Abstract
A control arrangement of a multiple-stator machine, comprising a frequency converter for each of the plurality of stators and a controller for each frequency converter, wherein a controller of a frequency converter is realized to generate control signals for that frequency converter on the basis of current values relating to that stator, and to generate a compensation current value for a further controller on the basis of the received current values in the event of an open-circuit fault in a frequency converter; to receive a compensation current value from a further controller; and to compute a voltage reference for a subsequent transform stage of the controller on the basis of the received current values is provided. The invention further describes a current control module of a frequency converter controller of such a multi-stator machine; a multi-stator machine; and a method of performing fault-tolerant control of a multi-stator machine.
Claims
1. A control arrangement of a multiple-stator machine, comprising a frequency converter for each of the plurality of stators and a controller for each frequency converter, wherein a controller of a frequency converter is realized to generate control signals for that frequency converter on the basis of current values relating to that stator, and to generate a compensation current value for a further controller on the basis of the received current values; to receive a compensation current value from a further controller; and to compute a voltage reference for a subsequent transform stage of the controller on the basis of the received current values.
2. The control arrangement according to claim 1 for a dual stator machine, comprising a first controller for the frequency converter of a first stator; and a second controller for the frequency converter of a second stator; wherein the first controller receives a first compensation current value from the current control module of the second controller and generates a compensation current value for the current control module of the second controller; and the second controller receives a second compensation current value from the current control module of the first controller and generates a compensation current value for the current control module of the first controller.
3. The control arrangement according to claim 1, comprising a fault diagnosis module realized to detect the occurrence of an open-circuit fault in a frequency converter, and to generate a compensation current activation signal in the event of an open-circuit fault.
4. The current control module in a controller of a frequency converter assigned to one stator of a multiple-stator machine, which current control module comprises a number of inputs for receiving a current value and a reference current value relating to that stator, and a comparator for determining a difference current value on the basis of the received current values; a compensation current computation module for computing an output compensation current value on the basis of the difference current value; and a reference voltage computation unit realized to compute a voltage reference for a subsequent transform stage on the basis of the received current values (I.sub.dq1, I*.sub.dq1, I.sub.dq2, I*.sub.dq2) and an input compensation current value received from a current control module of another frequency converter controller.
5. The current control module according to claim 4, wherein the compensation current computation module comprises a filter arrangement arranged to process the difference current value.
6. The current control module according to claim 5, wherein the filter arrangement comprises a number of adaptive filters.
7. The current control module according to claim 5, wherein the filter arrangement comprises a low-pass filter and/or a notch filter.
8. The current control module according to claim 4, wherein the compensation current computation module comprises a delay unit for introducing a time delay on the output compensation current value (I.sub.dq.sub._.sub.comp1, I.sub.dq.sub._.sub.comp2).
9. The current control module according to claim 4, comprising an input filter realized to filter the reference current value.
10. A multiple-stator machine, wherein said multiple-stator machine is a multiple-stator permanent magnet synchronous generator, comprising a frequency converter for each of the plurality of stators, and a control arrangement according to claim 1 for controlling the frequency converters.
11. A wind turbine comprising a multiple-stator machine according to claim 10.
12. A method of performing fault-tolerant control of a multiple-stator machine comprising a frequency converter for each of the plurality of stators, and a controller for each frequency converter, which method comprises the steps of: providing a controller with current values relating to its stator; generating a compensation current value on the basis of the current values received by a controller; monitoring the frequency converters to detect an occurrence of an open-circuit fault (F1, F2, F3) in a frequency converter; exchanging compensation current values between controllers in the event of an open-circuit fault; and computing a voltage reference for a transform stage of a controller on the basis of the received current values.
13. The method according to claim 12, comprising the step of generating a compensation current activation signal to enable the exchange of compensation current values between frequency converter controllers in the event of an open-circuit fault.
14. The method according to claim 12, wherein the steps of performing fault-tolerant control are carried out during uninterrupted operation of the multi-stator machine.
15. The method according to claim 12, comprising a step of derating the power output of a multi-stator generator in the event of a fault, wherein the power output is preferably de-rated by at most 20% of rated power.
