Title of Invention | A TRANSFORMER SYSTEM FOR AN ELECTRICAL ARC FURNACE HAVING THREE ELECTRODES |
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Abstract | A transformer system (1) for an electrical arc furnace (2) comprising three electrodes (202). The transformer system (1) has at least two three-phase transformers (100), which are connected in parallel to the electrodes (202), and the at least two three-phase transformers (100) can be switched on and off time-synchronously in an interdependent manner. |
Full Text | FIELD OF THE INVENTION The invention relates to a transformer system for an arc furnace with three electrodes. BACKGROUND OF THE INVENTION Electrical arc furnaces are predominantly used in modern plants for steel production. Arcs, which represent the radiation source for heat production, form between at least three electrodes and the molten mass. Industrial plants currently melt a quantity of crude steel generally in the range of 100-200 t within less than an hour. The molten crude steel, often chiefly comprising scrap iron and aggregate, then undergoes chemical analysis, and if required is tapped off after other required aggregates are fed in. The arc furnace is switched off prior to tapping, consequently partly or completely emptied, refilled and switched back on. These industrial plants for steel production accordingly must be switched off and on several times per day. Switching-off and switching-on procedures take place however not only prior to the tapping procedure and after the filling procedure, respectively, but the electrical arc furnace must also be switched off and on several times to remove metal samples and for other reasons within one melting cycle. The high current of up to in the region of 100 kA flowing between the electrodes and the material to be melted is usually drawn from medium voltage mains supply via a transformer. With the extreme currents on the secondary side switching in the heavy-current circuit is technically not possible, or only so with difficulty, and thus the furnace must be switched on the primary medium voltage side. The secondary voltage is likewise adjusted at the primary winding of the transformer via tap changers. The medium voltage switches common in power supply are limited on the current side and also have only a limited service life of approximately 10,000 switching cycles. A rise in switching capacity leads to a sometimes drastic reduction in the service life of the switchgear assemblies. Electrical arc furnaces are thus subject to certain technical restraints with respect to a rise in output. One possibility for boosting the melting performance of an individual furnace, and thus being able to produce more steel within a melting cycle, is to use a number of electrode/transformer systems in one furnace. An electrical arc furnace is known for example from DE 30 24 223 C2 in which up to four sets each of three electrodes and in each case one three-phase transformer are arranged in one furnace above the molten bath. Each transformer system is connected in terms of a three-phase delta connection. SU 1149446 A furthermore also discloses an electrical arc furnace having a number of electrode/transformer systems, in which two sets, in each case comprising three electrodes, lie opposite one another in a longitudinal furnace. The wiring of the respective phases is done such that the phase sequences of both transformers are opposite one another in reverse order. Since the output of an electrical arc furnace is restricted on the input side by the switchgear and transformer assemblies, several electrode/transformer systems are thus deployed in one furnace. This requires not only the furnace to be a special shape, but also an electrically symmetrical arrangement of the electrodes sets. In the case of metallurgical non-uniformities or in the case of inadequate symmetry of the electrodes the desired power often cannot be applied to the furnace as a result. In addition, the use of several electrode/transformer systems, as evident also from DE 30 24 223 C2, severely restricts the functionality of the furnace, above all with respect to tapping. OBJECT OF THE INVENTION It is therefore the object of the present invention to provide a transformer system for an electrical arc furnace with three electrodes, in which the output of the electrical arc furnace can be substantially increased in terms of increased melting quantity. SUMMARY OF THE INVENTION This object is achieved by the transformer system according to the features of the invention. Further advantages configuration of the invention are described hereinafter. According to the present invention a transformer system for an electrical arc furnace is provided with three electrodes, which transformer system has at least two three-phase transformers. The three phases of the three-phase transformers are in each case connected in parallel to one of the electrodes. The three-phase transformers furthermore can be switched on or off in a manner dependent on one another. The inventive transformer system therefore has the advantage of being able to supply an increased electrical power to an individual arc furnace with only three electrodes. In the process, dependent switching takes over the switching of the individual transformers in terms of synchronous switching. Even though several transformers are connected in parallel on the secondary side, any danger to staff and plant is obviated, especially from voltages