Harmonic direct torque control of an electric machine

Field of invention

The present invention relates to a method and to an arrange ment of generating a converter control signal for a generator side converter portion, in particular of a wind turbine, be ing coupled to a generator. Further, the present invention relates to a power generation system comprising the arrange ment as well as at least one generator and at least one con verter .

Art Background

When operating a generator for example of a wind turbine, in particular a permanent magnet synchronous machine, undesired harmonics of a fundamental electric frequency disturb an ef ficient operation. The prior art discusses harmonic control for vector control drives.

Concerning torque ripple control in control of electrical ma chines with DTC (Direct Torque Control), prior art addresses mainly estimation of electromagnetic torque, aiming to im prove estimation accuracy by means of more complex/accurate machine models and respective torque equations. These presum ably high fidelity models require a large number of parame ters, which depend on load/saturation and rotor position and are difficult to measure/estimate. Therefore, conventional methods apply a number of simplifications for implementation and tuning, for instance, neglecting given parameters and/or load variation in given operating ranges. As feedback source an accelerometer located in the stationary ring of the gener ator main bearing has been used. Conventional harmonic con- trol in FOC (Field Oriented Control) is largely based on Id and Iq currents.

However, it has been observed that the reduction of harmonics is not satisfactory when conventional methods are applied.

Thus, there may be a need for a method and an arrangement of generating a converter control signal for a generator side converter portion, wherein harmonic control is improved. Fur thermore, there may be a need for a method of controlling at least one generator side converter portion in which the con verter control signal is generated according to a method ac cording to an embodiment of the present invention and is sup plied to a respective converter or generator side converter portion .

Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to an embodiment of the present invention it is provided a method of generating a converter control signal for a generator side converter portion, in particular of a wind turbine, being coupled to a generator, in particular a permanent magnet synchronous machine, the method comprising: deriving at least one harmonic torque reference (e.g. Th* ) , in particular based on a harmonic torque demand and/or a torque indicating feedback signal (e.g. Fb) ; deriving at least one harmonic flux reference (e.g. psih*), in particular based on a harmonic stator voltage demand and/or a stator voltage indicating feedback signal (e.g. Vrms); adding all of the at least one harmonic torque reference (e.g. Th* ) to a fundamental torque reference (e.g. Te*) and subtracting an estimated generator torque (e.g. Te ^A ) to derive a torque er ror; adding all of the at least one harmonic flux reference (e.g. psih*) to a fundamental flux reference (e.g. psis*) and subtracting an estimated generator flux (e.g. psis ^A ) to de rive a flux error; and deriving the converter control signal (e.g. Sabc) based on the torque error and the flux error.

The method may be implemented in software and/or hardware.

The method may in particular be performed by a wind turbine controller or in general a converter controller controlling the generator side converter portion which is coupled to the generator .

The converter control signal may for example define a switch ing state of plural controllable switches which are comprised in the generator side converter portion. The converter con trol signal may comprise pulse width modulation (PWM) sig nals. The method may be applicable to any number of harmon ics, one or more i.e. multiples of a fundamental electrical frequency. Thus, the method may be applied in parallel to several different harmonics of a fundamental electric fre quency .

The harmonic torque reference may be derived by a torque rip ple controller (TRC) . The harmonic torque reference may be derived such that when the converter portion is supplied with the respective converter control signal, the torque complies with the harmonic torque demand which may for example be zero for one or more harmonics. The torque indicating feedback signal may comprise for example a sensor signal, such as de rived from an accelerometer or a microphone, or a strain gauge .

The harmonic flux reference may for example be derived by a harmonic voltage controller (HVC) . The harmonic flux refer ence may be derived such that the stator voltage complies with the harmonic stator voltage demand. The flux may relate to the magnetic flux of magnetic fields through the coils of the generator. The flux includes the flux generated by the permanent magnet and the flux generated by the armature reac tion .

The fundamental torque reference may relate to a desired dc torque. The fundamental flux reference may relate to a de sired flux at the fundamental frequency when observed in the stationary reference frame, i.e., stator flux.

