METHOD AND SYSTEM FOR CONTROLLING AN ELECTRIC ARC FURNACE

Abstract

 The present disclosure relates to a method of operating an electric arc furnace by means of an electric arc furnace operation system connectable with the electric arc furnace to form an electric arc furnace system, and to a corresponding system. The electric arc furnace operation system comprises a converter device connectable with the AC grid for supplying the electric arc furnace with electric power, an electrode controller controlling electrodes of the electric arc furnace, and a converter controller. The method includes the following operations:
- receiving, at the converter controller a reference impedance comprising a reference inductance and a reference resistance, a reference voltage, and an electrode voltage;
- determining, by means of the converter controller, a converter reference voltage on basis of at least the reference voltage, the reference impedance, the electrode voltage, and a physical inductance of the electric arc furnace system; and
- controlling the converter device to generate an output voltage on basis of the converter reference voltage.

Description

TECHNICAL FIELD

[0001] The present disclosure generally relates to a method and a system for controlling an electric arc furnace.

BACKGROUND

[0002] An Electric Arc Furnace (EAF) may be supplied by a power supply system having a converter device, such as a Static Frequency Converter (SFC), which connects the Alternating Current (AC) grid to the EAF. An advantage of such a power supply system is to optimize the production of melted metal without requiring a tap-changer on the EAF transformer nor additional series reactors as in previous power supply systems, which decrease the electrode consumption. A prior art solution using a converter device is dislosed in the patent US10470259B2, which shows an electric arc furnace system with a Multi-Modular Converter (MMC) arranged to supply power from the AC grid to the EAF. The MMC is controlled by a control device, which receives a reference signal constituting a reference current value and/or a reference voltage value from an electrode controller. The control device is configured to control the MMC to generate an output current feeding the EAF, via the EAF transformer, to meet the reference current and/or reference voltage.

[0003] This prior art power supply system is considered to be less efficient than expected.

SUMMARY

[0004] The present disclosure seeks to at least partly remedy the above discussed issues. To achieve this, a method of operating an electric arc furnace and an electric arc furnace operation system, as defined by the independent claims are provided. Further embodiments are provided in the dependent claims.

[0005] According to an aspect of the present disclosure, there is provided a method of operating an electric arc furnace by means of an electric arc furnace operation system connectable with the electric arc furnace to form an electric arc furnace system, wherein the electric arc furnace operation system comprises a converter device connectable with the AC grid for supplying the electric arc furnace with electric power, an electrode controller controlling electrodes of the electric arc furnace, and a converter controller, the method comprising:

• receiving, at the converter controller a reference impedance comprising a reference inductance and a reference resistance, a reference voltage and an electrode voltage;
• determining, by means of the converter controller, a converter reference voltage on basis of at least the reference voltage, the reference impedance, the electrode voltage, and a physical inductance of the electric arc furnace system; and

controlling the converter device to generate an output voltage on basis of the converter reference voltage.

[0006] According to an embodiment of the method it may comprise measuring a load voltage and a load current between the converter device and the electric arc furnace. The operation of controlling the converter may be further based on the load voltage and the load current.

[0007] According to an embodiment of the method it may comprise comparing the load current with a threshold, and determining, when the electrode current exceeds the threshold, a virtual resistance arranged to decrease the electrode current below the threshold.

[0008] According to an embodiment of the method it may comprise generating the reference impedance and the reference voltage by means of the electrode controller.

[0009] According to an embodiment of the method the physical inductance may be predetermined.

[0010] According to another aspect of the present disclosure there is provided an electric arc furnace operation system connectable to an electric arc furnace to form an electric arc furnace system. The electric arc furnace operation system comprises:

• a converter device connectable with the AC grid for supplying the electric arc furnace with electric power;
• an electrode controller for controlling electrodes of the electric arc furnace; and
• a converter controller controlling the converter device;

wherein the converter controller is configured to receive a reference voltage, a reference impedance comprising a reference inductance and a reference resistance, and an electrode voltage, wherein the converter controller is configured to determine a converter reference voltage on basis of at least the reference voltage, the reference impedance, the electrode voltage, and a physical inductance of the electric arc furnace system, and to control the converter device to generate an output voltage on basis of the converter reference voltage.

