Apparatus and method for supplying or absorbing power to or from a load

Abstract

An apparatus is provided that is configured to supply power to, or absorb power from, a load (1) connected to a power system (2), the load being connected to, or connectable to, a load conductor (3). The apparatus comprises a power supply and / or absorption device (6) configured to selectively supply power to, or absorb power from, the load conductor (3), and a control unit (7) configured to control the power supply and / or absorption device (7). The control unit (7) determines a voltage reference value for the power supply and / or absorption device (6) based on at least one value indicative of the voltage of the load conductor (3) and the virtual impedance of the power supply and / or absorption device (6), and controls the power supply and / or absorption device (6) to supply power to the load (1) by supplying power to the load conductor (3), or absorb power from the load (1) by absorbing power from the load conductor (3), based on the determined voltage reference value.

Description

[Technical Field]

[0001] Technical Field The present invention relates to devices configured to supply power to or absorb power from a load connected to an electric power grid. [Background technology]

[0002] background Electric power systems, such as transmission and / or distribution systems and / or power grids, can be used to power equipment in many industries. In this context, the electric power system can be referred to as a power supply network or system, and the equipment can be referred to as a load. In at least some industries, flicker in the power system voltage, i.e., rapid fluctuations in the power system voltage and the generation of harmonic frequencies in the power system voltage due to loads, can be a concern. For example, in many electric arc furnace (EAF) steelmaking facilities, flicker and the generation of harmonic frequencies in the power grid voltage, i.e., “harmonics,” are concerns with the power grid. Flicker can be reduced by measuring EAF current and compensating for harmonic frequencies and negative sequence currents in the power grid voltage using a device such as a static synchronous compensator (STATCOM). To further improve compensation for harmonic frequencies in the power grid voltage, one can focus on compensating for low-order harmonics, primarily caused by the EAF. Summary of the Invention [Means for solving the problem]

[0003] overview Devices such as static synchronous compensators (STATCOMs) can be used to regulate the voltage at a point of connection to a power system or grid by providing or absorbing reactive power. For example, in the case of a STATCOM used in an electric arc furnace (EAF) application, two control methods can operate simultaneously. One control method can be so-called flicker control, which can function as open-loop control and can counteract sudden changes in load (e.g., EAF) current. Flicker control can compensate for reactive power, negative-sequence current in the load, and possibly some low-order harmonics in the power system or grid voltage. Another control method can be power factor control, which can function as closed-loop control and can compensate for the power factor of the connecting power system or grid. Power factor control can be based on sensed or measured or calculated current in the power system or grid and can provide active and reactive power at the point of connection to the power system or grid.

[0004] Flicker control algorithms may aim to correct negative-sequence currents in a load, which can cause flicker in the load current. However, harmonic compensation for low-order harmonics (e.g., harmonics below the fifth order) may be quite limited. Other control algorithms may be used to improve compensation for low-order harmonics. However, such other control algorithms may require that devices used to regulate voltage at the connection point to the power system or grid by providing or absorbing reactive power (e.g., devices based on STATCOMs or voltage source converters (VSCs)) be relatively large because a relatively large number of converter cells may be required to enable such control algorithms to compensate for low-order harmonics. Examples of such other control algorithms are shown in WO 98 / 27476. However, increasing the size of the device by increasing the number of converter cells generally increases the cost of the device and, therefore, the overall system cost.

[0005] In view of the above, it is a concern of the present invention to provide a means for reducing or avoiding the need to increase the size of devices used to regulate the voltage at a point of connection to an electrical grid or power supply by providing or absorbing reactive power, while compensating for low order harmonics generated by loads such as EAFs in the voltage of the electrical grid or power system supplying the load.

[0006] To address this and / or other concerns, an apparatus and method are provided according to the independent claims. Preferred embodiments are defined by the dependent claims.

[0007] According to a first aspect of the present invention, there is provided an apparatus configured to supply power to or absorb power from a load connected to an electric power system. The load is connected to or connectable to a load conductor. The electric power system may be connected to or connectable to the 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 to selectively supply power to or absorb power from the load conductor. The power supplied to or absorbed from the load conductor by the power supplying and / or absorbing device is regulated by at least a voltage reference value of the power supplying and / or absorbing device. The apparatus comprises a control unit configured to control the power supplying and / or absorbing device, for example, to control operation of the power supplying and / or absorbing device. The control unit is configured to obtain at least one value indicative of a voltage of the load conductor. The control unit is configured to determine a voltage reference value for the power supplying and / or absorbing device based on at least one value indicative of a voltage of the load conductor and a virtual impedance of the power supplying and / or absorbing device. The control unit is configured to control the power supplying and / or absorbing device to supply power to the load by supplying power to the load conductor or absorb power from the load by absorbing power from the load conductor based on the determined voltage reference value. The virtual impedance of the power supplying and / or absorbing device is related to a virtual reactance and a virtual resistance. The power supplying and / or absorbing device is configured such that the value of the virtual reactance is higher than the value of the reactance of the power supplying and / or absorbing device and the value of the virtual resistance is lower than the value of the virtual reactance.

