OPERATING PROCEDURE FOR AN ARC FURNACE

Description

field of technology

[0001] The present invention is based on an operating method for an arc furnace, wherein a control device of the arc furnace initially controls a power supply device of the arc furnace with first control values ​​in a melting phase and then in a flat bath phase following the melting phase, so that the power supply device draws electrical energy from a supply network and supplies electrodes of the arc furnace via a furnace transformer, and further controls a positioning device of the arc furnace with second control values, so that the positioning device positions the electrodes in the melting phase relative to steel-containing material in a solid state located in a furnace vessel of the arc furnace, so that arcs form between the electrodes and the steel-containing material in the melting phase, by means of which arcs the steel-containing material is melted into a steel melt, and positions them relative to the steel melt in the flat bath phase,so that in the shallow bath phase, arcs form between the electrodes and the molten steel, by means of which the molten steel is further heated, wherein the control device determines both the first control values ​​and the second control values ​​during the melting phase in such a way that electrical parameters of the electrical energy supplied to the electrodes are approximated as closely as possible to the desired values ​​corresponding to the desired values, wherein the control device determines the first control values ​​during the shallow bath phase in such a way that the electrical parameters are approximated as closely as possible to the corresponding desired values.

[0002] The present invention further relates to a control program for a control device of an arc furnace, wherein the control program comprises machine code that can be processed by the control device, wherein the processing of the machine code by the control device causes the control device to operate an arc furnace according to such an operating method.

[0003] The present invention further relates to a control device of an arc furnace, wherein the control device is programmed with such a control program, so that the control device operates the arc furnace according to such an operating method.

[0004] The present invention further relates to an arc furnace, wherein the arc furnace comprises a furnace vessel to which steel-containing material in a solid state can be fed, wherein the arc furnace comprises a power supply device and electrodes as well as a furnace transformer, wherein the power supply device is connected on the input side to a supply network and on the output side to the electrodes via the furnace transformer, wherein the arc furnace comprises a positioning device by means of which the electrodes can be positioned in a melting phase relative to the steel-containing material and in a flat bath phase following the melting phase relative to a steel melt produced by melting the steel-containing material, wherein the arc furnace comprises a control device,of which the energy supply device can be controlled with first control values ​​and the positioning device can be controlled with second control values ​​both in the melting phase and in the flat bath phase, the control device being designed as explained above. State of the art

[0005] The aforementioned subject matter is generally known. For example, reference can be made to WO 2015 / 176 899 A1. EP 1 026 921 A1 and EP 3 124 903 A1 can also be mentioned in this context.

[0006] An operating method for an arc furnace is also known from WO 2019 / 207 611 A1. In this operating method, the power supply device for the arc furnace electrodes is designed as an intermediate circuit converter. The intermediate circuit converter appears to be arranged downstream of the furnace transformer. WO 2019 / 207 611 A1 does not elaborate on the position control of the electrodes.

[0007] EP 3 124 903 A1 discloses an operating method for an arc furnace in which an energy supply device for the electrodes and a positioning device for the electrodes are controlled jointly as a function of electrical operating variables of the arc furnace.

[0008] US 5 115 447 A discloses an operating method for an arc furnace during the so-called drilling phase, in which the electrodes are individually checked for short circuits and arc interruption and, if such a condition occurs, the electrode position is adjusted. Summary of the invention

[0009] When melting 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 grid via a medium-voltage transformer. The furnace transformer provides several voltage levels. For constant power and other high-current applications, the respective voltage level can be selected on the furnace transformer. Fine control within a specific voltage level can be achieved, for example, using impedance control.

[0010] With this approach, only a few voltage levels are possible, and the electrode currents are subject to strong fluctuations. To reduce these fluctuations, the electrode positions are mechanically controlled, usually via hydraulic adjustment devices. The mechanical adjustment of the electrodes exhibits significantly less dynamic response than the actual behavior of the arcs. The fluctuations can therefore only be inadequately compensated for. Furthermore, the fluctuations lead to considerable stress on the components, for example, the high-current cables, the current-carrying support arms, the hydraulic cylinders, etc. The fluctuations occur both in the melting phase and in the flat bath phase.

