Fractional Harmonic Control

Abstract

A method is described for controlling a fractional slot electric machine (865), particularly for a wind turbine, comprising a stator and a rotor rotatable relative to the stator to process at least one subharmonic vibration, the method comprising determining a subharmonic electrical angular position (θ_h) of the rotor corresponding to a subharmonic, and controlling the machine (865) based on the subharmonic angular position (θ_h).

Description

[Technical Field]

[0001] The present invention relates to a method and corresponding apparatus for controlling a fractional slot electric machine comprising a stator and a rotor rotatable relative to the stator to deal with at least one, or even in particular multiple, subharmonic vibrations occurring within the machine or in nearby components.

[0002] Background technology Harmonic control in electric drivetrains, especially for wind turbine applications, can be very important in terms of noise / vibration reduction, efficiency improvement, etc. Harmonics can refer to current, voltage, torque, vibration, or acoustics. While much technology is established in dealing with harmonics that are integer multiples of the machine-electrical frequency, little has been addressed about the effects from fractional harmonics typically associated with fractional slot electric machines.

[0003] By convention, electric machines (e.g., three-phase permanent magnet synchronous motors) are usually controlled with field-oriented techniques or vector control. Electrical signals in a stationary reference frame are transformed into a synchronous rotating reference frame (often referred to as the dq frame), where the main signals of interest are primarily DC quantities and standard controllers such as PI can be easily applied. One typical example is fundamental current control, or FCC (Fundamental Current Control). "Fundamental" means that the signal has a frequency of 1f in the stationary frame and becomes DC after transformation to the synchronous rotating frame.

[0004] For the above transformation, an angle is required. This can be from a position sensor (e.g., an encoder) or a sensorless observer, and typically appears as a sawtooth waveform with a period of the fundamental cycle, or repeating at a frequency of 1f. For integer harmonic signals, the angle can simply be a multiple of 1f angle (or theta 0).

[0005] In integer slot machines, the harmonics are of integer orders. However, in fractional slot machines, i.e., machines with a fractional ratio between the number of stator slots and the number of rotor poles, there are fractional harmonics in the air-gap flux distribution, e.g., 2.4 and 4.8 times the machine-electrical frequency (denoted 2.4f and 4.8f, respectively).

[0006] In an ideal machine, some effects from fractional harmonic flux distribution, such as torque ripple, can be canceled out. However, in reality, some fractional torque ripple appears due to manufacturing tolerances and possible displacement of the magnets attached to the rotor housing. Measurements on prototype generators have shown that the level of fractional torque ripple is very high, which can excite generator structural resonances when operating at low speeds. The presence of fractional harmonics can cause acoustic noise when operating at high speeds and can affect DC link voltage usage and therefore efficiency. Therefore, harmonic current injection at fractional orders is required to suppress harmonic effects.

[0007] For fractional harmonic control, the corresponding angles are required. If we form the fractional harmonic angles from the fundamental angle and then apply the same conventional techniques for harmonic control as for integer harmonics, the results will be inaccurate, resulting in distorted sine and cosine signals when used in frame transformations.

[0008] Thus, conventionally, simply multiplying the fundamental electrical angle by the harmonic number results in an erroneous fractional harmonic angle position, which results in erroneous control of the electric machine.

[0009] Therefore, there may be a need for a method and corresponding apparatus for controlling a fractional slot electric machine to process at least one sub-harmonic vibration, which ensures reliable determination of the accurate electrical angle for the considered sub-harmonic, and improves further control of the machine.

[0010] Summary of the Invention This need can be met by the subject matter of the independent claims. Advantageous embodiments of the invention are set forth in the dependent claims.

[0011] According to one embodiment of the present invention, there is provided a method of controlling a fractional slot electric machine, particularly a wind turbine, comprising a stator and a rotor rotatable relative to the stator for managing at least one subharmonic vibration, the method comprising determining a subharmonic electrical angular position of the rotor corresponding to a subharmonic, and controlling the machine based on the subharmonic angular position.

[0012] The method can be implemented in software and / or hardware and can be performed, for example, by part or module of an electric machine controller, in particular a wind turbine controller.A fractional slot electric machine is one in which the ratio between the number of winding slots and the number of magnetic poles is rational, but not integer.

[0013] The electric machine may be a synchronous machine, in particular a permanent magnet synchronous machine, although in other embodiments the electric machine may be an asynchronous machine, such as an induction machine.

[0014] The electric machine may be configured as a generator to generate electrical energy upon rotation of the rotor relative to the stator. The stator of the electric machine may include one or more multi-phase winding sets, e.g., one or two or more winding sets, each winding set providing multiple phases, e.g., three phases.

[0015] A machine may be designed or operated to operate at a particular fundamental electrical frequency, to which a corresponding fundamental angular position of the rotor is associated. In fractional-slot electric machines, subharmonic vibrations may also occur, which can be handled by embodiments of the present invention. In particular, to convert electrical quantities, such as current and / or voltage, and even mechanical quantities, such as torque, from a stationary A, B, C reference frame or a synchronously rotating reference frame to a coordinate frame that rotates according to a fractional harmonic order, a corresponding fractional harmonic electrical angular position may be required to perform the transformation. Embodiments of the present invention provide accurate and reliable fractional harmonic electrical angular positions to improve control of fractional-slot electric machines, particularly with respect to control of at least one subharmonic vibration.