occurring on the primary side due to corresponding back-transformation. The dependent switching can furthermore also continue to operate the electrical arc furnace at reduced output without any danger in the event of malfunction by one of the three-phase transformers by the inactive transformers in question being disconnected. According to the advantageous embodiment of the present invention, the three- phase transformers are switched on or off within a time window of a maximum of 100 ms, largely avoiding dangerous voltages in the transformer system. Connection of the heavy-current lines on the secondary side is therefore advantageously not required. It can be provided that the three-phase transformers are already internally triangulated. The respective two terminals of the three secondary coils of a three-phase transformer are already connected within the transformer in terms of a three-phase delta connection, and in each case only three heavy-current lines need to be routed from each transformer to the electrodes. According to a further advantageous embodiment of the present invention, the lines exist from the three-phase transformers at a right angle to a housing wall of the transformer. The three phases of all the three-phase transformers in question are furthermore connected to one another by heavy-current rails. The three-phase transformers in question can thus be connected to one another advantageously using rigid, and thus solid, lines. The heavy-current rails are preferably water-cooled. The electrical paths are as short as possible here, which is an advantage in particular with respect to the high secondary currents and the high magnetic alternating fields therefore also occurring, and which minimizes the furnace reactance. The individual transformers of the transformer system can thus advantageously be arranged spatially such that the distance from magnetic parts, thus for example parts of the transformers themselves, is as great as possible and a sufficiently large iron-free space around the heavy- current rails can generally be maintained. In a further embodiment of the present invention, the heavy current rails run between the at least two three- phase transformers in three planes one above the other. The terminals for a phase thus leave a three-phase transformer in each case at a different height. The three-phase transformers can thus advantageously be connected to one another by heavy-current rails electrically symmetrically and without any intersections. Furthermore, it can be provided that in each case two of the heavy-current rails of a phase meet each other at an angle between 45° and 180°. In this way, the transformer system can be implemented in the most advantageous possible manner, with the three-phase transformers in question being arranged spatially such that the electrical paths are short and the distance from magnetic materials is as great as possible. According to a further embodiment in each case one phase of a three-phase transformer is connected by flexible lines to the corresponding phase of the remaining three-phase transformers of the transformer system. The flexible heavy-current lines are preferably water-cooled and enable the inventive parallel connection of several three-phase transformers for one electrical arc furnace with just minimal restrictions with respect to the arrangement of the transformers in question and of the furnace. The inventive increase in output for an electrical arc furnace can thus be realized, even in unfavorable spatial circumstances. It can furthermore be provided that the flexible lines of in each case one phase of the three-phase transformers in question are connected conductively in each case to one of the three electrodes of the arc furnace. The need for electrical contacting can thus be advantageously minimized. This is advantageous in particular, since even the smallest transfer resistances at high currents lead to major performance losses. In addition, in terms of this advantageous embodiment of the present invention there is as far as possible independence from the spatial circumstances with respect to the structure of the transformer system. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Preferred embodiments of the present invention are explained hereinbelow in greater detail by means of the accompanying drawings, in which Figure 1 shows a schematic illustration of an electrical arc furnace with a transformer system according to a first embodiment of the present invention in plan view; Figure 2a shows a detailed view of a transformer system according to a second embodiment of the invention in plan view; Figure 2b shows a frontal view of a transformer according to the second embodiment of the invention; and Figure 3 shows a schematic illustration of an electrical arc furnace with a transformer system according to a third embodiment of the present invention. DETAIL DESCRIPTION OF THE INVENTION In schematic plan view Figure 1 shows the transformer system 1 with the arc furnace 2. Arranged in the electrical arc furnace 2 are three electrodes 202, as a rule with one electrode arm (not shown), above the arc furnace receptacle holding the molten material. The transformer system 1 also has a switchgear assembly 120 and, as shown here, two three-phase transformers 100, also designated as transformers below. In terms of the present invention more than two transformers 100 can generally also be fed from one switchgear assembly 120 and