ARC FIREWORK AND METHOD FOR OPERATING A ARC FIREWORK

Description

[0001] The invention relates to an electric arc furnace and a method for operating an electric arc furnace.

[0002] WO 2018 / 050332 A1 describes an electric arc furnace according to the preamble of claim 1. The electrodes of the electric arc furnace can be mechanically moved downwards and upwards as required towards the bottom of a furnace vessel of the electric arc furnace in order to ignite the electric arc between the electrodes and the charge material introduced therein, in particular steel scrap, and then to adjust the arc voltage, the current and thus the coupled power via the distance between the electrode and the charge material in the furnace vessel of the electric arc furnace.

[0003] International patent application WO 2018 / 233833 A1 discloses an electric arc furnace supplied with electrical energy by a matrix converter. The voltage, current, and frequency can be individually adjusted for each electric arc.

[0004] Patent application WO 2019 / 207609 A1 discloses an electric arc furnace in which a power converter provides the current for the electrodes. The power converter is controlled by a control unit.

[0005] Utility model DE 297 13 666 U1 discloses a device for monitoring the slag condition and arc stability in an electric arc furnace. The device is designed to control the electric arc furnace by monitoring the slag condition.

[0006] Compared to conventional electric arc furnaces, it is desirable to improve the power quality characteristics of the electrical grid supplying the furnace. Furthermore, it is desirable to operate the electric arc furnace more efficiently than is possible through control using the conventional mechanical movement of the electrodes.

[0007] The object of the invention is therefore to at least partially reduce the disadvantages of arc furnaces known from the prior art, in particular to improve the network quality characteristics of the power grid supplying the arc furnace and to enable efficient operation of the arc furnace.

[0008] The aforementioned problem is solved by the subject matter of the claims, in particular by an electric arc furnace according to claim 1 and a method according to claim 10. Further advantages and details of the invention will become apparent from the dependent claims, the description, and the drawings. Features and details disclosed in connection with the electric arc furnace according to the invention naturally also apply in connection with the method according to the invention, so that, with regard to the disclosure of the individual aspects of the invention, there is always, or can always be, a reciprocal reference to the invention.

[0009] The problem is solved according to a first aspect by an electric arc furnace with a furnace vessel, three electrodes arranged in the furnace vessel, a transformer connected to the three electrodes, and a power connection for connecting to a power grid to provide current to the electrodes, wherein the electric arc furnace has a converter and a control system, wherein the control system is configured to control at least one parameter of the electric arcs generated by at least one of the three electrodes by means of the converter.

[0010] According to the invention, an electric arc furnace is provided whose arcs generated by the electrodes are controlled by means of the control system and converter. This makes it possible to improve the power quality characteristics of the power grid supplying the electric arc furnace and to make the operation of the electric arc furnace more efficient.

[0011] The electric arcs are generated by the electrodes between them and the conductive material. The material is metallic, which is the basis of its conductive properties. Examples of suitable materials include scrap steel or sponge iron, which are melted in the furnace vessel of the electric arc furnace by the electric arcs generated by the electrodes.

[0012] At least one parameter can be, in particular, the current, voltage, or frequency of the electric arcs. If the arc furnace is a three-phase arc furnace supplied with three-phase current, the current refers to the amplitude of the three-phase current, the voltage to the amplitude of the alternating voltage, and the frequency to the frequency of the alternating voltage.

[0013] The furnace vessel, in which the electrodes are arranged, can be made of a steel and / or brick structure, for example. The furnace vessel can be lined with a refractory lining to withstand the molten material. Furthermore, the furnace vessel can have a pouring spout, thus functioning as a ladle. This allows the molten material to be easily poured from the ladle. The electrodes can be, for example, graphite electrodes.

[0014] Preferably, the control system is configured to regulate at least one parameter of the electric arcs generated by at least one of the three electrodes independently of the other electrodes. This enables individual electrode-specific control of the electric arcs. As a result, the melting of the charge material in a local environment of the electrodes can be individually controlled, thus enabling particularly efficient operation of the electric arc furnace.