[0011] During the shallow bath phase, relatively low voltages are generally applied to the electrodes, and the electrodes are positioned relatively close to the surface of the molten steel. This results in high currents. At the same time, heat losses are reliably shielded by the foamed slag. However, depending on the power level of the arc furnace or in the production of certain steels (particularly stainless steels), the arcs are only partially or not at all enveloped by the foamed slag. This reduces the energy efficiency of the arc furnace.

[0012] When adjusting the electrode voltage via the voltage levels of the furnace transformer, the positioning of the electrodes must be continuously adjusted. This 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. These fluctuations are further magnified by wave motions and currents in the molten steel. As a result, the energy input into the molten steel is not optimal.

[0013] Prior art documents, in particular WO 2015 / 176 899 A1 and EP 3 124 903 A1, and to a limited extent also EP 1 026 921 A1, disclose procedures in which the electrode voltages can be continuously adjusted. These embodiments offer significant advantages over adjusting the electrode voltage via voltage steps of the furnace transformer. Firstly, the electrode voltages can be varied continuously rather than just in steps. Secondly, the furnace transformer can be designed more simply because it does not have to provide multiple voltage steps. Furthermore, these embodiments enable additional types of control.

[0014] The object of the present invention is to create possibilities by means of which a fast and high-quality control of the arcs is possible in a simple and reliable manner during the flat bath phase.

[0015] The object is achieved by an operating method having the features of claim 1. Advantageous embodiments of the operating method are the subject of dependent claims 2 to 8.

[0016] According to the invention, an operating method of the type mentioned at the outset is designed in that the control device determines the second control values ​​during the flat bath phase either completely independently of the electrical parameters or only as a function of the electrical parameters if the control device detects the risk of an arc break and / or a short circuit based on the electrical parameters.

[0017] The first control values ​​are determined by the control device during the flat bath phase – as in the prior art – in such a way that the electrical parameters are as close as possible to the corresponding target values. The second control values, however, are determined independently of the electrical parameters – except in the case of the risk of special operating conditions, which must be avoided at all costs. As a result, the electrode voltages and electrode currents are adjusted exclusively by adjusting the control of the power supply device.

[0018] The electrical parameters of the electrical energy supplied to the electrodes can be determined as required. For example, the electrical parameters can be the electrode currents. Electrode currents are particularly suitable as active currents. In individual cases, however, the electrical parameters can also be the reactive currents and / or the apparent currents. Alternatively, the electrical parameters can be the electrical power. Active power is particularly suitable as power. In individual cases, however, the electrical parameters can also be the reactive power and / or the apparent power.

[0019] The voltages applied to the electrodes, and thus also the currents supplied to the electrodes, are generally alternating quantities, i.e., alternating voltages and alternating currents. Alternating quantities can be characterized by their amplitude, frequency, and their waveform during a period (e.g., sinusoidal, triangular, sawtooth, rectangular, etc.). The temporal waveform is preferably sinusoidal.

[0020] The amplitude must always be adjusted appropriately. The frequency can be kept constant in some cases. In other cases, however, it is preferable for the control system to determine the initial control values ​​during the flat bath phase in such a way that, in order to bring the electrical parameters closer to the corresponding target values, the frequency of the electrode currents supplied to the electrodes and / or the electrode voltages applied to the electrodes is also varied. This approach offers greater flexibility in optimizing arc furnace operation.

[0021] Preferably, the frequency of the electrode currents supplied to the electrodes and / or the electrode voltages applied to the electrodes during the shallow bath phase is lower than a base frequency of the supply network. This approach has proven particularly advantageous in experiments.

