Arc furnace with more than a three-phase feed

Abstract

An arc furnace (2) has a number of electrodes (3) which can be provided in a furnace vessel (4) of the arc furnace (2) such that energy can be introduced into a metal material (6) located in the furnace vessel (4) via the electrodes (3). Each of the electrodes (3) is connected to one of a plurality of phases (9) of a feeding supply system (8). Phase currents (I1 to In) supplied to the electrodes (3) for the phases (9) can be set independently of one another by the feeding supply system (8) with the exception of one phase (9). Alternatively, phase voltages (U1 to Un) present at the electrodes (3) can be set independently of one another by the feeding supply system for the phases (9) with the exception of one phase (9). The number of phases (9) of the feeding supply system (8) is greater than three. Furthermore, the supply system (8) comprises a transformer assembly (10) which is connected directly or indirectly to the electrodes (3) on the secondary side, and a converter assembly (11) is provided upstream of the transformer assembly (10).

Description

[0001] Description

[0002] Title of the invention

[0003] Electric arc furnace with more than three-phase feed

[0004] field of technology

[0005] The present invention relates to an energy input device for an electric arc furnace,

[0006] - wherein the electric arc furnace has a number of electrodes that can be arranged in a furnace vessel of the electric arc furnace, so that energy can be introduced into metallic material located in the furnace vessel via the electrodes,

[0007] - wherein the electrodes are each connected to one of several phases of a feeding supply system,

[0008] - wherein phase currents supplied to the electrodes can be adjusted independently of each other by the supplying system for the phases except one phase, or phase voltages present at the electrodes can be adjusted independently of each other by the supplying system for the phases except one phase,

[0009] - where the number of phases of the feeding supply system is greater than three,

[0010] - wherein the supply system comprises a transformer arrangement which is directly or indirectly connected to the electrodes on the secondary side.

[0011] State of the art

[0012] Such electric arc furnaces and the associated energy input devices are known, for example, from JP 2004 096 897 A. Similar disclosures can be found in CN 217 483 230 U and EP 1 848 248 A1.

[0013] Summary of the invention

[0014] In an electric arc furnace, scrap metal is melted. The scrap is placed in the furnace vessel. Then, typically three electrodes are lowered into the vessel, and an electric arc is generated between the scrap and each electrode. The arc heats the scrap until it melts and may also heat the molten metal further. The electric arc furnace is usually supplied with three-phase alternating current via a step-down transformer. Typical electrode phase voltages range from several hundred volts to just over 1 kV, and in extreme cases, up to 2 kV. Typical power ratings for an industrial electric arc furnace are between 50 MW and 300 MW. The step-down transformer is switchable, allowing the heating power of the electric arc furnace to be adjusted by switching the transformer – possibly in conjunction with the positioning of the electrodes over the scrap or the molten metal.

[0015] The constantly flickering electric arc causes feedback effects on the power supply network. These effects, commonly referred to as flicker, can, for example, cause fluctuations in brightness in lighting fixtures connected to the power supply network. Avoiding flicker is an important aspect of operating an electric arc furnace.

[0016] Various approaches exist that attempt to standardize power consumption from the supply network as much as possible. However, all state-of-the-art approaches suffer from certain shortcomings.

[0017] In the case of an energy input device of the type mentioned above, the problem arises, for example, that if one of the arcs in a typical three-electrode arc furnace breaks, the currents through the two remaining electrodes must be inversely proportional to each other and thus, in particular, equal in magnitude. However, these currents can only be maintained for a very short time (in the range of a few milliseconds) using a conventional transformer arrangement. Afterward, a zero crossing of the currents is inevitable. Consequently, the power consumption fluctuates considerably. Furthermore, the arc interruption itself often lasts a significant amount of time. Therefore, network disturbances cannot be avoided if one of the arcs breaks.

[0018] It is also known to arrange a converter on the secondary side of the transformer assembly. In this case, if one of the arcs breaks, a direct current can flow through the two remaining electrodes until the extinguished arc reignites. While the power of the arc furnace may have to be reduced, at least a zero crossing of the power can be avoided. However, a disadvantage of this solution is that the converter must be able to handle the high secondary-side current, which can be well over 100 kA.