[0015] Furthermore, it is preferred that the control system is configured to regulate at least one parameter of the electric arcs generated by at least one of the three electrodes, using the inverter, according to the state and / or position of the charge material in the furnace vessel. The control system can be configured to detect the state and / or position of the charge material, particularly by means of the inverter. This will be explained in more detail later. This enables the electric arc furnace to react efficiently in the respective operating phase, which is determined in particular by the state and position of the charge material. This, too, will be explained in more detail later.

[0016] One possible state of the feed material is the form in which it is typically fed into the furnace vessel, such as scrap steel or sponge iron. Another state is molten material, in which the feed material has already been melted by the electric arcs. Further possible states of the feed material include "bears" or "icebergs," in which the feed material is fused together and can float on the molten metal. Bears and icebergs are particularly dangerous because contact with the electrodes can damage them, especially causing them to break.

[0017] Consequently, the charge material can occupy different positions within the furnace vessel. For example, the charge material, when molten, can settle at the bottom of the furnace vessel, forming a melt pool. Clumps or clumps can float on the melt and be located at the edge of the furnace vessel, in the center, and / or between the electrodes. The position of these clumps and clumps can be further or closer to the electrodes, and this, along with their size, determines the risk of electrode damage.

[0018] It is also preferred that the converter be designed as a matrix converter. This enables particularly efficient control via the control system. Furthermore, each electrode can be connected to its own converter, which allows the arcs generated by the respective electrode to be controlled.

[0019] It is also preferred that the electric arc furnace is designed as a three-phase arc furnace. It is also preferred that the electrodes are arranged circularly within the furnace vessel. In other words, the electrodes can be arranged on an imaginary circle. This allows the molten charge material to be stirred in the three-phase arc furnace. This is advantageous when the melt is subjected to metallurgical processes. These metallurgical processes can include, for example, the addition of alloys, the adjustment of the carbon content in the melt, the reduction of phosphorus and / or sulfur in the melt, and the like. Stirring ensures that the chemical processes and transformations are introduced into the melt homogeneously and efficiently.

[0020] It is also preferred that the control system is configured to lower or increase, as at least one parameter, the voltage and / or the current of the arcs generated by at least one of the three electrodes relative to the other three electrodes by means of the inverter.

[0021] This is particularly advantageous for influencing the radiation symmetry in the main melting phase and the shallow bath phase. The main melting phase begins when the electrodes have been lowered, the electrode arcs ignited, and a drill crater forms in the charge material, which deepens and lengthens the arcs. The shallow bath phase begins with the melting of the last charge material, or scrap basket, in which it is placed for the arc furnace.

[0022] Primarily, the melting power during the main melting phase and flat bath phase should be introduced into the furnace vessel as symmetrically as possible, so that the melting process runs at the same speed in all areas of the furnace vessel, and the wear pattern of the electric arc furnace appears uniform, and resulting repair-related downtimes can be planned.

[0023] However, various effects influence the radiation symmetry and complicate precise process and maintenance planning. For example, the insertion of charge material into the furnace vessel via a crane and a scrap basket often results in an uneven load within the furnace. This inevitably leads to premature melting in furnace zones with less solid, i.e., not yet melted, charge material, while other areas still contain solid charge material. Further melting at high electrical power accelerates the earlier exposure of these exposed furnace zones.

[0024] During the shallow bath phase, the melting process is more stable, and the applied radiant power depends on the electrode current and voltage setpoints as well as the three partial reactances of the electric arc furnace. Efforts are made to standardize the wear pattern of the three zones of the electric arc furnace in the long term by setting the parameters differently for each electrode and, if necessary, additionally supporting the process with a so-called foamed slag operation. However, many other parameters influence the shallow bath phase and make a stable, consistently uniform melting process difficult. These include the varying rates of electrode burn-off, which alters the individual reactances of the furnace zones. Another factor is that a furnace extraction system, in conjunction with a slag door, creates turbulence and thus colder and hotter furnace zones.Fossil fuel burners and oxygen lances are also operated differently depending on the specific metallurgical requirements.

