Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND APPARATUS TO FACILITATE RESTRIKING
IN AN ARC-FURNACE
FIELD OF THE INVENTION
The present invention relates to arc-furnace equipment and more
specifically to means and methods to help an electrical arc to strike thus
improving productivity, reducing operating cost and reducing flicker.
BACKGROUND OF THE INVENTION
Industrial arc-furnaces are huge furnaces which are typically used to melt
different metallurgical elements such as bulk iron coming from scrap. The
bulk metal is melted by the intense heat radiating from a hot gas column
produced between an electrode and the scrap by an electric arc. The arc-
furnace is basically composed of a vat to retain the scrap and the melted
metal; a set of electrodes to spark the arcs; a set of actuators to control
the
electrodes distance from the scrap; and a large current power supply
(including a transformer equipped with a tap changer to select a voltage
level) to supply the arc currents. When the melting is completed, impurities
floating on the surface are skimmed or scraped from the surface and then,
the liquid metal is retrieved from the vat for further processing.
Creation of an electric arc requires an ignition normally performed by
making a contact between two electrodes: a cathode and an anode. The
cathode then emits electrons that are accelerated towards the anode by an
electric field applied between the electrodes. These electrons collide with
the gas molecules within the gap to generate positively charged ions and
negatively charged free electrons to form a conductive gas column
between the electrodes allowing the current to flow. A gas conductive
enough to allow a current to flow will be referred in this document as a
plasma. As the current increases, more collisions are made and more ions
and electrons are freed, thus increasing the conductivity and temperature
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of the plasma column. At the same time, the cathode is bombarded with
more ions and heats up thus maintaining electron emission. The anode
also heats up due to the impact of the incoming electrons. The emission,
the bombardments and the series of collisions generate a voltage-drop that
can be divided in three zones: the cathode voltage-drop; the anode
voltage-drop; and the plasma column voltage-drop. An arc-furnace arc has
a voltage-drop distributed, for the most part, along the plasma column.
Therefore, the arc voltage-drop will mainly increase with the arc length, will
diminish inversely to the plasma temperature, and will depend on the
plasma gas composition.
When the furnace electric arc is interrupted, it leaves the plasma column in
an initial ionized state whose lifetime is influenced by the rate of plasma
temperature drop and composition. The ignition-voltage required to re-
strike the electrical arc will increase with the degradation of the plasma
state. If the plasma is lost, a dielectric breakdown or a temporary electrical
contact will be required to recreate the plasma and restrike the arc.
The most commonly used arc-furnace is the three-phase AC current type.
The furnace comprises an electrode for each phase, all disposed according
to a triangular pattern in the vat. During operation, each electrode
produces an arc having its other end in contact with the load of metal. All
the electrodes of the AC arc-furnace are alternately anode and cathode. At
each half cycle, the arc current must pass through a zero point in order to
reverse. The intense heat radiating from each plasma column is
proportional to the arc current and therefore will fluctuate in a synchronous
manner. At the line frequency of 50 or 60 Hz and in a cold environment,
there is not enough heat inertia to maintain the plasma temperature to
preserve the ionized state. In this case, the plasma temperature will
fluctuate according to the current flow and will affect its conductivity. This
change in conductivity will then affect the voltage-drop as the current
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fluctuates. If we consider the state following a current peak while the arc
burns in a cold environment, there will be a progressive increase of the
voltage-drop at the electrodes end. This voltage-drop will rise up to the
extinction-voltage value where the current reaches zero and the arc
extinguishes. For the reverse arc current to ignite, the alternating voltage
supply must then, in the reverse polarity, exceed the ignition-voltage, which
is dependent on the plasma column ionized state (temperature) and on the
anode and cathode condition. After re-ignition, as the arc current increases
back, the gas column warms up again, and the voltage-drop progressively
regains, in reverse polarity, a lower value equivalent to the voltage-drop of
the precedent current peak. If we draw the evolution of the arc voltage, the
ignition-voltage will be higher than the extinction-voltage because in
between events, the plasma column has continued to cool down.
An AC arc at a frequency of 50 or 60 Hz and burning in a hot environment
behaves differently. The plasma column remains hot therefore sufficiently
ionized when the arc current reaches zero and extinguishes. The
extinction/ignition-voltage level will be weakly affected and the evolution of
the voltage-drop will show a shape between a sinusoidal and a square
wave.
An AC arc-furnace works with a sinusoidal voltage power supply. In order
to ignite the arc shortly after its extinction, the arc-furnace operates at a
lower power factor making the voltage leading the current due to the
leakage inductance in the supply path of the furnace. In many cases, a
series inductance is even inserted on the primary side of the furnace
transformer. Then, when the current reaches zero and the extinction
occurs, there is an immediate application of the reverse polarity voltage
from the supply source with the vanishing of the back emf in the
inductance. If the supply voltage is higher than the ignition-voltage at this
instant, the arc will strike immediately. If it is not the case, a delay will
be
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introduced until the voltage supply catches up the ignition-voltage level.
This delay introduces dead time periods in the arc current, which creates
current-less time intervals. Even the amplitude of the current, as well as its
RMS value is reduced in a way similar to a phase controlled dimmer. The
impact on the power input of the furnace is impressive.
The behavior of the arc-furnace depends strongly on the environment in
which the arc is burning. Normally, a melting process involves two phases.
In the first phase, subsequent loads of scrap are poured in the vat for
melting down. During that phase, the furnace operates mainly in a cold
environment. The arcs are not stable as they move erratically and jump
from one piece of scrap to another. Also, the continuous slipping and
melting of the scrap affects the arc length and generates frequent short-
circuits of the electrodes. The arc behavior continuously changes the
plasma column length, which also introduces a continuous variation in the
dead time period and the short circuits creates inrush currents in the
furnace high current power supply. If the dead time period is prolonged,
the ignition-voltage will eventually become too high for the furnace supply
voltage to strike an arc and the plasma will be lost. When a complete
extinction of an arc occurs, the electrode must then be moved towards the
scrap to make a contact and reinitiate the arc. The touch of the contact
generates a high inrush current until the electrode is moved away to have
enough plasma length for the current to reduce. For the second phase of
the process the arcs behave differently. The scrap is completely melted in
a hot liquid bath and the arcs are burning in a hotter and more stable
environment. Moreover, a foamy slag is used to improve arc stability.
Contrary to the first phase, the arc length is more stable and easier to
control even if the arc contains current-less time intervals.
