APPARATUS AND METHOD FOR SUPPLYING POWER TO OR ABSORBING POWER FROM A LOAD

Abstract

An apparatus is configured to supply power to a load or absorb power from the load, the load being connected or connectable to a load conductor. The apparatus comprises a power supplying and/or absorbing device, configured for selectively supplying power to the load conductor or absorbing power from the load conductor, and a control unit configured to control the power supplying and/or absorbing device. The control unit is configured to determine, based on at least one value indicative of voltage of the load conductor and a virtual impedance of the power supplying and/or absorbing device, a voltage reference value for the power supplying and/or absorbing device, and control the power supplying and/or absorbing device to supply power to the load conductor, and thereby supply power to the load, or absorb power from the load conductor, and thereby absorb power from the load, based on the determined voltage reference value.

Claims

1. An apparatus configured to supply power to a load connected with a power system or absorb power from the load, the load being connected or connectable to a load conductor, the apparatus comprising: a power supplying and/or absorbing device connected to the load conductor and configured for selectively supplying power to the load conductor or absorbing power from the load conductor, wherein the power supplied to the load conductor or absorbed from the load conductor by the power supplying and/or absorbing device is governed at least by a voltage reference value of the power supplying and/or absorbing device; and a control unit configured to control operation of the power supplying and/or absorbing device, the control unit being further configured to: obtain at least one value indicative of voltage of the load conductor; determine, based on the at least one value indicative of voltage of the load conductor and a virtual impedance of the power supplying and/or absorbing device, a voltage reference value for the power supplying and/or absorbing device; and control the power supplying and/or absorbing device to supply power to the load conductor, and thereby supply power to the load, or absorb power from the load conductor, and thereby absorb power from the load, based on the determined voltage reference value; wherein the virtual impedance of the power supplying and/or absorbing device is associated with a virtual reactance and a virtual resistance, wherein the power supplying and/or absorbing device is configured such that a value of the virtual reactance is higher than the value of a reactance of the power supplying and/or absorbing device, and such that a value of the virtual resistance is less than the value of the virtual reactance.

2. An apparatus according to claim 1, wherein the virtual reactance is a sum of the reactance of the power supplying and/or absorbing device and a selected percentage of the reactance of the power supplying and/or absorbing device, the selected percentage of the reactance of the power supplying and/or absorbing device being in a range between 1% and 50% of the reactance of the power supplying and/or absorbing device.

3. An apparatus according to claim 1, wherein the virtual reactance is such that, by the determined voltage reference value for the power supplying and/or absorbing device, the power supplying and/or absorbing device is capable of supplying power to the load conductor or absorbing power from the load conductor so as to keep any fluctuations in voltage of the load conductor, compared to an average voltage level of the voltage of the load conductor, over a period of time below a selected threshold voltage fluctuation level while at the same time keeping the virtual reactance as small as possible.

4. An apparatus according to claim 1, wherein the virtual resistance is such that a value of the virtual resistance is between 25% and 75% of the value of the virtual reactance.

5. An apparatus according to claim 1, wherein the control unit is further configured to: obtain a plurality of values indicative of current of the load at different time instants during a period of time; determine a change in current of the load over the period of time based on the plurality of values indicative of current of the load at the different time instants; and determine the voltage reference value for the power supplying and/or absorbing device further based on the determined change in current of the load over the period of time.

6. An apparatus according to claim 5, wherein the control unit is configured to determine the voltage reference value for the power supplying and/or absorbing device based on the plurality of values indicative of current of the load at the different time instants to decrease any fluctuations in the current of the load, compared to an average current level of the current of the load, over a period of time.

7. An apparatus according to claim 1, wherein the control unit is further configured to obtain at least one value indicative of current of the power system and to determine the voltage reference value for the power supplying and/or absorbing device further based on the least one value indicative of current of the power system.

8. An apparatus according to claim 7, wherein the control unit is configured to determine the voltage reference value for the power supplying and/or absorbing device based on the least one value indicative of current of the power system and the at least one value indicative of voltage of the load conductor to increase the power factor of the load.

9. An apparatus according to claim 1, wherein the power supplying and/or absorbing device is based on a Voltage Source Converter (VSC) based device, a Static Synchronous Compensator (STATCOM), and/or a multi-level converter.