[0008] According to a second aspect of the present invention, there is provided a method implemented in an apparatus configured to supply power to or absorb power from a load connected to an electric power system. The load is connected to or connectable to a load conductor. The electric power system may be connected to or connectable to the load conductor. The apparatus includes 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 to selectively supply power to or absorb power from the load conductor. The power supplied to or absorbed from the load conductor by the power supplying and / or absorbing device is regulated by at least a voltage reference value of the power supplying and / or absorbing device. The method includes obtaining at least one value indicative of a voltage of the load conductor. The method includes determining a voltage reference value for the power supplying and / or absorbing device based on the at least one value indicative of the voltage of the load conductor and a virtual impedance of the power supplying and / or absorbing device. The method includes controlling the power supplying and / or absorbing device to supply power to the load by supplying power to the load conductor or absorb power from the load by absorbing power from the load conductor based on the determined voltage reference value. A virtual impedance of the power supplying and / or absorbing device is related to a virtual reactance and a virtual resistance, and the power supplying and / or absorbing device is configured such that a value of the virtual reactance is higher than a value of the reactance of the power supplying and / or absorbing device and a value of the virtual resistance is lower than a value of the virtual reactance.

[0009] According to one or more embodiments of the present invention, power supplying and / or absorbing devices, which may be directly connected to load conductors, can be considered to appear as a voltage source behind a virtual impedance using grid-forming control. The virtual impedance of the power supplying and / or absorbing devices can be used to control the power supplying and / or absorbing devices to supply power to the load by supplying power to the load conductor or to absorb power from the load by absorbing power from the load conductor, as described above. This can be done to compensate for disturbances or voltage fluctuations in the voltage of the power grid connected to the load conductor or the load, and to reduce or avoid low-order harmonics, such as those below the fifth order, that may be generated by the load in the power grid voltage. When the power supplying and / or absorbing devices act as a specific impedance on the load conductor, changes in the voltage of the load conductor may also result in the power supplying and / or absorbing devices providing a corresponding current. Therefore, this manner of controlling the power supplying and / or absorbing devices can attenuate voltage fluctuations in the voltage of the load conductor and harmonics that may be generated by the load in the power grid voltage by removing a portion of the voltage fluctuations in the voltage of the load conductor.

[0010] Loads such as EAFs not only produce characteristic harmonics, but can also produce (e.g., nearly produce) relatively low frequency interharmonics. Interharmonics are a concern in many applications. The virtual impedance-based control of the power supply and / or absorbing devices described herein can attenuate both the characteristic harmonics and the interharmonics.

[0011] By using such virtual impedance based control of power supply and / or absorption devices as described herein to reduce or avoid harmonics that may be generated by loads in the power system voltage, the need to increase the size of the power supply and / or absorption devices to achieve a desired or required capacity for compensating for the harmonics can be reduced or avoided, thus keeping the cost of the power supply and / or absorption devices, and therefore the overall system cost, relatively low.

[0012] The virtual impedance-based control of power supply and / or absorption devices described herein can have a (much) shorter response time than that of "traditional" voltage control, i.e., voltage control in which the voltage of a power grid is controlled by measuring the voltage and regulating the voltage of the power grid toward a particular voltage set point using a control loop mechanism that uses a controller, such as a proportional integral (PI) controller, based on the measured voltage. Such virtual impedance-based control of power supply and / or absorption devices described herein can be thought of as causing the power supply and / or absorption devices to function as synchronous machines, providing stiffness to the power grid. The stiffer the power grid, the less sensitive the voltage of the power grid is to changes in load.

[0013] The virtual impedance based control of the power supply and / or absorption devices described herein allows all phases to be controlled independently of each other.

[0014] The virtual reactance may be, for example, the sum of the reactance of the power supply and / or absorbing device and a selected fraction of the reactance of the power supply and / or absorbing device. The selected fraction of the reactance of the power supply and / or absorbing device, sometimes referred to as a margin, may be, for example, in a range between 1% and 50% of the reactance of the power supply and / or absorbing device.