[0019] The object of the present invention is to create possibilities by means of which the disadvantages of the prior art can be avoided.

[0020] The problem is solved by an energy input device with the features of claim 1. Advantageous embodiments of the energy input device are the subject of dependent claims 2 to 12.

[0021] According to the invention, an energy input device of the type mentioned above is designed by placing a converter arrangement upstream of the transformer arrangement. Due to the use of more than three phases, there is redundancy of at least one phase, so that even if the arc breaks at one electrode, arcs are still present at at least three electrodes, thus maintaining a three-phase system. This means that even if no electrical power is drawn via the corresponding phase during the zero crossing of the alternating current at one of the at least three electrodes, the power draw as such can be maintained overall and even kept exactly or at least substantially constant by the current flow via the at least two remaining phases.

[0022] The converter arrangement is considered upstream of the transformer arrangement when the transformer arrangement is positioned between the converter arrangement and the electrodes. In this case, the converter arrangement can be designed as a multilevel converter. This allows for the use of significantly smaller cross-sections for current transmission than if the converter were downstream of the transformer arrangement. It is also considerably easier to avoid or suppress network feedback. Furthermore, smoothing a DC voltage used within the converter is significantly simpler because considerably smaller capacitances suffice at higher voltages. Comparably larger capacitances are practically impossible to achieve if the converter is positioned downstream of the transformer. To compensate, the transformer would need to have many secondary windings.Since this is unnecessary when the converter assembly is located upstream of the transformer assembly, the transformer assembly can be designed much more simply. Furthermore, the available installation space between a typical furnace transformer and the electrodes is usually very small. It presents considerable difficulties to install a converter assembly there.

[0023] In many cases, the transformer assembly is designed as a step-down transformer. In this case, the electrical energy supplied to the arc furnace can be adjusted by means of the corresponding switching stage.

[0024] In the prior art, the number of switching stages is often considerably greater than two. However, within the scope of the present invention, it is also possible for the transformer arrangement to be switchable only between two switching stages or not at all. This is made possible by the fact that the phase voltages and phase currents can be adjusted over a wide range by means of the upstream converter arrangement.

[0025] Furthermore, the transformer arrangement can be configured as required. For example, it is possible for the transformer arrangement to include at least one multiphase transformer. This also applies if the transformer arrangement is directly connected to the electrodes on the output side. In this case, the multiphase transformer can be designed for the number of phases of the supply system on both the input and output sides. It is also possible for the multiphase transformer to be designed for only three phases, but for the remaining phases of the supply system, at least one additional multiphase transformer or a corresponding number of single-phase transformers are provided. It is also possible to provide one single-phase transformer for each phase of the supply system.

[0026] On the input side, the converter arrangement can have a conventional three-phase configuration. On the output side, however, the converter arrangement must be able to provide the number of phases of the power supply system. Both of these requirements apply regardless of whether the converter arrangement is upstream or downstream of the transformer arrangement. Furthermore, the converter arrangement can be configured as required. Preferably, the converter arrangement comprises an input-side rectifier with a number of input phases (as already mentioned, typically three input phases) and an output-side converter with a number of output phases, thus functioning as a DC link converter. The number of output phases corresponds to the number of phases of the power supply system.

[0027] The input-side rectifier can be configured as either an uncontrolled or a controlled rectifier, depending on requirements. In both cases, the input-side rectifier and the output-side converter each have one phase arm for each input and output phase, respectively. If the rectifier is configured as a controlled rectifier, the phase arms can be identical, allowing any phase arm of the rectifier and any phase arm of the converter to be interchanged without affecting the performance of the converter arrangement.

