Method for determination of a location of a short circuit fault in a generator arrangement, generator arrangement, wind turbine, computer program and electronically readable medium

The invention relates to a method for determination of a lo cation of a short circuit fault in a generator arrangement, wherein the generator arrangement comprises an electrical ma chine and at least one channel , wherein the or each channel comprises a breaker, a converter unit and a set of stator windings of the electrical machine connected to the converter unit via the breaker, wherein upon an occurrence of a short circuit in a channel , the connection between the set of sta tor windings and the converter unit is interrupted by opening the breaker . Furthermore, the invention relates to a genera tor arrangement, a wind turbine, a computer program and an electronically readable medium.

In such a generator arrangement, short circuit faults can oc cur in different components of the arrangement . Generally, a short circuit fault causes a large current exceeding the cur rent during proper operation of the generator arrangement . It is known to detect and to stop such a short circuit current in an electric circuit using fuses or circuit breakers . How ever, the large current appears independently of the location of the short circuit fault in the electric circuit of the generator arrangement . Hence, upon occurrence of a short cir cuit fault in a channel of the generator arrangement, only the faulty channel can be identified.

Therefore, it is desirable that upon occurrence of a short circuit fault in an electric circuit, detailed information on the short circuit fault can be obtained .

It is therefore an object of the invention to provide a meth od for determination of a location of a short circuit fault that gives more precise information on the location of the short circuit fault .

According to the invention, this problem is solved by a meth od as initially described, wherein depending on at least one measured signal of a measurand, wherein the measured signal is measured by at least one sensor of the electric machine and wherein the measurand describes a torque ripple of the electrical machine, either the electrical machine or the con verter unit of the channel is determined as location of the short circuit fault .

Upon occurrence of the short circuit fault in a channel of the electrical machine, not only can the faulty channel be detected by the opened breaker, but also a determination whether the short circuit fault occurred in the converter unit of the channel or in the electrical machine becomes pos sible by evaluating the measurand or the at least one meas ured signal of the measurand, respectively . Hence, different sources of the short circuit fault can be differentiated by evaluation of the measurand .

This has the advantage that after occurrence of the short circuit fault, a faulty component can be identified, facili tating a repair by providing the location of the faulty com ponent which has to be repaired or replaced.

In a preferred embodiment of the invention, the electrical machine comprises two or more channels , wherein after occur rence of the short circuit fault in a channel , the converter unit of the channel is stopped and the at least one breaker of the channel is opened, or the converter units of all chan nels are stopped and the breakers of all channels are opened . Opening the breaker stops the large short circuit current in the channel . It is in particular possible that the breaker comprises one breaker per phase of the channel or that a mul ti-phase breaker, for example a three-phase breaker for a three-phase channel , is used, so that each stator winding of the set of stator windings is separated from the converter unit .

In the case that only the converter unit of the faulty chan nel , in which the short circuit fault occurs , is stopped and the breaker of the faulty channel is opened, the remaining channels of the electrical machine can be used for stopping the electrical machine or the rotation of its rotor, respec tively . It is also possible that all breakers in all channels of the electrical machine are opened and all converter units are stopped . In this case, a stopping of the rotation of the rotor of the electric machine can occur for instance using a mechanical brake or the like .

Preferably, the electrical machine comprises two or more channels , wherein after occurrence of the short circuit fault in a faulty channel of the channels , if the converter unit is determined as location, an operation of the generator ar rangement is continued or restarted with the channel or the channels except the faulty channel , wherein, if the electri cal machine is determined as location, the operation is stopped . A continuation of the operation of the electrical machine or a restarting of the operation of the generator ar rangement in case of an occurrence of the short circuit fault in one converter unit of a channel enables a usage of the generator arrangement in the time period until the faulty converter unit can be repaired or replaced . In case of an oc currence of the short circuit fault in the electrical ma chine, a continuation of the operation of the generator ar rangement is too dangerous , since the short circuit in the electrical machine may cause severe damage to the electrical machine or to the generator arrangement when the operation is continued, so that in this case the operation is stopped and a restarting of the operation of the generator arrangement can be prevented .

