TORQUE RIPPLE REDUCTION FOR A GENERATOR

Abstract

It is provided a method of controlling a generator to reduce a harmonic torque ripple, the method including: measuring a first value of an acceleration using a first accelerometer mounted at a first position of the generator; measuring a second value of an acceleration using a second accelerometer mounted at a second position of the generator; deriving a vibration signal based on a combination of the first value and the second value of the acceleration; deriving, based on the vibration signal, an amplitude and a phase of a reference harmonic current; injecting a current into the generator based on the reference harmonic current.

Claims

1. A method of controlling a generator to reduce a harmonic torque ripple, the method comprising: measuring a first value of an acceleration using a first accelerometer mounted at a first position of the generator; measuring a second value of an acceleration using a second accelerometer mounted at a second position of the generator; deriving a vibration signal based on a combination of the first value and the second value of the acceleration; deriving, based on the vibration signal, an amplitude and a phase of a reference harmonic current; and injecting a current into the generator) based on the reference harmonic current.

2. The method according to claim 1, wherein the vibration signal is based on a sum of the first value and the second value of the acceleration.

3. The method according to claim 1, wherein the first value of the acceleration and the second value of the acceleration relate to an acceleration in a circumferential direction of the generator.

4. The method according to claim 1, wherein the first position and the second position have a same radial position and a same circumferential position, but different axial positions, such that the first position and the second position are mirror symmetrically arranged.

5. The method according to claim 1, wherein the first accelerometer is mounted at a first stator plate and the second accelerometer is mounted at a second stator plate, the first stator plate and the second stator plate delimiting a stator towards an environment, further wherein the first stator plate and the second stator plate are annular flat plates.

6. The method according to claim 1, wherein the generator comprises a rotor having permanent magnets mounted thereon arranged in at least two rings in different axial positions being skewed relative to each other in a circumferential direction.

7. The method according to claim 1, further comprising: determining a value of an operating point of the generator; deriving, based on the vibration signal and the value of the operating point, the amplitude and the phase of the reference harmonic current;

8. The method according to claim 7, wherein the value of the operating point is determined based on a measured fundamental torque and a measured rotational speed of the generator or other measurements.

9. The method according to claim 1, wherein deriving the amplitude and the phase of the reference harmonic current comprises: filtering the vibration signal thereby reducing components of the vibration signal other than a particular harmonic to obtain a filtered vibration signal; time averaging an RMS value of the filtered vibration signal; looking up an initial amplitude and an initial phase associated with the value of the operating point from a storage; performing an optimization of the amplitude and phase based on the initial amplitude and the initial phase so that the vibration signal is reduced; and storing, associated with the value of the operating point, the optimized amplitude and optimized phase in a storage.

10. The method according to claim 1, wherein injecting the current into the generator comprises: determining a reference harmonic voltage based on the reference harmonic current and an actual current in at least one stator winding, each of the reference harmonic current and the actual current being represented by components in a dq-coordinate system; forming a sum of the reference harmonic voltage and a reference fundamental voltage; and supplying the sum as reference voltage to a control input of a converter having power input terminals connected to power output terminals of the generator.

11. The method according to claim 1, further comprising: determining the reference fundamental voltage based on the actual current in at least one stator winding of the generator and a reference fundamental current.

12. An arrangement for controlling a generator to reduce a harmonic torque ripple, the arrangement comprising: a first accelerometer mountable at a first position of the generator and adapted to measure a first value of an acceleration; a second accelerometer mountable at a second position of the generator and adapted to measure a second value of an acceleration; a processor adapted: to derive a vibration signal based on a combination of the first value and the second value of the acceleration, and to derive, based on the vibration signal, an amplitude and a phase of a reference harmonic current; and a driver adapted to inject a current into the generator based on the reference harmonic current.

13. A wind turbine, comprising: a shaft with rotor blades connected thereto; a generator mechanically coupled with the shaft; and an arrangement according to claim 1, the driver being configured as a converter that is controlled based on the reference harmonic current.

