Operating method for an arc furnace

Abstract

A furnace vessel (1) of an arc furnace is loaded with metal (2) in a solid state of matter. Afterward, an energy supply device (3) of the arc furnace draws electrical energy from a supply network (4) and feeds the drawn electrical energy to electrodes (6) of the arc furnace via a furnace transformer (5), with the result that arcs (12) form between the electrodes (6) and the metal (2) or the metal melt (13), the arcs causing the metal (2) to be melted to form the metal melt (13). Finally, the metal melt (13) is removed from the furnace vessel (1). The number of electrodes (6) is at least three. For at least two of the electrodes (6), the energy supply device (3) individually sets the operating frequency (fa, fb) of the relevant electrode (6a, 6b). The current (Ic) through the third electrode (6c) is defined by the current (Ia, Ib) through the first and the second electrodes (6a, 6b). With respect to electrodes (6) of a totality of electrodes (6) of the arc furnace, the fact of which of them is the first electrode {6a), which of them is the second electrode (6b) and which of them is the third electrode (6c) is assigned to the electrodes dynamically.

Description

[0001] Description

[0002] Title of the invention

[0003] Operating procedure for an arc furnace

[0004] field of technology

[0005] The present invention is based on an operating method for an arc furnace, wherein an energy supply device of the arc furnace draws electrical energy from a supply network between charging a furnace vessel of the arc furnace with metal in a solid aggregate state and removing a molten metal from the furnace vessel and supplies the drawn electrical energy via a furnace transformer to at least a first, a second and a third electrode of the arc furnace, so that arcs are formed between the electrodes and the metal or the molten metal, by means of which arcs the metal is melted into the molten metal, wherein the energy supply device individually sets an operating frequency of the first electrode and an operating frequency of the second electrode, so that the current through the third electrode is determined by the current through the first and the second electrode.

[0006] The present invention is further based on a control program for a control device of an arc furnace, wherein the control program comprises machine code which can be processed by the control device, wherein the processing of the machine code by the control device causes the control device to operate the arc furnace according to such an operating method.

[0007] The present invention is further based on a control device of an arc furnace, wherein the control device is programmed with such a control program, so that the processing of the machine code by the control device causes the control device to operate the arc furnace according to such an operating method.

[0008] The present invention further relates to an arc furnace,

[0009] - wherein the arc furnace has a furnace vessel which can be charged with metal and from which a molten metal can be removed,

[0010] - the arc furnace comprising a power supply device and electrodes as well as a furnace transformer,

[0011] - the energy supply device is connected on the input side to a supply network and on the output side via the furnace transformer to the electrodes,

[0012] - wherein the arc furnace has a control device by which at least the energy supply device can be controlled,

[0013] - wherein the control device is designed as explained above.

[0014] State of the art

[0015] The objects mentioned are known, for example, from WO 2021 / 115 573 A1.

[0016] Summary of the invention

[0017] When melting metal - especially steel - in an arc furnace, the electrical energy is supplied to the arc furnace electrodes via a furnace transformer. The furnace transformer is often connected to the power supply via a medium-voltage transformer. The furnace transformer provides several voltage levels. For constant power ranges and other high-current ranges, the respective voltage level can be selected on the furnace transformer. Fine adjustment within a specific voltage level can be achieved, for example, using impedance control. With this approach, only a few voltage levels are possible, and the electrode currents are subject to strong fluctuations. To reduce these fluctuations, the positioning of the electrodes is controlled mechanically, usually via hydraulic adjustment devices.The mechanical adjustment of the electrodes exhibits significantly less dynamic response than the actual behavior of the arcs. Therefore, the fluctuations can only be compensated for inadequately. Furthermore, the fluctuations lead to considerable stress on the components, such as the high-current cables, the current-carrying support arms, the hydraulic cylinders, etc. The fluctuations occur both in the melting phase and in the subsequent flat bath phase.