In a DC arc-furnace, there is no change in the direction of the arc current
so that only the dead time periods described above do not exist. However,
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in a similar way to the AC arc-furnace, the erratic movement of the arc in
the first phase may stretch the arc length to a limit where the furnace
voltage supply can no longer maintain the current in the plasma because
the voltage-drop gets to high. When this occurs, the plasma current
decreases, thus cooling the arc and reducing its conductivity thus causing
the current decrease to run away until the arc is interrupted and the plasma
is lost. Here too, the electrodes have to be lowered in order to make a
contact with the scrap for striking a new arc with the accompanied inrush
current. These arc interruptions are most likely to happen only in the first
phase.
The operation of an arc-furnace causes supply current fluctuations in the
utility line. The largest current fluctuations are produced in the first phase
by both the AC and the DC arc-furnace. In an AC arc-furnace, the erratic
movement of the arcs, the dead time periods, the inrush currents and the
frequent extinction of the arcs create these current fluctuations. In the DC
furnace, the inrush currents, the continuous change in the firing angle of
the rectifier valves to compensate for the erratic movements of the arc as
well as arc interruptions are the source of the supply current fluctuations.
These fluctuations are the source of voltage fluctuations in the utility
network. The utility company, to a certain extent, tolerates a part of this
disturbance, known as flicker. The flicker is defined as the low frequency
component of the voltage fluctuation encountered on the utility grid that
cause disturbance to the eyes on such equipment as a light bulb. The
amount of flicker is related to the ratio between the short circuit power of
the supply network and the short circuit power of the arc-furnace. Unless
this power ratio is sufficiently high, the furnace working point must be
adapted during the process in order to constrain the flicker level within
permissible limits. The flicker level can be reduced with the aid of static
power compensators or by inserting a large inductor on the primary side of
the arc-furnace transformer. Unfortunately, these apparatus are costly and
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the modifications to the arc-furnace supply are significant. Often, the arc-
furnace operates with a low supply voltage and with the electrodes closer
to the scrap. This will reduce the injected power until the scrap phase is
completed and then, the power is increased as the burning arcs are more
stable and the flicker is reduced.
An important aspect of an arc-furnace is its productivity. An arc-furnace is
operated to produce the greatest number of heats possible. It is strongly
related to the amount of power that can be transmitted to melt the scrap in
a given time. The frequent complete extinction of the arcs, the dead time
period encountered between each extinction and ignition, and the limited
amount of flicker that can be tolerated, all contribute negatively to the arc-
furnace production since these events all extend the melting process time.
Another important aspect of an arc-furnace is the production cost. For a
fixed plasma current, the plasma voltage and heating capacity are
proportional to the arc length. A longer arc will allow a plasma current
reduction for a same amount of injected power. A smaller current has the
advantage of reducing the electrode deterioration and consumption and
also reduces the Joule losses in the supply circuit. Consequently, they will
reduce the production cost.
These advantages will reduce the melting process time and the arc-furnace
operating and maintenance cost and they will improve productivity.
A method to precipitate the striking of the electrical arc was disclosed in
the
international PCT application publication number WO 94/22279 (inventors
Paulsson and Angquist). In this document, the apparatus improves the arc
burning behavior by supplying to the electrodes a voltage pulse in
connection with an interruption of the arc. After an immediate extinction of
an arc, a voltage pulse is injected by discharging a capacitor or is induced
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in an inductor in the supply path by a temporary short-circuit to shorten the
current-less intervals of the arcs. Unfortunately, for maximum efficiency,
the apparatus requires the pulse to be injected at an optimum time delay
following an interruption of the arc. The ignition may be unreliable because
the striking of the main furnace arc may not happen or the main furnace arc
current may not reach sufficient amplitude to maintain the arc after the
voltage pulse has disappeared. (The main furnace arc is defined as the
electric arc current supplied in the plasma column by the arc-furnace
transformer). Moreover, during the delay preceding the injection of the
pulse and during the time elapsing after the pulse disappears without the
main furnace arc being struck, the plasma ionized state still continues to
degrade. Also, the controller unit must track the arc-furnace output current
to operate adequately. This method may prove to be reliable in the liquid
bath phase of a heat but is difficult to apply in the first phase where the
arc
is erratic and most of the problems are encountered. The apparatus
disclosed also requires a serial inductor at the output of the arc-furnace
supply. Knowing that the supplied current is enormous, the inductor size is
likely to be large. It is mentioned that the inductor could be avoided if the
inductance consists of the inductance of the network, the furnace
transformer and the connection lead. This option implies that part of the
voltage pulse will propagate into the transformer and into the utility
network,
which is generally not desired nor allowed by the arc-furnace owner or the
utility. If the inductor includes the furnace connection leads, then the power
electronics must be located close to the electrodes where the environment
conditions are extremely severe and where maintenance is problematic
and must require a furnace shutdown.
SUMMARY OF THE INVENTION
An improvement of the arc-furnace can be accomplished by the application
of a method to precipitate the striking of the electrical arc or avoid its
interruption. This method offers the multiple following advantages:
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The arc length or current can be increased;
the mean cyclic extinction period can be lowered;
the number of complete extinction event can be reduced;
the electrodes consumption can be reduced;
the Joule losses in the electrical circuit can be lowered;
the power factor can be increased;
and the flicker level can be lowered or the power can be increased.
It is an object of the present invention to provide a novel method and
apparatus aimed to facilitate arc restriking in an arc-furnace, hence to
obtain the advantages mentioned in the above background description
without the drawbacks of the previous art.
It is a secondary object of the present invention to provide an apparatus to
work in parallel with an arc-furnace without making major change to the
arc-furnace structure and power supply.
It is another secondary object of the present invention to provide means to
avoid excessive voltage amplitudes to be applied to the arc-furnace
components.
It is another secondary object of the present invention to provide an
apparatus in which the power electronics and the control unit are not
exposed to the arc-furnace severe environment and can be accessed for
maintenance without requiring an interruption of the furnace operation.
It is another secondary object of the present invention to provide an
apparatus in which the controllability can be made simple and does not
require an optimal time interval to act in order to be effective.
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In accordance with a first aspect of the invention, there is provided an
apparatus for improving re-striking in an arc-furnace having a large current
conductor with a high-current power supply connected to one end of the
conductor and an electrode connected to the other end of the conductor to
produce an electrical arc for melting metal, the apparatus comprising a
second quasi-continuous energy supply for maintaining a plasma link
between the electrode end and the melting metal following an interruption
of the electrical arc.
