How to operate an electric arc furnace

Abstract

During the melting stage, the metal (2) present in the furnace vessel (1) of the arc furnace is melted. The control device (9) controls the target variable (X) of the electrical energy to be supplied by the electrodes (6). * ) based on the provisional voltage target value (U1 * ) is determined. * When a voltage (U) corresponding to the target variable (X) is applied to the electrode (6), the first actual variable (X) of the electrical energy supplied from the electrode (6) is * The control device (9) adjusts the final voltage target value (U2 * ) to control the energy supply device (3). * ) is applied to the electrode (6). The energy supply device (3) then draws electrical energy from the power grid (4) and supplies it to the electrode (6) via the furnace transformer (5). During the initial melting stage, the control device (9) sets a provisional voltage target value (U1 * ) to the maximum allowable value (Umax) to * The maximum allowable value (Umax) is less than or equal to the maximum possible value (U0) that can be applied to the electrodes (6) by the energy supply device (3).

Description

[Technical Field]

[0001] The invention is based on a method of operating an electric arc furnace, in which, during a melting stage, the metal present in the furnace vessel of the electric arc furnace is melted, and the control device of the electric arc furnace comprises: - determining a provisional voltage target value based on an electrical target variable of the electrical energy to be supplied by the electrode, so that when a voltage corresponding to the provisional voltage target value is applied to the electrode, a first actual electrical variable of the electrical energy supplied by the electrode comes as close as possible to the electrical target variable; -Controlling an energy supply device of the arc furnace based on the final voltage target value, so that a voltage corresponding to the final voltage target value is applied to the electrodes, and the energy supply device draws electrical energy from the power grid and supplies it to the electrodes via the furnace transformer.

[0002] The invention is further based on a control program for a control device of an electric arc furnace, the control program including machine code that is processable by the control device, and processing of the machine code by the control device causes the control device to operate the electric arc furnace according to such an operating method.

[0003] The invention is further based on a control device for an electric arc furnace, which is programmed with a control program such that the control device operates the electric arc furnace according to such an operating method.

[0004] The invention is further based on an electric arc furnace, the electric arc furnace has a furnace vessel, the furnace vessel being capable of being charged with metal; The arc furnace comprises an energy supply device and electrodes as well as a furnace transformer, the energy supply device is connected on the input side to the supply grid and on the output side to the electrodes via the furnace transformer, the arc furnace has a control device capable of controlling the energy supply device, The control device is configured as described above. [Background technology]

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

[0006] When melting metals, especially steel, in an electric arc furnace, the electrodes of the electric arc furnace are supplied with electrical energy via a furnace transformer, which is often connected to the grid via a medium voltage transformer. The furnace transformer provides several voltage steps. Each voltage step can be selected in the furnace transformer for a certain power range and for other high current ranges. Fine adjustment within a particular voltage step can be achieved, for example, by impedance control.

[0007] In this procedure, only small voltage steps are possible, and the electrode current is subject to strong fluctuations. To reduce these fluctuations, the electrode positioning is controlled mechanically, usually through a hydraulic adjustment device. The mechanical adjustment of the electrode has significantly lower dynamics than the actual behavior of the arc. Therefore, the fluctuations can only be compensated for insufficiently. Furthermore, the fluctuations cause significant loads on components such as high-current cables, current-carrying support arms, and hydraulic cylinders. The fluctuations occur both during the melting phase and the subsequent flat-bath phase.

[0008] When adjusting the electrode voltage through the voltage steps of the furnace transformer, the electrode positioning must be continually readjusted. This readjustment can be done, for example, to control a specific impedance or a specific power output. However, because the dynamics of the positioning device are relatively small compared to changes in the arc's electrical system, some uncompensable fluctuations remain, which results in less than optimal energy input to the molten metal.