Description

BRIEF DESCRIPTION

[0046] Some of the embodiments will be described in detail, with references to the following Figures, wherein like designations denote like members, wherein:

[0047] FIG. 1 schematically illustrates a wind turbine according to an embodiment of the present invention including an arrangement for controlling a generator according to an embodiment of the present invention;

[0048] FIG. 2 schematically illustrates an arrangement for controlling a generator according to an embodiment of the present invention;

[0049] FIG. 3 schematically illustrates a generator as may be comprised in the wind turbine illustrated in FIG. 1 according to an embodiment of the present invention;

[0050] FIG. 4 schematically illustrates another generator which may be comprised in a wind turbine according to an embodiment of the present invention;

[0051] FIG. 5 illustrates a first graph for illustrating embodiments of a control method;

[0052] FIG. 6 illustrates a second graph for illustrating embodiments of a control method;

[0053] FIG. 7 illustrates a third graph for illustrating embodiments of a control method;

[0054] FIG. 8 illustrates a third graph for illustrating embodiments of a control method;

[0055] FIG. 9 illustrates a first graph for explaining embodiments of the present invention;

[0056] FIG. 10 illustrates a second graph for explaining embodiments of the present invention;

[0057] FIG. 11 illustrates a third graph for explaining embodiments of the present invention; and

[0058] FIG. 12 illustrates a fourth graph for explaining embodiments of the present invention.

DETAILED DESCRIPTION

[0059] The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit.

[0060] FIG. 1 illustrates in a schematic form a wind turbine 100 which provides electric energy to a utility grid 101. The wind turbine comprises a hub 103 to which plural rotor blades 105 are connected. The hub is mechanically connected to a main shaft 107 whose rotation is transformed by an optional gear box 108 to a rotation of a secondary shaft 109, wherein the gear box 108 may be optional in which case the wind turbine may be a direct drive wind turbine. The main shaft 107 or the secondary shaft 109 drives a generator 111 which may be in particular a synchronous permanent magnet generator providing a power stream in the three phases or windings 113, 115 and 117 to a converter 119 which comprises a AC-DC portion 121, a DC-link 123 and a DC-AC portion 125 for transforming a variable AC power stream to a fixed frequency AC power stream which is provided in three phases or windings 127, 129, 131 to a wind turbine transformer 133 which transforms the output voltage to a higher voltage for transmission to the utility grid 101.

[0061] The converter 119 is controlled via a converter command 135 (Vdref, Vqref) which is derived and supplied from a control arrangement 150 according to an embodiment of the present invention, which receives at least one input signal 137, such as including at least a vibration signal and optionally including stator winding currents and/or one or more reference values and/or one or more quantities indicative of the operation of the generator 111 or any component of the wind turbine 100.

[0062] The generator in FIG. 1 comprises a single three-phase stator winding or multiple three-phase stator windings. Thereby, the winding 113 carries the stator current I.sub.a, the winding 115 carries the stator current I.sub.b and the winding 117 carries the stator current I.sub.c. The control arrangement 150 controls the converter 119.

[0063] FIG. 2 schematically illustrates an arrangement 250 for controlling a generator, for example the generator 111 as illustrated in FIG. 1, to reduce a harmonic torque ripple according to an embodiment of the present invention.

[0064] The arrangement 250 illustrated in FIG. 2 comprises an input port 241 for receiving a vibration signal 243 indicating a measured mechanic vibration of the generator, for example generator 111 illustrated in FIG. 1.

[0065] Furthermore, the arrangement 250 comprises a processor 245 which is adapted to determine a value 247 of an operating point of the generator, for example represented by the two values (T.sub.n, .sub.m), one of a plurality of predetermined fundamental torques and one of a plurality of predetermined rotational speeds of the generator.

[0066] The processor 245 is further adapted to derive, based on the vibration signal 243 and optionally also based on the value 247 of the operating point, an amplitude 251 (e.g. A.sub.q6f for a 6.sup.th harmonic of the basic or fundamental frequency 0 and a phase 253 (for example .sub.q6f, for the 6.sup.th harmonic of the fundamental frequency) of a reference harmonic current (for example I.sub.q6f for a reference current of a 6.sup.th harmonic), wherein the harmonic current is also indicated by reference sign 255.