[0015] In general, it may be desirable to minimize the virtual reactance. For example, the virtual reactance may be such that a determined voltage reference value for the power supplying and / or absorbing device enables the power supplying and / or absorbing device to supply power to or absorb power from a load conductor such that fluctuations in the voltage of the load conductor are kept below a selected threshold voltage fluctuation level compared to the average voltage level of the voltage of the load conductor over a period of time, while keeping the virtual reactance as small as possible. Preferably, the selected percentage of the reactance of the power supplying and / or absorbing device may be, for example, within a range between 1% and 20%, more preferably between 1% and 10%, of the reactance of the power supplying and / or absorbing device.

[0016] The virtual resistance may be, for example, a virtual resistance whose value is between 25% and 75% of the value of the virtual reactance, preferably between 35% and 65%, more preferably between 45% and 55%, such as at or about 50%. The virtual resistance may govern the degree and speed of damping of transients after the occurrence of a disturbance in the power system. The virtual resistance may provide damping to approach or reach a steady-state condition after the occurrence of a disturbance in the power system. The selection of the value of the virtual resistance may depend on the characteristics of the load and, in some cases, on reactive power compensation devices other than power supply and / or absorption devices, such as harmonic filters, that may be included in the system. A less damped load may require a larger virtual resistance.

[0017] As mentioned above, the load may include, for example, an EAF (or EAF installation). However, the load is not limited thereto and may instead or in addition to an EAF (installation) include other types of loads that may tend to introduce low-order harmonics (e.g., harmonics below the fifth order) into the voltage of the power system or grid supplying the load. The load may include, for example, a cycloconverter or some relatively large electric machine.

[0018] There may be several loads connected to the power system, each of which may be connected to a load conductor or may be connectable to a load conductor.

[0019] The load conductors may comprise, for example, buses or bus bars. The power system may comprise, for example, a transmission and / or distribution system. The power system may comprise, for example, a power grid, such as a transmission and / or distribution network.

[0020] The voltage reference value may be determined by multiplying the sensed voltage of the load conductor by the virtual impedance. Controlling the power supplying and / or absorbing device to supply power to or absorb power from the load conductor based on the determined voltage reference value may include adjusting or controlling the voltage output by the power supplying and / or absorbing device, for example, by comparing the voltage of the load conductor with the voltage reference value, so that the output voltage matches or approaches the voltage reference value.

[0021] The virtual impedance of a power supplying and / or absorbing device relates, for example, to a virtual reactance and a virtual resistance as components of the virtual impedance.

[0022] The power supplying and / or absorbing devices may be selected or configured to have a predetermined virtual impedance in such a way that the value of the virtual reactance is greater than the value of the reactance of the power supplying and / or absorbing device and the value of the virtual resistance is less than the value of the virtual reactance.

[0023] The reactance of the power supply and / or absorption device may be constituted by the "physical" reactance of the power supply and / or absorption device, in other words, the reactance of the power supply and / or absorption device may be the reactance of the power supply and / or absorption device resulting from the reactances of the individual elements or components of the power supply and / or absorption device.

[0024] As mentioned above, the control unit is configured to obtain at least one value indicative of the voltage of the load conductor. The at least one value indicative of the voltage of the load conductor may be or may have been measured or sensed by at least one sensor, such as at least one voltage transducer, for example. The control unit may be connected to the at least one sensor to obtain (the at least one value indicative of) the voltage of the load conductor. Such at least one sensor may be part of the device, and thus the device may include such at least one sensor.

[0025] In addition to the control based on the virtual impedance of the power supplying and / or absorbing devices described herein, the flicker control algorithm or method can be implemented, for example, in a control unit. To this end, the control unit can be configured to obtain multiple values ​​indicative of the current in the load at different times during a period of time and determine a change in the current in the load over the period of time based on the multiple values ​​indicative of the current in the load at the different times. The multiple values ​​indicative of the current in the load at different times during the period of time may be measured or sensed by, or may have been measured or sensed by, at least one sensor, such as at least one current transducer. The control unit can be connected to at least one sensor to obtain multiple (indicative) values ​​of the current in the load at the different times. Such at least one sensor can be part of the apparatus, and thus the apparatus can include such at least one sensor. The control unit can be configured to determine a voltage reference value for the power supplying and / or absorbing devices further based on the determined change in the current in the load over the period of time. The control unit may be configured to determine a voltage reference value for the power supply and / or absorption device based on a plurality of values ​​indicative of the load's current at different times so as to reduce fluctuations in the load's current relative to an average current level over a period of time. Such a configuration enables the power supply and / or absorption device to counteract sudden changes in the load current (sometimes referred to as flicker, as discussed above). This can be done in addition to attenuating voltage fluctuations in the load conductor voltage and harmonics that may be caused by the load in the power system voltage through control based on the virtual impedance of the power supply and / or absorption device as described herein.