[0022] At the beginning of the shallow bath phase, the electrodes are spaced away from the surface of the molten steel. Consequently, the arcs have a base length at the beginning of the shallow bath phase. In some situations, it is advantageous for the control device to move the electrodes towards the molten steel during the shallow bath phase, so that after moving towards the molten steel, the arcs only have a residual length that is shorter than the base length. To avoid the risk of a short circuit, however, a certain minimum length should not be undercut. For this reason, the residual length is preferably at least 20% of the base length.

[0023] The base length can be determined or at least estimated based on the electrical parameters present at the beginning of the shallow bath phase. This determination / estimation can be performed intellectually by a person, but it is preferably performed by the control device.

[0024] The object is further achieved by a control program having the features of claim 9. According to the invention, the processing of the machine code by the control device causes the control device to operate an arc furnace according to an operating method according to the invention.

[0025] The object is further achieved by a control device having the features of claim 10. According to the invention, the control device is programmed with a control program according to the invention, so that the control device operates the arc furnace according to an operating method according to the invention.

[0026] The object is further achieved by an arc furnace having the features of claim 11. According to the invention, the control device is designed as a control device according to the invention. Short description of the drawings

[0027] 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: FIG 1 a block diagram of an arc furnace, FIG 2 a furnace vessel during a melting phase, FIG 3 a flow chart, FIG 4 the operation of a control device in the melting phase, FIG 5 the furnace vessel during a shallow bath phase, FIG 6 the operation of the control device in the shallow bath phase, FIG 7 a modification of FIG 6 , FIG 8 a flowchart, FIG 9 a determination block, FIG 10 a determination block and a supplementary block, FIG 11 a time diagram, FIG 12 a modification of FIG 5 , FIG 13 a time diagram and FIG 14 a flow chart. Description of the embodiments

[0028] According to FIG 1 An arc furnace has a furnace vessel 1. The furnace vessel 1 can - see FIG 2 - steel-containing material 2 is fed into the furnace vessel 1. The steel-containing material 2 is fed into the furnace vessel 1 in a solid state. The steel-containing material 2 can be scrap, for example.

[0029] 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 2-digit kV range and a base frequency f0 (see FIG 11 ). The base frequency f0 is usually 50 Hz or 60 Hz. The supply network 4 is as shown in FIG 1 usually a three-phase network.

[0030] The arc furnace further comprises a furnace transformer 5 and electrodes 6. The power supply device 3 is connected on the output side via the furnace transformer 5 to the electrodes 6. As a rule, as shown in FIG 1 several electrodes 6 are present and the furnace transformer 5 is designed as a three-phase transformer. However, other designs are also possible, in particular a single-phase design. 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 in FIG 1 shown 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.

[0031] As a rule, switching devices are also provided by means of which the power supply device 3 can be separated from the supply network 4. Furthermore, switching devices can be provided by means of which the power supply device 3 can be separated from the furnace transformer 5 and / or the furnace transformer 5 from the electrodes 6. The switching devices perform purely binary switching operations, but do not adjust voltages or 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 FIG 1 (and also the other FIGS) are not shown for the sake of clarity.

[0032] 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. For this purpose, the energy supply device 3 generally has many semiconductor switches. 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 performing 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 - i.e., towards the furnace transformer 5. Analogous to the representation for the electrode voltages U, the electrode current I in FIG 1 also shown only for one of the electrodes 6.

[0033] Furthermore, the arc furnace has a positioning device 7. By means of the positioning device 7, the electrodes 6 can be positioned as shown in FIG 1 indicated by a double arrow 8 next to one of the electrodes 6. In the simplest case, the electrodes 6 are positioned together. However, the electrodes 6 can also be positioned individually. 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, however, the component in the vertical direction is the dominant component of the movement. The positioning device 7 can, for example, have one or more hydraulic cylinder units.

