Electric arc-furnace stabilization using voltage pulse injection

An arc furnace (1) has an electrode (13) and connection members (14) for connection to a power-supply network (20) for supplying an arc (16) at the electrode with current. The furnace is provided with a voltage-pulse generating member (3) adapted, in connection with an interruption in the arc, to supply voltage pulses to the furnace for striking the arc.

TECHNICAL FIELD 
The present invention relates to furnace equipment, comprising an arc 
furnace having at least one electrode and connection members for 
connecting the furnace to a power-supply network for supplying an arc at 
the electrode with current. 
BACKGROUND ART 
Electric arc furnaces have long been used, for example for steel 
manufacture. Such a furnace has one or more electrodes and a furnace 
container, in which the charge of the furnace is applied. The electrode or 
electrodes is or are connected to an electric power network, which feeds 
current to one or more arcs which are burning between the electrodes and 
the charge and/or between the electrodes. The electrical energy developed 
in the arcs results in the desired heating of the furnace charge. 
Arc furnaces are usually supplied with alternating voltage, but also arc 
furnaces supplied with direct voltage occur. A typical arc furnace for use 
during steel manufacture has three electrodes arranged above the charge, 
which are each connected to their own phase in a three-phase power-supply 
network. The electrodes can be provided in a known manner with devices for 
position control of the electrodes, that is, for control of their 
distances to the charge and hence of the length of the arcs. The furnace 
is usually connected to the network via a furnace transformer, which is 
normally provided with an on-load tap changer for control of the voltage 
supplied to the furnace. 
Arc furnaces are often designed for very high power, and the effect of such 
a furnace on the power-supply network, as well as its effect on other 
consumers connected to this network, may therefore be strong. 
An arc furnace exhibits, especially during certain types of operation such 
as, for example, during melting of a charge, unstable operating 
conditions. Short circuits between the electrodes and the charge 
frequently occur, with surge currents resulting therefrom. Also, it may 
occur that an arc is extinguished, whereby the current in the phase in 
question ceases altogether. Usually, these disturbances are unsymmetrical 
in such a way that two phases become short-circuited and one phase 
currentless. These phenomena give rise to considerable load variations in 
the power-supply network, and the load variations cause voltage variations 
in the network. 
Low-frequency components of the above voltage variations can be reduced 
with the aid of equipment for controllable static reactive-power 
compensation. However, a considerable part of the voltage variations 
caused by an arc furnace lie within a frequency range--about 4-8 Hz--where 
the voltage variations cause so-called flicker, which is disturbing to the 
eye, that is, variations in light intensity of light bulbs and other light 
sources, as well as other disturbances, for example changes in the 
appearance of the picture in TV receivers, while at the same time the 
frequency of the disturbances is so high that it is not possible to reduce 
the disturbances to the desirable extent with the aid of controllable 
reactive-power compensation. 
Unless the short-circuit power of the electric power-supply network is very 
large in relation to the rated power of the furnace, an arc furnace will, 
for the above reasons, cause inconvenient disturbances for the other 
subscribers. 
After the meltdown phase, the arcs of the furnace burn in a more stable 
manner, which reduces the above-mentioned disturbances. During this phase, 
an arc is extinguished at each zero crossing of the electrode current and 
is regularly restruck after a certain time during the next half-cycle of 
the electrode voltage, when the voltage with the new polarity has reached 
a certain value sufficient for firing. A currentless interval appears 
between each extinction and the following firing, that is, once every 
half-cycle. These currentless intervals reduce the average power developed 
in the arcs and hence the production capacity of the furnace. It is 
therefore desirable to reduce the length of the currentless intervals, if 
possible. 
It is previously known that, by connecting additional inductances into the 
supply lines of the electrodes, faster restrikings after the zero 
crossings of the current can be obtained. This results in a reduction of 
the length of the currentless intervals. However, for the furnace 
equipment as a whole, the method entails a lower power factor and a lower 
current amplitude but, on the other hand, it results in longer arcing 
times and hence a possibility of increased furnace production. At the same 
time, the costs of the furnace equipment, and the losses therein, 
increase. 
SUMMARY OF THE INVENTION 
The invention aims to provide equipment of the kind described in the 
introductory part of the specification, in which, without increasing the 
losses of the equipment or the reactivepower consumption, a reduction of 
the disturbances (e.g. flicker), caused by the furnace, on the 
power-supply network as well as a reduction of the length of the 
currentless intervals of the arcs can be achieved and hence a high 
production capacity of the furnace be attained. 