[0011] According to an embodiment of the electric arc furnace operation system, it comprises a voltage measurement device configured to measure a load voltage between the converter device and the electric arc furnace, and a current measurement device configured to measure a load current between the converter device and the electric arc furnace, wherein the load voltage and the load current measurement devices are connected with the converter controller, and wherein the converter controller is configured to additionally base the converter control on the load voltage and the load current.

[0012] According to an embodiment of the electric arc furnace operation system, the converter controller is configured to compare the load current with a threshold, wherein the converter controller is configured to determine, when the load current exceeds the threshold, a virtual resistance which is arranged to decrease the load current below the threshold.

[0013] According to an embodiment of the electric arc furnace operation system, it comprises a supply voltage measurement device and a supply current measurement device for measuring a supply voltage and a supply current. The converter controller is configured to additionally base the control of the converter device on the measured supply voltage and supply current.

[0014] According to an embodiment of the electric arc furnace operation system, the converter device has an input and an output and comprises a first converter part connected with the input, a second converter part connected with output, and a mid converter part interconnecting the first and second parts.

[0015] According to an embodiment of the electric arc furnace operation system, the converter device is a back-to-back converter

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Exemplifying embodiments of the present disclosure will be described below with reference to the accompanying drawings, in which:

Figure 1 is a block diagram of an embodiment of an electric arc furnace operation system according to the present disclosure;

Figure 2 is a schematic diagram of the electric arc furnace operation system;

Figure 3 is a block diagram of a part of the electric arc furnace operation system according to the present disclosure;

Figures 4 and 5 illustrate examples of converters; and

Figure 6 is a flowchart illustrating an embodiment of a method of operating an electric arc furnace according to the present disclosure.

DETAILED DESCRIPTION

[0017] The present disclosure will be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments are shown. The present disclosure should however not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this description will convey the scope of the present disclosure to those skilled in the art.

[0018] An exemplifying embodiment of the present electric arc furnace operation system 100 comprises a converter device 101, connected with an electric arc furnace (EAF) 102 for supplying electric power to the EAF 102, an electrode controller 103, connected with electrodes 104 of the EAF 102 for controlling the position of the electrodes 104, and a converter controller 105, connected with the converter device 101 and controlling the converter device 101. Further, the converter device 101 is connected to an AC grid 106, at an input 110 of the converter device 101, and more particularly to grid power lines 107 via a grid transformer 108 and intermediate power lines 109 extending between the grid transformer 108 and the converter device 101. The converter controller 105 is, additionally, connected with the electrode controller 103. More particularly, the converter device 101 is connected with the EAF 102 at an output 111 of the converter device 101 via load power lines 112, an EAF transformer 113 and electrode power lines 114 extending between the EAF transformer 113 and the electrodes 104.

[0019] The total system comprising the electric arc furnace operation system 100 and the electric arc furnace 104 is defined as an electric arc furnace system 150.

[0020] According to an embodiment of the present method of operating an electric arc furnace, the electric arc furnace operation system 100 is operated as follows. Generally, the converter device 101 supplies electric power from the AC grid 106 to the EAF 102, and more particularly to the electrodes 104 of the EAF 102. The electrode controller 103 controls the arc impedance by means of adjusting the electrode positions in the EAF 102 to obtain an efficient melting process in the EAF 102, which requires different impedances at different stages of the melting process. The converter controller 105 controls the converter device 101 to provide the required output voltage based on input from at least the electrode controller 103.