In a generator arrangement with an electrical machine that comprises more than one channel , only the faulty channel may remain disconnected if the converter unit of the faulty chan nel caused has been determined as location of the short cir cuit fault . Other channels of the electrical machine that comprise converter units which are still working properly can be used to continue the operation of the generator arrange ment . In this case, it is possible to continue the operation of the generator arrangement and to prevent a shut-down of the entire generator arrangement .

However, in the case that the short circuit fault occurs in the electrical machine of the generator arrangement, a fur ther operation of the entire generator arrangement should not occur, since the short circuit fault in the electrical ma chine may cause damage to the electrical machine or to other components of the generator arrangement, respectively . Conse quently, if the electrical machine is determined as location of the short circuit fault, a further operation of the gener ator arrangement can be prevented .

Having the ability to continue the operation of a generator arrangement comprising more than one channel is in particular advantageous for a generator arrangement which is a part of facilities generating electrical energy like wind turbines or the like . In this case, a short circuit fault in a converter unit of one channel does not cause a stopping of the entire facility until the faulty component is repaired .

The electrical machine can be in particular an electrical ma chine with a multi-phase stator, which comprises several sets of stator windings each connected to a separate converter unit . The set of stator windings or the sets of stator wind ings can be three-phased stator windings generating a three- phase electrical current in the stator during a rotation of a rotor or operation of the electrical machine as generator, respectively . In particular, the rotor can comprise permanent magnets or rotor coils , which generate a magnetic field also after occurrence of the short circuit fault . Due to a magnetic coupling between the rotating rotor and the stator of the electrical machine, after occurrence of a short circuit fault, a torque ripple in the torque of the electri cal machine can occur . This torque ripple is measured by the at least one sensor of the electrical machine, wherein the sensor quantifies at least one measurand, which describes the torque ripple of the electrical machine . As measurand, a physical quantity, which is influenced by the torque ripple of the electrical machine, can be used . The measurand can for instance exhibit a ripple corresponding to and/or proportion al to the torque ripple of the electrical machine .

The torque ripple of the electrical machine is different for short circuits , depending on whether they appear in the con verter unit or in the generator . Hence, by evaluation of the torque ripple of the electrical machine, a determination of the location of the short circuit fault becomes possible .

Upon occurrence of a short circuit of the channel , the con nection between the set of stator windings and the converter unit is interrupted by opening of the breaker . The breaker may comprise for instance a switch coupled to a current sen sor . Preferably, the breaker is a circuit breaker, which in terrupts the connection between the converter unit and the set of stator windings when a short circuit or a large short circuit current appears . Preferably, also the converter unit is stopped upon occurrence of the short circuit, for instance by switching it off or by stopping its operation, respective ly.

However, the short circuit fault may have occurred either in the converter unit or in the generator . The short circuit fault in the converter unit can occur, for instance, due to faulty components in the converter . A short circuit fault in the generator can occur for instance due to an insulation breakdown inside the electrical machine . However, the connec tion between the converter unit and the set of stator wind- ings is interrupted independently of the location of the short circuit for stopping the large short circuit current .

After interrupting the connection between the converter unit and the set of stator windings , the high short circuit cur rent flow between converter and machine in the faulty channel stops . Despite the short circuit fault, before opening the breaker, the behaviour of the generator arrangement does not depend on the location of the short circuit fault . However, after interrupting the connection between the converter unit and the set of stator windings , the rotor of the electrical machine is still rotating and the behaviour of the generator arrangement differs depending on the location of the short circuit fault .