[0018] When adjusting the electrode voltage via the voltage levels of the furnace transformer, the positioning of the electrodes must be continuously adjusted. The adjustment can be achieved, for example, by controlling for a specific impedance or power. However, since the dynamics of the positioning device are relatively low compared to the changes in the electrical system of the arc, certain fluctuations remain that cannot be compensated for. As a result, the energy input into the molten steel is not optimal.

[0019] Procedures in which the electrode voltages can be adjusted continuously are known from prior art documents, in particular from WO 2015 / 176 899 A1 and EP 3 124 903 A1 and, to a limited extent, also from EP 1 026 921 A1. These embodiments offer considerable advantages over adjusting the electrode voltage via voltage steps of the furnace transformer. The electrode voltages can be varied continuously, rather than just step by step. Furthermore, the furnace transformer can be designed more simply because it does not have to provide multiple voltage steps. Furthermore, the electrode voltages can be adjusted with considerably greater dynamics than the positioning of the electrodes. Finally, these embodiments enable further types of control.

[0020] Despite the flexibility in operating the arc furnace, which is achieved through the ability to continuously adjust the electrode voltages, the operation of the arc furnace is often still not optimal in the current state of the art. For example, suboptimal energy input into the metal or molten metal and arc interruptions can still occur.

[0021] The object of the present invention is to create possibilities by means of which the disadvantages of the prior art can be avoided.

[0022] The problem is solved by an operating method having the features of claim 1. Advantageous embodiments of the operating method are the subject of dependent claims 2 to 11.

[0023] According to the invention, an operating method of the type mentioned at the outset is designed in that electrodes are dynamically assigned to a totality of electrodes of the arc furnace, which of them is the first electrode, which of them is the second electrode and which of them is the third electrode.

[0024] This allows the respective operating frequency to be optimally adjusted for the specific operation of the respective electrode, at least for the first and second electrode.

[0025] It is possible for the operating frequencies of the first and second electrodes to be the same. However, this is only the case if the same values ​​arise by chance due to the individual setting of the operating frequencies. As a rule, however, the operating frequencies for the first and second electrodes are different from one another. It is possible for the operating frequency of the first electrode to be constant over time. As a rule, however, it leads to better operation of the arc furnace if the operating frequency of the first electrode varies over time. Preferably, the operating frequency of the first electrode varies depending on a process state of the arc furnace. The process state can be determined in particular by evaluating electrical or acoustic operating variables of the arc furnace. The same procedure can of course also be taken for the second electrode.

[0026] It is possible for the operating frequency of the second electrode to be set independently of the operating frequency of the first electrode. However, it is often advantageous if the operating frequency of the second electrode is set taking the operating frequency of the first electrode into account. For example, the operating frequency of the second electrode can be set such that it is a certain distance from the operating frequency of the first electrode. Also, although the operating frequency of the second electrode can initially be determined independently of the operating frequency of the first electrode, the operating frequency determined in this way can only be adopted if it is at least the certain distance from the operating frequency of the first electrode. Otherwise, the operating frequency of the second electrode can be set so that it is the certain distance from the operating frequency of the first electrode.

[0027] In the present invention, the respective status as first, second, and third electrode is therefore not statically determined once and for all—considered for the period between charging the furnace vessel with metal and removing the molten metal from the furnace vessel—but rather changes from time to time. One and the same electrode of the arc furnace can therefore be the first electrode at one point in time, the second electrode at another point in time, and neither the first nor the second electrode at yet another point in time.If, in the case of a total of three electrodes in the arc furnace, at a certain time x electrode a is operated at a frequency fa and electrode b at a frequency fb and the current through electrode c is determined by the current through electrodes a and b, then at a later time y, for example, electrode b can be operated at the frequency fa and electrode c at the frequency fb and consequently the current through electrode a can be determined by the current through electrodes b and c. The transitions can be sudden or continuous, in the latter case in particular sinusoidal.

