POWER SUPPLY DEVICE FOR A METALLURGICAL DEVICE

Description

[0001] The invention relates to a power supply device for a metallurgical device to be operated with electrical energy with a non-linear load, such as an electric arc furnace with a power consumption greater than or equal to 180 MVA.

[0002] Power supply devices for metallurgical devices to be operated with electrical energy are known from EP 3 943 853 A1 (US 11,320,203 B2), WO 2021 / 130791 A1, US 11,346,605 B2, US 2022 / 0412651 A1, WO 2021 / 161355 A1, EP 4 110 015 A1, WO 2015 / 176899 A1, EP 1 026 921 B1 and FR 2 926 182 A1. WO 2007 / 014535 A1 discloses a transformer system for an electric arc furnace with three electrodes, which has at least two three-phase transformers that are internally triangulated, connected in parallel to the electrodes and can be switched on and off independently of one another.

[0003] In the course of decarbonization, the European steel industry has begun a transformation from an integrated route of steel production with blast furnaces and converters to an electric arc steel route with electric arc furnaces for scrap and H2-based direct reduction iron / sponge iron (also called DRI = Direct Reduction Iron) in briquetted form (also called HBI = Hot Briquetted Iron). Currently, the share of the electric arc steel route in Germany, for example, is around 30%. Due to the targeted CO2 reduction, this share is to increase to 100% by the mid-2030s. The current tapping masses of the converters are around 150 t to 250 t and cannot be changed due to the necessary retention of the installed ladles and casting systems and the necessary productivity. This has consequences for the design of the electric arc furnaces that are to replace the converters.

[0004] It is an object of the invention to improve the power supply for a metallurgical device to be operated with electrical energy and having a non-linear load, such as an electric arc furnace, in particular for an electric arc furnace to be used in an existing plant with a blast furnace and / or converter.

[0005] This object is achieved by a power supply device according to claim 1.

[0006] Further developments of the invention are specified in the dependent claims.

[0007] In the power supply device according to claim 1, two furnace transformers which are small in relation to the required total power are connected in parallel.

[0008] Structurally equivalent transformers mean that the transformers do not differ in their operating characteristics and that their mass, dimensions, and connections are largely the same. This is naturally the case for identical transformers. Therefore, structurally equivalent transformers are preferably identical. However, transformers with minor differences that do not result in different operating characteristics are considered structurally equivalent transformers. Minor differences in mass, dimensions, or connections that do not impede the interchangeability of the transformers are also considered structurally equivalent.

[0009] The two structurally equivalent furnace transformers are designed with an open secondary winding and are externally connected in a delta (circuit group Diii0).

[0010] The two external delta connections are connected in parallel at their corners, i.e., connected in a specific way via additional conductors. Therefore, the conductors carry only half the electrode current on each side until these conductors are connected.

[0011] The conductor routing of the low-voltage parallel circuit, i.e. the arrangement or routing of the conductors in the high-current system, can therefore be dimensioned smaller than in the case in which the entire electrode current must be guided through one conductor over its entire length.

[0012] Such a design is achieved, for example, by simulating the electrical network using a special method for calculating electromagnetic fields. This allows the impedances of the entire high-current system to be calculated with great accuracy, including all current displacement effects, and designed symmetrically.

[0013] Such a symmetrical design leads on average to the same properties of the three arcs (power, radiation, length) and has significant advantages in operation.

[0014] Since the two structurally equivalent furnace transformers are designed with an open secondary winding and externally connected in a delta (vector group Diii0), the furnace transformers are structurally simpler and therefore more reliable, lighter and smaller.

[0015] Another advantage of such a parallel connection of two structurally equivalent or identical smaller furnace transformers compared to one with twice the power is that the production of furnace transformers with 90 to 150 MVA is routine compared to the production of individual furnace transformers with a power of 180 MVA and above. Several manufacturers can build such standard sizes up to 120 MVA, and some up to 150 MVA.

[0016] Such transformers, with comparatively lower but still high power outputs greater than or equal to 90 MVA, are also less complicated because, for example, they only have an iron core with windings and no booster. The entire internal circuitry is simpler. This improves reliability compared to a single transformer with the required power output.

[0017] The dimensions and mass of individual transformers, with comparatively lower but still high power ratings greater than or equal to 90 MVA, are significantly smaller than those of a single large furnace transformer. A 100 MVA furnace transformer has a total mass of approximately 130 t and is relatively easy to transport, install, and remove, usually using the charging crane at the steelworks.

[0018] Furnace transformers that are more or less standard models are more cost-effective than custom-made and very large transformers. Manufacturing costs do not scale linearly with power or mass.