[0028] The phase arms are therefore preferably interchangeable or compatible with one another. This simplifies both the electrical and mechanical design of the phase arms and the control of semiconductor switches located within them. Furthermore, spare parts inventory is simplified, as the same semiconductor switches can be installed in a phase arm of the input-side rectifier or in a phase arm of the output-side converter as needed. Another advantage is that, despite the identical design of the phase arms, the fact that the power supply system ultimately needs to be larger on the output side than on the input side can be accommodated. This larger design results from the fact that the number of phases in the power supply system can be greater than the number of phases on the input side.

[0029] As a rule, the energy input device includes a control unit that manages the power supply system. Preferably, the control unit is given a setpoint for the total power to be supplied to the electrodes. This allows the control unit to adjust the power supply system as closely as possible to the setpoint, ensuring that the total power supplied to the electrodes, as defined by the instantaneous values ​​of the phase currents and phase voltages applied to the electrodes, is as close as possible to the setpoint.

[0030] The phrase "where possible" was chosen because it is possible for too many arcs to break. If only two arcs remain, a constant power output cannot be maintained, especially at the zero crossing of the current. However, it is quite possible for the control unit to maintain the control of the power supply system, so that the total power supplied to the electrodes is as close as possible to the setpoint, even if one of the phases is not carrying current during an arc break. It is even possible for the control unit to maintain the control of the power supply system, so that the total power supplied to the electrodes is as close as possible to the setpoint, as long as at least three of the phases are carrying current.

[0031] If the power supply system has four phases, the two aforementioned situations are identical. However, if the power supply system has five or more phases, the two situations differ, since even when two arcs break, at least three phases will still be carrying current.

[0032] In a conventional three-phase arc furnace, the electrodes are arranged to form the vertices of an equilateral triangle. An analogous configuration involves arranging the electrodes in a regular polygon when there are more than three electrodes, as is the case here. However, with more than three electrodes, it is also possible, for example, for one electrode to form a central electrode around which the remaining electrodes are arranged in a regular polygon. Alternatively, the electrodes in the arc furnace can be arranged in a row, either one behind the other or side by side.

[0033] Brief description of the drawings

[0034] The properties, features, and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more readily understandable in connection with the following description of an exemplary embodiment, which is explained in more detail in conjunction with the drawings. These drawings show:

[0035] FIG 1 shows an energy input device and an electric arc furnace, FIG 2 shows an energy input device,

[0036] FIG 3 a converter arrangement,

[0037] FIG 4 a supply system and a control device,

[0038] FIG 5 a flowchart and

[0039] FIGS. 6 to 8 show possible arrangements of electrodes.

[0040] Description of the embodiments

[0041] According to FIG. 1, an energy input device 1 for an electric arc furnace 2 has a number of electrodes 3. Of the electrodes 3, only one is labeled with its reference numeral in FIG. 1. The electrodes 3 can be arranged in a furnace vessel 4 of the electric arc furnace 2. In particular, as indicated by a double arrow 5 in FIG. 1, they can be lowered from above onto a metallic material 6 (for example, scrap metal) or lifted off the metallic material 6 again. Energy can be introduced into the material 6 via the electrodes 3 – more precisely, via electric arcs 7 that form between the material 6 and the electrodes 3. This heats the material 6 so that it melts. After melting, the temperature of the now molten material 6 can be further increased by the electric arcs 7. Again, only one of the electric arcs 7 is labeled with its reference numeral in FIG. 1.

[0042] The electrodes 3 are supplied with electrical energy via a power supply system 8. The power supply system 8 thus feeds the electrodes 3. To supply the electrodes 3, each electrode 3 is connected to a phase 9 of the power supply system 8. Typically, there is a 1:1 relationship between the electrodes 3 and the phases 9, so that exactly one electrode 3 is connected to each phase 9. However, it is also possible for several electrodes 3 to be connected to one, several, or even all phases 9. The number of phases 9 of the supplying power supply system 8 is greater than three. FIG. 1 shows a configuration in which the number n of phases 9 is five. However, the number n of phases 9 can also be four (minimal configuration) or greater than five.The supply system 8 generates phase voltages U1 to Un (here with n = 5) for the phases 9, so that (after the ignition of the arcs 7) phase currents 11 to In (here with n = 5) flow via the phases 9, which are supplied to the electrodes 3.