[0025] However, by individually controlling the electrode arcs using the inverter, the radiation symmetry and distribution can be actively influenced. Independently adjusting the currents and voltages at each electrode allows for the creation of a three-phase asymmetrical melting power or radiation output. If, after extended furnace operation between repair cycles, a stable, recurring wear pattern develops in the arc furnace, the inverter can be used to perform a targeted, fixed pre-trimming of the radiation symmetry, thus counteracting uneven wear.Furthermore, if the operator (also called smelter) of the electric arc furnace or the control system detects during the flat bath phase that there are still deposits of feed material on one furnace zone, a melting process can be initiated by temporarily adjusting the radiation with the help of the converter, without unduly burdening the other furnace zones.

[0026] Furthermore, this is particularly advantageous when the feedstock is in the form of sponge iron, which is to be melted down. Sponge iron is iron ore that occurs in small spheres approximately 15 mm in diameter. Unlike scrap iron, this material can be easily transported logistically by ship and then by conveyor belt. Steel production is also possible in this way in countries with limited scrap or rich iron ore resources.

[0027] Iron sponge is continuously fed into the furnace vessel via metering devices, falling into the melt bath through a furnace lid. The aim is to align the jet of iron sponge, or rather the point of impact, precisely in the center between the three electrodes. However, the position of the point of impact depends on the feed rate, which can lead to significant interference with the arcs of individual electrodes if the point of impact moves too close to that electrode. This phenomenon has two disadvantages. First, the melt bath area of ​​the less affected electrodes overheats, with the risk of the molten metal boiling over. Second, the electrical power of the affected arcs of the respective electrodes is reduced, increasing the risk of the iron sponge spheres clumping together and thus forming icebergs.The power reduction stems from a significant disturbance of the arc caused by splashing molten metal and falling iron sponge at the arc. The inverter can detect the shift in the iron sponge's point of impact in the molten metal, as the harmonics in the current roughly reflect the arc penetration. The electrical power can then be selectively increased or decreased at the respective electrode without altering the overall target power. This also prevents icebergs from striking the electrodes and breaking them. This avoids production downtime, increased operating costs, and eliminates the need for post-treatment of the molten metal due to excessive carbon content.

[0028] It is also preferred that the control system is configured to lower or raise the voltage and / or current of the arcs generated by at least one of the three electrodes, depending on the result of a harmonic analysis of the electrodes and / or a temperature analysis of the furnace lining. This allows the detection of prematurely cleared furnace zones during the main melting phase via harmonic analysis and the temperature profile of the furnace lining or wall. In these zones, excessive furnace wear can be avoided by temporarily reducing the melting power of the electrode assigned to that zone.

[0029] It is also preferred that the control system is configured to reduce the voltage and / or current of the arcs generated by at least one of the three electrodes by means of the inverter, as at least one parameter, when an increased current is registered at at least one of the three electrodes by means of the inverter.

[0030] This is particularly advantageous during the main melting phase. The electric arcs transfer the melting power (also radiation) from the electrodes to the surrounding material. This heats the material, causing it to collapse and melt into a molten pool within the furnace. During this process, pieces of the material, such as scrap metal, regularly fall against the electrodes because the slowly softening structure of the material, which is usually piled up, becomes unstable. The impact of these material pieces causes numerous short circuits between the electrodes and the material.

[0031] The condition of the material being processed, in the form of pieces, can be determined here by means of the inverter based on the short circuits, since an increased current occurs at these short circuits due to the voltage drops across the arcs. Furthermore, the position relative to the three electrodes can be determined by observing at which of the three electrodes the short circuits occur, particularly if they are numerous.

[0032] Accordingly, reducing the voltage and / or current of the arcs generated by the electrodes that are subject to frequent short circuits, and thus to increased currents, can yield numerous advantages in the operation of the arc furnace. For example, the risk and frequency of electrode damage, particularly breakage, can be reduced. Thermal overload of the arc furnace's electrical components can also be avoided. Furthermore, the service life of the circuit breakers upstream of the electrodes can be extended. Finally, undesirable network disturbances such as flickering can be prevented, and voltage stability can be improved.

