ELECTRICAL POWER SYSTEM OF A WIND TURBINE

Abstract

An electrical power system of a wind turbine is provided. A power converter of the wind turbine is configured to convert electrical power generated by an electrical generator of the wind turbine and to provide the converted electrical power to a power grid. The electrical power system includes an inductor unit coupled between a grid side converter section of the power converter and the power grid. Electrical power provided by the power converter towards the power grid causes a voltage drop across the inductor unit. The power system further includes an inductance adjustment unit coupled to the inductor unit. The inductance adjustment unit is configured to adjust an inductance value of the inductor unit to thereby adjust the voltage drop across the inductor unit.

Claims

1. An electrical power system of a wind turbine, wherein a power converter of the wind turbine is configured to convert electrical power generated by an electrical generator of the wind turbine and to provide the converted electrical power to a power grid, wherein the electrical power system comprises: an inductor unit coupled between a grid side converter section of the power converter and the power grid wherein electrical power provided by the power converter towards the power grid causes a voltage drop across the inductor unit; an inductance adjustment unit coupled to the inductor unit, wherein the inductance adjustment unit is configured to adjust an inductance value of the inductor unit to thereby adjust the voltage drop across the inductor unit; and a controller configured to receive a monitoring signal that is indicative of at least one of a voltage on a DC link of the power converter, a voltage at a grid side output of the power converter, a voltage on a connection from the inductor unit to the power grid, or a voltage on the power grid, wherein the controller is configured to control the inductance value of the inductor unit by means of the inductance adjustment unit based on the voltage indicated by the monitoring signal, wherein the controller: is configured to compare the voltage indicated by the monitoring signal to a respective voltage threshold, wherein if the voltage exceeds the voltage threshold, the controller controls the inductance adjustment unit so as to reduce the inductance value of the inductor unit.

2. The electrical power system according to claim 1, further comprising a filter configured to filter electrical power converted by the power converter and provided towards the power grid, wherein the inductor unit forms part of the filter.

3. The electrical power system according to claim 2, wherein the filter is a harmonic filter.

4. The electrical power system according to claim 1, wherein the inductance adjustment unit comprises one or more switches coupled to the inductor unit and configured to be switchable to electrically bypass part of the inductor unit to change the inductance value of the inductor unit.

5. The electrical power system according to claim 4, wherein the inductance adjustment unit comprises one or more taps coupled to at least one inductor of the inductor unit, wherein a switch of the one or more switches is electrically connected to a respective tap such that by closing the switch, a part of the inductor is electrically bypassed.

6. The electrical power system according to claim 5, wherein the one or more switches comprise a further switch connected in series with the inductor, the further switch being connected to a terminal (36) of the inductor part that is being bypassed by closing the switch associated with the tap, the tap being connected at the other end of the inductor part that is being bypassed.

7. The electrical power system according to claim 4, wherein the inductor unit comprises at least two distinct inductors connected in series between the grid side converter section of the power converter and the power grid, wherein a switch of the one or more switches is connected to a point between the two inductors such that by closing the switch, one or more of the at least two inductors are electrically bypassed.

8. The electrical power system according to claim 7, wherein the at least two distinct inductors are coils.

9. The electrical power system according to claim 4, wherein the one or more switches comprise a power electronic switch and/or comprise an electromechanical switch.

10. The electrical power system according to claim 1, wherein the inductor unit is connected between the grid side converter section and a transformer of the wind turbine.

11. The electrical power system according to claim 1, wherein the controller is configured to obtain the voltage on the DC link of the power converter or at a grid side output of the power converter, and is further configured to control the inductance adjustment unit so as to reduce the inductance value of the inductor unit the voltage on the DC link or the voltage at the grid side output of the power converter, respectively, exceeds a respective threshold voltage to thereby reduce the voltage on the DC link of the power converter.

12. The electrical power system according to claim 1, wherein the controller is implemented in a converter controller of the power converter or is implemented in a wind turbine controller of the wind turbine.

