METHOD FOR COMPENSATION OF THE REACTIVE POWER OF AN ELECTRIC SMELTING FURNACE OPERATED WITH THREE-PHASE CURRENT

METHODE DE COMPENSATION DE L'ENERGIE REACTIVE D'UN FOUR ELECTRIQUE DE FUSION FONCTIONNANT AU COURANT TRIPHASE

English Abstract

ABSTRACT
A method for compensation of the reactive power of an
electric smelting furnace, operated with three-phrase current,
to which electrical power is supplied through a three-phrase
transformer or through three single-phase transformers
interconnected to a three phase system comprises the supply
of electrical power through high-tension lines, contact-jaws,
and electrodes, the connection of compensating capacitors
to separate high-tension lines, wherein three-phase voltage
potential, present at the electrode, is tapped off above the
contact-jaws and is used to feed the compensating capacitors,
and the flow of current from the contact-jaws to the
compensating device in a direction opposite to that of the
current flowing to the electrode from the furnace transformer,
for the purpose of decreasing the inductance. A device is
especially adapted to carry out this method.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of compensating for the reactive power
of an electric smelting furnace operated with three-phase
current, electrical power being supplied to the furnace
through a three-phase transformer or through three single-
phase transformers interconnected through high-tension
lines, contact-jaws, and electrodes, to a three-phase
system, which method comprises the steps of connecting
compensating capacitors to separate high-tension lines,
tapping off a three-phase voltage potential present at
the electrode above the contact-jaws thereof and applying
said three-phase voltage potential to the compensating
capacitors, so that the current from the contact-jaws
to the compensating device flows in a direction opposite
to that of the current flowing to the electrode from the
furnace transformer, thereby decreasing the inductance.
2. A system for compensating for the reactive power
of an electric smelting furnace operated with three-phase
current, electrical power being supplied to said electric
smelting furnace through a three-phase transformer or
through three-single phase transformers interconnected
through high tension lines, contact-jaws, and electrodes,
to a three-phase system, comprising compensating capacitors
connected to respective separate high tension lines, means
for tapping off a three-phase voltage potential, present
at the electrode, above the contact-jaws thereof, and
circuit means for applying the three-phase voltage
potential to the compensating capacitors so that the
current from the contact-jaws to the compensating device
flows in a direction opposite to that of the current
flowing to the electrode from the furnace transformer,
thereby decreasing the inductance.

3. A system according to claim 2, having at least
one additional contact provided above the contact-jaws
surrounding the electrode, the said additional contact
being electrically connected to at least one of said
compensating capacitors.
4. A system according to claim 2, including at least
one additional contact provided on the electrode above
the contact-jaws surrounding the said electrode, and
an additional transformer, said additional contact being
electrically connected to the additional transformer
and at least one said compensating capacitor being
electrically connected to the secondary side thereof.
5. A system according to claim 3 or claim 4, wherein
said means for tapping off the three-phase voltage
potential are the contact-jaws surrounding the electrode.

Description

11~5~ 7~
Method for Compensation of the Reactive
Power of an Electric Smelting Furnace
. . .
Operated with Three-Phase Current
The invention relates to a method for compensation of
the reactive power of an electric smelting furnace, operated
with three-phase current, to which electrical power is
supplied through a three-phase transformer, or through three
single-phase transformers interconnected to a three-phase
system, through high-tension lines and contact-jaws,
compensating capacitors being connected to separate high-
tension lines.
Energy-conversion in the electric smelting furnace is
effected, under the electrodes, as Joule's heat in the so-
called hearth-resistance. As the size of the furnace increases,
there is a sharp drop in ohmic resistance, due to a sharp
increase in the cross-sections and volumes of current-carrying
structures and electrical:Ly-active, current-carrying hearth
parts, and there is a sharp increase in inductive resistance.
The result of this is that, as the size of the furnace
increases, the elctrical power to be installed increases to a
substantlally greater degree than the thermal power to be
expected from the structural expenditure.
Efforts have therefore been made to reduce inductive
resistances, e.g. by the use of bifilar conductors and/or by
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compensating for the reactive powers, caused by inductive
resistances, by means of capacitors.
It is known to decrease inductive resistances, e.g. by
using bifilar eonductors (elektrow~rme international vol~s2~
1972: Dr. Mollenkamp, Dr. Kallfelz "Modern eleetric smelting
furnaces for recovering ferro-alloys, pig-iron and calcium-
earbide"). However, the bifilar arrangement of the
conduetors, because of the furnace circuitry, is possible only
in the section of line between the furnace-transformer and
the flexible part of the conductor (high-tension line)
permitting movement of the electrodes. The general laws
governing uncompensated three-phase eonductors (R.S.T.) apply
to the remaining sections of the eurrent-eonductors, namely
the high-tension line along the electrode phase-conductor,
through which current is passed to the eleetrodes by means of
eontaet-jaws, and to the eurrent-carrying eleetrode part
thereunder. The latter seetions of the high-tension conductor
system aeeount for about 70~ of the total furnaee reaetive
power.
It is therefore the pupose of the invention to reduee
the total reaetive power of an eleetrie smelting furnaee of
the type mentioned at the beginning hereof and, more partieularly,
to relieve several eomponents, used to supply power, of
reaetive power. The deviee for the exeeution of the method
aeeording to the invention is to be of simple design. Aeeording
to the invention, this purpose is aehieved by the features
set forth in the eharaeterizing portion of elaim 1. The
sub-elaims eontain advantageous designs of the deviee for the
exeeution of the method aeeording to the invention.
With the method aeeording to the invention, the reactive
power of the furnaee is largely redueed, by eapaeitative

