Method, computer program and computer-readable medium for avoiding an unintended fall of a blade of a wind turbine, and energy conversion system

Abstract

A method for avoiding an unintended fall of a blade (50) of a wind turbine (20) when the blade (50) or another blade (50) of the wind turbine (20) is arranged at a rotor hub (51) of the wind turbine (20) is provided. The wind turbine (20) comprises the rotor hub (51), a rotating electrical machine (22) mechanically coupled to the rotor hub (51), a machine-side converter (24, 44) electrically coupled to the rotating electrical machine (22) and comprising several semiconductor switches (70, 72, 74), a DC-link (26, 46) electrically coupled to the machine-side converter (24, 44), and a grid-side converter (28, 48) electrically coupled to the DC-link (26, 46) and to an electrical grid. The method comprises: generating a short circuit at the DC-link (26, 46) by activating all of the semiconductor switches (70, 72, 74) at the same time.

Description

[0001]DESCRIPTION METHOD, COMPUTER PROGRAM AND COMPUTER-READABLE MEDIUM FOR AVOIDING AN UNINTENDED FALL OF A BLADE OF A WIND TURBINE, AND ENERGY CONVERSION SYSTEMFIELD OF THE INVENTION The invention relates to the field of wind turbines. In particular, the invention relates to a blade assembly during which a blade of the wind turbine is arranged at a rotor hub of thewind turbine. In particular, the invention relates to a method, a computer program, and acomputer-readable medium for avoiding an unintended fall of a blade of a wind turbine, to an energy conversion system of the wind turbine, and to a master controller for the energy conversion system.BACKGROUND OF THE INVENTIONConventional wind turbines, e.g. gearless wind turbines, comprise three or more blades mechanically coupled to a rotatable rotor hub of the wind turbine. The rotor hub is mechanically coupled to an energy conversion system of the wind turbine. The energy conversion system comprises amongst others a rotating electrical machine. In a generatormode, the rotating electrical machine is configured for being rotated by the blades via therotor hub and for generating electric energy upon being rotated. In this context, the rotating electrical machine may be referred to as generator. The electric energy may be supplied to an electrical grid to which the energy conversion system is electrically coupled. The rotating electrical machine may be coupled to the electrical grid by one or more converters,e.g. one or more 3-Level Neutral Point Clamped (3LNPC) converters, of the energyconversion system in order to convert the energy generated by the rotating electrical machine such that it is appropriate for being supplied to the grid. When the wind turbine is installed at its predetermined location in the field, a tower of the wind turbine is arranged at the predetermined location first. Then, the blades are installedat the rotor hub. For example, the blades may be arranged at the rotor hub on the ground.Then, the rotor hub with the blades may be lifted up to an installation interface for the rotor hub near a top of the tower. However, this solution needs a lot of space, in particular on the ground. Additionally, as the blades are becoming bigger and bigger with proceeding technical development, it may be hard or even impossible to lift the weight of three or more blades at a height of 100 meters or more with cranes currently available on the market. Alternatively, the rotor hub may be installed at the tower before installing the tower at thepredetermined location and the blades may be lifted up and coupled to the rotor hub, usuallyhorizontally, one after the other (“single blade installation”) afterwards. Normally, a specific “blade lifting tool” is needed for this approach. Such tools may be provided by a manufacturer of the wind turbine and may be designed to fit a specific blade model only. In addition, for this single blade installation, as each of the blades is usually installedhorizontally, the rotor hub has to be rotated after one of the blades is installed to have anempty socket in the right position for the next blade. This procedure has to be repeated until all blades are installed. A second tool may be needed to turn the rotor hub to a predetermined position in which the next one of the blades shall be installed. This second tool may be referred to as "rotor turning tool" or "turning gear". Those turning tools are usually of ahydro-mechanical typ. A power source may be connected to a pump generating a hydro-pressure needed to rotate the gears of the wind turbine with the help of such a hydro- mechanical turning tool. These turning tool may add complexity and costs to the blade installation process. US 2009 / 0162202 A1 describes a method for positioning a rotor of a wind turbine at adesired position, e.g., before engaging a locking mechanism, e.g. for maintenance purposes.The corresponding wind turbine comprises an electrical generator adapted to be selectively used as a motor, a control unit adapted for switching between a generator operation and a motor operation of the electrical generator and for controlling the torque generated by the generator when used as a motor. The control unit monitors a sensor signal indicating anangular position of the rotor in its main rotation plane. Energy may be fed into the generatorto use it as a motor and thereby to move the rotor to the predetermined angular position. The energy for the turning of the rotor may be taken from the power grid. US 2013 / 0328309 A1 describes a wind turbine comprising a generator connected to a rotor of the wind turbine via a shaft. The generator is a rotational machine that may be alsoused as a motor and, for example, a rotational machine of permanent magnet type. A positiondetector detects a rotational position of the wind turbine rotated by wind power. A controller of the wind turbine performs a position control process, so that the rotational position of the wind turbine coincides with a target position. A wind power generator includes a speed detector in addition to the above-described generator and the position detector. The speed detector detects the rotational speed of the generator and outputs the detected rotational speed value to a matrix converter apparatus for wind-power generation as a detected speed. US 2016 / 0195069 A1 describes a wind turbine comprising means allowing the generatoracting as a motor receiving power from a suitable source such as an Uninterruptible PowerSupply (UPS) device available in the own wind turbine or an electricity grid to which the turbine is connected. Thus, the generator can be used as a rotor driving means. The wind turbine has a system for positioning a rotor of the wind turbine in a predetermined position so that different tasks can be carried out. EP 2 574774 A1 describes a method to rotate a rotor of a wind turbine and means to beused in this method, e.g., for bringing a hub of the wind turbine in a predetermined angular rotational position to establish a connection between the hub and a rotor blade of the rotor. The actuator means are constructed and arranged to rotate the hub of the wind turbine in respect to a stator of an electrical generator of the wind turbine, wherein the actuator meansis the electrical generator and wherein the electrical generator of the wind turbine is used asa motor to rotate the hub of the wind turbine. DESCRIPTION OF THE INVENTION It is an object of the present invention to provide a method for avoiding an unintendedfall of a blade of a wind turbine, and to provide a corresponding computer program fordriving the wind turbine, both of which enabling a safe blade assembly. It is another object of the present invention to provide a master controller for an energy conversion system of the wind turbine, which is able to carry out the method, to provide the energy conversion system comprising the master controller, and to provide a computer-readable mediumstoring the computer program.These objects are achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description. An object is achieved by a method for avoiding an unintended fall of a blade of a windturbine when the blade or another blade of the wind turbine is arranged at the rotor hub ofthe wind turbine. The wind turbine comprises a rotor hub, a rotating electrical machine mechanically coupled to the rotor hub, a machine-side converter electrically coupled to the rotating electrical machine and comprising several semiconductor switches, a DC-link electrically coupled to the machine-side converter, and a grid-side converter electrically coupled to the DC-link and to an electrical grid. The method comprises generating a short circuit at the DC-link by activating all of the semiconductor