What characterizes equipment according to the invention in more detail will 
become clear from the appended claims. 
According to one embodiment of the invention, the voltage pulse is 
generated by establishing a short-circuit path in parallel with the 
furnace, whereupon the short-circuit path is broken. A voltage pulse can 
in this way, that is, by breaking up an inductive circuit, be generated in 
a simple manner and with a minimum of necessary additional equipment. The 
short-circuit path is preferably arranged by firing a thyristor, 
preferably a gate turn-off thyristor (GTO), which is included in a 
thyristor circuit capable of being turned off and arranged in parallel 
with the furnace, and the short-circuit path is broken by turning off the 
mentioned thyristor. 
It has proved that an increase of the advantages provided by the invention 
can be obtained by generating the voltage pulse for firing with a certain 
delay in relation to the interruption of the arc. In many cases, it has 
proved that the magnitude of this delay can influence the magnitude and 
the frequency spectrum of the disturbances generated by the furnace as 
well as the amplitude of the furnace current and the length of the 
currentless intervals. According to one embodiment of the invention, 
therefore, the delay is controlled in dependence on some suitable sensed 
quantity, or function of sensed quantities, in such a way that an optimum 
relation is obtained between the quantities influenced by the delay.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows in the form of a single-line diagram furnace equipment 
according to one embodiment of the invention. It comprises a schematically 
shown arc furnace 1 which has a furnace container 10 with a bottom contact 
11, which makes electrical contact with a charge 12 located in the furnace 
container. A furnace electrode 13 is arranged above the charge. The 
electrode has a connection lead 14 and the bottom contact a connection 
lead 15. During operation of the furnace, an arc 16 is burning between the 
electrode and the charge. 
The furnace is supplied from a power network 20 via a furnace transformer 
21 provided with an on-load tap changer. The secondary winding of the 
transformer is connected, via an inductance 22, to the connection lead 14 
of the furnace. In parallel with the furnace, a pulse generator 3 is 
connected and adapted to apply voltage pulses across the furnace in a way 
which will be described below. 
A voltage-measuring member 23, for example a voltage transformer, is 
connected to the furnace and delivers to the pulse generator a signal "u" 
corresponding to the instantaneous value of the furnace voltage. A 
current-measuring device 24, for example a current transformer, is 
connected in the supply lead to the furnace and is adapted to supply a 
signal "i" which corresponds to the instantaneous value of the current 
supplied from the transformer to the furnace. The signals "u" and "i" are 
used in a manner which will be described below for controlling the pulse 
generator 3 such that current pulses with a suitable polarity and a 
suitable phase position are applied to the furnace. 
The furnace may be a single-phase furnace and be supplied from a 
single-phase power network, and the equipment then closely corresponds to 
that shown in FIG. 1. Alternatively, the furnace may be a three-phase 
furnace and be supplied from a three-phase power network via a three-phase 
furnace transformer. In this case, the furnace may have no bottom contact, 
and the pulse-generating member 3 may then be connected to the connection 
leads of the three electrodes and apply voltage pulses to the furnace with 
a suitable polarity and a suitable phase position between the electrodes 
instead of, as shown in FIG. 1, between the electrode and a bottom 
contact. 
In a known manner, the inductance 22 can consist of a discrete inductor 
connected between the furnace transformer and the furnace. Since a 
considerable advantage which is obtained with the invention is that stable 
operation can be achieved with a lower series inductance than what has 
been possible earlier, a discrete inductor may possibly be completely 
avoided, and the inductance 22 in FIG. 1 then consists of the inductances 
of the network, the furnace transformer and the connection lead 14. 
The furnace 1 is only schematically shown and may be provided with 
additional suitable or necessary items of equipment or features, known per 
se, such as control devices for position control of the electrodes and of 
the on-load tap changer of the transformer. 
The pulse generator 3 is connected to the electrode connection 14 of the 
furnace. It can supply voltage pulses of both polarities, and it comprises 
for each polarity a capacitor, 31a and 31b, respectively, a charging 
member for the capacitor in the form of a rectifier, 33a and 33b, 
respectively, supplied from an alternating-voltage source, and a gate 
turn-off thyristor (GTO), 32a and 32b, respectively, connected in series 
with the respective capacitor. Each one of the two rectifiers consists of 
a controllable rectifier, the current and voltage of which are controlled 
in a known manner such that charging of the capacitor with a suitable 
speed and to a suitable voltage is obtained. Further, the pulse generator 
3 comprises a resistor 34 with a low resistance so chosen that the 
discharging current of the capacitors through the thyristors is limited to 
a permissible value. Further, the pulse generator comprises an overvoltage 
protective device 35 in the form of a varistor for protection of the pulse 
generator and the components included therein, and of the rest of the 
furnace circuit and particularly the furnace transformer. Finally, the 
pulse generator includes a control device 36, which is supplied with the 
measured quantities "i" and "u" and which delivers firing and turn-off 
pulses to the thyristors 32. 