[0009] From the prior art, in particular from US Pat. No. 5,629,499 and US Pat. No. 5,629,499, and to a lesser extent from US Pat. No. 5,629,499, procedures are known that allow the electrode voltage to be continuously adjusted. These embodiments offer significant advantages over adjusting the electrode voltage through voltage steps in the furnace transformer: on the one hand, it is possible to vary the electrode voltage continuously, not just in steps; on the other hand, the furnace transformer can be simpler, since it does not have to provide multiple voltage steps. Furthermore, these embodiments allow for additional types of control.

[0010] As is known from the prior art, when the electrode voltage can be continuously adjusted, the supply of electrical energy is often carried out with constant power or constant current. The power or current can be maximum, i.e., as high as the design allows ("energy supply, supply what you can"). In this case, the voltage applied to the electrodes is used to adjust the desired power or current. Especially in the early stages of melting in an arc furnace, the electrodes are located close to the arc furnace cover, but still far above it, so that an arc can form from the electrode towards the cover rather than towards the metal. This not only significantly reduces the energy input to the metal, but also significantly increases the wear of the cover, and can even cause it to break down in a short time. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 2015 / 176899 Brochure [Patent Document 2] European Patent Application Publication No. 1026921 [Patent Document 3] European Patent Application Publication No. 3124903 Summary of the Invention [Problem to be solved by the invention]

[0012] The object of the present invention is to provide a possibility to avoid the drawbacks of the prior art. [Means for solving the problem]

[0013] This 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 5.

[0014] According to the invention, an operating method of the type mentioned at the outset is configured in that the control device determines a final voltage target value by limiting the provisional voltage target value to a maximum permissible value, at least in the initial stage of the melting phase, which is less than or equal to the maximum possible value that can be applied to the electrodes by the energy supply device.

[0015] For example, if the design of the energy supply device allows a maximum of 1200 V to be applied to the electrodes and 5000 A is to flow through them, in the prior art, the voltage is adjusted as needed to the maximum of 1200 V to set the desired current of 5000 A. In contrast, in the present invention, such adjustment is only made to values below 1200 V, for example, up to 700 V. If a voltage value above 700 V is required to set the current of 5000 A, deviations in the current from the original desired value (= 5000 A) are tolerated. This reduces the energy supplied to the arc furnace, but ensures that the supplied energy is introduced into the metal to be melted and not into the components of the arc furnace itself. The values of 700 V, 1200 V, and 5000 A are given as illustrative examples. In practice, larger or smaller values may occur. In particular, the current is often significantly larger.

[0016] In the simplest embodiment, the maximum permissible value is fixed. In this case, it is possible that the maximum permissible value is taken into account only in the early stages of the melting phase and not in the later parts of the melting phase. Alternatively, it is also possible that the maximum permissible value is taken into account during the entire melting phase but is no longer taken into account in the subsequent flat bath phase.

[0017] In other embodiments, the maximum allowed value may be dynamic.

[0018] For example, the control device can receive an input value from an operator and determine the maximum allowable value based on the input value. Alternatively, the input value can always determine the maximum allowable value. That is, if the input value is "500V," the maximum allowable value is also always 500V. Alternatively, the input value can be specified only in terms of an upper limit. In this case, the control device can first determine the maximum allowable value in a different way. If the value determined in this way is lower than the voltage value determined by the input value (e.g., only 450V compared to 500V), the value determined by the control device (450V) is used as the maximum allowable value. If the value determined by the control device is higher than the voltage value determined by the input value (e.g., 550V), the voltage value determined by the input value (500V) is used as the maximum allowable value. Again, the numerical values 450V, 500V, and 550V given are purely exemplary for illustrative purposes. In practice, larger or smaller values may occur.

[0019] The input values may alternatively be specified continuously or in steps.

[0020] In another embodiment, the control device can determine the maximum permissible value based on the time elapsed since the start of the melting phase, either as an alternative or in addition to taking into account the input values. For example, the maximum permissible value can initially have a relatively low value and, after a certain waiting time, for example, 5 minutes, be increased in one step, in several steps, or continuously (linearly or non-linearly). The maximum permissible value can be increased in a later section, i.e., after at least the initial stage of the melting phase, up to the maximum possible value (theoretically even higher). In this case, the voltage limitation according to the present invention is no longer active once the maximum possible value is reached. The 5-minute period mentioned is merely an example. In practice, larger or smaller values may occur.