[0028] The electrodes of the arc furnace are generally height-adjustable. In this case, the electrodes can be height-adjustable either jointly or independently of one another. In the case of independent adjustability, the dynamic assignment of the respective "role" to the electrodes can be combined with a procedure in which the positioning of the respective electrode of the arc furnace is determined depending on whether it is the first, second, or third electrode. The term "dependent" does not imply that the "role" of the respective electrode completely determines the positioning of the respective electrode. Rather, it is sufficient that the "role" of the respective electrode is also taken into account.

[0029] To achieve individual height adjustment, it may be sufficient if, in addition to a joint adjustment of the electrodes, only a slight overlap of additional movements of the individual electrodes is possible. In this case, with n electrodes, only n-1 devices are required to effect the additional movements.

[0030] Between the charging of the furnace vessel with metal and the removal of the molten metal from the furnace vessel, the arc furnace is operated first in a melting phase and then in a shallow bath phase. In the melting phase, the metal is melted into the molten metal. The melting phase can in turn be divided into an initial phase and a final phase. The initial phase is often referred to as the drilling phase. The majority of the melting of the metal takes place in the final phase. In the shallow bath phase, the molten metal is further heated.

[0031] In some cases, the charging of the furnace vessel with the metal is very inhomogeneous, so that larger pieces of metal are located in the area below one electrode and smaller pieces of metal are located in the area below another electrode. In this case, it can be advantageous to determine the operating frequency of the first electrode during the melting phase as a function of the size of the pieces of metal to be melted down using the first electrode. The operating frequency of the first electrode can be determined during the melting phase in particular such that the larger the pieces of metal to be melted down using the first electrode, the lower the frequency. Furthermore, the operating frequency of the first electrode during the melting phase or during a final phase of the melting phase is preferably higher than a mains frequency at which the supply network is operated, in particular at least 10 Hz higher.The same procedure can of course also be used for the second electrode.

[0032] Preferably, the electrical energy supplied to the electrodes is determined in such a way that the electrodes each introduce the same amount of energy into the metal or the molten metal on average over time. The time average is not formed over a single period of the respective operating frequency of the respective electrode, but over the entire operation of the electrodes between the charging of the furnace vessel with metal and the removal of the molten metal from the furnace vessel or at least over one operating phase of the arc furnace (initial phase of the molten phase, final phase of the molten phase and shallow bath phase or molten phase and shallow bath phase). This makes it possible in particular to uniform the temperature of the molten metal.

[0033] The object is further achieved by a control program having the features of claim 12. According to the invention, the processing of the machine code by the control device causes the control device to operate the arc furnace according to an operating method according to the invention.

[0034] The object is further achieved by a control device having the features of claim 13. According to the invention, the control device is programmed with a control program according to the invention, so that it operates the arc furnace according to an operating method according to the invention.

[0035] The object is further achieved by an arc furnace having the features of claim 14. According to the invention, the control device is designed as a control device according to the invention.

[0036] Short description of the drawings

[0037] The above-described properties, features, and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood in connection with the following description of the embodiments, which are explained in more detail in conjunction with the drawings. Herein, in schematic representation:

[0038] FIG 1 is a block diagram of an arc furnace, FIG 2 is a furnace vessel during a melting phase, FIG 3 is a flow diagram,

[0039] FIG 4 the furnace vessel during a flat bath phase,

[0040] FIG 5 a flow chart,

[0041] FIG 6 a timing diagram,

[0042] FIG 7 another timing diagram and

[0043] FIG 8 a functional dependency. Description of the embodiments

[0044] According to FIG. 1, an arc furnace has a furnace vessel 1. The furnace vessel 1 can be charged with metal 2 (see FIG. 2). The metal 2 is supplied to the furnace vessel 1 in a solid state during charging. The metal 2 can be, for example, steel, and in the case of steel, in particular scrap.

[0045] The arc furnace further comprises a power supply device 3. The power supply device 3 is connected on the input side to a supply network 4. The supply network 4 is generally a medium-voltage network with a nominal voltage in the double-digit kV range and is operated at a network frequency f0. The network frequency f0 is generally 50 Hz or 60 Hz. The supply network 4 is generally a three-phase network, as shown in FIG. 1.

