Patent Description:
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 efficient operation. The prior art discusses harmonic control for vector control drives, see for example <NPL>.

Concerning torque ripple control in control of electrical machines with DTC (Direct Torque Control), prior art addresses mainly estimation of electromagnetic torque, aiming to improve estimation accuracy by means of more complex/accurate machine models and respective torque equations. These presumably high fidelity models require a large number of parameters, 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 generator main bearing has been used. Conventional harmonic control 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. Furthermore, there may be a need for a method of controlling at least one generator side converter portion in which the converter control signal is generated according to a method according to an embodiment of the present invention and is supplied to a respective converter or generator side converter portion.

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^) to derive a torque error; 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^) to derive 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 switching state of plural controllable switches which are comprised in the generator side converter portion. The converter control signal may comprise pulse width modulation (PWM) signals. The method may be applicable to any number of harmonics, 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 frequency.

The harmonic torque reference may be derived by a torque ripple 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 derived 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 reference 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 reaction.

The fundamental torque reference may relate to a desired dc torque. The fundamental flux reference may relate to a desired 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 output 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 controllable switches comprised in the generator side converter portion 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 converter portion may comprise two controllable switches connected in series.

According to an embodiment of the present invention, the stator 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 configured 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. θpsis).

The hysteresis controller may output plus or minus one or plus or minus a constant depending on whether the input exceeds 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 switching 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 currents) 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 particular 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 particular also to a harmonic flux controller (e.g., HFC) in parallel, in particular operating in one or more reference frames, which derive a second voltage reference, wherein the converter 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 harmonic 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-αβ-transformation module that outputs a total voltage reference (e.g. uab*), wherein the voltage reference (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 calculation modules may be utilized.

According to an embodiment of the present invention, the stator 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 estimated generator torque (e. g Te^) and the estimated generator flux (e.g. psis^) 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 example is given below: <MAT> <MAT>.

According to an embodiment of the present invention, the fundamental 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 indicate the actual torque at the considered harmonic.

According to an embodiment of the present invention, the harmonic torque reference (Th*) may alternatively be derived from a look-up table based on an operating point of the generator, 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 turbine, being coupled to at least one generator, in particular a permanent magnet synchronous machine, the method comprising: 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 controlling the generator side converter portion. Thereby, torque and/or voltage ripples of the generator may be effectively reduced. harmonic flux may be generated for voltage 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 generator 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 converter portion, in particular of a wind turbine, being coupled to a generator, in particular a permanent magnet synchronous 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.

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.

The illustration in the drawings is in schematic form. It is noted that in different figures, elements similar or identical 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 described in one embodiment may be taken from a description of this element with respect to another embodiment.

<FIG> illustrates in a schematic form a wind turbine <NUM> according to an embodiment of the present invention as example of a power generation system, which provides electric energy to a utility grid <NUM>.

The wind turbine comprises a hub <NUM> to which plural rotor blades <NUM> are connected. The hub is mechanically connected to a main shaft <NUM> whose rotation is transformed by a gear box <NUM> to a rotation of a secondary shaft <NUM>, wherein the gear box <NUM> may be optional. The main shaft <NUM> or the secondary shaft <NUM> drives a generator <NUM> which may be in particular a synchronous permanent magnet generator providing a power stream in the three phases or windings <NUM>, <NUM> and <NUM> to a converter <NUM> which comprises a generator side portion (AC-DC portion) <NUM>, a DC-link <NUM> and a grid side portion (DC-AC portion) <NUM> for transforming a variable AC power stream to a fixed frequency AC power stream which is provided in three phases or windings <NUM>, <NUM>, <NUM> to a wind turbine transformer <NUM> which transforms the output voltage to a higher voltage for transmission to the utility grid <NUM>.

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

The generator in <FIG> comprises a single three-phase stator winding. Thereby, the winding <NUM> carries the stator current Ia, the winding <NUM> carries the stator current Ib and the winding <NUM> carries the stator current Ic.

The arrangement <NUM> is adapted to counteract torque and voltage harmonics (for example a harmonic corresponding to six times the electrical frequency of the generator <NUM>). The generator <NUM>, the converter <NUM> and the arrangement <NUM> together form a generator system according to an embodiment of the present invention.

The arrangement <NUM> for controlling a generator side converter portion as included in the wind turbine power generation system <NUM> illustrated in <FIG> receives inputs signals <NUM> which may relate to stator voltage, stator current, may relate to a feedback signal, may relate to an operating point, as will be detailed with reference to <FIG>. The control signal <NUM> in particular is a converter control signal for controlling the generator side converter portion <NUM> of the converter <NUM>.