The desired torque change as well as the desired flux change may be supplied as inputs to a switching table which may out put a switching state, as an example of the converter control signal. According to an embodiment of the present invention, the converter control signal represents a switching state signal defining a switching state of plural controllable switches of the generator side converter portion.

The switching state signal may define which of the controlla ble switches comprised in the generator side converter por tion are to be in a conducting state or in a non-conducting state. Thereby, conventional control methods for a converter portion are supported. The method may for example support a three-phase permanent magnet generator being coupled to the generator side converter portion. For each phase, the con verter portion may comprise two controllable switches con nected in series.

According to an embodiment of the present invention, the sta tor voltage indicating feedback signal (e.g. Vrms) is derived from a measured DC-link voltage (e.g. Vdc) and the switching state, in particular involving applying an adaptive band pass filter tuned to the harmonic at interest. The reconstructed stator voltage is used in the case when reference voltages (or demand voltages) are not directly available.

A power generation system may comprise, beside the generator and the generator side converter portion also a DC link and a grid side converter portion. The generator side converter portion may substantially be adapted to convert a variable frequency AC power stream to a substantially DC power stream at the DC link. The grid side converter portion may be con- figured for converting the DC power stream to a substantially fixed frequency AC power stream. From the DC link voltage and the switching state, the stator voltage may be estimated. Thereby, a simple stator voltage indicating feedback signal may be provided.

According to an embodiment of the present invention, the torque error and the flux error are supplied to respective hysteresis controllers whose outputs (i.e., the desired torque change and flux change) are supplied to a switching table that outputs the switching state, wherein the switching table in particular outputs the switching state further based on a stator flux position (e.g. 6psis) .

The hysteresis controller may output plus or minus one or plus or minus a constant depending on whether the input ex ceeds a first threshold or is below a second threshold. The hysteresis controllers may also be referred to as a bang-bang controller. Thereby, a simple manner for deriving the switch ing state may be provided.

According to an embodiment of the present invention, the torque error and the flux error (and in particular also total torque and flux references as well as stator voltage and cur rents) are both supplied to a predictive torque control, in particular including model-based prediction and cost function minimization, that derives the switching state.

According to an embodiment of the present invention, the torque error is supplied to a torque controller, and in par ticular also to a harmonic torque controller (e.g. HTC) in parallel, in particular operating in one or more reference frames, which derive a first voltage reference, wherein the flux error is supplied to a flux controller, and in particu lar also to a harmonic flux controller (e.g., HFC) in paral lel, in particular operating in one or more reference frames, which derive a second voltage reference, wherein the convert- er control signal (e.g. Sabc) is derived based on the first voltage reference and the second voltage reference.

As the references for harmonic torque and harmonic flux are normally zero, the torque error and flux error to the harmon ic controllers would be mostly derived from the harmonic feedbacks .

For above both methods (i.e., hysteresis based control and predictive control), the stator voltage indicating feedback signal (e.g. Vrms) is derived from a measured DC-link voltage (e.g. Vdc) and the switching state.

According to an embodiment of the present invention, the first voltage reference and the second voltage reference are both supplied to a dq-c^-transformation module that outputs a total voltage reference (e.g. uab*), wherein the voltage ref erence (e.g. uab*) is supplied to a space vector modulator that derives the switching state signal (e.g. Sabc) . Thereby, conventional methods may be supported and conventional calcu lation modules may be utilized.

According to an embodiment of the present invention, the sta tor voltage indicating feedback signal (e.g. Vrms) is derived from the total voltage reference (e.g. uab*, or udq* ) .

According to an embodiment of the present invention, the es timated generator torque (e.g Te ^A ) and the estimated genera tor flux (e.g. psis ^A ) are derived based on the stator voltage (e.g. vs), in particular reference stator voltage, and stator current (e.g. is), in particular measured.

A variety of flux/torque estimators may be employed, an exam ple is given below:

According to an embodiment of the present invention, the fun damental torque reference (e.g. Te*) and the fundamental flux reference (e.g. psis*) may be derived for example based on the stator voltage, in particular reference stator voltage, and/or stator current, in particular measured, and/or DC-link voltage .

