Method of controlling a multi-channel multi-phase electrical machine

Abstract

Provided is a method of controlling a multi-channel multi-phase electrical machine including a plurality of channels each with a set of phase windings connected to a converter, which method includes the steps of operating the converters to electrically phase-shift the channels; computing harmonic injection currents for a dominant harmonic on the basis of electrical quantities in a rotating reference frame; determining harmonic voltage references for the dominant harmonic on the basis of the harmonic injection currents; and regulating the AC output voltages of the channels according to the fundamental voltage references and the harmonic voltage references. Also provided is a control arrangement of a multi-channel multi-phase electrical machine; a wind turbine; and a computer program product.

Claims

1. A method of controlling a multi-channel multi-phase electrical machine comprising a plurality of channels each with a set of phase windings connected to a converter the method comprising: operating the converters to electrically phase-shift the channels; and, for each channel: computing harmonic injection currents for a dominant harmonic on a basis of electrical quantities in a rotating reference frame, wherein the injection currents are computed on a basis of a target ripple value for the dominant harmonic, the target ripple value comprising a target power ripple component and a target voltage ripple component; determining harmonic voltage references for the dominant harmonic on a basis of the harmonic injection currents; and regulating the AC output voltages of that channel of the multi-channel multi-phase electrical machine according to fundamental voltage references and the harmonic voltage references.

2. The method according to claim 1, wherein the harmonic injection currents are computed using a model that relates generator electrical values to generator speed.

3. The method according to claim 1, wherein the injection currents are computed using a feedforward control method.

4. The method according to claim 3, wherein the injection currents are computed from a target ripple specified for the machine output voltage and power.

5. The method according to claim 1, wherein the harmonic injection currents are computed using a decoupled feedback control method.

6. The method according to claim 1, wherein the harmonic injection currents are computed using a multivariable feedback control method.

7. The method according to claim 1, wherein the steps of the method are used to control a dual three-phase electrical machine with a 30° phase-shift between the two channels of the machine.

8. The method according to claim 7, wherein the dominant harmonic is the sixth harmonic.

9. A control arrangement of a multi-channel multi-phase electrical machine, comprising: a voltage reference generator configured to generate fundamental voltage references for the machine frequency; a harmonic voltage reference generator configured to generate harmonic voltage references for a dominant harmonic of the machine frequency; and an output voltage controller configured to control a machine output voltage on a basis of the fundamental voltage references and the harmonic voltage references; and wherein the harmonic voltage reference generator comprises, computation modules configured to compute a generator power value and a generator voltage value on the basis of electrical quantities in a rotating reference frame; and an injection current computation module configured to compute injection currents for the dominant harmonic on the basis a target ripple value for the dominant harmonic; and a harmonic current controller configured to compute the harmonic voltage references from the harmonic injection currents.

10. The control arrangement according to claim 9, wherein the injection current computation module comprises a ripple minimization module.

11. The control arrangement according to claim 9, wherein the injection current computation module comprises a harmonic power regulator and a harmonic voltage regulator.

12. The control arrangement according to claim 9, wherein the injection current computation module comprises a multivariable regulator.

13. A wind turbine comprising a multi-channel multi-phase generator; and a wind turbine controller comprising the control arrangement according to claim 9.

14. A computer program product comprising a computer readable hardware storage device having computer readable program code stored therein, said program code executable by a processor of a computer system to implement the method of claim 1, wherein the computer readable program code is directly loadable into a memory of a control arrangement of a multi-channel multi-phase electrical machine and which comprises program elements for computing harmonic voltage references.

Description

BRIEF DESCRIPTION

(1) Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

(2) FIG. 1 shows a simplified electrical diagram of a dual three-phase electrical machine;

(3) FIG. 2 illustrates an electromagnetic phase-shift between windings of the first channel and windings of the second channel of the electrical machine of FIG. 1;

(4) FIG. 3 shows an exemplary frequency spectrum of a dual three-phase electrical machine;

(5) FIG. 4 shows a simplified block diagram of the inventive control arrangement;

(6) FIG. 5 shows a block diagram of a first embodiment of an injection current computation module for the control arrangement of FIG. 4;

(7) FIG. 6 shows a block diagram of a second embodiment of an injection current computation module for the control arrangement of FIG. 4;

(8) FIG. 7 shows a block diagram of a harmonic regulator implemented in the control arrangement of FIG. 6;

(9) FIG. 8 shows a block diagram of a third embodiment of an injection current computation module for the control arrangement of FIG. 4;

(10) FIG. 9 shows 6f ripple waveforms resulting from the inventive control method;

(11) FIG. 10 shows a 6f ripple waveform observed in a conventional art control method;

(12) FIG. 11 shows a 6f ripple waveform observed in a conventional art control method;

(13) FIG. 12 shows a 6f ripple waveform observed in a conventional art control method; and

(14) FIG. 13 shows a block diagram of a conventional art controller.