[0016] Embodiments can be designed to control multiple subharmonic vibrations, such as two, three, four, or even more subharmonic vibrations, that may occur simultaneously during operation of an electric machine. For example, conventional methods of determining integer harmonic electrical angular positions from a fundamental angular position may fail, resulting in incorrect or erroneous subharmonic electrical angular positions. Thus, conventional methods may also be unable to control subharmonic vibrations occurring within an electric machine.

[0017] The method may include, inter alia, performing vector control including at least one transformation from a stationary coordinate frame to (or from) a rotating frame that rotates according to a subharmonic order or frequency or subharmonic frequency.

[0018] According to one embodiment of the present invention, a machine is controlled to control vibrations having a subharmonic frequency, the subharmonic frequency corresponding to a fundamental electrical frequency multiplied by a fractional harmonic order that is a rational number and different from an integer number.

[0019] The vibrations may be related to, for example, electrical vibrations, torque vibrations, or mechanical vibrations. The vibrations may be measured, for example, by a microphone or by measuring torque fluctuations, current fluctuations, or voltage fluctuations. Subharmonic orders are different from integer harmonic orders as traditionally considered. Embodiments of the present invention address the control of subharmonic vibrations, which traditional methods have not addressed.

[0020] According to one embodiment of the present invention, determining the fractional harmonic electrical angle position includes determining a number of cycles based on the fractional harmonic order, unwrapping the fundamental electrical angle according to the number of cycles, multiplying the unwrapped fundamental angle by the fractional harmonic order, and wrapping the result of the multiplication.

[0021] The number of cycles may be an integer indicating how many sawtooth sections of the base electrical angle should be unwrapped. Unwrapping the base electrical angle may then include converting the multiple sawtooth sections into straight lines having the same slope as each slope line of each sawtooth section. The unwrapped base electrical angle has an angle range that is a multiple of 360°, where the multiple corresponds to the number of cycles.

[0022] The subharmonic number can be obtained, for example, as the ratio between the subharmonic frequency and the fundamental frequency.

[0023] Wrapping the result of the multiplication may include, among other things, bringing the result into the range of 0 to 360 degrees by subtracting a multiple of 360 degrees to different parts of the result of the multiplication.

[0024] According to one embodiment of the present invention, the minimum required number of cycles (e.g., N_cycles) is determined according to N_cycles=360*M / GCD(360*M, 360*M*(fractional harmonic order)), where GCD(X, Y) denotes the greatest common divisor of X and Y. M may be an integer to ensure that 360*M*(fractional harmonic order) is an integer value.

[0025] This allows for a simple derivation of the number of cycles. The number M is needed to ensure the correct functioning of the GCD function.

[0026] According to one embodiment of the present invention, unwrapping the base electrical angle according to the number of cycles includes converting the sawtooth signal, which represents the base angle and covers a number of angular ranges from 0° to 360°, into a straight line that covers an angular range of 0° to 360°*(number of cycles).

[0027] According to one embodiment of the present invention, the unwrapped fundamental electrical angle is multiplied by a fractional harmonic order, and then the angle resulting from the multiplication is wrapped by subtracting a multiple of 360° from the result, so that the wrapped result is in the range of 0 to 360°, and the wrapped result has a sawtooth shape.

[0028] According to one embodiment of the present invention, the method further includes fractional harmonic initial angle determination, particularly in the case of open-loop harmonic current reference generation where torque and / or noise feedback is not used by the closed-loop controller, comprising: determining a plurality of candidate positions for the initial angle of the fractional harmonic angular position; selecting an initial angle from the plurality of candidate positions; and deriving the fractional harmonic angular position as the sum of the angular position that varies in a sawtooth pattern at the fractional harmonic frequency and the selected initial angle, wherein determining the plurality of candidate positions particularly comprises applying the following function mod([0:360:(N cycles-1)*360]*(fractional harmonic order),360) and evaluating the result.

[0029] The final fractional harmonic angular position is the sum of the sawtooth-varying angular position at the fractional harmonic frequency and the selected initial angle, and one of the multiple candidate positions is the correct position corresponding to the correct initial angle associated with the fractional harmonic.

[0030] A selection step selects the candidate position corresponding to the true initial angle from among the multiple candidate positions, thereby eliminating the ambiguity and determining an accurate final sub-harmonic angular position, which can be used for sub-harmonic transformation and control.

[0031] According to one embodiment of the present invention, selecting an initial angle from a plurality of candidates includes at least one of: evaluating all candidate positions during operation based on damping performance such as torque ripple control; evaluating all candidate positions during operation based on a controller output comparison with a historical controller output; and evaluating all candidate positions during operation by calculating the initial angle from dq voltages in a sub-harmonic reference frame as output by a current controller.