[0021] Generally, the electrode positioning changes over time. The electrode positioning may be known to the controller. For example, the controller may determine the electrode positioning, or the electrode positioning may be externally provided to the controller. Thus, in one embodiment of the present invention, the controller may determine the maximum allowable value based on the electrode positioning.

[0022] As with the determination based on the time elapsed since the start of the melting stage, the determination based on the positioning of the electrodes may initially have a relatively low value and may be increased in one step, in multiple steps, or continuously (linearly or non-linearly) when a certain position is reached, i.e., as a result of the later sections of the melting stage, up to and even beyond the maximum possible value.

[0023] In another embodiment of the invention, the control device can determine the progress of the melting of the metal in the arc furnace by evaluating the time course of a second actual electrical variable of the electrical energy supplied to the electrodes and / or by evaluating an actual acoustic variable of the arc furnace, and determine the maximum permissible value based on the determined progress.

[0024] The term "second real electrical variable" is used to distinguish it from the first real electrical variable purely formally. It is intended to express that the real electrical variable used by the control device to determine the progress of the melting of the metal in the arc furnace is not necessarily the same as the real electrical variable that is brought as close as possible to the target electrical variable. This is possible, but not absolutely necessary. For example, the first real electrical variable may be the electrode current, while the second real electrical variable may be the voltage applied to or the power flowing through the electrode. However, in individual cases, these may also be the same real electrical variable.

[0025] As with the determination based on the time elapsed since the start of the melting stage, when the determination is based on the determined progress, the maximum allowable value may initially have a relatively low value and be increased in one step, multiple steps, or continuously (linearly or non-linearly) when the process reaches a certain progress stage. The maximum allowable value may be raised up to and even beyond the maximum possible value.

[0026] The problem is further solved by a control program having the features of claim 6. According to the invention, processing of the machine code by the control device causes the control device to operate the arc furnace according to the operating method according to the invention.

[0027] The problem is further solved by a control device with the features of claim 7. 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 the operating method according to the invention.

[0028] The problem is further solved by an arc furnace with the features of claim 8. According to the invention, the control device is configured as a control device according to the invention.

[0029] The above-mentioned features, characteristics and advantages of the present invention, as well as the manner in which they are obtained, will become clearer and more easily understood in connection with the following description of an embodiment, which is set out in more detail in conjunction with the drawings, in which: [Brief explanation of the drawings]

[0030] [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. 1 shows the furnace vessel at the flat bath stage. [Figure 5] FIG. [Figure 6] Location map. [Figure 7] FIG. DETAILED DESCRIPTION OF THE INVENTION

[0031] According to Figure 1, the electric arc furnace has a furnace vessel 1. The furnace vessel 1 can be charged with a metal 2 (see Figure 2). The metal 2 is charged into the furnace vessel 1 in the form of a solid mass. The metal 2 can be, for example, steel, and in the case of steel, in particular scrap.

[0032] The arc furnace further comprises an energy supply device 3, which is connected on the input side to a supply network 4. The supply network 4 is typically a medium voltage network, has a nominal voltage in the double-digit kV range and operates at a fundamental frequency f0, which is typically 50 Hz or 60 Hz. As shown in Figure 1, the supply network 4 is typically a three-phase supply network.

[0033] The arc furnace further comprises a furnace transformer 5 and electrodes 6. The energy supply device 3 is connected on the output side to the electrodes 6 via the furnace transformer 5. As shown in FIG. 1, there are typically several electrodes 6, and the furnace transformer 5 is configured as a three-phase transformer. However, other embodiments, particularly single-phase embodiments, are also possible. Regardless of the specific embodiment, however, the electrode voltage U applied to the electrodes 6 is significantly lower than the nominal voltage of the supply network 4. The electrode voltage U is shown in FIG. 1 for only one of the electrodes 6. In most cases, the electrode voltage U is in the range of several hundred volts. In individual cases, voltages above 1 kV are also possible. However, it generally does not exceed 2 kV.

