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 for operating an electric arc furnace, during which, during a melting phase, the metal present in the furnace vessel of the electric arc furnace is melted, the control device of the electric arc furnace comprising: - determining a provisional voltage setpoint based on an electrical setpoint variable of the electrical energy to be supplied by the electrode, so that when a voltage corresponding to the provisional voltage setpoint is applied to the electrode, a first actual electrical variable of the electrical energy supplied by the electrode approaches the electrical setpoint variable as closely as possible; - 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 supply 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 arc furnace, the control program comprising machine code processable by the control device, the processing of the machine code by the control device causing the control device to operate the 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 a furnace transformer, the arc furnace has a control device capable of controlling the energy supply device, The control device is constructed as described above. [Background technology]

[0005] The above-mentioned subject matter is generally known. Reference may be made, for example, to US Pat. No. 5,399,433. In this connection, reference may be made to US Pat. No. 5,499,433 and US Pat. No. 5,523,666.

[0006] When melting metals, especially steel, in an electric arc furnace, the supply of electrical energy to the electrodes of the electric arc furnace takes place via a furnace transformer, which is often connected to the supply grid via a medium voltage transformer. The furnace transformer supplies several voltage steps. For a certain power range and for other high current ranges, the respective voltage steps can be selected in the furnace transformer. Fine adjustment within a certain voltage step can take place, for example, by impedance control.

[0007] In this procedure, only some small voltage steps are possible and the electrode current is subject to strong fluctuations. To reduce these fluctuations, the positioning of the electrode is controlled mechanically, usually through a hydraulic adjustment device. The mechanical adjustment of the electrode has significantly lower dynamics than the real behavior of the arc. The fluctuations can therefore only be compensated poorly. Furthermore, the fluctuations result in significant loads on components such as high-current cables, current-carrying support arms, hydraulic cylinders, etc. The fluctuations occur both in the melting phase and in 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 to a specific impedance or a specific power output. However, since the dynamics of the positioning device are relatively small compared to the changes in the electrical system of the arc, some uncompensable fluctuations remain. This results in less than optimal energy input to the molten metal.

[0009] From the prior art, in particular from US Pat. No. 5,399,623 and US Pat. No. 5,493,637, and to a lesser extent from US Pat. No. 5,493,627, procedures are known that allow the electrode voltage to be adjusted continuously. These embodiments offer significant advantages over the adjustment of the electrode voltage through voltage steps of the furnace transformer: on the one hand, it is possible to vary the electrode voltage not only stepwise but also continuously; on the other hand, the furnace transformer can be of simpler construction, since it does not have to provide several voltage steps; moreover, these embodiments allow further 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 performed 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 cover of the arc furnace, but still far above, 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 destroy it 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 disadvantages of the prior art. [Means for solving the problem]

[0013] This problem is solved by a method for operating having the features of claim 1. Advantageous embodiments of the method for operating are the subject of dependent claims 2 to 5.

[0014] According to the invention, an operating method of the type mentioned at the beginning is configured in that the control device determines a final voltage setpoint by limiting the provisional voltage setpoint, at least in the initial phase of the melting phase, to a permissible maximum value, 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, if necessary, to a maximum of 1200 V in order to set the desired current of 5000 A. In contrast, in the present invention such adjustments are only made to values ​​below 1200 V, for example up to a maximum of 700 V. If a voltage value of more than 700 V is required to set a current of 5000 A, deviations in the current from the original desired value (=5000 A) are accepted. 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 figures 700 V, 1200 V and 5000 A given are exemplary values ​​for the purpose of illustration. In practice, larger or smaller values ​​may also occur. In particular the currents are 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 part of the melting phase and not in the later part 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, it is possible for the control device to receive an input value from an operator and for the control device to determine the maximum allowed value based on the input value. Alternatively, the input value can always determine the maximum allowed value, i.e., for example, if the input value is "500V", then the maximum allowed value is always 500V as well. Alternatively, the input value can be specified only in the sense of an upper limit value. In this case, the control device can first determine the maximum allowed value in a different way. If the value determined in this way is lower than the voltage value determined by the input value (for example, only 450V as opposed to 500V by the input value), then the value determined by the control device (450V) is used as the maximum allowed value. If the value determined by the control device is higher than the voltage value determined by the input value (for example, 550V), then the voltage value determined by the input value (500V) is used as the maximum allowed value. Here again, the numerical values ​​450V, 500V and 550V given are purely exemplary values ​​for the purpose of illustration. In practice, larger or smaller values ​​may also occur.