[0032] Selecting multiple candidate initial angles can be implemented in different ways that can be selected based on the particular application. Damping performance such as torque ripple can be evaluated, for example, by measuring or determining whether vibrations due to sub-harmonic orders or frequencies are reduced when controlling the machine.

[0033] The historical controller output may have been previously collected and stored in relation to a corresponding operating point, which may be characterized, for example, by the rotational speed and output power, as well as the correct initial angle of the fractional harmonic. The controller output may include, for example, a current output and / or a voltage output. The controller may include, for example, a current controller that can receive a current error signal and output a voltage signal (e.g., in a dq frame, particularly in a coordinate frame that rotates at the fractional harmonic order frequency). By passing through the angle candidates and using one of them, the dq voltage in the fractional harmonic reference frame may correspond to or be equal to the expected or stored voltage from the historical controller output. This last-mentioned procedure does not require any additional sensors to be permanently installed, thus reducing system complexity.

[0034] According to one embodiment of the present invention, evaluating all candidate positions during operation based on damping performance such as torque ripple control includes continuously applying candidate positions during operation, determining, for each applied candidate position, an output quantity indicative of vibration according to a sub-harmonic frequency, and selecting a candidate position based on an evaluation of the output quantity, wherein the output quantity is torque and / or noise.

[0035] Applying the candidate positions may include using corresponding subharmonic electrical angular positions in one or more transducer modules or sections. The output quantity indicative of the vibration may be measured, for example, by a microphone. The sensor output signal may be filtered, for example, by a bandpass filter to reduce the amplitude of vibrations having frequencies substantially different from the subharmonic frequency of interest.

[0036] According to one embodiment of the present invention, selecting a candidate location based on the output amount includes comparing the output amount to an expected output amount and selecting a candidate location whose output amount is closest to the expected output amount.

[0037] According to one embodiment of the present invention, the expected power output may be zero, meaning that there is substantially no vibration with subharmonic frequencies, in which case the candidate location with the lowest power output intensity or amplitude and the smallest vibration with subharmonic frequencies may be selected.

[0038] According to one embodiment of the present invention, evaluating all candidate positions during operation based on controller output by comparison with historical controller output includes retrieving historical controller output data for a current operating point where a true initial angular position was available, successively applying candidate positions during operation, and for each candidate position applied, collecting controller output data and comparing the collected controller output data with the historical controller output data, wherein the candidate position whose associated collected controller output data deviates least from the historical controller output data is selected.

[0039] In this embodiment, no extra sensors are required for permanent installation, however, historical controller output data may need to be stored, for example, in electronic storage, and particularly in a database associated with corresponding operating points or load points, which may be characterized, for example, by rotor rotational speed and power output.

[0040] According to one embodiment of the present invention, evaluating all candidate positions during operation by calculating an initial angle includes setting a fractional harmonic current reference to zero, setting an initial angle to zero, operating the machine according to the settings, calculating the initial angle from a voltage signal output by a current controller that receives the current error signal, and selecting the candidate position closest to the calculated initial angle.

[0041] The above procedure is applied only once after machine control is enabled.

[0042] Also, this embodiment does not require extra sensors, which can simplify the equipment or reduce the complexity of the system.

[0043] This provides several different selection methods that can be applied depending on the particular application.

[0044] According to one embodiment of the present invention, controlling the machine includes at least one of performing vector control including at least one transformation of at least one electrical quantity into or from a fractional harmonic rotating coordinate frame, and using the electrical fractional harmonic angular position as an input to a transformation module to transform at least one electrical quantity from a stationary or synchronous rotating coordinate frame into a fractional harmonic coordinate frame or vice versa, thereby supporting vector control as traditionally applied, but now also applicable to fractional harmonic control.

[0045] It is to be understood that the features disclosed, described, illustrated or provided for, individually or in any combination, for a method of controlling a fractional slot electric machine may also be applied or provided for, in particular, individually or in any combination, for an apparatus for controlling a fractional slot electric machine according to embodiments of the present invention, and vice versa.

[0046] According to one embodiment of the present invention, there is provided an apparatus for controlling a fractional slot electric machine, in particular a wind turbine, comprising a stator and a rotor rotatable relative to the stator for processing at least one fractional harmonic, the apparatus comprising: a determination portion adapted to determine a fractional harmonic electrical angular position of the rotor corresponding to the fractional harmonic; and a control portion adapted to control the machine based on the fractional harmonic angular position.

[0047] The device may be implemented in software and hardware and may for example be part of a machine controller, in particular a wind turbine controller.

[0048] According to an embodiment of the present invention, there is provided a wind turbine comprising a fractional slot electric machine, in particular a synchronous machine, comprising a stator, a rotor rotatable relative to the stator and having a plurality of rotor blades attached thereto, and in particular a converter, and a device according to the above-mentioned embodiment connected to control the machine.

[0049] An optional converter can be connected to the output terminals of the machine, which can operate in generator or motor mode. Control of the electric machine can be achieved via the converter. In particular, the device can provide a control signal to the converter, based on which the converter can adjust the conductance states of the plurality of controllable switches, which can result in control of the electric machine.