13. A wind turbine, comprising: a power converter configured to convert electrical power generated by an electrical generator of the wind turbine and to provide the converted electrical power to a power grid; and the electrical power system according to claim 1.

14. A method of controlling the operation of an electrical power system of a wind turbine, wherein the wind turbine includes a power converter configured to convert electrical power generated by an electrical generator of the wind turbine and to provide the converted electrical power to a power grid, and wherein an inductor unit is coupled between a grid side converter section of the power converter and the power grid, wherein electrical power provided by the power converter towards the power grid causes a voltage drop across the inductor unit, wherein the method comprises: adjusting, by means of an inductance adjustment unit coupled to the inductor unit, an inductance value of the inductor unit to thereby adjust the voltage drop across the inductor unit, wherein adjusting the inductance value comprises: obtaining a monitoring signal that is indicative of at least one of a voltage on a DC link of the power converter, a voltage at a grid side output of the power converter a voltage on a connection from the inductor unit to the power grid, or a voltage on the power grid; and controlling the inductance value of the inductor unit by means of the inductance adjustment unit based on the obtained monitoring signal by comparing the voltage indicated by the monitoring signal to a respective voltage threshold, wherein if the voltage exceeds the voltage threshold, the inductance adjustment unit is controlled to reduce the inductance value of the inductor unit.

Description

BRIEF DESCRIPTION

[0039] Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

[0040] FIG. 1 is a schematic diagram showing loss of reactive power providing capability of a wind turbine at higher grid voltages;

[0041] FIG. 2 is a schematic drawing showing an electrical power system comprising an inductance adjustment unit according to an embodiment of the invention;

[0042] FIG. 3 is a schematic drawing showing an electrical power system comprising an inductance adjustment unit according to an embodiment of the invention;

[0043] FIG. 4 is a schematic drawing showing an electrical power system comprising an inductance adjustment unit according to an embodiment of the invention; and

[0044] FIG. 5 is a schematic flow diagram illustrating a method of controlling the inductance value of an inductor unit according to an embodiment of the invention.

DETAILED DESCRIPTION

[0045] In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of the embodiments is given only for the purpose of illustration and is not to be taken in a limiting sense. It should be noted that the drawings are to be regarded as being schematic representations only, and elements in the drawings are not necessarily to scale with each other. Rather, the representation of the various elements is chosen such that their function and general purpose become apparent to a person skilled in the art. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted.

[0046] As indicated above, FIG. 1 is a schematic diagram that illustrates how the maximum reactive power capability, the DC link voltage of a power converter and the grid side converter current I.sub.r behave if the grid voltage changes. The inventors of embodiments of the present invention have found that the DC link voltage reaching its upper limit relatively quickly upon the grid voltage reaching or exceeding nominal voltage values is also caused by the inductor comprised in the filter, in particular harmonic filter, that filters the output of the power converter. Examples of such filter 16 with an inductor unit 30 are for example shown in FIGS. 2 to 4. Current grid code requirements may only be met if the inductor has a relatively high inductance value, resulting in a significant voltage drop across the filter inductor, so that the power converter needs to be operated at higher voltages when supplying electric power to the grid. Consequently, the DC link voltage quickly reaches its upper limit if grid voltage increases. For example, a filter inductance of 400.Math.H at 60 Hz grid frequency results in a reactance of 151 mΩ for the inductor. If the maximum voltage UDC.sub.max on the DC link, including a safety margin, is 1175 V, then the maximum RMS operating voltage is 830.85 V. At a rated grid side current at the power converter output of 400 A, the voltage drop across the inductor is 151 mΩ × 400 A = 60.4 V. Consequently, when operating at the upper limit, the power converter can only deliver at maximum 830.85 V - 60.4 V = 770.45 V towards the power grid, corresponding to 1.116Pu at a nominal operating voltage of 690 V on the connection towards the transformer (LV side of transformer). It should be clear that the voltage limit will be reached much faster upon the use of higher inductance values. In accordance with embodiments of the invention, the inductance value is now modified and in particular reduced to increase the maximum reactive power providing capability of the power converter and thus of the wind turbine. The inductance value of the inductor of the filter is adapted so that the DC link voltage is reduced and more reactive power can be provided. The electrical stability of the power grid is thereby improved by extending, for a short period of time, the reactive power capabilities. During such time, and in particular during electrical transient events, the grid code requirements may allow that some requirements are not met, and in particular that harmonic emissions may be increased during such short period of time. Reactive power capability is thus increased at the expense of the emission of harmonics, since such harmonic emissions of the wind turbine are increased if the filter inductance value is reduced.