According to an embodiment of the present invention, the torque indicating feedback signal (Fb) comprises a sensor measurement signal, in particular obtained by a torque sensor and/or a microphone, and/or an accelerometer. Thereby, the feedback signal may for example be bandpass-filtered to indi cate the actual torque at the considered harmonic.

According to an embodiment of the present invention, the har monic torque reference (Th*) may alternatively be derived from a look-up table based on an operating point of the gen erator, the operating point in particular defining rotational speed and power output. The operating point may for example be measured by measuring for example the rotational speed and the power output, and/or torque, etc.

According to an embodiment of the present invention it is provided a method of controlling at least one generator side converter portion, in particular of at least one wind tur bine, being coupled to at least one generator, in particular a permanent magnet synchronous machine, the method compris ing: performing a method of generating a converter control signal according to any one of the preceding embodiments; supplying the converter control signal to the generator side converter portion.

Thereby, the converter control signal is utilized for con trolling the generator side converter portion. Thereby, torque and/or voltage ripples of the generator may be effec tively reduced. E.g. harmonic flux may be generated for volt age ripple reduction. It should be understood, that features, individually or in any combination, described, explained, provided or applied to a method of generating a converter control signal for a gen erator side converter portion are also, individually or in any combination, applicable to an arrangement for controlling a generator side converter portion according to an embodiment of the present invention and vice versa.

According to an embodiment of the present invention it is provided an arrangement for controlling a generator side con verter portion, in particular of a wind turbine, being cou pled to a generator, in particular a permanent magnet syn chronous machine, the arrangement being adapted to carry out a method according to any one of the preceding embodiments.

Furthermore, according to an embodiment, it is provided a power generation system, in particular a wind turbine or a wind park, comprising: at least one generator; at least one converter comprising a generator side converter portion, a DC-link and a utility grid converter portion, the generator side converter portion being coupled to the generator; and at least one arrangement according to the preceding embodiment.

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ment but to which the invention is not limited.

Brief Description of the Drawings

Embodiments of the present invention are now described with reference to the accompanying drawings. The invention is not restricted to the illustrated or described embodiments. Fig. 1 schematically illustrates a power generation system, here implemented as a wind turbine, according to an embodi ment of the present invention;

Figs. 2, 3, 4 schematically illustrate an arrangement for controlling a generator side converter portion according to different embodiments of the present invention;

Figs. 5, 6 and 7 illustrate implementations of harmonic con trollers which may be employed in the arrangements illustrat ed in Figs. 2 to 4; and

Fig. 8 illustrates simulation results.

Detailed Description

The illustration in the drawings is in schematic form. It is noted that in different figures, elements similar or identi cal in structure and/or function are provided with the same reference signs or with reference signs, which differ only within the first digit. A description of an element not de scribed in one embodiment may be taken from a description of this element with respect to another embodiment.

Fig. 1 illustrates in a schematic form a wind turbine 100 ac cording to an embodiment of the present invention as example of a power generation system, 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 a gear box 108 to a rotation of a secondary shaft 109, wherein the gear box 108 may be optional. The main shaft 107 or the sec ondary shaft 109 drives a generator 111 which may be in par ticular 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 generator side portion (AC-DC portion) 121, a DC-link 123 and a grid side portion (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.

The converter 119 is controlled via a converter control sig nal 135 which is derived and supplied from an arrangement 150 for controlling a generator side converter portion according to an embodiment of the present invention, which receives at least one input signal 137, such as one or more reference values or one or more quantities indicative of the operation of the generator 111 or any component of the wind turbine 100.

The generator in Fig. 1 comprises a single three-phase stator winding. Thereby, the winding 113 carries the stator current la, the winding 115 carries the stator current lb and the winding 117 carries the stator current I _c .

The arrangement 150 is adapted to counteract torque and volt age harmonics (for example a harmonic corresponding to six times the electrical frequency of the generator 111) . The generator 111, the converter 119 and the arrangement 150 to gether form a generator system according to an embodiment of the present invention.

