Process and apparatus for measuring the instability of an arc in an electrical arc furnace for processing liquid metal

According to this process and this device: PA1 a signal representing the derivative of the intensity of the current in the arc is sensed, PA1 the sensed signal is amplified, PA1 the amplified signal is directed simultaneously into two band pass filters (6, 7) respectively having a high frequency broad band (6) and having a narrow band centered on a low fundamental frequency (7), PA1 the signals emanating from the filters (6, 7) are transmitted to effective value extractors (8, 9), PA1 by means of a divider module (11) an energy signal is formulated, which is proportional to the ratio of the effective value of the signal emanating from the broad band filter (6) and the effective value of the signal emanating from the narrow band filter (7), PA1 and the proportional energy signal obtained is displayed on a scale (12), where it is expressed as a percentage, this percentage being indicative of the behaviour of the arc, signifying the formulation phase. The signal supplied by the display element (12) permits the regulation of the furnace in real time by the control operator.

The present invention relates to a process and a device for measuring the 
instability of an arc in an electrical furnace for processing liquid 
metal, in particular in a steel works. 
It is known that the characteristics of the arc of an electrical furnace 
are liable to vary during a liquid metal processing. 
Moreover, the slag which floats on top of the liquid metal bath undergoes, 
during the refining of the steel, a foaming caused by the chemical 
reactions in the liquid metal and in the slag, which are consequent upon 
the injection of oxygen and of carbon through blast pipes respectively 
into the liquid metal and into the slag. In fact, the injection of oxygen 
causes the formation of iron oxide which passes into the slag, where the 
injection of charcoal causes the formation of carbon oxide which causes 
the slag to foam. Thus, the arc is to a greater or lesser extent immersed 
in the layer of slag during the refining process. 
Now, it has been found that there is a correlation between the distortion 
of the intensity of the current in the electric arc and the foaming of the 
slag. In fact, the foaming has an effect on the characteristics of the 
electric arc, namely its voltage and the intensity of the current. 
Thus, the object of the invention is also to permit the management of the 
process in the furnace on the basis of the development of the foaming of 
the slag. 
The process according to the invention is thus characterized in that: 
a signal representing the derivative of the intensity of the current in the 
arc is sensed, 
the sensed signal is amplified, 
the amplified signal is directed simultaneously into two band pass filters 
respectively having a high frequency broad band and having a narrow band 
centered on a low fundamental frequency, 
the signals emanating from the filters are transmitted to effective value 
extractors, 
by means of a divider module, an energy signal is formulated, which is 
proportional to the ratio of the effective value of the signal emanating 
from the broad band filter and the effective value of the signal emanating 
from the narrow band filter, 
and the proportional energy signal obtained is displayed on a scale where 
it is expressed as a percentage, this percentage being indicative of the 
behaviour of the arc, signifying the formulation phase. 
In the case where the layer of slag actually undergoes a foaming during the 
process, the arc thus being immersed in the foam, the proportional energy 
signal obtained is utilized as an indicator of the degree of immersion of 
the arc in the foaming slag, the value of the said signal tending towards 
a minimum when the foaming of the slag causes a maximum immersion of the 
arc. 
In this manner, a measurement is made of the harmonic distortion of the 
current, which constitutes an image of the stability of the arc, which 
stability is improved when the arc is immersed in the foam. The operators 
managing the operation of the furnace then have available an indicator 
permitting them to undertake regulation operations in real time, to 
regularize the setpoint values between the teams, and to supply an SPC 
(statistical process control) indicator. 
The device for carrying out this process comprises: 
a sensor of a signal representing the derivative of the intensity of the 
current passing in the arc, 
an amplifier of the said signal, 
two band pass filters connected to the amplifier, one of which has a high 
frequency broad band and the other of which has a narrow band centered on 
a low fundamental frequency, 
two effective value extractors connected respectively to the said filters, 
a divider module connected to the extractors and capable of formulating an 
energy signal proportional to the ratio of the effective value of the 
signal emanating from the broad band filter and the effective value of the 
signal emanating from the narrow band filter, 
an element for the display in the form of a percentage of the proportional 
energy signal delivered by the divider module, 
and a possible link with a computer. 
Further particular features and advantages of the invention will become 
evident in the course of the description which will follow, which is made 
with reference to the accompanying drawing which illustrates an embodiment 
thereof by way of non-limiting example.