[0019] Since it is highly unlikely that both structurally equivalent furnace transformers will fail at the same time, one structurally equivalent reserve transformer is sufficient for the usual stockpiling of a replacement transformer. This means, for example, instead of two gigantic 200 MVA furnace transformers, one for operation and one as a reserve, three 100 MVA standard furnace transformers, two for operation and one as a reserve, are sufficient. The cost of three smaller standard furnace transformers is lower than the cost of two comparatively very large non-standard furnace transformers. Furthermore, replacing a defective transformer with the replacement transformer is also much easier than with a single transformer with twice the power (≥ 180 MVA).

[0020] Arranging the two furnace transformers so that they face each other with the low-voltage side connections saves space and in turn makes replacement even easier in the event of a defect.

[0021] Further features and advantages will become apparent from the description of exemplary embodiments based on the figures. The figures show: Fig. 1 shows a power supply device for a metallurgical device to be operated with electrical energy and having a non-linear load, namely an electric arc furnace, according to a first embodiment with a medium-voltage furnace busbar and a power consumption of 200 MVA; Fig. 2 shows a power supply device for a metallurgical device to be operated with electrical energy and having a non-linear load, namely an electric arc furnace, according to a second embodiment with two medium-voltage furnace busbars and a power consumption of 280 MVA; Fig. 3 shows a schematic representation of the electrical connections of part of a power supply device for an electric arc furnace with two structurally equivalent furnace transformers connected in parallel and an electric arc furnace in one embodiment; Fig. 4 shows a secondary-side electrical equivalent circuit diagram (ESB) with furnace transformers connected in parallel; Fig.5 shows an electrically symmetrical arrangement of water-cooled high-current conductors in the secondary-side high-current system of a low-voltage parallel connection of the output-side low-voltage connections of two structurally equivalent arc furnace transformers with symmetrical external delta connections; and Fig. 6 shows a schematic representation of the electrical connections of part of a power supply device corresponding to the representation in . Fig. 3 with reference symbol from Fig. 5 corresponds.

[0022] Fig. 1 shows a power supply device for a metallurgical device to be operated with electrical energy and having a non-linear load, namely an electric arc furnace, according to a first embodiment in a simplified single-line view. In the single-line view, the three phases of the three-phase voltage are represented by only one line instead of three. Throughout the application and in all embodiments of the invention, the input and output voltages of the transformers are always the three phases of a three-phase voltage, offset by 120°, unless expressly stated otherwise or necessarily different, such as the output voltage for a single-phase output.

[0023] The power supply device has a high-voltage supply rail 500, which is connected to the general electrical power supply, such as a 400 kV line 520 or two 220 kV lines 520a, 520b. The general electrical power supply supplies a three-phase voltage (in each case) on the 400 kV line 520 or on the two 220 kV lines 520a, 520b. The power supply device has at least two structurally equivalent supply transformers 160, 260, each of which has an input-side high-voltage connection 160h, 260h for the three phases and an output-side medium-voltage connection 160m, 260m for the three phases. The inputs of the input-side high-voltage connection 160h, 260h are connected to the high-voltage supply rail 500.

[0024] The power supply device has a medium-voltage furnace busbar 140. The outputs of the output-side medium-voltage connection 160m, 260m of the supply transformers 160, 260 are connected to the medium-voltage furnace busbar 140.

[0025] The power supply system comprises two structurally equivalent, here identical, three-phase arc furnace transformers 100, 200.

[0026] Fig. 3 schematically shows the electrical connections of a part of the power supply system for an electric arc furnace with two furnace transformers connected in parallel on the output side and an electric arc furnace in one embodiment. Each of the two three-phase arc furnace transformers 100, 200 has an input-side three-phase medium-voltage connection 1U-100, 1V-100, 1W-100 or 1U-200, 1V-200, 1W-200, as shown in Fig. 3 Each of the two three-phase arc furnace transformers 100, 200 has a delta connection D on the input side. The medium-voltage furnace busbar 140 is connected to the input-side three-phase medium-voltage terminals / inputs 1U-100, 1V-100, 1W-100 and 1U-200, 1V-200, 1W-200 of the arc furnace transformers 100, 200.

[0027] Such a medium-voltage furnace busbar is a medium-voltage switchgear, as is generally available in the state of the art, but in this case for two furnace transformers.

[0028] Each of the two three-phase arc furnace transformers 100, 200 has an output-side three-phase low-voltage connection 2U1-100, 2U2-100, 2V1-100, 2V2-100, 2W1-100, 2W2-100 or 2U1-200, 2U2-200, 2V1-200, 2V2-200, 2W1-200, 2W2-200 and an external interconnection (iii) on the output side. This means that the two winding ends—i.e., the beginning of the winding U1, V1, W1 and the end of the winding U2, V2, W2—of all three windings U, V, W on the secondary side / output side (low-voltage side) of the three-phase arc furnace transformers 100, 200 are led out of the transformer, as indicated by iii. This type of connection or output is also referred to as open windings.