The arrangement 150 for controlling a generator side convert er portion as included in the wind turbine power generation system 100 illustrated in Fig. 1 receives inputs signals 137 which may relate to stator voltage, stator current, may re late to a feedback signal, may relate to an operating point, as will be detailed with reference to Figs. 2 to 4. The con trol signal 135 in particular is a converter control signal for controlling the generator side converter portion 121 of the converter 119. Fig. 2 schematically illustrates an arrangement 250 for con trolling a generator side converter portion and Figs. 3 and 4 illustrate respective embodiments 350, 450 of the arrangement for controlling a generator side converter portion.

The inputs to the arrangement 250 are collectively denoted with reference sign 237. The arrangement 250 comprises a torque ripple controller (TRC) 239 which receives as inputs a feedback signal 241 and/or an optional signal 243 indicating the operating point. The torque ripple controller 239 derives therefrom a harmonic torque reference 245 (Th*) .

The arrangement 250 further comprises a harmonic voltage con troller (HVC) 247 which receives as input a stator voltage indicating feedback signal 249 and derives therefrom a har monic flux reference 251 (yϊi*) .

The torque ripple controller 239 and the harmonic voltage controller 247 operate on a particular harmonic of a funda mental frequency. If more than one harmonic of the fundamen tal frequency are to be treated, for each harmonic, a respec tive torque ripple controller 239 and a respective harmonic voltage controller 247 may be provided. In case of several harmonics to be treated, all of the harmonic torque refer ences 245 are added at an addition element 254 to a fundamen tal torque reference 253 (Te*) and an estimated generator torque 255 is subtracted to derive a torque error 257 . All of the harmonic flux references 251 are added at an addition element 254 to a fundamental flux reference 259 (ye*) and an estimated generator flux 261 is subtracted to derive a flux error 263. The desired flux change difr and desired torque change dTe are obtained as output of the hysteresis control lers 268, 269.

A converter control signal 265 is derived based on the de sired torque change dTe and the desired flux change difr.

Thereby, the converter control signal 265 represents a switching state signal defining a switching state of plural controllable switches of the generator side converter portion 121 (illustrated in Fig. 1) .

The stator voltage indicating feedback signal 249 is derived from a measured DC link voltage Vdc and the switching states 265 using a calculation module 267. The calculated stator voltage may be bandpass-filtered to eliminate harmonics dif ^¬ ferent from the harmonic the harmonic voltage controller 247 is working on. According to the embodiment 250 illustrated in Fig. 2, the torque error 257 and the flux error 263 are sup ^¬ plied to respective hysteresis controllers 268, 269 whose outputs, the desired flux change difr and desired torque change dTe, are supplied to a switching table 271 that outputs the switching state 265. Furthermore, the switching table 271 re ^¬ ceives the stator flux position Qye (273) and derives the switching states 265 also based on the stator flux position 273. The estimated generator torque 255 and the estimated generator flux 261 are derived by a torque and flux estimator 275 based on the stator voltage vs, in particular reference stator voltage, and stator current is.

The fundamental torque reference 253 and the fundamental flux reference 259 are derived by a fundamental torque/flux refer ^¬ ence calculation module 277 based on one or several of the following quantities: stator voltage 'vs', stator current 'is' and DC link voltage Vdc. As can be taken from Fig. 2, the harmonic torque reference 245 (Th*) is derived by the torque ripple controller 239 further based on an operating point 243 of the generator 111.

The arrangements 350, 450 illustrated in Figs. 3 and 4 com ^¬ prise modules similar or identical to modules of the arrange ^¬ ment 250 illustrated in Fig. 2 which modules are labelled with reference signs differing only in the first digit. How ^¬ ever, the arrangement 350 for controlling a generator side converter portion according to an embodiment of the present invention does not comprise the hysteresis controllers 268, 269 and the switching table 271 but instead comprises a pre dictive torque controller 377 which receives the torque error 357 and the flux error 363 and derives therefrom the convert er control signal 365.