switches at the same time.This causes a three-phase short circuit at the machine-side terminals of the machine-sideconverter and a corresponding generator short circuit torque. This generator short circuit torque may prevent the unintended fall of the blade. This enables to move the blade smoothly to a vertical blade position. A corresponding short circuit steady state current may be in the range of a rated current, e.g. due to a high generator impedance ofapproximately 1 pu). A start of the method may be triggered with respect to a fallprevention mode as described below. A rotor hub of the wind turbine may be brought in a predetermined position for arranging one of the blades of the wind turbine at the rotor hub. The rotating electrical machine may be operated in a blade assembly mode in which the rotating electricalmachine is configured for rotating the rotor hub such that the rotor hub is brought in thepredetermined position by the rotating electrical machine, wherein the predetermined position of the rotor hub is given by a predetermined rotor mechanical angle reference of the rotor hub. The wind turbine may be a gearless wind turbine. The wind turbine comprises an energyconversion system, wherein the rotating electrical machine, the rotor hub, the machine-sideconverter, the DC-link, and the grid-side converter are components of the energy conversion system. The rotating electrical machine may be operated as a generator, e.g. a synchronous generator, or as a motor. The mode in which the rotating electrical machine is operated as the generator may be referred to as generator mode. The mode in which therotating electrical machine is operated as the motor may be referred to as motor mode. Therotating electrical machine may have one or more independent winding systems. The rotating electrical machine may be operated in the blade assembly mode by a master controller of the wind turbine. The master controller may be electrically coupled to the rotating electrical machine and may be configured for operating the rotating electricalmachine. Further, the master controller may be electrically coupled to the machine-sideconverter and / or to the grid-side converter. In this case, in the generator mode, the master controller may be configured for modulating the machine-side converter for converting an AC voltage of the rotating electrical machine into a DC voltage supplied to the DC-link and for modulating the grid-side converter to convert a DC-link voltage at the DC-link into an AC output voltage to be supplied to the electrical grid. In contrast, in the blade assembly mode or motor mode, the master controller may be configured for modulating the grid-side converter to convert an AC input voltage supplied from the electrical grid into a DC-linkvoltage at the DC-link and for modulating the machine-side converter for converting theDC voltage supplied to the DC-link into an AC voltage for operating the rotating electrical machine. The master controller may modulate the machine-side converter and the grid-side converter by one or more control signals generated by the master controller and supplied to semiconductor switches of the machine-side converter and, respectively, the grid-sideconverter by the master controller.So, during normal operation, the rotating electrical machine is used as a generator for producing power which is transferred to the electrical grid. In this context, the rotating electrical machine may be simply referred to as “generator”. The master controller may include a torque controller, which may be configured for following a torque referenceprovided by external loops of the master controller during the normal operation. Incontrast, in the proposed blade assembly mode of operation, the flow of the power is reversed. In particular, the electrical grid supplies the electrical power which, via the proposed method, is used to operate the generator as a motor and to orientate the rotor hub to the predetermined position for assembling one or more of the blades to the rotor hub. Inthis context, the electrical grid may be optionally represented by an external generator, e.g.a diesel generator, that provides the required electrical power for the rotating electrical machine during the blade assembly and / or motor mode, for example as long as the “normal” or public electrical grid is not available. In this case, a transformer with a very small rated power may be used to connect the conversion lines to the diesel generator. The machine-side converter may be an active rectifier. The grid-side converter may bean inverter. The machine-side converter and the grid side converter may form a converter line, referred to in the following simply as converter. The wind turbine may comprise a further converter with a further machine-side converter, a further DC-link and a further grid-side converter. The further converter may be arranged in a further rotating electricalmachine winding or in parallel to the converter. At least one of the machine-sideconverters may be a converter with a neutral point, e.g. a three-level NPC converter, e.g. an active NPC or a T-Type converter. At least one of the grid-side converters may be a neutral point clamped converter, e.g. a 3LNPC converter. In the case of two machine windings and two back-to-back converters the energy conversion system may comprise a wye / delta-wye transformer, which connects the converter(s) to the grid. Operating the rotating electrical machine as the motor for bringing the rotor hub in the predetermined position for assembling the blade enables to assemble the blade without anyexternal turning tool and without the use of any additional expensive equipment, such ase.g. a pump for generating a hydro-pressure needed to rotate the gears of the wind turbine by a hydro-mechanical turning tool. By means of this proposed approach a reduced complexity of and reduced costs for the corresponding blade assembling process are achieved. When the wind turbine is installed at its predetermined location in the field, a tower ofthe wind turbine including the rotor hub is arranged at the predetermined location first. Then, the blades may be lifted up and coupled to the rotor hub, usually horizontally, one after the other (“single blade installation”), e.g. with the help of a specific “blade lifting tool”. Such a tool may be provided by a manufacturer of the wind turbine and may bedesigned to fit a specific blade model. In addition, for this single blade installation, as eachof the blades is usually installed horizontally, the rotor hub has to be rotated after one of the blades is installed to have an empty socket in the right position for the next blade, preferably by the method explained above and in the following. This procedure has to be repeated until all blades are installed. So, the above method may be carried out two ormore times, e.g. three time, in order to install all blades, e.g. three blades, at the rotor hub.The blade assembly mode may comprise operating the rotating electrical machine in the motor mode of the rotating electrical machine, wherein, in the motor mode, the rotating electrical machine is configured for rotating the rotor hub by electric energy from the electrical grid supplied to the rotating electrical machine via the grid-side converter, theDC-link, and the machine side converter. In this context, the rotating electrical machinemay be simply referred to as “motor”. So, the blade assembly mode may comprise or may be identical to the motor mode. An actual angle signal may be received which is representative for an actual rotor mechanical angle of the rotor hub, wherein the actual rotor mechanical angle isrepresentative for an actual position, in particular an actual angular position, of the rotorhub; and operating the rotating electrical machine in the blade assembly mode depending on the actual rotor mechanical angle of the rotor hub. The actual position of the rotor hub refers to an angular position of the rotor hub. The actual position may be measured by an appropriate sensor, e.g. an angular position encoder, simply referred to as “encoder”. The rotating electrical machine may be operated in the blade assembly mode depending on the actual rotor mechanical angle of the rotor hub by determining an actual position error between the predetermined rotor mechanical angle reference and the actual rotormechanical angle, determining a rotor mechanical speed reference depending on the actualposition error, and operating the rotating electrical machine depending on the rotor mechanical speed reference. One critical aspect of the blade assembly mode is the calculation of the rotor mechanical speed of the rotor hub and thereby of the already installed blades. A simplestraight-forward calculation