[0021] Therefore, the electrode controller 103 additionally determines a reference voltage U_REF, and a reference impedance Z_REF comprising a reference inductance L_REF and a reference resistance R_REF, which provides a desired electrode current I_EL through the electrodes 104. An advantage of the present solution is that the reference impedance and the reference voltage can be individually controlled. The reference impedance Z_REF may be constructed as a virtual impedance representing a total impedance of the electric arc furnace system 150. Correspondingly, the reference inductance L_REF may be constructed as a virtual inductance, reference resistance R_REF may be constructed as a virtual resistance, and the reference voltage U_REF may be constructed as a virtual voltage which, when applied across the whole electric furnace system, generates a desired electrode current I_EL. In order to determine the reference voltage U_REF and reference impedance Z_REF the electrode controller 103 may, for example, use a predetermined schedule which is related to the stage at which the melting process in the electric arc furnace presently is. The present electrode voltage U_EL and the present electrode current I_EL may be measured by means of suitable voltage and current measurement devices 115, 116, connected with the electrode controller 105. The electrode controller 103 uses the measurements to adjust the reference impedance Z_REF and/or the reference voltage U_REF. The electrode controller 103 may emulate a tap-changer system, where different tap-changer positions are related with different source impedances and voltages, to be represented by a virtual controller impedance and a physical impedance of, inter alia, the converter device 101 and the EAF transformer 113, and by a converter voltage.

[0022] The converter controller 105 receives, from the electrode controller 103, the reference voltage U_REF, and the reference impedance Z_REF. The reference inductance L_REF may be regarded to comprise a virtual control inductance L_C and a physical inductance L_F of the electric arc furnace system 150, such that the sum of the physical inductance L_F and the virtual control inductance Lc equals the reference inductance L_REF, or, if desired, matches some other appropriate relation between the virtual control inductance L_C and the reference inductance L_REF.

[0023] The converter controller 105 is configured to determine a converter reference voltage U_CREF on basis of at least the reference voltage U_REF, the reference impedance Z_REF, both received from the electrode controller 103, the physical inductance L_F, which is predetermined, and an electrode voltage U_EL, which can be the above-mentioned electrode voltage and is received from the electrode controller 103 as well or measured in some other way. The converter controller 105 controls the converter device 101 to generate an output voltage U_CON on basis of the converter reference voltage U_CREF.

[0024] Figure 2 shows a schematic diagram of the electric arc furnace system 150 illustrating the control principle. The physical inductance L_F of the electric arc furnace system 150 is represented in the diagram by a first inductor denoted L_F. The physical inductance L_F of the electric arc furnace system 150 is a sum of physical inductances of the converter device 101, the EAF transformer 113, the electrodes 104, electrode power cables, etc. The virtual control inductance Lc is represented by a second inductor denoted Lc. The reference resistance R_REF is represented by a resistor denoted R_REF.

[0025] As mentioned above, the operations performed by the converter controller 105 include determining the converter reference voltage U_CREF This determination can be expressed mathematically as follows.

[0026] The converter current is given by:

where 's' is the Laplace operator and all voltages and currents are expressed in vector representation, i.e. in terms of a vector with a magnitude, which rotates at the angular nominal frequency.

[0027] The converter voltage reference U_CREF is defined as the converter voltage output behind the physical reactance, or the converter voltage at no load, and it can be expressed as follows:

[0028] Inserting Eq. 1 in Eq. 2 results in:

[0029] It should be noted that in these calculations it is assumed that the measured electrode voltage U_EL is the electrode voltage measured at the secondary side of the EAF transformer 114, as shown in Figure 2. Furthermore, it should be noted that a converter output voltage U_CON at the output 111 of the converter device 101 is the converter reference voltage U_CREF minus a voltage drop across phase reactors at the output end of the converter device 101, such as for example illustrated by inductors 127 in Figure 4.