Description
BRIEF DESCRIPTION
[0029] Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:
[0030] FIG. 1 shows an embodiment of the control arrangement;
[0031] FIG. 2 shows an embodiment of a current control module;
[0032] FIG. 3 shows a first embodiment of a compensation current computation module in the current control module of FIG. 2;
[0033] FIG. 4 shows graphs of current and torque achieved using a control arrangement;
[0034] FIG. 5 shows graphs of current and torque achieved using a control arrangement;
[0035] FIG. 6 shows a second embodiment of a compensation current computation module of a current control module;
[0036] FIG. 7 shows a fault scenario for a multi-stator machine;
[0037] FIG. 8 shows a further fault scenario for a multi-stator machine;
[0038] FIG. 9 shows a block diagram of a conventional wind turbine;
[0039] FIG. 10 shows an embodiment of a conventional control arrangement;
[0040] FIGS. 11 and 12 show graphs of current and torque achieved using a conventional control method.
[0041] In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.
DETAILED DESCRIPTION
[0042] FIG. 1 shows an embodiment of the control arrangement 1 according to embodiments of the invention in use with a dual-stator PMSG 2. Each of the two stators has a set of windings W_1, W_2. The generator 2 has two frequency converters 20_1, 20_2, one for each stator. Each frequency converter 20_1, 20_2 is controlled by its own controller 1_1, 1_2. Each controller 1_1, 1_2 is supplied with appropriate input signals such as a power reference, rotor position θ.sub.e, a DC voltage reference, measured values of winding current i.sub.abc1, i.sub.abc2, generator torque, rotational velocity etc., as will be known to the skilled person. On the basis of various input signals 130_1, 130_2, 140_1, 140_2 an Iq reference computation block 13 and an Id reference computation block 14 determine a current reference I*.sub.dq1, I*.sub.dq2 for the current control module 10 in each case. The purpose of each current control module 10 is to compare values of observed current I.sub.dq1, I.sub.dq2 (obtained by the usual transformation of the observed current i.sub.abc1, i.sub.abc2) and current reference I*.sub.dq1, I*.sub.dq2. Any error between these is used to generate a voltage reference V*.sub.dq1, V*.sub.dq2 which undergoes an inverse Park transformation in block 11 (to transform the input from a synchronous rotating dq0 reference frame to a stationary abc reference frame by using an appropriate transformation matrix) before being used by a subsequent pulse-width modulation (PWM) block 12 which will generate the switching signals 120_1, 120_2 for the frequency converter 20_1, 20_2 in order to control the winding currents.
[0043] FIG. 2 shows an embodiment of two current control modules 10 of a dual-stator machine. Each current control module 10 comprises a comparator 101 to compare the values of observed current I.sub.dq1, I.sub.dq2 and current reference I*.sub.dq1, I*.sub.dq2. Any error ΔI.sub.dq1, ΔI.sub.dq2 between these is used to generate the voltage reference V*.sub.dq1, V*.sub.dq2 for the subsequent transformation block 11. Each current control module 10 is provided with a value of observed current I.sub.dq1, I.sub.dq2 (obtained by the usual transformation of the observed current i.sub.abc1, i.sub.abc2), a reference current value I*.sub.dq1, I*.sub.dq2 (from the reference computation blocks 13, 14), and a compensation current I.sub.dq.sub._.sub.comp1, I.sub.dq.sub._.sub.comp2 from the other current control module 10.
[0044] In a conventional current control module, the reference voltage V*.sub.dq1, V.sup.*.sub.dq2 would be calculated by feeding the error ΔI.sub.dq1, ΔI.sub.dq2 to an appropriate feedback controller 103 such as a proportional integral (PI) controller 103. The inventive current control module 10 goes beyond the conventional reference voltage computation, and exchanges information with another current control module 10. To this end, a first current control module 10 (at the top in the diagram) receives an input compensation current value I.sub.dq.sub._.sub.comp1 from a second current control module 10 (at the bottom in the diagram), and generates an output compensation current value I.sub.dq.sub._.sub.comp2 to send to the second current control module 10. A compensation current value can be regarded as being essentially equal to the difference between the actual observed current and the reference current. An adder 102 adds the received input compensation current value to the locally computed difference, and the total is given to the feedback controller which computes the reference voltage. In this way, the reference voltage V*.sub.dq1, V*.sub.dq2 of a current control module 10 is no longer based only on the observed and reference currents of its own frequency converter, but also on any error current of another frequency converter.
