Operating method for an electric arc furnace

Abstract

The control device (9) of the electric arc furnace controls the energy supply device (3) during the melting phase and thereafter during the flat bath phase by a first control value (A1), which supplies electric energy to the electrode (6) of the electric arc furnace via the furnace transformer (5). The control device further controls the positioning device (7) during both phases by a second control value (A2), which positions the electrode (6) relative to the still unmelted steel-containing material (2) during the melting phase and relative to the molten steel (15) during the flat bath phase. As a result, an electric arc (14) is formed during both phases, whereby the steel-containing material (2) is melted or the molten steel (15) is further heated. During the melting phase, both the first control value (A1) and the second control value (A2) are determined such that the electrical parameters (U, I, P) of the electrical energy supplied to the electrodes (6) are as close as possible to the corresponding target variables (U*, I*, P*). In the flat bath phase, this applies only to the first control value (A1). In contrast, the second control value (A2) is either determined completely independently of the electrical parameters (U, I, P) or is determined depending on the electrical parameters (U, I, P) only if a risk of an electric arc breakdown and / or a short circuit is identified on the basis of the electrical parameters (U, I, P).

Description

[Technical Field]

[0001] The present invention has as its starting point a method of operation for an electric arc furnace, comprising: - the control device of the electric arc furnace activates, using a first activation value, a power supply device of the electric arc furnace first in a melting stage and thereafter in a flat bath stage following the melting stage, the power supply device being adapted to draw electric energy from a supply system and to supply electric energy to electrodes of the electric arc furnace via a furnace transformer; the control device of the electric arc furnace further activates, using a second activation value, a positioning device of the electric arc furnace being adapted to position the electrodes relative to the steel-containing material in a solid agglomerated state, the steel-containing material being positioned in the furnace vessel of the electric arc furnace in the melting stage, an electric arc being formed between the electrodes and the steel-containing material in the melting stage, whereby the steel-containing material is melted to form a steel melt; the steel-containing material being positioned relative to the steel melt in the flat bath stage, whereby an electric arc is formed between the electrodes and the steel melt in the flat bath stage, whereby the steel melt is further heated; - the control device determines both the first and second start-up values ​​during the melting phase in such a way that the electrical parameters of the electrical energy supplied to the electrodes are as close as possible to the corresponding target values, - using an operating method in which the control device determines a first start-up value during the flat bus phase in such a way that the electrical parameter is as close as possible to the corresponding target value.

[0002] The present invention further takes as its starting point a control program for a control device of an electric arc furnace, the control program comprising machine code that can be executed by the control device, the execution of the machine code by the control device causing the control device to operate the electric arc furnace according to an operating method of this type.

[0003] The invention further uses as a starting point a control device for an electric arc furnace, which is programmed with a control program of this type and which is adapted to operate the electric arc furnace according to an operating method of this type.

[0004] The present invention has as its starting point an electric arc furnace, the electric arc furnace has a furnace vessel, and the steel-containing material can be fed to the furnace vessel in a solid agglomerate state; - the electric arc furnace has a power supply device and electrodes, and also has a furnace transformer; - the power supply device is connected on the input side to the supply system and on the output side to the electrodes via the furnace transformer; the electric arc furnace has a positioning device by which the electrodes can be positioned relative to the steel-containing material in the melting stage and relative to the steel melt produced by melting the steel-containing material in the flat bath stage following the melting stage; the electric arc furnace has a control device by which the power supply device can be activated using a first activation value and the positioning device can be activated using a second activation value both in the melting phase and in the flat bath phase; The control device further uses an electric arc furnace, which is designed as described above. [Background technology]

[0005] The above-mentioned subject matter is generally known. Reference can be made, for example, to US Pat. No. 5,629,999. Also, US Pat. No. 5,629,999 and US Pat. No. 5,629,999 can be mentioned in this context.

[0006] Also known from US Pat. No. 5,629,999 is an operating method for an electric arc furnace. In this operating method, a power supply device is designed for the electrodes of the electric arc furnace as an indirect converter. The indirect converter appears to be downstream of the furnace transformer. US Pat. No. 5,629,999 does not cover the position control of the electrodes in more detail.

[0007] An operating method for an electric arc furnace is known from US Pat. No. 5,623,999, in which a power supply device for the electrodes and a positioning device for the electrodes are activated together as a function of the electrical operating values ​​of the electric arc furnace.