[0047] FIG. 2 schematically illustrates a power system 10 that may comprise a harmonic filter 16 that may include the inductor unit 30 comprising or consisting of the inductor 31. The power system 10 may further comprise the inductance adjustment unit 40 that may be configured to change the inductance value of inductor unit 30, so that the inductance value may be set to the most advantageous value for the respective circumstances. For the sake of simplicity, other components of the harmonic filter 16 are not shown.

[0048] The inductor unit 30 and the inductance adjustment unit 40 may be employed in any power system configuration, such as in the DFIG configuration of FIG. 2 of the full converter topology of FIG. 3. In FIG. 2, a DFIG 11 may include a rotor 12 and a stator 13, wherein the stator may be coupled, via a power transformer 15, to power grid 100, as common for such topologies. A power converter 20 may include a grid side converter section 21, a generator side converter section 22 and an intermediate DC link 23 connecting these sections may be connected to the rotor 12 of DFIG 11. Reference numeral 14 indicates the generator inductance. Inductor unit 30 may be connected in series between the grid side converter section 21 and the power grid 100, it may in particular be connected between the power converter output and the transformer 15. The AC output of the grid side converter section 21 may be in particular connected, for example directly connected, to an input terminal 35 of the inductor 31. An output terminal 36 of the inductor 31 may be coupled to the power grid 101, it may for example be connected to a lower voltage side of the wind turbine transformer 15. Accordingly, converted electrical power provided by power converter 20 to power grid 100 may pass through inductor 31 and may thereby be filtered. Power converter 20 may handle between 20 and 50% of electrical power generated by DFIG 11, for example about 30%.

[0049] To change the inductance value of inductor unit 30, one or more taps 45 may be provided on the inductor 31. Accordingly, parts of the inductor 31, in particular windings of the coil that provides the inductor 31, may be taken out of the circuit so that the converted electrical power does not pass through the respective coil windings. Consequently, the voltage drop across the remaining parts of the inductor 31 may be reduced (as the inductance value and thus the reactance is reduced), so that power converter 20 can operate at a lower DC link voltage. Switch 41 may be associated with tap 45 and may be connected such that by closing the switch 41, the respective part of the inductor may be electrically bypassed. A further switch 42 may be connected in series with the inductor 31, in particular between a second terminal (output terminal) 36 and the transformer 15. The further switch 42 may be closed during operation with nominal inductance value (i.e. if the inductance value is not adjusted). Upon connecting tap 45 by closing switch 41, the further switch 42 may be opened to thereby avoid that a short circuit is formed through the bypassed part of the inductor. High-short circuit currents and unnecessary power dissipation may thereby be avoided, since currents will still be induced in the bypassed part of inductor 31, as the magnetic field produced by the active part of inductor 31 may still extend into the bypassed part, thereby causing respective induction. It should be clear that in other configurations, the further switch 42 may be connected to the input terminal 35 of inductor 31, and switch 41 may be connected between tap 45 and the connection to the power converter so that the other part of the inductor 31 may be bypassed.