[0045] The correction only takes effect during an actual open-circuit fault in a frequency converter. To this end, each frequency converter 20_1, 20_2 comprises a fault diagnosis module 21_1, 21_2 that generates an enable or activation signal FTC_en1, FTC_en2 to activate fault-tolerant control of the frequency converters 20_1, 20_2. For example, when an open-circuit fault occurs in the first frequency converter 20_1, the activation signal FTC_en1 will toggle from logic low (“0”) to logic high (“1”). The first current control module 10 of control arrangement 1_1 then generates a non-zero output compensation current signal I.sub.dq.sub._.sub.comp2 to send to the second current control module, which in this case is current control module 10 of controller 1_2.
[0046] If the frequency converter 20_2 of control arrangement 1_2 is healthy, its output compensation current I.sub.dq.sub._.sub.comp1 is zero and does not have any effect on the reference voltage V*.sub.dq1 computed by the feedback controller 103 of the current control module 10 in the first controller 1_1.
[0047] The output compensation current signal I.sub.dq.sub._.sub.comp1, I.sub.dq.sub._.sub.comp2 provided by a current control module 10 is computed in a compensation current computation module 100. An embodiment of such a compensation current computation module 100 is shown in FIG. 3, showing the signals relating to the first controller 1_1. The input ΔI.sub.dq1 is first filtered in a low-pass filter 105 and then by a notch filter 106. The resulting filtered signal can be given a time delay by an appropriate time delay compensation unit 107. These steps can be performed continuously, whether or not there is a fault in a frequency converter. When an open-circuit fault does in fact occur in this frequency converter, the activation signal FTC_en1 generated by the fault diagnosis module allows the filtered (and optionally also delayed) compensation I.sub.dq.sub._.sub.comp2 signal to be sent to the current control module 10 of the second frequency converter 20_2. Otherwise, the output compensation current I.sub.dq.sub._.sub.comp2 will have a zero value and will not have any affect on the converter control signals 120_2 of the second converter 20_2.
[0048] FIG. 4 shows per unit graphs of the winding currents i.sub.abc1 of a first stator of a dual-stator PMSG, and the winding currents i.sub.abc2 of the second stator. The diagram also shows the generator torque 40 achieved using the control arrangement according to embodiments of the invention. The experimental values were observed over a fault-free interval N and a fault-tolerant control interval FTC. During normal fault-free operation, the winding currents i.sub.abc1, i.sub.abc2 of each stator have an essentially steady sinusoidal form. At time t.sub.F, an open-circuit fault occurs in one switch of one phase of the generator-side frequency converter of the first stator. The winding currents i.sub.abc1 of the first stator become erratic. By activating the exchange of compensation current values between the current control modules of the control arrangement, the negative effect of the fault is cancelled out to a large extent, and the torque 40 does not exhibit any pronounced ripple following the fault. This favourably steady behaviour allows the generator to remain connected to deliver power to the grid. Of course, the generator may need to be de-rated by up to 20% of rated power, since the DC-link voltages may exhibit additional harmonics during fault-tolerant control, and the generator phase currents may assume higher maximum values.
[0049] FIG. 5 also shows per unit graphs of current i.sub.abc1, i.sub.abc2 and generator torque 50 over a fault-free interval N and a fault-tolerant control interval FTC following open-circuit faults in both switches of one phase at time t.sub.F. Here also, by activating the exchange of compensation current values between the current control modules, the negative effect of the fault is shared, and the torque 50 remains favourably steady following the fault.