[0034] Finally, the arc furnace has a control device 9. The control device 9 controls (at least) the energy supply device 3 and the positioning device 7. The control device 9 thus generates first control values ​​A1, with which it controls the energy supply device 3, and second control values ​​A2, with which it controls the positioning device 7. The energy supply device 3 and the positioning device 7 are operated according to the respective control values ​​A1, A2.

[0035] The control device 9 is designed as a software-programmable control device. This is FIG 1 indicated by the designation "µP" (for microprocessor-controlled). The effect and mode of operation of the control device 9 is thus 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 conjunction with the other FIGS.

[0036] First, the furnace vessel 1 is prepared according to FIG 3 in a step S1 with the steel-containing material 2. This process can, but does not have to, be carried out under the control of the control device 9. The step S1 is therefore in FIG 3 only shown in dashed lines.

[0037] After charging with the steel-containing material 2, the arc furnace enters a melting phase. The melting phase comprises steps S2 to S4. The melting phase is followed by a shallow bath phase. The shallow bath phase comprises steps S5 to S7.

[0038] In the melting phase, the control device 9 determines in step S2 the first control values ​​A1 for the energy supply device 3 and the second control values ​​A2 for the positioning device 7. The determination is carried out according to FIG 4 in corresponding determination blocks 12 and 13. In step S3, the control device 9 controls the energy supply device 3 and the positioning device 7 according to the determined control values ​​A1, A2.

[0039] The determination of the first control values ​​A1 is carried out in such a way that the energy supply device 3 draws electrical energy from the supply network 4 due to the corresponding control and supplies it to the electrodes 6 via the furnace transformer 5. The determination of the second control values ​​A2 is carried out in such a way that the positioning device 7 positions the electrodes 6 relative to the steel-containing material 2. The determination of the first control values ​​A1 and the second control values ​​A2 by the control device 9 is coordinated with one another in such a way that arcs 14 (see FIG 2 ) form. The steel-containing material 2 is melted by the arcs 14 and thus gradually a steel melt 15 ( FIG 5 ) is generated.

[0040] To determine the first control values ​​A1 and the second control values ​​A2, the control device 9 is supplied with FIG 4 Parameters U, I, P of the electrical energy supplied to the electrodes 6 are supplied. The parameters U, I, P can be, for example, the electrode voltages U and / or the electrode currents I and / or values ​​derived therefrom. A derived value is, for example, the instantaneous power P (= the product of electrode voltage U and electrode current I).

[0041] A further derived value can result from the temporal progression of electrode voltages U and electrode currents I. Such values ​​are, for example, the active current, the active power, the apparent power, the reactive current and the reactive power. The characteristic variables can alternatively be given or derived for all of the electrodes 6 or individually for the respective electrode 6. To determine the first control values ​​A1 and the second control values ​​A2, the control device 9 is further supplied with target values ​​U*, I*, P* for the characteristic variables U, I, P, for example target values ​​U*, I* for the electrode voltages U and / or the electrode currents I or other suitable target values ​​(for example a target value P* for the power P). Both the characteristic variables U, I, P and the target variables U*, I*, P* are supplied to both determination blocks 12, 13 during the melting phase.

[0042] Based on the characteristic variables U, I, P and the associated target variables U*, I*, P*, the control device 9 determines the first control values ​​A1 and the second control values ​​A2. The determination is performed in both determination blocks 12, 13 such that the electrical characteristic variables U, I, P are as close as possible to the corresponding target variables U*, I*, P*. This procedure and thus the implementation of step S2 are generally known to those skilled in the art. Therefore, it does not need to be explained in more detail.

[0043] In step S4, the control device 9 checks whether the melting phase is finished. The melting phase is finished when the steel melt 15 is as shown in FIG 5 has completely or at least essentially formed a continuous horizontal surface. Thus, either the steel-containing material 2 has completely melted, or the not yet melted elements of the steel-containing material 2 are located completely below the surface of the molten steel 15, or the not yet melted elements of the steel-containing material 2 only protrude insignificantly above the surface of the molten steel 15. Furthermore, a slag layer 16 may have formed on the surface of the molten steel 15.