In accordance to another aspect of the invention, there is provided a
method for melting metal in an arc-furnace using an electrical arc,
comprising the steps of feeding a high current, from a high current power
supply, using a large current conductor and an electrode, to the electrical
arc, between the electrode and the melting metal of the arc-furnace and
maintaining a plasma link between the electrode and the melting metal for
a duration of an extinction of the electrical arc until a voltage of the high
current power supply regains a value that will reestablish the electrical arc.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood by way of the following description
of specific preferred embodiments, together with the accompanying
drawings, in which:
Fig. 1 shows a single line diagram of a basic AC arc-furnace as known in
the art;
Fig. 2 shows is a typical construction of a three-phase AC arc-furnace as
known in the art;
Fig. 3a and Fig. 3b show the evolution of the arc in the furnace according to
the variation of the voltage and the current; in Fig. 3a, the arc intensity
changes as the current decreases to 0; in Fig. 3b, which shows the
preferred embodiment of the present invention, an HF current supply is
injected in the plasma;
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Fig. 4 shows a schematic circuit diagram of an apparatus to inject an HF
electrical energy in the plasma column at the electrode end in order to re-
strike an arc; the circuit comprises a high current power supply, a capacitor,
an inductor, an HF current source with a capacitor;
Fig. 5 shows an HF voltage source with its capacitor;
Fig. 6 shows a resonant capacitive circuit that separates the line frequency
voltage from the HF voltage; the resonant capacitive circuit comprises a
bypass inductor and two different capacitors;
Fig. 7a and Fig. 7b show an apparatus using a cooled coaxial cable which
allows the HF source to be installed at a remote location; in Fig. 7a, the HF
power supply is a current source; in Fig. 7b, the HF power supply is a
voltage source;
Fig. 8 shows a cross section of the coaxial cable used to install the HF
source at a remote location; a support member is surrounded by a first
conductor layer; a second conductor layer surrounds the first conductor
layer and is separated from the first layer by a dielectric layer;
Fig. 9a and 9b shows another configuration where the HF source is located
at a remote location and where a snubber circuit is used to cut off harmonic
components; in Fig. 9a, the HF source is a current source; in Fig. 9b, the
HF source is a voltage source;
Fig. 10a and 10b shows in more detail the circuitry for the HF voltage and
current sources respectively; Fig. 10a comprises a voltage generator, an
HF voltage inverter and an optional HF voltage transformer; Fig. 10b
comprises a current generator, an HF current inverter, and an optional HF
current transformer;
Fig. 10c and 10d show in more detail an HF inverter circuitry, comprising
an HF sense circuit, a controller unit, gate drivers and an H-Bridge; in Fig.
10c, the HF sense unit senses a current and the input of the circuit is a DC
voltage; in Fig. 10d, the HF sense unit senses a voltage and the input of
the circuit is a DC current;
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Fig. 11 a, 11 b and 11 c show voltage or current plots which illustrate the
operating principle of the present invention; Fig. 11a shows the HF voltage
produced by an inverter; Fig. 11 b shows the electrode voltage; and Fig. 11 c
shows the current in the furnace arc;
Fig. 12 shows the measured signal of the apparatus for one embodiment of
the present invention; more precisely, the dark trace represents the current
in the plasma column and the gray trace represents the voltage drop in the
plasma column;
Fig. 13 shows the integration of one embodiment of the present invention
into a three-phase arc-furnace; it comprises two HF current transformers,
two HF current inverter and two current generator;
Fig. 14 shows the installation of all the components of the apparatus of one
embodiment of the present invention in a three-phase arc-furnace.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Fig. 1, there is shown a schematic of an AC arc-furnace.
It comprises a vat 1 for holding the element 2 to be melted. An electrode 3
is clamped to a conductive and mobile supporting member 12 with the aid
of a clamp-release contact 5. An actuator 10 is fixed under the supporting
member 12 to provide a vertical mobility to the electrode. A large current
power supply 4 is connected to the conductive supporting member 12 with
a flexible and high current conductor 6 to allow the movement of the
supporting member 12. The large current power supply 4 comprises a step
down transformer 8 connected via a voltage tap changer 9 to the utility
network 7. In operation, electrode 3 is lowered in the vat 1 by the actuator
10 to strike an arc with the metal 2. Then, the actuator 10 acts on the arc
length by positioning the height of the electrode and the voltage tap
changer 9 is switched to control the mean current supplied to the arc. The
arc current follows a return path through a bottom electrode in the vat 1 or
through another arc produced between the metal 2 and another electrode.
As the arc burns, the electrode is consumed and, when it becomes
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necessary, a new electrode 3 is attached to the upper end and slid to
ensure a continuous feeding.
In an other type of arc-furnace apparatus, the large current power supply 4
includes a rectifier which is inserted at the output of the transformer to
convert the AC current into a DC current, therefore to generate a DC
plasma current at the electrode end.
Referring now to Fig. 2, there is shown a typical construction of a three-
phase AC arc-furnace. Except for the network supply, every part of the
schematic draft of Fig. 1 is identified by like reference numerals.
According to the present invention, the object of the method and apparatus
is to maintain a plasma link between the electrode end and the melting
metal during the current-less time intervals. The plasma link is conditioned
by a quasi-continuous energy power supply that generates new ions and
new free electrons within the plasma. The quasi-continuous energy power
supply supplies energy to the plasma in a way that may be continuous, or if
not, may be oscillatory or in the form of repetitive pulses at a frequency
faster than the vanishing time constant of the plasma. With the method
and apparatus of the present invention, the current-less time intervals
occurring in the furnaces of the prior art will be reduced by lowering the
ignition-voltage necessary to the large current power supply to strike the
main furnace arc. The arc furnace will be able to operate in conditions
where the arcs will be more stable than without the use of this invention.
According to the present invention, one embodiment of the method and
apparatus is to maintain a plasma link of a length greater than the
maximum length of the main furnace arc. This way, a plasma link will be
maintained in some encountered conditions where the furnace supply
voltage cannot exceed the ignition-voltage. These conditions include the
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stretch of the plasma link caused by the movement of the scrap in the
furnace vat or by the movement of the arc itself, which may both cause the
interruption of the main furnace arc. Then, with the motion of the
electrodes or with the erratic movement of the plasma itself, the plasma link
will get shorter enough for the voltage of the high current power supply to
exceed the ignition voltage and strike the main furnace arc. This will
reduce the number of events involving the complete loss of the main
furnace arc.
