. 
     EP 1 651 865 B1 describes a method for regulating the pitch of blades of a wind turbine on detection of a malfunction of the electricity network. Thereby, the pitch is regulated continuously or in steps in order to reduce the power production of the wind turbine and thus protect the components of the wind turbine from overheating. 
     EP 1 819 023 A2 solves the problem of overheating components of a wind turbine in case of an electricity network malfunction by switching the power output from the turbine to a dump load as soon as the malfunction has been rectified or until the wind turbine has been stopped in a controlled manner. 
     There may be a need for providing a control for the rotational speed of a wind turbine which is impeded to export electrical power to an electricity network, wherein the control changes a characteristic operational parameter being indicative for the rotational speed of the rotor in such a manner that mechanical loads on the tower and yaw system of the wind turbine can be reduced. 
     SUMMARY OF THE 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 device for controlling the rotational speed of a rotor of a wind turbine for feeding electrical power into an electricity network. The provided rotational speed controlling device comprises (a) a first input node for receiving a first signal being indicative for an actual rotational speed of the rotor, (b) a second input node for receiving a second signal being indicative for a reference rotational speed for the rotor, (c) a detection unit for detecting a defective operational situation in which the wind turbine is impeded to export electrical power to the electricity network, (d) a control unit for providing a reference signal for a characteristic operational parameter of the wind turbine based on the first signal and based on the second signal, the characteristic operational parameter being indicative for the rotational speed of the rotor, wherein the control unit comprises a proportional control element and an integral control element, and (e) a pulse generator, which is connected to the detection unit and to the integral control element and which is adapted to send a kick pulse to the integral control element in response to the detection of the defective operational state. 
     The described rotational speed controlling device is based on the idea that by sending a kick pulse to the integral control element of the control unit, which represents a proportional plus integral controller (PI controller), the time rate of a change of the reference signal will be increased. This may mean that the magnitude of the change of the reference signal in response to the detection of the defective operational state will be much stronger as compared to known speed controllers having no pulse generator connecting an electricity network detection unit with the integral control element of a control element. 
     According to the basic principles of automatic control engineering, by changing the reference signal to a large extend the response time for adapting respectively changing the characteristic operational parameter of the wind turbine will be reduced significantly. This means that the rotational speed controlling device will cause the wind turbine to quickly respond to the detection of the defective operational state by adapting the reference signal for the characteristic operational parameter in such a manner, that the rotational speed of the rotor will be adapted accordingly. Thereby, damages to the wind turbine in particular in connection with an over speed situation may be avoided effectively. 
     The described kick pulse may be applied to the integral control element immediately after the defective operational state has been detected. This ensures a fast response of the rotational speed controlling device to the detection of the defective operational state. 
     The defective operational state may be associated with at least one component of the wind turbine and/or may be associated with the condition of the electricity network. For instance the frequency of the electrical power being provided by the wind may be out of tune with respect to the frequency of the electricity network. Such a mistuning may be caused for instance by a trip of the wind turbines frequency converter. However, also other failures, which occur in other hardware of the wind turbine, may suddenly make it impossible for the wind turbine to exports its electrical power to the electricity network. 
     The described rotational speed controlling device may be realized by means of a computer program respectively by means of software. However, the rotational speed controlling device may also be realized by means of one or more specific electronic circuits respectively hardware. Furthermore, the control system may also be realized in a hybrid form, i.e. in a combination of software modules and hardware modules. 
     According to an embodiment of the invention the rotational speed controlling device further comprises a third input node for connecting the device to the electricity network. Thereby, (a) the detection unit is connected to the third input node and (b) the detection unit is adapted for detecting an electrical failure on the electricity network. This may provide the advantage that a malfunction of the electricity network, which makes it impossible for the wind turbine to export its generated electrical power, can be detected in a quick and reliable way. 
     According to a further embodiment of the invention the detection unit comprises a voltage detector. This may provide the advantage that voltage drops of the electricity network can easily be detected by the detection unit. Since most malfunctions of electricity networks are associated with voltage drops, the voltage detector will be able to detect almost any type of failures of the electricity network. 
     In this respect it is mentioned that the electrical power P is associated with the voltage U and the current I by the known equation. P=U*·I. Therefore, if the voltage U is very low, then the power P will also be low, because the current I will be saturated to a nominal current. 