10. An apparatus according to claim 1, wherein the power supplying and/or absorbing device is directly connected to the load conductor.

11. A method implemented in an apparatus configured to supply power to a load connected with a power system or absorb power from the load, the load being connected or connectable to a load conductor, the apparatus comprising a power supplying and/or absorbing device connected to the load conductor and configured for selectively supplying power to the load conductor or absorbing power from the load conductor, wherein the power supplied to the load conductor or absorbed from the load conductor by the power supplying and/or absorbing device is governed at least by a voltage reference value of the power supplying and/or absorbing device, the method comprising: obtaining at least one value indicative of voltage of the load conductor; determining, based on the at least one value indicative of voltage of the load conductor and a virtual impedance of the power supplying and/or absorbing device, a voltage reference value for the power supplying and/or absorbing device; and controlling the power supplying and/or absorbing device to supply power to the load conductor, and thereby supply power to the load, or absorb power from the load conductor, and thereby absorb power from the load, based on the determined voltage reference value; wherein the virtual impedance of the power supplying and/or absorbing device is associated with a virtual reactance and a virtual resistance, wherein the power supplying and/or absorbing device is configured such that a value of the virtual reactance is higher than the value of a reactance of the power supplying and/or absorbing device, and such that a value of the virtual resistance is less than the value of the virtual reactance.

12. A non-transitory computer-readable medium having a computer program comprising instructions stored thereon, wherein the instructions, when executed by one or more processors comprised in a control unit, cause the control unit to perform a method according to claim 11.

13. A system comprising: a power system; a load connected with the power system; a load conductor, wherein the load is connected or connectable to the load conductor; and an apparatus according to claim 1 configured to supply power to the load or absorb power from the load.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Exemplifying embodiments of the present invention will be described below with reference to the accompanying drawings.

[0040] FIG. 1 is a schematic view of a system according to an embodiment of the present invention.

[0041] FIG. 2 is a schematic flowchart of a method according to an embodiment of the present invention.

[0042] All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate embodiments of the present invention, wherein other parts may be omitted or merely suggested.

DESCRIPTION WITH REFERENCE TO THE DRAWINGS

[0043] The present invention will now be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments of the present invention set forth herein; rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the present invention to those skilled in the art.

[0044] FIG. 1 is a schematic view of a system according to an embodiment of the present invention, which system comprises an apparatus according to an embodiment of the present invention.

[0045] In accordance with the embodiment of the present invention illustrated in FIG. 1, the system comprises a power system 2, a transformer 4, a load 1 that is connected with the power system 2, and a load conductor 3. The load 1 is connected or connectable to the load conductor 3, possibly via a transformer 5. The transformer 5, which may be referred to as a load transformer, may be omitted. The power system 2 is connected or connectable to the load conductor 3 via the transformer 4. The load conductor 3 may for example comprise a bus, or busbar. The power system 2 may for example comprise a power transmission and/or distribution system. The power system 2 may for example comprise a power grid, e.g., a power transmission and/or distribution grid. The load 1 may for example include an electrical arc furnace (EAF) but is not limited thereto. The transformer 4 is connected between the power system 2 and the load 1, and as such the transformer 4 could, according to one or more embodiments of the present invention, be referred to (or comprising) as a step-down transformer. The transformer 4 may be omitted.

[0046] As illustrated in FIG. 1, the load 1 may be connectable to the load conductor 3 by means of a switch 8, which may be normally closed (e.g., so that the switch 8 conducts current therethrough) when or whenever the load 1 is in operation.

[0047] The system comprises an apparatus that is configured to supply power to the load 1 connected with the power system 2 or absorb power from the load 1. The apparatus may be configured to supply power to the load 1, e.g., in addition to any power supplied to the load 1 from the power system 2.

[0048] The apparatus comprises a power supplying and/or absorbing device 6. The power supplying and/or absorbing device 6 is connected to the load conductor 3 and is configured for selectively supplying power to the load conductor 3 or absorbing power from the load conductor 3. The power supplied to the load conductor 3 or absorbed from the load conductor 3 by the power supplying and/or absorbing device 6 is governed at least by a voltage reference value of the power supplying and/or absorbing device 6.