[0044] It is possible for the control device 9 to evaluate actual measured variables of the arc furnace as part of the check to determine whether the melting phase has ended. 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 generated noise. Alternatively, it is possible for an operator (not shown) to indicate to the control device 9 that the melting phase has ended.

[0045] If the melting phase is not yet complete, the control device 9 returns to step S2. However, if the melting phase is complete, the control device 9 proceeds to the flat bath phase and thus to step S5.

[0046] In the flat bath phase, the control device 9 determines the first control values ​​A1 for the energy supply device 3 and the second control values ​​A2 for the positioning device 7 in step S5. In step S6, the control device 9 controls the energy supply device 3 and the positioning device 7 according to the determined control values ​​A1, A2.

[0047] The first control values ​​A1 are determined by the energy supply device 3 drawing electrical energy from the supply network 4 based on the corresponding control and supplying it to the electrodes 6 via the furnace transformer 5. The second control values ​​A2 are determined by the positioning device 7 positioning the electrodes 6 relative to the molten steel 15. In this respect, the procedure in steps S5 and S6 corresponds to the procedure in steps S2 and S3.

[0048] The procedure of steps S5 and S6 also corresponds to the procedure of steps S2 and S3 in that the first control values ​​A1 and the second control values ​​A2 are coordinated with one another in such a way that arcs 14 are formed. However, the arcs 14 form in the shallow bath phase as shown in FIG 5 between the electrodes 6 and the molten steel 15. The arcs 14 further heat the molten steel 15.

[0049] The control device 6 is provided with FIG 6 the characteristic variables U, I, P of the electrical energy supplied to the electrodes 6 and the associated target variables U*, I*, P* are also still supplied. However, the characteristic variables U, I, P and the associated target variables U*, I*, P* are only supplied to the determination block 12 within the control device 9. Thus, the control device 9 continues to determine the first control values ​​A1 in such a way that the electrical characteristic variables U, I, P are brought as close as possible to the corresponding target variables U*, I*, P*.

[0050] However, the detection block 13 is deactivated in the shallow bath phase. Instead, according to FIG 6 a determination block 17 is activated. By means of the determination block 17, the control device 9 determines the second control values ​​A2 in the flat bath phase. In particular, it is possible for the control device 9 to determine the second control values ​​A2 according to the illustration in FIG 3 determined completely independently of the electrical parameters U, I, P. In this case, it is possible that the determination block 17 is assigned the value as shown in FIG 6 the electrical parameters U, I, P are not supplied at all. Instead, the control device 9 can determine the second control values ​​A2 based on another internal determination or based on external specifications V (for example, specifications originating from an operator).

[0051] In step S7, the control device 9 checks whether the flat bath phase has ended. It is possible for the control device 9 to evaluate actual measured values ​​of the arc furnace as part of the check to determine whether the flat bath phase has ended. Alternatively, the operator can indicate to the control device 9 that the flat bath phase has ended.

[0052] If the shallow bath phase is not yet completed, the control device 9 returns to step S5. If, however, the shallow bath phase is completed, the control device 9 proceeds to step S8. In step S8, the produced steel melt 15 is removed from the furnace vessel 1, for example, poured into a ladle (not shown). This process can, but does not have to, be controlled by the control device 9. Step S8 is therefore FIG 3 - analogous to step S1 - shown only in dashed lines.

[0053] With the execution of step S8, a complete cycle in the operation of the arc furnace is completed. A new cycle can therefore be started, beginning with step S1.

[0054] In the simplest embodiment, the determination of the second control values, as already mentioned, is carried out independently of the electrical parameters U, I, P. Alternatively, it is possible that the second control values ​​A2 are normally determined by the determination block 17 independently of the electrical parameters U, I, P, but are nevertheless taken into account under special circumstances. In this case, the determination block 17 is assigned, as shown in FIG 7 the corresponding electrical parameters U, I, P are supplied. However, it is not necessary to also supply the corresponding target values ​​U*, I*, P*.