[0067] Further, the arrangement 250 comprises a driver 257 (e.g. configured as converter 119 in FIG. 1) which is adapted to inject a current into the generator based on the reference harmonic current (I.sub.qref), Idref being in particular set to zero.

[0068] For performing these functions, the arrangement 250 comprises an auto-tuning controller 259 which receives the vibration signal 243 as well as the value 247 of the load point and further receives an enable signal 249 which is derived by a load point detection module 261 which derives the value of the load point 247 based on the torque T.sub.g of the generator and the electrical frequency .sub.e of the electric generator.

[0069] The reference harmonic current is labelled in FIG. 2 also as Iqref, i.e. a harmonic current reference or a q-component of a harmonic current reference.

[0070] The arrangement 250 further comprises a harmonic current regulator 263 which receives (e.g. a representation of) the reference harmonic current (Iqref) as well as the d-component Idref of the reference harmonic current which is usually zero. Furthermore, the harmonic current regulator 263 receives the harmonic currents Id, Iq (derived e.g. from Ia, Ib, Ic by Transformation into the dq-system) of at least one set of stator windings of the generator, such as generator 111. The d-component and the q-component of the stator current are for example derived based on the three phase currents Ia, Ib, Ic by performing a park transformation.

[0071] The harmonic current regulator 263 comprises circuitry to derive from the input values a reference harmonic voltage Vdac, Vqac, i.e. components in the d/q-coordinate system which are supplied to addition elements 265. Using the addition elements 265, a sum of a reference fundamental voltage Vddc, Vqdc with the reference harmonic voltage Vdac, Vqac is calculated and output as a reference voltage Vdref, Vqref which is supplied to the driver 257, which may for example be configured as a converter.

[0072] FIG. 2 shows the torque ripple controller 244 which is used in the direct drive permanent magnet synchronous generator. The harmonic current references on the d- and q-axes are given into the harmonic current regulator for minimizing the torque ripple in the generator. In the harmonic current reference calculation module, the harmonic current reference on the d-axis is e.g. set as 0; the harmonic current reference on the q-axis is given as a harmonic sinusoidal signal, the amplitude and phase angle of this signal are both obtained by the auto tuning controller.

[0073] In FIG. 2, the load point detection module 261 will give the enable/disable signal and the load point information (Tn, m) to the auto tuning controller. The scheme of load point detection can be expressed as:

[00001]$\{\begin{array}{c}\left|T-T_1\right|.Math..Math.T\\ \left|-{}_1\right|\end{array}.fwdarw.\mathrm{Enable}\&.Math..Math.\left(T_1,{}_1\right).Math..Math.\{\begin{array}{c}\left|T-T_1\right|.Math..Math.T\\ \left|-{}_2\right|\end{array}.fwdarw.\mathrm{Enable}\&.Math..Math.\left(T_1,{}_2\right).Math..Math.\mathrm{.Math.}.Math..Math.\{\begin{array}{c}\left|T-T_1\right|.Math..Math.T\\ \left|-{}_m\right|\end{array}.fwdarw.\mathrm{Enable}\&.Math..Math.\left(T_1,{}_m\right).Math..Math.\mathrm{.Math.}.Math..Math.\{\begin{array}{c}\left|T-T_n\right|.Math..Math.T\\ \left|-{}_m\right|\end{array}.fwdarw.\mathrm{Enable}\&.Math..Math.\left(T_n,{}_m\right)$

[0074] The arrangement 250 comprises the harmonic current reference calculation module or processor 242 which harbours the auto-tuning controller 259 and the limitation element 252.

[0075] The arrangement 250 further comprises a fundamental current regulator 267 which receives the stator winding currents Id, Iq as well as fundamental current references Iddcref, Iqdcref based on which the fundamental voltage references Vddc, Vqdc are calculated.