[0026] In some cases or applications, it may be difficult or even impossible to obtain (a value indicative of) the current of a load. For example, sensing or measuring the current of a load may be difficult or even impossible for some reasons. Furthermore, as mentioned above, several loads may be connected to an electric power system, and each load may be connected or connectable to a load conductor. However, sensing or measuring the current of one or more of those loads may be difficult or even impossible for some reasons. For example, it may only be possible to sense or measure the current of one load, but not the other loads. In such cases, it may be difficult or even impossible to establish the load current to be used in a flicker control algorithm or method. However, in such cases, the virtual impedance-based control of power supplying and / or absorbing devices described herein can still be used.

[0027] Additionally, use of the virtual impedance-based control of power sourcing and / or absorbing devices described herein can facilitate or enable relaxation of requirements for any current sensing devices that may be installed on a feeder connecting a load (e.g., an EAF) to another component, such as a substation, or other component, and that may be configured to sense the load's current. Such current sensing devices may include, for example, current transformers that may saturate when DC current is generated by a load, such as an EAF. However, in such cases, the virtual impedance-based control of power sourcing and / or absorbing devices described herein can still be used.

[0028] In addition to the virtual impedance-based control of power supply and / or absorption devices described herein (and possibly the flicker control described herein), a power factor control algorithm or method can be implemented, for example, in a control unit. To this end, the control unit may be configured to obtain at least one value indicative of a current in the power grid and to determine a voltage reference value for the power supply and / or absorption device further based on the at least one value indicative of the current in the power grid. The control unit may be configured to determine a voltage reference value for the power supply and / or absorption device based on the at least one value indicative of the current in the power grid and at least one value indicative of a voltage of a load conductor so as to increase the power factor of the load.

[0029] The (at least one value indicative of the) current in the power grid may be determined directly, e.g., by measuring or sensing the current in the power grid, e.g., using a current transducer connected to the power grid, or may be determined indirectly. The (at least one value indicative of the) current in the power grid may be determined indirectly, e.g., based on measuring or sensing the current in the power supply and / or absorption devices and the current in the load, e.g., using one or more current transducers connected to the power supply and / or absorption devices and / or loads. The control unit may be connected to such one or more current transducers to obtain the at least one value indicative of the current in the power grid.

[0030] The power supply and / or absorption devices may be based on, for example, devices based on a voltage source converter (VSC), a static synchronous compensator (STATCOM), and / or a multilevel converter. The STATCOM may have a delta topology. However, the STATCOM is not limited thereto and may instead have, for example, a wye topology. The multilevel converter may comprise, for example, a three-level converter.

[0031] As previously mentioned, the power supplying and / or absorbing devices may be directly connected to the load conductors. However, in other exemplary embodiments, the power supplying and / or absorbing devices may be indirectly connected to the load conductors (i.e., via one or more intermediate components).

[0032] According to a third aspect of the present invention there is provided a computer program comprising instructions which, when executed by one or more processors included in a control unit, cause the control unit to carry out a method according to the second aspect of the present invention.

[0033] According to a fourth aspect of the present invention, there is provided a processor-readable medium having loaded thereon a computer program comprising instructions which, when executed by one or more processors included in or constituting a control unit, cause the control unit to carry out a method according to the second aspect of the present invention.

[0034] According to a fifth aspect of the present invention, there is provided a system comprising an electric power system, a load connected to the electric power system, and a load conductor, the load being connected to or connectable to the load conductor, and the system comprising an apparatus according to the first aspect of the present invention configured to supply power to or absorb power from the load.

[0035] The control unit may include or consist of, for example, any suitable central processing unit (CPU), microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), field programmable gate array (FPGA), etc., or any combination thereof. The control unit may optionally be capable of executing software instructions stored, for example, on a computer program product in the form of a memory. The memory may be, for example, any combination of read and write memory (RAM) and read only memory (ROM). The memory may comprise persistent storage, which may be, for example, magnetic memory, optical memory, solid state memory, or remotely mounted memory, or any combination thereof.

[0036] Each or any of the one or more processors may comprise, for example, a CPU, microcontroller, DSP, ASIC, FPGA, etc., or any combination thereof.