[0008] A method of operation for an electric arc furnace during what is known as the meltdown phase is known from US Pat. No. 5,623,999, in which the electrodes are individually checked for short circuits and breakdown of the electric arc and, if such a condition occurs, the electrode position is corrected. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2015 / 176899 Brochure [Patent Document 2] European Patent Application Publication No. 1 026 921 [Patent Document 3] European Patent Application Publication No. 3 124 903 [Patent Document 4] International Publication No. 2019 / 207611 Brochure [Patent Document 5] U.S. Patent Application Publication No. 5,115,447 Summary of the Invention [Problem to be solved by the invention]

[0010] During the melting of steel in an electric arc furnace, the supply of electrical energy to the electrodes of the electric arc furnace is carried out via a furnace transformer. In many cases, the furnace transformer is connected to the supply system via a medium voltage transformer. The furnace transformer provides multiple voltage steps. For certain power ranges and other high current ranges, each voltage step can be selected in the furnace transformer. Fine control within a particular voltage step can be achieved, for example, by impedance control.

[0011] With this approach, only a few voltage steps are possible, and the electrode current is subject to strong fluctuations. To reduce the fluctuations, the electrode positioning is mostly controlled mechanically by a hydraulic adjustment device. The mechanical adjustment of the electrode has much smaller dynamics than the real behavior of the electric arc. Therefore, the fluctuations can only be compensated for poorly. Furthermore, the fluctuations result in considerable loads on the components (e.g., high-current cables, current-carrying brackets, hydraulic cylinders, etc.). Fluctuations occur both in the melting phase and in the flat bath phase.

[0012] In the flat bath stage, a relatively small voltage is generally applied to the electrodes, which are positioned relatively close to the surface of the steel melt. As a result, a high current is set up. At the same time, heat loss is reliably shielded by the foamy slag. However, depending on the performance level of the electric arc furnace or during the production of certain steels (especially stainless steels and high-grade steels), the electric arc is only partially or not at all surrounded by the foamy slag. As a result, the energy efficiency of the electric arc furnace drops.

[0013] During the adjustment of the electrode voltage via the voltage step of the furnace transformer, the positioning of the electrodes must be continuously readjusted. The readjustment can be performed, for example, in such a way that control is performed up to a specific impedance or a specific power. Since the dynamics of the positioning device are relatively low compared to the changes in the electrical system of the electric arc, certain fluctuations remain, which cannot be compensated for. The fluctuations are again increased by the waves and flow of the steel melt. As a result, the energy input into the steel melt is not optimal.

[0014] Approaches in which the electrode voltage can be continuously adjusted are known from prior art documents, in particular from US Pat. No. 5,629,499 and US Pat. No. 5,629,499, and to a limited extent from US Pat. No. 5,629,499. These embodiments offer considerable advantages compared to adjusting the electrode voltage by voltage steps of the furnace transformer. On the one hand, the electrode voltage can be changed not only step by step, but also continuously. On the other hand, the furnace transformer can be designed more simply, since it does not need to provide multiple voltage steps. Furthermore, further types of control are possible with these embodiments.

[0015] The aim of the present invention is to create an option that allows fast and high-quality control of the electric arc in a simple and reliable manner during the flat bath phase. [Means for solving the problem]

[0016] This object is achieved by an operating method having the features of claim 1. Advantageous embodiments of the operating method are the subject matter of dependent claims 2 to 8.

[0017] According to the invention, an operating method of the type described in the introduction is configured such that the control device determines the second activation value during the flat bus phase completely independently of the electrical parameter or depending on the electrical parameter only if the control device detects a risk of an electric arc breakdown and / or a short circuit based on the electrical parameter.

[0018] Thus, the first activation value is 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. In contrast, the second activation value is determined independently of the electrical parameters (except in the case of a specific operating state danger that must be absolutely avoided). As a result, correction of the electrode voltage and electrode current is therefore performed exclusively by adapting the activation of the power supply device.

[0019] The electrical parameter of the electrical energy supplied to the electrode can be determined as needed. For example, the electrical parameter can be the electrode current. In particular, the active current can be the electrode current. However, in certain cases, the electrical parameter can also be the reactive current and / or the apparent current. Alternatively, the electrical parameter can be the power. In particular, the active power can be the power. However, in certain cases, the electrical parameter can also be the reactive power and / or the apparent power.