     It has to be mentioned that the voltage detector may not only be able to detect voltage drops. The voltage detector may also be sensitive to voltage dips. This means that the voltage detector may not only be sensitive to the voltage level but also to the time rate of change of the voltage level. Thereby, an even higher sensitivity of the voltage detector may be achieved. This holds in particular for short fluctuations of the voltage level of the electricity network, during which it is not possible for the wind turbine to export its generated electric power. 
     As has already been mentioned above, when the voltage of the electricity network drops below a predefined threshold value, the wind turbine may not be able any more to export electrical power to the electricity network. As a consequence, the efficiency of the wind turbine with respect to its ability to extract energy from a surrounding wind flow has to be decreased in order to avoid an over speed situation. An over speed situation, which is outside the mechanical and/or electrical specifications of the wind turbine, may cause significant damage to the wind turbine. Therefore, in case of a voltage drop and/or a voltage dip of the electricity network the characteristic operational parameter of the wind turbine should be changed in such a manner that the rotational speed of the wind turbines rotor will be reduced. 
     By preventing the wind turbine from passing over in an over speed situation by means of the described rotational speed controlling device the maximum tower bending moment can be reduced by approximately 50% as compared to the usage of a known rotational speed controlling device having no kick pulse causing a faster response of the wind turbine to the defective operational state. Further, when the described rotational speed controlling device is used the yaw moment can be reduced by approximately 30%. These improvements of the mechanical loads, which are caused by the described kick pulse, have been shown by the inventor by means of numerical simulations of the behavior of a concrete wind turbine. 
     According to a further embodiment of the invention the characteristic operational parameter of the wind turbine is a blade pitch angle of at least one blade of the rotor. This may provide the advantage that the degree of efficiency of the wind turbine can be reduced easily simply by adjusting the blade pitch angle to a position being different with respect to the optimal blade pitch angle. 
     In particular, when a voltage drop in the electricity network voltage is detected by the detection unit, the kick pulse will be sent to the integral control element of the PI-controller. The following integration of this kick pulse leads to a step on the integrator output which forces a fast movement of the blade pitch angle. 
     Applying the pitch kick has the same effect on the reference signal, which in this embodiment represents a blade pitch reference output, as a large negative speed error. It therefore acts to further reduce the rotational speed of the rotor and thus effectively prevents an over speed situation. 
     According to a further embodiment of the invention the pulse generator is adapted to send an inverse kick pulse to the integral control element. Such an inverse kick pulse may be generated by the pulse generator if an electrical failure on the electricity network, which has been detected by the detection unit, has been overcome within a predefined time interval after the electrical failure has been detected. The predefined time interval may have a duration of between 0.02 s and 10 s. An inverse pitch kick may be send for instance if the predefined time duration is below 1.5 s. 
     Generally speaking, if the electricity network malfunction lasts only for a short duration of time, the described rotational speed controlling device will not issue a stop command for shutting down the operation of the wind turbine. Instead the described inverse kick pulse is sent to the integral control element of the PI controller when the electricity network condition has returned to normal. This forces a negative step on the PI controller integral element and will eliminate the kick pulse that was sent when the electricity network fault occurred. 
     In this respect it is pointed out that after the issue of the inverse kick pulse the described rotational speed controlling device will remain in operation. Therefore, if the malfunction of the electricity network lasts only for a comparatively short duration of time, a need for a new start up procedure of the rotational speed controlling device is eliminated. 
     According to a further embodiment of the invention the rotational speed controlling device further comprises a timer unit, which is connected to the detection unit and which is adapted to provide a stop command signal to other components of the device. This may provide the advantage that if the defective operational state lasts for a time duration, which is longer than a predefined threshold of for instance 100 ms, the wind turbine can be shut down in a defined manner. This may facilitate a later restart of the wind turbine. 
     In particular, the stop command signal may cause a minimum blade pitch angle to be initiated to the actual blade pitch angle and increase the minimum blade pitch and as a function of time since the stop command was initiated. Thereby, a blade pitch angle of approximately 0° may be considered as to represent the optimal blade pitch angle when both the wind turbine and the electricity network are in an error free operational state in which the wind turbine is ready for producing a maximal electric power. By contrast thereto, a blade pitch angle of 90° may mean that the blade(s) of the rotor is (are) oriented perpendicular to the wind direction. 
     Further, the stop command signal may cause that, after the rotational speed has fallen below the reference rotational speed, the reference rotational speed is ramped down to zero. This may provide the advantage that the wind turbine can shut down its operation in a highly reliable way. This holds in particular for the adjustment of the blade pitch angle. 