[0037] The processor-readable medium may include, for example, a Digital Versatile Disk (DVD) or a floppy disk, or any other suitable type of processor-readable means or processor-readable (digital) medium, such as, but not limited to, memory, such as non-volatile memory, a hard disk drive, a Compact Disc (CD), flash memory, magnetic tape, a Universal Serial Bus (USB) memory device, a Zip drive, etc.

[0038] Further objects and advantages of the present invention are described below by way of exemplary embodiments. It should be noted that the present invention relates to all possible combinations of the features set forth in the claims. Further features and advantages of the present invention will become apparent upon review of the appended claims and the description herein. Those skilled in the art will understand that various features of the present invention can be combined to produce embodiments other than those described herein.

[0039] BRIEF DESCRIPTION OF THE DRAWINGS Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings. [Brief explanation of the drawings]

[0040] [Figure 1] 1 is a schematic diagram of a system according to one embodiment of the present invention. [Figure 2] 1 is a schematic flow chart of a method according to one embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0041] All figures are schematic, not necessarily to scale, and generally show only those parts necessary to explain embodiments of the invention; other parts may be omitted or only suggested.

[0042] Description of the drawings The present invention will now be described below with reference to the accompanying drawings, which show exemplary embodiments of the invention. However, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments of the invention set forth herein; rather, these embodiments are provided as examples so that the scope of the invention will be conveyed to those skilled in the art through this disclosure.

[0043] FIG. 1 is a schematic diagram of a system according to an embodiment of the present invention, the system comprising an apparatus according to an embodiment of the present invention.

[0044] According to an embodiment of the present invention shown in FIG. 1 , the system includes a power grid 2, a transformer 4, a load 1 connected to the power grid 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 grid 2 is connected or connectable to the load conductor 3 via the transformer 4. The load conductor 3 may comprise, for example, a bus or bus bar. The power grid 2 may comprise, for example, a transmission and / or distribution system. The power grid 2 may comprise, for example, a power grid, such as a transmission and / or distribution grid. The load 1 may include, for example, but is not limited to, an electric arc furnace (EAF). The transformer 4 is connected between the power grid 2 and the load 1, and thus, according to one or more embodiments of the present invention, the transformer 4 may be referred to as (or comprise) a step-down transformer. The transformer 4 may be omitted.

[0045] As shown in FIG. 1 , load 1 may be connectable to load conductor 3 by switch 8, which may be normally closed (e.g., such that switch 8 conducts current through switch 8) when load 1 is operating or whenever load 1 is operating.

[0046] The system comprises a device configured to supply power to and absorb power from a load 1 connected to a power grid 2. The device may be configured to supply power to the load 1 in addition to power supplied to the load 1 from the power grid 2, for example.

[0047] 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 configured to selectively supply power to or absorb power from the load conductor 3. The power supplied to or absorbed from the load conductor 3 by the power supplying and / or absorbing device 6 is regulated by at least a voltage reference value of the power supplying and / or absorbing device 6.

[0048] The apparatus comprises a control unit 7. The control unit 7 controls the power supply and / or absorption device 6 The apparatus comprising the power supply and / or absorption device 6 and the control unit 7 is configured to supply power to or absorb power from the load 1.

[0049] The power supply and / or absorption device 6 may be, for example, a device based on a voltage source converter (VSC), a static synchronous compensator (STATCOM), and / or a multilevel converter. The STATCOM may have, for example, a delta topology. However, the STATCOM is not limited thereto and may instead have, for example, a wye topology. The multilevel converter may, for example, comprise a three-level converter.

[0050] For example, if the power supply and / or absorption device 6 is based on a STATCOM having a delta topology, the power supply and / or absorption device 6 may be connected to the load conductors 3 at a corner point (e.g., terminal) of the STATCOM configured in the delta topology, or may be connected to the load conductors 3 via a conductor connected to a corner point (e.g., terminal) of the STATCOM configured in the delta topology.

[0051] The power supply and / or absorption device 6 may be directly connected to the load conductor 3 as shown in FIG. 1, or may be indirectly connected to the load conductor 3 (e.g., via one or more intermediate devices or components).

[0052] The control unit 7 is configured to obtain at least one value indicative of the voltage of the load conductor 3, and to determine a voltage reference value for the power supplying and / or absorbing device 6 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. The control unit 7 is configured to control the power supplying and / or absorbing device 6 based on the determined voltage reference value to supply power to the load 1 by supplying power to the load conductor 3, or to absorb power from the load 1 by absorbing power from the load conductor 3. The virtual impedance of the power supplying and / or absorbing device 6 relates to, for example, a virtual reactance and a virtual resistance as components of the virtual impedance. The power supplying and / or absorbing device is configured (or is configured) such that the value of the virtual reactance is higher than the value of the reactance of the power supplying and / or absorbing device 6, and the value of the virtual resistance is lower than the value of the virtual reactance.