[0020] The voltages applied to the electrodes, and thus the currents supplied to the electrodes, are generally alternating quantities, i.e., AC voltages and currents. Alternating quantities can be characterized by their amplitude over a period of time, their frequency, and their curve (e.g., sine wave, triangular wave, sawtooth wave, square wave, etc.). The time curve is preferably sinusoidal.

[0021] The amplitude must always be set in an appropriate manner. The frequency can, in some cases, be kept constant. However, in other cases, it will be preferable for the control device to determine the first start-up value during the flat bath phase in such a way that the frequency of the electrode current supplied to the electrodes and / or the frequency of the electrode voltage applied to the electrodes are also changed in order to bring the electrical parameters closer to the corresponding target values. This approach offers greater flexibility in optimizing the operation of the electric arc furnace.

[0022] Preferably, in the flat bath phase, the frequency of the electrode current supplied to the electrodes and / or the frequency of the electrode voltage applied to the electrodes is less than the base frequency of the supply system, an approach that has proven particularly advantageous in experiments.

[0023] At the start of the flat bath phase, the electrode is spaced from the surface of the steel melt. The electric arc consequently has a base length at the start of the flat bath phase. In some situations, it is advantageous for the control device to move the electrode toward the steel melt during the flat bath phase so that after the movement toward the steel melt, the electric arc still has a residual length that is smaller than the base length. However, to avoid the risk of short circuits, a certain minimum length should not be dropped below. For this reason, the residual length is preferably at least 20% of the base length.

[0024] The base length can be determined or at least estimated based on the electrical parameters as they exist at the start of the flat bus phase. This determination / estimation can be done intelligently by a human, but it is preferably done by a control device.

[0025] Furthermore, this object is achieved by a control program having the features of claim 9. According to the invention, execution of the machine code by the control device preferably causes the control device to operate the electric arc furnace according to the operating method according to the invention.

[0026] Furthermore, this object is 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, and the control device is adapted to operate the electric arc furnace according to the operating method according to the invention.

[0027] Furthermore, this object is achieved by an electric arc furnace with the features of claim 11. According to the invention, the control device is designed as a control device according to the invention.

[0028] The above-mentioned characteristics, features and advantages of the present invention, as well as the manner in which they are realized, will become more apparent and more clearly understood in connection with the following description of exemplary embodiments, which are set forth in more detail in connection with the drawings, in which: [Brief explanation of the drawings]

[0029] [Figure 1] FIG. 1 is a block diagram of an electric arc furnace. [Figure 2] FIG. 1 shows the furnace vessel during the melting stage. [Figure 3] FIG. [Figure 4] FIG. 10 illustrates the mode of action of the control device during the melting stage. [Figure 5] FIG. 1 shows the furnace vessel during the flat bath stage. [Figure 6] FIG. 10 illustrates the mode of action of the control device in the flat bus phase. [Figure 7] FIG. 7 is a diagram showing a modification of FIG. 6. [Figure 8] FIG. [Figure 9] FIG. 1 illustrates a decision block. [Figure 10] FIG. 1 illustrates a decision block and a trailer block. [Figure 11] FIG. [Figure 12] FIG. 6 is a diagram showing a modification of FIG. 5. [Figure 13] FIG. [Figure 14] FIG. DETAILED DESCRIPTION OF THE INVENTION

[0030] According to Figure 1, the electric arc furnace has a furnace vessel 1. A steel-containing material 2 (see Figure 2) can be fed to the furnace vessel 1. The steel-containing material 2 is fed to the furnace vessel 1 in the form of solid agglomerates. The steel-containing material 2 can be, for example, scrap.

[0031] Furthermore, the electric arc furnace has a power supply device 3. The power supply device 3 is connected on the input side to a supply system 4. The supply system 4 is typically a medium voltage system, which has a nominal voltage in the double-digit kV range and is operated at a base frequency f0 (see FIG. 11). The base frequency f0 is typically 50 Hz or 60 Hz. According to the illustration in FIG. 1, the supply system 4 is typically a three-phase system.

[0032] The electric 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. Generally, according to the illustration in FIG. 1, there are multiple electrodes 6, and the furnace transformer 5 is further designed as a three-phase transformer. However, other embodiments are also possible, in particular single-phase embodiments. Regardless of the actual embodiment, the electrode voltage U applied to the electrodes 6 is, however, clearly below the nominal voltage of the supply system 4. The electrode voltage U is illustrated in FIG. 1 only for one of the electrodes 6. In most cases, the electrode voltage U is in the range of several hundred volts. In certain cases, voltages above 1 kV are also possible. However, 2 kV is generally not exceeded.