[0034] Generally, there is also a switching device present, which can disconnect the energy supply device 3 from the supply network 4. There may also be a switching device present, which can disconnect the energy supply device 3 from the furnace transformer 5 and / or the furnace transformer 5 from the electrodes 6. The switching device performs a purely binary switching process, but does not regulate the voltage and current. Furthermore, active or passive filter devices may be arranged on the primary or secondary side of the furnace transformer 5. Since the switching and filter devices are of secondary importance for the functioning according to the invention, they are not shown in FIG. 1 (and other figures) for the sake of clarity.

[0035] 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 a number of semiconductor switches for this purpose. Possible embodiments of the energy supply device 3 are described in WO 2007 / 024994 ("Gold Standard"). Alternatively, for example, embodiments according to WO 2007 / 024994 or WO 2007 / 024994 can be used. However, regardless of the specific embodiment of the energy supply device 3, the energy supply device 3 can, on the output side, i.e. towards the furnace transformer 5, perform a quasi-continuous staging of the electrode voltage U applied to the electrodes 6 and / or the electrode current I supplied to the electrodes 6. As with the electrode voltage U, the electrode current I is also shown in FIG. 1 for only one of the electrodes 6.

[0036] The electrode voltage U can reach a maximum value U0 (see Figures 5 to 7). The value U0, i.e. the maximum possible value of the electrode voltage U, is determined by the nominal voltage of the supply network 4, the configuration of the energy supply device 3 and the configuration of the furnace transformer 5. For example, the value U0 can be 1200 V.

[0037] Furthermore, the arc furnace has a positioning device 7, by means of which the electrodes 6 can be positioned, as shown by the double arrow 8 next to the electrodes 6 in FIG. 1. In the simplest case, joint positioning of the electrodes 6 is performed. However, individual positioning of the electrodes 6 can also be performed. The direction of movement in which the electrodes 6 are positioned can be vertical. Alternatively, the direction of movement can be slightly inclined to the vertical. However, also in this case the vertical component is the dominant component of the movement. The positioning device 7 can have, for example, one or more hydraulic cylinder units.

[0038] Finally, the arc furnace has a control device 9. The control device 9 controls at least the energy supply device 3. The control device 9 generates a control value A1 that is used to control the energy supply device 3. The energy supply device 3 operates according to the control value A1.

[0039] The positioning device 7 is often also controlled by the control device 9, in which case the control device 9 further generates control values A2 used to control the positioning device 7. In this case, the positioning device 7 operates in accordance with these control values A2. However, such control of the positioning device 7 itself is not the subject of the present invention, and therefore will not be described in detail.

[0040] The controller 9 is configured as a software programmable controller, which is indicated in Figure 1 by the notation "μP" (for microprocessor controlled). The function and method of operation of the controller 9 are determined by a control program 10 into which the controller 9 is programmed. The control program 10 includes machine code 11 processable by the controller 9. Processing of the machine code 11 by the controller 9 causes the controller 9 to operate the arc furnace according to a method of operation as will be described in detail below in connection with other figures.

[0041] First, according to Fig. 3, in step S1, metal 2 is charged into the furnace vessel 1. The charging can be performed under the control of the control device 9. However, it does not have to be performed under the control of the control device 9. Therefore, step S1 is shown by a dashed line in Fig. 3.

[0042] The charging of the metal 2 into the furnace vessel 1 is followed by a melting stage of the arc furnace. In the melting stage, the metal 2 is melted to form molten metal 12. The melting stage comprises steps S2 to S6. The melting stage is followed by a flat bath stage. The flat bath stage comprises steps S7 to S11.