connected to the electrical arc furnace 2. The transformers 100 are connected on the primary side to the switchgear assembly 120 via conductive connections 123. In the switchgear assembly 120 all the transformers 100 are switched on or off via medium voltage switches 121 of a switching unit 122, in a manner dependent on one another. The switching unit 122 enables the medium voltage switches 121 to be switched on or off time- synchronously, preferably within a time window of 100 ms. Furthermore, the switchgear assembly 120 also permits risk-free operation of the arc furnace 2 with only some of the provided transformers 100, for example in the event of malfunction by one or more transformers 100. In conjunction with the medium voltage switches 121 the switching unit 122 ensures operation of the transformer system 1 with several transformers 100 in terms of excluding danger to staff and plant. Switching of the transformers 100 on the secondary side is not practicable in the region of 100 kA due to extreme secondary currents, and all the transformers 100 in question are thus connected permanently to one another on the secondary side by contact points 105. When only some of the transformers 100 provided in the transformer system 1 are operating, e.g. when only one of the two transformers 100 shows in Figure 2 is operating, a back-transformed voltage on the primary side is applied to the inactive transformers 100 due to interconnection on the secondary-side. The medium voltage switches 121 of the switchgear assembly 120 then disconnect the primary side if required, and a dangerous voltage cannot leave the switchgear assembly 120. The transformer 100 are advantageously internally triangulated. This means that all six terminals of the three coils of the three-phase transformer 100 are already connected internally in terms of a three-phase delta connection. A housing wall 110 of the transformers 100 accordingly has only three heavy-current terminals u, v and w on the secondary side. In terms of the first embodiment of the present invention these three phases are connected to one another in parallel by heavy-current rails 101 at contact points 105. Other heavy-current rails can first lead from these contact points 105 to the arc furnace 2, or, as shown here, the electrodes 202 of the arc furnace 2 can be connected by flexible heavy- current lines 102 to the contact points 105. Both the heavy current rails 101 and the flexible heavy-current lines 102 are advantageously water-cooled. According to the first embodiment of the present invention the heavy-current rails 101 exit at a right angle to the housing wall 110 of the transformers 100. They meet at the contact points 105 at an angle β . In this way, the transformers 100 are arranged at an angle α = 180° - β. According to an advantageous embodiment this angle α is in a range of 45° to 180°. The individual transformers 100 of the transformer system 1 can accordingly be arranged spatially in terms of the shortest possible electrical paths. At the same time, in this embodiment the greatest possible distances of the magnetic fields induced by the high currents from magnetic materials are ensured. An added advantage of this embodiment is the design of the transformer system 1 with several transformers 100 in the most electrically symmetrical possible arrangement. Figure 2a shows a detailed view of the transformers 100 according to a second embodiment of the present invention. In a variant of the embodiment of diagram 1 the transformers have a housing wall 111, from which heavy-current rails 101 exit. As a result view of a transformer 100 in Figure 2b shows, three connecting units 115 are provided on the housing wall 111 for each phase u, v and w. According to this second embodiment of the present invention these three connecting units 115 are arranged in three planes one above the other. The advantage of this embodiment is that the three phases can be connected to one another at contact points 105 by heavy-current rails 101 without any intersections, and the length of the rails 101 can be configured as short as possible. The transformers 100 in question, such as the two transformers 100 shown here, can thus be arranged as closely to one another as possible. Figure 3 schematically shows a transformer system 1 with an electrical arc furnace 2 according to a third embodiment of the present invention. As already described for Figure 1, the transformer system 1 has two, generally however several, three-phase transformers 100 and a switchgear assembly 120. The transformers 100 in question are connected to the switchgear assembly 120 on the primary side by conductive connections 123. A switching unit 122 in the switchgear assembly 120 actuates several medium voltage switches 121 in a manner dependent on one another. This dependent switching by the switching unit 122 takes place similarly as in the first embodiment in Figure 1, again in terms of preventing danger to staff and plant. According to this third embodiment of the present invention the transformers 100 have a housing wall 112, at which flexible lines 102 are connected to connecting units, not shown. With these flexible lines 102 the three phases u, v and w of the transformers 100 in question are connected in parallel to the electrodes 202 of the arc furnace 2. The electrodes 202, located over a metal bath 201 of the arc furnace 2 can be rearranged on an electrode arm (not shown). The advantage of this third embodiment