FIG. 2 shows the configuration of the control device 36. The signals "i" 
and "u" are supplied to level-sensing circuits 361, 362 and 363, 
respectively. The circuit 361 is adapted to deliver an output signal i0 
which is a "1" when the furnace current is zero, that is, in practice when 
it is lower than a low limit value. The circuit 362 delivers a signal u+ 
which is "1" when the furnace voltage is positive, that is, when it 
exceeds a low positive value. The circuit 363 delivers a signal u- which 
is "1" when the furnace voltage is negative, that is, when it is lower 
than a low negative value. The signal i0 is supplied to one of the inputs 
of two AND circuits 364 and 365, and the signals u+ and u- are supplied to 
the other inputs of the AND circuits. The output signal p+ of the circuit 
364 passes from "0" to "1" when the condition "i" zero and "u" positive is 
fulfilled. The signal is supplied to a differentiating input of a delay 
circuit 366 with the delay "t". The latter circuit thus delivers a pulse 
t+ at the time "t" after the above-mentioned condition has been fulfilled. 
This pulse is supplied to a control pulse device 368 and causes this to 
emit a firing signal to the firing electrode 321a of the thyristor 32a 
and, at a predetermined time thereafter, a turn-off signal to the turn-off 
electrode 322a of the same thyristor. 
In similar manner, the output signal p- of the circuit 365 passes from "0" 
to "1" when the condition "i" zero and "u" negative is fulfilled. The 
signal is supplied to a differentiating input of a delay circuit 367 with 
the delay "t". The latter circuit thus delivers a pulse t- at the time "t" 
after the above-mentioned condition has been fulfilled. This pulse is 
supplied to a control pulse device 369 and causes this to deliver a firing 
signal to the firing electrode 321b of the thyristor 32b and, at a 
predetermined time thereafter, a turnoff signal to the turn-off electrode 
322b of the same thyristor. 
The time delay in the circuits 366 and 367 can suitably be of the order of 
magnitude of 1 ms. The time between firing and turn-off of each one of the 
thyristors 32a and 32b may, for example, be 100 .mu.s. 
When, for example after a half-cycle with negative voltage and current, the 
arc 16 is extinguished, the sensed current will be zero and the furnace 
voltage positive. With the delay "t", the thyristor 32ais turned on and is 
shortly thereafter turned off (in this example 100 .mu.s later), the 
capacitor 32athus becoming discharged through the thyristor and a short 
voltage pulse with a high amplitude being supplied to the electrode. The 
arc is thereby caused to become ignited earlier than what would otherwise 
be the case. In a corresponding way, the furnace is supplied with a 
negative voltage pulse with the aid of the thyristor 32bafter a half-cycle 
with a positive voltage and current. 
In a corresponding way, the equipment according to the invention operates 
if an interruption of the arc occurs at other times than at the end of the 
half-cycles of the furnace current. An interruption is indicated by the 
signal i0 becoming "1", which causes a voltage pulse to be supplied to the 
furnace and results in a rapid and reliable restriking of the arc. As in 
the case described above, the circuits 362 and 363 ensure a correct 
polarity to the voltage pulse. 
The charging voltage of the capacitors and hence the amplitude of the 
applied voltage pulses may, for example, be of the order of magnitude of 
one or a few kV. 
Each thyristor is suitably provided, in a known manner, with a so-called 
snubber circuit, that is, an RC circuit, connected in parallel with the 
thyristor, or another circuit for reducing the rate of rise of, and for 
limiting the maximum amplitude of, the thyristor voltage when turning off 
the thyristor. 
The voltage pulses applied according to the invention typically give rise 
to a certain increase of the amplitude of the furnace current, a 
considerable reduction of the dead time (the time between extinction and 
restriking of the arc) and a more rapid growth of the current after 
striking of the arc. An important advantage of the invention is therefore 
the considerable increase which is obtained of the current-time area 
during each half-cycle of the furnace current, and hence the considerable 
increase of the production capacity of the furnace. By the chosen delay 
between the current zero-crossing and the voltage pulse, the advantageous 
effects of the invention are strengthened. Especially advantageous is the 
described delay when the furnace current is low and the dead time long. 