<FIG> schematically illustrates an arrangement <NUM> for controlling a generator side converter portion and <FIG> and <FIG> illustrate respective embodiments <NUM>, <NUM> of the arrangement for controlling a generator side converter portion.

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

The arrangement <NUM> further comprises a harmonic voltage controller (HVC) <NUM> which receives as input a stator voltage indicating feedback signal <NUM> and derives therefrom a harmonic flux reference <NUM> (ψh*).

The torque ripple controller <NUM> and the harmonic voltage controller <NUM> operate on a particular harmonic of a fundamental frequency. If more than one harmonic of the fundamental frequency are to be treated, for each harmonic, a respective torque ripple controller <NUM> and a respective harmonic voltage controller <NUM> may be provided. In case of several harmonics to be treated, all of the harmonic torque references <NUM> are added at an addition element <NUM> to a fundamental torque reference <NUM> (Te*) and an estimated generator torque <NUM> is subtracted to derive a torque error <NUM>. All of the harmonic flux references <NUM> are added at an addition element <NUM> to a fundamental flux reference <NUM> (ψs*) and an estimated generator flux <NUM> is subtracted to derive a flux error <NUM>. The desired flux change dψ and desired torque change dTe are obtained as output of the hysteresis controllers <NUM>, <NUM>.

A converter control signal <NUM> is derived based on the desired torque change dTe and the desired flux change dψ. Thereby, the converter control signal <NUM> represents a switching state signal defining a switching state of plural controllable switches of the generator side converter portion <NUM> (illustrated in <FIG>).

The stator voltage indicating feedback signal <NUM> is derived from a measured DC link voltage Vdc and the switching states <NUM> using a calculation module <NUM>. The calculated stator voltage may be bandpass-filtered to eliminate harmonics different from the harmonic the harmonic voltage controller <NUM> is working on. According to the embodiment <NUM> illustrated in <FIG>, the torque error <NUM> and the flux error <NUM> are supplied to respective hysteresis controllers <NUM>, <NUM> whose outputs, the desired flux change dψ and desired torque change dTe, are supplied to a switching table <NUM> that outputs the switching state <NUM>. Furthermore, the switching table <NUM> receives the stator flux position Θψs (<NUM>) and derives the switching states <NUM> also based on the stator flux position <NUM>. The estimated generator torque <NUM> and the estimated generator flux <NUM> are derived by a torque and flux estimator <NUM> based on the stator voltage vs, in particular reference stator voltage, and stator current is.

The fundamental torque reference <NUM> and the fundamental flux reference <NUM> are derived by a fundamental torque/flux reference calculation module <NUM> 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>, the harmonic torque reference <NUM> (Th*) is derived by the torque ripple controller <NUM> further based on an operating point <NUM> of the generator <NUM>.

The arrangements <NUM>, <NUM> illustrated in <FIG> and <FIG> comprise modules similar or identical to modules of the arrangement <NUM> illustrated in <FIG> which modules are labelled with reference signs differing only in the first digit. However, the arrangement <NUM> for controlling a generator side converter portion according to an embodiment of the present invention does not comprise the hysteresis controllers <NUM>, <NUM> and the switching table <NUM> but instead comprises a predictive torque controller <NUM> which receives the torque error <NUM> and the flux error <NUM> and derives therefrom the converter control signal <NUM>.

Instead of the predictive torque control <NUM> or the hysteresis controllers <NUM>, <NUM> and the switching table <NUM>, the arrangement <NUM> illustrated in <FIG> comprises a harmonic torque controller <NUM> and a fundamental torque controller <NUM> to which the torque error <NUM> is supplied and the outputs are added together using the addition element <NUM> to derive a first voltage reference <NUM>. Furthermore, the arrangement <NUM> comprises a fundamental flux controller <NUM> and a harmonic flux controller <NUM> to which the flux error <NUM> is supplied and whose outputs are added by an addition element <NUM> to arrive at a second voltage reference <NUM>. The first voltage reference <NUM> and the second voltage reference <NUM> are supplied to a dq-αβ-transformation module <NUM> that outputs a total voltage reference <NUM> uab*. The total voltage reference <NUM> is supplied to a space vector modulation module <NUM> which derives therefrom the switching state <NUM>. As can be taken from <FIG>, the stator voltage indicating feedback signal <NUM> is derived by the module <NUM> based on the total voltage reference.