I f the short circuit fault is caused by a fault inside the converter unit, the current in the channel will go down to zero, since the switching of the converter unit is stopped and the connection between the converter unit and the set of stator windings is interrupted . I f the short circuit fault is caused by an internal fault in the electrical machine, even after interruption of the connection between the converter unit and the set of stator windings the magnetic coupling by the electromotive force (back-EMF) in the windings of the electrical machine would generate a fault current in the electrical machine until the rotation of the rotor is

stopped . Upon occurrence of a short circuit fault in a set of stator windings , even after opening of the breaker, a closed loop remains due to the short circuit . In this closed loop, a fault current is induced by the rotating rotor of the elec trical machine . However, such a fault current cannot be meas ured directly . The invention is based on the idea to measure such a fault current indirectly by evaluating a measured sig nal of a measurand, which describes a torque ripple of the electrical machine, since the presence of a fault current in fluences the torque ripple of the electrical machine . Conse quently, by evaluating the at least one measured signal of the measurand related to the torque ripple of the electrical machine, a determination of a location of the short circuit becomes possible.

Preferably, as the at least one sensor of the electrical ma chine, a sensor measuring a vibration and/or an acceleration of at least one component of the electrical machine is used . By a sensor measuring the vibration of at least one component of the electrical machine, the measurand can be, for in stance, a periodic displacement of the at least one component of the electrical machine . The displacement can correspond to the torque ripple, since a larger torque ripple of the elec trical machine may cause a larger vibration of the at least one component of the electrical machine . Additionally or al ternatively it is possible that a sensor, which measures an acceleration of at least one component of the electrical ma chine, is used, wherein correspondingly a larger torque rip ple of the electrical machine may cause a larger acceleration ripple amplitude of at least one component of the electrical machine .

In a preferred embodiment of the invention, an accelerometer coupled to a main bearing of the electrical machine and/or to at least one stator plate of the electrical machine is used as sensor and/or a microphone measuring sound inside the electrical machine is used as sensor and/or a short circuit torque controller is used as sensor . The main bearing can be arranged between the rotor and the stator of the electrical machine to support the rotating rotor against the stator, so that a torque ripple of the electrical machine causes a vi bration or a periodic deflection of the main bearing or com ponent of the main bearing, which can be measured by an ac celerometer coupled to the main bearing . A sensor can be cou pled, for instance, to a stationary ring of the main bearing . It is possible that a sensor provided for monitoring a bear ing condition is used .

It is also possible that an accelerometer coupled to a stator plate is used . The stator plate is an annular flat plate which delimits the stator towards the environment and which can vibrate and/or exhibit a displacement depending on a torque ripple of the electrical machine .

Additionally or alternatively, a microphone measuring sound inside the electrical machine can be used as a sensor since the appearance of a torque ripple in the electrical machine may cause the generation of sound into the air and/or as a mechanical vibration inside the electrical machine . It is al so possible that a short circuit torque controller is used as a sensor, wherein the short circuit torque controller can be configured to reduce the appearance of torque ripple during the occurrence of a short circuit in the electrical machine to minimize noise generation and to prevent damage to the electrical machine in case of a short circuit fault . Such a short circuit torque controller comprises a sensor which measures a measurand describing a torque ripple of the elec trical machine to apply counter measures reducing the torque ripple . Advantageously, the measurand measured by the short circuit torque controller can be used also for the determina tion of the location of the short circuit .

Preferably, a comparison value related to an amplitude of the torque ripple is derived from the at least one measured sig nal of the measurand, wherein the comparison value is com pared to a threshold value, wherein the electrical machine is determined as location of the short circuit fault if the com parison value is above the threshold value, and the converter unit is determined as location of the short circuit fault if the comparison value is below the threshold value . The com parison value can be derived from one or more measured sig nals of the measurand, wherein the at least one measured sig nal can comprise in particular a measurement over a short pe riod of time to describe the effect of the torque ripple on the measurand . From this at least one measured signal , a com parison value is derived and compared to a threshold value . A larger comparison value corresponds to a larger torque rip ple, which appears as described above in case of a short cir- cuit fault in the electrical machine . Hence, a comparison value above the threshold indicates a short circuit in the electrical machine and a comparison value below the threshold value indicates a short circuit in the converter unit . The threshold value, which is used for the comparison to the com parison value, can be a constant or a variable and it can be derived from measurements or it can be determined empirical ly . The threshold value can depend in particular on the type of the electrical machine and/or the type of the generator arrangement or facility in which the generator arrangement is used .