[0033] Typically, further switching devices are present, by means of which the power supply device 3 can be disconnected from the supply system 4. Further switching devices can be present, by means of which the power supply device 3 can be disconnected from the furnace transformer 5 and / or the furnace transformer 5 can be disconnected from the electrodes 6. The switching devices perform a purely binary switching operation but do not perform voltage and current regulation. 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 functionality according to the invention and therefore are also not shown in FIG. 1 (and in other figures) for the sake of clarity.

[0034] The power supply device 3 is capable of drawing electrical energy from the supply system 4 and supplying the drawn electrical energy to the electrodes 6 via the furnace transformer 5. The power supply device 3 typically has a number of semiconductor switches for this purpose. Possible embodiments of the power supply device 3 are described in US Pat. No. 5,629,499 ("Absolute Reference"). Alternatively, for example, embodiments according to US Pat. No. 5,629,499 or US Pat. No. 5,629,499 can be used. Regardless of the actual embodiment of the power supply device 3, however, on the output side (i.e., towards the furnace transformer 5), the power supply device 3 is capable of almost continuous stepping of the electrode voltage U applied to the electrodes 6 and / or the electrode current I supplied to the electrodes 6. Similar to the illustration for the electrode voltage U, the electrode current I is likewise illustrated in FIG. 1 only for one of the electrodes 6.

[0035] Furthermore, the electric arc furnace has a positioning device 7, by means of which the electrodes 6 can be positioned, as shown by the double-headed arrow 8 in FIG. 1 next to one of the electrodes 6. In the simplest case, several electrodes 6 are positioned together. However, individual positioning of the electrodes 6 is also possible. The direction of movement in which the electrodes 6 are positioned can be vertical. Alternatively, the direction of movement can be slightly inclined relative to the vertical. However, even in this case, the vertical component is the dominant component of the movement. The positioning device 7 can, for example, have one or more hydraulic cylinder units.

[0036] Finally, the electric arc furnace has a control device 9. (At least) the power supply device 3 and the positioning device 7 are controlled by the control device 9. Thus, the control device 9 generates a first activation value A1 (using which the control device 9 activates the power supply device 3) and a second activation value A2 (using which the control device 9 activates the positioning device 7). The power supply device 3 and the positioning device 7 are operated according to the respective activation values ​​A1, A2.

[0037] The control device 9 is designed as a software-programmable control device, which is indicated in FIG. 1 by the information "μP" (for microprocessor controlled). The action and mode of operation of the control device 9 are therefore 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 executed by the control device 9. 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, as will be explained in more detail below in connection with further figures.

[0038] First, the furnace vessel 1 is fed with steel-containing material 2 in step S1 according to Figure 3. This process can be, but does not have to be, under the control of the control device 9. Therefore, step S1 is simply shown in dashed lines in Figure 3.

[0039] A melting stage in the electric arc furnace follows the feeding of the steel-containing material 2. The melting stage comprises steps S2 to S4. The melting stage is followed by a flat bath stage. The flat bath stage comprises steps S5 to S7.

[0040] During the melting phase, the control device 9 determines in step S2 a first activation value A1 for the power supply device 3 and a second activation value A2 for the positioning device 7. The determination is performed according to Fig. 4 in corresponding decision blocks 12 and 13. In step S3, the control device 9 activates the power supply device 3 and the positioning device 7 according to the determined activation values ​​A1, A2.

[0041] The determination of the first activation value A1 is performed in such a way that, by corresponding activation, the power supply device 3 draws electrical energy from the supply system 4 and supplies it to the electrode 6 via the furnace transformer 5. The determination of the second activation value A2 is performed in such a way that the positioning device 7 positions the electrode 6 with respect to the steel-containing material 2. The determination of the first activation value A1 and the determination of the second activation value A2 by the control device 9 are coordinated with respect to each other in such a way that an electric arc 14 (see FIG. 2) is formed between the electrode 6 and the steel-containing material 2. Due to the electric arc 14, the steel-containing material 2 is melted, and thus a steel melt 15 (FIG. 5) is gradually produced.