[0043] The phase currents 11 must satisfy the condition that the (signed) sum of the phase currents 11 to In is zero at any given time. This can be the case, for example, if the neutral point within the supply system 8 is present but not grounded, or not present at all. In any case, however, the phase currents 11 to In supplied to the electrodes 3 can only be set independently of one phase 9 by the supplying system 8. The ability to set them independently is indicated in FIG. 1 by the fact that setpoint values ​​11* to In* for the phase currents 11 to In can be specified to the supply system 8, whereby the setpoint In* for the phase current In is placed in parentheses, since this setpoint In* cannot be specified independently.

[0044] Similarly, the phase voltages U1 to 11n for phases 9 may have to satisfy the condition that the (signed) sum of the phase voltages U1 to 11n equals zero at any given time. This can be the case, for example, if (in a generalization of a delta connection) the n-corner voltages are tapped. Just as with the phase currents 11 to 11, in such cases the phase voltages U1 to 11n applied to the electrodes 3 can also be set independently of each other by the supply system 8 for phases 9, with the exception of one phase 9. The ability to set them independently is indicated in FIG. 1 by the fact that setpoint values ​​U1* to 11n* for the phase voltages U1 to 11n can be specified to the supply system 8, although the setpoint value 11n* for phase voltage 11n is enclosed in parentheses because this single setpoint 11n* cannot be specified independently.

[0045] According to FIG. 2, the supply system 8 comprises a transformer assembly 10. The transformer assembly 10 is connected directly or indirectly to the electrodes 3 on the secondary side. The transformer assembly 10 can, for example, be designed as a non-switchable transformer assembly. Furthermore, a converter assembly 11 is arranged upstream of the transformer assembly 10. The transformer assembly 10 can be designed as a non-switchable or as a switchable transformer assembly, i.e., as a step-down transformer. It may be sufficient if the transformer assembly 10 is switchable only between a few switching stages—in particular, two switching stages.

[0046] The specific configuration of the transformer arrangement 10 can be chosen as required. For example, the transformer arrangement 10 can include at least one multiphase transformer and / or at least one single-phase transformer.

[0047] The converter arrangement 11, as shown in FIG. 2, has a number of input phases 12. The number of input phases 12 can be the usual three. However, the converter arrangement 11 has more than three output phases 15. The number of output phases 15 of the converter arrangement 11 corresponds to the number of phases 9 of the supply system 8. The further configuration of the converter arrangement 11 can be as required. In particular, the converter arrangement 11 can have an input rectifier 13 and an output converter 14, as shown in FIG. 3. In this case, the rectifier 13 has the input phases 12, and the converter 14 has the output phases 15. The input rectifier 13 can be configured as an uncontrolled rectifier. Preferably, however, it is designed as a controlled rectifier, as shown in FIG. 3.

[0048] The input-side rectifier 13 has phase arms 16, one for each input-side phase 12. The phase arms 16 can be identical to each other, so that they could be interchanged without affecting the performance of the converter arrangement 11. For example, the phase arms 16 can each be designed for a nominal voltage of 110 kV, a nominal current of 3 kA, and a nominal power of 100 MW (or generally the same nominal voltage, the same nominal current, and the same nominal power). The interchangeability of the phase arms 16 is indicated in FIG. 3 by the dashed arrows connected above the rectifier 13.

[0049] Similarly, the output-side converter 14 also has phase arms 17, one for each output-side phase 15. The phase arms 17 can be identical to one another, so that they could be interchanged without affecting the performance of the converter arrangement 11. For example, the phase arms 17 – like the phase arms 16 – can each be designed for a nominal voltage of 110 kV, a nominal current of 3 kA, and a nominal power of 100 MW (or generally the same nominal voltage, the same nominal current, and the same nominal power). The interchangeability of the phase arms 17 is indicated in FIG. 3 by the dashed arrows connected to each other above the converter 14.