The energy supply source can be an apparatus such as, for example, a
laser beaming through a hole in the center of the electrode (e.g. an axial
borehole) or, in the preferred embodiment of the present invention, it can
be an electrical power supply providing a current to the plasma. The
electrical power supply operates by injecting a quasi-continuous electrical
energy, more particularly by injecting an HF (High Frequency) AC
oscillating electrical energy at a frequency faster than the time constant of
the plasma to vanish.
It is another object of one embodiment of the present invention to inject an
HF AC oscillating electrical energy in the plasma at the electrode end to
facilitate restriking in an arc-furnace.
Also according to the present invention, another object of the method is to
create an HF electrical energy discharge at the electrode end with an HF
voltage when the plasma link has been lost'after an arc interruption. This
HF electrical energy discharge will initiate a new plasma link with a reduced
ignition-voltage for the main furnace arc to strike. The method will avoid
having to make a contact between the electrode and the metal in order to
strike an arc, which cause an inrush in the furnace high current power
supply.
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It is another object of one embodiment of the present invention to create an
HF current discharge at the electrode end to facilitate the restriking in arc-
furnace.
In an arc-furnace, the typical carrying conductors used to carry the current
from the transformer up to the electrodes are constructed using copper in
the form of a long conduit having a wall thickness close to the line
frequency skin depth. The conduit is required to cool the conductor by
forcing circulation of water through its opening. An arc-furnace design uses
large diameter conduit easily exceeding 30cm. At higher frequencies, the
skin depth becomes very thin. A current at this frequency range, flowing in
the conductor, will be concentrated in the outer periphery of the conductor.
The large diameter of the arc-furnace conductors offers a long periphery for
the HF current to distribute itself around. It results in a very low
resistance
for the conductor. Also, the arc-furnace conductors may run for at least a
dozen meters from the transformer output up to the electrodes. The
resulting parasitic inductance of the conductor combined with its low
resistance forms an inductor typically ranging from lOpH to 30NH and
having a good quality factor around the 100kHz range. According to the
present invention, a resonant capacitor is connected to the supply of the of
the arc-furnace close to the electrode side. Therefore, this capacitor in
conjunction with a major section of this inductor can be forced in to
resonance within a certain frequency range.
According to the present invention, the HF oscillating electrical energy can
be injected in the plasma at the electrode end using the resonant circuit
and an HF power source. An HF current is injected in the plasma when the
HF current source oscillating near or at the resonant frequency feeds the
electrode besides the resonant capacitor. Then, the resonant circuit at the
output goes into resonance and builds up an oscillating voltage at the
electrode in accordance with the plasma conductivity to generate an HF
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current flow through the plasma column. The HF current will follow a path
in the plasma column to maintain a plasma link. The voltage buildup at the
electrode can be higher than the arc-furnace supplied voltage so that it is
possible to maintain a longer plasma link.
To better illustrate the impact of the present invention on the arc-furnace,
Fig. 3 shows the evolution of the furnace arc voltage and current
accompanied by a series of drawings of the electrode arc. In Fig. 3a, the
arc intensity changes as the current decreases to zero. At zero current, the
instant reapplied voltage is lower than the ignition-voltage and introduces a
delay until the voltage source catches up the ignition voltage and the main
furnace arc is restriken. In Fig. 3b, the apparatus of the present invention
injects an HF current in the plasma which can be viewed on the arc current
waveform. In the Figure, the frequency of the HF current has been
intentionally reduced for purpose of clarity. The drawing of the arc at the
electrode shows an HF arc at the zero current point which reduces the
ignition-voltage and allows the main furnace arc to strike sooner. The
impact results in a higher current under area thus increasing the amount of
power injected in the furnace vat.
Also according to the present invention, an HF electrical energy discharge
can be produced at the electrode end if there is not enough plasma for the
HF current to flow through. The resonant circuit still goes into resonance
and builds up an oscillating voltage at the electrode. This oscillating
voltage can be made high enough to initiate a dielectric breakdown and
generate an HF current discharge.
Referring now to Fig. 4, there is shown a schematic diagram of an HF
resonant circuit integrated with an arc-furnace according to the present
invention. It comprises: a capacitive circuit 13 which comprises a resonant
capacitor 21 connected to the arc-furnace large current conductor at a
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junction point 17 close to the electrode and an HF controlled current source
19 which is also connected close to the electrode and in parallel to the
capacitor 21. The arc-furnace also comprises an HF bypass impedance
preferably composed of a capacitor 15 connected to a supply path at the
junction point 16 on the same conductor and close to the arc-furnace
current supply output 4. As previously mentioned, the conductive section
located between the end connections 16 and 17 has a parasitic self-
inductance 14 referred to as the resonant inductor. The inductor 14 and
the capacitor 21 create with the bypass capacitor 15, a resonant circuit
having a resonant frequency of:
~r 12~ LC
where L and C are respectively the inductor 14 inductance and capacitor
21 capacitance values. Considering that the size of the capacitor 21 is
growing with its NF value, it is possible to reduce this value by inserting a
ferromagnetic material 35 around the arc current carrying conductor that
will increase the inductance value 14. This ferromagnetic part is designed
to saturate slightly above the maximum resonant current. Typically, the
resonant current will be of a few hundreds of amperes compared to a few
tens of thousands of amperes for the furnace supply current. Thus, the
ferromagnetic part will not interfere with the furnace supply path impedance
as it saturates rapidly as the main furnace arc current ignites.
According to the present invention, the apparatus is characterized in that
the inductor element of the resonant circuit is constituted mainly by the
parasitic self-inductance of the conductor intended to carry the plasma
current to the electrode end. This avoids having to use external inductors
that are difficult to insert due to the large geometry of the arc-furnace
conductors.
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The injected HF current in the plasma can be increased in the present
invention if the resonant capacitor and the HF power supply are connected
at a distance from both the bypass impedance 15 and the electrode 3.
Inductor 14 is now split in two (shown in Fig. 4), each part belonging to a
distinct loop, both loops sharing the capacitor 21 as a common branch.
When the main furnace arc is burning, the two inductor 14 parts appear in
parallel to the high frequency components. Then, if the HF power supply is
tuned to the resonant frequency, of this oscillatory circuit, a part of the
resonant current with increased amplitude will flow through the plasma.