The harmonic torque controller <NUM> in <FIG> and the torque ripple controller <NUM>, <NUM>, <NUM> in <FIG>, <FIG>, <FIG> are distinct controllers, but they may have the same structure (such as any of <FIG>) in case suitable feedback signal are available. 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 <NUM> and the flux controller <NUM> in <FIG>) may be PI controllers for fundamental torque and flux.

The converter control signal in <FIG> is derived based on the inputs to the transformation module dq/αβ by Inverse Park transformation plus voltage modulator.

High performance harmonic control in an electric drive is important as this is a requirement for permanent magnet generators for several reasons: (<NUM>) meet noise standards; (<NUM>) prevent excitation of structural modes and accelerated fatigue; (<NUM>) optimized DC link voltage usage and system efficiency. The torque ripple controller (TRC) illustrated in <FIG>, <FIG> and <FIG> 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 <FIG>, <FIG> and <FIG> generates 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 control of torque and flux/voltage. Moreover, the simplicity of implementation of the LUT-based approach is obvious from <FIG>, since the inner flux and torque controller (hysteresis controllers <NUM>, <NUM>) provide high bandwidth control without the need for additional controllers in parallel.

The arrangement <NUM> illustrated in <FIG> is another attractive 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>, the TRC and HVC are integrated into the direct torque control with space vector modulation. As opposed to the embodiments illustrated in <FIG> and <FIG>, in <FIG>, voltage references are readily available at the modulator input, namely at the signal <NUM>, but the lower bandwidth of torque and flux controllers may require additional parallel controllers (HTC and HFC in <FIG>) for achieving high performance 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^<NUM> + Vq^<NUM>) or Vrms = sqrt( Vα^<NUM> + Vβ^<NUM>). Vdq are readily available (sum of torque/flux controllers' outputs) in DTC-SVM, whereas for DTC-LUT voltages are reconstructed using switching states (Sabc) and dc-link voltage (Vdc): <MAT>.

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 <FIG>, <FIG>, <FIG> as TRC and HVC, which modify the reference torque and flux required by any DTC based control method. Accordingly, the different examples presented in <FIG>, <FIG>, <FIG> (LUT, predictive, SVM) are given for the sake of completeness. However, fast inner control loops may be achieved in other manners.

<FIG>, <FIG> illustrate harmonic controllers as implementations of the control blocks HVC, TRC, HTC, HFC illustrated in <FIG>, <FIG> and <FIG>.

In particular, the torque ripple controllers <NUM>, <NUM>, <NUM> illustrated in <FIG>, <FIG>, <FIG> may receive a harmonic torque demand signal <NUM>, <NUM>, <NUM>, respectively, representing the desired torque at the harmonic at consideration. This harmonic torque demand may for example be zero. Similarly, the harmonic voltage controller may receive as an input a harmonic voltage demand signal <NUM>, <NUM>, <NUM> in <FIG>, <FIG>, <FIG>, respectively. 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 <FIG>, <FIG>, a harmonic error <NUM> is calculated from the demand harmonic value <NUM> and a feedback signal <NUM>. In the implementation illustrated in <FIG>, the error <NUM> is <NUM>° shifted by a <NUM>° phase-shifter <NUM> and the output signal is supplied to a frame transformation module <NUM>. The frame transformation module <NUM> transforms the error and the <NUM>° shifted error according to a coordinate system rotating with the considered harmonic. The output is supplied to PI regulators <NUM> which derive an output signal such that the error reduces to zero. Downstream the regulators <NUM>, a further frame transformation module is arranged which back-transforms to obtain harmonic reference <NUM>.

In <FIG>, the harmonic error <NUM> is supplied to trigonometric function <NUM>, <NUM> and the output is again supplied to PI regulators <NUM> whose output is multiplied with another trigonometric functions <NUM>, <NUM> and added together to obtain the harmonic reference <NUM>.

In the implementation illustrated in <FIG>, the harmonic error <NUM> is supplied to a resonant regulator <NUM>, to obtain the harmonic reference <NUM>.