from a derivative of the actual angular position may beproblematic at very low speeds since, depending on a resolution of the encoder, it may take very long until a new value of the actual angular position is sent to the master controller. This results in a big delay in the rotor mechanical speed calculation, which in turn may cause undesired rotor mechanical speed and / or angular position oscillations for thecorresponding closed-loop control. The following embodiments may appropriately handlethis issue, wherein the corresponding method steps may be carried out by a speed controller of the wind turbine. The speed controller may be implemented within the master controller or may communicate with the master controller. The rotating electrical machine may be operated depending on the rotor mechanicalspeed reference by receiving an actual speed signal which is representative for an actualrotor mechanical speed of the rotor hub, determining an actual speed error between the rotor mechanical speed reference and the actual rotor mechanical speed, and operating the rotating electrical machine depending on the actual speed error. The rotating electrical machine may be operated in the blade assembly mode dependingon the actual rotor mechanical angle of the rotor hub by determining an estimated rotormechanical angle depending on the actual rotor mechanical angle, determining an estimated position error between the predetermined position and the estimated rotor mechanical angle, determining a rotor mechanical speed reference depending on the estimated position error, and operating the rotating electrical machine depending on therotor mechanical speed reference. The estimated rotor mechanical angle may bedetermined depending on the actual rotor mechanical angle by a PLL. The rotating electrical machine may be operated depending on the rotor mechanical speed reference by determining an estimated rotor mechanical speed depending on the actual rotor mechanical angle and the estimated rotor mechanical angle, determining an estimated speed error between the rotor mechanical speed reference and the estimated rotor mechanical speed, and operating the rotating electrical machine depending on theestimated speed error. The estimated rotor mechanical speed may be determined dependingon the actual rotor mechanical angle and the estimated rotor mechanical angle by a PLL. The PLL may be integrated within the controller of the wind turbine or at least may communicate with the controller of the wind turbine. Optionally, the estimated rotor mechanical speed may be filtered by a low pass filter. Inthis case, the estimated speed error may be determined as the difference between the rotormechanical speed reference and the filtered estimated rotor mechanical speed. The filtered estimated rotor mechanical speed may be sent to a speed controller of the wind turbine. Optionally, to have more degrees of freedom in a fine-tuning of the speed control by the speed controller, different time constants ^^pand ^^imay be used for the lowpass filtering ofthe parts calculated by a corresponding proportional and integral controller respectively,wherein they optionally may be set to the same value. The speed controller may be implemented within the master controller or at least may communicate with the master controller. The proportional and / or the integral controller may be implemented within the master controller or at least may communicate with the master controller. The estimated rotor mechanical angle may be determined depending on the estimatedrotor mechanical speed. For example, the estimated rotor mechanical angle may be determined depending on the estimated rotor mechanical speed by integrating the estimated rotor mechanical angle. This integration may be carried out at a very high sampling rate, e.g. in a range of 0.1 MHz to 10 MHz, e.g.0.5 MHz to 5 MHz, e.g. about 1MHz. The PI controller for determining the estimated rotor mechanical speed may beimplemented at a much lower sampling rate, e.g. in the range of some kHz, in particular without compromising the dynamic performance of the blade assembly mode. The rotating electrical machine may be operated depending on the actual speed error or the estimated speed error by determining a temporary torque reference depending on theactual speed error or, respectively, the estimated speed error, and operating the rotatingelectrical machine depending on the temporary torque reference. The rotating electrical machine may be operated depending on the temporary torque reference by estimating a blade torque of one or more blades already arranged at the rotor hub, adjusting the temporary torque reference by the blade torque, and operating the rotating electrical machine depending on a correspondingly adjusted torque reference. In this context, for the calculation of the adjusted torque reference, a feedforward termcorresponding to the blade torque may be added to an output of the speed controller. Thefeedforward term may correspond to an estimated blade torque generated by already mounted blades. The estimated blade torque may be a function of the actual angular position of the rotor hub and may depend on the number of the blades which are already mounted at the rotor hub. The consideration of the torque of the already mounted blades and the correspondinginclusion of the feedforward term allows a very good, e.g. optimal, tuning of the speed controller which may generate a relatively small torque reference only necessary for the acceleration / deceleration, as well as for a compensation of wind gusts. The adjusted torque reference may be sent to a torque controller of the wind turbine. The torque controller maybe implemented within or may communicate with the master controller. The torquecontroller may consist of a linear current controller or of a non-linear flux controller, as they are known in the art. The temporary torque reference may be adjusted by the blade torque by adding the blade torque to the temporary torque reference, and the adjusted torque referencecorresponds to the sum of the blade torque and the temporary torque reference.The machine-side converter comprises several semiconductor switches and a short circuit is generated at the DC-link by activating all of the semiconductor switches at the same time. This causes a three-phase short circuit at the machine-side terminals of the machine-side converter and a corresponding generator short circuit torque. This generatorshort circuit torque may prevent an unintended fall of an already installed one of theblades. This enables to move the corresponding blade smoothly to a vertical blade position, after the blade has been installed in a horizontal blade position. A corresponding short circuit steady state current may be in the range of a rated current, e.g. due to a high generator impedance of approximately 1 pu). Activating all of the semiconductor switchesat the same time may be referred to as fall prevention mode. The fall prevention mode maybe triggered when the rotor hub 51 is not locked mechanically and if one or more of the converters does not function properly. For example, in case of a fault of one or more of the converters one or more converter protection functions may be activated, as they are known in the art, and at least one of these converter protection functions may trigger the fall prevention mode. Alternatively or additionally, the fall prevention mode may be triggered, when a control of the blades is lost. This lost of control may be determined depending on the actual position of the blade and / or depending on a position signal of the encoder. Another object is achieved by the master controller for the energy conversion system ofthe wind turbine. The master controller is adapted / configured for performing the above- described method. Another object is achieved by the energy conversion system of the wind turbine. The energy conversion system comprises the rotating electrical machine, the at least onemachine-side converter coupled to the rotating electrical machine, the at least one DC-linkcoupled to the machine-side converter; the at least one grid-side converter, which is configured for coupling the DC-link to a grid, and the above-mentioned master controller. Another object is achieved by a computer program for the above energy conversion system, in particular for the above master controller. The computer program, when beingexecuted by a processor of the master controller, is adapted for performing the abovemethod. Another object is achieved by a computer-readable medium, in which the above computer program is stored. The computer-readable medium may be a floppy disk, a hard disk, an USB storage device, a RAM, a ROM, an EPROM or a FLASH memory. Thecomputer readable medium may also be a data communication network, e.g. the Internet,which allows downloading a program code. In general, the computer-readable medium may be a non-transitory or transitory medium. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. It has to be understood that in order toprovide a clear and concise description and in order not to provide unnecessarilyrepetitions, some features, embodiments, and / or advantages of the present invention are described in this description with respect to only one aspect of the invention, e.g. only with respect to the method for bringing the rotor hub in the predetermined position, only with respect to the master controller, only with respect to the energy conversion system, onlywith respect to the computer program, only with respect the computer-readable medium,and / or only with respect to the method for avoiding the unintended fall of the blade. However, these features, embodiments, and / or, respectively, advantages may be simply transferred from one aspect to another of the aspects, without leaving the scope of the present invention, as may be realised by a person skilled in the corresponding art. BRIEF DESCRIPTION OF THE DRAWINGS The subject matter of the invention will be explained in more detail in the following textwith reference to exemplary embodiments which are illustrated in the attached drawings. Fig. 1 shows an exemplary embodiment of an energy conversion system of a windturbine; Fig. 2 shows an exemplary embodiment of a converter of the energy conversionsystem of figure 1;Fig. 3 shows an example of a first control diagram of the wind turbine;Fig. 4 shows an example of a second control diagram of the wind turbine;Fig. 5 shows an example of a third control diagram of the wind turbine;Fig. 6 shows an example of a blade of the wind turbine at a rotor hub of the windturbine in two different positions;Fig. 7 shows a flowchart of an exemplary embodiment of a method for avoiding anunintended fall of the blade of the wind turbine of figure 6. The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with thesame reference symbols in the figures.DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Fig.1 shows an exemplary embodiment of an energy conversion system 18 of a wind turbine 20. The wind turbine 20 may be a gearless wind turbine. The wind turbine 20 maycomprise one or more, e.g. three blades 50, coupled a rotor hub 51 of the wind turbine 20.The rotor hub 51 is mechanically coupled to the energy conversion system 18. The energy conversion system 18 comprises a rotating electrical machine 22 mechanically coupled to the rotor hub 51, a first machine-side converter 24, a first DC-link 26, a first grid-side converter 28, a master controller 30 (see figure 3) and a transformer 40. The first machine-side converter 24 and the first grid-side converter 28 form a first converter line. The first machine-side converter 24, the first DC-link 26 and the first grid- side converter 28 implement a first converter. The first converter may be a back-to-back converter. The first machine-side converter 24 may be a converter with a neutral point ,e.g. a 3LNPC converter. The first DC-link 26 comprises a first capacitor 32 and a secondcapacitor 34 arranged in series, wherein the node between the first and second capacitor 32, 34 is a neutral point NP of the first converter. The first grid-side converter 28 may be a converter with a neutral point , e.g. a 3LNPC converter. The energy conversion system 20 further comprises a second converter, e.g. a secondconverter line, with a second machine-side converter 44, a second DC-link 46 and a secondgrid-side converter 48. The second converter may be a back-to-back converter. The second machine-side converter 44 may be a converter with a neutral point, e.g. a 3LNPC converter. The second DC-link 46 may comprise a third capacitor 52 and a fourth capacitor 54 arranged in series, wherein a node between the third and fourth capacitor 52, 54 may bea neutral point NP of the second converter. The second grid-side converter 48 may be aconverter with a neutral point , e.g. a 3LNPC converter. The transformer 40 may be a wye / delta-wye transformer, which connects the first converter to an electrical grid. The transformer 40 may comprise first transformer secondary winding inductances 60 connected to the first converter, second transformersecondary winding inductances 62 connected to the second converter, transformer primarywinding inductances 64 each being coupled to a grid inductance 66 and a grid source 68. The rotating electrical machine 22 may be operated in a generator mode and as such as an electrical generator, e.g. a synchronous generator, or in a motor mode and as such as an electric motor. In the generator mode, which may be referred to as “normal” mode, theblades 50 and thereby the rotor hub 51 can be rotated by wind in the surroundings of thewind turbine 20 and the corresponding kinetic energy may be converted into electric energy by the energy conversion system 18. In contrast, in the motor mode, which may be referred to as “blade assembly mode” in this description, electric energy supplied to the wind turbine 20 from an electric grid may be converted into kinetic energy by the energyconversion system 18 such that the rotor hub 51 can be rotated by the electric energythereby generating the kinetic energy. The first machine-side converter 24 may be used as an active rectifier, if the rotating electrical machine 22 is in the generator mode, or as an inverter, if the rotating electrical machine 22 is in the motor mode. The first grid-side converter 28 may be used as an inverter, if the rotating electrical machine 22 is in the generator mode, or as a rectifier, e.g. an active rectifier, if the rotating electrical machine 22 is in the motor mode. The second machine-side converter 44 may be used as a rectifier, e.g. an active rectifier, if the rotatingelectrical machine 22 is the generator mode, or as an inverter, if the rotating electricalmachine 22 is in the motor mode. The second grid-side converter 48 may be used as an inverter, if the rotating electrical machine 22 is in the generator mode, or as a rectifier, e.g. an active rectifier, if the rotating electrical machine 22 is in the motor mode. The rotating electrical machine 22 may have two or more independent winding systems.In the generator mode, the rotating electrical machine 22, e.g. a first winding system of therotating electrical machine 22, may generate a three-phase input current with partial currents. The partial currents may be supplied to the machine-side converter 24. The machine-side converter 24 may convert the AC voltage of the rotating electrical machine 22 into a DC voltage supplied to the DC-link 26 and to the grid-side converter 28. Thegrid-side converter 28 may convert the DC voltage into an AC output voltage providing athree-phase output current with partial currents. The second converter may be arranged in a second winding of the rotating electrical machine 22. In the generator mode, the rotating electrical machine 22, e.g. the second winding system of the rotating electrical machine 22, also generates a three-phase inputcurrent with corresponding partial currents. These partial currents may be supplied to thesecond machine-side converter 44. The second machine-side converter 44 may convert the AC voltage of the rotating electrical machine 22 into a second DC voltage supplied to the second DC-link 46 and to the second grid-side converter 48. The second grid-side converter 48 may convert the second DC voltage into an AC output voltage providing athree-phase output current with partial currents.The master controller 30 is configured for controlling the energy conversion system 20. The master controller 30 may comprise a processor and a computer-readable medium. The computer-readable medium may store a computer program for controlling the energy conversion system 18. Alternatively or additionally, the master controller 30 may comprisea circuitry, e.g. an FPGA or ASIC, which is configured for controlling the energyconversion system 18. The computer-readable medium may be a memory unit of the energy conversion system, e.g. a floppy disk, a hard disk, an USB (Universal Serial Bus) storage device, a RAM (Random Access Memory), a ROM (Read Only Memory), an EPROM (Erasable Programmable Read Only Memory) or a FLASH memory. The computer readable medium may also be a data communication network, e.g. the Internet, which allows downloading a program code. In general, the computer-readable medium may be a non-transitory or transitory medium. In particular, in the generator mode, the controller 30 may be configured for modulatingthe first machine-side converter 24 and optionally the first