[0046] The arc furnace further comprises a furnace transformer 5 and electrodes 6. The power supply device 3 is connected on the output side to the electrodes 6 via the furnace transformer 5. In the context of the present invention, at least three electrodes 6 are present. Often, exactly three electrodes 6 are present. Furthermore, the furnace transformer 5 is generally designed as a three-phase transformer. Regardless of the specific design, the electrode voltages U applied to the electrodes 6 are significantly below the nominal voltage of the supply network 4. The electrode voltage U is shown in FIG. 1 for only one of the electrodes 6. The electrode voltages U are usually in the range of several hundred V. In individual cases, voltages above 1 kV are also possible. However, 2 kV is generally not exceeded.

[0047] As a rule, switching devices are also provided by means of which the energy supply device 3 can be separated from the supply network 4. Furthermore, switching devices can be provided by means of which the energy supply device 3 can be separated from the furnace transformer 5 and / or the furnace transformer 5 from the electrodes 6. The switching devices carry out purely binary switching operations, but no adjustment of voltages and currents. Furthermore, active or passive filter devices can be arranged on the primary or secondary side of the furnace transformer 5. The switching devices and also the filter devices are of secondary importance for the functioning according to the invention and are therefore not shown in FIG. 1 (or the other FIGS) for the sake of clarity.

[0048] The energy supply device 3 can draw electrical energy from the supply network 4 and supply the drawn electrical energy to the electrodes 6 via the furnace transformer 5. The energy supply device 3 generally has many semiconductor switches for this purpose. Possible embodiments of the energy supply device 3 are described in WO 2015 / 176 899 A1 ("gold standard"). Alternatively, the embodiments according to EP 3 124 903 A1 or EP 1 026 921 A1 can also be used. Regardless of the specific embodiment of the energy supply device 3, the energy supply device 3 is, however, capable of carrying out a quasi-continuous gradation of the electrode voltages U applied to the electrodes 6 and / or the electrode currents I supplied to the electrodes 6 on the output side - that is to say towards the furnace transformer 5.Analogous to the representation for the electrode voltages U, the electrode current I in FIG 1 is also shown for only one of the electrodes 6. A respective operating frequency f, with which the electrode voltages U applied to the electrodes 6 or the electrode currents I supplied to the electrodes 6 vary, can also be set by means of the energy supply device 3. The respective operating frequency f can be above or below the mains frequency f 0 as required. The arc furnace also has a positioning device 7. By means of the positioning device 7, the electrodes 6 can be positioned, as indicated in FIG 1 by double arrows 8 next to the electrodes 6. In the simplest case, the electrodes 6 are positioned together. Preferably, however, the electrodes 6 are positioned individually. This is indicated in FIG 1 by the fact that the double arrows 8 are of different lengths.The direction of movement in which the electrodes 6 are positioned can be vertical. Alternatively, the direction of movement can also be slightly inclined relative to the vertical. In this case, too, the vertical component is the dominant component of the movement. The positioning device 7 can, for example, comprise one or more hydraulic cylinder units.

[0049] Finally, the arc furnace has a control device 9. At least the energy supply device 3 is controlled by the control device 9. The control device 9 generates control values ​​Al, with which it controls the energy supply device 3. The energy supply device 3 is operated in accordance with the control values ​​Al.

[0050] Often, the control device 9 also controls the positioning device 7. In this case, the control device 9 generates additional control values ​​A2 with which it controls the positioning device 7. In this case, the positioning device 7 is operated according to these control values ​​A2.

[0051] The control device 9 is designed as a software-programmable control device. This is indicated in FIG. 1 by the designation "pP" (for microprocessor-controlled). The effect and mode of operation of the control device 9 is determined by a control program 10 with which the control device 9 is programmed. The control program 10 comprises machine code 11 which can be processed by the control device 9. The processing of the machine code 11 by the control device 9 causes the control device 9 to operate the arc furnace according to an operating method as explained in more detail below in connection with the other FIGS.