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

[0020] In another embodiment, the control device can determine the maximum allowable value, alternatively or additionally to the consideration of the input values, based on the time that has elapsed since the beginning of the melting phase. For example, the maximum allowable 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). In a later section, i.e. after at least the initial part of the melting phase, the maximum allowable value can be raised up to (and theoretically even higher than) the maximum possible value. In this case, the voltage limitation according to the invention is no longer active once the maximum possible value has been reached. The time of 5 minutes given is also merely an example. In practice, larger or smaller values ​​can occur.

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

[0022] As with the determination based on the time elapsed since the start of the melting phase, in the case of the determination based on the positioning of the electrodes, the maximum allowable value 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 in the later sections of the melting phase. The maximum allowable value may be raised up to and even beyond the maximum possible value.

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

[0024] The notion "second electrical real variable" is used to distinguish it purely formally from the first electrical real variable. It is intended to express that the electrical real variable used by the control device to determine the progress of the melting of the metal in the arc furnace is not necessarily identical to the electrical real variable which is brought as close as possible to the electrical target variable. This is possible, but not absolutely necessary. For example, the first electrical real variable may be the electrode current, while the second electrical real variable may be the voltage applied to the electrode or the power flowing through it. But also, in individual cases, these may be the same electrical real 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, in multiple steps, or continuously (linearly or non-linearly) when the process reaches a certain progress state. The maximum allowable value may be raised up to and even beyond the maximum possible value.

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

[0027] The object is furthermore achieved 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 object is furthermore achieved by an electric 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 invention and 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 described in more detail in conjunction with the drawings, in which: [Brief description of the drawings]

[0030] [Figure 1] FIG. 1 is a block diagram of an electric arc furnace. [Diagram 2] FIG. 2 shows the furnace vessel during the melting stage. [Diagram 3] FIG. [Figure 4] FIG. 2 shows the furnace vessel at the flat bath stage. [Diagram 5] FIG. [Figure 6] Location map. [Figure 7] FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0032] The arc furnace further comprises an energy supply device 3. The energy supply device 3 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 kV range and operates at a fundamental frequency f0. The fundamental frequency f0 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 generally several electrodes 6, and the furnace transformer 5 is further configured as a three-phase transformer. However, other embodiments are also possible, in particular single-phase embodiments. Regardless of the specific embodiment, however, the electrode voltage U applied to the electrodes 6 is clearly lower than the nominal voltage of the supply network 4. The electrode voltage U is shown in FIG. 1 only for one of the electrodes 6. In many cases, the electrode voltage U is in the range of several hundreds of volts. In individual cases, voltages of more than 1 kV are also possible. However, in general, it does not exceed 2 kV.

[0034] Generally, there is furthermore a switching device, which can disconnect the energy supply device 3 from the supply network 4. Furthermore, there may be a switching device, 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 devices and filter devices are of secondary importance for the function according to the invention, they are not shown in FIG. 1 (and in the 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 EP 1 339 636 ("Gold Standard"). Alternatively, for example, the embodiments according to EP 1 339 636 or EP 1 339 636 can be used. However, independently of the concrete 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 phasing 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 Figs. 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 may be 1200V.

[0037] Furthermore, the arc furnace has a positioning device 7, by means of which the electrodes 6 can be positioned, as indicated in FIG. 1 by a double arrow 8 next to the electrodes 6. In the simplest case, a joint positioning of the electrodes 6 is performed. However, also an individual positioning of the electrodes 6 can 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 for example have one or more hydraulic cylinder units.

[0038] Finally, the arc furnace has a control device 9. By means of the control device 9 at least the energy supply device 3 is controlled. The control device 9 generates control values ​​A1 which are used to control the energy supply device 3. According to the control values ​​A1 the energy supply device 3 is operated.

[0039] The positioning device 7 is often also controlled by the control device 9, which in this case further generates control values ​​A2 used to control the positioning device 7. In this case, the positioning device 7 operates according to these control values ​​A2. However, such control of the positioning device 7 itself is not the subject of the present invention, and will not be mentioned 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 manner of operation of the controller 9 is 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 more detail below in connection with the other figures.

[0041] First, according to Fig. 3, in step S1, the furnace vessel 1 is charged with metal 2. 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 in Fig. 3 by a dashed line.

[0042] The charging of the metal 2 into the furnace vessel 1 is followed by a melting stage of the arc furnace. During the melting stage, the metal 2 is melted to form a 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 * 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 variable X* (if necessary also taking into account a first actual electrical 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 bring the actual current or the actual power as close as possible to the target current or the target power. However, the provisional voltage target value U1 determined in this way *is limited upwards to a maximum permissible value Umax in step S3. The maximum permissible value Umax (see Figs. 5 to 7) is below the maximum possible value U0. The maximum permissible value Umax is usually between 50% and 75% of the maximum possible value U0. If the maximum possible value U0 is 1200V, then the maximum permissible value Umax is for example between 600V and 900V.