[0050] The above-defined and further aspects of the present invention will be apparent from and will be explained with reference to the example embodiments described hereinafter. The present invention is explained in more detail below with reference to example embodiments, but the invention is not limited thereto.

[0051] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the accompanying drawings, in which: FIG. [Brief explanation of the drawings]

[0052] [Figure 1] 10 illustrates unwrapping of a base electrical angle applied according to an embodiment of the present invention. [Figure 2] 1 shows a conventional method for obtaining harmonic angles. [Figure 3] 10 illustrates the results of a method for determining a subharmonic angle according to an embodiment of the present invention. [Figure 4] 1 illustrates a module for determining a subharmonic angle according to an embodiment of the present invention. [Figure 5] 1 illustrates schematically an apparatus for controlling a fractional slot electric machine according to an embodiment of the present invention; [Figure 6] 1 illustrates schematically an apparatus for controlling a fractional slot electric machine according to an embodiment of the present invention; [Figure 7] 1 illustrates schematically an apparatus for controlling a fractional slot electric machine according to an embodiment of the present invention; [Figure 8] 1 illustrates schematically an apparatus for controlling a fractional slot electric machine according to an embodiment of the present invention;

[0053] MODE FOR CARRYING OUT THE INVENTION The drawings are shown in schematic form. It should be noted that in different figures, elements that are similar or identical in structure and / or function are given the same reference numerals or reference numerals that differ only in the first digit. The description of an element not described in one embodiment can be taken from the description of this element for another embodiment.

[0054] 1, 2 and 3 show a coordinate system having an abscissa 100 indicating a fundamental electrical angle, an ordinate 101 indicating a fundamental electrical angle, an ordinate 102 indicating a harmonic angle, and an ordinate 103 indicating a harmonic electrical angle in accordance with an embodiment of the present invention.

[0055] The embodiments shown in Figures 1 and 3 illustrate determining the harmonic electrical angle, particularly the fractional harmonic electrical angle, when the fractional harmonic order is 2.4. Those skilled in the art will understand that the method can be appropriately adapted for other fractional harmonic orders. Therefore, those skilled in the art will understand how the method should be implemented to determine the fractional harmonic electrical angle for any fractional harmonic order.

[0056] In this case, the number of cycles is predetermined to be 5. The number of cycles is determined based on the fractional harmonic order. In the next method step, the fundamental electrical angle represented by sawtooth segments 104a, 104b, 104c, d, and e is unwrapped according to the number of cycles. Note that FIG. 1 shows five sawtooth segments 104a, b, c, d, and e unwrapped to yield unwrapped electrical angle 105. Unwrapped fundamental electrical angle 105 is represented by a straight line with the same slope as the slope of sawtooth segments 104a, b, c, d, and e. Thus, a particular sawtooth signal or sawtooth segments 104a, b, c, d, and e covering an angle range of 0° to 360° is converted to a straight line 105 covering an angle range of 0° to 360° (number of cycles), i.e., 360° * 5 = 1800°.

[0057] In the next method step, the unwrapped fundamental angle 105 is multiplied by a fractional harmonic order, in this embodiment 2.4, to arrive at the result of the multiplication as shown by the line bearing reference sign 106. The result of the multiplication 106 is only partially shown in Figure 1 due to the steepness of its slope being 2.4 times that of the unwrapped fundamental angle 105. However, the result of the multiplication 106 is further extended to have values ​​up to an x-axis position of 1800°.

[0058] 3, the result of the multiplication 106 is wrapped to arrive at the thus determined fractional harmonic angle 107, which is shown as a sawtooth pattern. Wrapping the result of the multiplication 106 involves subtracting multiples of 360° from the result 106, such that the wrapped result 107 is in the range of 0 to 360° and the wrapped result 107 has a sawtooth shape, as shown in FIG.

[0059] Figure 2 shows an incorrectly determined harmonic fractional angle according to the conventional method as curve 108. Thereby, sawtooth portions 104a, b, c, and d, representing the fundamental electrical angle, are simply multiplied by the fractional harmonic number, i.e., 2.4. However, this results in an incorrect fractional harmonic angle 108, resulting in improper control of the electric machine.

[0060] Embodiments of the present invention can be configured from the following techniques. 1) Formation of fractional harmonic angles 2) Handling of angle ambiguity 3) Control using transformations in the subharmonic reference frame

[0061] A. Forming angles for fractional harmonics The problems presented by conventional methods arise primarily from the derivation of the angles of the harmonic signals from the wrapped fundamental angles.

[0062] In Figure 1, angle 105 is calculated by first unwrapping base angles 104a, b, c, d, and e. Angle 107 in Figure 3 is obtained by multiplying angle 105 by 2.4, resulting in angle 106, which is then wrapped.

[0063] In a practical implementation, the angle cannot be unwrapped forever. The solution is to unwrap the fundamental angle 104 within a given number of cycles, for example, every 5 cycles for the 2.4f subharmonic. This is mod([0:360:10*360]*2.4,360)=[0 144 288 72 216 0 144 288 72 216 0] In other words, it is calculated based on the logic that the pattern repeats every five cycles.