[0050] A multi-stator machine may comprise more than two stators. FIG. 6 shows a second embodiment of a compensation current computation module 100 of a current control module according to embodiments of the invention for use in a machine that comprises n stators. Similarly to FIG. 3, the error current value ΔI.sub.dq1 is filtered in a filter arrangement 105, 106. Since the compensation current will be sent to n−1 current control modules, the output of the filter arrangement is divided by n−1. In the event of a fault in a frequency converter, compensation currents I.sub.dq.sub._.sub.comp2, I.sub.dq.sub._.sub.compn are sent to the other n−1 current control modules. If required, the compensation current signal can have been delayed as appropriate in delay units 107, wherein the added delay may be different in each case.
[0051] FIG. 7 shows a fault scenario for a multi-stator machine. For the sake of simplicity, the multi-stator machine is assumed to be a dual-stator machine with isolated neutral points and no spatial shift between windings. The diagram shows a simplified representation of two generator-side frequency converters 20_1, 20_2 of a dual-stator machine 2. Each frequency converter 20_1, 20_2 comprises an upper and lower set of three n-channel IGBTs 200, one for each winding current phase. Control signals (not shown) are applied to the gates of the IGBTs 200, and each generator-side frequency converter 20_1, 20_2 is connected via a DC link to a grid-side frequency converter (not shown). The diagram shows an open-circuit fault F1 in the upper switch of one phase, and another open-circuit fault F2 in the lower switch of the same phase. Such open-circuit faults F1, F2 involve failure of the transistor, so that current can no longer flow from collector to emitter. However, the switch diode still offers a path for the phase current in the opposite direction.
[0052] FIG. 8 shows a further fault scenario for a multi-stator machine. Here, all of the upper switches for all three phases of the first converter 20_1 have failed. The functionality of the faulty converter 20_1 is restricted, and is referred to as a “half-controlled rectifier”. Even with this serious fault F3, the inventive method allows the generator to continue operating with a favourably low level of torque ripple, since the disturbances to the phase currents of the faulty first converter 20_1 are compensated by introducing complementary disturbances into the phase currents of the healthy second converter 20_2.
[0053] FIG. 9 shows a block diagram of a conventional wind turbine 8. In this very simplified diagram, the wind turbine 8 is represented by a rotor hub 80 with three blades, arranged to turn a main shaft. The main shaft is connected to a gearbox 81, which converts the slow, high-torque rotation of the main shaft into a faster rotation of a dual-stator generator 2. A control arrangement for each stator comprises two DC/AC converters in a back-to-back topology, namely a generator-side converter 9A (for controlling the stator currents) connected via a DC-link to a grid-side converter 9B (for controlling the transformer currents). The control arrangement is the interface between the multi-stator generator 2 and the transformer 90, allowing the output power to be fed into the grid 91.
[0054] A block diagram of the generator-side converters 9A, 9B of the conventional control arrangement is shown in FIG. 10. Here, each generator-side converter 9A, 9B comprises reference computation blocks 13, 14; a current control module 84; a transformation unit 11 and a modulator 12. In the conventional control arrangement, there is no exchange of information between the current control modules 84. An erratic winding current, arising from an open-circuit fault F1, F2, F3 (as described in FIG. 7 or FIG. 8) in the corresponding frequency converter is not corrected or compensated. The erratic winding current(s) result in a pronounced level of torque ripple. If a fault-tolerant control is not possible, the wind turbine 8 must be disconnected from the grid 91, resulting in significant loss of revenue.
[0055] FIG. 11 demonstrates the outcome in the absence of any fault-tolerant control. Here also, the diagram shows per unit graphs of current i.sub.abc1, i.sub.abc2 for the windings W_1, W_2 of a dual-stator PMSG, and the generator torque 87. The experimental values were observed over a fault-free interval N and a fault interval F commencing at time t.sub.F after an open-circuit fault in one switch of one phase in a generator-side frequency converter. The fault interval F demonstrates the outcome in the absence of any fault-tolerant control: the winding currents i.sub.abc2 of the second stator remain steady since the generator-side frequency converter of the second stator is healthy, but the generator torque exhibits pronounced ripple as can be seen by the distinct peaks in this interval F.
[0056] FIG. 12 is similar to FIG. 11, and shows the outcome of two open-circuit faults F1, F2 in one of the generator-side frequency converters. In this scenario, the generator torque 88 exhibits severe ripple as can be seen in the interval F.
[0057] 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.
[0058] 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.