[0053] The reactance of the power supply and / or absorption device 6 may be the “physical” reactance of the power supply and / or absorption device 6. In other words, the reactance of the power supply and / or absorption device 6 may be the reactance of the power supply and / or absorption device 6 resulting from the reactance of the individual elements or components of the power supply and / or absorption device 6. For example, a STATCOM configured in a delta topology may have three converter arms, each of which may include multiple converter cells connected in series with a reactor and may also include other components. If the power supply and / or absorption device 6 is based on a STATCOM having a delta topology, the reactor in each of the converter arms may at least partially define the reactance of the power supply and / or absorption device 6. Similarly, if the power supply and / or absorption device 6 is based on a STATCOM having a topology other than a delta topology, the reactor in each of the multiple converter arms of the STATCOM may at least partially define the reactance of the power supply and / or absorption device 6.

[0054] The power supplying and / or absorbing device 6 may be disconnected from the load conductor 3 by the switch 9 when it is not in operation, and the switch 9 may be normally closed when the power supplying and / or absorbing device 6 is in operation or whenever the power supplying and / or absorbing device 6 is in operation (e.g., such that the switch 9 conducts current through the switch 9).

[0055] At least one value indicative of the voltage of the load conductor 3 may be or may have been measured or sensed by at least one sensor, such as, for example, at least one voltage transducer. According to the embodiment of the invention shown in Figure 1, a sensor 13 including a voltage transformer is provided for sensing the voltage of the load conductor 3. As shown in Figure 1, the control unit 7 may be connected to the sensor 13 to obtain (at least one value indicative of) the voltage of the load conductor 3. The sensor 13 may be part of the device, and thus the device may include the sensor 13.

[0056] According to the embodiment of the present invention shown in FIG. 1, the control unit 7 includes first, second, third and fourth sub-units 15-18, respectively.

[0057] The first subunit 15 receives as input (at least one value indicative of) the voltage of the load conductor 3, which is provided by the sensor 13 according to the embodiment of the present invention shown in FIG. 1 . Thus, the first subunit 15 can obtain (at least one value indicative of) the voltage of the load conductor 3 from the sensor 13. The first subunit 15 performs control based on the virtual impedance of the power supply and / or absorption device 6, as described above in this section of the specification and elsewhere herein. The first subunit 15 is configured to determine a first voltage reference value for the power supply and / or absorption device 6 based on (at least one value indicative of) the voltage of the load conductor 3 and the virtual impedance of the power supply and / or absorption device 6. The output from the first subunit 15 is the first voltage reference value, which is transmitted to the fourth subunit 18. The first voltage reference value may have a component corresponding to active power and a component corresponding to reactive power.

[0058] The second sub-unit 16 receives as input at least one value indicative of the current of the power system 2 and (at least one value indicative of) the voltage of the load conductor 3. Similar to the first sub-unit 15, (at least one value indicative of) the voltage of the load conductor 3 is provided by a sensor 13, according to the embodiment of the present invention shown in FIG. 1. Furthermore, according to the embodiment of the present invention shown in FIG. 1, (at least one value indicative of) the current of the power system 2 is provided by a sensor 11, which may include, for example, a current transducer that may be connected to the power system 2 as shown in FIG. 1. Thus, (at least one value indicative of) the current of the power system 2 can be determined directly. It can also be determined indirectly, for example, based on measuring or sensing the current of the power supply and / or absorption device 6 and the current of the load 1. As shown in FIG. 1, the control unit 7 may be connected to the sensor 11 to obtain (at least one value indicative of) the current of the power system 2. The sensor 11 may be part of the apparatus, and thus the apparatus may include the sensor 11. In this manner, the second subunit 16 obtains (at least one value indicative of) the current of the power grid 2 from the sensor 11 and (at least one value indicative of) the voltage of the load conductor 3 from the sensor 13. The second subunit 16 is configured to determine a second voltage reference value for the power supply and / or absorption device 6 based on (at least one value indicative of) the current of the power grid 2 and (at least one value indicative of) the voltage of the load conductor 3 to increase the power factor of the load 1. An output from the second subunit 16 is the second voltage reference value, which is transmitted to the fourth subunit 18. The second voltage reference value may have a component corresponding to reactive power (e.g., may have only a component corresponding to reactive power). In this manner, the second subunit 16 can implement a power factor control algorithm or method.