The resonant frequency must be faster than the time constant for the
plasma to disappear. In a preferred embodiment of the present invention,
the resonant frequency is located in the tens of kilohertz frequency range,
close to 100kHz. Typically, the resonant inductor value lies between 10pH
to 20pH. Therefore, the resonant capacitor has a value in the hundreds of
nF range. The resonance is created by operating the HF source 19 at a
frequency near the resonant frequency. The resonant voltage at the
electrode can increase as high as 5kV to 10kV, and is much higher than
the maximum output voltage of the furnace power supply, which is about 1
kV. This difference in voltage allows the apparatus to maintain a longer
plasma link compared to the plasma column length that the furnace power
supply can maintain. The current source 19 may be sinusoidal;
trapezoidal; pulsed or quasi-resonant pulsed type. In all cases, it is
necessary that the fundamental component frequency of the HF current
source 19 oscillates close to or at the resonant frequency. Preferably, the
HF current source peak power output is about 100 kW with a RMS supply
current between 10A to 100A. When the arc-furnace has more than one
phase (most arc-furnaces work with three phase arc currents), the HF
current sources may work asynchronously or synchronously with a
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predetermined phase shift. Preferably, they work synchronously in phase
with their output oscillating voltage.
In operation, the resonant circuit can inject an HF current in the plasma
from ten to a few hundreds of amperes, and can produce an arc power of
several tens of kilowatts.
In Fig. 4, the HF bypass capacitor 15 acts as a short circuit at the resonant
frequency range to bypass the furnace current supply 4 in order for the
resonant circuit to operate adequately. It is also used to avoid the
propagation of the resonant current and voltage into the power supply of
the arc-furnace (typically the transformer inner winding). The bypass
capacitor 15 can also be connected to the transformer conductive
enclosure to prevent excessive voltage from appearing across the
insulation between the transformer windings and the enclosure. Generally,
arc-furnace transformers are not designed to meet dielectric specifications
for recurrent application of an HF voltage in the inner parts of their coils.
Introduction of a voltage or current at a high frequency may result in rapid
deterioration of the dielectric material and consequently will lead to a
shorter lifetime. Considering the large price of this equipment, proper
protection is important. The HF bypass capacitor 15 must be large enough
to limit the perturbation of the resonant current and voltage to within a
maximum value. Preferably the voltage is limited to a few tens of volts and
the bypass capacitor has about ten to a hundred of pF.
Fig. 5 shows another configuration of the capacitive circuit 13 according to
the present invention for injecting a high frequency current at the electrode.
Instead of connecting a current source in parallel with the capacitor 21, the
same effect can be obtained by inserting an HF voltage source in series
with the capacitor 21. The voltage source 19 may be, as explained above,
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sinusoidal, trapezoidal, pulsed or quasi-resonant pulsed type at the
frequency near or equal to the resonant frequency.
The capacitive circuit 13 in Fig. 4 has an advantage over the capacitive
circuit of Fig. 5 if the HF supply 19 generates harmonics. The capacitive
circuit of Fig. 4 will generate less EMI interference since the harmonics are
restrained within the HF current supply and the resonant capacitor 21. In
counter part, the voltage harmonics of the HF voltage source 19 of Fig. 5
will propagate to the inductor 14 and will generate EMI interference through
the leaking magnetic field. The circuit of Fig. 4 is the preferred embodiment
of the present disclosed apparatus.
Referring back to Fig. 4, the arc-furnace plasma current supply may be
either DC or AC at the line frequency of the utility network. The apparatus
of the present invention must take into account the presence of the plasma
current supply frequency. During operation of the furnace, the resonant
capacitor 21 will withstand the sum of the HF voltage and the arc-furnace
supplied voltage. The LF (low frequency) voltage component will add stress
on the capacitor that must be accounted in the capacitor design. The
design criteria been different for an LF and an HF capacitor, it is cost
effective to separate both frequency components.
Referring now to Fig. 6, there is illustrated a different way to provide the
capacitor 21 of Fig. 6 in order to reduce the voltage stress on the HF
capacitor and the associated costs. The capacitor 21 of Fig. 6 contains
elements to separate the line frequency voltage (or DC) from the resonant
frequency voltage. The capacitor 21 comprises: an HF capacitor 26; an LF
bypass inductor 24 connected in parallel to the HF capacitor 26; and an LF
blocking capacitor 27 connected in series with the HF capacitor 26 and the
inductor 24. The HF range includes the resonant frequency. The
characteristics of both the inductor 24 and the capacitor 27 force the HF
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voltage to appear mainly across the capacitor 26 and the LF voltage to
appear mainly across the capacitor 27.
An important factor to consider for the present apparatus is the operating
environment conditions. Any device that must be located on the supporting
member 12 of the arc-furnace will be exposed to intense heat and open
flames, will encounter frequent vibrations, will be splashed by metal in
fusion, and will be submitted to the magnetic field produced by tens of
thousands of amperes that flows in the nearby furnace conductors. The
capacitor 21 must be placed in close proximity to the furnace vat for the
apparatus to operate adequately. The capacitor 21 is a dense and passive
component that can be easily packaged into a water-cooled housing for
protection. So, its operation does not cause a major problem and needs no
maintenance. In case of a defect, its replacement can be made easily on
the next scheduled shutdown. On the other hand, the HF source is
composed of semiconductor elements, of electronic and digital elements
and needs initial tuning. The harsh environment conditions represent a
serious challenge for the HF source that, once rugged, will be costly to
manufacture. Also, the tuning, probing and debugging represents a
problem. A person is not allowed to access the arc-furnace when it is
operating. An unplanned shutdown is extremely costly to the furnace
owner so that in case of a failure in the HF source, the apparatus will
remain inoperative until the next planned shutdown.
According to the present invention, the HF power source can be located in
an area not exposed to the harsh environment of the arc-furnace. The HF
source uses an HF supply cable 50 to connect the HF supply to the circuit
far enough from the vat. The choice of location will be made to guaranty an
access without interrupting the operation of the furnace. This represents a
major advantage.
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Fig. 7a shows a schematic illustrating the circuit of Fig. 4. The difference
is
that the HF current source 19 is connected to the node point 17 and in
parallel to the capacitor 21 from a distant location via an HF supply cable
50. Fig. 7b applies the same technique, as Fig. 7a, for the HF voltage
source of Fig. 5. The cable connects, in series, the capacitor 21 and the
voltage source 19 via the nodal point 20. In both configurations, shown in
Fig. 7a and 7b, the cable 50 is preferably coaxial and is dedicated to carry
the supply current or voltage over a distance far enough from the arc-
furnace vat.