[0029] The output-side low-voltage connections 2U1-100, 2U2-100, 2V1-100, 2V2-100, 2W1-100, 2W2-100, 2U1-200, 2U2-200, 2V1-200, 2V2-200, 2W1-200, 2W2-200 of the arc furnace transformers 100 and 200 are connected in a low-voltage parallel circuit 400 externally, i.e., outside the transformers, in a delta configuration so that the current flows are symmetrical. For this purpose, the impedances of the entire high-current system, i.e., the low-voltage connections in which large (high) currents flow, are calculated with all current displacement effects and designed symmetrically.

[0030] These external delta connections are realized by means of water-cooled high-current conductors connected to three electrodes 11, 12, 13 of the metallurgical device (10), as will be described in more detail below.

[0031] In detail, as in Fig. 3 is shown, a (first) winding start 2U1-100 of a first secondary-side winding 2U of the first arc furnace transformer 100 of the two arc furnace transformers 100, 200 with a (third) winding end 2W2-100 of a third secondary-side winding 2W of the first arc furnace transformer 100, a (first) winding end 2U2-100 of the first secondary-side winding 2U of the first arc furnace transformer 100 with a (second) winding start 2V1-100 of a second secondary-side winding 2V of the first arc furnace transformer 100, and a (second) winding end 2V2-100 of the second secondary-side winding 2V of the first arc furnace transformer 100 with a (third) winding start 2W1-100 of the third secondary winding 2W of the first arc furnace transformer 100.

[0032] In the same way, in the second arc furnace transformer 200 of the two arc furnace transformers 100, 200, as also shown in Fig. 3 shown, a (first) winding start 2U1-200 of a first secondary-side winding 2U of the second arc furnace transformer 200 with a (third) winding end 2w2-200 of a third secondary-side winding 2W of the second arc furnace transformer 200, a (first) winding end 2U2-200 of the first secondary-side winding 2U of the second arc furnace transformer 200 with a (second) winding start 2V1-200 of a second secondary-side winding 2V of the second arc furnace transformer 200, and a (second) winding end 2V2-200 of the second secondary-side winding 2V of the second arc furnace transformer 200 with a (third) winding start 2W1-200 of the third secondary winding 2W of the second arc furnace transformer 200.

[0033] The correspondingly pairwise interconnected low-voltage terminals or winding starts and winding ends 2U1-100 and 2W2-100, 2U2-100 and 2V1-100, 2V2-100 and 2W1-100 of the first arc furnace transformer 100 are connected in parallel (connected) to the correspondingly pairwise interconnected low-voltage terminals or winding starts and winding ends 2U1-200 and 2W2-200, 2U2-200 and 2V1-200, 2V2-200 and 2W1-200 of the second arc furnace transformer 200, iethe winding start 2U1-100 and the winding end 2W2-100 are connected in parallel in a node 2 with the winding start 2U1-200 and the winding end 2W2-200, the winding end 2U2-100 and the winding start 2V1-100 are connected in parallel in a node 1 with the winding end 2U2-200 and the winding start 2V1-200, and the winding end 2V2-100 and the winding start 2W1-100 are connected in parallel in a node 3 with the winding end 2V2-200 and the winding start 2W1-200, as shown in . Fig. 3 is shown. In this low-voltage parallel circuit 400 outside the transformers, the corresponding voltages are at nodes 1, 2, and 3.

[0034] An electric arc furnace 10, operated with three-phase current, has three electrodes 11, 12, 13. These nodes 1, 2, 3 of the parallel circuit 400 are connected to the electrodes 11, 12, 13 of the electric arc furnace 10 via lines 111, 112, 113.

[0035] The parallel circuit 400 and the electric arc furnace 10 are in Fig. 1 shown schematically. In Fig. 1 According to the illustration in single-line view, only one output connection of the furnace transformers 100, 200 and one electrode of the arc furnace 10 and one line are shown, but of course there are three of each according to the three-phase operation.

[0036] Fig. 4 shows a secondary electrical equivalent circuit diagram of the parallel connected furnace transformers from Fig. 1 and 3 . For each delta branch of the external secondary transformer circuit, there are two voltage sources U 12-1 , U 12-2 , and U 23-1 , U 23-2 , and U 31-1 , U 31-2 , instead of just one voltage source as is normally the case with only one furnace transformer. Fig. 4 The boxes shown next to the voltage sources represent the decoupled impedances of the high-current conductors. The boxes labeled I 11, I 12, and I 13 represent the impedances of the corresponding electrodes 11, 12, and 13 and their connecting lines 111, 112, and 113 to nodes 1, 2, and 3. The node labeled 0 represents the electrical star point in the melt, which is conductively connected to the earthing of the furnace vessel of the arc furnace 10.