[0052] According to FIG. 3, the furnace vessel 1 is first charged with the metal 2 in a step S 1. FIG. 2 shows the state after charging the furnace vessel 1 immediately after the ignition of arcs 12 (see FIGS. 2 and 4) by appropriate control of the energy supply device 3 by the control device 9.

[0053] The actual operation of the arc furnace then takes place in a step S2. During step S2, the electrode voltages U are applied and the electrode currents I are supplied to the electrodes 6. As a result, the arcs 12 initially form between the electrodes 6 and the metal 2, by means of which the metal 2 is melted into a molten metal 13. This operating phase of the arc furnace is generally referred to as the melting phase. It can be divided into an initial phase and a final phase, with the initial phase usually being referred to as the drilling phase and the final phase as the main melting phase. The arcs 12 then form between the electrodes 6 and the molten metal 13, so that the latter is heated further. This operating phase of the arc furnace is generally referred to as the shallow bath phase. This state is shown in FIG. 4.In the shallow bath phase, the molten metal 13 may be covered on its upper side by a slag layer 14. The slag layer 14 may be a foamed slag.

[0054] Finally, in a step S3, the metal melt 13 produced is removed from the furnace vessel 1, for example poured into a ladle (not shown).

[0055] Step S2, i.e. the actual operation of the arc furnace, is carried out by appropriately controlling the energy supply device 3. Steps S1 and S3 can also be carried out under the control of the control device 9. However, they do not have to be carried out under the control of the control device 9. Steps S1 and S3 are therefore only shown in dashed lines in FIG. 3.

[0056] Below, in conjunction with FIG 5, step S2, i.e. the actual operation of the arc furnace between charging the furnace vessel 1 with the metal 2 and removing the molten metal 13 from the furnace vessel 1, is explained in more detail. For this purpose, step S2 is divided into steps S11 to S20. Furthermore, in the further explanation, a distinction is made between the individual electrodes 6. For this purpose, the electrodes 6 are each assigned a lowercase letter according to their sequence if a distinction is to be made between the individual electrodes 6. In this case, the electrodes 6 are referred to as the first electrode 6a, the second electrode 6b and the third electrode 6c. If, on the other hand, the electrodes 6 are only referred to in general terms, only the general reference symbol 6 is used.The same applies to the electrode-specific quantities such as the electrode voltages U, the electrode currents I and the operating frequencies f .

[0057] According to FIG 5, the control device 9 determines a state Z of the arc furnace in step S 11. The state Z indicates the extent to which the operation of the arc furnace has progressed between the charging with the metal 2 and the removal of the molten metal 13. In the simplest case, the state Z of the control device 9 is specified by an operator 15 (see FIG 1). Alternatively, it is possible for the control device 9 to determine the state Z directly on the basis of the time t that has elapsed since the first ignition of the arcs 12 after the charging of the furnace vessel 1. Preferably, however, the control device 9 evaluates actual variables of the arc furnace that have been measured in step S 12. For example, it is possible for the control device 9 to evaluate the electrode currents I and / or the electrode voltages U, in particular their fluctuations.The control device 9 can also evaluate acoustic variables of the arc furnace, for example the noise level or the acoustic spectrum of the noise generated.

[0058] In step S 12 the control device 9 determines which of the electrodes 6 is the first electrode 6a, the second electrode 6b and the third electrode 6c. The determination can be made in direct dependence on the time t that has elapsed since the first ignition of the arcs 12 after the charging of the furnace vessel 1. Alternatively, it can be made in direct dependence on the time that has elapsed since the start of the respective state Z. In the last two cases the electrodes 6 are dynamically assigned which of them is the first electrode 6a, which of them is the second electrode 6b and which of them is the third electrode 6c. The electrodes 6 can therefore swap roles from time to time.