Fig. 8 shows the preferred embodiment for the cable construction of the
present invention. It comprises a center support member 54 surrounded by
a first conductor layer 55. The support member 54 has an external
diameter sufficiently large to ensure enough conducting perimeter for the
conductor layer 55 to carry the HF current of the HF supply source with
reduced Joule losses. The support member 54 may be of any type of
flexible material. In a preferred embodiment, the support member 54 is
made of polytetrafluoroethylene or polyethylene and comprises at least one
opening to allow a cooling liquid to flow through, in one or two direction. In
Fig. 8, there is shown 4 different openings 60, 61, 62, 63; in the openings
60, 61, 63 the cooling liquid flows in one direction; in the opening 62, the
cooling liquid flows in another direction. The cooling liquid, preferably
water, can be used to cool down the cable, the resonant capacitor 21, or
other parts requiring cooling. The conductor layer 55 is preferably made
with a copper braid to ensure flexibility and have a braid thickness greater
than the skin depth of the HF supply source frequency to ensure a
maximum current distribution within the conductor section. A second
conductor layer 57, of similar construction to the first conductor layer 55,
surrounds the conductor layer 55 and is separated by a dielectric layer 56
of sufficient thickness to sustain the voltage. The dielectric layer 56 is
preferably a low loss dielectric material such as polytetrafluoroethylene or
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polyethylene and is flexible. A heat-shield and insulated flexible jacket 58
covers the cable and ensures its protection against its environment.
In a typical arc-furnace installation, a cable of 25 to 50 meters is long
enough to safely locate the HF power supply. The HF source supply
frequency of the present invention (close to a hundred kilohertz) has a
wavelength, in the cable, of a few kilometers, which is much greater then
the cable length itself. Therefore, the cable shunt and series impedance
can be considered as lumped elements at this frequency. In the circuit
configuration of Fig. 7a, the cable parasitic capacitance dominates over the
inductance and must be added to the value of the capacitor 21.
Conversely, in the circuit of Fig. 7b, the cable parasitic inductance
dominates and must also be added to the inductor 14 inductance value of
the furnace conductors. In both cases, the cable contribution to the circuit
impedance remains low and does not seriously affect the resonant current
and voltage frequency in the L/C circuit.
The circuits of Figs. 9 are a modified version of the circuit shown in Fig. 7
and are used when the HF power supply produces harmonics like the HF
power supplies of Fig. 10. Referring now to Fig. 9a, the HF current source
19 is connected to the arc-furnace conductor 17 via the HF coaxial cable
50 and a snubber circuit 59. Referring to Fig. 9b, the HF voltage source 19
is remotely connected, in series with the resonant capacitor 21 at the
connecting point 20 which is connected in parallel to a snubber circuit 59
via the HF coaxial cable 50. In both configurations, when the harmonic
wavelength is of the same order as the cable length, the coaxial cable 50
acts as a transmission line having a propagation time for the wave to reach
the other end of the cable. Depending on the cable ends' impedance, a
reflection can be created successively at both ends. This "back and forth"
wave from one end to the other creates an undesired oscillation. The
snubber circuit 59 is intended to attenuate these reflections.
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From the HF source side of the cable, the transient impedance seen by the
source prior to the return of any reflection is called the surge impedance
and is equal to:
Surge impedance = L
Where Lc is the cable's inductance per unit length and Cc is the cable's
capacitance per unit length. An injected current transient into the cable will
generate a surge voltage proportional to the current amplitude times the
surge impedance during the period preceding the return of the first
reflection. If a voltage wave is injected instead, a current surge will be
generated in proportion to the voltage amplitude divided by the surge
impedance.
Referring back to Fig. 9a, the snubber circuit 59 comprises a resistor 51
connected in series with an HF coupling capacitor 52, and it also comprises
an LF (Low Frequency) bypass inductor 53 connected in parallel to the
resistor 51 and the HF coupling capacitor 52. The inductor 53 and the
capacitor 52 values are chosen so that the snubber circuit 59 almost acts
as a short-circuit for LF components (including the resonant frequency) and
appears as only resistor 51 for frequencies higher than the resonant
frequency. In operation, the harmonic waves coming from the HF supply
source encounter the snubber circuit 59 as a terminal impedance since the
capacitor 21 behaves as an short-circuit. These harmonic waves have a
frequency content higher than the resonant frequency and will therefore
see resistor 51. By matching the resistor 51 to the cable surge impedance,
the reflections will be strongly attenuated. Dues to inductor 53, the current
oscillating at the resonant frequency sees the snubber circuit as a short-
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circuit thus avoiding the creation of unnecessary Joule losses in the
snubber circuit.
The snubber circuit of Fig. 9b is different from the snubber circuit of Fig.
9a
since the HF voltage source is connected in series with the resonant
capacitor 21. The snubber circuit 59 comprises a resistor 51 connected in
series to an HF coupling capacitor 52 and an LF bypass inductor 53
connected in parallel to the resistor 51. The inductor 53 and the capacitor
52 are chosen so that the impedance of the snubber circuit is close to an
open-circuit for LF components (including the resonant frequency) and
appears as only the resistor 51 for frequencies higher than the resonant
frequency. In operation, the harmonic waves incoming from the HF voltage
source encounter the snubber circuit 59 as a terminal impedance. These
harmonic waves have a frequency content higher than the resonant
frequency and will therefore see the resistor 51 equaled to the surge
impedance and the reflections will be strongly attenuated. The inductor 53
serves to bypass the resistor 51 for lower frequencies in order to avoid
unnecessary Joule losses.
The preferred embodiment uses the circuit configuration of Fig. 9a. The
coaxial cable has a parasitic inductance below 0.05NH/m, a parasitic
capacitance below 500pF/m and a surge impedance bellow 2052. With this
surge impedance, the surge voltage produced by the injection of a high
current harmonic content is moderate and avoids excessive stresses on the
HF current supply.
In a preferred embodiment, the HF coaxial cable 50 can be installed along
the arc-furnace conductor 6 and 12 without perturbing the arc-furnace
conductors movement. Also, the HF cable can be inserted into a flexible
metal pipe running along the arc-furnace conductor in the region close to
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the vat in order to protect the HF cable from the furnace open flames and
from the splashes of metal in fusion.