FIGS. 3aand 3bshow an example of the effects of the invention. The figures 
show the supply voltage u.sub.n (the secondary voltage of the 
transformer), the furnace voltage u (approximately equal to the arc 
voltage) and the furnace current i. FIG. 3ashows, for one 
alternating-voltage cycle, the conditions during operation in the manner 
previously known, that is, without the voltage pulses according to the 
invention. FIG. 3bshows, also for one cycle, the conditions during 
operation according to the invention. The voltage pulses are marked "SP". 
As can be seen, a considerable reduction of the dead time dt is obtained 
with the aid of the invention, which results in a considerable increase of 
the power factor of the furnace equipment. Similarly, a certain increase 
of the current amplitude and a considerable increase of the current-time 
area, and hence of the productivity of the furnace, are obtained. In this 
operating case, the furnace is supplied with 50 Hz alternating voltage 
with 400 V amplitude. The inductance 22 in FIG. 1 is 10 .mu.H. The delay 
"t" of the circuits 366 and 367 was 1 ms. As is clear from the figure, an 
increase of the amplitude of the furnace current from about 25 kA to about 
40 kA and a reduction of the dead time from about 2 ms to about 1 ms were 
obtained. Thus, in a simple and economical way, considerable improvements 
of the operating properties of the furnace can be obtained. The energy in 
the voltage pulses will for the most part be supplied to the arc and hence 
to the furnace, so the power losses in the voltage-pulse generating 
circuit according to the invention are small. 
In those cases where the furnace is operated with medium-high or high 
current and already has a short dead time, it may be advantageous to 
reduce or completely eliminate the delay described. The delay in the 
circuits 366 and 367 may therefore advantageously be made variable in 
dependence on the operating conditions of the furnace. This is shown in 
FIG. 2 by supplying a control signal S (dashed line) to the circuits, for 
controlling the magnitude of the delay "t". This control signal can be 
manually influenced for adjusting the delay to a value which gives optimum 
operating conditions. Alternatively, the delay can be controlled 
automatically in dependence on one or more sensed operating variables. 
FIG. 2 shows how a plurality of sensed operating variables a1, a2, a3, a4 
are supplied to a control unit 360, which calculates and generates the 
control signal S which in each case gives the optimum delay as a 
predetermined function of the sensed operating variables. Examples of 
suitable operating variables for controlling the delay are, as mentioned 
above, the mean value and the dead time of the furnace current. 
It has proved that the advantageous effect of the invention, in an often 
complicated and incompletely predictable way, depends on the value of the 
above-mentioned time delay. Alternatively, therefore, the control unit 360 
can be designed as an optimization unit, in a manner known per se within 
control engineering. It is then supplied continuously with--or determines 
itself based on sensed operating variables--one or more of the quantities 
which are influenced by the delay and which are desired to be optimized 
with the aid of the invention. These quantities may, for example, be the 
flicker amplitude within a certain frequency range, the reactive-power 
consumption and the mean current of the furnace. The control unit 
calculates an optimization variable according to a predetermined function 
of these variables. The function is chosen such that the operation, if the 
optimization variable is maintained at a minimum (or a maximum), is 
considered optimal. For example, the function may be a simple linear 
relationship 
EQU T=k1.multidot.a1+k2.multidot.a2+k3.multidot.a3+. . . 
where a2, a2, etc., are the different operating variables, where k1, k2, 
etc., are the chosen weighing factors for the respective operating 
variable, and where T is the optimization variable. The control unit in a 
known manner automatically and continuously adjusts the control signal S 
and hence the set delay, and the unit senses the changes of the variable T 
caused by the delay. This is done in such a way that the variable T is 
always maintained at a minimum (or maximum) and thereby the operation of 
the furnace is always optimal in accordance with the criteria 
predetermined by the optimization function. 