[0030] Additionally, a load voltage U_L and a load current I_L may be measured, by means of suitable voltage and current measurement devices 128, 129, between the converter output 111 and the EAF 102. For instance, as shown in Figure 2, the measurements are performed between the converter device 101 and the EAF transformer 113. The voltage and current measurement devices 128, 129 are connected with the converter controller 105, which uses the values of the load voltage U_L and load current I_L for feedback control of the converter output voltage U_CON, and thus of the converter reference voltage U_CREF. The load voltage U_L may be the same as the above-mentioned electrode voltage U_EL, and the load current I_L may be the same as the above-mentioned electrode current I_EL.

[0031] Thus, referring to Figure 6, the method operating an electric arc furnace may contain the following main operations. Receiving a reference impedance comprising a reference inductance and a reference resistance, a reference voltage and an electrode voltage, box 601; determining a converter reference voltage on basis of at least the reference voltage, the reference impedance, the electrode voltage, and a physical inductance of the electric arc furnace system, box 602; and controlling the converter device to generate an output voltage on basis of the converter reference voltage, box 603.

[0032] The converter device 101 may be a Back to Back (B2B) converter, as shown in Figure 3. While being an AC-AC converter as a whole, looking at the input 110 and output 111 of the converter device 101, on a more detailed level it is an AC-DC-AC converter having a first Voltage Source Converter (VSC) part 117 connected with the input 110 of the converter device 101, a second VSC part 118, connected with the output 111 of the converter device 101, and an intermediate DC part 119 interconnecting the first and second VSC parts 117, 118. An advantage of this kind of converter is that it can be used for controlling reactive power on the AC grid side to minimize flicker, voltage variations, etc. Furthermore, it enables the EAF frequency to be equal to the AC grid frequency, which is not possible with, for instance, an MMMC described below. In this case, the converter controller 105 additionally uses measurements of the supply voltage Is and the supply current Us, measured by means of suitable supply voltage and supply current measurement devices 120, 121. Furthermore, the converter controller 105, in addition to voltage magnitudes controls the phase angle of the voltages in order to obtain the reactive power control, such that the converter device 101, and more particularly the first VSC part 117 in conjunction with the second DC part 118, either consumes reactive power from or generates reactive power to the AC grid 106.

[0033] In Figure 4 an example of a B2B converter is shown in more detail. Each VSC part 117, 118 comprises two converter units 122 arranged in parallel in two branches, which in turn are connected with the DC part 119. Each converter unit 122 has three modules 123 each. The modules 123 of the first VSC part 117 are connected with one phase each of the 3-phase AC grid 106 via phase reactors 127, and the modules 123 of the second VSC part 118 are connected with one phase each of the output lines via phase reactors 127. Each module 123 comprises several cells 124 connected in series, where each cell 124 comprises several switches 125 and a capacitor element 126. Each cell 124 can be switched between a state where it is inserted and contributes to a total voltage across the module 123 and a state where it is bypassed and provides no contribution.

[0034] The converter device 101 may be a Matrix Modular Multi-level Converter (MMMC) 201 as shown in Figure 5. The MMMC 201 comprises three converter units 202 connected in parallel. Each converter unit 202 comprises three modules 203, one for each phase. An input end of each module 203 is connected to a phase line 206 at the input 204 of the MMMC 201, and an output end of each module 203 is interconnected with the output ends of the other two modules of the converter unit 202 and connected to one of the phase lines 207 at the output 205 of the MMMC. Thus, the converter units 202 are connected to one phase each at the output 205 of the MMMC 201.

[0035] The converter controller 105 comprises the ordinary units for detailed control of the converter device 101, i.e. cell selection units, such as e.g. one or more pwm units for determining how many cells to insert or bypass in each module, one or more cell selector units for determining which cells within each module to insert or bypass, gate units for operating the individual switches of the cells, etc., in order to generate the converter output voltage U_CON. Since this is well known to the person skilled in the art no further description thereof will be made.