[0045] Next, the control device 9 determines the control value A1 of the energy supply device 3 in step S4. 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 setpoint 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 phase has ended. The melting phase 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 the elements of the metal 2 that are not yet melted are completely below the surface of the molten metal 12, or that the elements of the metal 2 that are 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 phase within the melting phase has been completed, for example, that the drilling phase has been completed. However, regardless of the configuration of the end condition, the end condition is generally only met a few minutes after the start of the melting phase.

[0047] The control device 9 can evaluate measurement-technically detected actual variables of the arc furnace within the scope of the check as to whether an 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 its fluctuations. The control device 9 can also evaluate acoustic variables of the arc furnace, for example the noise level or the sound spectrum of the generated noise. Alternatively, an operator 15 (see FIG. 1) can also indicate to the control device 9 that an end condition has been reached.

[0048] If the end condition has not yet been reached, the control device 9 returns to step S2 (alternatively to step S3). If, on the other hand, the end condition has been reached, the control device 9 proceeds to step S7 and transitions to a further operation phase of the arc furnace. The further operation phase may, for example, be a flat bath phase. Alternatively, the further operation phase may be a melting phase plus a flat bath phase.

[0049] In the further operation phase of the arc furnace, the control device 9 calculates the target variable X * 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 control device 9 sets the provisional voltage target value U1 in step S8. * The final voltage target value U2 * The alternative is to directly accept it as such, i.e. not to limit it to the maximum permitted value Umax.

[0051] In step S11, the control device 9 checks whether a further operating phase of the arc furnace has ended. Analogous 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 operating phase has not yet been completed, the control device 9 returns to step S7 (alternatively step S8). If, on the other hand, the further operating phase has been completed, 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). The 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. Step S12 is therefore shown in FIG. 3 with dashed lines, as is step S1.

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

[0054] Regarding the determination of the maximum allowable value Umax, there are various possibilities.

[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, the 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 the time t1, the maximum permissible value Umax generally has a minimum value. Thereafter, the maximum permissible value Umax can be increased to higher values ​​continuously, alternatively through one or more steps, as shown in FIG. 5. In particular, the maximum possible value U0 can be reached at the time t2. In the case of the embodiment according to FIG. 5, the control device 9 determines the maximum permissible value Umax directly based on the time that has elapsed since the start of the melting phase: if the time that has 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 positioning p of the electrode 6, as shown in Fig. 6. Fig. 6 is depicted such that the positioning 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 positioning p (i.e. the minimum distance from the cover 15) and rises 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 can be at its lowest value up to a first predetermined positioning p and reach the maximum possible value U0 at a second predetermined positioning p. In the case of the embodiment according to Fig. 5, the control device 9 determines the maximum allowable value Umax based on the positioning p of the electrode 6.

[0057] Obviously, also in the embodiment according to Fig. 6, the maximum permitted value Umax is consequently dependent on time t, but the relevant functional relationship is not known in advance, and in particular the case may arise where the positioning p does not rise continuously but temporarily takes on a smaller value again.

[0058] Likewise, it is possible for the control device 9 (similar to the procedures of steps S6 and S11) to determine the progress of the melting of the metal 2 in the arc furnace by evaluating the course over time of the electrical actual variables of the electrical energy supplied to the electrodes 6 and / or by evaluating the acoustic actual 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 on the basis of 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, and at a relatively high value (but still below the maximum possible value U0) during the remaining part of the melting phase, and leave it unconsidered 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 Fig. 1, it is also possible for the controller 9 to receive an input value E from the operator 16. In this case, the controller 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 appropriate, or can be taken into account in addition to any of the procedures of Figs. 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 other variations may be devised by those skilled in the art 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 At time 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, comprising the steps of: during a melting stage, a metal (2) present in a furnace vessel (1) of the electric arc furnace is melted; and a control device (9) of the electric arc furnace comprises: - the electrical target variable (X ＊ ), a provisional voltage target value (U1 ＊ ), 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 the energy supply device (3) of the arc furnace, thereby controlling 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 provisional voltage target value (U1 ＊ ) to the maximum allowable value (Umax), thereby ＊ ) wherein the permissible maximum 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).

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

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

4. 4. The method according to claim 1, characterized in that a position (p) of an 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) on the basis of the position (p) of the electrode (6).

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

6. A control program for a control device (9) of an arc furnace, the control program comprising a 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 according to any one of claims 1 to 5.

7. A control device for an electric arc furnace, 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. 1. An electric arc furnace, comprising: the electric arc furnace comprises a furnace vessel (1) which can be charged with metal (2), the arc furnace comprises an energy supply (3) and electrodes (6) as well as 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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