The electrical furnace of FIG. 1 comprises an electrode 2 and contains a 
liquid metal bath M. On top of the liquid metal M there is during the 
process a layer L of slag, which layer is traversed to a variable degree 
by the electric arc 3. 
Blast pipes (not shown),-which are symbolized by the arrows passing through 
the wall of the furnace 1, permit the injection respectively of oxygen 
into the liquid metal M and of charcoal into the slag L. These injections 
cause chemical reactions in the bath and in the slag, which, in their 
turn, cause a foaming of the latter, within which the arc 3 is therefore 
immersed to a greater or lesser extent; this has an effect on the voltage 
and the current in the arc. 
In order to monitor the development of the foaming of the slag L and thus 
to permit interventions in the course of the process, the invention 
provides an electrical system, the circuit of which is shown in FIG. 2. 
This measuring device thus comprises: 
a sensor 4 of a signal representing the derivative di/dt of the intensity 
of the current passing through the arc, measured on one of the conductors 
of the secondary circuit of the furnace, 
an amplifier 5 of the signal supplied by the sensor 4, 
two band pass filters connected to the amplifier 5, one 6 of which has a 
high frequency broad band and the other 7 of which has a narrow band 
centered on a low fundamental frequency, 
two effective value extractors 8, 9 connected respectively to the said 
filters 6 and 7, 
a divider module 11 connected to the extractors 6, 7 and capable of 
formulating an energy signal proportional to the ratio of the effective 
value of the signal emanating from the broad band filter 6 and the 
effective value of the signal emanating from the narrow band filter 7; 
and finally an element 12 for the display in the form of a percentage of 
the proportional energy signal delivered by the divider module 11. 
The sensor 4 is of the inductive type, for example a Robowski torus, that 
is to say an electromagnetic ammeter which is capable of measuring the 
derivative of the current, and its harmonics. The use of a sensor of this 
type exhibits, in particular, the advantage of increasing the accuracy of 
the measurement of the fluctuations of the current, since its very broad 
pass band avoids a peak limiting of the high frequency components. The 
amplifier 5 is an isolation amplifier, for example using galvanic 
separation. The filters 6 and 7 dissociate the original signal into two 
branches in each filter. The filter 6 may thus be a band pass of 500 to 
1,000 Hz with a gradient close to 50 dB/octave, while the filter 7 has a 
narrow band centered on a low frequency, for example 50 Hz in the case of 
a supply of the furnace with alternating current at industrial frequency. 
The two filters 6 and 7 thus effect the separation between the fundamental 
frequency component at 50 Hz and the frequency components situated within 
the band 500 to 1,000 Hz; this frequency band having proved to be of 
particular interest, as regards the accuracy of the measurement and of the 
processing of the signal, for carrying out the process. 
The energy value of the two signals emanating from the two filters 6, 7 is 
obtained by the two effective value extractors 8, 9, the characteristics 
of which are identical ("root mean square"). The extractors 8 and 9 
receive alternating signals at the input and deliver direct-current 
signals, and must have strictly the same integration characteristics. 
The divider 11 computes the ratio of the energy values of the signals 
supplied by the extractors 8 and 9, the result of this computation being 
displayed on the element 12. 
The signal supplied by the divider 11 may be considered as an indicator of 
foaming of the slag L on account of the fact that its value tends towards 
a minimum when the foaming of the slag causes a maximum immersion of the 
arc 3. Thus, the value of the quotient delivered by the divider 11 is a 
minimum when the foaming is a maximum (the arc 3 being entirely enveloped 
in foam) and vice versa. 
This may be understood by the fact that the development of the foaming 
causes a diminution of the harmonics of the current, which causes, in its 
turn, a diminution of the ratio of the energy signal delivered by the 
extractor 8 to the energy signal delivered by the extractor 9. 
Of course, the 500 -1,000 Hz zone given hereinabove is supplied only by way 
of example, since for a liquid metal bath M it is the richest in 
information. Outside this band, the closer is the approach to the 
fundamental frequency the greater is the necessity to use a steep-gradient 
filter. For this reason, an appropriate frequency window is selected The 
ceiling of 1,000 Hz proved, in the example given, to be appropriate, since 
beyond a certain frequency (which may be different from 1,000 Hz, for 
example 2,000 Hz) the additional information obtained is no longer useful. 
The results supplied by the indicator 12 may be used in the following 
manner: 
1. For the regulation, in real time, of the furnace 1 by the control 
operator. In fact, if the signal from the element 12 is a maximum, this 
means that the arc 3 is immersed to a very small extent in the foam and 
that it is necessary to implement a regulation of the operation of the 
furnace, for example a different positioning of the blast pipes for the 
injection of oxygen and of charcoal, or a regulation of the injection 
throughputs. 
2.Optimization of the operation of the furnace. It is possible to supply to 
the indicator 12 setpoint values with which there will be associated 
specified types of intervention, for the regulation of certain parameters 
as a function of the recorded values. 
In the absence of foaming of the slag L, the process according to the 
invention permits the monitoring of the behaviour of the arc 3, which is 
influenced by parameters other than the foaming. Thus, for example during 
the phases of fusion of the ladles of scrap iron with effect from a given 
percentage, for example 50 %, of the indicator 12, it will be known that 
the furnace is at the end of fusion, and that it is then necessary to 
recharge it in order to maintain a maximum yield. The process may thus be 
utilized in order to maintain the characteristics of the arc at constant 
values. 
It will be noted that the filter 6, in the indicative example given 
hereinabove, may in fact be constituted by a succession of two filters 
having two cells, the first being 1,000 Hz low pass and the second being 
500 Hz high pass. The gain is regulated to 0 or 20 dB, only the first cell 
being regulated to 20 dB for the measuring of the signals at the divider 
11. 
Nevertheless, a single filter 6 may be sufficient, with a first cell 
regulated to 1,000 Hz and a second cell regulated to 500 Hz, and the gain 
of one of the two to 20 dB. 
The process and the device described hereinabove by way of example in the 
case of an alternating current furnace, may likewise be applied to direct 
current furnaces, the low fundamental frequency under consideration then 
being virtually zero, and the narrow band filter then being for example a 
1 Hz low pass filter, it being possible for the high frequency band to be 
possibly broadened, for example, to 50-1,000 Hz.