Preferably, a root-mean-square value of the value or a peak- to-peak value of the measured signal is used as comparison value or an absolute value of the measured signal is used . A root-mean-square value and/or a peak-to-peak-value can be used to describe an amplitude of a periodic signal or a peri odic component of the measured signal of the measurand corre sponding to the torque ripple of the electric machine, re spectively . Due to the rotation of the rotor, also the torque ripple can exhibit a periodicity, in particular corresponding to the rotational speed of the rotor . As absolute value, for instance, a maximum value or an average value within an aver aging period of the measured signal can be used .

In a preferred embodiment of the invention, the threshold value is determined depending on an average torque of the electrical machine and/or a rotational speed of the electri cal machine . A determination of the threshold value depending on an average torque of the electrical machine can account for the fact that a larger operating torque of the electrical machine may cause a larger torque ripple in the case of a short circuit fault . An average torque after opening of the breakers can be used in particular in a multi-channel genera tor comprising two or more sets of stator windings . Since the amplitude of the torque ripple may depend on the frequency of rotation as well , also a dependence of the threshold value on a rotational speed of the electrical machine can be advanta- geous . The rotational speed of the electrical machine and/or the average torque of the electrical machine can be deter mined for instance by a torque sensor and/or an rotation sen sor of the electric machine and/or a speed observer unit im plemented in the operating channels of a multi-channel ma chine, in particular when the short circuit fault occurs or in a time interval shortly after the occurrence of the short circuit fault, respectively, so that the present state of the machine can be considered for the determination of the loca tion of the short circuit fault .

In particular, the implementation of the described method is feasible in a wind turbine even though a stop of the wind turbine operation is triggered upon occurrence of the short circuit, since shortcircuit detection and breaker opening are typically accomplished in the ms range (below 300 ms ) , where as the turbine stop takes several seconds (above 10 s ) . Ac cordingly, a sufficient time window between breaker opening and turbine stop is available for fault discrimination .

Preferably, the at least one measured signal of the measurand is bandpass filtered, wherein the lower cutoff frequency and/or the upper cutoff frequency of the bandpass filtering are determined in dependence of a rotational speed of the electrical machine . By bandpass filtering of a measured sig nal of the measurand, a certain frequency or a certain fre quency band corresponding to the passband of the bandpass filter can be extracted for evaluation of the measurand . By determining the lower cutoff frequency and/or the upper cut off frequency of the bandpass filtering, it becomes possible to filter the measured signal , so that a measured signal , which corresponds to a certain harmonic of a torque ripple can be obtained . It is for instance possible that a torque ripple at a frequency, which is twice the frequency of the rotation of the electrical machine, is extracted . Also other harmonics , like a fourth harmonic or a sixth harmonic, can be extracted using the bandpass filtering and the determination of the lower cutoff frequency and the upper cutoff frequency depending on a rotational speed of the electrical machine. Choosing different harmonics of the torque ripple may account for different geometries and/or features of electrical ma chines and can depend on the type of the electrical machine which is used in the generator arrangement .

A generator arrangement according to the invention comprises an electrical machine, a control unit and at least one chan nel , wherein the or each channel comprises a breaker, a con verter unit and a set of stator windings of the electrical machine connected to the converter unit via the breaker, wherein the electrical machine comprises at least one sensor that measures a measurand describing a torque ripple of the electrical machine, wherein the control unit is configured to perform a method according to the invention .

Preferably, the electrical machine can comprise two or more channels , which can be, for instance, three-phase channels comprising each a three-phase set of stator windings . The control unit can be connected to the converter unit and/or to the breaker . The control unit is connected to the at least one sensor that measures the measurand . Furthermore, the con trol unit can be connected to further sensors of the electri cal machine, wherein the further sensorsmeasure, for in stance, an average torque of the electrical machine and/or a rotational speed of the electrical machine . Additionally or alternatively, it is possible that an average torque of the electrical machine and/or a rotational speed of the electri cal machine are calculated and/or estimated by the control unit, for instance by evaluating currents and/or voltages of the electrical machine .