Control blocks HVC, TRC, HTC, HFC in <FIG>, <FIG>, <FIG> can all be implemented similarly and are next called harmonic controllers. Different options of harmonic controllers are shown in <FIG>, <FIG>. In all options, the harmonic error is calculated and a close loop regulation is made. Thus, the regulators are used to control harmonic error to <NUM>. The harmonic regulator based on vector control principle is shown in <FIG>, <FIG>, enabling the implementation of simple PI controllers. <FIG> shows the harmonic regulator using resonant regulator with a typical transfer function as below, where f1 is the resonant frequency and ξ1 is the damping of the controller. The block diagrams shown in <FIG>, <FIG> assume that adaptive bandpass filters (BPF) are applied to reference/feedback signals whenever needed and therefore only the harmonic order of interest is given to the controller input. Alternatively, BPFs may be implemented in the harmonic error (Vn_error) in <FIG>, <FIG>.

Application to PM machines may allow to reduced noise and vibration 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 control 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 overmodulation, 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 possibilities: (<NUM>) increase of average stator flux reference; or (<NUM>) reduction of dc-link voltage. In other words, HVC allows to extend the operating range on MTPA (Maximum Torque Per Ampere), narrowing the flux weakening range, and therefore optimizing 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> illustrates simulation results of the modified space vector modulator based direct torque control as is illustrated in <FIG>. 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 weakening 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 limit). Regarding harmonic control, measured or inferred torque ripple is used as feedback signal together with the controller in <FIG> for the torque ripple controller and the harmonic voltage control uses voltage ripple in Vrms as input and a controller structure as in <FIG>.

The abscissas <NUM> denote the time and the ordinates <NUM> the strength of the signal. The curve <NUM> illustrates the feedback torque, the curve <NUM> illustrates the estimated torque, the curve <NUM> depicts the modulation index and the curve <NUM> indicates the estimated flux. The torque ripple controller is enabled at the time point t=<NUM>, i.e. at the time point <NUM>. 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 imposed in the estimated/referenced torque. The harmonic voltage control is enabled at the time point <NUM>, i.e. at t=<NUM>, reducing the voltage ripple by imposing a sixth harmonic in the reference flux. Accordingly, the converter operates further from the flux weakening and/or overmodulation regions.

According to an embodiment of the present invention, the fundamental references for the current/torque/flux are calculated by means of controllers or look-up tables, some examples are speed, power, torque, flux, voltage controllers and maximum torque per ampere methods. Such controllers provide Te* and ψs* which are usually DC signals during steady state operation.

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 ψh* which is a sinusoidal signal varying at given frequencies or a combination of sinusoidal signals with different frequencies. 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 controllers implemented by means of PI controllers in the harmonic reference frame, proportional-resonant controllers or any other suitable methods.

Park transformation may transform between stationary frame (αβ) and synchronous rotating frame (dq) and vice versa. Similar techniques may be applied for transformations between synchronous rotating frame and harmonic reference frame.

Voltage modulator uses the reference voltages in the stationary frame for generating PWM (pulse width modulation) pattern. The PWM signals are used to control power electronic switches such as IGBT in the generator side converter portion. Hysteresis (bang-bang controller) may be considered as controllers used to define LUT entries in DTC-LUT, <NUM> and/or <NUM> level controllers are typically used. Controllers output determine if torque/flux is to increase, decrease or remain unchanged.

Switching table determines the optimized voltage according to the desired action stated at the LUT entries by the hysteresis 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 instead 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 direct torque control drives. The harmonic control may include torque ripple control and voltage ripple control. A control method for improving harmonic control performance in electrical 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 robustness. 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 reduce noise and vibration and increasing voltage control range and drive efficiency. A control method suitable for implementation in the controller of a frequency converter may be provided.

Claim 1:
Method of generating a converter control signal (<NUM>) for a generator side converter portion (<NUM>) of a wind turbine (<NUM>), being coupled to a generator (<NUM>) being a permanent magnet synchronous machine, characterised by the method comprising:
deriving at least one harmonic torque reference (<NUM>, Th*) based on a harmonic torque demand (<NUM>) and a torque indicating feedback signal (<NUM>, Fb);
deriving at least one harmonic flux reference (<NUM>, psih*) based on a harmonic stator voltage demand (<NUM>) and a stator voltage indicating feedback signal (<NUM>, Vrms);
adding all of the at least one harmonic torque reference (<NUM>, Th*) to a fundamental torque reference (<NUM>, Te*) and subtracting an estimated generator torque (<NUM>, Te^) to derive a torque error (<NUM>);
adding all of the at least one harmonic flux reference (<NUM>, psih*) to a fundamental flux reference (<NUM>, psis*) and subtracting an estimated generator flux (<NUM>, psis^) to derive a flux error (<NUM>); and
deriving the converter control signal (<NUM>, <NUM>, Sabc) based on the torque error (<NUM>) and the flux error (<NUM>).