[0050] For example, one to ten, or one to five taps may be provided on inductor 31. The taps 45 may include a tap that completely bypasses the inductor 31, but, the tap at which the lowest inductance value is achieved may still correspond to a certain percentage of the maximum inductance value, such as 10% or more. Voltage drop across inductor 31 may depend on the inductance value (without adjustment) and the nominal operating voltage and frequency on the power grid. The taps may for examples be chosen such that between 5% and 10% of voltage variation of the power grid voltage from the nominal grid voltage may be compensated by changing the inductance value using inductance adjustment unit 40. As example, five taps may be provided, each tap switching about 2% of the nominal grid voltage. Accordingly, if the grid voltage rises by 2%, a tap may be switched, thereby returning the voltage at the output of power converter 20 to the nominal operating value.

[0051] The switching may be accordingly performed such that voltage changes of the power grid voltage may be compensated by complementary changes of the voltage drop across the inductor unit 30.

[0052] The one or more switches 41, 42 of inductance adjustment unit 40 may be power electronic switches or electromechanical switches. In the example of FIG. 2, each switch may be implemented as a switching valve comprising of two anti-parallel connected thyristors. Other power electronic devices, such as IGBTs or the like may also be used for switching. When implemented as electromechanical switch, vacuum circuit breakers, switching relays or the like may be used. The switches may be on load switches and may be configured to provide switching with a response time of less than 10 seconds. The advantage of power electronic switches is that the switching times may be in the millisecond range. The switching time may be less than 1 second, in particular less than 100 ms, e.g. about 5-50 ms (a typical switching time would be 10 ms). Mechanical switches may further suffer from a limited number of possible switching cycles.

[0053] The electrical power system 10 may further comprise a controller 50 configured to control the inductance adjustment unit 40. It may also comprise a converter controller 25, wherein in the example of FIG. 2, controller 50 may be implemented as part of converter controller 25. Other solutions are also conceivable, in which controller 50 may for example be implemented in a wind turbine controller, may be a separate stand alone controller or may be distributed among plural controllers. The controller 50 may control the one or more switches 41, 42 to adjust the inductance value of inductor unit 30. For this purpose, it may obtain a monitoring signal that may indicate a voltage value which may have an impact on the operating voltage of power converter 20. For example, using a voltage sensor 61, the DC link voltage may be measured and provided as a monitoring signal to controller 50. Using sensor 62, a voltage at the AC output of the grid side converter section 21 may be measured and provided as monitoring signal. Sensor 63 may measure a voltage between the inductor unit 30 and a coupling point 101 to the power grid 100, e.g. between inductor unit 30 and transformer 15, between transformer 15 and coupling point 101 or at coupling point 101. The respective voltage measurement may be indicative of the grid voltage and may be supplied as monitoring signal to controller 50. Further, grid voltage may also directly be measured using sensor 64 and provided as a monitoring signal to controller 50. Based on the received monitoring signal(s), the controller 50 may control the inductance adjustment unit 40. It should be clear that one of the monitored voltage signals may be sufficient to provide the respective control, although a combination of signals may be used. Furthermore, it should be clear that the power system 10 may not need to comprise sensors 61 to 64, but that the respective information may be obtained by communication with a respective information source, such as obtaining grid voltage values from a grid operator, obtaining power converter DC link voltage or AC output voltage from a converter controller or the like.

[0054] Controller 50 may provide feed-forward control, for example by basing the control on a voltage signal that may not be directly influenced by the setting of inductance adjustment unit 40, for example by making use of measurements from sensors 63 or 64. Controller 50 may include a number of respective voltage thresholds that may correspond to the different available taps 45 of inductance adjustment unit 40. If the voltage value reaches or exceeds a first threshold, the first tap may be switched, thus decreasing the inductance value by one step. If the voltage increases further and reaches a second threshold, a respective second tap may be switched, thus reducing the inductance value by a further step. The voltage difference between these thresholds may in particular correspond to the change in voltage drop across the inductor 31 caused by switching the respective taps, so that the voltage increase of the grid voltage may be compensated.