Instead of the predictive torque control 377 or the hystere sis controllers 268, 269 and the switching table 271, the ar rangement 450 illustrated in Fig. 4 comprises a harmonic torque controller 479 and a fundamental torque controller 481 to which the torque error 457 is supplied and the outputs are added together using the addition element 480 to derive a first voltage reference 483. Furthermore, the arrangement 450 comprises a fundamental flux controller 485 and a harmonic flux controller 487 to which the flux error 463 is supplied and whose outputs are added by an addition element 480 to ar rive at a second voltage reference 489. The first voltage reference 483 and the second voltage reference 489 are sup plied to a dq-c^-transformation module 491 that outputs a to tal voltage reference 492 uab* . The total voltage reference 492 is supplied to a space vector modulation module 493 which derives therefrom the switching state 465. As can be taken from Fig. 4, the stator voltage indicating feedback signal 449 is derived by the module 467 based on the total voltage reference .

The harmonic torque controller 479 in Fig. 4 and the torque ripple controller 239, 339, 439 in Figs. 2, 3, 4 are distinct controllers, but they may have the same structure (such as any of Figs. 5-7) in case suitable feedback signal are avail able. However, in case feedback signal is not available for torque ripple controller, it can be a look-up table having the operating point (OP) as input.

The torque controller 481 and the flux controller 485 in Fig. 4) may be PI controllers for fundamental torque and flux. The converter control signal in Fig. 4 is derived based on the inputs to the transformation module dq/ b by Inverse Park transformation plus voltage modulator.

High performance harmonic control in an electric drive is im portant as this is a requirement for permanent magnet genera tors for several reasons: (1) meet noise standards; (2) pre vent excitation of structural modes and accelerated fatigue; (3) optimized DC link voltage usage and system efficiency.

The torque ripple controller (TRC) illustrated in Figs. 2, 3 and 4 generates the harmonic torque reference (Th*) which is added to the fundamental torque reference (Te*), modifying the reference torque to contain the errors in the estimated torque at the harmonic frequencies of concern. The harmonic voltage controller (HVC) illustrated in Figs. 2, 3 and 4 gen erates a harmonic flux reference to minimize the respective harmonic voltage.

Direct torque control merits are expected to be retained for the TRC and HVC, namely fast dynamics due to a decoupled con trol of torque and flux/voltage . Moreover, the simplicity of implementation of the LUT-based approach is obvious from Fig. 2, since the inner flux and torque controller (hysteresis controllers 268, 269) provide high bandwidth control without the need for additional controllers in parallel.

The arrangement 350 illustrated in Fig. 3 is another attrac tive solution including the predictive torque control (PTC) method which is endowed with high bandwidth flux and torque control like the LUT-based DTC (inner loop composed of blocks for model-based prediction and cost function minimization) .

In Fig. 4, the TRC and HVC are integrated into the direct torque control with space vector modulation. As opposed to the embodiments illustrated in Figs. 2 and 3, in Fig. 4, voltage references are readily available at the modulator in put, namely at the signal 492, but the lower bandwidth of torque and flux controllers may require additional parallel controllers (HTC and HFC in Fig. 4) for achieving high per- formance reference tracking of harmonic content and zero steady-state errors. Some of the approaches may be resonant controllers and PI controllers in multiple reference frames.

The calculation of Vrms for the illustrated control method is as follows: Vrms = sqrt ( Vd ^A 2 + Vq ^A 2) or Vrms = sqrt ( VcA2 + nb ^L 2) . Vdq are readily available (sum of torque/flux control lers' outputs) in DTC-SVM, whereas for DTC-LUT voltages are reconstructed using switching states (Sabc) and dc-link volt age (Vdc) :

The harmonic of interest is extracted from Vrms by means of an adaptive BPF at the selected harmonic. It is emphasized that embodiments focus on the outer harmonic control loops shown in Figs. 2, 3, 4 as TRC and HVC, which modify the ref erence torque and flux required by any DTC based control method. Accordingly, the different examples presented in Figs. 2, 3, 4 (LUT, predictive, SVM) are given for the sake of completeness. However, fast inner control loops may be achieved in other manners.