DETAILED DESCRIPTION

(15) FIG. 1 shows a simplified electrical diagram of a dual three-phase electrical generator. The two channels C1, C2 of the generator are indicated on the right. The terminal voltages of each channel C1, C2 are controlled by a machine-side converter M1, M2. A DC link capacitor D1, D2 is arranged in the DC link between a machine-side converter M1, M2 and a grid-side converter G1, G2. The grid-side converters G1, G2 are connected to a transformer T via line reactors R1, R2.

(16) FIG. 2 illustrates a 30° phase shift between the first channel and the second channel of FIG. 1, by overlaying phasor diagrams of the channel currents on a simplified representation of the stator. The first channel C1 is represented by three windings WA1, WB1, WC1 and the second channel C2 is represented by three windings WA2, WB2, WC2. Here, the windings are connected in a star configuration (a delta configuration is equally possible). The 30° phase shift between the first channel C1 and the second channel C2 has been shown to have various advantages, one of which is that 6f torque ripple is effectively cancelled out.

(17) FIG. 3 shows an exemplary frequency spectrum of a dual three-phase electrical machine controlled using a conventional control approach. The diagram indicates the fundamental f.sub.0 (at the machine electrical frequency) and a number of harmonics. In a dual three-phase electrical machine, the sixth harmonic 6f (at six times the machine electrical frequency) is the largest (dominant) and therefore also the most problematic harmonic. The amplitude of a harmonic in the frequency spectrum (relative to the amplitude of the fundamental f.sub.0) corresponds to the amplitude of the ripple component that is overlaid on the output voltage or output power.

(18) FIG. 4 shows a simplified block diagram of the inventive control arrangement 1. A transformation has been performed on the measured current to obtain vector I.sub.dq in the rotating dq0 reference frame, whilst V.sub.dq is the voltage vector derived from an I.sub.dq current controller (not shown), as will be known to the skilled person. The vectors I.sub.dq, V.sub.dq are passed to a fundamental controller 11 that generates voltage references V.sub.d*, V.sub.q* for a PWM control unit 12 that determines or regulates the output voltages V.sub.C or terminal voltages V.sub.C of that channel. The current vector I.sub.dq shall be understood to comprise a d-axis component I.sub.d and a q-axis component I.sub.q in the rotating reference frame. The same applies for the voltage vector V.sub.dq, which shall be understood to also comprise a d-axis component V.sub.d and a q-axis component V.sub.q in the rotating reference frame. A conventional control arrangement generally only comprises a fundamental controller and a PWM control unit that determines or regulates the terminal voltages V.sub.C using only the fundamental voltage references V.sub.d*, V.sub.q*.

(19) In the inventive control arrangement, the vectors I.sub.dq, V.sub.dq are also passed to a harmonic voltage reference computation module 10 that can be realized in one of several ways as will be explained below, and which comprises a 6f reference computation module that provides harmonic voltage references V.sub.d6*, V.sub.q6* to be added by the PWM control unit to the fundamental voltage references V.sub.d*, V.sub.q*. The harmonic voltage reference computation module 10 is realized to provide 6f voltage references V.sub.d6*, V.sub.q6*, i.e. voltage references that will result in a minimization of the 6f ripple on the machine output voltage and output power. In the inventive control arrangement 1, the PWM control unit 12 for that channel determines the generator output voltage V not only on the basis of the fundamental voltage references V.sub.d*, V.sub.q*, but also by taking into consideration the harmonic voltage references V.sub.d6*, V.sub.q6*, so that the dominant harmonic ripple on the output power and voltage of that channel can be minimized or even eliminated.

(20) FIGS. 5, 6 and 7 show various possible embodiments of the 6f reference computation module 102 of the harmonic voltage reference computation module 10. In each case, a generator power computation module 101_P computes a value of the generator power sixth harmonic P6 on the basis of the vectors I.sub.dq, V.sub.dq, and a generator voltage computation module 101_V computes a value of the generator voltage sixth harmonic V6 on the basis of the vectors I.sub.dq, V.sub.dq. Each computation module 101_P, 101_V includes a speed-dependent band-pass filter to only pass the sixth harmonic frequency. Injection references I.sub.d6*, I.sub.q6* are computed in an injection current computation module 102 and passed to a harmonic current controller 103 which in turn generates the 6f voltage references V.sub.d6*, V.sub.q6*.