[0036] The converter controller 105 may additionally receive a reference frequency F_REF from the electrode controller 103. The converter controller 105 is configured to use the reference frequency F_REF for optimizing the melting down process of the EAF 102 and for reducing the electrode power consumption. For example, the frequency may be increased at the beginning of the process and then a lower frequency can be used when the arc is more stable to increase the metal production.

[0037] The converter controller 105 may additionally be configured to compare the load current I_L with a threshold, and to determine whether the load current I_L exceeds the threshold or not. When the load current I_L exceeds the threshold, the converter controller 105 will determine a virtual resistance which is added to the reference resistance to decrease the electrode current below the threshold.

[0038] While the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive.

[0039] Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements.

[0040] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A method of operating an electric arc furnace by means of an electric arc furnace operation system connectable with the electric arc furnace to form an electric arc furnace system, wherein the electric arc furnace operation system comprises a converter device connectable with the AC grid for supplying the electric arc furnace with electric power, an electrode controller controlling electrodes of the electric arc furnace, and a converter controller, the method comprising:

- receiving, at the converter controller a reference impedance comprising a reference inductance and a reference resistance, a reference voltage and an electrode voltage;

- determining, by means of the converter controller, a converter reference voltage on basis of at least the reference voltage, the reference impedance, the electrode voltage, and a physical inductance of the electric arc furnace system; and

- controlling the converter device to generate an output voltage on basis of the converter reference voltage.

2. The method according to claim 1, comprising:

- measuring a load voltage and a load current between the converter device and the electric arc furnace;

said controlling the converter being further based on the load voltage and the load current.

3. The method according to claim 2, comprising:

- comparing the load current with a threshold; and

- determining, when the electrode current exceeds the threshold, a virtual resistance arranged to decrease the electrode current below the threshold.

4. The method according to any one of the preceding claims, comprising

- generating the reference impedance and the reference voltage by means of the electrode controller.

5. The method according to any one of the preceding claims, comprising generating the reference impedance and the reference voltage by means of the electrode controller.

6. The method according to any one of the preceding claims, wherein the physical inductance is predetermined.

7. An electric arc furnace operation system connectable to an electric arc furnace to form an electric arc furnace system, the electric arc furnace operation system comprising:

- a converter device connectable with the AC grid for supplying the electric arc furnace with electric power;

- an electrode controller for controlling electrodes of the electric arc furnace; and

- a converter controller controlling the converter device;

wherein the converter controller is configured to receive a reference voltage, a reference impedance comprising a reference inductance and a reference resistance, and an electrode voltage, wherein the converter controller is configured to determine a converter reference voltage on basis of at least the reference voltage, the reference impedance, the electrode voltage, and a physical inductance of the electric arc furnace system, and to control the converter device to generate an output voltage on basis of the converter reference voltage.

8. The electric arc furnace operation system according to claim 7, comprising a voltage measurement device configured to measure a load voltage between the converter device and the electric arc furnace, and a current measurement device configured to measure a load current between the converter device and the electric arc furnace, wherein the load voltage and the load current measurement devices are connected with the converter controller, and wherein the converter controller is configured to additionally base the converter control on the load voltage and the load current.

9. The electric arc furnace operation system according to claim 7 or 8, wherein the converter controller is configured to compare the load current with a threshold, wherein the converter controller is configured to determine, when the load current exceeds the threshold, a virtual resistance which is arranged to decrease the load current below the threshold.

10. The electric arc furnace operation system according to any one of claims 7 to 9, comprising a supply voltage measurement device and a supply current measurement device for measuring a supply voltage and a supply current wherein the converter controller is configured to additionally base the control of the converter device on the measured supply voltage and supply current.

11. The electric arc furnace operation system according to any one of claims 7 to 10, wherein the converter device has an input and an output and comprises a first converter part connected with the input, a second converter part connected with output, and a mid converter part interconnecting the first and second parts.

12. The electric arc furnace operation system according to claim 11, wherein the converter device is a back-to-back converter.

Drawing