[0043] In the melting stage, the control device 9 first calculates the target variable X in step S2. * Receive the target variable X * is an electrical variable related to the electrical energy to be supplied to the electrode 6. * can be, for example, a target current or a target power.

[0044] In step S3, the control device 9 sets the final voltage target value U2 * Determine the final voltage target value U2 * To determine this, the control device 9 first determines a provisional voltage setpoint U1 based on the electrical setpoint X* (and, if necessary, also taking into account a first electrical actual variable X characteristic of the electrical energy supplied to the electrode 6). * Determine the provisional voltage target value U1 * The determination of the provisional voltage target value U1 * When a voltage U corresponding to the first electrical actual variable X is applied to the electrode 6, the first electrical actual variable X corresponds to the corresponding target variable X. * That is, for example, an attempt is made to make the actual current or actual power as close as possible to the target current or target power. However, the provisional voltage target value U1 determined in this way *is limited upward to the maximum allowable value Umax in step S3. The maximum allowable value Umax (see Figures 5 to 7) is less than the maximum possible value U0. The maximum allowable value Umax is usually between 50% and 75% of the maximum possible value U0. If the maximum possible value U0 is 1200V, the maximum allowable value Umax will be between 600V and 900V, for example.

[0045] Next, in step S4, the control device 9 determines the control value A1 of the energy supply device 3. This determination is performed based on the final voltage target value U2 * In step S5, the control device 9 controls the energy supply device 3 according to the determined control value A1. Based on the corresponding control, the final voltage target value U2 * A voltage U corresponding to is applied to the electrode 6. The energy supply 3 thereby draws electrical energy from the supply network 4 and supplies it to the electrode 6 via the furnace transformer 5. As a result, an arc 13 is formed.

[0046] In step S6, the control device 9 checks whether an end condition has been reached. The end condition may be that the melting stage has ended. The melting stage ends when the molten metal 12 forms a completely or at least approximately continuous horizontal surface, as shown in FIG. 4 . This means that the metal 2 is completely melted, or that elements of the metal 2 that have not yet melted are completely below the surface of the molten metal 12, or that elements of the metal 2 that have not yet melted slightly protrude above the surface of the molten metal 12. Furthermore, a slag layer 14 may have formed on the surface of the molten metal 12. Alternatively, the end condition may be, for example, that an initial stage within the melting stage has been completed, such as the completion of the drilling stage. However, regardless of the configuration of the end condition, the end condition is generally met only a few minutes after the start of the melting stage.

[0047] The control device 9 can evaluate actual variables of the arc furnace that are detected by measurement technology within the scope of the check as to whether the end condition has been reached. 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 variables of the arc furnace, such as the noise level or the sound spectrum of the generated noise. Alternatively, an operator 15 (see FIG. 1) can indicate to the control device 9 that the end condition has been reached.

[0048] If the end condition has not yet been reached, the control device 9 returns to step S2 (alternatively, step S3). On the other hand, if the end condition has been reached, the control device 9 proceeds to step S7, where the arc furnace enters a further operating phase. The further operating phase may be, for example, a flat bath phase. Alternatively, the further operating phase may be a melting phase followed by a flat bath phase.

[0049] In a further operation phase of the arc furnace, the control device 9 calculates the target variable X in step S7. * Furthermore, the control device 9 receives the final voltage target value U2 in step S8. * Next, in step S9, the control device 9 determines a control value A1 for the energy supply device 3. In step S10, the control device 9 controls the energy supply device 3 in accordance with the determined control value A1.

[0050] Steps S7 to S10 basically correspond to steps S2 to S5. The only difference is that the controller 9 sets the provisional voltage target value U1 in step S8. * The final voltage target value U2 * The alternative is to accept it directly as Umax, i.e., not to limit it to the maximum allowed value Umax.