[0033] It is also preferred that the control system is configured to initiate the lifting of at least one of the three electrodes at which the increased current was detected. This is advantageous because the formation of "bears" or "icebergs" floating on the meltwater can be dangerous. These "bears" or "icebergs" can collide with the electrodes and cause a short circuit. Upon detection of such a short circuit, the respective electrode can be lifted. Subsequently, the "bear" or "iceberg" now located below the electrode can be melted by means of the electrode's arcs.

[0034] Furthermore, it is preferred that the control system is configured to increase the frequency of the arcs generated by at least one or all of the three electrodes using the inverter, as at least one parameter. In a three-phase arc furnace, this allows for arc stabilization by increasing the furnace's reactance. This is because, in a three-phase arc furnace, the arc extinguishes at each zero crossing of the current. The arc is reignited as soon as a required ignition voltage is reached. However, this results in a brief current gap, i.e., a distortion of the sinusoidal waveform of current and voltage. This effect leads to an undesirably high level of feedback on the power grid.The size of the current gap after the zero crossing depends on the melting phase (main melting or flat bath phase), the temperature of the electrodes and the charge material, and the overall reactance of the electric arc furnace, since a high reactance leads to a faster voltage increase after a current change. Because the reactance of the electric arc furnace is linearly dependent on the frequency of the three-phase current, increasing the frequency via the inverter also increases the reactance of the electric arc furnace. This reduces the undesirable effects of the current gaps, especially during the main melting phase. The result is a homogeneous melting process, which has a positive impact on the availability and production quality of the electric arc furnace.

[0035] The three electrodes are arranged in a circle, and the control system is configured to alternately increase and decrease the voltages and / or currents of the arcs generated by all three electrodes along their circular arrangement using the inverter, in order to stir the molten metal in the furnace vessel. During the flat bath phase, the metallurgical processes mentioned above are carried out, for which stirring the molten metal is necessary. A basic movement of the molten metal, or rather the molten bath, originates from the three-phase system of the three-phase arc furnace itself, namely through induction, analogous to the rotation of a rotor in an asynchronous motor. This circular movement of the molten bath around the three electrodes can be significantly increased by additionally imparting suitable mechanical pulses to the molten bath.Here, the jet effect of the electric arcs can be utilized, whereby a current from the electrode, via the arc, into the melt pool exerts a force of up to 5 tons on the melt pool, pressing it inwards. The jet effect is linearly dependent on the current intensity.

[0036] The inverter is used to variably adjust the current and / or voltage of the individual electrodes, which may be arranged in a circle, so that the melt pool beneath the electrodes is alternately compressed. By setting a suitable frequency and direction of rotation for the compression sequence, the natural rotational movement of the melt pool can be significantly enhanced. A further, additional local movement of the melt pool is achieved by the individual pulse of the increased jet effect beneath each electrode during the compression and release of the pool.

[0037] The problem initially posed is solved according to a second aspect by a method for operating an electric arc furnace with a furnace vessel, three electrodes arranged in the furnace vessel, a transformer connected to the three electrodes, and a power connection for connecting to a power grid to provide electrical power to the electrodes, wherein the electric arc furnace further comprises a converter and the method comprises the step: controlling at least one parameter of the electric arcs generated by at least one of the three electrodes by means of the converter.

[0038] The inventive method thus offers the same advantages as the inventive arc furnace. The inventive arc furnace can also incorporate the control system, which can be configured to regulate at least one parameter of the electrical power at at least one of the three electrodes by means of the inverter and can also regulate further process steps. However, this is not mandatory. These process steps can also be performed or initiated by the operator of the arc furnace. For this purpose, the operator can access corresponding values ​​supplied by the arc furnace, in particular by the inverter, and regulate the inverter accordingly.

[0039] Preferably, at least one parameter is the voltage and / or the current of the arcs generated by at least one of the three electrodes, which is lowered or increased by means of the converter compared to the other three electrodes.

[0040] Furthermore, it is preferred that the voltage at and / or the current of the arcs generated by at least one of the three electrodes is reduced by means of the inverter when an increased current is registered at at least one of the three electrodes by means of the inverter.

[0041] It is preferred that at least one of the three electrodes at which the increased current was registered is raised.