grid-side converter 28. The master controller 30 may modulate the first machine-side converter 24 for converting the AC voltage of the rotating electrical machine 22 into the DC voltage supplied to the first DC-link 26. The master controller 30 may modulate the first grid-side converter 28 toconvert the first DC-link voltage at the first DC-link 26 into the AC output voltage to besupplied to the transformer 40 and further to the electrical grid. Further, in the generator mode, the controller 30 may be configured for modulating the second machine-side converter 44 and optionally the second grid-side converter 48. The master controller 30 may modulate the second machine-side converter 44 for convertingthe AC voltage of the rotating electrical machine 22 into the second DC voltage supplied tothe second DC-link 46. The master controller 30 may modulate the second grid-side converter 48 to convert the second DC-link voltage at the second DC-link 46 into the AC output voltage to be supplied to the transformer 40 and further to the electrical grid. So, during normal operation, the rotating electrical machine is used as a generator forproducing power which is transferred to the electrical grid. In this context, the rotatingelectrical machine may be simply referred to as “generator”. The master controller may include a torque controller, which may be configured for following a torque reference provided by external loops of the master controller during the normal operation. However, when the wind turbine 20 is installed at its predetermined location in thefield, a tower 16 (see figure 6) of the wind turbine 20 including the rotor hub 51 may bearranged at the predetermined location first. Then, the blades 50 may be lifted up and coupled to the rotor hub 51, usually horizontally orientated, one after the other (“single blade installation”), e.g. with the help of a specific “blade lifting tool”. In addition, for this single blade installation, as each of the blades 50 is usually installed horizontally, the rotorhub 51 has to be rotated after one of the blades 50 is installed to have an empty socket ofthe rotor hub 51 in the right position for the next blade 50. This procedure may have to be repeated until all blades 50 are installed. So, the method carried out during the blade assembly mode and described in the following may be carried out two or more times, e.g. three time, in order to install all of the blades 50, e.g. the three blades 50, at the rotor hub 51. A method for bringing the rotor hub 51 of the wind turbine 20 in a predetermined position for arranging the (next) blade 50 of the wind turbine 20 at the rotor hub 51comprises operating the rotating electrical machine 22 in the blade assembly mode inwhich the rotating electrical machine 22 is configured for rotating the rotor hub 51 such that the rotor hub 51 is brought in the predetermined position by the rotating electrical machine 22, wherein the predetermined position of the rotor hub 51 is given by apredetermined rotor mechanical angle reference ^^^^^^^^^ (see figures 3 and 4) of the rotor hub51. The rotor mechanical angle reference ^^^^^^^^^ may vary from blade 50 to blade 50.In particular, the blade assembly mode may comprise operating the rotating electrical machine 22 in the motor mode of the rotating electrical machine 22, wherein, in the motor mode, the rotating electrical machine 22 is configured for rotating the rotor hub 51 by electric energy from the electrical grid supplied to the rotating electrical machine 22 via thegrid-side converters 28, 28, the DC-links 26, 46, and the machine-side converters 24, 48.In more detail, in the motor mode, in particular in the blade assembly mode, the master controller 30 may be configured for modulating the grid-side converters 28, 48 to convert the AC input voltage supplied from the electrical grid into the corresponding DC-link voltage at the DC-link and for modulating the machine-side converters 24, 44 forconverting the DC voltage supplied to the DC-link into the corresponding AC voltage foroperating the rotating electrical machine 22. The master controller 30 may modulate the machine-side converters 24, 44 and the grid-side converters 28, 48 by one or more control signals generated by the master controller 30 and supplied to semiconductor switches 70, 72, 74 (see figure 2) of the machine-side converters 24, 44 and, respectively, the grid-sideconverters 28, 48 by the master controller 30. In this context, the rotating electricalmachine 22 may be simply referred to as “motor”. So, the blade assembly mode 22 may comprise or may be identical to the motor mode. So, in the proposed blade assembly mode of operation, the flow of the power is reversed with respect to the normal mode of operation. In particular, the electrical grid supplies theelectrical power which, via the proposed method, is used to operate the generator as amotor and to orientate the rotor hub 51 to the predetermined position for assembling one or more of the blades 50 to the rotor hub 51. In this context, the electrical grid optionally may be represented by an external generator, e.g. a diesel generator, that provides the required electrical power for the rotating electrical machine 22 during the blade assembly and / or motor mode. In this case, another transformer with a very small rated power may be used to connect the converter lines to the diesel generator. Fig. 2 shows an exemplary embodiment of one of the converters of the energyconversion system 18 of figure 1. The converter may comprise the first or second machine- side converter 24, 48 and / or, respectively, the first, second, third, and / or fourth capacitor 32, 34, 52, 54. The converter may further comprise amongst others a first switch 70, a second switch 72, and a third switch 74. The switches 70, 72, 74 may be individuallyswitched by corresponding control signal of the master controller 30, e.g. to provide themodulation of the corresponding converter. The first switch 70 may be an ideal switch for the mathematical modelling of semiconductor switches of the converter in a first phase. The second switch 72 may be an ideal switch for the mathematical modelling of the semiconductor switches in a second phase. The third switch 74 may be an ideal switch forthe mathematical modelling of the semiconductor switches in a third phase.In the following the motor mode, in particular the blade assembly mode, is described in detail with the help of the block diagrams shown in figures 3 to 5, wherein steps of the method for bringing the rotor hub 51 in the predetermined position may correspond to the functions carried out by the individual blocks of the block diagrams. In other words, thefollowing block diagrams may be representative for the steps carried out by carrying outthe method for bringing the rotor hub 51 in the predetermined position. Fig.3 shows an example of a first control diagram of the wind turbine 20. The first control diagram may represent at least in part the master controller 30. As such, the master controller 30 may comprise a position controller 75, a speed controller 76, and a torque controller 78. An actual angle signal which is representative for an actual rotor mechanical angle ^^^of the rotor hub 51 is received by the master controller 30. The actual rotor mechanical angle ^^^may be measured by a corresponding position sensor, e.g. an encoder, and may be sent from the corresponding position sensor to the master controller 30. The actual rotor mechanical angle^^^is representative for an actual position of the rotor hub 51. Then, therotating electrical machine 22 is operated in the blade assembly mode depending on theactual rotor mechanical angle ^^^of the rotor hub 51. The actual position of the rotor hub 51 may refer to an angular position of the rotor hub 51. In particular, the rotating electrical machine 22 may be operated in the blade assembly mode depending on the actual rotor mechanical angle ^^^of the rotor hub 51 bydetermining an actual position error ^^^^^^^^^ -^^^ between the predetermined rotor mechanicalangle reference ^^^^^^^^^ and the actual rotor mechanical angle ^^^ (see very left node in figure3), by determining a rotor mechanical speed reference ^^^^^^^^^^ depending on the actualposition error ^^^^^^^^^ -^^^, e.g. by a position controller 75, and by operating the rotatingelectrical machine 22 depending on the rotor mechanical speed reference ^^^^^^^^^^ .Further, the rotating electrical machine 22 may be operated depending on the rotormechanical speed reference ^^^^^^^^^^ by receiving an actual speed signal which isrepresentative for an actual rotor mechanical speed ^^^^ of the rotor hub 51, by determiningan actual speed error ^^^^^^^^^^ - ^^^^ between the rotor mechanical speed