[0055] Additionally or alternatively, controller 50 may implement feedback control by basing the control on a monitored voltage that may be influenced by the change of the inductance value, for example by making use of voltage values measured by sensors 61 or 62. For example, a single upper threshold above a nominal operating voltage value may be employed, and if the monitored voltage exceeds or reaches such upper threshold, the controller may cause inductance adjustment unit 40 to reduce the inductance value. Consequently, the voltage monitored by sensor 61 or 62 may drop towards the nominal operating voltage due to the reduced voltage drop across inductor 31. If grid voltage further increases, the monitored voltage will again reach the upper threshold, upon which the controller may cause switching to a next tap, thus again reducing the voltage monitored by sensors 61, 62. This may be performed until the last tap is reached. The threshold may be selected such that the monitored voltage does not reach or exceed a respective maximum operating voltage.

[0056] A respective lower threshold may be defined so that upon the monitored voltage dropping below the lower threshold, controller 50 may cause adjustment unit 40 to increase the inductance value again. Thereby, the system may return to normal operation upon the grid voltage returning to its nominal value.

[0057] Likewise, in case of the feed-forward control, if the voltage monitored by sensors 63 or 64 drops again below the defined thresholds, the controller 50 may cause adjustment unit 40 to again increase the inductance value by switching to the next tap until the whole inductor 31 again forms part of the circuit.

[0058] A respective example for feedback control (e.g. using sensor 61 or 62) is illustrated in FIG. 5. In step S1, the voltage on the DC link and/or at the grid side converter output may be monitored by controller 50. In step S2, it may be checked if the monitored voltage exceeds the upper threshold. If this is the case, then it may be checked in step S3 if the inductance adjustment unit 40 is already operating at the last tap. If not, then the tap of the inductor 31 may be changed so as to decrease the inductance value (step S4). Monitoring of the respective voltage may then continue in step S1. If the last tap has already been reached in step S3, then other mitigation measures may be taken in the optional step S5. As an example, the maximum output current of the power converter may be reduced in order to prevent damage to the converter, thereby limiting the reactive power capability. Also, the converter may be operated according to an over-modulating strategy according to which the converter can be driven to higher output voltage levels. Also, to avoid damage to the converter, the converter may be disconnected.

[0059] If the monitored voltage does not exceed the upper threshold in step S2, it may be checked in step S6 if it has dropped below a lower threshold. If this is not the case, then the monitored voltage is within range and operation may continue in step S1. If the voltage has dropped below the lower threshold, it may be checked in step S7 if the inductance adjustment unit 40 is already operating without any reduction of the inductance value (i.e. at “tap zero”). If this is not the case, then the inductor tap may be changed in step S8 to increase the inductance value. Operation may then continue in step S1. If it is determined in steps S7 that the inductor unit 40 is already operating without any reduction of the inductance value, then other mitigation measures may be taken in the optional step S9. Such mitigation measures may for example include under-voltage ride-through measures or the like. Operation may then continue in step S1. It should be clear that the example of FIG. 5 only illustrates the control of the inductance adjustment unit 40 by controller 50, and does not include further operating steps performed by the electrical power system 10, such as the converter control.

[0060] It should also be clear that the method may likewise be performed in a feed-forward mode in which plural thresholds are employed for switching the taps based on voltages measured by sensors 63, 64, as outlined above.

[0061] By such power system and such method, the power converter 20 may, at higher grid voltages, accordingly be operated below the maximum allowable DC link voltage. In other words, when looking at the diagram of FIG. 1, the curve 202 may reach the maximum voltage value of 1200 V only at higher voltages of the grid voltage V.sub.Grid, for example only at 1.05 or 1.1pu, depending on the inductor size and the minimum inductance value that is retained in the circuit. The grid side converter current I.sub.r indicated by curve 201 may thereby be maintained at higher values and the power converter may provide an improved reactive power capability 203, i.e. more reactive power can be provided at higher grid voltages. The grid voltage range over which the wind turbine can operate efficiently can thereby be increased and in particular, the reactive power providing capabilities are improved. Also, the electrical performance during grid transient events may be improved, such as voltage stability, over-voltage ride-through and the like. Furthermore, a significant performance increase may be achieved while the costs of implementing the power system 10 may be only moderate.

