Apparatus and method for supplying power to a load or absorbing power 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 an apparatus configured to supply power to a load connected to a power system or absorb power from the load.

Background Art

[0002] Background Power systems such as transmission and / or distribution systems and / or power grids can be used to supply power to equipment in many industries. In this context, the 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 voltage of the power system, i.e., rapid fluctuations in the voltage of the power system, and the generation of harmonic frequencies in the voltage of the power system due to the load can be a concern. For example, in many electric arc furnace (EAF) steelmaking facilities, flicker and the generation of harmonic frequencies, i.e., "harmonics," in the voltage of the power supply network are concerns regarding the power supply network. Flicker can be reduced by measuring the EAF current and compensating for harmonic frequencies and negative sequence current in the voltage of the power supply network using a device such as a static synchronous compensator (STATCOM). To further improve the compensation of harmonic frequencies in the voltage of the power supply network, focus can be placed on compensating for low-order harmonics mainly generated by the EAF.

Summary of the Invention

Means for Solving the Problems

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

[0004] The flicker control algorithm can aim to correct the negative sequence current of loads that may generate flicker in the load current. However, harmonic compensation for low-order harmonics (for example, harmonics below the 5th order) can be extremely limited. To improve the compensation of low-order harmonics, other control algorithms may be used. However, such other control algorithms may require a relatively large number of converter cells to enable the compensation of low-order harmonics by such control algorithms, and thus a device (for example, a device based on a STATCOM or a voltage source converter (VSC)) used to regulate the voltage at the connection point to the power system or the feeder network by providing or absorbing reactive power may need to be relatively large. An example of such other control algorithms is 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 thus the cost of the entire system.

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

[0006] To address at least one of this concern and other concerns, an apparatus and method according to the independent claims are provided. Preferred embodiments are defined by the dependent claims.

[0007] According to a first aspect of the present invention, an apparatus is provided. The apparatus is configured to supply power to a load connected to a power system or to absorb power from the load. The load is connected to or connectable to a load conductor. The power system may be connected to or connectable to the load conductor. The apparatus comprises a power supply and / or absorption device. The power supply and / or absorption device is connected to the load conductor. The power supply and / or absorption device is configured to selectively supply power to the load conductor or to absorb power from the load conductor. 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. The apparatus comprises a control unit configured to control the power supply and / or absorption device, such as by controlling the operation of the power supply and / or absorption device. The control unit is configured to obtain at least one value indicative of the voltage of the load conductor. The control unit is configured to determine a voltage reference value for the power supply and / or absorption device based on at least one value indicative of the voltage of the load conductor and a virtual impedance of the power supply and / or absorption device. The control unit is configured to control 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. The virtual impedance of the power supply and / or absorption device is related to a virtual reactance and a virtual resistance. The power supply and / or absorption device is configured such that a value of the virtual reactance is higher than a value of a reactance of the power supply and / or absorption device and a value of the virtual resistance is smaller 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 a load connected to a power system or to absorb power from the load. The load is connected to or connectable to a load conductor. The power system may be connected to or connectable to the load conductor. The apparatus comprises a power supply and / or absorption device. The power supply and / or absorption device is connected to the load conductor. The power supply and / or absorption 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 supply and / or absorption device is regulated at least by a voltage reference value of the power supply and / or absorption device. The method includes obtaining at least one value indicative of the voltage of the load conductor. The method includes determining a voltage reference value for the power supply and / or absorption device based on at least one value indicative of the voltage of the load conductor and a virtual impedance of the power supply and / or absorption device. The method includes controlling 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. The virtual impedance of the power supply and / or absorption device is related to a virtual reactance and a virtual resistance, and the power supply and / or absorption device is configured such that a value of the virtual reactance is higher than a value of the reactance of the power supply and / or absorption device and a value of the virtual resistance is smaller than the value of the virtual reactance.

[0009] According to one or more embodiments of the present invention, a power supply and / or absorption device that may be directly connected to a load conductor can be considered as a voltage source behind a virtual impedance using grid-forming control. The virtual impedance of the power supply and / or absorption device can be used to control the power supply and / or absorption device so as 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, as described above. This can be done to compensate for voltage disturbances or voltage fluctuations in the voltage of the load conductor or the power grid connected to the load, and to reduce or avoid harmonics that may be generated by the load in the voltage of the power grid, such as lower-order harmonics below the fifth order. When the power supply and / or absorption device acts as a specific impedance on the load conductor, a change in the voltage of the load conductor can also result in the corresponding current provided by the power supply and / or absorption device. Therefore, this way of controlling the power supply and / or absorption device can attenuate the voltage fluctuations in the voltage of the load conductor and the harmonics that may be generated by the load in the voltage of the power grid by removing a part of the voltage fluctuations in the voltage of the load conductor.

[0010] Loads such as EAFs can not only generate characteristic harmonics but also generate inter-harmonics at relatively low frequencies (e.g., almost generate). Inter-harmonics are a concern in many applications. By the control based on the virtual impedance of the power supply and / or absorption device described herein, both characteristic harmonics and inter-harmonics can be attenuated.

[0011] By using such control based on the virtual impedance of the power supply and / or absorption device described herein to reduce or avoid harmonics that may be generated by the load in the voltage of the power grid, the need to increase the size of the power supply and / or absorption device to achieve the desired or necessary capacity for compensating harmonics can be reduced or avoided, and thus the cost of the power supply and / or absorption device, and hence the cost of the entire system, can be kept relatively low.

[0012] The control based on the virtual impedance of the power supply and / or absorption device described herein can have a (much) shorter response time than the response time of "traditional" voltage control, i.e., the voltage of the power grid is controlled by using a control loop mechanism that measures the voltage and uses a controller such as a proportional integral (PI) controller based on the measured voltage to adjust the voltage of the power grid towards a specific voltage setpoint. By such control based on the virtual impedance of the power supply and / or absorption device described herein, the power supply and / or absorption device can be considered to function as a synchronous machine and provide rigidity to the power grid. The higher the rigidity of the power grid, the less affected the voltage of the power grid is by changes in the load.

[0013] By the control based on the virtual impedance of the power supply and / or absorption device described herein, all phases can be controlled independently of each other.

[0014] The virtual reactance can be, for example, the sum of the reactance of the power supply and / or absorption device and a selected ratio of the reactance of the power supply and / or absorption device. The selected ratio of the reactance of the power supply and / or absorption device, which may also be called a margin, can be within the range of 1% to 50% of the reactance of the power supply and / or absorption device, for example.

[0015] Generally, it may be desirable to make the virtual reactance as small as possible. For example, the virtual reactance can be such that, based on a determined voltage reference value for the power supply and / or absorption device, the power supply and / or absorption device can supply power to or absorb power from the load conductor while keeping the voltage fluctuations of the load conductor below a selected threshold voltage fluctuation level compared to the average voltage level of the voltage of the load conductor over a certain period, and at the same time, the virtual reactance can be kept as small as possible. Preferably, the selected percentage of the reactance of the power supply and / or absorption device can be in the range between, for example, 1% and 20% of the reactance of the power supply and / or absorption device, more preferably between 1% and 10%.

[0016] The virtual resistance can be, for example, a virtual resistance such that the value of the virtual resistance is between 25% and 75% of the value of the virtual reactance, preferably between 35% and 65%, more preferably between 45% and 55%, for example 50% or about 50%. The virtual resistance can govern the degree and speed of attenuation of transient phenomena after the occurrence of a disturbance in the power system. The virtual resistance can provide attenuation for approaching or reaching steady-state conditions after the occurrence of a disturbance in the power system. The selection of the value of the virtual resistance can depend on the characteristics of the load and, in some cases, on reactive power compensation devices other than the power supply and / or absorption device, such as harmonic filters that may be included in the device. A load with less attenuation may require a larger virtual resistance.

[0017] As described above, the load may include, for example, an EAF (or an EAF facility). However, the load is not limited thereto and can include, instead of or in addition to the EAF (facility), another type or other types of loads that tend to generate low-order harmonics (e.g., harmonics below the 5th order) in the voltage of the power system or electrical network that supplies power to the load. The load can include, for example, a cycloconverter or certain relatively large electrical machines.

[0018] Several loads may be connected to the power system. Each load may be connected to, or connectable to, a load conductor.

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

[0020] A voltage reference value can be determined by multiplying the sensed voltage of the load conductor by a virtual impedance. Based on the determined voltage reference value, controlling the power supply and / or absorption device to supply power to, or absorb power from, the load conductor may include adjusting or controlling the voltage output by the power supply and / or absorption device, for example, by comparing the voltage of the load conductor with the voltage reference value, such that the output voltage matches, or approaches a match with, the voltage reference value.

[0021] The virtual impedance of the power supply and / or absorption device is related to, for example, a virtual reactance and a virtual resistance as components of the virtual impedance.

[0022] The power supply and / or absorption device 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 supply and / or absorption 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 derived from the reactances of the individual elements or components of the power supply and / or absorption device.

[0024] As described 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 measured or sensed by at least one sensor, such as at least one voltage transducer, or may have been measured or sensed. The control unit may be connected to at least one sensor in order to obtain the voltage of the load conductor (the at least one value indicative thereof). 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 supply and / or absorption device described herein, a flicker control algorithm or method can be implemented, for example, in a control unit. For this purpose, the control unit can be configured to obtain a plurality of values indicating the current of the load at different points in time during a period, and to determine the change in the current of the load during the period based on the plurality of values indicating the current of the load at different points in time. The plurality of values indicating the current of the load at different points in time during a period can be measured or sensed, for example, by at least one sensor such as at least one current transducer, or can be measured or sensed. The control unit can be connected to at least one sensor to obtain a plurality of (indicated) values of the current of the load at different points in time. Such at least one sensor can be part of the device, and thus the device can include such at least one sensor. The control unit can be further configured to determine a voltage reference value for the power supply and / or absorption device based on the determined change in the current of the load during the period. The control unit can be configured to determine a voltage reference value for the power supply and / or absorption device based on a plurality of values indicating the current of the load at different points in time such that, for a period, the variation in the current of the load is made small compared to the average current level of the current of the load. With such a configuration, the power supply and / or absorption device can cancel out sudden changes in the load current (which may also be called flicker as described above). This can be done in addition to the attenuation by the control based on the virtual impedance of the power supply and / or absorption device described herein of the voltage fluctuations in the voltage of the load conductor and the harmonics that can be generated by the load in the voltage of the power grid.

[0026] In some cases or applications, it may be difficult or even impossible to obtain the value indicating the current of the load. For example, it may be difficult or even impossible to sense or measure the current of the load for several reasons. Further, as described above, several loads may be connected to the power system, and each load may be connected to or connectable to a load conductor. However, it may be difficult or even impossible to sense or measure the current of one or more of those loads for several reasons. For example, it may be possible to sense or measure only the current of one load, and it may be impossible to sense or measure other loads. In such a case, it may be difficult or even impossible to establish the load current used in the flicker control algorithm or method. However, in such a case, the control based on the virtual impedance of the power supply and / or absorption device described in this specification can still be used.

[0027] Also, the use of the control based on the virtual impedance of the power supply and / or absorption device described in this specification may be installed, for example, in a feeder connecting a load (e.g., EAF) to another component such as a substation or other components, and can facilitate or enable the relaxation of the requirements of any current sensing device that may be configured to sense the current of the load. Examples of such a current sensing device include a current transformer that may saturate when a DC current is generated by a load such as an EAF. However, in such a case, the control based on the virtual impedance of the power supply and / or absorption device described in this specification can still be used.

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

[0029] The current of the power grid (at least one value indicating it) can be directly determined, for example, by using a current transducer connected to the power grid, for example, by measuring or sensing the current of the power grid, or can be indirectly determined. The current of the power grid (at least one value indicating it) can be indirectly determined, for example, based on using one or more current transducers connected to the power supply and / or absorption device and / or load, for example, by measuring or sensing the current of the power supply and / or absorption device and the current of the load. The control unit may be connected to such one or more current transducers to obtain at least one value indicating the current of the power grid.

[0030] The power supply and / or absorption device 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 a delta connection form. However, the STATCOM is not limited to this and, instead, can have, for example, a Y connection form. The multilevel converter may include, for example, a three-level converter.

[0031] As described above, the power supply and / or absorption device may be directly connected to the load conductor. However, in another exemplary embodiment or other exemplary embodiments, the power supply and / or absorption device may be indirectly connected to the load conductor (i.e., via one or more intermediate components).

[0032] According to a third aspect of the present invention, a computer program is provided. The computer program includes instructions that, when executed by one or more processors included in the control unit, cause the control unit to execute the method according to the second aspect of the present invention.

[0033] According to a fourth aspect of the present invention, a processor-readable medium is provided. The processor-readable medium has a computer program loaded thereon, and the computer program includes instructions that, when executed by one or more processors included in or constituting the control unit, cause the control unit to execute the method according to the second aspect of the present invention.

[0034] According to a fifth aspect of the present invention, a system is provided. The system includes a power grid, a load connected to the power grid, and a load conductor. The load is connected to or connectable to the load conductor. The system includes 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, 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, or may be constituted thereby. Optionally, the control unit may be capable of executing software instructions stored, for example, in the form of a computer program product in a memory. The memory may be, for example, any combination of a read and write memory (RAM) and a read only memory (ROM). The memory may include, for example, a permanent storage device such as a magnetic memory, an optical memory, a solid state memory, or a remote mount memory, or any combination thereof.

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

[0037] The processor-readable medium may include, for example, a digital versatile disc (DVD) or a floppy (registered trademark) disk, or, without being limited thereto, for example, a memory such as a non-volatile memory, a hard disk drive, a compact disc (CD), a flash memory, a magnetic tape, a universal serial bus (USB) memory device, a Zip drive, or any other suitable type of processor-readable means or processor-readable (digital) medium.

[0038] Further objects and advantages of the present invention will be described below by way of exemplary embodiments. It should be noted that the present invention relates to all possible combinations of the features described in the claims. Further features and advantages of the present invention will become apparent upon consideration 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 be described below with reference to the accompanying drawings.

Brief Description of the Drawings

[0040]

Figure 1

Figure 2

Best Mode for Carrying Out the Invention

[0041] All the figures are schematic diagrams and are not necessarily to scale. Generally, only the parts necessary for explaining the embodiments of the present invention are shown, and other parts are omitted or only suggested.

[0042] Description of the Drawings Next, the present invention will be described below with reference to the accompanying drawings showing exemplary embodiments of the present invention. However, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments of the present invention described herein. Rather, these embodiments are provided by way of example so that the scope of the present invention can be conveyed to those skilled in the art by this disclosure.

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

[0044] According to the embodiment of the present invention shown in FIG. 1, the system includes a power system 2, a transformer 4, a load 1 connected to the power system 2, and a load conductor 3. The load 1 is connected to or connectable to the load conductor 3, perhaps via a transformer 5, which may also be called a load transformer and may be omitted. The power system 2 is connected to or connectable to the load conductor 3 via the transformer 4. The load conductor 3 may include, for example, a bus or a busbar. The power system 2 may include, for example, a power transmission and / or distribution system. The power system 2 may include, for example, a power grid such as a power transmission and / or distribution network. 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 system 2 and the load 1, and thus, according to one or more embodiments of the present invention, the transformer 4 may be called (or include) a step-down transformer. The transformer 4 may be omitted.

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

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

[0047] The device includes a power supply and / or absorption device 6. The power supply and / or absorption device 6 is connected to the load conductor 3 and is 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 supply and / or absorption device 6 is adjusted at least by the voltage reference value of the power supply and / or absorption device 6.

[0048] The device includes a control unit 7. The control unit 7 is configured to control the operation of the power supply and / or absorption device 7. The device including the power supply and / or absorption device 6 and the control unit 7 is configured to supply power to the load 1 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 multi-level converter. The STATCOM may have, for example, a delta connection form. However, the STATCOM is not limited thereto and may alternatively have, for example, a Y connection form. The multi-level converter may include, for example, a three-level converter.

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

[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 indicating the voltage of the load conductor 3 and determine a voltage reference value for the power supply and / or absorption device 6 based on at least one value indicating the voltage of the load conductor 3 and the virtual impedance of the power supply and / or absorption device 6. The control unit 7 is configured to control the power supply and / or absorption device based on the determined voltage reference value 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. The virtual impedance of the power supply and / or absorption device 6 is related to, for example, a virtual reactance and a virtual resistance as components of the virtual impedance. The power supply and / or absorption 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 supply and / or absorption device 6 and the value of the virtual resistance is smaller 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 may be the reactance of the power supply and / or absorption device 6 derived from the reactances of the individual elements or components of the power supply and / or absorption device 6. For example, a STATCOM configured in a delta connection form may include three converter arms, each converter arm may include a plurality of converter cells connected in series with a reactor, and may optionally further include another component or other components. When the power supply and / or absorption device 6 is based on a STATCOM having a delta connection form, the reactor of each converter arm among the converter arms can 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 connection form other than the delta connection form, the reactor of each converter arm among the plurality of converter arms of the STATCOM can at least partially define the reactance of the power supply and / or absorption device 6.

[0054] When the power supply and / or absorption device 6 is non-operational, it may be disconnected from the load conductor 3 by the switch 9, and the switch 9 may be normally closed when the power supply and / or absorption device 6 is operating, or always (e.g., such that the switch 9 conducts the current passing through the switch 9) when the power supply and / or absorption device 6 is operating.

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

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

[0057] The first subunit 15 receives, as an input, at least one value indicating the voltage of the load conductor 3 provided by the sensor 13 according to an embodiment of the present invention shown in FIG. 1. Thus, the first subunit 15 can obtain at least one value indicating 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 also explained elsewhere in the specification. 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 indicating 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 and 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 indicating the current of the power system 2 and at least one value (indicating the voltage) of the load conductor 3. Similar to the first sub-unit 15, the at least one value (indicating the voltage) of the load conductor 3 is provided by the sensor 13 according to the embodiment of the present invention shown in FIG. 1. Further, according to the embodiment of the present invention shown in FIG. 1, the at least one value indicating the current of the power system 2 can be provided by a sensor 11 which can include, for example, a current transducer connected to the power system 2 as shown in FIG. 1. Thus, the current (at least one value indicating) of the power system 2 can be directly determined. This can also be indirectly determined, for example, based on the measurement or sensing of 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 the current (at least one value indicating) of the power system 2. The sensor 11 may be part of the device, and thus the device may include the sensor 11. In this way, the second sub-unit 16 obtains the current (at least one value indicating) of the power system 2 from the sensor 11 and the voltage (at least one value indicating) 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 supply and / or absorption device 6 based on the current (at least one value indicating) of the power system 2 and the voltage (at least one value indicating) of the load conductor 3 in order to increase the power factor of the load 1. The output from the second sub-unit 16 is the second voltage reference value and is transmitted to the fourth sub-unit 18. The second voltage reference value may have a component corresponding to the reactive power (for example, may have only the component corresponding to the reactive power). In this way, the second sub-unit 16 can implement a power factor control algorithm or method.

[0059] The third sub-unit 17 receives, as inputs, a plurality of values indicating the current of load 1 at different times during a certain period. According to the embodiment of the present invention shown in FIG. 1, the plurality of values indicating the current of load 1 at different times during a certain period can be provided, for example, by a sensor 12 that can include a current transducer that may be connected to 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 a plurality of values indicating the current of load 1 at different times during a certain period. The sensor 12 may be part of the 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 indicating the current of load 1 at different times during a certain period. The third sub-unit 17 determines the change in the current of load 1 during a certain period based on the plurality of values indicating the current of load 1 at different times, and based on the determined change in the current of load 1 during a certain period, in a certain period, in order to reduce the fluctuation of the current of load 1 compared to the average current level of the current of load 1, it is configured to determine a third voltage reference value for the power supply and / or absorption device 6. The output from the third sub-unit 17 is the third voltage reference value and 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 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 can be combined (for example, summed) into a voltage reference value for the power supply and / or absorption device 6. The output from the fourth sub-unit 18, that is, 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. 1.

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

[0062] If the third sub-unit 17 is omitted but the second sub-unit 16 is not, the outputs from the first sub-unit 15 and the second sub-unit 16 can be transmitted to the fourth sub-unit 18, where they can be combined into the 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. In this case, the sensor 12 may be omitted.

[0063] If the second sub-unit 16 is omitted but the third sub-unit 17 is not, the outputs from the first sub-unit 15 and the third sub-unit 17 can be transmitted to the fourth sub-unit 18, where they can be combined into the 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. In this case, the sensor 11 may be omitted.

[0064] It should 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] Figure 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 or absorb power from a load connected to a power system. The load is connected to or connectable to a load conductor. The apparatus includes a power supply and / or absorption device. The power supply and / or absorption device is connected to the load conductor. The power supply and / or absorption 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 supply and / or absorption device is adjusted by at least the voltage reference value of the power supply and / or absorption device.

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

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

[0068] At 33, based on the determined voltage reference value, the power supply and / or absorption device is controlled 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.

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

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

[0071] Although the present invention has been illustrated in the accompanying drawings and the foregoing description, such illustration should be considered as illustrative or exemplary and not restrictive, and the present invention is not limited to the disclosed embodiments. By studying the drawings, the disclosure, and the appended claims, those skilled in the art will be able to understand and achieve other variations of the disclosed embodiments in the practice of the claimed invention. In the appended claims, the term "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. The mere fact that certain means are recited in mutually different dependent claims does not indicate that a combination of these means cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. An apparatus configured to supply power to or absorb power from a load (1) connected to a power system (2), wherein the load is connected to 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 the load conductor or absorbed from the load conductor by the power supply and / or absorption device is regulated at least by a voltage reference value of the power supply and / or absorption device; a control unit (7) configured to control the operation of the power supply and / or absorption device; wherein the control unit acquires at least one value indicating the voltage of the load conductor; determines a voltage reference value for the power supply and / or absorption device based on the at least one value indicating the voltage of the load conductor and a virtual impedance of the power supply and / or absorption device; and controls the power supply and / or absorption 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; wherein the virtual impedance of the power supply and / or absorption device is related to a virtual reactance and a virtual resistance, and the power supply and / or absorption device is configured such that a value of the virtual reactance is greater than a value of a reactance of the power supply and / or absorption device and a value of the virtual resistance is smaller than the value of the virtual reactance.

2. The apparatus according to claim 1, wherein the virtual reactance is a sum of a reactance of the power supply and / or absorption device and a selected ratio of the reactance of the power supply and / or absorption device, and the selected ratio of the reactance of the power supply and / or absorption device is in a range between 1% and 50% of the reactance of the power supply and / or absorption device.

3. The virtual reactance is a virtual reactance such that, based on the determined voltage reference value for the power supply and / or absorption device, the power supply and / or absorption device can supply power to or absorb power from the load conductor so as to keep the voltage fluctuation of the load conductor below a selected threshold voltage fluctuation level compared to the average voltage level of the voltage of the load conductor over a certain period, and at the same time, the virtual reactance can be kept as small as possible. The apparatus according to claim 1 or 2.

4. The virtual resistance is a virtual resistance such that the value of the virtual resistance is between 25% and 75% of the value of the virtual reactance. The apparatus according to any one of claims 1 to 3.

5. The control unit acquires a plurality of values indicating the current of the load at different time points during a certain period, determines the change in the current of the load during the period based on the plurality of values indicating the current of the load at the different time points, and further determines the voltage reference value for the power supply and / or absorption device based on the determined change in the current of the load during the period. The apparatus according to any one of claims 1 to 4, further configured as such.

6. 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 indicating the current of the load at the different time points so as to make the current fluctuation of the load smaller compared to the average current level of the current of the load for a certain period. The apparatus according to claim 5.

7. The control unit further acquires at least one value indicating the current of the power grid, and is further configured to determine the voltage reference value for the power supply and / or absorption device based on the at least one value indicating the current of the power grid. The apparatus according to any one of claims 1 to 6.

8. 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 indicating the current of the power grid and the at least one value indicating the voltage of the load conductor so as to increase the power factor of the load. The apparatus according to claim 7.

9. The power supply and / or absorption device is a device based on a voltage source converter (VSC), a static synchronous compensator (STATCOM), and / or a multilevel converter, and is the device according to any one of claims 1 to 8.

10. The power supply and / or absorption device is the device according to any one of claims 1 to 9, and is directly connected to the load conductor.

11. A method (30) implemented in a device configured to supply power to a load (1) connected to a power system (2) or to absorb power from the load, wherein the load is connected to or connectable to a load conductor (3), and the device is connected to the load conductor and is configured to selectively supply power to or absorb power from the load conductor, and includes a power supply and / or absorption device (6) configured to supply power to or absorb power from the load conductor, and the power supplied to or absorbed from the load conductor by the power supply and / or absorption device is regulated at least by a voltage reference value of the power supply and / or absorption device, and the method includes: acquiring (31) at least one value indicating the voltage of the load conductor; determining (32) a voltage reference value for the power supply and / or absorption device based on the at least one value indicating the voltage of the load conductor and a virtual impedance of the power supply and / or absorption device; and 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. and the virtual impedance of the power supply and / or absorption device is related to a virtual reactance and a virtual resistance, and the power supply and / or absorption device is configured such that a value of the virtual reactance is greater than a value of a reactance of the power supply and / or absorption device, and a value of the virtual resistance is smaller than the value of the virtual reactance. Method (30).

12. A computer program comprising instructions which, when executed by one or more processors included in the control unit (7), cause the control unit to execute the method according to claim 11. **Claim 13** A power system (2), a load (1) connected to the power system, a load conductor (3) to which the load is connected or can be connected, and a device according to any of claims 1 to 10 configured to supply power to or absorb power from the load A system comprising:

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