[0042] To determine the first and second activation values ​​A1 and A2, the parameters U, I, P of the electrical energy supplied to the electrodes 6 are supplied to the control device 9 according to Fig. 4. The parameters U, I, P can be, for example, the electrode voltage U and / or the electrode current I and / or values ​​derived therefrom. The derived value is, for example, the instantaneous power P (= the product of the electrode voltage U and the electrode current I). Further derived values ​​can result from the time curves of the electrode voltage U and the electrode current I. Values ​​of this type are, for example, the active current, the active power, the apparent power, the reactive current, and the reactive power. The parameters can alternatively be given or derived for the electrodes 6 as a whole or for each electrode 6 individually. To determine the first and second activation values ​​A1 and A2, the setpoint values ​​U*, I*, P* for the parameters U, I, P are further supplied to the control device 9, e.g. setpoint values ​​U*, I* for the electrode voltage U and / or electrode current I or other suitable setpoint values ​​(e.g. setpoint value P* for the power P). Both the parameters U, I, P and the setpoint values ​​U*, I*, P* are supplied to both decision blocks 12, 13 during the melting phase.

[0043] Based on the parameters U, I, P and the associated setpoint values ​​U*, I*, P*, the control device 9 determines a first start-up value A1 and a second start-up value A2. The determination is performed in both decision blocks 12, 13 in such a way that the electrical parameters U, I, P are as close as possible to the corresponding setpoint values ​​U*, I*, P*. This approach, and thus the implementation of step S2, is generally known to those skilled in the art. Therefore, it does not need to be described in more detail.

[0044] In step S4, the control device 9 checks whether the melting stage is complete. The melting stage is complete when the steel melt 15 according to the illustration in Fig. 5 has completely or at least substantially formed a continuous horizontal surface. Thus, the steel-containing material 2 is either completely melted, or the not-yet-melted elements of the steel-containing material 2 are located completely below the surface of the steel melt 15, or the not-yet-melted elements of the steel-containing material 2 still protrude only slightly above the surface of the steel melt 15. Furthermore, a slag layer 16 may have formed on top of the surface of the steel melt 15.

[0045] The control device 9 can evaluate actual values ​​of the electric arc furnace that are detected metrologically in the context of checking whether the melting stage is complete. For example, the control device 9 can evaluate the electrode current I and / or the electrode voltage U, in particular their fluctuations. The control device 9 can also evaluate acoustic values ​​of the electric arc furnace, for example the noise level or the acoustic spectrum of the generated noise. Alternatively, an operator (not shown) can determine for the control device 9 that the melting stage is complete.

[0046] If the melting phase is not yet complete, the control device 9 then returns to step S2. In contrast, if the melting phase is complete, the control device 9 moves to the flat bath phase and thus to step S5.

[0047] In the flat bus phase, in step S5, the control device 9 determines a first activation value A1 for the power supply device 3 and a second activation value A2 for the positioning device 7. In step S6, the control device 9 activates the power supply device 3 and the positioning device 7 according to the determined activation values ​​A1, A2.

[0048] The determination of the first activation value A1 is performed in such a way that, by corresponding activation, the power supply device 3 draws electrical energy from the supply system 4 and supplies it to the electrode 6 via the furnace transformer 5. The determination of the second activation value A2 is performed in such a way that the positioning device 7 positions the electrode 6 relative to the steel melt 15. In this respect, the procedure of steps S5 and S6 matches the procedure of steps S2 and S3.

[0049] The sequence of steps S5 and S6 also matches the sequence of steps S2 and S3 insofar as the first activation value A1 and the second activation value A2 are adjusted relative to each other in such a way that an electric arc 14 is formed. However, the electric arc 14 is formed in the flat bath phase between the electrode 6 and the steel melt 15, as shown in Figure 5. The steel melt 15 is further heated due to the electric arc 14.

[0050] Furthermore, the parameters U, I, P and the associated setpoint values ​​U*, I*, P* of the electrical energy supplied to the electrodes 6 are also supplied to the control device 9 according to Fig. 6. However, the parameters U, I, P and the associated setpoint values ​​U*, I*, P* are supplied only to the decision block 12 inside the control device 9. The control device 9 therefore further determines the first activation value A1 in such a way that the electrical parameters U, I, P are as close as possible to the corresponding setpoint values ​​U*, I*, P*.