[0051] In step S11, the control device 9 checks whether a further operating phase of the arc furnace has ended. Similar to the check in step S6, the control device 9 can evaluate actual variables of the arc furnace detected by measurement technology within the scope of the check in step S11. Alternatively, the operator 15 can also indicate to the control device 9 that a further operating phase has ended.

[0052] If the further operational phase has not yet ended, the control device 9 returns to step S7 (alternatively, step S8). On the other hand, if the further operational phase has ended, the control device 9 proceeds to step S12. In step S12, the formed molten metal 12 is removed from the furnace vessel 1 and poured, for example, into a ladle (not shown). Removal of the molten metal 12 may be under the control of the control device 9. However, it does not have to be under the control of the control device 9. Therefore, step S12 is shown in FIG. 3 with a dashed line, just like step S1.

[0053] It is also possible to omit steps S6 to S10 and instead return from step S11 to step S2 or step S3. In this case, consideration of the maximum allowable value Umax is carried out throughout the entire operation of the arc furnace.

[0054] There are various possibilities for determining the maximum allowable value Umax.

[0055] For example, the maximum permissible value Umax can be determined by the control device 9 as a function of time t, as shown in FIG. 5. In this case, time t1 corresponds to the start of the melting phase, i.e., in principle, to the first ignition of the arc 13 after the furnace vessel 1 is charged with the metal 2. At time t1, the maximum permissible value Umax generally has a minimum value. Thereafter, the maximum permissible value Umax can increase to higher values continuously, alternatively in one or more steps, as shown in FIG. 5. In particular, it can reach the maximum possible value U0 at time t2. In the embodiment according to FIG. 5, the control device 9 determines the maximum permissible value Umax directly based on the time elapsed since the start of the melting phase: if the time elapsed since the start of the melting phase is known, the associated currently valid maximum permissible value Umax can also be determined.

[0056] Alternatively, the maximum allowable value Umax can be determined by the control device 9 as a function of the position p of the electrode 6, as shown in FIG. 6. FIG. 6 is depicted such that the position p corresponds to the distance of the underside of the electrode 6 from the cover 15 of the reactor vessel 1. In general, the maximum allowable value Umax has its minimum value at the minimum position p (i.e., the minimum distance from the cover 15) and increases to higher values continuously, alternatively in one or more steps, with increasing distance, as shown in FIG. 6. In particular, the maximum allowable value Umax is at its lowest value up to a first predetermined position p and can reach the maximum possible value U0 at a second predetermined position p. In the embodiment according to FIG. 5, the control device 9 determines the maximum allowable value Umax based on the position p of the electrode 6.

[0057] Obviously, also in the embodiment according to Fig. 6, the maximum permissible value Umax consequently depends on the time t, but the relevant functional relationship is not known in advance, and in particular it may happen that the positioning p does not continuously increase, but temporarily takes on a smaller value again.

[0058] Similarly, the control device 9 can determine the progress of the melting of the metal 2 in the arc furnace (similar to steps S6 and S11) by evaluating the time course of the actual electrical variable of the electric energy supplied to the electrodes 6 and / or by evaluating the actual acoustic variables of the arc furnace. For example, the control device 9 can thus recognize the transition from the piercing phase to the remaining part of the melting phase and from the melting phase to the flat bath phase. In this case, the control device 9 can determine the maximum allowable value Umax based on the determined progress. For example, the control device 9 can maintain the maximum allowable value Umax at a relatively low value during the piercing phase, maintain it at a relatively high value (but still below the maximum possible value U0) during the remaining part of the melting phase, and leave it unaccounted for during the flat bath phase (or alternatively set it to a value equal to or above the maximum possible value U0), as shown in FIG. 7.

[0059] Similarly, as shown in Figure 1, it is also possible for the control device 9 to receive an input value E from the operator 16. In this case, the control device 9 can determine the maximum allowable value Umax based on the input value E. The input value E can be used to determine the maximum allowable value Umax alone, as needed, or can be taken into account in addition to any of the procedures of Figures 5 to 7.