FIG. 4 shows an alternative embodiment of furnace equipment according to 
the invention. It differs from that shown in FIG. 1 by the design of the 
voltage-pulse generating member 3. This is considerably simplified in that 
the capacitors and their charging devices have been eliminated. Instead, 
shortcircuit paths 30a, 30b are connected in parallel with the furnace and 
with the aid thereof the furnace can be shortcircuited. For closing and 
opening, respectively, of the short-circuit paths, the turn-off thyristors 
32aand 32bare connected into the paths. When a voltage pulse is to be 
generated, for example a positive pulse, the corresponding thyristor is 
fired, in this case 32a. The short-circuit path 30 is then closed, and a 
rapidly increasing short-circuit current is driven by the supply voltage 
of the furnace through the inductance 22 and the short-circuit path. After 
a time interval, so selected that the current has time to reach a high 
value but not a value exceeding the current capability of the thyristor, 
the thyristor is turned off. The short-circuit path is then opened, and a 
voltage is induced in the inductance which gives a positive voltage pulse 
of a high amplitude across the furnace. The interval between turn-on and 
turn-off may as an example, under the operating conditions described with 
reference to FIG. 3, be about 500 .mu.s, which means that a maximum 
short-circuit current of the order of magnitude of 10 kA is built up prior 
to the turn-off of the thyristor. To some extent depending on the 
dimensioning and design of the snubber circuits, this is sufficient to 
generate a voltage pulse of sufficient amplitude--typically a few kV--to 
give a rapid and reliable restriking of the arc. Each thyristor is 
provided with a snubber circuit consisting of a capacitor 324a, 324b in 
series with a resistor 325a, 325b. 
The resistor is connected in parallel with a diode 323a, 323b with the same 
conduction direction as the thyristor, the reason being to facilitate the 
turn-off in a known manner by providing a slower voltage rise during the 
turn-off process. 
The control device 36 can be designed in the same way as that shown in FIG. 
2, and the function and the effect of the equipment according to FIG. 4 
are, in all essentials, the same as described with reference to FIGS. 1-3. 
The varistor 35 limits the pulse and thus protects the pulsegenerating 
equipment and its components as well as the furnace circuit and especially 
the furnace transformer. In the control device shown in FIG. 2, each 
thyristor is turned on for a predetermined period of time, in the example 
described 500 .mu.s. Alternatively, the current through the turned-on 
short-circuiting thyristor can be measured and the thyristor be turned off 
when the current has reached a pre-determined value, for example a value 
within the interval 1 kA-10 kA, depending on, inter alia, the current 
capability of the short-circuiting thyristors. 
The embodiments of the invention described above with reference to FIGS. 
1-4 can be used, with appropriate alterations, both for single-phase and 
multi-phase furnaces. Furnaces of the kind described occurring in practice 
are normally of three-phase design. Such a furnace often has no bottom 
contact, and in these cases the charge of the furnace is not electrically 
accessible. In such furnaces the items of equipment described above are 
not directly applicable. 
FIG. 5 shows equipment for supplying a three-phase arc furnace. The power 
supply network 20 is a three-phase network, and the furnace transformer 21 
is a three-phase transformer. In the figure the furnace is symbolically 
shown as three single-phase furnaces 1r, 1s, 1t but in reality consists of 
one single furnace container with a charge, above which the three 
electrodes 13r, 13s 13t are arranged. The equipment generates voltage 
pulses according to the principle described above with reference to FIG. 
4. However, in this case the short-circuit paths are established between 
the three connection leads 14r, 14s, 14t of the furnace. This is done with 
the aid of the thyristor connections 32sr-32rs, 32ts-32st, 32tr-32rt 
connected between the connection leads. Each such connection consists of 
two antiparallel-connected turn-off thyristors. The thyristors are 
provided with snubber circuits, which for the sake of simplicity are not 
shown in the figure, for example of the same type as the snubber circuits 
shown in FIG. 4. 
The control equipment for the thyristors can be designed in a manner 
analogous to the equipment shown in FIG. 2. An example of that part of the 
equipment which controls the thyristors 32rs and 32sr is shown in FIG. 6. 
The current ir in phase r is sensed, and a signal ir0 ="1" is delivered by 
a level detector 361rs if the absolute value of the current is below a 
predetermined low level. The signal is supplied to the AND circuits 364rs 
and 365rs. The voltage urs between the phases r and s is sensed and 
supplied to level detectors 362rs and 363rs. The former detector delivers 
a signal urs+="1" if the voltage is positive, and the latter detector 
delivers a signal urs-="1" if the voltage is negative. These signals are 
supplied to the AND circuits 364rs and 365rs. In a manner analogous to 
that shown with reference to FIG. 2, the circuit 364rs, when both its 
input signals are "1", delivers a signal p+ for firing the thyristor 32rs. 