[0049] The apparatus comprises a control unit 7. The control unit 7 is configured to control operation of the power supplying and/or absorbing device 7. The apparatus, comprising the power supplying and/or absorbing device 6 and the control unit 7, is configured to supply power to the load 1 or absorb power from the load 1.

[0050] The power supplying and/or absorbing device 6 may for example be based on a Voltage Source Converter (VSC) based device, a Static Synchronous Compensator (STATCOM) and/or a multi-level converter. The STATCOM may for example have a delta topology. However, the STATCOM is not limited thereto, and could in alternative have, e.g., a wye topology. The multi-level converter may for example comprise a three-level converter.

[0051] For example, in case the power supplying and/or absorbing device 6 is based on a STATCOM having a delta topology, the power supplying and/or absorbing device 6 may be connected to the load conductor 3 at, or via a conductor connected to, a corner point (e.g., terminal) of the delta topology-configured STATCOM.

[0052] The power supplying and/or absorbing device 6 may be directly connected to the load conductor 3, as illustrated in FIG. 1, or may be indirectly connected to the load conductor 3 (e.g., via one or more intermediate devices or components).

[0053] The control unit 7 is configured to obtain at least one value indicative of voltage of the load conductor 3, and determine, based on the at least one value indicative of voltage of the load conductor 3 and a virtual impedance of the power supplying and/or absorbing device 6, a voltage reference value for the power supplying and/or absorbing device 6. The control unit 7 is configured to control the power supplying and/or absorbing device to supply power to the load conductor 3, and thereby supply power to the load 1, or absorb power from the load conductor 3, and thereby absorb power from the load 1, based on the determined voltage reference value. The virtual impedance of the power supplying and/or absorbing device 6 is associated with a virtual reactance and a virtual resistance, e.g., as components of the virtual impedance. The power supplying and/or absorbing device is (or has been) configured such that a value of the virtual reactance is higher than the value of a reactance of the power supplying and/or absorbing device 6, and such that a value of the virtual resistance is less than the value of the virtual reactance.

[0054] The reactance of the power supplying and/or absorbing device 6 may be the physical reactance of the power supplying and/or absorbing device 6. In other words, the reactance of the power supplying and/or absorbing device may be the reactance of the power supplying and/or absorbing device 6 resulting from the reactance of individual elements or components of the power supplying and/or absorbing device 6. For example, a delta topology-configured STATCOM may comprise three converter arms wherein each converter arm may include a plurality of converter cells connected in series with a reactor and possibly also another or other components. In case the power supplying and/or absorbing device 6 is based on a STATCOM having a delta topology, the reactors in the respective one of the converter arms may define at least in part the reactance of the power supplying and/or absorbing device 6. Similarly, in case the power supplying and/or absorbing device 6 would be based on a STATCOM having a topology other than delta topology, any reactors in the respective one of a plurality of converter arms of the STATCOM may define at least in part the reactance of the power supplying and/or absorbing device 6.

[0055] In case the power supplying and/or absorbing device 6 would be out of service it may be disconnected from the load conductor 3 by means of a switch 9, which may be normally closed (e.g., so that the switch 9 conducts current therethrough) when or whenever the power supplying and/or absorbing device 6 is in service.

[0056] The at least one value indicative of voltage of the load conductor 3 may for example be or have been measured or sensed by at least one sensor such as at least one voltage transducer. In accordance with the embodiment of the present invention illustrated in FIG. 1, a sensor 13, including a potential transformer, is provided for sensing voltage of the load conductor 3. As illustrated in FIG. 1, the control unit 7 may be connected with the sensor 13 in order to obtain the (at least one value indicative of the) voltage of the load conductor 3. The sensor 13 may be part of the apparatus, which hence may include the sensor 13.

[0057] In accordance with the embodiment of the present invention illustrated in FIG. 1, the control unit 7 includes first, second, third and fourth sub-units 15 to 18, respectively.