[0050] Furthermore, the identity preferably exists not only between the phase arms 16 among themselves and between the phase arms 17 among themselves, but also across the entire range of phase arms 16 and 17. Thus, it is also possible to exchange a phase arm 16 of the rectifier 13 with a phase arm 17 of the converter 14, as before, without impairing the performance of the converter arrangement 11. This is indicated in FIG. 3 for a single phase arm 16 of the rectifier 13 and a single phase arm 17 of the converter 14 by two dashed arrows connected below the converter arrangement 11.

[0051] According to the illustration in FIG. 3, the phase arms 16, 17 each have only two switches 18. However, this illustration is only meant to be generic. The "switches 18" can represent the arms of a multilevel converter.

[0052] The energy input device 1, as shown in FIG. 4, has a control unit 19. The power supply system 8 is controlled by the control unit 19. In particular, the inverter arrangement 11 is also controlled by the control unit 19. In a preferred embodiment of the present invention, a setpoint value P* is specified to the control unit 19. The setpoint value P* corresponds to a power that is to be supplied to the electrodes 3 in their entirety. In this case, the control unit 19 preferably determines the setpoint according to the relationship P*.

[0053] P = U1 - I1 + U2 - 12 + ... + Un - In (1) Based on the current instantaneous phase voltages U1 to Iln and phase currents 11 to In, the respective current instantaneous active power P is determined. Furthermore, it determines control signals C for the supply system 8 and controls the supply system 8 according to the control signals C. The determination of the control signals C is carried out, if possible, in such a way that the control unit 19 controls the supply system 8 at all times in such a way that the total power P supplied to the electrodes 3 is approximated as closely as possible to the setpoint P*. Other types of control, for example, quantities averaged over full or half periods of a supply network, such as the (full or at least half-period averaged) active power, reactive power, apparent power, or cos φ, are not used. <p, kann abgesehen werden.

[0054] The operating mode of the control unit 19 is explained in more detail below in conjunction with FIG. 5. It should be noted beforehand that the control unit 19 executes its control procedure cyclically. The cycle time is generally in the range of less than 1 ms to a maximum of a few milliseconds, usually in the range between 0.1 ms and 1.0 ms.

[0055] According to FIG. 5, the control unit 19 receives the setpoint P* for the power P in step S1. In step S2, the control unit 19 is informed of the current instantaneous phase voltages U1 to Un and the current instantaneous phase currents 11 to In. For example, the phase voltages U1 to Un and the phase currents 11 to In can be measured and the corresponding measured values ​​received by the control unit 19. In step S3, the control unit 19 determines the respective current instantaneous active power P according to equation 1.

[0056] In step S4, the control unit 19 checks whether one of the arcs 7 has been extinguished or broken. Essentially, the check in step S4 involves verifying whether the phase currents 11 to In have a value of 0. However, it must be taken into account that the phase currents 11 to In are alternating currents and therefore exhibit brief zero crossings even when the corresponding arc 7 is lit. Such a zero crossing must be distinguished from the extinguishing of an arc 7. This fact is generally known to those skilled in the art, and a distinction between a zero crossing of a phase current 11 to In and the extinguishing of an arc 7 is readily possible. For example, the breaking of an arc 7 can be detected if the corresponding phase current 11 to In has a value of 0 for a sufficiently long period of time.When all arcs 7 are lit, i.e., no arc 7 has extinguished, the control unit 19 proceeds to step S5. In step S5, the control unit 19 determines the phase voltages U1 to Un and / or the phase currents 11 to In for phases 9, which are to be set in the next cycle. The determination is carried out in such a way that the total power P supplied to the electrodes 3 is approximated as closely as possible to the setpoint P*.

[0057] If, however, the control unit 19 detects in step S4 that one of the arcs 7 has extinguished, the control unit proceeds to step S6. In step S6, the control unit 19 checks whether another of the arcs 7 has extinguished. Essentially, the check in step S6 is analogous to the check in step S4.