of the present invention is that the greatest possible flexibility with respect to the spatial construction of the transformers 100 in question on the arc furnace 2 is ensured. This embodiment thus also allows the inventive use of several transformers 100 on one arc furnace 2, even in the event of unfavorable spatial circumstances. An arrangement of several transformers 100 one above the other which can be connected by flexible lines 102 to the electrodes 202 of the arc furnace 2, advantageously with the shortest possible lines 102 and without any intersections, is also conceivable. WE CLAIM: 1. A transformer system (1) for an electrical arc furnace (2) comprising three electrodes (202), characterized in that the transformer system (1) has at least two three-phase transformers (100), which are connected in parallel to the electrodes (202), and the at least two three-phase transformers (100) can be switched on and off in an interdependent manner. 2. The transformer system (1) as claimed in claim 1, wherein the three- phase transformers (100) can be switched on or off within a time window of a maximum of 100 ms. 3. The transformer system (1) as claimed in claim 1 or 2, wherein the three- phase transformers (100) are internally triangulated. 4. The transformer system (1) as claimed in claim 3, wherein the three- phase transformers (100) have a housing wall (110, 111), and heavy-current rails (101) for in each case one phase exit at a right angle to the housing wall (110, 111). 5. The transformer system (1) as claimed in claim 3 or 4, wherein in each case one phase of one of the at least two three-phase transformers (100) is connected by the heavy-current rails (101) to the corresponding phase of the remaining three-phase transformers (100). 6. The transformer system (1) as claimed in claim 5, the heavy-current rails (101) run between the at least two three-phase transformers (100) in three planes for in each case one phase without any intersections. 7. The transformer system (1) as claimed in any one of claims 4 to 6, wherein in each case two of the heavy-current rails (101) of a phase meet each other at an angle between 45° and 180°C 8. The transformer system (1) as claimed in any one of claims 1 to 3, wherein in each case one phase of one of the at least two three-phase transformers (101) is connected by flexible lines (102) to the corresponding phase of the remaining three-phase transformers (100). 9. The transformer system (1) as claimed in claim 8, wherein the flexible lines (102) of in each case one phase of the at least two three-phase transformers (100) are connected conductively in each case to one of the three electrodes (202). ABSTRACT Indian Patent Application No. 444/KOLNP/2008 of 31.1.2008 in the name of SIEMENS AG TITLE: A TRANSFORMER SYSTEM FOR AN ELECTRICAL ARC FURNACE HAVING THREE ELECTRODES A transformer system (1) for an electrical arc furnace (2) comprising three electrodes (202). The transformer system (1) has at least two three-phase transformers (100), which are connected in parallel to the electrodes (202), and the at least two three-phase transformers (100) can be switched on and off time-synchronously in an interdependent manner. |
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00444-kolnp-2008-correspondence others.pdf
00444-kolnp-2008-description complete.pdf
00444-kolnp-2008-international publication.pdf
00444-kolnp-2008-international search report.pdf
00444-kolnp-2008-pct request form.pdf
00444-kolnp-2008-translated copy of priority document.pdf
444-KOLNP-2008-(04-09-2012)-CORRESPONDENCE.pdf
444-KOLNP-2008-(05-06-2012)-ABSTRACT.pdf
444-KOLNP-2008-(05-06-2012)-AMANDED CLAIMS.pdf
444-KOLNP-2008-(05-06-2012)-CORRESPONDENCE.pdf
444-KOLNP-2008-(05-06-2012)-DRAWINGS.pdf
444-KOLNP-2008-(05-06-2012)-FORM-3.pdf
444-KOLNP-2008-(05-06-2012)-OTHERS.pdf
444-KOLNP-2008-(05-06-2012)-PA-CERTIFIED COPIES.pdf
444-KOLNP-2008-(19-11-2012)-ABSTRACT.pdf
444-KOLNP-2008-(19-11-2012)-CORRESPONDENCE.pdf
444-KOLNP-2008-(19-11-2012)-DRAWINGS.pdf
444-KOLNP-2008-CANCELLED COPY.pdf
444-KOLNP-2008-CORRESPONDENCE OTHERS 1.1.pdf
444-KOLNP-2008-CORRESPONDENCE.pdf
444-KOLNP-2008-EXAMINATION REPORT.pdf
444-KOLNP-2008-GRANTED-ABSTRACT.pdf
444-KOLNP-2008-GRANTED-CLAIMS.pdf
444-KOLNP-2008-GRANTED-DESCRIPTION (COMPLETE).pdf
444-KOLNP-2008-GRANTED-DRAWINGS.pdf
444-KOLNP-2008-GRANTED-FORM 1.pdf
444-KOLNP-2008-GRANTED-FORM 2.pdf
444-KOLNP-2008-GRANTED-FORM 3.pdf
444-KOLNP-2008-GRANTED-FORM 5.pdf
444-KOLNP-2008-GRANTED-SPECIFICATION-COMPLETE.pdf
444-KOLNP-2008-INTERNATIONAL PUBLICATION.pdf
444-KOLNP-2008-INTERNATIONAL SEARCH REPORT & OTHERS.pdf
444-KOLNP-2008-PETITION UNDER RULE 137.pdf
444-KOLNP-2008-REPLY TO EXAMINATION REPORT.pdf
Patent Number | 255527 | ||||||||||||
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Indian Patent Application Number | 444/KOLNP/2008 | ||||||||||||
PG Journal Number | 09/2013 | ||||||||||||
Publication Date | 01-Mar-2013 | ||||||||||||
Grant Date | 28-Feb-2013 | ||||||||||||
Date of Filing | 31-Jan-2008 | ||||||||||||
Name of Patentee | SIEMENS AKTIENGESELLSCHAFT | ||||||||||||
Applicant Address | WITTELSBACHERPLATZ 2, 80333 MUNCHEN | ||||||||||||
Inventors:
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PCT International Classification Number | H05B 7/00 | ||||||||||||
PCT International Application Number | PCT/DE2005/001361 | ||||||||||||
PCT International Filing date | 2005-08-02 | ||||||||||||
PCT Conventions:
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