[0051] In contrast, decision block 13 is deactivated in the flat bus phase. Instead, decision block 17 is activated according to Fig. 6. The control device 9 determines the second activation value A2 by means of decision block 17 in the flat bus phase. In particular, according to the illustration in Fig. 3, it is possible for the control device 9 to determine the second activation value A2 completely independently of the electrical parameters U, I, P. In this case, according to the illustration in Fig. 6, it is possible for the electrical parameters U, I, P not to be supplied to decision block 17 at all. Instead, the control device 9 can determine the second activation value A2 based on other internal decisions or based on external specifications V (e.g., specifications coming from an operator).

[0052] In step S7, the control device 9 checks whether the flat bath phase is complete. It is possible for the control device 9 to evaluate metrologically detected actual values ​​of the electric arc furnace in the context of checking whether the flat bath phase is complete. Alternatively, it is possible for the operator to specify for the control device 9 that the flat bath phase is complete.

[0053] If the flat bath phase is not yet complete, the control device 9 returns to step S5. In contrast, if the flat bath phase is complete, the control device 9 proceeds to step S8. In step S8, the resulting steel melt 15 is removed from the furnace vessel 1 and poured, for example, into a ladle (not shown). This process can be, but does not have to be, performed under the control of the control device 9. Therefore, step S8, like step S1, is simply illustrated in dashed lines in Figure 3.

[0054] The execution of step S8 completes a complete cycle of operation of the electric arc furnace, so that a new cycle can be initiated starting from step S1.

[0055] In the simplest embodiment, the determination of the second activation value A2 is performed independently of the electrical parameters U, I, P, as previously mentioned. Alternatively, the second activation value A2 is generally determined by the decision block 17 independently of the electrical parameters U, I, P, although these may actually be taken into account under certain circumstances. In this case, the corresponding electrical parameters U, I, P are supplied to the decision block 17 in accordance with the illustration in FIG. 7. It is not, in contrast, necessary to supply the corresponding target values ​​U*, I*, P*.

[0056] In this case, decision block 17 (and consequently control device 9, since decision block 17 is a component of control device 9) checks whether the electrical parameters U, I, P satisfy predetermined conditions. In particular, decision block 17 checks in this case whether it detects a risk of an electric arc breakdown and / or short circuit based on the electrical parameters U, I, P. Only then does decision block 17 take the electrical parameters U, I, P into account in determining the second activation value A2. However, even in this case, they are taken into account only if a risk of an electric arc breakdown and / or short circuit exists. If the risk no longer exists, the determination of the second activation value A2 is performed again, again independently of the electrical parameters U, I, P. This is explained in more detail below in connection with FIG. 8.

[0057] Figure 8 shows the procedure in the flat bath stage. The procedure in the melting stage does not need to be changed.

[0058] According to FIG. 8, the control device 9 first proceeds from step S4 to step S11. In step S11, the control device 9 checks whether it detects a risk of electric arc breakdown. In the context of the check in step S11, the control device 9 evaluates the electrical parameters U, I, and P. If the control device 9 detects a risk of electric arc breakdown, it proceeds to step S12. In step S12, the control device 9 determines the first start-up value A1 and the second start-up value A2 so that the risk of electric arc breakdown is eliminated. For example, the control device 9 can change the first start-up value A1 so that the electrode voltage U is increased, and can change the second start-up value A2 so that the electrode 6 is lowered in the direction towards the steel melt 15.

[0059] If the control device 9 does not detect a risk of an electric arc breakdown in step S11, the control device 9 proceeds to step S13. In step S13, the control device 9 checks whether it detects a risk of a short circuit. In the context of the check in step S13, the control device 9 also evaluates the electrical parameters U, I, and P. If the control device 9 detects a risk of a short circuit, it proceeds to step S14. In step S14, the control device 9 determines the first start-up value A1 and the second start-up value A2 so that the risk of a short circuit is eliminated. For example, the control device 9 can change the first start-up value A1 so that the electrode voltage U is reduced, and can change the second start-up value A2, among other things, so that the electrode 6 is lifted away from the steel melt 15.

[0060] If in step S13 the control device 9 does not detect a risk of a short circuit, the control device 9 proceeds to step S5, in which the determination of the first and second activation values ​​A1 and A2 is carried out as already explained in connection with FIG.

[0061] Regardless of whether the control device 9 has performed step S12, step S14, or step S5, the control device 9 then proceeds to step S6, in which the control device 9 activates the power supply device 3 and the positioning device 7 in accordance with the determined first and second activation values ​​A1, A2. The control device then proceeds to step S7, from which either there is a transition to step S8 or the control device 9 returns to step S11.