Figs. 5, 6 and 7 illustrate harmonic controllers as implemen tations of the control blocks HVC, TRC, HTC, HFC illustrated in Figs. 2, 3 and 4.

In particular, the torque ripple controllers 239, 339, 439 illustrated in Figs. 2, 3, 4 may receive a harmonic torque demand signal 240, 340, 440, respectively, representing the desired torque at the harmonic at consideration. This harmon ic torque demand may for example be zero. Similarly, the har- monic voltage controller may receive as an input a harmonic voltage demand signal 246, 346, 446 in Figs. 2, 3, 4, respec tively. This signal may represent a harmonic voltage demand and this may be zero according to different applications.

In all the implementations of the controller illustrated in Figs. 5, 6 and 7, a harmonic error 504 is calculated from the demand harmonic value 502 and a feedback signal 503. In the implementation illustrated in Fig. 5, the error 504 is 90° shifted by a 90° phase-shifter 506 and the output signal is supplied to a frame transformation module 508. The frame transformation module 508 transforms the error and the 90° shifted error according to a coordinate system rotating with the considered harmonic. The output is supplied to PI regula tors 510 which derive an output signal such that the error reduces to zero. Downstream the regulators 510, a further frame transformation module is arranged which back-transforms to obtain harmonic reference 545.

In Fig. 6, the harmonic error 604 is supplied to trigonomet ric function 612, 614 and the output is again supplied to PI regulators 610 whose output is multiplied with another trigo nometric functions 616, 618 and added together to obtain the harmonic reference 645.

In the implementation illustrated in Fig. 7, the harmonic er ror 704 is supplied to a resonant regulator 714, to obtain the harmonic reference 745.

Control blocks HVC, TRC, HTC, HFC in Figs. 2, 3, 4 can all be implemented similarly and are next called harmonic control lers. Different options of harmonic controllers are shown in Fig. 5, 6, 7. In all options, the harmonic error is calculat ed and a close loop regulation is made. Thus, the regulators are used to control harmonic error to 0. The harmonic regula tor based on vector control principle is shown in Figs. 5, 6, enabling the implementation of simple PI controllers. Fig. 7 shows the harmonic regulator using resonant regulator with a typical transfer function as below, where fl is the resonant frequency and xΐ is the damping of the controller. The block diagrams shown in Figs. 5, 6, 7 assume that adaptive band pass filters (BPF) are applied to reference/ feedback signals whenever needed and therefore only the harmonic order of in terest is given to the controller input. Alternatively, BPFs may be implemented in the harmonic error (Vn_error) in Fig. 5, 6 and 7.

Application to PM machines may allow to reduced noise and vi bration and also to optimize hardware utilization. Noise and vibration reduction by means of minimization of torque ripple may be straight forward to understand.

On the other hand, increasing hardware usage by means of con trol of harmonic stator voltage/flux may not be so obvious. The latter is explained by the fact that in the presence of non-negligible voltage harmonics, a reduced (average) flux reference may need to be set to avoid converter over

modulation, resulting in entering the flux-weakening region at a lower speed level and operating with increased phase currents. The introduction of HVC enables the following pos sibilities: (1) increase of average stator flux reference; or (2) reduction of dc-link voltage. In other words, HVC allows to extend the operating range on MTPA (Maximum Torque Per Am pere) , narrowing the flux weakening range, and therefore op timizing drive efficiency.

One of the main advantages of the proposed control structure and respective feedback signals may be that the dependence on the accuracy of flux and torque estimators is eliminated with regards to harmonic control. It is to be noted that the input of TRC may not be limited to an accelerometer, and other signals such as microphone and different sensors may be options. Moreover, the closed-loop harmonic controller TRC may be replaced by a simple LUT with operating point (OP) information as input such as speed and torque, providing a cheap feed-forward solution.

