Title of Invention

A TRANSFORMER SYSTEM FOR AN ELECTRICAL ARC FURNACE HAVING THREE ELECTRODES

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.

Documents:

00444-kolnp-2008-abstract.pdf

00444-kolnp-2008-claims.pdf

00444-kolnp-2008-correspondence others.pdf

00444-kolnp-2008-description complete.pdf

00444-kolnp-2008-drawings.pdf

00444-kolnp-2008-form 1.pdf

00444-kolnp-2008-form 2.pdf

00444-kolnp-2008-form 3.pdf

00444-kolnp-2008-form 5.pdf

00444-kolnp-2008-gpa.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-form 18.pdf

444-KOLNP-2008-FORM 181.1.pdf

444-KOLNP-2008-GPA.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-OTHERS-1.1.pdf

444-KOLNP-2008-PETITION UNDER RULE 137.pdf

444-KOLNP-2008-REPLY TO EXAMINATION REPORT.pdf

abstract-00444-kolnp-2008.jpg


Patent Number 255527
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:
# Inventor's Name Inventor's Address
1 RALPH-HERBERT BACKES FALKENSTR. 59 91056 ERLANGEN
2 THOMAS MATSCHULLAT PETER-HENLEIN-STR. 15 90542 ECKENTAL
3 DIETER FINK BUSSARDSTR. 50, 91088 BUBENREUTH
PCT International Classification Number H05B 7/00
PCT International Application Number PCT/DE2005/001361
PCT International Filing date 2005-08-02
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA