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EP-4548451-B1 - APPARATUS AND METHOD FOR SUPPLYING POWER TO OR ABSORBING POWER FROM A LOAD

EP4548451B1EP 4548451 B1EP4548451 B1EP 4548451B1EP-4548451-B1

Inventors

  • HASLER, JEAN-PHILIPPE
  • RASMUSSEN, JON

Dates

Publication Date
20260513
Application Date
20220629

Claims (13)

  1. An apparatus configured to supply power to a load (1) connected with a power system (2) and absorb power from the load, the load being connected or connectable to a load conductor (3), the apparatus comprising: a power supplying and absorbing device (6) configured to be connected to the load conductor and configured for selectively supplying power to the load conductor or absorbing power from the load conductor, wherein the apparatus is configured such that the power supplied to the load conductor or absorbed from the load conductor by the power supplying and absorbing device is governed at least by a voltage reference value of the power supplying and absorbing device; and a control unit (7) configured to control operation of the power supplying and absorbing device, the control unit being further configured to: obtain at least one value indicative of voltage of the load conductor; determine, based on the at least one value indicative of voltage of the load conductor and a virtual impedance of the power supplying and absorbing device, a voltage reference value for the power supplying and absorbing device; and control the power supplying and absorbing device to selectively supply power to the load conductor, and thereby supply power to the load, or absorb power from the load conductor, and thereby absorb power from the load, based on the determined voltage reference value; characterized in that the virtual impedance of the power supplying and absorbing device is associated with a virtual reactance and a virtual resistance, wherein the power supplying and absorbing device is configured such that a value of the virtual reactance is higher than the value of a reactance of the power supplying and absorbing device, and such that a value of the virtual resistance is less than the value of the virtual reactance.
  2. An apparatus according to claim 1, wherein the virtual reactance is a sum of the reactance of the power supplying and absorbing device and a selected percentage of the reactance of the power supplying and absorbing device, the selected percentage of the reactance of the power supplying and absorbing device being in a range between 1% and 50% of the reactance of the power supplying and absorbing device.
  3. An apparatus according to claim 1 or 2, wherein the virtual reactance is such that, by the determined voltage reference value for the power supplying and absorbing device, the power supplying and absorbing device is capable of supplying power to the load conductor or absorbing power from the load conductor so as to keep any fluctuations in voltage of the load conductor, compared to an average voltage level of the voltage of the load conductor, over a period of time below a selected threshold voltage fluctuation level while at the same time keeping the virtual reactance as small as possible.
  4. An apparatus according to any one of claims 1-3, wherein the virtual resistance is such that a value of the virtual resistance is between 25% and 75% of the value of the virtual reactance.
  5. An apparatus according to any one of claims 1-4, wherein the control unit is further configured to: obtain a plurality of values indicative of current of the load at different time instants during a period of time; determine a change in current of the load over the period of time based on the plurality of values indicative of current of the load at the different time instants; and determine the voltage reference value for the power supplying and absorbing device further based on the determined change in current of the load over the period of time.
  6. An apparatus according to claim 5, wherein the control unit is configured to determine the voltage reference value for the power supplying and absorbing device based on the plurality of values indicative of current of the load at the different time instants to decrease any fluctuations in the current of the load, compared to an average current level of the current of the load, over a period of time.
  7. An apparatus according to any one of claims 1-6, wherein the control unit is further configured to obtain at least one value indicative of current of the power system and to determine the voltage reference value for the power supplying and absorbing device further based on the least one value indicative of current of the power system.
  8. An apparatus according to claim 7, wherein the control unit is configured to determine the voltage reference value for the power supplying and absorbing device based on the least one value indicative of current of the power system and the at least one value indicative of voltage of the load conductor to increase the power factor of the load.
  9. An apparatus according to any one of claims 1-8, wherein the power supplying and absorbing device is based on a Voltage Source Converter, VSC, based device, a Static Synchronous Compensator, STATCOM, and/or a multi-level converter.
  10. An apparatus according to any one of claims 1-9, wherein the power supplying and absorbing device is configured to be directly connected to the load conductor.
  11. A method (30) implemented in an apparatus configured to supply power to a load (1) connected with a power system (2) and absorb power from the load, the load being connected or connectable to a load conductor (3), the apparatus comprising a power supplying and absorbing device (6) connected to the load conductor and configured for selectively supplying power to the load conductor or absorbing power from the load conductor, wherein the power supplied to the load conductor or absorbed from the load conductor by the power supplying and absorbing device is governed at least by a voltage reference value of the power supplying and absorbing device, the method comprising: obtaining (31) at least one value indicative of voltage of the load conductor; determining (32), based on the at least one value indicative of voltage of the load conductor and a virtual impedance of the power supplying and absorbing device, a voltage reference value for the power supplying and absorbing device; and controlling (33) the power supplying and absorbing device to selectively supply power to the load conductor, and thereby supply power to the load, or absorb power from the load conductor, and thereby absorb power from the load, based on the determined voltage reference value; characterized in that the virtual impedance of the power supplying and absorbing device is associated with a virtual reactance and a virtual resistance, wherein the power supplying and absorbing device is configured such that a value of the virtual reactance is higher than the value of a reactance of the power supplying and absorbing device, and such that a value of the virtual resistance is less than the value of the virtual reactance.
  12. A computer program comprising instructions, which when executed by one or more processors comprised in a control unit (7) of an apparatus according to claim 1, cause the control unit to perform a method according to claim 11.
  13. A system comprising: a power system (2); a load (1) connected with the power system; a load conductor (3), wherein the load is connected or connectable to the load conductor; and an apparatus according to any of claims 1-10 configured to selectively supply power to the load or absorb power from the load.