For the following description that relates to Figs. 10 and 11, whenever the
term is followed by a term in parenthesis, the term applies to the series
voltage source of Fig. 5 and the term in parenthesis applies to the parallel
current source of Fig. 4. If it applies only for one case, it will be
specified.
Referring now to Figs. 10a (10b), there is shown the preferred
embodiments of the HF voltage (current) source in accordance to the
present invention.
In Figs. 10a (10b) the HF voltage (current) source 19 constructed using an
HF voltage (current) power inverter 28 (31) supplied by a DC voltage
(current) source 29 (32). The inverter uses technology commonly found in
the known art and incorporates a controller and sensors to adjust the
frequency according to sensed conditions. Preferably, an HF voltage
(current) transformer circuit 30 (33) is inserted at the output of the HF
voltage (current) inverter in order to adapt the voltage and the current to
fully use the power handling capability of the semiconductor switches in the
inverter. The HF transformer 30 (33) comprises a magnetic coupling
transformer and may include decoupling elements to avoid propagation of
the furnace supply current into the HF source. Also, if the resonant
capacitor circuit of Fig. 6 is used in combination with the HF current source,
the current transformer 33 magnetizing impedance may be sized in such a
way that it replaces the inductor 24 thus reducing the number of
components.
Referring now to fig. 10c (10d), a more detailed schematic representing a
preferred embodiments of the HF voltage (current) sources are shown. The
HF voltage (current) inverter 28 (31) includes an H-bridge inverter 39 which
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includes semiconductor elements such as IGBTs and diodes. The gate-
controlled semiconductors are activated by gate drivers 40 and the
switching commands are sent to the gate drivers by a controller unit 41.
The switching commands are determined by the controller unit 41 in
relation to the resonant frequency. It can be a fixed frequency or a
frequency locked to the HF waveform produced by an HF current (voltage)
sense 42 connected to the resonant circuit. In a preferred embodiment
shown in Fig. 10c (10d), the HF sensor reads the current 43 (voltage 44) at
the output of the inverter 39. If the apparatus of the present invention
comprises the optional HF voltage (current) transformer 30 (33), it is
preferred to connect the HF sensor at the output of the transformer in order
to avoid possible interaction between the internal impedance of the
transformer and the resonant circuit operation and controllability.
The HF voltage (current) source can operate appropriately with the
resonant circuit when the HF sense unit is connected elsewhere in the
circuit. For example, it could be connected across the capacitor 21. It is
important for the controller to monitor an HF waveform produced by the
resonant circuit when excited by the HF voltage (current) source, in order to
command the phase difference between the HF voltage (current) source
and the sensed current or voltage. In this way, in one embodiment, it is
possible to synchronize the commutation of the inverter with a zero-
crossing event of the current (voltage) to reduce the switching losses in the
semi-conductors.
According to one particular embodiment of the present invention, when the
output of the HF voltage (current) inverter 28 (31) is connected to an HF
resonant circuit, the method to control the HF voltage (current) source
comprises the steps of: reading and sending to the controller unit 41 the HF
sensed current (voltage) waveform resulting from the interaction of the HF
voltage (current) source 28 (31) with the resonant circuit it is connected to;
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performing with the controller unit 41 a phase locking to the HF sensed
signal waveform and generating with the controller 41 the switching
command cycles for the gate drivers at a predetermined phase shift from
the read waveform signal in order to impose an HF voltage (current) at the
output at a determined phase-shift from the output current (voltage).
According to another embodiment of the present invention, when the output
of the HF voltage (current) inverter 28 (31) is connected to an HF resonant
circuit, the method to control the HF voltage (current) source comprises the
steps of: reading and sending to the controller unit 41 the HF sensed
current (voltage) waveform resulting from the interaction of the HF voltage
(current) 28 (31) with the resonant circuit it is connected to; comparing said
HF sensed current (voltage) with a current (voltage) threshold; if the
sensed current (voltage) is lower than the current (voltage) threshold, then
generating with the controller 41 the switching command cycles in order to
produce an HF voltage (current) at a standby frequency close to the
resonant frequency; if the sensed current (voltage) is higher than the
current (voltage) threshold, then performing with the controller 41 a phase-
lock to the HF sensed signal waveform, and generating with the controller
41 the switching command cycles for the gate-drivers, at a predetermined
phase-shift from the read waveform signal, in order to impose an HF
voltage (current) at the output at a determined phase-shift from the output
current (voltage).
In the present invention, the HF voltage (current) source 28 (31) is
operated in such a way to track the resonant frequency of the resonant
circuit. This action is performed by the controller unit 41 and is achieved by
forcing a phase-shift between the HF inverted voltage (current) and the
sensed waveform that results in a null difference in phase between the
inverted HF voltage (current) and the resulting HF output current (voltage).
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This condition is obtained when the HF voltage (current) source is
switching at the resonant frequency of the HF resonant circuit.
The inductance of the furnace conductor is changing when the electrodes
are in movement. Of course, this affects the resonant frequency that the
controller of the inverter will track.
When the inverter operates at the standby frequency, the standby
frequency is chosen at a convenient value in the range of all possible
resonance frequencies. The variation of the inductance of the furnace
conductors varies with the logarithm of the distance separating the
conductors, and the resonant frequency varies with the square root of the
conductors' inductance. When the arc-furnace operates, the electrodes
relative displacement is normally maintained small enough to avoid any
unbalance in the conductors' impedance that would create an unbalance in
the currents. Therefore, the resonant frequency is not expected to change
much on an arc-furnace installation and the standby frequency will be close
enough to the resonant frequency to build a resonance. When the
oscillating current (voltage) reaches the threshold, in one embodiment of
the present invention, it is preferred to lock the inverter frequency at the
resonant frequency in order to reduce the switching loss in the inverter and
achieve the highest resonance. In an other embodiment, the last frequency
locked can be held by the controller and used as the standby frequency
until the next threshold is attained. This has the advantage that, until the
next zero current point of the next half cycle, the furnace conductor
configuration will not move in a perceptible way to affect the resonant
frequency.
Fig. 11 is composed of three plots whose time scales are divided according
to the four time periods involved in the restriking process. The first plot
Fig.
11 a illustrates the evolution of the square wave HF voltage (current) source
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produced by an inverter such as the one described in Figs. 10. Fig. 11 b
shows the evolution of the resonant capacitor HF voltage. Of course Fig.
11 c shows the evolution of the HF current in the furnace plasma. The
component of the current and voltage created by the furnace power supply
were intentionally omitted for clarity of the description.