[0064] This can be demonstrated by the following example for the 2.4f harmonic, where a limited number of angle unwraps are applied with the correct number 5:

[0065] In each of the graphs in Figures 1-3, Figure 1 shows the fundamental angle and the unwrapped angle, Figure 2 shows the harmonic angle formed by direct multiplication of the harmonic order to the original wrapped fundamental angle, and Figure 3 shows the harmonic angle formed from the unwrapped angle, which is corrected by the appropriate number of angle unwraps on the fundamental angle.

[0066] In practice, the number of cycles for repetition, and therefore angle unwrapping, is determined by the fractional part of the harmonic order, which can be determined by the greatest common denominator (GCD) of 360 degrees and the fractional angle, 360 / GCD(360, 360*fraction), where the "fraction" is the fractional part of the harmonic order, but may also be the harmonic order itself.

[0067] For example, for 2.4f, the fractional part is 0.4, the angle fraction is 144 degrees, and the number of cycles of iteration is 5. In implementations, depending on the resolution of the fractional order, multiples of 360 may be used in the formula for calculating the number of cycles, i.e., 360M / GCD(360M,360M*fraction) where M is a multiple of 360 degrees.

[0068] 4 shows a schematic diagram of a method or module for determining a fractional harmonic angle, denoted as h·θ, without an initial angle determination. A calculation block 410 receives a fractional harmonic order 411, which may be, for example, 2.4. The calculation block outputs the number of cycles required to perform the unwrapping operation 412. In module 413, the fundamental angle, denoted as θ, is unwrapped to yield an unwrapped fundamental electrical angle, denoted as 405.

[0069] It should be understood that elements or modules in the figures are designated by reference numerals that differ only in the first digit. In one embodiment, the unwrapped electrical fundamental angle 405 can be represented as a line 105 as shown in FIG.

[0070] Next, in derivation module 414, multiplication and wrapping is performed, whereby the unwrapped fundamental electrical angle 405 is first multiplied by the fractional harmonic order, e.g., 2.4, as described with reference to Figures 1 and 3, and then the result of the multiplication is wrapped. Module 414 outputs the fractional harmonic angle h·θ.

[0071] B. Determining the initial angle of the fractional harmonic Unlike the angle treatment for integer harmonics, the initial angle for fractional harmonics is not deterministic for the same reason that a unique 1f angle cannot be derived from the 2f angle. In the case of 2.4f, each time the controller is enabled, corresponding to the zero position of the fundamental angle, the initial angle for the 2.4f harmonic can be one of five values, i.e., [0 72 144 216 288] degrees, and only one will provide the performance required for, say, 2.4f torque ripple control.

[0072] Clearly, this is not an issue for closed-loop control, such as TLC (torque loop control) and HVC (harmonic voltage control), where the applied phase angle is automatically adjusted for possible changes in the initial angle each time the generator control is enabled. Closed-loop TLC or HVC can be shown in Figure 5, where the reference can be set to zero, torque ripple feedback can be from a torque transducer or accelerometer, while voltage ripple feedback is simply from the command voltage. From the ripple signal, a pair of quadrature signals is formed and then transformed into a harmonic reference frame to enable regulation by a PI controller. The control output is transformed back into a synchronous rotating dq frame, providing a harmonic current reference for downstream HCC control. With these closed-loop controllers, the initial or "offset" angle in Figure 6 (described below) can be safely set to zero or any other value, such as θ_h = h·θ.

[0073] 5 illustrates schematically an apparatus for controlling a fractional-slot electric machine according to one embodiment of the present invention. Apparatus 520 has an input for receiving a harmonic torque or current or voltage reference 521 and an input for receiving a torque or voltage or current harmonic feedback 522 that is subtracted from reference 521 by subtraction element 523 to result in a fractional-harmonic error signal 524. Error signal 524 is received by a bandpass filter module 525 that attenuates any components of error 524 that are outside of a frequency band of interest.

[0074] The filtered output 526 is received by a 90° phase shifter 527, resulting in a phase-shifted error signal 528. The phase-shifted error signal 528 as well as the unshifted error signal 526 are received by a transformation module 529, which converts from the dq frame to a fractional harmonic coordinate frame. It can be seen that the shifted error signal 528 corresponds to the d-component of the error, and the unshifted error signal 526 corresponds to the q-component of the error. The transformation module 529 receives as a required input the fractional harmonic angle θ_h derived by the decision module 509, which may be designed or configured similarly to module 409 shown in FIG. 4.

[0075] The transformation module 529 outputs the d-component 530 and the q-component 531 to respective PI regulators 532, 533, respectively. The PI regulators 532, 533 output control signals 534, 535, which may be, for example, current signals, in the d-frame and the q-frame, respectively. For closed-loop control for torque ripple or voltage ripple, current references 534, 535 may be generated, which may in the next stage be applied to current controllers to generate, for example, the voltages required for converter control.

[0076] The signals 534, 535 are fed to another transformation module 536 which performs a transformation from the fractional harmonic coordinate frame to the dq coordinate frame. Again, the transformation module 536 requires as input the fractional harmonic angle θ_h and as outputs Idref and Iqref, i.e., the references to the currents in the d and q frames, respectively.