[0062] The parameters U, I, P can be selected in various ways. For example, according to the illustration in FIG. 9, the electrical parameter U, I, P can be the electrode current I, at least during the flat bus phase. Alternatively, according to the illustration in FIG. 10, the electrical parameter U, I, P can be the power P, at least during the flat bus phase. In this case, the trailer block 18 can be, for example, upstream of the decision block 12. In this case, for example, the electrode voltage U and the electrode current I can be supplied to the trailer block 18. In this case, for example, the trailer block 18 determines the instantaneous power or determines the average power over the period of the electrode voltage U and outputs the determined value as the electrical parameter P to the decision block 12.

[0063] During the flat bath phase, the control device 9 can determine the first activation value A1 in such a way that the frequency f of the electrode voltage U (or the corresponding frequency f of the electrode current I) is changed. This is shown in FIG. 11 in that the corresponding period T is changed. The change in period T, and correspondingly, the change in frequency f, is indicated in FIG. 11 by a double-headed arrow 19, with the aim of approximating the electrical parameters U, I, P to the corresponding target values ​​U*, I*, P*. The change in frequency f is preferably made in a range between 70% and 90% of the base frequency f, in particular between 75% and 85% of the base frequency f.

[0064] At the start of the flat bath phase, therefore, when the control device 9 passes from step S4 to step S5 (or, in the case of the embodiment according to FIG. 8, when passing to step S11), the electric arc 14 according to the illustration in FIG. 5 has a basic length L0. In some cases, it is advantageous if the control device 9 moves the electrode 6 towards the steel melt 15 during the flat bath phase. After the movement towards the steel melt 15, the electric arc 14 according to FIG. 12 still has only a residual length LR. The residual length LR is smaller than the basic length L0. However, according to the illustration in FIG. 13, it should be at least 20% of the basic length L0.

[0065] The basic length L0 can be known to the control device 9 in various ways. For example, the basic length L0 of the control device 9 can be specified by an operator. Alternatively, according to the illustration in FIG. 14, the control device 9 can first execute step S21 immediately after step S4. In this case, the control device 9 determines the basic length L0 in step S21 based on the electrical parameters U, I, P as they exist at the start of the flat bath phase. Corresponding approaches are known to those skilled in the art. Step S21, if present, is executed only once. Therefore, it is not included in the loop of steps S5 to S7. This also applies if steps S11 to S14 are present.

[0066] The control device 9 can determine the residual length LR based on the basic length L0. Alternatively, the control device 9 can determine only one minimum acceptable value for the residual length LR, or a corresponding minimum acceptable value for the residual length LR can be predetermined for the control device 9. In this case, the control device 9 can maintain the movement of the electrode 6 until the control device 9 detects an optimized operation of the electric arc furnace based on the evaluation of the parameters U, I, and P, or until the residual length RL reaches the minimum acceptable value. Regardless of the actual approach taken, step S5 is implemented in this case in such a way that the first activation value A1 is determined as previously described, while the second activation value A2 is determined in such a way that the length of the electric arc 14 is reduced starting from the basic length L0. Due to the reduction of the length of the electric arc 14 to the residual length LR, the energy efficiency of the electric arc furnace can be improved in certain operating conditions of the electric arc furnace.

[0067] The present invention has many advantages, but above all it allows the mechanical load on the positioning device 7 to be reduced and the energy efficiency during operation of the electric arc furnace to be further improved.

[0068] Although the present invention has been shown and described in more detail by means of preferred exemplary embodiments, the invention is not limited to the disclosed examples, and other variations can be deduced therefrom by those skilled in the art without departing from the scope of protection of the invention. [Explanation of symbols]

[0069] 1 Furnace vessel 2 Steel-containing materials 3 Power Supply Device 4. Supply System 5. Furnace transformer 6 electrodes 7 Positioning Device 8, 19 Double-headed arrow 9 Control Devices 10 Control Program 11 Machine Code 12, 13, 17 Decision Blocks 14 Electric Arc 15 Steel melt 16 Slag layer 18 Trailer Block A1, A2 startup values f frequency f0 base frequency I electrode current L0 Basic length LR Residual Length P power S1~S21 steps T period U electrode voltage U, I, P parameters U*, I*, P* target values V specification