[0055] In this case, the determination block 17 (and, because the determination block 17 is a component of the control device 9, thus the control device 9) checks whether the electrical parameters U, I, P meet predetermined conditions or not. In particular, the determination block 17 checks whether it detects the risk of an arc break and / or a short circuit based on the electrical parameters U, I, P. Only then does the determination block 17 take the electrical parameters U, I, P into account when determining the second control values ​​A2. However, in this case too, they are only taken into account as long as the risk of an arc break and / or a short circuit exists. If the risk no longer exists, the second control values ​​A2 are again determined independently of the electrical parameters U, I, P. This is explained below in connection with FIG 8 explained in more detail.

[0056] FIG 8 shows the procedure in the shallow bath phase. The procedure in the melt phase can remain unchanged.

[0057] According to FIG 8 From step S4, the control device 9 first proceeds to step S11. In step S11, the control device 9 checks whether it detects the risk of an arc breaking. As part of the check in step S11, the control device 9 evaluates the electrical parameters U, I, P. If the control device 9 detects the risk of an arc breaking, it proceeds to step S12. In step S12, the control device 9 determines the first control values ​​A1 and the second control values ​​A2 in such a way that the risk of an arc breaking is counteracted. For example, the control device 9 can vary the first control values ​​A1 in such a way that the electrode voltages U are increased and the second control values ​​A2 in such a way that the electrodes 6 are lowered towards the molten steel 15.

[0058] If the control device 9 does not detect the risk of an arc breaking off in step S11, the control device 9 proceeds to step S13. In step S13, the control device 9 checks whether it detects the risk of a short circuit. As part of the check in step S13, the control device 9 also evaluates the electrical parameters U, I, P. If the control device 9 detects the risk of a short circuit, it proceeds to step S14. In step S14, the control device 9 determines the first control values ​​A1 and the second control values ​​A2 such that the risk of a short circuit is counteracted. For example, the control device 9 can vary the first control values ​​A1 such that the electrode voltages U are reduced, and in particular vary the second control values ​​A2 such that the electrodes 6 are raised in the direction away from the molten steel 15.

[0059] If the control device 9 does not detect the risk of a short circuit in step S13, the control device 9 proceeds to step S5. In step S5, the first control values ​​A1 and the second control values ​​A2 are determined as already described in connection with FIG 6 was explained.

[0060] Regardless of whether the control device 9 has executed step S12, step S14, or step S5, the control device 9 next proceeds to step S6, in which it controls the energy supply device 3 and the positioning device 4 according to the determined first and second control values ​​A1, A2. The control device then proceeds to step S7. From there, it either proceeds to step S8 or the control device 9 returns to step S11.

[0061] The parameters U, I, P can be selected in various ways. For example, as shown in FIG 9 possible that - at least during the shallow bath phase - the electrical parameters U, I, P are the electrode currents I. Alternatively, according to the illustration in FIG 10 It is possible that - at least during the shallow bath phase - the electrical parameters U, I, P are the electrical powers P. In this case, for example, a supplementary block 18 can be arranged upstream of the determination block 12. In this case, for example, the electrode voltages U and the electrode currents I can be supplied to the supplementary block 18. In this case, the supplementary block 18 determines, for example, the instantaneous power instantly or the average electrical power over a period of the electrode voltages U and outputs the determined value as the electrical parameter P to the determination block 12.

[0062] It is possible for the control device 9 to determine the first control values ​​A1 during the shallow bath phase in such a way that a frequency f of the electrode voltages U (or, correspondingly, a frequency f of the electrode currents I) is varied. This is shown in FIG 11 indicated by varying a corresponding period T. The variation of the period T and correspondingly the frequency f is in FIG 11 indicated by a double arrow 19. It is carried out for the purpose of approximating the electrical parameters U, I, P to the corresponding target values ​​U*, I*, P*. The variation of the frequency f preferably takes place in a range lying between 70% and 90% of the base frequency f0, in particular between 75% and 85% of the base frequency f0.