     For ramping down the reference rotational speed appropriate hardware and/or software components may be used. Such appropriate components are well known by persons being skilled in the art of controlling the operational states of wind turbines. Therefore, for the sake of conciseness of this application no further details will be given about possible ways how such a ramping down can be realized. 
     According to a further embodiment of the invention the reference rotational speed of the rotor is (a) a time-independent rotational speed having a predefined nominal value, (b) a time-independent rotational speed having a zero value or (c) a time-dependent rotational speed having a predefined time dependency, which is started in response to the stop command signal. 
     The corresponding different second signals representing the different described reference rotational speeds may be selectively fed to the second input node via a controllable switch. This controllable switch may be connected directly or indirectly with the timer unit. 
     The predefined time dependency may be in particular a ramp function starting from the predefined nominal value and approaching the zero value. Thereby, any continuous function may be used. Probably the simplest function is a function having a constant slope. Such a function can also b called a ramp function. 
     According to a further embodiment of the invention the control unit comprises a subtracting unit for obtaining a rotational speed error by subtracting the first signal being indicative for the actual rotational speed from the second signal being indicative for the reference rotational speed. Further, the rotational speed error is provided to the proportional control element and to the integral control element. This may provide the advantage that the proportional control element and/or the integral control element may operate with a single electric variable. Thereby, this electric variable represents the actual rotational speed error. 
     According to a further aspect of the invention there is provided a wind turbine for generating electric power. The wind turbine comprises (a) a rotor having at least one blade, wherein the rotor is rotatable around a rotational axis and the at least one blade extends radial with respect to the rotational axis, (b) a generator being mechanically coupled with the rotor for generating electrical power from a rotational movement of the rotor, and (c) a device as described above for controlling the rotational speed of the rotor. 
     The described wind turbine is based on the idea that by embedding the above described rotational speed controlling device into a usual wind turbine, the wind turbine can quickly respond to the detection of a defective operational state by adapting the reference signal for the characteristic operational parameter in such a manner, that the rotational speed of the rotor will be adapted accordingly. Thereby, damages to the wind turbine in particular due to an over speed situation may be avoided effectively. 
     According to an embodiment of the invention the wind turbine further comprises an uninterruptible power supply which is connected to the device for controlling the rotational speed of the rotor. This may provide the advantage that the above described rotational speed controlling device may be able to control the wind turbines operation even during a fault of the electricity network. 
     According to a further aspect of the invention there is provided a method for controlling the rotational speed of a rotor of a wind turbine for feeding electrical power into an electricity network. The provided rotational speed controlling method comprises (a) receiving a first signal being indicative for an actual rotational speed of the rotor, (b) receiving a second signal being indicative for a reference rotational speed for the rotor, (c) detecting a defective operational situation in which the wind turbine is impeded to export electrical power to the electricity network, (d) providing a reference signal for a characteristic operational parameter of the wind turbine based on the first signal and based on the second signal, wherein the characteristic operational parameter is indicative for the rotational speed of the rotor, (e) sending a kick pulse from a pulse generator to an integral control element of a control unit, and (f) integrating the kick pulse such that the reference signal for the characteristic operational parameter of the wind turbine is changed in response to the detection of the defective operational state, wherein the change of the reference signal causes the rotor to change its rotational speed. 
     Also the described rotational speed controlling method is based on the idea that by sending a kick pulse to the integral control element the time rate of a change of the reference signal can be increased significantly. Thereby, the magnitude of the change of the reference signal in response to the detection of the defective operational state may be much stronger as compared to known speed controlling methods, which do not use a kick pulse in order to further stimulate the integral control element. Preferably, the integral control element may be a part of a control unit, which preferably also comprises a proportional control element. 
     According to a further aspect of the invention there is provided a computer program for controlling the rotational speed of a rotor of a wind turbine for feeding electrical power into an electricity network. The computer program, when being executed by a controlling device, is adapted for controlling the above described method. 
     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 element may be implemented as a 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. 
     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 application. 
     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 DRAWINGS 
         FIG. 1  shows a wind turbine comprising a rotational speed controlling device according to an embodiment of the present invention. 
         FIG. 2  shows a circuit diagram for a rotational speed controlling device, which comprises a generator for sending a kick pulse to an integral control element in response to a detection of a defective operational state in which the wind turbine is impeded to export electrical power to an electricity network. 
         FIG. 3  shows a diagram depicting a variation of a blade pitch angle for a rotor of a wind turbine in response to the detection of a malfunction of an electricity network the wind turbine is connected to. 