Description

TECHNICAL FIELD The present invention relates to an apparatus configured to supply power to a load that is connected with a power system or absorb power from the load. BACKGROUND A power system, such as a power transmission and/or distribution system and/or a power grid, may be used to provide power to equipment in many industries. In this context, the power system may be referred to as a feeding electrical network or system, and the equipment may be referred to as a load. At least in some industries, flicker in the voltage of the power system - i.e., rapid fluctuations in the voltage of the power system - and generation of harmonic frequencies by the load in the voltage of the power system may be a concern. For example, in many electrical arc furnace (EAF) steel-making facilities, flicker and generation of harmonic frequencies in the voltage of the feeding electrical network, or "harmonics", are a concern for the feeding electrical network. The flicker can be reduced by measuring the EAF current and compensating for harmonic frequencies in the voltage of the feeding electrical network and negative sequence currents using a device such as a Static Synchronous Compensator (STATCOM). To further improve compensation for harmonic frequencies in the voltage of the feeding electrical network it may be focused on compensating for the low-order harmonics mostly generated by the EAF. Examples of prior art may be found in an article entitled "An Adaptive Virtual Impedance Control for Improving Power Sharing Among Inverters in Islanded AC Microgrids", IEEE TRANSACTIONS ON SMART GRID, IEEE, USA, vol. 12, no. 4, 26 February 2021 (2021-02-26), pages 2991-3003, an article entitled "Analysis and implementation of virtual impedance for fixed-frequency control strategy in microgrid", IET GENERATION, TRANSMISSION&DISTRIBUTION, IET, UK, vol. 15, no. 15, 31 March 2021 (2021-03-31), pages 2262-2276, an article entitled "Power-Sharing Control in an Islanded Microgrid using Virtual Impedance", 2020 8TH INTERNATIONAL CONFERENCE ON SMART GRID (ICSMARTGRID), IEEE, 17 June 2020 (2020-06-17), pages 73-77, and an article entitled "Virtual-Impedance-Based Control for Voltage-Source and Current-Source Converters", IEEE TRANSACTIONS ON POWER ELECTRONICS, INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS, USA, vol. 30, no. 12, 1 December 2015 (2015-12-01), pages 7019-7037. SUMMARY A device such as a Static Synchronous Compensator (STATCOM) can be used to provide or absorb reactive power and thereby regulate the voltage at the point of connection to the power system or feeding electrical network. For example, for a STATCOM used in an electrical arc furnace (EAF) application, two control methods may operate concurrently. One control method may be so-called flicker control, which may work as an open-loop control, and which may counteract any rapid changes in the load (e.g., EAF) current. The flicker control may compensate for reactive power, negative sequence current of the load and possibly some low-order harmonics in the voltage of the power system or feeding electrical network. Another control method may be power factor control, which may work as a closed-loop control and which may compensate the power factor to the connecting power system or feeding electrical network. The power factor control may be based on sensed (or measured) or calculated current of the power system or feeding electrical network, which may give the active and reactive power at the point of connection to the power system or feeding electrical network. A flicker control algorithm may aim at correcting the negative sequence current of the load, which may produce flicker in the load current. Harmonics compensation for the low-order harmonics (e.g., below 5th order harmonics) may however be very limited. To improve the low-order harmonics compensation, other control algorithms may be used. However, such other control algorithms may require that the device, which is used to provide or absorb reactive power and thereby regulate the voltage at the point of connection to the power system or feeding electrical network (e.g., a STATCOM or a Voltage Source Converter (VSC) based device) is relatively large, due to that a relatively large number of converter cells may be needed in order for such control algorithms to be able to compensate for the low-order harmonics. An example of such other control algorithms is provided in WO 98/27476 A1. However, increasing the size of device by increasing the number of converter cells will in general increase the cost of the device - and thereby also the cost of the overall system. In view of the foregoing, a concern of the present invention is to provide means for compensating for any low-order harmonics generated by a load such as an EAF in the voltage of an electrical network or power system feeding the load, while reducing or even avoiding the need for increasing a size of a device used to provide or absorb reactive power and thereby regula