[0060] The invention has many advantages, in particular it ensures that flashover of the arc 13 onto the cover 16 of the reactor vessel 1 and the associated disadvantages are avoided.

[0061] Although the present invention has been illustrated and described in more detail through preferred embodiments, the present invention is not limited to the disclosed embodiments, and those skilled in the art may derive other modifications without departing from the scope of protection of the present invention. [Explanation of symbols]

[0062] 1 Furnace vessel 2 metal 3 Energy supply device 4 Supply network 5. Furnace transformers 6 electrodes 7 Positioning device 8 Double Arrow 9 Control Device 10 Control Program 11 Machine Code 12 Molten Metal 13. Arc 14 Slag layer 15 Operator 16 Cover A1, A2 control values E Input value f0 Fundamental frequency I electrode current p Positioning p1, p2 Predetermined positioning S1~S12 steps t time Time points t1 and t2 U electrode voltage U0 maximum possible value Umax Maximum allowable value X * goal variable X real variable

Claims

1. A method for operating an electric arc furnace, during which a metal (2) present in a furnace vessel (1) of the electric arc furnace is melted, the control device (9) of the electric arc furnace comprising: - the electrical target variable (X ＊ ), based on the provisional voltage target value (U1 ＊ ) is determined, thereby determining the provisional voltage target value (U1 ＊ When a voltage (U) corresponding to the voltage (U) is applied to the electrode (6), a first actual electrical variable (X) of the electrical energy supplied from the electrode (6) is equal to the electrical target variable (X ＊ ) as close as possible to - Final voltage target value (U2 ＊ ) based on which the energy supply device (3) of the arc furnace is controlled, thereby achieving the final voltage target value (U2 ＊ ) is applied to the electrodes (6), and the energy supply device (3) draws electrical energy from the supply network (4) and supplies it to the electrodes (6) via a furnace transformer (5), The control device (9) controls the temporary voltage target value (U1 ＊ ) to the maximum allowable value (Umax), thereby ＊ ) and the maximum permissible value (Umax) is equal to or less than the maximum possible value (U0) that can be applied to the electrodes (6) by the energy supply device (3).

2. 2. The method of claim 1, wherein the control device (9) receives an input value (E) from an operator (15), and the control device (9) determines the maximum permissible value (Umax) based on the input value (E).

3. 2. A method according to claim 1, characterized in that the control device (9) determines the maximum permissible value (Umax) based on the time elapsed since the start of the melting phase.

4. 2. The method according to claim 1, characterized in that the position (p) of the electrode (6) changes over time, the position (p) of the electrode (6) is known to the control device (9), and the control device (9) determines the maximum permissible value (Umax) based on the position (p) of the electrode (6).

5. 2. The method according to claim 1, characterized in that the control device (9) determines the progress of melting of the metal (2) in the arc furnace by evaluating the time course of a second actual electrical variable (U, I) of the electrical energy supplied to the electrodes (6) and / or by evaluating an actual acoustic variable of the arc furnace, and that the control device (9) determines the maximum permissible value (Umax) on the basis of the determined progress.

6. 6. A control program for a control device (9) of an arc furnace, the control program comprising machine code (11) processable by the control device (9), the processing of the machine code (11) by the control device (9) causing the control device (9) to operate the arc furnace according to the operating method of any one of claims 1 to 5.

7. A control device for an electric arc furnace, the control device being programmed with a control program (10) according to claim 6, thereby operating the electric arc furnace according to the operating method according to any one of claims 1 to 5.

8. An electric arc furnace, the arc furnace has a furnace vessel (1) which can be charged with metal (2), the arc furnace comprises an energy supply (3) and electrodes (6) and a furnace transformer (5); - said energy supply device (3) is connected on the input side to the supply network (4) and on the output side to said electrodes (6) via said furnace transformer (5), the arc furnace comprises a control device (9) capable of controlling said energy supply device (3), - an electric arc furnace, wherein the control device (9) is constructed as claimed in claim 7;

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