[0058] When all other arcs 7 are lit, meaning only a single arc 7 has extinguished, the control unit 19 proceeds to step S7. In step S7, the control unit 19 determines the phase voltages U1 to Un and / or the phase currents 11 to In for the phases 9, which are to be set in the next cycle. Step S7 essentially corresponds to step S5. However, when determining step S7, the control unit 19 takes into account that the current 11 to In for the extinguished arc 7 has the value 0. For example, if the arc 7 of the electrode 3 supplied via the nth phase 9 has extinguished, the control unit 19 determines the phase voltages U1 to Un-1 for the remaining phases 9 such that, in conjunction with the corresponding phase currents 11 to ln-1, the desired power P* is set. The phase voltage Un for the extinguished arc 7 can in principle be set arbitrarily.In practice, it will be adjusted in such a way as to favor the ignition of the extinguished arc 7. This can be achieved, for example, by setting the corresponding phase voltage Un to its minimum or maximum possible value.

[0059] The sequence of tests can continue until the control unit 19 checks in step S8 whether the third-to-last arc 7 has also extinguished. As long as three arcs 7 are still burning, the control unit 19 proceeds to step S9. In step S9, the control unit 19 determines the phase voltages U1 to Un and / or the phase currents 11 to In for the phases 9 of the still-burning arcs 7, which are to be set in the next cycle. The determination is carried out in such a way that the total power P supplied to the electrodes 3 is approximated as closely as possible to the setpoint P*. In principle, the phase voltages U1 to Un for the extinguished arcs 7 can be set arbitrarily. In practice, they will be set in such a way as to favor the ignition of the extinguished arcs 7.

[0060] If, however, the control unit 19 detects in step S8 that the third-to-last arc 7 has also extinguished, the control unit 19 proceeds to step S10. Now, only (exactly) two arcs 7 are available, so that the currents 11 to In for the two corresponding phases 9 must necessarily be equal in magnitude and have opposite signs. In this case, it is no longer possible to regulate the current, instantaneous active power P to the corresponding setpoint P*.

[0061] Step S9, i.e., the procedure in the case where exactly three arcs 7 are still burning (more precisely: where exactly three phase currents 11 to In can still have values ​​other than 0), can, for example, be designed as detailed in the applicant's two European patent applications 23 21 8774.0 and 23 21 8588.4, both filed on December 20, 2023. These procedures can also be generalized for more than three phases (step S5, and optionally also step S7 and, if applicable, steps not shown prior to the examination of step S8). The corresponding procedure is outlined below for the usual case where the sum of the phase currents 11 to In must always equal 0.

[0062] Assume a system with N phases. If the total number of phases of the supply system 8 is, for example, n = 5, the following explanations are valid for step S5 with N = 5 and for step S7 with N = 4.

[0063] The relationship always applies

[0064] P = Ul -Il + U2-I2 + ... + UN-IN . (2)

[0065] The relationship always remains valid.

[0066] Il + I2 + ... + IN = Q . (3)

[0067] We now define an orthonormal basis with respect to a normal vector v0. The normal vector v0 is given by

[0068] For N = 4, the corresponding basis vectors v1, v2 and v3 can be, for example, by or through be given.

[0069] The corresponding orthonormal basis can be readily determined. Every possible vector x in N-dimensional space that can be represented as a linear combination of the basis vectors is orthogonal to the normal vector v0. For N = 4, therefore, every vector x that can be represented by the relation x = xl • vl + x2 • v2 + x3 • v3 (7) is orthogonal to the normal vector v0.

[0070] In the following, x denotes a space vector. The space vector x must be distinguished from the vector x. In particular, the space vector x has dimension N-1, while the vector x has dimension N.

[0071] It is evident that, given an orthonormal basis, the space vector x and the vector x can be interconverted. This requires the corresponding orthonormal transformation matrix B, which can be readily calculated. For the orthonormal basis according to equations 5, the transformation matrix B is, for example, given by defined. For the orthonormal basis according to equations 6, the transformation matrix B is, for example, by defined.

[0072] For orthonormal matrices, and therefore also for the transformation matrices B according to equations 8 and 9, the following relationship holds:

[0073] B T = B~ . (10)

[0074] For the transformation matrix B according to equation 9, the following relationship also clearly holds.