[0058] The first sub-unit 15 takes as input the (at least one value indicative of the) voltage of the load conductor 3, which in accordance with the embodiment of the present invention illustrated in FIG. 1 is provided by the sensor 13. Thus, the first sub-unit 15 may obtain the (at least one value indicative of the) voltage of the load conductor 3 from the sensor 13. The first sub-unit 15 implements the virtual impedance-based controlling of the power supplying and/or absorbing device 6 as described in the foregoing in this section of the description and elsewhere herein. The first sub-unit 15 is configured to determine, based on the (at least one value indicative of the) voltage of the load conductor 3 and the virtual impedance of the power supplying and/or absorbing device 6, a first voltage reference value for the power supplying and/or absorbing device 6. The output from the first sub-unit 15 is the first voltage reference value, which is transmitted to the fourth sub-unit 18. The first voltage reference value may have a component corresponding to active power and a component corresponding to reactive power.

[0059] The second sub-unit 16 takes as inputs at least one value indicative of current of the power system 2 and the (at least one value indicative of the) voltage of the load conductor 3. As for the first sub-unit 15, the (at least one value indicative of the) voltage of the load conductor 3 is, in accordance with the embodiment of the present invention illustrated in FIG. 1, provided by the sensor 13. Further in accordance with the embodiment of the present invention illustrated in FIG. 1, the at least one value indicative of current of the power system 2 is provided by a sensor 11, which for example may include a current transducer, which as illustrated in FIG. 1 may be connected to the power system 2. Thus, the (at least one value indicative of the) current of the power system 2 may be determined directly. It could be determined indirectly for example based on measuring or sensing current of the power supplying and/or absorbing device 6 and current of the load 1. As illustrated in FIG. 1, the control unit 7 may be connected with the sensor 11 in order to obtain the (at least one value indicative of the) current of the power system 2. The sensor 11 may be part of the apparatus, which hence may include the sensor 11. Thus, the second sub-unit 16 obtains the (at least one value indicative of the) current of the power system 2 from the sensor 11 and the (at least one value indicative of the) voltage of the load conductor 3 from the sensor 13. The second sub-unit 16 is configured to determine a second voltage reference value for the power supplying and/or absorbing device 6 based on the (at least one value indicative of the) current of the power system 2 and the (at least one value indicative of the) voltage of the load conductor 3 to increase the power factor of the load 1. The output from the second sub-unit 16 is the second voltage reference value, which is transmitted to the fourth sub-unit 18. The second voltage reference value may have (e.g., only have) a component corresponding to reactive power. In this way, the second sub-unit 16 may implement a power factor control algorithm or method.

[0060] The third sub-unit 17 takes as input a plurality of values indicative of current of the load 1 at different time instants during a period of time. In accordance with the embodiment of the present invention illustrated in FIG. 1, the plurality of values indicative of current of the load 1 at different time instants during a period of time are provided by a sensor 12, which for example may include a current transducer, which as illustrated in FIG. 1 may be connected to the load 1. As illustrated in FIG. 1, the control unit 7 may be connected with the sensor 12 in order to obtain the plurality of values indicative of current of the load 1 at different time instants during a period of time. The sensor 12 may be part of the apparatus, which hence may include the sensor 12. Thus, the third sub-unit 17 obtains plurality of values indicative of current of the load 1 at different time instants during a period of time from the sensor 12. The third sub-unit 17 is configured to determine a change in current of the load 1 over the period of time based on the plurality of values indicative of current of the load 1 at the different time instants, and to determine a third voltage reference value for the power supplying and/or absorbing device 6 based on the determined change in current of the load 1 over the period of time to decrease any fluctuations in the current of the load 1, compared to an average current level of the current of the load 1, over a period of time. The output from the third sub-unit 17 is the third voltage reference value, which is transmitted to the fourth sub-unit 18. The third voltage reference value may have a component corresponding to active power and a component corresponding to reactive power. In this way, the third sub-unit 17 may implement a flicker control algorithm or method.

[0061] In the fourth sub-unit 18, the respective outputs from the first sub-unit 15, the second sub-unit 16 and the third sub-unit 17 may be combined (e.g., summed) into a voltage reference value for the power supplying and/or absorbing device 6. The output from the fourth sub-unit 18i.e., the voltage reference value for the power supplying and/or absorbing device 6is transmitted to the power supplying and/or absorbing device 6 as illustrated in FIG. 1.