[0037] The power supply device for a metallurgical device to be operated with electrical energy made of Fig. 1 It features two structurally equivalent, preferably identical, supply transformers 160 and 260, each with a power rating of 100 MVA, based on the ONAN (Oil Natural Air Natural) cooling type. The ONAF (Oil Natural Air Forced) cooling type, with which these transformers are also equipped (installed fans), increases the drawable power by a specified value, e.g., +20% or +30%.

[0038] A third structurally equivalent, preferably identical, supply transformer 360 is preferably installed as a reserve so that if one of the other two supply transformers 160, 260 fails, it can be replaced by the third supply transformer 360 with minimal downtime.

[0039] The supply transformers 160 and 260 transform the input voltage of 400 kV or 220 kV to a medium voltage of 33 kV to 35 kV, typical for furnace transformer operation, at their output-side medium-voltage terminals 160m and 260m for the three phases. These are connected to the medium-voltage furnace busbar 140.

[0040] Such a feeder uses only standard components, a clear advantage for electrical maintenance. A semiconductor-based reactive current compensator is connected to the medium-voltage furnace busbar 140 to compensate for reactive currents. It is designed, for example, as a STATCOM compensation 150 with 220 MVA (Static Synchronous Compensator = STATCOM). The STATCOM 150 is connected to the medium-voltage furnace busbar 140 and can, for example, optionally and advantageously also be used to compensate for the reactive currents of the general busbar (rolling mills, etc.) to a certain extent if this is connected to the same high-voltage busbar as the arc furnaces. The STATCOM is a proven technology and capable of meeting the requirements for limit values ​​of the power supply (flicker, harmonics, asymmetry).

[0041] One or more ladle furnaces (LF) can also be connected to the medium-voltage furnace busbar 140. More specifically, one or more corresponding ladle furnace transformers for the ladle furnace(s) may be connected to the medium-voltage furnace busbar 140. Depending on the power of the ladle furnaces, the power of the supply transformers 160, 260 may need to be adjusted.

[0042] The power supply system comprises two structurally equivalent, preferably identical, three-phase arc furnace transformers 100, 200, each with a power output of 100 MVA. The arc furnace transformers 100, 200 are OFWF (oil-forced water-forced) cooled. A third structurally equivalent, preferably identical, three-phase arc furnace transformer 300 is preferably installed as a backup.

[0043] Each of the arc furnace transformers 100, 200 is fed separately and has an outgoing switch on the common medium-voltage furnace busbar 140 as well as a furnace switch 110, 210.

[0044] Ideally, a furnace switch is used to prevent the strong inrush current that otherwise always occurs during transformer inrush. This extends the lifespan of the transformers and reduces the load on the entire supply system.

[0045] In the embodiment shown, the two three-phase arc furnace transformers 100, 200 are each connected to the medium-voltage furnace busbar 140 via a furnace switch such as a VCB switch (VCB = Vacuum Circuit Breaker) 110, 210.

[0046] Common three-phase arc furnace transformers have tap changers that allow the transformer's secondary voltages to be adjusted on the high-current system under load. Synchronization of the tap changers of both arc furnace transformers must be ensured. This is not a problem from a control engineering perspective.

[0047] The two furnace transformers are preferably arranged so that the low-voltage side connections are opposite each other between the furnace transformers, as shown in Fig. 3 The furnace transformers are positioned on a lateral side of the arc furnace 10. This not only saves space but also facilitates replacement. Fig. 3 Rails 100s are shown for one furnace transformer 100, by means of which the corresponding furnace transformer 100 can be moved into its position. In Fig. 3 Rails 200s are shown for the other furnace transformer 200, by means of which the corresponding other furnace transformer 200 can be moved into its position opposite the position of the one furnace transformer 100 as described on one lateral side of the furnace 10. In Fig. 3 are labeled with 100k and 200k cooling systems. Fig. 3 In the furnace transformers 100, 200 shown, the cooling systems 100k and 200k are arranged on one lateral side of the furnace transformers 100, 200, but it is of course also possible to use furnace transformers 100, 200 in which cooling systems are present on both sides or at another location.

[0048] Theoretically, the furnace transformers could also be placed side by side in a conventional configuration. This would require considerably more structural width.

[0049] The mass of a 100 MVA furnace transformer is approximately 130 t. Therefore, such a 100 or 200 MVA furnace transformer is relatively easy to transport, install, and remove, for example, using a standard truck-mounted crane and, in most cases, even the charging crane of the existing steelworks.

[0050] Since the ladle size of integrated steelworks is around 180 t to 250 t, e.g. 220 t of liquid steel, it is necessary for the planned replacement of the converters to also tap 220 t of liquid steel from a single electric arc furnace (EAF) 10. In this case, very high electrical power is needed to melt scrap and DRI (HBI) in order to achieve the required TTT (tap-to-tap time = time between two tappings), especially since converters are usually faster than EAFs. When operating an EAF with 180 t to 250 t of tapping mass, electrical power of 180 MVA and more is required. A 180 MVA furnace transformer, for example, would be dimensioned for a maximum current of 115 kA, which would require electrodes with a diameter of 750 mm.

[0051] A single 180 MVA furnace transformer would already have a mass of at least 200 t and a maximum current of approximately 115 kA. The required conductor cross-sections in the transformer for currents of 100 kA and above are problematic due to current displacement and cooling.

[0052] The described parallel connection of two identical furnace transformers with external parallel delta connection Diii halves the current in the corresponding parallel conductors up to nodes 1, 2, 3.

[0053] An electrically symmetrical configuration of the parallel furnace transformer system is possible. The goal is for each transformer winding to carry the same delta branch current, so that the transformer windings are loaded symmetrically at the same electrode currents. The high-current system of the arc furnace must be included in the final design of the line geometry. Such a design is achieved, for example, by simulating the electrical network using a special method for calculating electromagnetic fields. This allows the impedances of the entire high-current system to be calculated with high accuracy, including all current displacement effects, and designed symmetrically. One example of such a method is the finite network method (FNM) (see, for example, Abbas Farschtschi, "New Calculation System Solves Electromagnetic Problems in Electric Arc Furnaces," Stahl und Eisen 131 (2011) No.6 / 7, pages 93 to 104, or Abbas Farschtschi, "An advanced computation system to solve electromagnetic problems in arc fumaces" www.steeltimesint.com, Steel Times International, September 2011). Electrically symmetrical here means ≤3%. The electrical symmetry in percent is calculated from the equivalent star reactances of the conductor arrangement of the high-current system X 1 , X 2 , X 3 as Ux = (X_max - X_min) / X_average, where X denotes the reactance in mOhm. The value Ux is based on the standard configuration of the high-current system, which has identical electrode lengths below the holders.

[0054] Fig. 5 shows an electrically symmetrical arrangement of water-cooled high-current conductors in the secondary-side high-current system for a low-voltage parallel connection of the output-side low-voltage connections of two arc furnace transformers with symmetrical external delta connections, as shown in Fig. 3 which is designed by simulating the currents in the conductors. The same elements as in Fig. 3 are designated by the same reference numerals and their description is not repeated. The water-cooled high-current conductors are typically Cu tubes designed for an average current density of 6 A / mm 2 . The wall thicknesses and diameters of the Cu tubes are selected accordingly. For example, tubes with wall thicknesses of 10 to 15 mm, such as 12.5 mm, and diameters of 140 mm to 250 mm, such as 140 mm, 160 mm, 180 mm, 200 mm, 220 mm, or 240 mm, can be selected depending on the current intensities. The tubes are preferably laid horizontally in parallel in at least two parallel horizontal planes and vertically in at least two parallel vertical planes, both for cable suspension and to maintain appropriate spacing between the tubes while keeping the lengths as equal as possible. Fig. 5 There are four parallel horizontal planes and five parallel vertical planes. However, the number of these planes is not crucial and can be chosen according to the design.

[0055] In detail, as in Fig. 5 As shown, the (first) winding start 2U1-100 of the first secondary-side winding 2U of the first arc furnace transformer 100 of the two arc furnace transformers 100, 200 is connected via a Cu tube 21-100 and a Cu tube 22-100 connected thereto to the (third) winding end 2W2-100 of the third secondary-side winding 2W of the first arc furnace transformer 100, the (first) winding end 2U2-100 of the first secondary-side winding 2U of the first arc furnace transformer 100 is connected via a Cu tube 11-100 and a Cu tube 12-100 connected thereto to the (second) winding start 2V1-100 of the second secondary-side winding 2V of the first arc furnace transformer 100,and the (second) winding end 2V2-100 of the second secondary-side winding 2V of the first arc furnace transformer 100 is connected via a Cu tube 31-100 and a connected Cu tube 32-100 to the (third) winding start 2W1-100 of the third secondary-side winding 2W of the first arc furnace transformer 100.