[0059] The third sub-unit 17 receives as input a plurality of values ​​indicative of the current in the load 1 at different times during a period of time. According to the embodiment of the present invention shown in FIG. 1 , the plurality of values ​​indicative of the current in the load 1 at different times during a period of time are provided by a sensor 12 which may, for example, include a current transducer which may be connected to the load 1 as shown in FIG. 1 . As shown in FIG. 1 , the control unit 7 may be connected to the sensor 12 to obtain the plurality of values ​​indicative of the current in the load 1 at different times during a period of time. The sensor 12 may be part of a device, and thus the device may include the sensor 12. In this way, the third sub-unit 17 obtains from the sensor 12 a plurality of values ​​indicative of the current in the load 1 at different times during a period of time. The third subunit 17 is configured to determine a change in current in the load 1 over a period of time based on a plurality of values ​​indicative of the current in the load 1 at different times, and to determine a third voltage reference value for the power supplying and / or absorbing device 6 based on the determined change in current in the load 1 over a period of time, compared to an average current level of the current in the load 1 over the period of time, to reduce fluctuations in the current in the load 1. The output from the third subunit 17 is the third voltage reference value, which is transmitted to the fourth subunit 18. The third voltage reference value may have a component corresponding to active power and a component corresponding to reactive power. In this manner, the third subunit 17 can implement a flicker control algorithm or method.

[0060] 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 supply and / or absorption device 6. The output from the fourth sub-unit 18, i.e., the voltage reference value for the power supply and / or absorption device 6, is transmitted to the power supply and / or absorption device 6 as shown in FIG.

[0061] It should be understood that each of the second subunit 16 and the third subunit 17 is optional, and one or both of the second subunit 16 and the third subunit 17 may be omitted. If both the second subunit 16 and the third subunit 17 are omitted, the fourth subunit 18 may also be omitted, and the output from subunit 15 may be sent directly to the power supplying and / or absorbing device 6 rather than to the fourth subunit 18 as shown in FIG. 1. In other words, in that case, the first voltage reference value output by the first subunit 15 may constitute the voltage reference value for the power supplying and / or absorbing device 6 that is sent to the power supplying and / or absorbing device 6 as shown in FIG. 1.

[0062] If the third subunit 17 is omitted but the second subunit 16 is not omitted, the outputs from the first subunit 15 and the second subunit 16 can be sent to the fourth subunit 18, where they can be combined into a voltage reference value for the power supply and / or absorption device 6. The output from the fourth subunit 18, i.e., the voltage reference value for the power supply and / or absorption device 6, is sent to the power supply and / or absorption device 6. In this case, the sensor 12 may be omitted.

[0063] If the second subunit 16 is omitted but the third subunit 17 is not omitted, the outputs from the first subunit 15 and the third subunit 17 can be sent to the fourth subunit 18, where they can be combined into a voltage reference value for the power supply and / or absorption device 6. The output from the fourth subunit 18, i.e., the voltage reference value for the power supply and / or absorption device 6, is sent to the power supply and / or absorption device 6. In this case, the sensor 11 may be omitted.

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

[0065] 2 is a schematic flow chart of a method 30 according to one embodiment of the present invention. The method 30 is implemented in an apparatus configured to supply power to or absorb power from a load connected to an electric power grid. The load is connected to or connectable to a load conductor. The apparatus includes 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 to selectively supply power to or absorb power from the load conductor. The power supplied to or absorbed from the load conductor by the power supplying and / or absorbing device is regulated by at least a voltage reference value of the power supplying and / or absorbing device.

[0066] The method 30 includes, at 31, obtaining at least one value indicative of a voltage on a load conductor.

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

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

[0069] The virtual impedance of the power supplying and / or absorbing device is related to a virtual reactance and a virtual resistance, and the power supplying and / or absorbing device is configured such that the value of the virtual reactance is higher than the value of the reactance of the power supplying and / or absorbing device and the value of the virtual resistance is lower than the value of the virtual reactance.

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

[0071] While the present invention has been illustrated in the accompanying drawings and in the foregoing description, such illustrations are to be considered illustrative or exemplary rather than restrictive, and the present invention is not limited to the disclosed embodiments. From a study of the drawings, the disclosure, and the appended claims, those skilled in the art will understand and effect other variations to the disclosed embodiments in practicing the claimed invention. 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 are not to be construed as limiting the scope.