reference ^^^ ^^ ^^^^^and theactual rotor mechanical speed ^^^^(see node between the position controller 75 and a speed controller 76 of the master controller 30), and operating the rotating electrical machine 22depending on the actual speed error ^^^^^^^^^^ - ^^^^. The actual rotor mechanical speed ^^^^ of therotor hub 51 may be measured by an appropriate sensor, e.g. the above encoder or anothersensor coupled to the rotor hub 51. Furthermore, the rotating electrical machine 22 may be operated depending on theactual speed error ^^^^^^^^^^ - ^^^^ by determining a temporary torque reference ^^^ ^^ ^^^^^, depending on the actual speed error ^^^^ and operating the rotating electrical machine22 depending on the temporary torque reference ^^^^^^^^, ^^^^^^ . The temporary torque reference^^^^^^^^, ^^^^^^ may be determined by the speed controller 76. For example, the rotating electricalmachine 22 may be operated depending on the temporary torque reference ^^ byestimating a blade torque of one or more of the blades 50 already arranged at the rotor hub51, by adjusting the temporary torque reference ^^ by the blade torque, and operatingthe rotating electrical machine depending on a correspondingly adjusted torque reference^^^^^^^^^^ . In this context, for the calculation of the adjusted torque reference ^^^ ^^ ^^^^^, a bladetorque feedforward term ^^^^^^^^, ^^ corresponding to the blade(s) torque may be added to anoutput of the speed controller 76 (see node between speed controller 76 and a torquecontroller 78 of the master controller 30). The blade torque feedforward term ^^^^^^^^,^^ maycorrespond to an estimated blade torque generated by already mounted ones of the blades 50. The estimated blade torque may be a function of the actual angular position of the rotor hub 51 and may depend on the number of the blades 50 which are already mounted at therotor hub 51. The blade torque feedforward term ^^^^^^^^, ^^ may be estimated by calculationor simulation or may be determined empirically. The temporary torque reference ^^^^^^^^, ^^^^^^ may be adjusted by the blade torquefeedforward term ^^^^^^^^, ^^^^^^ by adding the blade torque feedforward term ^^^ ^^ ^^^^^, ^^^^to thetemporary torque reference ^^^^^^^^, ^^^^^^ , and the adjusted torque reference ^^^ ^^ ^^^^^maycorresponds to the sum of the blade torque feedforward term ^^^^^^^^, ^^ and the temporarytorque reference ^^^^^^^^, ^^^^^^ . The adjusted torque reference ^^^ ^^ ^^^^^may be sent to the torquecontroller 78.The controller 78 determines an electromagnetic torque Tedepending on the adjustedtorque reference ^^^^^^^^^^ . For example, if a current control is applied on the rotating electricalmachine 22, then the adjusted torque reference ^^^^^^^^^^ , which may be referred to aselectromagnetic torque reference, may be translated to a respective d-axis and q-axiscurrent reference in a rotating d-q reference frame that is aligned to a permanent magnetflux, as e.g. described in the paper “Decoupled vector control scheme for dual three-phase permanent magnet synchronous machines” by J. Karttunen, S. Kallio, P. Peltoniemi, P. Silventoinen, O. Pyrhönen, IEEE Transactions on Industrial Electronics, vol.61, no.5, pp. 2185-2196, 2013. The d-q current references may be sent to a corresponding currentcontroller which ensures that they are followed. By following the desired currentreferences the required electromagnetic torque Teis generated by the torque controller 78. Another option may be to use a non-linear stator flux controller for the torque control, e.g. as described in EP 22182749.6 (see description of figures 3 to 5). Then, the adjustedtorque reference ^^^^^^^^^^ may be sent to a corresponding stator flux controller in a stationaryreference frame. By tracking a certain flux trajectory the desired electromagnetic torque Temay generated by the torque controller 78. A difference between the electromagnetic torque Teand a load torque, which is the sum of the blade torque, a wind gusts torque, friction torque etc., may cause a rotation of the wind turbine rotor. The wind turbine rotor speed is equal to the integral of the aforementioned torque difference multiplied by a gain, where is the moment of inertia,according to Newton’s second law for rotation, as implied by the symbols in the block representing the rotating electrical machine 22. The integral of the speed is the change of the rotor angle (position). However, one critical aspect of the blade assembly mode may be the calculation of the actual rotor mechanical speed^^^^of the rotor hub 51 and thereby of the already installedblades 50. The embodiments described in the following with respect to figures 4 and 5 mayappropriately handle this issue. Fig.4 shows an example of a second control diagram of the wind turbine 20. The second control diagram may widely correspond to the first circuit diagram. Therefore, to provide a concise description of the present invention, only those features of the secondcontrol diagram are explained in the following, in which the second control diagram differsfrom the first control diagram. The actual rotor mechanical angle ^^^may be measured by a position sensor 82 of the wind turbine 20. The position sensor 82 may be a part of the energy conversion system 18 and / or of the master controller 30. The actual rotor mechanical angle ^^^ may be sent from the position sensor 82 to anestimation block 80. The estimation block 80 may be configured for determining an estimated rotor mechanical angle ^^^^depending on the measured actual rotor mechanical angle ^^^. The estimation block 80 is further explained in more detail below with respect tofigure 5. An estimated position error ^^^^^^^^^ - ^^^^ between the predetermined position, i.e. therotor mechanical angle reference ^^^^^^^^^ and the estimated rotor mechanical angle ^^^ may bedetermined (see very left node in figure 4). Then, a rotor mechanical speed reference ^^^^^^^^^^may be determined depending on the estimated position error ^^^^^^^^^ - ^^^^, e.g. by the positioncontroller 75. Then, the rotating electrical machine 22 may be operated depending on therotor mechanical speed reference Further, the rotating electrical machine 22 may be operated depending on the rotormechanical speed reference ^^^^^^^^^ by determining an estimated rotor mechanical speed ^^^^depending on the actual rotor mechanical angle ^^^and the estimated rotor mechanical angle ^^^^, by determining an estimated speed error ^^ ^ ^^ ^^^^^- ^^^^ between the rotor mechanicalspeed reference ^^^^^^^^^^ and the estimated rotor mechanical speed ^^^^ (see node betweenposition controller 75 and speed controller 76 of figure 4), and by operating the rotatingelectrical machine 22 depending on the estimated speed error ^^^^^^^^^^ - ^^^^. Furthermore, the rotating electrical machine 22 may be operated depending on theestimated speed error - by determining the temporary torque refere^^^^^^, ^^^^ ^^^^ nce ^^ ^^depending on the estimated speed error - ^^^^, and operating the rotating electricalmachine depending on the temporary torque reference ^^^^^^^^, ^^ . For example, the rotatingelectrical machine 22 may be operated depending on the temporary torque reference by estimating the blade torque feedforward term of one or more of theblades 50 already arranged at the rotor hub 51, by adjusting the temporary torque reference (see node between the speedcontroller 76 and the torque controller 78 of figure 4), and by operating the rotatingelectrical machine 22 depending on the correspondingly adjusted torque reference ^^^^^^^^^^ . Inparticular, the temporary torque reference ^^^^^^^^, ^^^^^^ may be adjusted by the blade torquefeedforward term ^^ by adding the blade torque feedforward term ^^^ ^^ to thetemporary torque reference ^^^^^^^^, ^^^^^^ , and the adjusted torque reference ^^^ ^^ ^^^^^corresponds tothe sum of the blade torque feedforward term ^^^^^^^^, ^^^^^^ and the temporary torque reference^^^^^^^^, ^^^^^^ .Fig.5 shows an example of a third control diagram of the wind turbine 20. The third control diagram may correspond to the estimation block 80 shown in figure 4. From figure 5 it may be seen that the estimated rotor mechanical angle ^^^^may be determined depending on the actual rotor mechanical angle ^^^by a PLL, wherein the PLL maycomprise an angular speed calculation block 84, a PI controller 86, and an anglecalculation block 88. In particular, the estimated rotor mechanical speed ^^^^may be determined depending on the actual rotor mechanical angle ^^^and the estimated rotor mechanical angle ^^^^by the PI controller 86 and the angle calculation block 88. Optionally, the estimated rotor mechanical speed ^^^^may be filtered by a low pass filter90. In this case, the estimated speed error ^^^^ may be determined as the