[0056] In the same way, in the second arc furnace transformer 200 of the two arc furnace transformers 100, 200, as also shown in Fig. 3 shown, the (first) winding start 2U1-200 of the first secondary-side winding 2U of the second arc furnace transformer 200 is connected via a Cu tube 21-200 and a Cu tube 22-200 connected thereto to the (third) winding end 2W2-200 of the third secondary-side winding 2W of the second arc furnace transformer 200, the (first) winding end 2U2-200 of the first secondary-side winding 2U of the second arc furnace transformer 200 is connected via a Cu tube 11-200 and a Cu tube 12-200 connected thereto to the (second) winding start 2V1-200 of the second secondary-side winding 2V of the second arc furnace transformer 200, and the (second) winding end 2V2-200 of the second secondary-side winding 2V of the second arc furnace transformer 200 is connected via a Cu tube 31-200 and a Cu tube 32-200 connected thereto to the (third) winding start 2W1-200 of the third secondary-side winding 2W of the second arc furnace transformer 200.

[0057] The corresponding pairwise interconnected Cu tubes or winding starts and winding ends 2U1-100 and 2W2-100, 2U2-100 and 2V1-100, 2V2-100 and 2W1-100 of the first arc furnace transformer 100 are connected in parallel with the corresponding pairwise interconnected Cu tubes or winding starts and winding ends 2U1-200 and 2W2-200, 2U2-200 and 2V1-200, 2V2-200 and 2W1-200 of the second arc furnace transformer 200 by connecting a Cu tube 2-100 with the Cu tubes 21-100 and 22-100 and the node 2 and a Cu tube 2-200 with the Cu tubes 21-200 and 22-200 and the node 2 by connecting a Cu-pipe 1-100 to the Cu-pipes 11-100 and 12-100 and the node 1 and a Cu-pipe 1-200 to the Cu-pipes 11-200 and 12-200 and the node 1 by connecting a Cu-pipe 3-100 to the Cu-pipes 31-100 and 32-100 and the node 3 and a Cu-pipe 3-200 to the Cu-pipes 31-200 and 32-200 and the node 3, as shown in Fig. 5 is shown.

[0058] Fig. 6 shows the same schematic diagram of the electrical connections of the part of the power supply device that is Fig. 3 shown, with reference numerals from Fig. 5 , to clarify the correspondence of the electrical cables from Fig. 3 and the copper pipes from Fig. 5 .

[0059] Fig. 2 shows a power supply device for a metallurgical device to be operated with electrical energy and having a non-linear load, such as an electric arc furnace, according to a second embodiment. The same elements as in Fig. 1 are designated by the same reference numerals and the description thereof is not repeated.

[0060] The second embodiment is designed for a very large EAF with, for example, 220 t tapping mass and a power consumption of up to 280 MVA. The electrodes therefore have diameters of 750 mm to 800 mm. Larger electrodes are not available on the market. However, if the furnace sizes and current strengths require it and larger electrodes with a diameter of, for example, 850 mm were available, these could be used if necessary. As can be seen from Fig. 2 As can be seen, the two structurally equivalent, preferably identical, arc furnace transformers 100 and 200 each have a power output of 140 MVA. The two supply transformers 160 and 260 each have a power output of 150 MVA. The high-voltage supply busbar 500 is connected to the general electrical power supply, for example, via a 400 kV line 520 (or two 220 kV lines).

[0061] Unlike the first embodiment from Fig. 1 , are in the second embodiment in Fig. 2 Two medium-voltage furnace busbars 140a, 140b, each with separate semiconductor-based reactive current compensators (SVC+) 150a, 150b, each rated at 140 MVA, are provided. These can also be designed, for example, as STATCOM compensation systems, each rated at 140 MVA.

[0062] Of the two supply transformers 160, 260, one is connected with its output medium voltage connection 160m, 260m to a corresponding one of the medium voltage furnace rails 140a, 140b, as shown in Fig. 2 can be clearly seen, i.e. supply transformer 160 with medium-voltage furnace busbar 140a and supply transformer 260 with medium-voltage furnace busbar 140b.

[0063] The design of the two structurally equivalent, preferably identical arc furnace transformers 100, 200 and the low-voltage parallel circuit 400 has already been Fig. 1 , 3 , 4, 5 explained.

[0064] The power supply device is usually operated with a medium voltage in the range of 25 kV to 40 kV, preferably 30 kV to 35 kV, more preferably 33 kV to 35 kV, or the furnace transformers are designed for this purpose.

[0065] The power supply device is typically operated with a low voltage in the range of 1000 V to 2000 V, preferably from 1100 V to 1800 V, more preferably from 1100 V to 1600 V, such as in a range of 1000 V to 1400 V or in a range of 1100 V to 1600 V, or the furnace transformers are designed for this. The voltage range for the low voltage refers to the possible settings of the furnace transformers via tap changers.