[0062] The electrical power system 10 may comprise the controller 50, the inductor unit 30 and the inductance adjustment unit 40. It may further comprise the power converter 20. Further, it may comprise the generator 11 and/or the transformer 15. The electrical power system 10 may be comprised in a wind turbine 200. A rotor (not shown) of the wind turbine may be rotationally coupled to the generator 11 to transfer rotational mechanical energy. Power grid 100 may be a wind farm grid, yet it may also be a utility power grid. As such, it should be clear that further intervening components may be provided between power grid 100 and transformer 15, such as further transformer, further power converters, a sub-station and the like. Power system 10 may comprise further components such as circuit breakers and the like that are common to wind turbine power systems.

[0063] Controller 50 may comprise a processor 51, in particular a microprocessor, an ASIC, a DSP or the like and may further comprise a memory 52, such as RAM, ROM Flash-Memory, a hard-disk drive or the like. The memory may comprise control instructions which when executed by the processor of controller 50 perform any of the control methods described herein.

[0064] FIG. 3 illustrates an embodiment that is a modification of the embodiment of FIG. 2, so only differences will be described. In the embodiment of FIG. 3, the power system 10 may be implemented in a wind turbine that may comprise a full converter solution. Generator 11 may for example be a permanent magnet synchronous generator (PMSG), or an asynchronous generator. All power generated by generator 11 may substantially pass through the power converter 20. Again, the inductor unit 30 may be series-connected between the power converter 20 and the power grid 100, so that the generated and converted electrical power may pass through the inductor unit 30. The configuration of the inductance adjustment unit 40 and the controller 50 may be similar to the one described above with respect to FIGS. 2 and 5.

[0065] FIG. 4 illustrates a modification of the inductor unit 30 and the inductance adjustment unit 40 that may be implemented in the power topologies of FIGS. 2 and 3 and that may be implemented additionally or alternatively to the taps used in these embodiments. In the embodiment of FIG. 4, the inductor unit 30 may comprise two or more distinct inductors 31, 32. A switch 43 may be connected via connection 46 to the connection between the two inductors 31, 32. By closing the switch 43, the second inductor 32 may be electrically bypassed (i.e. converted electrical power provided to power grid 100 does not pass through inductor 32). In this case, no switch corresponding to the further switch 42 needs to be provided, since no magnetic field exists in the separate second inductor 32, so that no short circuit currents are induced. In correspondence to the number of taps described with respect to FIG. 2, a respective number of separate inductors and connections 46 may be provided, thus providing the same inductance switching capabilities. Switching may then again occur in accordance with the above described feedback or feed-forward control (see for example FIG. 5), although no taps may be switched, but distinct inductors 31, 32, ... may be switched. The above explanations are thus equally applicable.

[0066] Further, it is certainly possible to combine the switching of taps and the switching of whole distinct inductors, i.e. one or more of the inductors 31, 32, ... may be provided with respective taps. This may allow the fast switching of the inductance value over a wide range.

[0067] Again, as mentioned above, switch 43 may be implemented as a power electronic switch (e.g. anti-parallel thyristors), or as an electromechanical switch.

[0068] Other ways of switching the inductance value may be certainly conceivable and may be implemented with embodiments of the present invention. For example, the inductance value may be changed by changing the position of an inductor core, changing the relative angular position (and thus the orientation of the magnetic field) of two inductors, in correspondence to a variometer, using a sliding contact on a coil or the like. However, such mechanical solutions may suffer from longer switching times and a reduced number of switching cycles, so that the switching by power electronic switches is preferred.

[0069] Although the present invention has been disclosed in the form of 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.

[0070] 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.