     
    
    
     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 wind turbine  100  according to a preferred embodiment of the invention. The wind turbine  100  comprises a mast  120 , which is mounted on a non-depicted fundament. On top of the mast  120  there is arranged a housing  122 . In between the mast  120  and the housing there is provided a rotor angle adjustment device  121 , which is capable of rotating the housing  122  around a non depicted vertical axis, which is aligned with the longitudinal extension of the mast  120 . By controlling the rotor angle adjustment device  121  in an appropriate manner it can be made sure, that during operation of the wind turbine  100  the housing  122  is always properly aligned with the current wind direction. 
     The wind turbine  100  further comprises a rotor  110  having three blades  114 . In the perspective of  FIG. 1  only two blades  114  are visible. The rotor  110  is rotatable around a rotational axis  110   a . The blades  114 , which are mounted at a driving collar  112 , extend radial with respect to the rotational axis  110   a . 
     In between the driving collar  112  and a blade  114  there is respectively provided a blade adjustment device  116  in order to adjust the blade pitch angle of each blade  114  by rotating the respective blade  114  around a non depicted axis being aligned parallel with the longitudinal extension of the blade  114 . By controlling the blade adjustment device  116  the blade pitch angle of the respective blade  114  can be adjusted in such a manner, that depending on the current wind conditions a maximum wind power can be retrieved from the available wind power. 
     As can be seen from  FIG. 1 , within the housing there is provided a gear box  124  in order to convert the number of revolutions of the rotor  110  into a higher number of revolutions of a shaft  125 , which is coupled in a known manner to a generator  128 . Further, a brake  126  is provided in order to stop the operation of the wind turbine  100  for instance in case of an emergency and/or in case of strong wind conditions, which might harm the wind turbine  100 . 
     The wind turbine  100  further comprises a rotational speed controlling device  140 , which is connected in a non depicted manner to a rotational speed sensor  118 , which in operation measures the rotational speed of the rotor  110 . Further, according to the embodiment described here the rotational speed controlling device  140  is connected to an uninterruptible power supply  135 , which itself is connected an electricity network  190  by means of a power line  135 a. A further power line, which for the sake of clarity is not depicted in  FIG. 1 , connects the generator  128  with the electricity network  190 . Via this non depicted power line the electrical power, which is produced by the wind turbine  100 , is transferred to the electricity network  190 . 
       FIG. 2  shows a circuit diagram for the rotational speed controlling device  140 , which is now denominated with reference numeral  240 . The rotational speed controlling device  240  comprises three input nodes. A first input node  241  is used for receiving a signal being indicative for the actual rotational speed of the rotor of a wind turbine. A second input node  242  is used for receiving a signal being indicative for a reference rotational speed for the rotor. A third input node  243  is used for connecting the device  240  to an electricity network  290 . The electricity network  290  is the same network in which the electrical power is fed, which is generated by the wind turbine. 
     A can be seen from  FIG. 2 , the reference signal being applied to the second input node  242  can represent (a) a time-independent rotational speed having a predefined nominal value (SpeedNom), (b) a time-independent rotational speed having a zero value (SpeedZero) or (c) a trigger signal which is used for initiating a ramping down of a speed reference being provided by a speed reference unit  248  starting from the predefined nominal value (SpeedNom) and ending at a zero value (SpeedZero). As can be further seen from  FIG. 2 , a switching unit  247  is provided in order to feed one of the three signals (a), (b) or (c) to a speed reference unit  248 . A ramp function representing the ramp down behavior in case the trigger signal (c) is provided to the speed reference unit  248  is stored in the speed reference unit  248 . 
     As can be furthermore seen from  FIG. 2 , a detection unit  245  is connected to the third input node  243 . The detection unit  245  comprises a voltage detector  246 , which is capable of detecting the voltage of the electricity network  290 . In case there is detected a voltage drop, which is larger than a predefined dip, the detection unit  245  outputs a corresponding signal indicating the voltage drop. This signal is fed both to a timer unit  270  and to a pulse generator  260 . 
     Immediately after receiving this signal indicating the voltage drop of the electricity network  290 , the pulse generator  260  outputs a kick pulse  260   a , which is supplied to an integral control element  256  of a control unit  250 . In a parallel arrangement with respect to the integral control element  256  the control unit  250  also comprises a proportional control element  254 . 