[0063] At the beginning of the flat bath phase, i.e. when the control device 9 moves from step S4 to step S5 (or in the case of the embodiment according to FIG 8 to step S11), the arcs 14 have the same shape as shown in FIG 5 a base length L0. In some cases, it is advantageous if the control device 9 moves the electrodes 6 towards the molten steel 15 during the flat bath phase. After the movement towards the molten steel 15, the arcs 14 have FIG 12 only a remaining length LR. The remaining length LR is smaller than the base length L0. However, it should be as shown in FIG 13 at least 20% of the base length L0.

[0064] The base length L0 can be made known to the control device 9 in various ways. For example, the base length L0 can be specified to the control device 9 by the operator. Alternatively, it is possible for the control device 9 to be configured as shown in FIG 14immediately after step S4, a step S21 is first executed. In this case, in step S21, the control device 9 determines the base length L0 based on the electrical parameters U, I, P as they exist at the beginning of the flat bath phase. Corresponding procedures are known to those skilled in the art. Step S21, if present, is only executed once. It is therefore not included in the loop of steps S5 to S7. This also applies analogously if steps S11 to S14 are present.

[0065] It is possible for the control device 9 to determine the remaining length LR based on the base length L0. Alternatively, it is possible for the control device 9 to determine only a minimum permissible value for the remaining length LR, or for a corresponding minimum permissible value for the remaining length LR to be specified for the control device 9. In this case, it is possible for the control device 9 to continue moving the electrodes 6 until the control device 9 detects optimized operation of the arc furnace based on an evaluation of the parameters U, I, P, or until the remaining length RL reaches the minimum permissible value. Regardless of the specific procedure taken, step S5 is implemented in this case such that the first control values ​​A1 are determined as already explained, but the second control values ​​A2 are determined such that the length of the arcs 14 is reduced, starting from the base length L0.By reducing the length of the arcs 14 to the residual length LR, the energy efficiency of the arc furnace can be improved in some operating states of the arc furnace.

[0066] The present invention has many advantages. In particular, the mechanical load on the positioning device 7 can be reduced and the energy efficiency during operation of the arc furnace can be improved.

[0067] 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 can be derived therefrom by those skilled in the art without departing from the scope of the invention. List of reference symbols

[0068] 1Furnace vessel 2Steel-containing material 3Energy supply device 4Supply network 5Furnace transformer 6Electrodes 7Positioning device 8, 19Double arrows 9Control device 10Control program 11Machine code 12, 13, 17Determination blocks 14Arcs 15Steel melt 16Slag layer 18Supplementary block A1, A2Control values ​​fFrequency f0Base frequency IElectrode currents L0Base length LRResidual length PElectrical powers S1 to S21Steps TPeriod duration UElectrode voltages U, I, PCharacteristics U*, I*, P*Target values ​​VSpecifications