[0076] The vibration signal 243 illustrated in FIG. 2 is derived based on a combination (derived by combination module 246) of a first value 240 of an acceleration measurement and a second value 244 of an acceleration measurement. Thereby, the first value 240 is measured by a first accelerometer 271 and the second value 244 of the acceleration is measured by a second accelerometer 273, which are both mounted on a generator, as is schematically illustrated in FIG. 3.

[0077] Thereby, FIG. 3 schematically illustrates a generator 311 which may be used as a generator 111 in the wind turbine 100 as illustrated in FIG. 1. For ease of illustration, only a stator 275 is illustrated, a not illustrated rotor would rotate around a rotation axis 277 also defining the axial direction. The radial direction 279 (y) is perpendicular to the axial direction (x) and also the circumferential direction (z) is perpendicular to the radial direction (y) and the axial direction (x). The stator 275 comprises a not illustrated stator yoke comprising teeth and slots which are spaced apart in the circumferential direction z. Around the teeth, plural not illustrated conductor windings are arranged.

[0078] At axial end faces, the generator 275 is covered with a first stator plate 281 and a second stator plate 283, respectively, which represent end plates of a generator housing. The first accelerometer 271 is fixed and mounted at the first stator plate 281 at a radial position r1 and a circumferential direction 1, while the second accelerometer 273 is mounted at the second stator plate 283 at a radial position r2 and at a circumferential position 2, wherein r1=r2 and 1=2. However, the two accelerometers 271, 273 are mounted at two different axial positions a1 and a2 which are measured along the axial direction x or 277.

[0079] According to this embodiment of the present invention, the acoustic noise signal for some specific harmonics (e.g. 6f, 12f) is represented by using vibration sensors (for example accelerometers) on the generator stator plates 281 and 283. At least two accelerometers 271, 273 are needed and mounted on two stator plates, respectively. The accelerometers need to be mounted on each stator plate 281, 283 in a mirror-symmetric manner, wherein a mirror plane 285 is perpendicular to the axial direction x or 277 and is arranged in the center (e.g. at (a2-a1)/2) between the stator plates 281 and 283.

[0080] It is in particular useful to employ at least two accelerometers in the case, that a direct drive permanent magnet generator employs a rotor-skewing design, as is exemplary illustrated in FIG. 4. Thereby, FIG. 4 illustrates a generator 411 which may be comprised in the wind turbine 100 as illustrated in FIG. 1 wherein additionally to a stator 475, also a rotor 487 is schematically illustrated. The rotor 487 comprises plural rings 489a, . . . , 489g of permanent magnets which are skewed relative to each other in the circumferential direction. While for example the ring 489a (no. 1) is at a circumferential direction =0, the rings 489b, 489g (no. 2, 3, 4, 5, 6 and 7) are skewed relative to the first ring by equal circumferential angle offsets such that ring 489g (no. 7) is offset by an angle . This rotor skewing design may reduce a cogging torque.

[0081] According to the design illustrated in FIG. 4, injection of harmonic current may have a different effect for different permanent magnets, i.e. different rings 489a, . . . , 489g which are skewed relative to each other. For example, when the 6f harmonic stator magnetic field is reducing the torque ripple from the ring 489a (no. 1), it may increase the torque ripple causing by the ring 489g (no. 7) at the same time. Therefore, the torque ripples and vibrations on stator plates 481, 483 will be different. By using the two symmetrically arranged accelerometers 471 and 473 mounted at the first stator plate 481 and the second stator plate 483, respectively, and adding the tangential direction (circumferential direction z) acceleration signals together, an overall torque ripple of the generator may be described. Moreover, this overall torque ripple signal may match the IEC location (international standard IEC61400-11) noise signal.

[0082] Embodiments of the present invention may provide an accurate and reliable feedback solution for the wind generator torque ripple and noise control. No turbine individually tuning for the torque ripple controller may be required.

[0083] The accelerometer may give a faster and more stable signal response compared to a microphone sound detection signal. The accelerometer signal may be friendlier to the turbine controller.