[0066] The power supply device is usually operated with a high voltage in the range of 400 kV to 150 kV, preferably 400 to 380 kV or 180 kV to 220 kV, or the supply transformers are designed for this purpose.

[0067] With the described power supply system, it is possible to operate very large EAFs with tapping masses ≥ 180 t, such as 200 t or 220 t or 250 t, while avoiding the disadvantages of very large furnace transformers.

[0068] In principle, furnace transformers for three-phase arc furnaces with outputs greater than 180 MVA are feasible, and several are already installed. The largest furnace transformer built has an output of around 300 MVA. However, such gigantic units have significant disadvantages: 1) Manufacturing is not routine and very complicated. Very few manufacturers are capable of reliably building such large-scale transformers. Their size is limited by the crane capacity at the manufacturing plant and the size of the active part drying ovens. Furthermore, such furnace transformers are specialized products that require particular engineering and manufacturing expertise, and the market for such furnace transformers is small. 2) Such high outputs always require an intermediate circuit design (booster), which has two iron cores with windings and is therefore very large and heavy. 3) The total mass of such transformers is at least 240 t. Their large external dimensions are also problematic. This makes transport from the manufacturer to the steelworks very complicated.4) The required conductor cross-sections of the windings for electrode currents of more than 100 kA require correspondingly large windings, making reliable cooling difficult. 5) The current intensities at the medium-voltage level become so high (e.g. 3500 A with 200 MVA at 33 kV) that two parallel furnace switchgear may be necessary. The medium-voltage cables must be dimensioned accordingly. 6) The transport, installation, and removal of such units are even more problematic. Due to the integration of new arc furnace steelworks into existing integrated steelworks, available space is often very limited. This makes replacing a furnace transformer at a later date very difficult, if not impossible. For example, a 1000 t mobile crane might be required to lift a transformer over the roof of the hall. The availability of such special cranes is very limited, and waiting times of 6 months are typical.7) The larger and more complex a transformer, the greater the susceptibility to faults. Simpler is more reliable. Experience shows that furnace transformers can fail at any time or must be shut down before a fatal fault occurs in order to remain repairable in principle. In this case, the rapid installation of a reserve is necessary. An identical reserve is therefore required. However, the giants among furnace transformers can practically only be repaired on site, as transporting them back to the transformer factory is too costly, especially if the transformer comes from abroad. An on-site repair or replacement of windings and the subsequent high-voltage test would be very complex. In the special case of an installed reserve, a very large transformer house would be required, which would have to support around 480 t.

[0069] The significant advantages of connecting two structurally equivalent, preferably identical, smaller furnace transformers in parallel are therefore evident. The production of, for example, 100...120 MVA furnace transformers is routine in comparison. There are more manufacturers capable of building such standard sizes. Such transformers are obviously less complicated, having, for example, only an iron core with windings (no booster). The entire internal wiring is simpler. This improves reliability. The dimensions and weight are significantly smaller. A 100 MVA furnace transformer has a total mass of approximately 130 t and is relatively easy to transport, install, and remove. Furnace transformers that conform to a standard are more cost-effective. Because the furnace transformers are designed with an open secondary winding and externally connected in a delta configuration (vector group Diii0), they are structurally simpler, more reliable, lighter, and smaller.The two external delta connections are connected in parallel at their corner points. Until these conductors are connected, each side carries only half the electrode current. The arrangement and routing of the conductors in the high-current system can be designed using simulation so that the entire high-current system is electrically symmetrical. This results in the same average properties of the three arcs (power, radiation, length) and has significant operational advantages. It is very unlikely that both furnace transformers will fail at the same time. Therefore, a structurally equivalent, preferably identical, reserve is sufficient.

[0070] The maximum power consumption of the metallurgical device is preferably less than or equal to 320 MVA, more preferably less than or equal to 280 MVA, even more preferably less than or equal to 240 MVA.

[0071] The Fig. 1 und 2The arrangements described for the high-voltage medium-voltage supply can also be replaced by other arrangements, since the specific design of the arrangements for the high-voltage medium-voltage supply is not important for the described and claimed low-voltage side design of the furnace transformers and their interconnection.

[0072] It is explicitly emphasized that all features disclosed in the description and / or the claims are to be considered separate and independent of each other for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, regardless of the feature combinations in the embodiments and / or the claims. It is explicitly stated that all range specifications or specifications of groups of units disclose every possible intermediate value or subgroup of units for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, in particular also as a limit of a range specification.