(21) In FIG. 5, the injection current computation module 102 implements as a ripple control module 1021 (or “ripple minimization module”) using a feedforward approach. At a given operating point with a certain speed and a certain load, the 6f power ripple P.sub.6 can be expressed as
P.sub.6=3/2ω[I.sub.qψ.sub.pm6a cos(6θ)+I.sub.q6ψ.sub.pm0 cos(6θ+δ.sub.q6)−I.sub.dψ.sub.pm6b sin(6θ)−6L.sub.qI.sub.qI.sub.q6 sin(6θ+δ.sub.q6)−6L.sub.dI.sub.dI.sub.d6 sin(6θ+δ.sub.d6)]  (1)
where ω is the speed or electrical angular frequency of the machine; I.sub.d, I.sub.q, V.sub.d and V.sub.q are the d-axis and q-axis components of the vectors I.sub.dq, V.sub.dq; and I.sub.d6 and I.sub.q6 are the harmonic current vectors that will be injected at the respective phase angles of δ.sub.d6 and δ.sub.q6 for the injection currents. ψ.sub.pm0 is the DC value of flux linkage from the permanent magnets, and ψ.sub.pm6a and ψ.sub.pm6b are derived from the 6f harmonic values in the d and q-axis permanent magnet flux linkage according to
ψ.sub.pm6a=6ψ.sub.pm6q+ψ.sub.pm6d  (1.1)
ψ.sub.pm6b=6ψ.sub.pm6d+ψ.sub.pm6q  (1.2)

(22) At that operating point, the 6f voltage ripples v.sub.d6, v.sub.q6 can be expressed as
v.sub.d6=−ψψ.sub.pm6b sin(6θ)−ωL.sub.qI.sub.q6 cos(6θ+δ.sub.q6)−6ωL.sub.dI.sub.d6 sin(6θ+δ.sub.d6)  (2)
v.sub.q6=ωψ.sub.pm6a cos(6θ)−6ωL.sub.gI.sub.q6 sin(6θ+δ.sub.q6)+ωL.sub.dI.sub.d6 cos(6θ+δ.sub.d6)  (3)
and the rms voltage 6f ripple v.sub.rms6 can be expressed as

(23) $\begin{array}{cc}{v}_{\mathrm{rms}6}=\frac{1}{v_{\mathrm{rms}0}}\left(V_d{v}_{d6}+V_q{v}_{q6}\right)& \left(4\right)\end{array}$
where v.sub.rms0 is the fundamental rms (root mean square) voltage. A current injection vector I.sub.inj can then be defined as:

(24) $\begin{array}{cc}{I}_{\mathrm{inj}}=\left[\begin{array}{c}{I}_{d6}\cos \left({\delta }_{d6}\right)\\ I_{d6}\sin \left({\delta }_{d6}\right)\\ I_{q6}\cos \left({\delta }_{q6}\right)\\ I_{q6}\sin \left({\delta }_{q6}\right)\end{array}\right]& \left(5\right)\end{array}$

(25) The 6f power ripple P.sub.6 and 6f rms voltage ripple v.sub.rms6 can also be expressed as
v.sub.rms6=V.sub.6_cos cos(6θ)+V.sub.6_sin sin(6θ)  (6)
p.sub.6=P.sub.6_cos cos(6θ)+P.sub.6_sin sin(6θ)  (7)
in which the relationship between the current injection vector and the output ripple vector is expressed as:
R.sub.6=A.Math.I.sub.inj+B  (8)
where the matrices A and B are related to the machine parameters and the fundamental electrical quantities only, and can be derived by using the equations presented above. For example,

(26) $A=\left[\begin{array}{cccc}0& -6L_d{I}_q& {\psi }_{pm0}& -6L_q{I}_q\\ -6L_d{I}_d& 0& -6L_q{I}_q& -{\psi }_{pm0}\\ L_d{V}_q& -6L_d{V}_d& -L_q{V}_d& -6L_q{V}_q\\ -6L_q{V}_q& -L_d{V}_q& -6L_q{V}_q& L_q{V}_d\end{array}\right]$$B=\left[\begin{array}{c}{I}_q{\psi }_{pm6a}\\ -I_d{\psi }_{pm6b}\\ V_q{\psi }_{pm6a}\\ -V_d{\psi }_{pm6b}\end{array}\right]$

(27) An output ripple vector R.sub.6 can be put together from the sine and cosine terms of the 6f power ripple P.sub.6 and 6f voltage ripple V.sub.6:

(28) $\begin{array}{cc}{R}_6=\left[\begin{array}{c}\begin{array}{c}\begin{array}{c}{P}_{6\mathrm{\_cos}}\\ P_{6\mathrm{\_sin}}\end{array}\\ V_{6\mathrm{\_cos}}\end{array}\\ V_{6\mathrm{\_sin}}\end{array}\right]& \left(9\right)\end{array}$
allowing the terms P.sub.6_cos, P.sub.6_sin, V.sub.6_cos, V.sub.6_sin to be established for equation (6) and equation (7). For example, if the target 6f power ripple and target 6f voltage ripple are each zero, the ripple vector is a 4×1 vector of null entries. With the ripple vector set up, and the closed form of matrices A and B derived from the machine parameters and the fundamental electrical quantities, values for the 6f power ripple P.sub.6 and the 6f rms voltage ripple V.sub.rms6 can be calculated.

(29) Subsequently, using equation (1), the required harmonic currents I.sub.d6, I.sub.q6 can be calculated from the target power ripple and target rms voltage ripple. Because power ripple can also be expressed in terms of voltage or current ripple, this machine parameter dependency may be removed. FIG. 6 shows a block diagram of a second embodiment of the injection current computation module 102 for the control arrangement of FIG. 4. Here, the harmonic currents I.sub.d6, I.sub.q6 are calculated using a pair of harmonic power and voltage regulators 102_P, 102_V connected in a feedforward arrangement. A harmonic power regulator 102_P receives the 6f power ripple P.sub.6 from the generator power computation module 101_P, and a power reference P6_ref (e.g. zero), and computes a d-axis current reference component I.sub.d6* and a q-axis current reference component I.sub.q6*. Since dominant harmonic power ripple is to be minimized, the value of the power reference P6_ref may be zero.

(30) A harmonic voltage regulator 102_V receives the 6f voltage ripple V.sub.6 from the generator voltage computation module 101_V, and a voltage reference V6_ref (e.g. zero), and computes a d-axis current reference component I.sub.d6* and a q-axis current reference component I.sub.q6*. In this case also, since dominant harmonic voltage ripple is to be minimized, the value of the voltage reference V6_ref may be zero.

(31) The d-axis components are summed to obtain the d-axis current reference I.sub.d6*. The q-axis components are summed to obtain the q-axis current reference I.sub.q6*. The current references I.sub.d6*, I.sub.q6* are then passed to the harmonic current controller 103 which generates the 6f voltage references V.sub.d6*, V.sub.q6*.

(32) FIG. 7 shows an exemplary block diagram of the harmonic power regulator 102_P of FIG. 5 (the harmonic voltage regulator 102_V is constructed identically, and only the relevant signals must be substituted). A feedback signal P6 is subtracted from the reference signal P6_ref. The result is passed to a 90° phase-shifter 1021 and also to a frame transformation module 1022, which performs a transformation of the non-phase-shifted with the phase-shifted signals from a dq rotating reference frame to a frame rotating at the 6f frequency. The outputs of the frame transformation module 1022 are passed to two proportional-integral controllers 1023, whose outputs are in turn passed to a second phase transformation module 1024 that generates the d-axis current reference component I.sub.d6* and the q-axis current reference component I.sub.q6*.

(33) FIG. 8 shows a block diagram of a third embodiment of the injection current computation module 102 for the control arrangement of FIG. 4. Here, a multivariable regulator 1028 receives the 6f power ripple P.sub.6 from the generator power computation module 101_P and the 6f voltage ripple V.sub.6 from the generator voltage computation module 101_V. The multivariable regulator 1028 is also given weighting factors λ, μ. The multivariable regulator 1028 is realized to optimize the following equation:

(34) $\begin{array}{cc}y=\sqrt{{\lambda \left(\frac{P_6}{P_0}\right)}^2+{\mathrm{.Math.}\left(\frac{V_6}{V_0}\right)}^2}& \left(10\right)\end{array}$
where y is the objective signal that is derived from the feedback of power ripple and voltage ripple (P.sub.6, V.sub.6) and from the DC values in the power and voltage (P.sub.0, V.sub.0). Since the objective is to minimize y, the technique of regulation shown in FIG. 6 can be applied, and the required values of I.sub.d6* and I.sub.q6* can be generated and then passed to the HCC controllers to compute the voltage demands.