[0075] The conversion from the space vector x in (N1)-dimensional space to the vector x in N-dimensional space is performed according to the relationship

[0076] The reverse conversion from the vector x in N-dimensional space to the space vector x in (Nl)-dimensional space is performed according to the relationship

[0077] The value xO is referred to below as the zero component.

[0078] The preceding equations apply equally to the phase currents 11 to IN and the phase voltages U1 to UN. If I denotes the current vector in N-dimensional space, the current vector I corresponds to a space vector I and a zero component IO. Due to the condition of equation 3, the zero component IO has the value 0. Analogously, U can denote the voltage vector in N-dimensional space. The voltage vector U corresponds to a space vector U and a zero component UO. The zero component UO can have the value 0 or a non-zero value.

[0079] Equation 2 can also be written as the scalar product of the current vector I and the voltage vector U: = EF -7 . (14)

[0080] It can easily be shown that the scalar product of the space vectors U and I yields the same value. This is because it also holds true that...

[0081] Because 10 = 0, the product of the zero components UO and 10 vanishes. Therefore, the following relationship also holds:

[0082] P = lf -I_ . (16)

[0083] It is thus possible to construct the space vector U for the voltage vector U from two components, starting from the space vector I for the current vector I. One component runs parallel to the space vector I for the current vector I and is referred to below as the parallel component. The length of the parallel component must be determined such that the dot product of the space vector I for the current vector I with the parallel component yields the desired power P*. The other component is determined within the (N-1)-dimensional space in which the space vectors U and I are defined, within the subspace that is orthogonal to the space vector I for the current vector I. This component runs orthogonally to the space vector I for the current vector I and is therefore referred to below as the orthogonal component. The dot product of the space vector I for the current vector I with the orthogonal component always yields the value 0 and therefore does not contribute to the power P.The orthogonal component can therefore be determined as needed. As already explained, the zero component UO also does not contribute to the power P. It, too, can therefore be determined as needed. For example, the orthogonal component and the zero component UO can be determined in such a way that the transformer arrangement 10 does not enter the saturated range. However, other possibilities for determining the orthogonal component and the zero component UO are also available.

[0084] The arrangement of the electrodes 3 in the electric arc furnace 2 can be as required. For example, the electrodes 3 in the electric arc furnace 2 can be arranged in the form of a regular polygon, as shown in FIG. 6; in the case of a total of five electrodes 3, for example, in the form of a regular pentagon. Alternatively, as shown in FIG. 7, one of the electrodes 3 can be arranged centrally, forming a central electrode 3. In this case, the remaining electrodes 3 can be arranged around the central electrode 3 in the form of a regular polygon. In the case of a total of five electrodes 3, the remaining electrodes 3 can therefore be arranged at the corners of a square, in the center of which is the central electrode 3. Alternatively, as shown in FIG. 8, the electrodes 3 in the electric arc furnace 2 can be arranged in a row, one behind the other or side by side.

[0085] The present invention has many advantages. In particular, even if a limited number of arcs 7 are interrupted, it is still possible to regulate the output to a constant or only slowly changing target power P*. This is possible as long as at least three arcs 7 are still burning. Consequently, even if an arc 7 should be interrupted for an extended period, the arc furnace 2 can continue to be supplied with a constant or at least only slowly changing electrical power P.

[0086] Although the invention has been further illustrated and described in detail by the preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived from them by the person skilled in the art without leaving the scope of protection of the invention.