A wind turbine according to the invention comprises a genera tor arrangement according to the invention .

A computer program according to the invention comprises in structions which, when the program is executed by a control unit of a generator arrangement comprising an electrical ma- chine, a control unit and at least one channel, wherein the or each channel comprises a means for detecting an occurrence of a short circuit in the channel , a converter unit and a set of stator windings of the electrical machine connected to the converter unit, wherein the electrical machine comprises at least one sensor that measures a measurand describing a torque ripple of the electrical machine, causes the control unit to carry out a method according to the invention .

An electronically readable medium according to the invention has a computer program according to the invention stored thereon .

All features and advantages described in relation to the method according to the invention apply correspondingly for the generator arrangement according the invention, the wind turbine according to the invention, the computer program ac cording to the invention and the electronical readable medium according to the invention .

Additional advantages and details of the invention become ev ident from the embodiments discussed below as well as from the figures . The figures are schematical figures and show :

Fig . 1 a generator arrangement according to the invention,

Fig . 2 a diagram depicting the torque of an electrical ma chine in case of a short circuit fault in a con verter unit,

Fig . 3 a diagram depicting the torque of an electrical ma chine in case of a short circuit fault in the elec trical machine,

Fig . 4 a block diagram of a first embodiment of a method according to the invention, and Fig. 5 a block diagram of a second embodiment of a method according to the invention .

In fig. 1 , a generator arrangement 1 according to the inven tion is shown . The generator arrangement 1 comprises an elec trical machine 2 , a first channel 3 and a second channel 4. The first channel 3 comprises a converter unit 5 and a break er 6. Correspondingly, the second channel 4 comprises a con verter unit 7 as well as a breaker 8.

The converter unit 5 of the first channel 3 is connected to a set 9 of stator windings of the electrical machine 2 via the breaker 6. Correspondingly, the second channel 4 comprises a set 10 of stator windings , which is connected to the control unit 7 via the breaker 8. The sets 9, 10 of stator windings are three-phase sets of stator windings . During rotation of the electrical machine 2 , hence during a generator operation of the electrical machine 2 , a three-phase electrical current is generated in each of the channels 3 , 4. The generated three-phase electrical current is converted by the converter units 5, 7 into a DC current . The DC-side of the converter units 5, 7 can be connected to other electrical components and/or electrical circuits and/or a power grid . In this em bodiment, on the DC-side of the converters 5, 7 of each chan nel 3 , 4 , a capacitor 11 , 12 is arranged for frequency fil tering . However, further components on the DC-side are omit ted in fig. 1. It is in particular possible that the genera tor arrangement 1 is used in a power generating device, for instance in a wind turbine .

The breaker 6 in the first channel 3 can be opened to inter rupt the connection between the converter unit 5 and the set 9 of stator windings in case of a short circuit in the first channel 3. Correspondingly, the breaker 8 in the second chan nel 4 can be opened to interrupt the connection between the converter unit 7 and the set 10 of stator windings in case of a short circuit in the second channel 4. Preferably, the breakers 6 and 8 may interrupt simultaneously their respec- tive connections in case of a short circuit in one of the channels 3, 4.

The generator arrangement 1 comprises further a control unit 13 which is connected to the converter units 5, 7 and the breakers 6, 8. Furthermore, the control unit 13 is connected to a sensor 14 of the electrical machine 2, wherein the sen sor 14 measures a measurand describing a torque ripple of the electrical machine 2. The control unit 13 comprises a memory unit for storing of data and/or programs , a communication unit for sending and receiving data and a processor unit for processing the data and/or for executing the programs and/or for controlling the communication unit .

In this embodiment, the sensor 14 is an accelerometer coupled to a main bearing of the electrical machine 2. By the sensor 14 , a vibration caused by the appearance of torque ripple in the electrical machine 2 can be measured, since the vibration measurable by the sensor 14 in the main bearing corresponds to a torque ripple of the electrical machine 2. Additionally or alternatively, also an accelerometer coupled to at least one stator plate of the electrical machine 2 and/or a micro phone measuring sound inside the electrical machine 2 and/or a short circuit torque controller can be used as sensor 14.