Claims

1. An operating method for an electric arc furnace, - a control device (9) of the electric arc furnace, initially in a melting phase and after that in a flat-bath phase that follows the melting phase, activating a power supply device (3) of the electric arc furnace using first activation values (A1), so that the power supply device (3) draws electrical energy from a supply system (4) and supplies it via a furnace transformer (5) to electrodes (6) of the electric arc furnace, and furthermore activating a positioning device (7) of the electric arc furnace using second activation values (A2), so that the positioning device (7) positions the electrodes (6) relative to steel-containing material (2) in solid aggregate state, which is located in a furnace vessel (1) of the electric arc furnace, in the melting phase, so that electric arcs (14) form between the electrodes (6) and the steel-containing material (2) in the melting phase, by means of which the steel-containing material (2) is melted to form a steel melt (15), and is positioned relative to the steel melt (15) in the flat-bath phase, so that in the flat-bath phase electric arcs (14) form between the electrodes (6) and the steel melt (15), by means of which the steel melt (15) is heated further, - the control device (9) determining both the first activation values (A1) and the second activation values (A2) during the melting phase in such a manner that electrical parameters (U, I, P) of the electrical energy supplied to the electrodes (6) are approximated as far as possible to corresponding target values (U*, I*, P*), - the control device (9) furthermore determining the first activation values (A1) in such a manner during the flat-bath phase that the electrical parameters (U, I, P) are approximated as far as possible to the corresponding target values (U*, I*, P*), characterized in that, the control device (9) determining the second activation values (A2) either completely independently of the electrical parameters (U, I, P) or depending on the electrical parameters (U, I, P) only if the control device (9) detects the danger of an electric arc breakdown and / or a short circuit on the basis of the electrical parameters (U, I, P).

2. The operating method as claimed in claim 1, characterized in that, at least during the flat-bath phase, the electrical parameters (U, I, P) are the electrode currents (I).

3. The operating method as claimed in claim 1, characterized in that, at least during the flat-bath phase, the electrical parameters (U, I, P) are the electric powers (P).

4. The operating method as claimed in claim 1, 2 or 3, characterized in that the control device (9) determines the first activation values (A1) during the flat-bath phase in such a manner that to approximate the electrical parameters (U, I, P) to the corresponding target values (U*, I*, P*), a frequency (f) of electrode currents (I) supplied to the electrodes (6) and / or of electrode voltages (U) applied to the electrodes (6) is varied.

5. The operating method as claimed in claim 4, characterized in that the frequency (f) of the electrode currents (I) supplied to the electrodes (6) and / or the electrode voltages (6) applied to the electrodes (6) in the flat-bath phase is smaller than a base frequency (f0) of the supply system (4).

6. The operating method as claimed in one of the preceding claims, characterized in that the electric arcs (14) consequently have a basic length (L0) at the start of the flat-bath phase and in that the control device (9) moves the electrodes (6) toward the steel melt (15) during the flat-bath phase, so that after the moving toward the steel melt (15), the electric arcs (14) still have a residual length (LR) that is smaller than the basic length (L0).

7. The operating method as claimed in claim 6, characterized in that the residual length (LR) is at least 20% of the basic length (L0).

8. The operating method as claimed in claim 6 or 7, characterized in that the control device (9) determines the basic length (L0) on the basis of the electrical parameters (U, I, P) as they are present at the start of the flat-bath phase.

9. A control program for a control device (9) of an electric arc furnace, the control program comprising machine code (11) that can be executed by the control device (9), the execution of the machine code (11) by the control device (9) causing the control device (9) to operate an electric arc furnace according to an operating method as claimed in one of the preceding claims.

10. A control device of an electric arc furnace, the control device being programmed with a control program (10) as claimed in claim 9, so that the control device operates the electric arc furnace according to an operating method as claimed in one of claims 1 to 8.

11. An electric arc furnace, - the electric arc furnace having a furnace vessel (1), to which steel-containing material (2) can be supplied in solid aggregate state, - the electric arc furnace having a power supply device (3) and electrodes (6) and also a furnace transformer (5), - the power supply device (3) being connected at the input side to a supply system (4) and at the output side via the furnace transformer (5) to the electrodes (6), - the electric arc furnace having a positioning device (7), by means of which the electrodes (6) can be positioned relative to the steel-containing material (2) in a melting phase and relative to a steel melt (15), which is created by melting the steel-containing material (2), in a flat-bath phase that follows the melting phase, - the electric arc furnace having a control device (9), by which, both in the melting phase and in the flat-bath phase, the power supply device (3) can be activated using first activation values (A1) and the positioning device (7) can be activated using second activation values (A2), - the control device (9) being designed as claimed in claim 10.