[0087] Reference symbol list

[0088] 1 Energy input device

[0089] 2 Electric arc furnace

[0090] 3 electrodes

[0091] 4 Ovenware

[0092] 5 Double Arrow

[0093] 6 metallic material

[0094] 7 arcs

[0095] 8 Supply system

[0096] 9 phases

[0097] 10 transformer arrangement

[0098] 11 Inverter arrangement

[0099] 12 input-side phases

[0100] 13 rectifiers

[0101] 14 inverters

[0102] 15 output-side phases

[0103] 16, 17 Phase arms

[0104] 18 switches

[0105] 19 Control unit

[0106] C Control signals

[0107] 11 to In phase currents

[0108] 11* to In* Setpoint values ​​for the phase currents

[0109] P current, instantaneous active power

[0110] P* Setpoint for the current, instantaneous active power

[0111] S1 to S10 steps

[0112] U1 to Un phase voltages

[0113] U1* to Un* setpoint values ​​for the phase voltages

Claims

Claims 1. Energy input device for an electric arc furnace (2), - wherein the electric arc furnace (2) has a number of electrodes (3) that can be arranged in a furnace vessel (4) of the electric arc furnace (2), such that energy can be introduced into metallic material (6) located in the furnace vessel (4) via the electrodes (3), - wherein the electrodes (3) are each connected to one of several phases (9) of a supply system (8), - wherein the phase currents (11 to In) supplied to the electrodes (3) by the supplying system (8) are independently adjustable for the phases (9) with the exception of one phase (9), or the phase voltages (U1 to Un) applied to the electrodes (3) are independently adjustable by the supplying system for the phases (9) with the exception of one phase (9), - where the number of phases (9) of the feeding supply system (8) is greater than three, - wherein the supply system (8) comprises a transformer arrangement (10) which is directly or indirectly connected to the electrodes (3) on the secondary side, characterized in that a converter arrangement (11) is arranged upstream of the transformer arrangement (10).

2. Energy input device according to claim 1, characterized in that the transformer arrangement (10) is designed as a step-down transformer.

3. Energy input device according to claim 2, characterized in that the transformer arrangement (10) can only be switched between two switching stages.

4. Energy input device according to claim 1, 2 or 3, characterized in that the transformer arrangement (10) comprises at least one multiphase transformer.

5. Energy input device according to one of the above claims, characterized in that the transformer arrangement (10) comprises at least one single-phase transformer.

6. Energy input device according to one of the above claims, characterized in that the converter arrangement (11) comprises an input-side rectifier (13) with a number of input-side phases (12) and an output-side converter (14) with a number of output phases (15) and that the number of output phases (15) matches the number of phases (9) of the supply system (8).

7. Energy input device according to claim 6, characterized in that the input-side rectifier (13) is designed as a controlled rectifier.

8. Energy input device according to claim 7, characterized in that the input-side rectifier (13) and the output-side converter (14) each have a phase arm (16, 17) for each input-side and output-side phase (12, 15) respectively, and that the phase arms (16, 17) are identical, so that any phase arm (16) of the rectifier (13) and any phase arm (17) of the converter (14) could be exchanged without impairing the performance of the converter arrangement (11).

9. Energy input device according to one of the above claims, characterized in that the energy input device has a control device (19) by which the supply system (8) is controlled, that a setpoint (P*) for a total power (P) to be supplied to the electrodes (3) is specified to the control device (19), and that the control device (19) controls the supply system (8) as far as possible at all times in such a way that the total power (P) supplied to the electrodes (3), as defined by the instantaneous values ​​of the phase currents (11 to In) supplied to the electrodes (3) and the phase voltages (U1 to Un) applied to the electrodes (3), is approximated to the setpoint (P*) as closely as possible.

10. Energy input device according to claim 9, characterized in that the control device (19) maintains the control of the supply system (8) so that the total power (P) supplied to the electrodes (3) is approximated as closely as possible to the setpoint (P*), even when one of the phases (9) does not carry current during an arc interruption.

11. Energy input device according to claim 10, characterized in that the control device (19) maintains the control of the supply system (8) so that the total power (P) supplied to the electrodes (3) is approximated as closely as possible to the setpoint (P*), as long as at least three of the phases (9) carry a current.

12. Energy input device according to one of the above claims, characterized in that the electrodes (3) are arranged in the arc furnace (2) in the form of a regular polygon, or one of the electrodes (3) forms a central electrode around which the remaining electrodes (3) are arranged in the form of a regular polygon, or the electrodes (3) are arranged in a row, one behind the other or next to the other.

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