differencebetween the rotor mechanical speed reference ^^^^^^^^^^ and the correspondingly filteredestimated rotor mechanical speed ^^^^,^^^^^^^^. The filtered estimated rotor mechanical speed ^^^^,^^^^^^^^may be sent to the speed controller 76. Optionally, to have more degrees of freedom in a fine-tuning of the speed control by the speed controller 76, different time constants ^^pand ^^i may be used for the lowpass filtering of the parts calculated by a correspondingproportional and integral controller respectively (see low-pass filter 90), wherein they optionally may be set to the same value. Optionally, the estimated rotor mechanical angle^^^^may be determined depending on the estimated rotor mechanical speed For example, the estimated rotor mechanical angle^^^^may be determined depending on the estimated rotor mechanical speed^^^^,^^^^^^^^byintegrating the estimated rotor mechanical speed ^^^^,^^^^^^^^. This integration may be carriedout at a very high sampling rate, e.g. in a range of 0.1 MHz to 10 MHz, e.g.0.5 MHz to 5 MHz, e.g. about 1 MHz. The PI controller 86 for determining the estimated rotor mechanical speed^^^^,^^^^^^^^may be implemented at a much lower sampling rate, e.g. in the range of some kHz, in particular without compromising the dynamic performance of theblade assembly mode.Fig.6 shows an example of the blade 50 of the wind turbine 20 at the rotor hub 51 of the wind turbine 20 in two different positions. In particular, in a first position of the blade 50, the blade 50 extends basically horizontally, wherein the blade 50 in the first position is indicated in figure 6 by the solid line. In contrast, in a second position of the blade 50, theblade 50 extends basically vertically, wherein the blade 50 in the second state is indicatedin figure 6 by the dashed line. The tower 16 of the wind turbine 20 comprising the rotor hub 51 may be already installed at the final location of the wind turbine 20. Then, the rotor hub 51 may be positioned such that the blade 50 may be installed, e.g. by the method explained above, wherein normally the blade 50 is mounted to the rotor hub 51 in the firstposition, i.e. when the blade 50 is oriented horizontally. Then, the next blade 50 has to bemounted to the rotor hub 51 and the rotor hub 51 has to be rotated such that the next blade 50 can be mounted when it is in the first position. However, a weight of the already installed blade 50 generates a torque on the rotor hub 51, which causes the blade 50 to move down, and may cause an unintended and / oruncontrolled rotation of the rotor hub 51 and thereby an unintended and / or uncontrolledtrip of the blade 50, when the rotor hub 51 is not locked mechanically at a certain angular position. In this scenario, a worst case of a corresponding blade fall may be that only one of the blades 50 is mounted on the rotor hub 51 and the fall starts at exact the position where the blade 50 has been mounted, e.g. when the blade 50 is oriented horizontallyand / or in the first state. A correspondingly uncontrolled fall of the blade 50 may representa danger for service or maintenance personal mounting the blades 50 to the rotor hub 51. In addition, the uncontrolled fall of the blade 50 may have as consequence uncontrolled oscillations around the equilibrium exposing to danger the personal and the equipment. A solution to this issue may be to create a short circuit in the dc-link 26, 46 of the first and / or second converter by activating all of the semiconductor switches 70 of the corresponding 3LNPC converter on the generator side, as explained with respect to figure7. In particular, after an initial transient of the short circuit, a non-oscillatingelectromagnetic torque is generated which counteracts the torque caused by the blade 50. The result is a smooth movement of the blade 50 to a vertical position of the blade 50, in particular such that the blade 50 is orientated vertically and / or in the second state. Fig.7 shows a flowchart of an exemplary embodiment of a method for avoiding theunintended fall of the blade 50 of the wind turbine 20 of figure 6. The method enables toavoid an unintended fall of the blade 50 of the wind turbine 20 when the blade 50 or another blade 50 of the wind turbine 20 is arranged at the rotor hub 51 of the wind turbine 20. In a step S2, a prevent unintended fall mode may be entered, when it is planned tomechanically unlock the rotor hub 51 outside of the normal operation of the wind turbine.For example, the prevent unintended fall mode may be entered, when one or more of the blades 50 are mounted to the rotor hub 51, when one or more of the blades 50 are removed from the rotor hub 51, and / or when carrying out one or more maintenance tasks at the wind turbine 20, in particular at one or more of the blades 50 or the rotor hub 51. Optionally, theprevent unintended fall mode may be triggered, e.g. automatically, when entering the bladeassembly mode, explained above. In a step S4, a short circuit is generated at the DC-link 26, 46 by activating all of the semiconductor switches 70, 72, 74 of the machine-side converter 24, 44 at the same time. This causes a three-phase short circuit at machine-side terminals of the machine-sideconverter 24, 44 and a corresponding generator short circuit torque. This generator shortcircuit torque prevents an uncontrolled fall and afterwards an unintended fall of an already installed one of the blades 50. A corresponding short circuit steady state current may be in the range of a rated current, e.g. due to a high generator impedance of approximately 1 pu). The functions of the circuit diagrams explained with respect to figures 3 to 5 and / or themethod explained with respect to figures 6 and 7 may be embodied as hardwarecomprising several electronic components and / or dedicated micro-chips, e.g. FPGAs, as software, e.g. in the form of one or more computer programs, or as a combination of hardware and software. For example, a computer program for the above energy conversion system 18, in particular for the above master controller 30, may be provided, wherein the computer program, when being executed by the processor of the master controller 30, may be adapted for performing the functions of one or more of the above circuit diagramsand / or of the above method. Further, a computer-readable medium may be provided, inwhich the above computer program is stored. The computer-readable medium may be a floppy disk, a hard disk, an USB storage device, a RAM, a ROM, an EPROM or a FLASH memory. The computer readable medium may also be a data communication network, e.g. the Internet, which allows downloading a program code. In general, the computer-readablemedium may be a non-transitory or transitory medium.While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by thoseskilled in the art and practising the claimed invention, from a study of the drawings, thedisclosure, and the appended claims. For example, the rotating electrical machine 22 may output only two currents for one converter and, in case of two converters, two for the first converter and two for the second converter, wherein every converter may comprise a machine-side converter and a grid-side converter. In the claims, the word “comprising” does not exclude other elements or steps, and theindefinite article “a” or “an” does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs inthe claims should not be construed as limiting the scope. LIST OF REFERENCE SYMBOLS16 tower18 energy conversion system20 wind turbine22 rotating electrical machine24 first machine-side converter26 first DC-link28 first grid-side converter30 master controller32 first capacitor34 second capacitor40 transformer44 second machine-side converter46 second DC-link48 second grid-side converter50 blade51 rotor hub52 third capacitor54 fourth capacitor60 first transformer secondary winding inductances62 second transformer secondary winding inductances64 transformer primary winding inductances66 grid inductances68 grid source70 first switch72 second switch74 third switch75 position controller76 speed controller78 torque controller80 estimation block82 position sensor84 angular speed calculation block86 PI controller88 angle calculation block90 low-pass filters^^^ actual rotor mechanical angle actual position errorrotor mechanical angle referencerotor mechanical speed referenceactual rotor mechanical speed^^^^^^^^^^ - ^^^^ actual speed error^^^^ estimated rotor mechanical angle^^^^^^^^^ - ^^^^ estimated position errorestimated rotor mechanical speed^^^^^^^^^^ - ^^^^ estimated speed error^^^^^^^^, ^^^^^^ temporary torque reference^^^^^^^^, ^^^^^^ blade torque feedforward term^^^^^^^^^^ adjusted torque reference^^^^,^^^^^^^^ filtered estimated rotor mechanical speedTeelectromagnetic torque