The circuit 365rs delivers a signal p- for firing the thyristor 32sr when 
both its input signals are "1". The signals p+ and p- are supplied to the 
delay circuits 366rs, 367rs with differentiating inputs and with 
controllable delay. The output signals t+ and t- from these circuits are 
supplied to the control devices 368rs, 369rs, which deliver turn-on and 
turn-off signals to the thyristors 32rs and 32sr, respectively. 
The control circuit shown in FIG. 6 and described above operates in a way 
which is completely analogous to that described above with reference to 
FIGS. 1-4. When the current in one phase (r) becomes zero, one of the two 
thyristors (32rs, 32sr), which are connected to the phase (s) which is 
positioned immediately after in the phase sequence (rst) of the supply 
alternating voltage, is fired, with a possible delay. Only one of these 
thyristors can carry current, depending on which of the phases mentioned 
has the highest potential. The level detectors (362rs, 363rs) ensure that 
this very thyristor is fired (and turned off after having worked up a 
short-circuit current of a suitable magnitude). During the next 
half-cycle, the other of the two mentioned thyristors is fired. 
The configuration and the mode of operation of the control circuits for the 
other two thyristor pairs are completely analogous to that described 
above. 
FIG. 7 shows the mode of operation of the equipment shown in FIGS. 5 and 6. 
In FIG. 7, the three phase currents ir, is, it of the furnace are shown as 
a function of time t. Furthest to the left in the diagram, ir is positive 
and declining. When ir becomes zero, the thyristor 32sr is fired at t=t1, 
and the supply voltage drives a short-circuit current through this 
thyristor and the inductances 22r, 22s (the voltage in phase s is positive 
in relation to the voltage in phase r). Part of the furnace current in 
phase s passes during this interval through the thyristor 32sr. At t=t2, 
this current has reached a sufficiently high value, and the thyristor 32sr 
is turned off. This causes a negative voltage pulse to be induced into the 
phase r of the furnace, which immediately fires the arc in this phase, and 
a high current immediately starts to flow. During the process described, 
the current in phase t is influenced only to an insignificant extent. 
At t=t3, the thyristor 32tr is fired, and at the turn-off thereof, at t=t4, 
a positive voltage pulse is supplied to the phase t of the furnace, 
whereby the arc in this phase is fired. 
At t=t5, the thyristor 32ts is fired, and at the turn-off thereof, at t=t6, 
a negative voltage pulse is supplied to the phase s of the furnace, 
whereby the arc in this phase is fired. 
At t=t7, the thyristor 32rs is fired, and at the turn-off thereof, at t=t8, 
a positive voltage pulse is supplied to the phase r of the furnace, 
whereby the arc in this phase is fired. 
At t=t9, the thyristor 32rt is fired, and at the turn-off thereof, at 
t=t10, a negative voltage pulse is supplied to the phase r of the furnace, 
whereby the arc in this phase is fired. At t=t11, the thyristor 32st is 
fired, and at the turn-off thereof, at t=t12, a positive voltage pulse is 
supplied to the phase t of the furnace, whereby the arc in this phase is 
fired. 
In FIG. 7 it has been assumed, for the sake of simplicity, that the 
short-circuit thyristor is fired simultaneously with the furnace current 
becoming zero, that is, the time delay of the delay circuits 366rs and 
367rs are assumed to be zero. The function is analogous to that shown if 
the delay in these circuits is greater than zero. 
The embodiments of the invention described above relate to a.c. furnaces. 
However, the invention offers considerable advantages also in connection 
with d.c. furnaces, because also in such furnaces interruptions in the arc 
occur, especially when melting down a charge. Since the invention provides 
a rapid and efficient restriking of the arc, a smoother operation is 
obtained also in this case, that is, a reduction of the effect of the 
furnace on the network, as well as an increase of the mean current of the 
furnace and therefore an increase of the production capacity of the 
furnace. 
The embodiments of the invention described above show the case where one 
single voltage pulse is supplied to the furnace in connection with the 
zero crossing of the furnace current, that is, in case of an interruption 
in the arc. Firing of the arc according to the invention can alternatively 
take place by supplying to the furnace, instead of one single pulse, two 
or more pulses, for example in the form of a high-frequency pulse train. 
Similarly, in the embodiments of the invention described above, only one 
single voltage pulse is supplied to the furnace in connection with each 
zero crossing of the furnace current (each interruption of the arc). 
Although this is a preferred embodiment, it is possible within the scope 
of the invention to modify the control circuits such that, for example in 
dependence on some predetermined criterion, voltage pulses for striking 
the arc are supplied only in case of certain kinds of interruptions in the 
arc.