[0059] In step S 13, the control device 9 determines a frequency range F in which the electrode currents I and / or the electrode voltages U should lie. The frequency range F can be dependent in particular on the state Z. For example, according to the illustration in FIG 6, it is possible for the frequency range F during the final phase of the melting phase (state Z = 2) to exclusively comprise frequencies f that are greater than the mains frequency f 0 . It is also possible for the frequency range F during the shallow bath phase (state Z = 3) to exclusively comprise frequencies f that are lower than the mains frequency f 0 . Finally, it is possible for the frequency range F during the initial phase of the melting phase (state Z = 1) to alternatively exclusively comprise frequencies f that are greater than the mains frequency f 0 , or exclusively comprise frequencies f that are lower than the mains frequency f 0 .

[0060] Preferably, the frequency range F always has a certain minimum distance from the mains frequency. The minimum distance can, for example, be between 7 Hz and 13 Hz, in particular around 10 Hz. If the frequency range F is above the mains frequency fO, a lower limit frequency of the frequency range F at a mains frequency fO of 50 Hz is therefore preferably "somewhere" between 57 Hz and 63 Hz, for example 60 Hz. Analogously, if the frequency range F is below the mains frequency fO, an upper limit frequency of the frequency range F at a mains frequency fO of 50 Hz is therefore preferably "somewhere" between 37 Hz and 43 Hz, for example 40 Hz. For a mains frequency fO of 60 Hz, 10 Hz must be added to each of the limit frequencies mentioned.

[0061] Preferably, according to the illustration in FIG 7, the power P* to be introduced into the metal 2 or the molten metal 13 is also set as a function of time t, specifically as a function of state Z. FIG 7 shows the preferred embodiment, according to which the power P* to be introduced has a relatively low value during the initial phase of the melting phase (state Z = 1), a relatively high value during the final phase of the melting phase (state Z = 2) and a value between the value of the initial phase and the final phase of the melting phase during the shallow bath phase (Z = 3).

[0062] In step S14, the control device 9 determines an operating frequency fa for the first electrode 6a. The determination is made such that the operating frequency fa lies within the currently valid frequency range F. The operating frequency fa can be static. Alternatively, it can be static within the respective state Z, but depend on the state Z. Alternatively again, it can - see FIG. 6 - also depend on time within the respective state Z, for example on the process progress. FIG. 6 shows possible time profiles of the operating frequency fa for the state Z = 1, both for the frequency range F above and for the frequency range F below the mains frequency f0. In practice, of course, only one of the two operating frequencies fa is valid. In step S15, the control device 9 determines an operating frequency fb for the second electrode 6b.The determination is individual in that the operating frequency fb does not necessarily have to coincide with the operating frequency fa. The operating frequency fb can therefore be different from the operating frequency fa.

[0063] It is possible for the determination of step S15 to be carried out independently of the determination of step S14. However, a certain dependency may also exist. For example, it may be required that the operating frequency fb of the second electrode 6b maintains a minimum distance from the operating frequency fa of the first electrode 6a. Alternatively, it may be required that the operating frequency fb of the second electrode 6b lies within the currently valid frequency range F. Otherwise, the explanations for determining the operating frequency fa of the first electrode 6a can be applied analogously.

[0064] For the third electrode 6c, it is no longer possible to determine an operating frequency. This is because the operating mode of the third electrode 6c is determined by the fact that the currents I through the electrodes 6a, 6b, 6c must add up to zero at all times (taking into account the sign of the currents I).

[0065] In step S 16, the control device 9 determines the control values ​​Al for the energy supply device 3. When determining the control values ​​Al, the control device 9 takes into account the operating frequencies fa, fb of the first and second electrodes 6a, 6b.

[0066] If the control device 9 also controls the positioning device 7, the control device 9 determines positionings pa, pb, pc for the first, the second and the third electrode 6a, 6b, 6c in step S17. In this case, the control device 9 determines the associated further control values ​​A2 in step S18. It is possible for a uniform determination to be made for all electrodes 6. However, it is also possible for the positionings pa, pb, pc to be made individually depending on whether the respective electrode 6 is the first, the second or the third electrode 6a, 6b, 6c.