(35) FIG. 9 illustrates the simultaneous minimization of all three 6f ripples when the inventive method is applied in the control of a dual three-phase machine in which the two channels C1, C2 are electromagnetically phase shifted by 30°. The upper part of the diagram shows the 6f torque ripple T.sub.61 of the first channel C1 and the 6f torque ripple T.sub.62 of the second channel C2. The 6f torque ripple T.sub.61, T.sub.62 in each case lies within the range ±45 kNm. Since the two channels C1, C2 have been phase-shifted by 30°, the 6f torque ripples T.sub.61, T.sub.62 cancel each other out, so that the net 6f torque ripple T.sub.6 is 0 Nm.

(36) The middle part of the diagram shows the 6f power ripple P.sub.6 and the lower part of the diagram shows the 6f Vrms voltage ripple V.sub.6 of either one of the two channels C1, C2. With the inventive method, using any of the three approaches described above with the aid of FIGS. 4-7, the 6f power ripple P.sub.6 has been reduced to a very favorable level that is significantly less than ±0.01 kW, and the 6f voltage ripple V.sub.6 has been reduced to a very favorable level close to zero volts.

(37) FIGS. 10-12 show typical waveforms that result when one type of ripple is minimized by I.sub.q harmonic current injection control, i.e. by harmonic current injection in the q-axis, as practiced in the conventional art. The machine being controlled is a dual three-phase generator of a wind turbine. For either one of the two channels C1, C2, the diagrams show the 6f ripple on each of torque, power and rms voltage against rotor electrical angle in radians. When only one type of 6f ripple is minimized, the other two 6f ripple types exhibit significantly higher levels:

(38) In FIG. 10, only the torque ripple is minimized by I.sub.q harmonic current injection. The 6f torque ripple T.sub.10 now lies within a favorably low range of −0.2-0.2 kNm. However, the 6f power ripple P.sub.10 is relatively high, reaching ±300 kW. Similarly, 6f voltage ripple V.sub.10 is also relatively high, reaching ±80 V.

(39) In FIG. 11, only power ripple is minimized by Iq harmonic current injection, and the 6f power ripple P.sub.11 lies within a favorably low range of ±3 kW. However, 6f voltage ripple V.sub.11 is also relatively high, reaching ±18 V. The 6f torque ripple T.sub.11 is relatively high, reaching ±45 kNm.

(40) In FIG. 12, only voltage ripple is minimized by Id harmonic current injection, and the 6f voltage ripple V.sub.12 now does not exceed ±0.02 V. However, the 6f power ripple P.sub.12 is also relatively high, reaching ±80 kW. The 6f torque ripple T.sub.12 is relatively high, reaching ±50 kNm.

(41) These diagrams illustrate that the known approaches to ripple reduction or elimination are only beneficial from the point of view of the reduced ripple, but the problems associated with the other two types of ripple may cancel out those benefits.

(42) FIG. 13 shows a simple block diagram of a conventional art controller for a first channel of a dual three-phase machine. An Id current controller 70d receives an Id reference Id1_ref and a measured Id value Id1, and computes a d-axis voltage reference Vd1*. An Iq current controller 70q receives an Iq reference Iq1_ref and a measured Iq value Iq1, and computes a q-axis voltage reference Vq1*.

(43) A harmonic current controller 71 provides harmonic voltage references Vqh1*, Vdh1* for a specific harmonic, for example the dominant harmonic. Inputs to the harmonic current controller 71 are received from three modules: a voltage ripple control module 710 that receives the generator Vrms value; a power ripple control module 711 that receives the generator power value; and a torque ripple control module 712 that receives the generator torque value. Each harmonic reference Vqh1*, Vdh1* is summed with the corresponding voltage reference Vq1*, Vd1* and the summed signals are passed to a PWM unit 72 that uses them to control the terminal voltages V.sub.C1 of the first channel.

(44) Although embodiments of the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of embodiments of the invention. Embodiments of the invention may be applied to electrical machines with different numbers of channels and different numbers of phases. For example, an electrical machine may have three channels each with three phases, and a 20° phase-shift between the channels. If the electrical machine has four channels each with three phases, a 15° phase-shift between the channels is used. For a three-phase machine, it is the 6f harmonic that is dominant and needs to be dealt with using the inventive method. Similarly, an electrical machine may have two/three/four channels each with five phases, and a 18°/12°/9° phase-shift between the channels. In this case, it is the 10f (tenth) harmonic that is dominant and needs to be dealt with using the inventive method. For an electrical machine with two/three/four channels each with seven phases, and a 12.86°/8.57°/6.42° phase-shift between the channels, it is the 14f (fourteenth) harmonic that is dominant and needs to be dealt with using the inventive method.

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

(46) For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module.