[0077] It should be noted that the apparatus 520 shown in FIG. 5 represents an example of closed loop harmonic control in which feedback 522 regarding subharmonics is received.

[0078] In other embodiments, the respective controller may be an open-loop controller for generating the current reference, i.e., a device that does not use harmonic feedback included as an input to a control element such as a PI regulator. Below, embodiments of the present invention are described in more detail and may be applied to an open-loop harmonic current reference generating device where determination of an initial angle of a fractional harmonic angle is required.

[0079] Determining the initial angle for fractional harmonics is only necessary when the current reference must be manually defined in open-loop control (Figure 6). For example, in the case of lookup table-based torque ripple control, where the amplitude and phase angle of the harmonic current reference are defined, the adjusted applied phase angle may not be valid at the next converter power-up. However, since the initial angle only needs to be determined once when the converter control is enabled, some simple methods can be adopted.

[0080] 1) Selection by scanning At any given load point, both the amplitude and phase angle of the harmonic reference current can be examined. Then, for the phase angle, an offset is added one at a time from a limited selection of candidate values, e.g., [0, 72, 144, 216, 288] degrees for the 2.4f harmonic. Using one of these values ​​should result in the lowest torque ripple (or accelerometer / noise measurement) at 2.4f. The selection can be done manually or automatically, but this method is only applicable when feedback signals, such as torque and vibration / noise, are available. To avoid large transients in harmonic control, a rate limiter can be added for the change between candidate values.

[0081] 2) Decision without extra sensors - Solution 1 At a given operating point of speed and load, the voltage ripple at a harmonic h (e.g., 2.4f) can be expressed as:

number

[0082] where Idh and Iqh are

number

number

number

[0083]

number

number

[0084] Next, the harmonic content of Vrms can be approximated by the following equation:

number

[0085] Typically, the q-axis harmonic currents are for torque ripple control and the d-axis harmonic currents are for voltage ripple control. For a given Iqh (in amplitude and phase), the Idh current is:

number

number

[0086] The initial angle determined by using one signal can be used to control all other signals of the same class of subharmonic orders, e.g., 2.4f and 4.8f.

[0087] 3) Decision without extra sensors - Solution 2 Following similar logic as "Solution 1" above, a much simpler approach is possible by taking advantage of the presence of harmonics in the PM flux linkage. In some cases, this new solution requires only a one-shot test and is largely non-intrusive. No extra sensors are required, even if only temporarily.

[0088] When the harmonic currents are controlled to be zero, the harmonic voltages in the basic dq frame (Equation 1) are as follows:

number

[0089] If the angles for harmonic control are defined by assuming zero initial values,

number

[0090] The harmonic voltages in the harmonic reference frame are

number

[0091] The actual initial angle of the fractional harmonic (

number

number

number

[0092] That is, the initial angle can be determined from the dq voltages in the fractional harmonic reference frame. These voltages are actually the output of the fractional harmonic current controller when the fractional harmonic reference current is set to 0 (Figure 9). To minimize the influence from the fundamental current, the Id and Iq demands can also be set to 0.

[0093] To minimize the influence from noise and therefore obtain better robustness, the candidate angles (

number

number

[0094] The angle of the subharmonic can then be determined and used for control.

number

[0095] The process of subharmonic angle determination can be summarized as follows: i) After synchronizing the fundamental position observer and enabling generator current control, fractional HCC control is enabled (e.g., at 2.4f), but the harmonic current references in both the d-axis and q-axis are set to 0. The angles for fractional harmonic control are temporarily set to

number

number

[0096] This solution is applicable to control structures that involve transformations to and from harmonic reference frames. In the case of direct harmonic control in the fundamental dq frame (e.g., with a resonant controller), the control voltages can be transformed into the harmonic frame for initial angle determination.

[0097] 6 illustrates an apparatus 620 for controlling a fractional-slot electric machine according to one embodiment of the present invention. A determination module 609 determines the fractional harmonic angle, for example, similar to module 409 shown in FIG. 4. However, the initial angle is still unknown. The fractional harmonic angle without the initial angle is denoted as h·θ.

[0098] At the top of Figure 6, an initial angle may be set to zero, as indicated by reference numeral 637. The zero offset 637 is added to the fractional harmonic angle h·θ. The result is fed to a control module 638, which may include a torque ripple closed-loop control 639 and a voltage harmonic closed-loop control 640. Module 638 outputs an automatically derived harmonic current reference, indicated by reference numeral 641.

[0099] At the bottom of FIG. 6, a candidate determination module 642 receives torque and / or vibration feedback 643 and then selects an initial angle 644 from a plurality of candidate angle positions. The full fractional harmonic electrical angle θ_h is input to a harmonic current reference determination module 645. Module 645 outputs a harmonic current reference 646, which is provided to a selection module 647 along with harmonic current reference 641. Selection module 647 allows for the selection of one of current references 641 or 646. The selected current reference is output by selection module 647 and is labeled with reference numeral 648. Furthermore, the selection module outputs a fractional harmonic electrical angle θ_h. These output values ​​648 and θ_h are provided to a current harmonic closed-loop control module 649.

[0100] Figure 7 schematically illustrates an apparatus 720 for controlling a fractional-slot electric machine according to one embodiment of the present invention. The apparatus 720 includes a fractional harmonic angle calculation module 750, which may include, for example, one or more of the modules shown in Figures 4 and 6. The apparatus 720 receives a generator operation request signal 751, which is received by an enable module 752. The enable module outputs a signal 753, which is received by a speed-position observer 754. The observer 754 further receives generator voltage and current 755 as inputs.

[0101] Module 754 outputs the fundamental electrical angle 704 and an enable signal 756, which are received by a fractional harmonic angle calculation module 750. Additionally, this module receives a harmonic number 757 and, optionally, a torque or acceleration input feedback signal 758. Fractional harmonic angle calculation module 750 includes storage for a "Harmonic Angle Without Initialization" determination block 759, a "Harmonic Initial Angle Determination" block 760, and a "Angle Candidates and Stored Harmonic Angle" module 761. Harmonic Initial Angle Determination module 760 receives the d and q components of the harmonic voltage as inputs. The initial angle θ determined by module 760 is added to the fractional harmonic angle without initialization h·θ, resulting in the full fractional harmonic angle θ_h. This fractional harmonic angle θ_h is received by a fractional harmonic control module 761, which may comprise a torque ripple control (TRC) module 762 and / or a harmonic voltage control (HVC) module 763 and / or a harmonic current control (HCC) module 764.

[0102] C. Synchronization process for generator electrical angle Figure 7 illustrates the generator electrical angle synchronization process, focusing on the calculation of fractional harmonic angles. After generator control is enabled, harmonic angle calculation begins, and the process is sequenced into two parts. First, fractional harmonic angles are derived from the fundamental angle (theta 0), as described in Section A, with the initial angle set to zero. This allows fractional harmonic control, such as HCC, TRC, and / or HVC, to be enabled while the generator is operating at a fixed, relatively low load point. Next, the initial angle determination process is initiated, where the correct angle is selected from several candidates or derived from a one-shot test, and fractional harmonic control can be fully enabled for TRC and HVC control. It may be noted in "Solution 2" in Section B.3 that only HCC control with a zero current reference is required in the initial angle determination.

[0103] FIG. 8 illustrates schematically an apparatus 820 for controlling a fractional slot electric machine 865 according to one embodiment of the present invention.

[0104] D. Subharmonic Control Existing structures for harmonic control can be applied, except that a fractional harmonic reference frame is introduced instead of an integer harmonic reference frame. The required fractional harmonic frequency is obtained by simply multiplying the fundamental frequency by the harmonic order. The fractional harmonic angle and its integer multiplier (e.g., 2) are derived using the techniques described above. This scheme can be seen in Figure 8, where h is a parameter that can be set to 2.4 for 2.4f harmonic control, for example.

[0105] The error signals are presented in the synchronously rotating dq reference frame. They can be harmonics of current, voltage, torque, or vibration. For current and voltage, the signals are derived by transforming from the stationary frame to the dq frame; for torque and vibration, the signals appear naturally in the dq frame.

[0106] The fractional harmonic angle can then be used to convert the signal in the dq frame to a positive and negative fractional harmonic frame, with one portion of the signal being DC and used as the input to the PI controller, and another portion being AC and filtered by a notch filter. The control signal derived in the fractional harmonic frame can then be converted back to the dq frame, with the signal appearing at a fractional harmonic frequency, e.g., 2.4f.

[0107] The output from the harmonic control can be a voltage or a current. For example, the output of the fractional harmonic current control is used as a voltage demand along with the output from another controller such as a fundamental current control, or applied as a current demand in a cascade control system; in the case of voltage ripple or torque ripple control, the output is a current demand supplied to the harmonic current controller.

[0108] The output signals Vdh, Vqh may be supplied to a fractional slot electric machine 865 which may comprise a converter, and in particular the output signals Vdh, Vqh are used to control the converter.

[0109] The present invention provides a technique for controlling subharmonics by introducing the concept of a subharmonic reference frame. Furthermore, a method for deriving the angle of the subharmonic is provided, and a further method for determining the initial angle with or without an additional sensor is disclosed. The embodiments may be applicable to the control of various subharmonics, such as current, voltage, torque ripple, and vibration.

[0110] This technique is also applicable to other harmonic control topologies, such as resonant controllers. Subharmonic control can work with other order harmonic control, for example integer harmonics.

[0111] It should be noted that the term "comprising" does not exclude other elements or steps, and "a" or "an" do not exclude a plurality. Also, elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

1. 1. A method of controlling a fractional slot electric machine (865), in particular for a wind turbine, comprising a stator and a rotor rotatable relative to said stator for processing at least one subharmonic, said method comprising: determining a subharmonic electrical angular position (θ_h) of the rotor corresponding to the subharmonic; and controlling the machine (865) based on the sub-harmonic angular position (θ_h).