Claims

CLAIMS1. A method for avoiding an unintended fall of a blade (50) of a wind turbine (20)when the blade (50) or another blade (50) of the wind turbine (20) is arranged at a rotorhub (51) of the wind turbine (20), the wind turbine (20) comprising the rotor hub (51), arotating electrical machine (22) mechanically coupled to the rotor hub (51), a machine-side converter (24, 44) electrically coupled to the rotating electrical machine (22) and comprising several semiconductor switches (70, 72, 74), a DC-link (26, 46) electrically coupled to the machine-side converter (24, 44), and a grid-side converter (28, 48)electrically coupled to the DC-link (26, 46) and to an electrical grid, the methodcomprising: generating a short circuit at the DC-link (26, 46) by activating all of the semiconductor switches (70, 72, 74) at the same time.

2. Method in accordance with claim 1, whereina three-phase short circuit at machine-side terminals of the machine-side converter (24, 44) and a corresponding generator short circuit torque are caused by the short circuit at the DC-link (26, 46), and the generator short circuit torque prevents the unintended fall of the blade (50).

3. Method in accordance with one of the preceding claims, whereina corresponding short circuit steady state current is in the range of a rated current.

4. Method in accordance with one of the preceding claims, whereina start of the method is triggered with respect to a fall prevention mode.

5. Method in accordance with claim 4, whereinthe fall prevention mode is triggered when the rotor hub (51) is not locked mechanically, if one or more of the converters (24, 44, 28, 48) does not function properly.

6. Method in accordance with claim 5, whereinin case of a fault of one or more of the converters (24, 44, 28, 48) one or more converter protection functions are activated, and at least one of these converter protection functions triggers the fall prevention mode.

7. Method in accordance with claim 5, whereinthe fall prevention mode is triggered, when a control of the blade (50) is lost, and this lost of control is determined depending on an actual position of the blade (50) and / or depending on a position signal of an encoder of the wind turbine (20).

8. A master controller (30) for an energy conversion system (18) for a wind turbine(20) adapted / configured for performing the method of one of the preceding claims.

9. An energy conversion system (18) of a wind turbine (20), comprising:a rotating electrical machine (22),at least one machine-side converter (24, 44) coupled to the rotating electrical machine (22), at least one DC-link (26, 46) coupled to the machine-side converter (24, 44); at least one grid-side converter (28, 48), which is configured for coupling the DC-link (26, 46) to an electrical grid, anda master controller (30) in accordance with claim 8.

10. A computer program for driving an energy conversion system (18) of a windturbine (20) in accordance with claim 9, which, when being executed by a processor of amaster controller (30) of the wind turbine (30) in accordance with claim 8, is adapted forperforming the method of one of claims 1 to 7.

11. A computer-readable medium in which a computer program according to claim 10is stored.

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