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compensation, in the area where it arises. The ind~ctive
reactive power of the furnace is furthermore reduced in that
the inductive resistances are lowered. The inductance,
i.e. the inductive resistance in the area of the vertical
high-tension line is reduced in that, in the part of the
electrode between the conventional contact-jaws, and the
additional compensating contact-jaws, the current flows in
a direction opposite to that in the high-tension line in this
area. As a result of this, the inductance-reducing effect
of coaxial conductors or of bifilar conductors comes into
effect.
The power-supply components, namely the high-tension
power-plant, the transformer, and the high-tension lines may
be made simpler and less expensive, since the reactive power
to be covered is less, i.e. the apparent and connected power
are less.
Furnace-operation can be improved by the use of a
S~derberg electrode, since the heat arising from the flow of
current in the electrode-shell above the contact-jaws may be
used for preheating the mass of the electrode.
Another advantage is that less experlsive mate~rials may
be used for the elect~ode phase~conductor (instead o non-
magnetizable steels, as heretofore) since, in the vicinity of
the vertical high-tension line, the inductance, and therefore
the induction-heating, produced in the structural parts, is
reduced.
The invention is explained hereinafter in greater detail,
in connection with the drawings attached hereto, wherein:

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Figure 1 shows an electrode in an electric smelting
furnace, with diagrammatically illustrated high-tension
lines, transformers and a device according to the invention
for compensating for the reactive power.
Figure 2 shows a modification of the compensating
device according to the invention.
Figure 3 is a diagrammatical representation, in plan
view, of three electrodes with the compensating device
according to the invention.
In Figures 1 and 2, the electrode is marked 2. The
high-tension line of conventional design marked 4 and 12,
having a flexible section 3,connects the secondary side of
a furnace-transformer 1 with contact-jaws through which the
current flows to electrode 2.
In the example illustrated, additional contact-jaws 7
are provided, above contact-jaws 5, through which the three-
phase voltage potential, in upper part 6 of electrode 2, is
tapped and passed, through a conventional three-phase circuit
(Figure 2), through a flexible conductor 8 and a rigid conductor
9, to a three-phase transformer 10. The ]atter trans.Eorms
the voltage tapped from the elc:ct::rotle into a higher, more
economical voltage wh:ich is fed to capacitors 11. The magni.tude
of the power converted in this arrangement is determined by
the level of the voltage-potential at conventional contact-jaws
5 and by the load-carrying ability of upper area 6 of
electrode 2.
The inductive resistance in the vici.nity of vertical high-
tension line 4 is reduced in that in area 6 of electrode 2,
between conventional contact-jaws 5 and additional contact-jaws

517~
7, the current flows in the opposite direction to that in
the said vertical high-tension line, thus producing the
effect of coaxial conductors.
At the secondary side of additional transformer 10, at
least one compensating capacitor 11 is connected to each
phase of the three-phase current.
As shown in Figure 2, the three-phase voltage-potential
at the electrode is tapped off by contact-jaws 5 and is
passed to three-phase transformer 10 through a flow-tube 13
and conductors 8 and 9.