2. the machine is controlled to control the vibrations having subharmonic frequencies; 2. The method of claim 1, wherein the fractional harmonic frequency corresponds to a fundamental electrical frequency multiplied by a fractional harmonic order (h) that is rational and different from an integer.

3. Determining the sub-harmonic electrical angular position (h*θ), particularly without an initial angle determination, comprises: determining a number of cycles (412) based on the fractional harmonic order (h); unwrapping the base electrical angle (θ, 104a, . . . c) according to the number of cycles; multiplying the unwrapped fundamental angle (105) by the fractional harmonic order (h); and wrapping the result of the multiplication (106).

4. The number of cycles (N_cycles, 412) is determined according to N_cycles=M / GCD(M,M*(fractional harmonic order)); GCD(X, Y) represents the greatest common divisor of X and Y, The method of claim 3 , wherein M is an integer.

5. Unwrapping the fundamental electrical angle (θ) according to the number of cycles (412) comprises:

5. The method according to claim 3, further comprising converting the sawtooth signals (104a, ..., e) representing the basic angles and covering an angle range of 0° to 360° into a straight line (105) covering an angle range of 0° to 360° * (number of cycles).

6. Wrapping the result (106) of the multiplication 6. The method of claim 3, comprising subtracting a multiple of 360 degrees from the result (106) such that the wrapped result (107) is in the range of 0 to 360 degrees and the wrapped result has a sawtooth shape.

7. In particular, for open loop current reference generation where torque and / or noise feedback is not used by the controller, determining a plurality of candidate positions (642) for an initial angle (θ 0 ) of the fractional harmonic angular position (θ_h); selecting the initial angle (θ0) from the plurality of candidate positions; deriving the sub-harmonic angular position (θ_h) as the sum of the sawtooth-changing angular position (h*θ) at the sub-harmonic frequency and the selected initial angle (θ0); Determining the plurality of candidate locations is in particular a function of: mod ([0:360: (N cycles - 1) * 360] * (fractional harmonic order), 360) and evaluating the result.

8. Selecting the initial angle (θ0) from the plurality of candidates includes: evaluating all candidate positions during operation based on damping performance, such as torque ripple control; evaluating all candidate positions during operation based on a controller output comparison with historical controller outputs; 8. The method of claim 7, comprising at least one of: calculating an angle from the dq voltages in the sub-harmonic reference frame as output by a controller; and looking up from all candidates for the initial angle, in particular based on the calculated angle from the dq voltages.

9. Evaluating all candidate locations during operation based on damping performance includes: continuously applying said candidate positions during operation; For each candidate position applied, determining an output quantity indicative of the vibration according to the subharmonic frequency; selecting the candidate location based on an evaluation of the output quantity; The method of claim 8 , wherein the output quantity is torque and / or noise / vibration.

10. Selecting the candidate location based on the output amount includes: comparing the output amount to an expected output amount; and selecting the candidate location whose output quantity is closest to the expected output quantity.

11. Evaluating all candidate positions during operation based on a controller output comparison with a historical controller output Retrieving historical controller output data for a current operating point for which a true initial angular position was available; continuously applying said candidate positions during operation; For each candidate position applied, collecting controller output data; comparing the collected controller output data with the historical controller output data; A method according to any one of claims 8 to 10, wherein the candidate location for which the associated collected controller output data deviates least from the historical controller output data is selected.

12. Evaluating all of the candidate positions during operation by calculating an initial angle includes: setting a fractional harmonic current reference to zero; setting the initial angle to zero; operating the machine in accordance with the settings; enabling the subharmonic current control; calculating the initial angle from a voltage signal output by a sub-harmonic current controller that receives a current error signal; and looking up and / or selecting the candidate position that is closest to the calculated initial angle.

13. Controlling the machine (865) performing vector control including at least one transformation of at least one electrical quantity into or out of a fractional harmonic rotating coordinate frame; and using the electrical sub-harmonic angular position as input to a transformation module to transform at least one electrical quantity from a stationary or synchronously rotating coordinate frame to a sub-harmonic coordinate frame, or vice versa.

14. 1. An apparatus (520, 620, 720, 820) for controlling a fractional slot electric machine (865), particularly of a wind turbine, comprising a stator and a rotor rotatable relative to the stator to counter at least one subharmonic vibration, the apparatus comprising: a determination section (409, 509) adapted to determine a sub-harmonic electrical angular position (h·θ) of the rotor corresponding to the sub-harmonic not involving an initial angle determination, said section (750) being in particular adapted to determine a sub-harmonic electrical angular position (θ_h) of the rotor corresponding to a sub-harmonic, also involving an initial angle determination; a control portion (529, 532, 533, 536) adapted to control the machine based on the fractional harmonic angular position (θ_h).

15. 1. A wind turbine comprising: a fractional slot electric machine (865), in particular a synchronous machine, comprising a stator and a rotor rotatable relative to the stator, the rotor having a plurality of rotor blades attached thereto; In particular, converters and 15. A wind turbine comprising: an apparatus (520, 620, 720, 820) according to claim 14 connected to control said machine (865).

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