Period T1 corresponds to an arc burning condition in the arc-furnace
operation. During that period, the plasma column carries the main furnace
arc current and the HF arc current shown in Fig. 11c. The HF voltage at the
resonant capacitor is considerably reduced in Fig. 11 b due to the shunting
effect provided by the high conductivity of the plasma column. This voltage
is not tracked by the controller unit, which then operates at the standby
frequency, not far from the resonant frequency. In a different embodiment,
the HF current source can be also turned off during that period since the
arc is burning.
The beginning of period T2 corresponds to an interruption of the furnace
arc. The loss of the arc causes the plasma column to gradually lose its
conductivity depending on the environment temperature and condition. As
the conductivity decreases, the resonant circuit increases its resonance
and builds an HF voltage on the resonant capacitor to maintain the HF arc
current shown in Fig. 11 c. Depending on decreasing rate of the plasma
conductivity, energy will be supplied by the HF voltage (current) source to
crank up the resonance. This will reduce the HF injected current in the
plasma. During the intensification of the resonance, if the threshold current
(voltage) is exceeded, the control unit triggers a phase lock loop circuit to
lock the voltage (current) source frequency at the resonant frequency in
order to limit the switching loss of the inverter. If the furnace arc re-
ignites,
the voltage drop will collapse and T4 start immediately without entering into
T3.
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The condition at the beginning of T2 may also correspond to an interruption
of the arc as well as a loss of the plasma column. Another condition that
would have caused a resonance could be the interruption of the arc
followed by the loss of the plasma column. In this case, the HF would be
null and the resonance will increase at the rate related to the amount of
energy supplied by the HF voltage (current) power supply and limited by
the dissipation factor of the resonant circuit. Of course, the plot of Fig. 11
c
would have shown no HF current in periods T2 and T3 until an ignition
occurs.
To limit the voltage at the electrode, a further aspect of this embodiment of
the present invention comprises a control strategy to limit the electrode
resonant voltage by controlling the way the inverter is switched. At the
beginning of period T3, if no ignition is obtained and the voltage reaches its
maximum permissible value on the resonant capacitor, the inverter starts to
cut out half cycles and lets the oscillating voltage decay. Once a lower limit
value is reached, the inverter injects the number of half cycles necessary to
bring the voltage back up to its upper limit, thus maintaining the oscillation
in an apparent steady state and keeping the resonant voltage present at
the electrode end. In this situation, the plasma conductivity may be
gradually lost. In that case, the HF arc current will decrease and will
eventually extinguish. But, the voltage will still be maintained and will help
to strike a new main furnace arc when the arc-furnace electrode will be
moved towards the scrap.
At the beginning of period T4, ignition occurs; the main furnace arc is re-
established; the voltage collapses; the HF current flows in the arc; and the
HF power supply inverter returns to the standby operating frequency before
entering again into period T1.
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A demonstration is shown in the recorded signal of the present apparatus
operating with a 60 Hz AC arc current in Fig. 12. The dark trace is the
current in the plasma column and the gray trace is the voltage drop. The
HF current in the plasma is superimposed on the 60 Hz current and is seen
as a thick dark trace due to its very fast variation. The same also applies to
the voltage drop. It can be observed that the HF resonant circuit combined
with the HF power supply produces the necessary voltage to force the flow
of HF current through the plasma column even if the plasma conductivity is
changing. The persistence of the HF plasma current can be observed
when the low frequency arc interrupts as pointed by the arrows. After the
loss of the 60Hz arc, the plasma starts to lose its conductivity and the
resonant circuit of the present invention progressively increases the
electrode voltage to inject the HF arc current in the plasma, thus
maintaining the production of ions and free electrons. When the 60 Hz
voltage catches up the reduced ignition-voltage, the low frequency arc
ignites and the voltage collapses. The change in the dead time period is
linked to the erratic behavior of the arc as mentioned earlier.
Fig. 13 shows a three-phase AC arc-furnace in accordance with the two
embodiments previously described of the present invention. The arc-
furnace uses two resonant capacitors 21 on a three-phase arc-furnace with
two HF current sources 19 that may either work asynchronously or
synchronously with a predetermined phase shift. This configuration has the
advantage that it reduces the number of required components compared to
using one resonant capacitor per phase. In this embodiment, the two HF
current inverters 31 are operated at the same frequency and in opposite
phase in reference to the dot mark at the inverter output 31. To maintain
the resonant capacitor 21 in contact at the closest point from the electrode
end, two sliding contacts 34 are added between each electrode's conductor
and on one end of a resonant capacitor 21 on two phases to compensate
for the movement of the electrodes. The other end of the two resonant
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capacitors 21 is permanently connected to the remaining electrode's
conductor. The sliding contact normally covers a maximum displacement
length. The displacement length can be sufficient to cover almost the
entire gap encountered in operation between the two electrodes. In this
way, the need is eliminated to make contact at very large gaps that occurs
only a very few times during the arc-furnace operation without significantly
affecting the advantages. obtained and thus reducing the slide contact
complexity.
Referring now to Fig. 14, there is illustrated a typical integration of the
apparatus on the arc-furnace according to Figs. 4 and 13. The resonant
capacitors 21, are mounted on the central electrode supporting member.
The HF current sources 19 are placed in a remote location as for instance,
in the transformer room. The HF sources 19 are connected to their
corresponding capacitor 21 via a pair of HF supply cable 50. In the
preferred embodiment, the coaxial cables run alongside the central arc-
furnace high current conductor. Each resonant capacitor 21 has one
terminal connected to the central electrode supporting member and the
other end connected to the corresponding lateral electrode supporting
member via a sliding contact (not shown) to cope with the relative
displacement of the electrodes. The high frequency bypass capacitor 15 is
connected at the output of the supply transformer.
Another advantage of the apparatus of the present invention resides in the
fact that little modifications are required to the arc-furnace equipment. The
apparatus operates in parallel with the arc-furnace and can be switched off
at any time and, in one embodiment of the present invention, the HF source
may be in a failure mode and under repair without interrupting the arc-
furnace operation.
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The preferred embodiment of the apparatus of the present invention, the
arc-furnace comprises a second electrical power supply to maintain or
initiate a plasma link between an electrode and the melting metal after an
interruption of the main furnace arc, with an apparatus located at a safe
distance from the severe environment condition where the plasma is
produced, using the inductive property of the furnace large current
conductor, and where the apparatus is of modest size.