[0067] In step S19, the control device 9 controls the energy supply device 3 according to the determined control values ​​A1. If the control device 9 also controls the positioning device 7, the control device 9 also controls the positioning device 7 with the additional control values ​​A2 in step S19.

[0068] In step S20, the control device 9 checks whether the respective cycle of operation of the arc furnace has been completed, ie, whether the metal 2 has completely melted into the molten metal 13 and whether the molten metal 13 has been further heated, if necessary. If this is the case, the control device 9 proceeds to step S3. Otherwise, the control device 9 returns to step S11.

[0069] FIG. 8 shows one possibility for determining the operating frequency fa of the first electrode 6a during the melting phase. The dependency according to FIG. 8 can alternatively exist during the entire melting phase or only during the initial phase or the final phase of the melting phase. According to FIG. 8, the control device 9 determines the operating frequency fa of the first electrode 6a as a function of the size G of the pieces of metal 2 to be melted by means of the first electrode 6. In particular, the operating frequency fa of the first electrode 6a can decrease monotonically or strictly monotonically with increasing size G of the pieces of metal 2. The operating frequency fa of the first electrode 6a is therefore preferably determined during the melting phase in such a way that the larger the pieces of metal 2 to be melted by means of the first electrode 6a, the lower the operating frequency fb. An analogous procedure can of course also be adopted for the second electrode 6b and its operating frequency fb.In the case of the procedure according to FIG. 8, different or identical operating frequencies fa, fb can result depending on the size G of the pieces of metal 2. For example, a statistical mean of the sizes of the individual pieces of metal 2 can be used as the size G. Alternatively, an extreme value (minimum or maximum) can be used.

[0070] The electrical energy supplied to the electrodes 6 - that is to say the integral of the electrical power supplied to the electrodes 6 - is preferably determined by the control device 9 in such a way that the electrodes 6 each introduce the same amount of energy into the metal 2 or the molten metal 13 on average over time. The time average is not formed over a single period of the respective operating frequency fa, fb, but over a large number of periods. Averaging over a respective operating phase of the arc furnace is particularly preferred, that is to say either over the initial phase of the melting phase, the final phase of the melting phase and the shallow bath phase or over the melting phase and the shallow bath phase.

[0071] The present invention has many advantages. In particular, the operation of the arc furnace can be flexibly adapted to the needs of the individual case. Furthermore, the specific energy required to produce a certain amount of molten metal 13 (e.g., kilowatt hours per ton) can often be reduced, and the cycle time can often also be reduced.

[0072] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples, and other variants may be derived therefrom by those skilled in the art without departing from the scope of the invention. List of reference symbols

[0073] 1 oven vessel

[0074] 2 Metal

[0075] 3 Energy supply facility

[0076] 4 Supply network

[0077] 5 Furnace transformer

[0078] 6 electrodes

[0079] 7 Positioning device

[0080] 8 double arrow

[0081] 9 Control device

[0082] 10 Control program

[0083] 11 Machine code

[0084] 12 arcs

[0085] 13 metal melt

[0086] 14 slag layer

[0087] 15 operators

[0088] A1 , A2 Control values ​​f , fa, fb Operating frequencies f 0 Mains frequency

[0089] F Frequency range

[0090] G size

[0091] I , la, Ib, Ic Electrode currents pa, pb, pc Positioning p* Power to be applied