[0084] Two accelerometers in a symmetrical position may describe an overall torque direction vibration which may cause the noise. Varying torque ripple due to the rotor-skewing design may properly be solved.

[0085] IEC position acoustic noise may be monitored in a real-time by using embodiments of the present invention.

[0086] The FIGS. 5 to 8 illustrate graphs in coordinate systems, wherein abscissas 1 denote the time, while the ordinates 3 denote amplitudes of a 6f harmonic vibration and noise (in FIG. 5). Thereby, the first column 5 relates to the case wherein no 6f current injection is performed, the column 7 (A and ) relate to the case, where the optimal amplitude and the optimal phase of the 6f current is injected, the column 9 (A+20) relates to the case, where a 6f current is injected which has the optimal amplitude increased by 20, the column 11 (A20) relates to the case, wherein a harmonic current is injected having the optimal amplitude reduced by 20, the column 13 (A and ) relates to the case where the optimal amplitude and optimal phase is injected, the column 15 (+20) relates to the case, where the optimal amplitude but the phase shifted by +20 relative to the optimal phase is injected and the column 17 (20) relates to the case, where the current having an optimal amplitude but having a phase which is reduced by 20 relative to the optimal phase is injected.

[0087] The trace 19 in FIG. 5 indicates a microphone derived signal for the different cases of harmonic current injection. As can be appreciated from FIG. 5, when the optimal amplitude in an optimum phase is injected (see column 7 (A and )), the resulting vibration is minimal, while the vibration increases when non-optimal values are used for injecting the harmonic current.

[0088] FIG. 6 illustrates the first value 21 of the acceleration as measured by the first accelerometer 271 and FIG. 7 illustrates the second value 23 of the acceleration, as is measured by the second accelerometer 273 (see FIG. 3). As can be seen in FIG. 6, also the non-optimal values defining the harmonic current injection in columns 9 and 17 (indicated by arrow 24) result in a relatively low vibration signal, although not the optimal current injection is performed.

[0089] Further, also FIG. 7 shows a relatively low vibration signal, when the non-optimal harmonic current is injected, as is indicated by arrows 25.

[0090] FIG. 8 now illustrates the detected vibration, when the first value 21 and the second value 23 are combined to result in a combination value 27 which may be the sum or the average of the first signal 21 and the second signal 23. As can be appreciated from FIG. 8, only for the columns 7 and 13, i.e. the optimal harmonic current injection, the derived vibration signal 27 is minimal, while for the non-optimal harmonic current injection, the vibration signal is considerably larger. Thereby, an effective damping of a particular harmonic or a number of particular harmonics may be achieved, when the harmonic current is calculated based on combination signal 27.

[0091] FIGS. 9 to 12 illustrate a further example how the first value 21 and the second value 23 of the measured acceleration can be combined to result in a combined value of vibration signal 27 (for example the sum or the average of the first signal and the second signal 21, 23), for effectively representing the vibration. Thereby, the columns AX denote injection of a current having an amplitude which is reduced by X relative to the optimal amplitude, the columns A+X represent the vibrations when a harmonic current is injected having an amplitude which is by X larger than the optimal amplitude. Analogous are the denomination of the injection having non-optimal phase .

[0092] FIG. 9 shows the trace 19 as obtained using a microphone, FIG. 10 shows the trace 21 of the first value of the acceleration, as obtained by the first accelerometer (e.g. 271 or 471 in FIGS. 3 and 4, respectively) and FIG. 11 shows the trace 23 of the second value of the acceleration, as obtained by the second accelerometer (e.g. 273 or 473 in FIGS. 3 and 4, respectively).

[0093] As can be taken from FIG. 12, the vibration signal 27 as derived by the combination of the first value 21 of the acceleration and the second value 23 of the acceleration have the minimum (indicated by an arrow 26) at those columns, which represent injection of the optimal harmonic current.

[0094] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0095] For the sake of clarity, it is to be understood that the use of a or an throughout this application does not exclude a plurality, and comprising does not exclude other steps or elements.