[0042] Furthermore, it may be provided to control at least one parameter of the arcs generated by at least one of the three electrodes independently of the other electrodes. It may also be provided to control at least one parameter of the arcs generated by at least one of the three electrodes according to the state and / or position of the charge material in the furnace vessel. It may also be provided to lower or raise the voltage and / or current of the arcs of at least one of the three electrodes as at least one parameter, depending on the result of a harmonic analysis of the electrodes and / or the result of a temperature analysis of a lining of the furnace vessel. Finally, it may be provided to increase the frequency of the arcs generated by at least one or all of the three electrodes by means of the converter as at least one parameter.Finally, the three electrodes are arranged in a circle, and the voltages and / or currents of the arcs generated by the three electrodes are alternately increased and decreased for each electrode along their circular arrangement by means of the inverter in order to stir a molten metal in the furnace vessel. This can be done by means of the control system and / or by the melter.

[0043] The invention is explained in more detail below with reference to the accompanying drawings. These show: FIG. 1 a schematic view of an embodiment of an electric arc furnace according to the invention, in which the charge material is in a first state and at a first position, FIG. 2 a schematic cross-sectional view of the electric arc furnace made of FIG. 1, in which the charge material is in a second state and at a second position, FIG. 3 a schematic top view of the electric arc furnace made of FIG. 1 , in which the charge material is in a third state, FIG. 4 a schematic cross-sectional view of the electric arc furnace made of FIG. 1 , in which the charge material is in a fourth state and in a fourth position, FIG. 5 a schematic top view of the electric arc furnace made of FIG. 1 , in which the charge material is in a fifth state and in a fifth position, and FIG. 6 a schematic cross-sectional view of the electric arc furnace made of FIG. 5 .

[0044] Elements with the same function and mode of operation are in the FIGURES 1 to 6Each element is provided with the same reference numerals. Several identical elements in a figure are numbered consecutively, with the consecutive numbering separated from its reference numeral by a period.

[0045] FIGURE 1 Figure 1 shows a schematic view of an embodiment of an electric arc furnace 10 according to the invention, in which the charge material 21 is in a first state and at a first position.

[0046] The electric arc furnace 10 is designed as a three-phase electric arc furnace. The electric arc furnace 10 has a furnace vessel 17 into which the charge material 21 is filled. FIGURE 1The electric arc furnace 10 is in the main melting phase, during which the charge material 21 is melted from its solid form. The charge material 21 floats on its own molten metal 22. The furnace vessel 17 is lined with a refractory lining 19 to withstand the molten metal 22. Furthermore, the furnace vessel 17 has a pouring spout 18.

[0047] Three electrodes 14, 15, 16 are embedded in the furnace vessel 17. Each electrode 14, 15, 16 is connected to a current converter 12. The converter 12 is connected to a transformer 11. The transformer 11 is connected to a power supply 40. A control system 13 is connected to or includes the converter 12. The control system 13 regulates the current or the arcs 20 of the electrodes 14, 15, 16 by means of the converter 12.

[0048] The electrodes 14, 15, 16 each generate arcs 20.1, 20.2, 20.3 between themselves and the feed material 21, which melt the feed material 21.

[0049] The current and / or voltage of the arcs of individual or all electrodes 14, 15, 16 can be limited by means of the control system 13 or by the melter if parts of the charge material 21 fall against the electrodes 14, 15, 16 due to the increasing instability of its bed and cause short circuits. These short circuits are detected by the converter 12 by the resulting increase in current.

[0050] FIGURE 2 shows a schematic cross-sectional view of the electric arc furnace 10. FIG. 1, in which the material 21 is in a second state and at a second position. For the sake of clarity, the apparatus of the electric arc furnace 10, comprising the converter 12, the transformer 11, and the control system 13, as well as the power grid 40, are not shown here and in the following figures.

[0051] A slug 23 (clumped-together insert material) has formed below the first electrode 14. The length of the arc 20.1 of the first electrode 14 is therefore shorter than that of the other arcs 20.2, 20.3 of the second and third electrodes 15, 16. Accordingly, the converter 12 can detect the slug 23. As a measure against a collision of the electrode 14 with the slug 23, the first electrode 14 can be mechanically lifted to prevent breakage of the first electrode 14.

[0052] FIGURE 3 shows a schematic top view of the electric arc furnace 10. FIG. 1, in which the operational material 21 is in a third state.

[0053] The feed material 21 is present in the flat bath phase and thus completely molten, i.e., as melt 22. The electrodes 14, 15, 16 are arranged in a circle. By supplying the electrodes 14, 15, 16 with three-phase current, the melt 22 is stirred according to an individual flow 24.1, 24.2, 24.3.

[0054] Furthermore, the voltages and / or currents of the arcs 20 generated by the three electrodes 14, 15, 16 are alternately increased and decreased for each electrode 14, 15, 16 along their circular arrangement by means of the converter 12. This intensifies the stirring of the melt 22 along the common flow 25.1, 25.2, 25.3. This results in even more homogeneous mixing during the metallurgical processes in the shallow bath phase.

[0055] FIGURE 4shows a schematic cross-sectional view of the electric arc furnace 10. FIG. 1 , in which the equipment is in a fourth state and in a fourth position.

[0056] There is a large amount of charge material in the furnace zone of the first electrode 14, little charge material in the furnace zone of the second electrode 15, and no charge material in the furnace zone of the third electrode 16. This can result, for example, from an uneven distribution of the charge material 21. If the electrodes 14, 15, and 16 are operated with the same electrical power, a high level of radiation 26 is emitted onto the refractory lining 19. Overall, the wear pattern of the electric arc furnace 10 becomes very inconsistent, which complicates repairs and makes repair times difficult to plan.

[0057] Therefore, it is possible to reduce the voltage and / or current of the arcs 20 generated by the third electrode 16 compared to the other electrodes 14, 15 using the converter 12, and also to reduce the voltage and / or current of the arcs 20 generated by the second electrode 15 compared to the first electrode 14. This results in more efficient melting of the charge material 21 and a more uniform wear pattern of the arc furnace 10.

[0058] FIGURE 5 shows a schematic top view of the electric arc furnace 10. FIG. 1 , in which the operational material 21 is in a fifth state and in a fifth position.

[0059] FIGURE 6 shows a to FIGURE 5 corresponding schematic cross-sectional view of the electric arc furnace 10 from FIG. 5 .

[0060] The deployment material 21 in the FIGURES 5 and 6The iron sponge is present as sponge iron. Sponge iron spheres 27 are poured into the furnace vessel 17 so that a jet 28 of sponge iron spheres 27 lands essentially in the middle between electrodes 14, 15, 16. However, the sponge iron spheres 27 do not have to land in the middle, but can land in the middle between electrodes 14, 15, 16 or elsewhere. The sponge iron spheres 27 thus form a sponge iron mound 29.

[0061] If the iron sponge mound 29 is driven towards one of the electrodes 14, 15, 16, this can be detected by the inverter 12, and a higher power can be applied to that electrode 14, 15, 16 accordingly, so that the iron sponge mound 29 does not collide with the electrode 14, 15, 16 and cause it to break. An iron sponge iceberg 30 has also formed at the edge of the furnace vessel 17 in the furnace zone of the third electrode 16. The inverter 12 can also detect the iron sponge iceberg 30, so that the control system 13 can provide appropriate control for faster melting and to avoid a collision. Reference symbol list

[0062] 10 Furnace vessel 11 Transformer 12 Inverter 13 Control system 14 First electrode 15 Second electrode 16 Third electrode 17 Furnace vessel 18 Pouring nozzle 19 Refractory lining 20 Arc 21 Charge material 22 Melt 23 Bear 24 Individual flow 25 Common flow 26 Radiation 27 Sponge iron sphere 28 Sponge iron jet 29 Sponge iron mountain 30 Sponge iron iceberg 40 Power grid

Claims

1. An electric arc furnace (10) with a furnace vessel (17), three electrodes (14, 15, 16) which are arranged in the furnace vessel (17), a transformer (11) which is connected to the three electrodes (14, 15, 16), and a current connection for connecting to a current network (40) for providing current to the electrodes (14, 15, 16), wherein the electric arc furnace (10) has a converter (12) and a control system (13), wherein the control system (13) is configured to control at least one parameter of electric arcs (20) generated by at least one of the three electrodes (14, 15, 16) by means of the converter (12), characterised in that the three electrodes (14, 15, 16) are circularly arranged and the control system (13) is configured to increase and lower, as parameters, the voltages and / or current intensities of the electric arcs (20) generated by the three electrodes (14, 15, 16) for each of the electrodes (14, 15, 16) alternately along their circular arrangement by means of the converter (12) in order to stir a melt (22) in the furnace vessel (17).