Claims

1. 1. A method of operation for an electric arc furnace, comprising: the control device (9) of the electric arc furnace firstly in the melting stage and then in the flat bath stage following the melting stage, activates the power supply device (3) of the electric arc furnace using a first activation value (A1), the power supply device (3) being adapted to draw electrical energy from a supply system (4) and to supply said electrical energy to the electrodes (6) of the electric arc furnace via a furnace transformer (5); the control device (9) of the electric arc furnace further activates the positioning device (7) of the electric arc furnace using a second activation value (A2), the positioning device (7) being adapted to position the steel-containing material (2) in a solid agglomerated state; the electrode (6) is positioned relative to the steel melt (15) in the flat bath stage, and an electric arc (14) is formed between the electrode (6) and the steel melt (2) in the melting stage, thereby melting the steel melt (2) to form a steel melt (15); the electrode (6) is positioned relative to the steel melt (15) in the flat bath stage, and an electric arc (14) is formed between the electrode (6) and the steel melt (15) in the flat bath stage, thereby further heating the steel melt (15); the control device (9) determines both the first activation value (A1) and the second activation value (A2) during the melting stage in such a way that the electrical parameters (U, I, P) of the electrical energy supplied to the electrode (6) are as close as possible to the corresponding target values ​​(U*, I*, P*), the control device (9) further determines the first activation value (A1) during the flat bath phase in such a way that the electrical parameters (U, I, P) are as close as possible to the corresponding target values ​​(U*, I*, P*), but determines the second activation value (A2) completely independently of the electrical parameters (U, I, P) or only in dependence on the electrical parameters (U, I, P) if the control device (9) detects a risk of an electric arc breakdown and / or a short circuit based on the electrical parameters (U, I, P).

2. 2. A method according to claim 1, characterized in that, at least during said flat bath phase, said electrical parameter (U, I, P) is the electrode current (I).

3. 2. A method according to claim 1, characterized in that, at least during said flat bus phase, said electrical parameter (U, I, P) is power (P).

4. 4. The method according to claim 1, 2 or 3, characterized in that the control device (9) determines the first start-up value (A1) during the flat bath phase in such a way that the frequency (f) of the electrode current (I) supplied to the electrode (6) and / or the frequency (f) of the electrode voltage (U) applied to the electrode (6) is changed in order to bring the electrical parameters (U, I, P) close to the corresponding target values ​​(U*, I*, P*).

5. 5. The method according to claim 4, characterized in that in the flat bath phase, the frequency (f) of the electrode current (I) supplied to the electrode (6) and / or the frequency (f) of the electrode voltage (U) applied to the electrode (6) is smaller than the base frequency (f0) of the supply system (4).

6. 4. The method according to claim 1, wherein the electric arc (14) consequently has a base length (L0) at the start of the flat bath phase, and the control device (9) moves the electrode (6) towards the steel melt (15) during the flat bath phase, such that after the movement towards the steel melt (15), the electric arc (14) still has a residual length (LR) that is smaller than the base length (L0).

7. 7. A method according to claim 6, characterized in that the residual length (LR) is at least 20% of the base length (LO).

8. 7. A method according to claim 6, characterized in that the control device (9) determines the basic length (L0) on the basis of the electrical parameters (U, I, P) as present at the start of the flat bus phase.

9. 4. A control program for a control device (9) of an electric arc furnace, the control program comprising machine code (11) executable by the control device (9), the execution of the machine code (11) by the control device (9) causing the control device (9) to operate the electric arc furnace according to the operating method of any one of claims 1 to 3.

10. A control device for an electric arc furnace, the control device being programmed by a control program (10) according to claim 9, the control device being adapted to operate the electric arc furnace according to the operating method according to any one of claims 1 to 3.

11. 1. An electric arc furnace, comprising: the electric arc furnace has a furnace vessel (1) into which the steel-containing material (2) can be fed in the form of solid agglomerates; said electric arc furnace comprises a power supply device (3) and electrodes (6) and also a furnace transformer (5); - said power supply device (3) is connected on the input side to a supply system (4) and on the output side to said electrodes (6) via said furnace transformer (5); the electric arc furnace has a positioning device (7) by means of which the electrode (6) can be positioned relative to the steel-containing material (2) during the melting stage and relative to the steel melt (15) produced by melting the steel-containing material (2) during the flat bath stage following the melting stage, the electric arc furnace has a control device (9) by means of which the power supply device (3) can be activated using a first activation value (A1) and the positioning device (7) can be activated using a second activation value (A2) both during the melting phase and during the flat bath phase; - an electric arc furnace, wherein the control device (9) is a control device according to claim 10;

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