Fig. 8 illustrates simulation results of the modified space vector modulator based direct torque control as is illustrat ed in Fig. 4. The control of the PM generator is accomplished by implementing a DC link voltage controller for calculation of the fundamental torque reference (Te*) and a flux weaken ing control for calculation of the fundamental stator flux reference (also called fundamental voltage controller, since it targets to keep the generator voltage below a given lim it) . Regarding harmonic control, measured or inferred torque ripple is used as feedback signal together with the control ler in Fig. 5 for the torque ripple controller and the har monic voltage control uses voltage ripple in Vrms as input and a controller structure as in Fig. 5.

The abscissas 816 denote the time and the ordinates 818 the strength of the signal. The curve 820 illustrates the feed back torque, the curve 822 illustrates the estimated torque, the curve 824 depicts the modulation index and the curve 826 indicates the estimated flux. The torque ripple controller is enabled at the time point t=3 s, i.e. at the time point 828. Thus, it results in an effective reduction of the measured torque ripple (remaining oscillations are at a non-controlled lower harmonic frequency) , whereas a sixth harmonic is im posed in the estimated/referenced torque. The harmonic volt age control is enabled at the time point 830, i.e. at t=5 s, reducing the voltage ripple by imposing a sixth harmonic in the reference flux. Accordingly, the converter operates fur ther from the flux weakening and/or overmodulation regions. According to an embodiment of the present invention, the fun damental references for the current/torque/ flux are calculat ed by means of controllers or look-up tables, some examples are speed, power, torque, flux, voltage controllers and maxi mum torque per ampere methods. Such controllers provide Te* and ye* which are usually DC signals during steady state op eration .

The torque ripple controller targets to control torque ripple by using a suitable sensor as feedback signal and generating a reference harmonic torque Th* which is a sinusoidal signal varying at a given frequency or a combination of sinusoidal signals with different frequencies. It may be composed of a variety of controllers (PI, search algorithms, etc.) and/or LUTs .

The harmonic voltage controller targets to control voltage ripple by using the modulus of the reference voltage (Vrms) as feedback and generating a reference harmonic flux i|rh* which is a sinusoidal signal varying at given frequencies or a combination of sinusoidal signals with different frequen cies. It may be composed of a variety of controllers.

The torque/flux controller may comprise PI controllers with given bandwidth, aiming to track DC content of torque and flux references.

The harmonic torque/flux controller (HTC and HFC) may be con trollers implemented by means of PI controllers in the har monic reference frame, proportional-resonant controllers or any other suitable methods.

Park transformation may transform between stationary frame (ab) and synchronous rotating frame (dq) and vice versa. Sim ilar techniques may be applied for transformations between synchronous rotating frame and harmonic reference frame. Voltage modulator uses the reference voltages in the station ary frame for generating PWM (pulse width modulation) pat tern. The PWM signals are used to control power electronic switches such as IGBT in the generator side converter por tion. Hysteresis (bang-bang controller) may be considered as controllers used to define LUT entries in DTC-LUT, 2 and/or 3 level controllers are typically used. Controllers output de termine if torque/flux is to increase, decrease or remain un changed .

Switching table determines the optimized voltage according to the desired action stated at the LUT entries by the hystere sis controllers. Stator flux angle is also an entry of the LUT, defining a given number of sectors.

Torque and flux estimators may employ machine models together with current measurements to estimate electromagnetic torque and stator flux. Reference voltages are typically used in stead of measured voltages. A variety of models and observer structures may be used for estimation purposes.

Embodiments of the present invention may provide a control method for incorporating harmonic control capability in di rect torque control drives. The harmonic control may include torque ripple control and voltage ripple control. A control method for improving harmonic control performance in electri cal drives may be provided. The harmonic control may have little dependency on the accuracy of torque ripple and flux linkage ripple estimation and thus may have a good robust ness. A control method may be well suited for the control of the permanent magnet generators for wind turbines which may provide an alternative to the more commonly employed vector control methods. Embodiments of the present invention may re duce noise and vibration and increasing voltage control range and drive efficiency. A control method suitable for implemen tation in the controller of a frequency converter may be pro vided . It should be noted that the term "comprising" does not ex clude other elements or steps and "a" or "an" does not ex clude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con strued as limiting the scope of the claims.