     According to the embodiment described here the control unit  250  is used for providing a reference signal for a blade pitch angle of the wind turbine based on the actual rotational speed of the rotor provided via the first input node  241  and based on a signal being indicative for a reference rotational speed for the rotor provided via the second input node  242 . A value indicating the actual rotational speed of the rotor is subtracted from the speed reference provided by the speed reference unit  248  by means of a subtracting unit  252 . The resulting value representing a speed error is fed both to the proportional control element  254  and the integral control element  256  of the control unit  250 . The output of the proportional control element  254  and the output of the integral control element  256  are added by means of an adding unit  258 . Thereby, a first pitch reference signal is generated, which is supplied to a selection unit  264 . 
     The reception of the kick pulse leads to a step on the output of the integral control element  256 . This step forces a fast movement of the blade pitch angle. In is mentioned that the kick pulse being applied to the integral control element has the same effect on the pitch reference output of the control unit  250  as a large negative speed error. The kick pulse  260   a  therefore acts to reduce the rotational speed of the rotor and thus contributes in preventing the wind turbine from an over speed situation. 
     As can be further seen from  FIG. 2 , the timer unit  270  is connected to a function generator  280 . If the failure respectively the under voltage situation of the electricity network  290  last longer than a predefined time duration of for instance a  4  seconds, the time unit will output a stop command both to the function generator  280  and to an AND gate  272 . 
     According to the embodiment described here, the AND gate  272  receives a second input from a comparator unit  274 , which detects whether the actual speed is larger than the nominal speed. If this is the case the above described ramp function in the speed reference unit  248  is activated by the switching unit  247 . 
     In response to the above described stop command the function generator  280  will output a time dependent function, which is indicative for a minimum pitch angle as a function of time after the reception of the stop command. This function, which is depicted in  FIG. 3  as a full line, represents a second pitch reference signal. 
     The output signal provided by the function generator  280  is also supplied to the selection unit  264 . The selection unit  264  selects the larger signal out of (a) the second pitch reference signal being provided by the function generator  280  and (b) the first pitch reference signal being provided by the adding unit as the final blade pitch reference signal (Pitch reference). This final blade pitch reference signal (Pitch reference) is present at an output node  288  of the rotational speed controlling device  240 . 
     It is mentioned that in case there is only a very short fault on the electricity network  290 , no stop command is issued. Instead an inverse kick pulse  260   b  is generated by the pulse generator  260  and is sent when the electricity network  290  condition has returned to normal. This forces a negative step on the integral control element  256  of the control unit  250  and will eliminate at least partially the blade pitch kick caused by the kick pulse  260   a  and having been sent when the electricity network  290  fault occurred. 
     It is noted that the rotational speed controlling device  240  may remain in operation even when a longer failure of the electricity network  290  occurs. This eliminates the need for a new start up procedure of the rotational speed controlling device  240  even if the wind turbine is completely stopped. 
       FIG. 3  shows a diagram depicting a variation of a blade pitch angle for a rotor of a wind turbine in response to the detection of a malfunction of an electricity network the wind turbine is connected to. As has already been mentioned above, the full line represents the second pitch reference signal provided by the function generator  280  depicted in  FIG. 2 . The dashed line represents the actual blade pitch angle. 
     As soon as the electricity network fault is detected the minimum pitch angle is slowly increased. At the same time the actual pitch angle is increased rapidly because of the pitch kick during net drop. If a wind gust causes the rotor speed to increase, the pitch angle increases again faster than the second pitch reference signal. If the rotor speed is decreasing faster than the speed reference, the blade pitch angle variation is limited by the minimum stop pitch angle. 
     As can be seen from  FIG. 3 , for the first 7 seconds after the electricity network fault has been detected, the minimum blade pitch angle varies relatively slowly. After 7 seconds the minimum blade pitch angle is varied more rapidly until the blade pitch angle reaches its final stop position approximately 18 seconds after the electricity network fault has been detected. 
     It should be noted that the term “comprising” does not exclude other elements or steps and “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. 
     In order to recapitulate the above described embodiments of the present invention one can state: 
     By generating a kick pulse as soon as an electricity network fault occurs, the integral control element  256  forces a fast movement of the blade pitch angle. 
     One advantage of the described rotational speed controlling device and the described rotational speed controlling method is that a very quick reaction on an electricity network fault can be achieved. A further advantage is that the rotational speed controlling device remains active for a period after the fault occurs. Therefore, if the fault lasts only for a short duration of time, a new and lengthy start up procedure for the rotational speed controlling device is not needed. 
     If the electricity network fault lasts long, the fact that the rotational speed controlling device remains active insures that the wind turbine can be stopped in a controlled manner. This may reduce significantly the mechanical loads on the tower and the yaw system of the wind turbine.