Claims

1. Power supply device for metallurgic equipment to be operated with electric energy with non-linear load comprising a maximum power consumption greater than or equal to 180 MVA, such as an electric arc furnace with a power consumption greater than or equal to 180 MVA, comprising at least two structurally equivalent three-phase furnace transformers (100, 200) with an output power greater than or equal to 90 MVA, each of the furnace transformers comprising an input-side three-phase medium voltage connector (1U-100, 1V-100, 1W-100; 1U-200, 1V-200, 1W-200) and a delta interconnection (D) on the input-side and an output-side three-phase low voltage connector (2U1-100, 2U2-100, 2V1-100, 2V2-100, 2W1-100, 2W2-100; 2U1-200, 2U2-200, 2V1-200, 2V2-200, 2W1-200, 2W2-200) and an external interconnection (iii) on the output side, at least one medium voltage furnace bar (140; 140a, 140b) connected to the input-side medium voltage connectors (1U-100, 1V-100, 1W-100; 1U-200, 1V-200, 1W-200) of the furnace transformers (100, 200), and a low voltage-side parallel circuit (400) of the output-side low voltage connectors (2U1-100, 2U2-100, 2V1-100, 2V2-100, 2W1-100, 2W2-100; 2U1-200, 2U2-200, 2V1-200, 2V2-200, 2W1-200, 2W2-200) of the furnace transformers (100, 200) with symmetrized external delta interconnections by means of water-cooled high current conductors, which low voltage-side parallel circuit is connectable to electrodes (11, 12, 13) of the metallurgic equipment (10).

2. Power supply device according to claim 1, further comprising a high voltage supply bar (500), and at least two structurally equivalent supply transformers (160, 260) with an output power greater than or equal to 90 MVA, each of the supply transformers comprising an input-side high voltage connector (160h, 260h) and an output-side medium voltage connector (160m, 260m), the high voltage inputs of which are connected to the high voltage supply bar (500), wherein the output-side medium voltage connectors (160m, 260m) of the supply transformers (160, 260) are connected to the medium voltage furnace bar (140; 140a, 140b).

3. Power supply device according to claim 1, comprising at least two medium voltage furnace bars (140a, 140b), each of which is connected to an input-side medium voltage connector (1U-100, 1V-100, 1W-100; 1U-200, 1V-200, 1W-200) of a corresponding one of the furnace transformers (100, 200).

4. Power supply device according to claim 3, further comprising a high voltage supply bar (500), and at least two structurally equivalent supply transformers (160, 260) with an output power greater than or equal to 90 MVA, each of the supply transformers comprising an input-side high voltage connector (160h, 260h) and an output-side medium voltage connector (160m, 260m), the high voltage inputs of which are connected to the high voltage supply bar (500), wherein an output-side medium voltage connector (160m, 260m) of each corresponding one of the supply transformers (160, 260) is connected to a corresponding one of the medium voltage furnace bars (140a, 140b).

5. Power supply device according to any one of claims 1 to 4, further comprising at least one non-active current compensation (150; 150a, 150b) per medium voltage furnace bar (140; 140a, 140b), of which one each is connected to a corresponding one of the medium voltage furnace bars (140; 140a, 140b), wherein the non-active current compensation(s) are preferably configured as semiconductor-based blind current compensators.

6. Power supply device according to any one of claims 1 to 5, wherein the low voltage-side parallel circuit (300) of the output-side low voltage connectors (2U1-100, 2U2-100, 2V1-100, 2V2-100, 2W1-100, 2W2-100; 2U1-200, 2U2-200, 2V1-200, 2V2-200, 2W1-200, 2W2-200) of the furnace transformers (100, 200) comprises water-cooled high current conductors for the electrically symmetrized external delta interconnection which are configured as water-cooled copper pipes (11-100, 12-100, 1-100, 11-200, 12-200, 1-200, 21-100, 22-100, 2- 100, 21-200, 22-200, 2-200, 31-100, 32-100, 3-100, 31-200, 32-200, 3-200) respectively connected for each of the three phases in one respective loop with predetermined length such that the predetermined lengths of the high current conductors within the three loops ensure the electrical symmetrisation.

7. Power supply device according to any one of claims 1 to 6, wherein the at least two three-phase furnace transformers (100, 200) are arranged opposing each other on a lateral side of the metallurgic equipment such as the electric arc furnace such that the output-side three-phase low voltage connectors oppose each other and the low voltage-side parallel circuit (400) of the output-side low voltage connectors is arranged between the furnace transformers (100, 200).

8. Power supply device according to any one of claims 1 to 7, wherein the at least two medium voltage furnace bars (140, 240) are respectively connected to an input-side medium voltage connector of a corresponding ladle furnace transformer.

9. Power supply device according to any one of claims 1 to 8, wherein the at least two structurally equivalent three-phase furnace transformers (100, 200) are of identical construction.

10. Power supply device according to any one of claims 1 to 9, wherein the at least two structurally equivalent three-phase furnace transformers (100, 200) are three-phase electric arc furnace transformers (100, 200).