I f a short circuit appears , for instance, in the first chan nel 3 , the breaker 6 is opened to stop the short circuit cur rent in the first channel 3. The opening of the breaker 6 can be detected and/or commanded for instance by the control unit 13. An operation of the converter unit 5 can be stopped by the control unit 13. The short circuit in channel 3 can occur in the converter unit 5, for instance due to a faulty compo nent in the converter unit 5. It is also possible that the short circuit fault appears in the electrical machine 2 , for instance due to an insulation breakdown inside the electrical machine 2 and/or in the set 9 of stator windings . Depending on the location in which the short circuit fault occurs , a different behaviour regarding a torque ripple of the electri- cal machine 2 occurs . Since the electrical machine 2 is in operation when the short circuit fault occurs , a rotor of the electrical machine 2 is rotating . The rotor comprises a means for generating of a magnetic field, for instance a plurality of permanent magnets , so that after the occurrence of the short circuit fault in the electrical machine 2, a current can be generated in the electrical machine 2 due to the mag netisation of the rotor . In this case, a fault current is flowing in the set 9 of stator windings .

In the case that the short circuit fault appears in the con verter unit 5, no fault current is generated since the con nection between the converter unit 5 and/or the stator wind ings of the set 9 of stator windings is disrupted by opening the breaker 6, and the switching in converter unit 5 is stopped .

The fault current in the electrical machine 2 causes a torque ripple of the torque of the electrical machine 2. Since the fault current in the electrical machine 2 cannot be measured directly, a measurement of the torque ripple can be used as an indirect measurement of the fault current . The torque rip ple can be for instance measured by the sensor 14 , which measures a vibration in the main bearing of the electrical machine 2 , since, if a torque ripple appears or increases in the torque of the electrical machine 2 , also a vibration of the main bearing of the electrical machine 2 can appear or increase, respectively .

In fig. 2 , a graph depicting the torque of the electrical ma chine 2 is shown . On the ordinate, the torque M is plotted, depending on the time t which is plotted on the abscissa . At the time t _f , a short circuit fault occurs in channel 3 of the generator arrangement 1. After a few initial peaks , a torque ripple is set up as discernible in the portion 15 of the curve . The curve in fig. 2 depicts the torque of the electri cal machine 2 in case of a short circuit fault in the con verter unit 5 of channel 3. The average torque after the oc- currence of the short-circuit fault is reduced to approxi mately half of the value prior to the fault .

In fig. 3 , the curve of the torque M of the electrical ma chine 2 for the occurrence of a short circuit fault in the electrical machine 2 is depicted . The short circuit fault ap pears correspondingly at t _f . However, in case of the occur rence of the short circuit fault in the electrical machine 2, the amplitude of the torque ripple in the portion 16 of the curve exhibits a larger amplitude than in the case of the oc currence of the short circuit fault in the converter unit 5 as depicted in fig. 2. Due to the different amplitude of the torque ripple of the torque of the electrical machine 2, a differentiation between the electrical machine 2 and the con verter unit 5 of the first channel 3 as location of the short circuit fault is possible .

In fig. 4 a block diagram of a first embodiment of a method according to the invention is shown . As input variable, a measured signal 17 of a measurand, which describes the torque ripple of the electric machine 2 , is used . The measured sig nal 17 can be measured by the sensor 14 of the electrical ma chine 2. The measurand can be, for instance, an acceleration measured by the sensor 14 at a stationary ring of a main bearing of the electrical machine 2.

In block 18 , a low pass filtering eliminating a DC bias of the measured signal 17 occurs . After the low pass filtering, the measured signal 17 is high pass filtered in block 19 to remove any HF noise . From the filtered measured signal 17 , an absolute value of a ripple in the measured signal 17 corre sponding to an absolute value of the torque ripple amplitude is determined in block 20. In block 21 , after determination of an absolute value of the ripple in the measured signal 17 , a comparison value is determined by averaging the obtained absolute value over a certain interval of time or over an av eraging window, respectively . The comparison value is related to an amplitude of the torque ripple or an amplitude of a ripple in the measured signal 17 corresponding to the torque ripple, respectively .