2. The electric arc furnace (10) according to claim 1, characterised in that the control system (13) is configured to control the at least one parameter of the electric arcs (20) generated by the at least one of the three electrodes (14, 15, 16) independently of the other electrodes (14, 15, 16).

3. The electric arc furnace (10) according to any one of the preceding claims, characterised in that the converter (12) is formed as a matrix converter.

4. The electric arc furnace (10) according to any one of the preceding claims, characterised in that the electric arc furnace (10) is formed as a three-phase current electric arc furnace (10).

5. The electric arc furnace (10) according to any one of the preceding claims, characterised in that the control system (13) is configured to lower or increase, as the at least one parameter, the voltage and / or the current intensity of the electric arcs (20) generated by the at least one of the three electrodes (14, 15, 16) relative to the others of the three electrodes (14, 15, 16) by means of the converter (12).

6. The electric arc furnace (10) according to claim 5, characterised in that the control system (13) is configured to lower or increase, as the at least one parameter, the voltage and / or the current intensity of the electric arcs (20) generated by the at least one of the three electrodes (14, 15, 16) depending on a result of a harmonic analysis of the electrodes (14, 15, 16) and / or a result of a temperature analysis of a lining (19) of the furnace vessel (17).

7. The electric arc furnace (10) according to claim 5 or 6, characterised in that the control system (13) is configured to lower the voltage and / or current intensity of the electric arcs (20) generated by the at least one of the three electrodes (14, 15, 16) by means of the converter (12) as the at least one parameter when an increased current intensity is registered at the at least one of the three electrodes (14, 15, 16) by means of the converter (12).

8. The electric arc furnace (10) according to claim 7, characterised in that the control system (13) is configured to initiate a lifting of the at least one of the three electrodes (14, 15, 16) at which the increased current intensity was registered.

9. The electric arc furnace (10) according to any one of the preceding claims, characterised in that the control system (13) is configured to increase, as the at least one parameter, the frequency of the electric arcs (20) generated by the at least one or all of the three electrodes (14, 15, 16) by means of the converter (12).

10. A method for operating an electric arc furnace (10) having a furnace vessel (17), three electrodes (14, 15, 16) which are arranged in the furnace vessel (17), a transformer (11) which is connected to the three electrodes (14, 15, 16), and a current connection for connecting to a current network (40) for providing electrical power to the electrodes (14, 15, 16), wherein the electric arc furnace (10) further comprises a converter (12) and a control system (13), and the method has the step of: controlling at least one parameter of the electric arcs (20) generated by at least one of the three electrodes (14, 15, 16) by means of the converter (12), characterised in that the three electrodes are circularly arranged and, as parameters, the voltages and / or current intensities of the electric arcs generated by the three electrodes are increased and lowered alternately for each of the electrodes along their circular arrangement by means of the converter in order to stir a melt in the furnace vessel.

11. The method according to claim 10, characterised in that the voltage and / or the current intensity of the electric arcs (20) generated by the at least one of the three electrodes (14, 15, 16) is lowered or increased by means of the converter (12) as the at least one parameter relative to the other of the three electrodes (14, 15, 16).

12. The method according to claim 11, characterised in that the voltage at and / or the current intensity of the electric arcs (20) generated by the at least one of the three electrodes (14, 15, 16) is lowered by means of the converter (12) as the at least one parameter when an increased current intensity is registered at the at least one of the three electrodes (14, 15, 16) by means of the converter (12).

13. The method according to claim 12, characterised in that the at least one of the three electrodes (14, 15, 16) at which the increased current intensity was registered is lifted.