Claims

1. 1. A device configured to supply or absorb power to or from a load (1) connected to an electric power system (2), said load being connected or connectable to a load conductor (3), comprising: a power supply and / or absorption device (6) connected to the load conductor and configured to selectively supply power to or absorb power from the load conductor, wherein the power supplied to or absorbed from the load conductor by the power supply and / or absorption device is regulated by at least a voltage reference value of the power supply and / or absorption device; a control unit (7) configured to control the operation of said power supply and / or absorption device; Equipped with The control unit obtaining at least one value indicative of a voltage on the load conductor; determining a voltage reference value for the power supply and / or absorbing device based on the at least one value indicative of a voltage of the load conductor and a virtual impedance of the power supply and / or absorbing device; and controlling the power supplying and / or absorbing device to supply power to the load by supplying power to the load conductor or to absorb power from the load by absorbing power from the load conductor based on the determined voltage reference value. further configured as follows:

1. An apparatus, wherein the virtual impedance of the power supplying and / or absorbing device is related to a virtual reactance and a virtual resistance, and the power supplying and / or absorbing device is configured such that a value of the virtual reactance is greater than a value of a reactance of the power supplying and / or absorbing device and a value of the virtual resistance is less than the value of the virtual reactance.

2. 2. The apparatus of claim 1, wherein the virtual reactance is the sum of the reactance of the power supplying and / or absorbing device and a selected fraction of the reactance of the power supplying and / or absorbing device, the selected fraction 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. 2. The apparatus of claim 1, wherein the virtual reactance is such that the determined voltage reference value for the power supply and / or absorption device enables the power supply and / or absorption device to supply power to or absorb power from the load conductor in a manner that keeps fluctuations in the voltage of the load conductor below a selected threshold voltage fluctuation level compared to an average voltage level of the voltage of the load conductor over a period of time, while keeping the virtual reactance as small as possible.

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

5. The control unit obtaining a plurality of values ​​indicative of current in the load at different times during a period of time; determining a change in current in the load over the period of time based on the plurality of values ​​indicative of current in the load at the different times; determining the voltage reference value for the power supply and / or absorption device further based on the determined change in current of the load during the period of time; The apparatus of claim 1 further configured to:

6. 6. The apparatus of claim 5, wherein the control unit is configured to determine the voltage reference value for the power supply and / or absorption device based on the plurality of values ​​indicative of the current in the load at the different times so as to reduce fluctuations in the current in the load compared to an average current level of the current in the load over a period of time.

7. 2. The apparatus of claim 1, wherein the control unit is further configured to obtain at least one value indicative of a current in the power grid and to determine the voltage reference value for the power supply and / or absorption device further based on the at least one value indicative of a current in the power grid.

8. 8. The apparatus of claim 7, wherein the control unit is configured to determine the voltage reference value for the power supply and / or absorption device based on the at least one value indicative of a current in the power grid and the at least one value indicative of a voltage in the load conductor so as to increase a power factor of the load.

9. 2. The apparatus of claim 1, wherein the power supply and / or absorption devices are based on devices based on Voltage Source Converters (VSCs), Static Synchronous Compensators (STATCOMs), and / or multilevel converters.

10. 10. The apparatus of claim 1, wherein the power supply and / or absorption device is connected directly to the load conductor.

11. A method (30) implemented in an apparatus configured to supply or absorb power to or from a load (1) connected to an electric power grid (2), comprising: The load is connected or connectable to a load conductor (3), the apparatus comprises a power supply and / or absorption device (6) connected to the load conductor and configured to selectively supply power to or absorb power from the load conductor, the power supplied to or absorbed from the load conductor by the power supply and / or absorption device being regulated by at least a voltage reference value of the power supply and / or absorption device, and the method comprises: obtaining (31) at least one value indicative of a voltage of said load conductor; determining (32) a voltage reference value for the power supply and / or absorbing device based on the at least one value indicative of the voltage of the load conductor and a virtual impedance of the power supply and / or absorbing device; controlling (33) the power supply and / or absorption device to supply power to the load by supplying power to the load conductor or to absorb power from the load by absorbing power from the load conductor based on the determined voltage reference value; Including, 10. A method (30) wherein the virtual impedance of the power supplying and / or absorbing device is related to a virtual reactance and a virtual resistance, and the power supplying and / or absorbing device is configured such that the value of the virtual reactance is greater than the value of the reactance of the power supplying and / or absorbing device and the value of the virtual resistance is less than the value of the virtual reactance.

12. A computer program comprising instructions that, when executed by one or more processors included in a control unit (7), cause said control unit to carry out the method of claim 11.

13. Power system (2) and A load (1) connected to the power grid; a load conductor (3) to which the load is connected or connectable; An apparatus according to any one of claims 1 to 10, configured to supply power to or absorb power from said load; A system comprising:

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