[0062] It is to be understood that each of the second sub-unit 16 and the third sub-unit 17 is optional, and that one or both of the second sub-unit 16 and the third sub-unit 17 may be omitted. In case both of the second sub-unit 16 and the third sub-unit 17 are omitted, the fourth sub-unit 18 may also be omitted, wherein the output from sub-unit 15 may be transmitted directly to the power supplying and/or absorbing device 6 and not to the fourth sub-unit 18 as illustrated in FIG. 1. In other words, in that case, the first voltage reference value output the by the first sub-unit 15 may constitute the voltage reference value for the power supplying and/or absorbing device 6 that is transmitted to the power supplying and/or absorbing device 6 as illustrated in FIG. 1.

[0063] In case the third sub-unit 17 is omitted but not the second sub-unit 16, the outputs from the first sub-unit 15 and the second sub-unit 16 may be transmitted to the fourth sub-unit 18 in which they may be combined into a voltage reference value for the power supplying and/or absorbing device 6. The output from the fourth sub-unit 18i.e., the voltage reference value for the power supplying and/or absorbing device 6is transmitted to the power supplying and/or absorbing device 6. In this case, the sensor 12 may be omitted. In case the second sub-unit 16 is omitted but not the third sub-unit 17, the outputs from the first sub-unit 15 and the third sub-unit 17 may be transmitted to the fourth sub-unit 18 in which they may be combined into a voltage reference value for the power supplying and/or absorbing device 6. The output from the fourth sub-unit 18i.e., the voltage reference value for the power supplying and/or absorbing device 6is transmitted to the power supplying and/or absorbing device 6. In this case, the sensor 11 may be omitted.

[0064] It is to be understood that the control unit 7, including the first to fourth sub-units 15 to 18, may be implemented in hardware and/or software. Each or any of the first to fourth sub-units 15 to 18 may be implemented in hardware and/or software.

[0065] FIG. 2 is a schematic flowchart of a method 30 according to an embodiment of the present invention. The method 30 is implemented in an apparatus configured to supply power to a load connected with a power system or absorb power from the load. The load is connected or connectable to a load conductor. The apparatus comprises a power supplying and/or absorbing device. The power supplying and/or absorbing device is connected to the load conductor. The power supplying and/or absorbing device is configured for selectively supplying power to the load conductor or absorbing power from the load conductor. The power supplied to the load conductor or absorbed from the load conductor by the power supplying and/or absorbing device is governed at least by a voltage reference value of the power supplying and/or absorbing device.

[0066] The method 30 comprises, at 31, obtaining at least one value indicative of voltage of the load conductor.

[0067] At 32 it is determined, based on the at least one value indicative of voltage of the load conductor and a virtual impedance of the power supplying and/or absorbing device, a voltage reference value for the power supplying and/or absorbing device.

[0068] At 33, the power supplying and/or absorbing device is controlled to supply power to the load conductor, and thereby supply power to the load, or absorb power from the load conductor, and thereby absorb power from the load, based on the determined voltage reference value.

[0069] The virtual impedance of the power supplying and/or absorbing device is associated with a virtual reactance and a virtual resistance, wherein the power supplying and/or absorbing device is configured such that a value of the virtual reactance is higher than the value of a reactance of the power supplying and/or absorbing device, and such that a value of the virtual resistance is less than the value of the virtual reactance.

[0070] In conclusion, an apparatus is provided, configured to supply power to a load connected with a power system or absorb power from the load, the load being connected or connectable to a load conductor. The apparatus comprises a power supplying and/or absorbing device, configured for selectively supplying power to the load conductor or absorbing power from the load conductor, and a control unit configured to control the power supplying and/or absorbing device. The control unit is configured to determine, based on at least one value indicative of voltage of the load conductor and a virtual impedance of the power supplying and/or absorbing device, a voltage reference value for the power supplying and/or absorbing device, and control the power supplying and/or absorbing device to supply power to the load conductor, and thereby supply power to the load, or absorb power from the load conductor, and thereby absorb power from the load, based on the determined voltage reference value.

[0071] While the present invention has been illustrated in the appended drawings and the foregoing description, such illustration is to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the appended claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.