[0092] S 1 to S20 steps t time

[0093] U Electrode voltages

[0094] Z condition

Claims

Claims 1. Operating method for an electric arc furnace, wherein an energy supply device (3) of the electric arc furnace draws electrical energy from a supply network (4) between the charging of a furnace vessel (1) of the electric arc furnace with metal (2) in a solid state and the removal of a molten metal (13) from the furnace vessel (1) and supplies the drawn electrical energy via a furnace transformer (5) to at least a first, a second and a third electrode (6a, 6b, 6c) of the electric arc furnace, so that an electric current is generated between the electrodes (6) and the metal (2) or the metal (2).the metal melting (13) form electric arcs (12) through which the metal (2) is melted to the metal melt (13), wherein the power supply device (3) individually sets an operating frequency (fa) of the first electrode (6a) and an operating frequency (fb) of the second electrode (6b), so that the current (Ic) through the third electrode (6c) is determined by the current (aa, Ib) through the first and the second electrode (6a, 6b), characterized in that electrodes (6) of a set of electrodes (6) of the electric arc furnace are dynamically assigned, which of them is the first electrode (6a), which of them is the second electrode (6b) and which of them is the third electrode (6c).

2. Operating method according to claim 1, characterized in that the operating frequency (fa, fb) for the first and the second electrode (6a, 6b) are different from each other.

3. Operating method according to claim 1 or 2, characterized in that the operating frequency (fa) of the first electrode (6a) varies over time.

4. Operating method according to claim 3, characterized in that the operating frequency (fa) of the first electrode (6a) varies depending on a process state of the electric arc furnace.

5. Operating method according to claim 4, characterized in that the process state is determined by evaluating electrical or acoustic operating parameters of the electric arc furnace.

6. Operating method according to one of the above claims characterized in that the operating frequency (fb) of the second electrode (6b) is set taking into account the operating frequency (fa) of the first electrode (6a).

7. Operating method according to one of the above claims characterized in that the electrodes (6) of the assembly of electrodes (6) of the electric arc furnace are height-adjustable independently of one another and that a positioning (pa, pb, pc) of the respective electrode (6) of the electric arc furnace is determined depending on whether it is the first, the second or the third electrode (6a, 6b, 6c).

8. Operating method according to one of the above claims characterized in that the electric arc furnace is operated first in a melting phase and then in a flat bath phase between charging the furnace vessel (1) with the metal (2) and removing the molten metal (13) from the furnace vessel (1), that the metal (2) is melted to form the molten metal (13) in the melting phase and the molten metal (13) is further heated in the flat bath phase, and that the operating frequency (fa) of the first electrode (6a) during the melting phase depends on the size (G) of the the pieces of metal (2) to be melted down at the first electrode (6a) are determined.

9. Operating method according to claim 8, characterized in that the operating frequency (fa) of the first electrode (6a) is determined during the melting phase such that it is smaller the larger the pieces of metal (2) to be melted by means of the first electrode (6a) are.

10. Operating method according to claim 8 or 9, characterized in that the supply network (4) is operated with a network frequency (fO) and that the operating frequency (fa) of the first electrode (6a) is greater than the network frequency (fO) during the melting phase or during a final phase of the melting phase.

11. Operating method according to one of the above claims characterized in that the electrical energies supplied to the electrodes (6) are determined in such a way that the electrodes (6) introduce the same amount of energy into the metal (2) or the molten metal (13) on average over time.

12. Control program for a control device (9) of an electric arc furnace, wherein the control program comprises machine code (11) that can be executed by the control device (9), wherein the execution of the machine code (11) by the control device (9) causes the control device (9) to operate the electric arc furnace according to an operating method according to one of the above claims.

13. Control device of an electric arc furnace, wherein the control device is programmed with a control program (10) according to claim 12, such that the execution of the machine code (11) through the control device (9) causes the control- The device (9) operates the electric arc furnace according to an operating method according to one of claims 1 to 11.

14. Electric arc furnace, - wherein the electric arc furnace has a furnace vessel (1) which can be charged with metal (2) and from which a molten metal can be drawn (13) can be removed, - wherein the electric arc furnace comprises a power supply unit (3) and electrodes (6) as well as a furnace transformer (5), - wherein the power supply device (3) is connected on the input side to a supply network (4) and on the output side to the electrodes (6) via the furnace transformer (5), - wherein the electric arc furnace has a control device (9) of which at least the power supply device (3) is controllable - wherein the control device (9) is designed according to claim 13.

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