In block 22 , the comparison value is compared to a threshold value, wherein the threshold value can be a fixed value or can be determined depending on an absolute torque or an aver age torque of the electrical machine 2. The absolute or aver age torque of the electrical machine 2 can be measured by a torque sensor connected to the control unit 13 or it can be calculated and/or estimated by control unit 13 based on cur rents and/or voltages in the electrical machine 2 measured and/or controlled by the control unit 13. If the comparison value exceeds the threshold value, the electrical machine 2 is chosen as location of the short circuit fault . Contrary, if the comparison value is below or equal to the threshold value, the converter unit 4 of the first channel 3 is deter mined as location of the short circuit fault . The functional ity of the blocks 18 to 22 can be implemented in the control unit 13 of the generator arrangement 1.

In fig. 5, a second embodiment of a method according to the invention is depicted . The method can be conducted by the control unit 13 of the generator arrangement 1. As input var iables , again the measured signal 17 measured by the sensor 14 is used . Additionally, a rotor speed signal 23, which is measured by a rotor speed sensor of the electrical machine 2 (not shown) or which is calculated and/or estimated by the control unit 13, is used . In block 24 , an adaptive bandpass filtering of the measured signal 17 occurs . Thereby, a lower cutoff frequency and/or an upper cutoff frequency of the bandpass filtering can be adjusted depending on the rotor speed signal 23. By ad usting the lower cutoff frequency and/or the upper cutoff frequency of the bandpass filtering, a certain harmonic of the rotation of the rotor can be fil tered from the measured signal . It is for instance possible that a second harmonic of the measured signal 17 is filtered . It is also possible that depending on a type of the electri cal machine 2 , a fourth harmonic or a sixth harmonic of the measured signal 17 is filtered. Additionally or alternative ly, also a centre frequency and/or a damping of the bandpass filtering can be adjusted.

In block 25, a root-mean-square is calculated from the meas ured signal 17 as a comparison value . Additionally or alter natively, a root-mean-square value, also the calculation of a peak-to-peak value of the measured signal 17 is possible . The comparison value can be determined depending on the rotor speed signal 23. The determined comparison value corresponds to the torque ripple of the electrical machine 2. The compar ison value determined in block 25 is compared to a threshold value in block 22 as described for the first embodiment of the method . It is possible that the threshold value used for the comparison in block 22 is determined depending on an av erage torque of the electrical machine 2 and/or depending on the rotor speed signal 23. The second embodiment of the meth od can also be implemented in the control unit 13.

In both embodiments of the method discussed in the foregoing, it is possible that, after the determination of the location of the short circuit fault, if the converter unit 5 has been determined as location of the short circuit fault, a restart ing of the generator arrangement 1 using the second channel 4 is performed, in particular if the breaker 8 was opened upon occurrence of the short circuit fault . It is also possible that in the case that only the breaker 6 of the first channel 3 has opened, an operation of the generator arrangement 1 is continued using the second channel 4. However, if the elec trical machine 2 has been determined as location of the short circuit fault, the operation of the generator arrangement 1 is stopped .

The method is conducted correspondingly if the fault occurs in the second channel 4 instead of the first channel 3. In the case of an electrical machine comprising only one chan nel , the location of the short circuit fault can be, for in stance, stored in a storage means of the control unit 13, so that prior to a repair of the generator arrangement 1, the localisation of the short circuit can be used to facilitate the repair . I f the method is applied in a generator arrangement compris ing more than two channels , the operation can be continued using all channels except the faulty channel if the converter unit of the faulty channel was determined as location of the short circuit fault . Correspondingly, if the electrical ma- chine 2 has been determined as location of the short circuit fault, the operation of all channels of the generator ar rangement 1 is prevented or stopped .

Although the present invention has been described in detail with reference to the preferred embodiment, the present in vention is not limited by the disclosed examples from which the skilled person is able to derive other variations without departing from the scope of the invention .