ARC EXPOSURE MONITOR

Abstract: A method of monitoring the degree of exposure of an electric arc within a furnace having a molten charge, eg an arc furnace or ladle furnace, in which characteristic frequency spectra of the waveforms from the supply associated with (a) arc-exposure and (b) arc-submersion are predetermined and in which signals (eg indicative of the arc current) emitted during operation are measured and processed and a comparison effected with a set level established from these characteristic signals whereby to provide an indication of the degree of arc exposure. The predetermined characteristic frequency spectra selected may be the harmonic content of the arc current.

Description: This invention relates to a method of, and apparatus for, monitoring the degree of exposure of an arc within a furnace, e.g. an electric arc furnace during meltdown of a charge. The current trend in arc furnace technology is to use one or more furnaces solely as melting units, and to perform steelmaking operations in a secondary vessel. The output rates of the equipment are matched to the throughput of a continuous casting machine. Large cost savings can be achieved if the melting requirements can be met by a single arc furnace. Furnace throughput is therefore of primary importance, and can be the limiting factor in the productivity of the plant. In order to achieve the highest output from an arc furnace, it is essential to use the full power capability of the transformer for as high a proportion of the melting time as is practical. However, large energy losses may be incurred unless precautions are taken to ensure that a high heat transfer efficiency from the arcs to the steel is maintained, particularly during the periods during and approaching melt-out. To achieve high efficiency, a technique has been developed for submerging the arcs in a deep foaming slag, thus containing the arc energy and transferring it into the steel melt. The normal bath decarburisation by oxygen lancing will often produce a deep foaming slag, but if foaming is to be maintained throughout the heat it is usually necessary to spray carbon or other foam inducing materials on to the slag furnace as the carbon boil subsides. On other occasions, where the bath metallurgy demands a high iron oxide slag content, it is not practical to maintain a foaming slag, and a lower transformer tap must be selected to reduce the arc length. It is clearly beneficial therefore, for the furnace operators to be made aware of the state of submersion/exposure of the arcs, so that they can take corrective action by spraying carbon, or by reducing the voltage tap. The state of submersion/exposure of an arc is also of interest in operating ladle furnaces where it is important to 'bury' the arcs beneath the slag to prevent erosion of the refractory lining. It has been proposed, for example in the article by Timmen and Grigat in Electrowarme International, February 1987, to use the Fourier transforms of current and voltage to indicate arc exposure, but this is expensive in computing resources and needs inputs of the arc voltage as well as the current. It is an object of this invention to monitor the degree of arc exposure for the purposes aforesaid. According to the present invention there is provided a method of monitoring the degree of exposure of an electric arc within a furnace having a molten charge in which a signal representative of the high-frequency content of the arc current is derived and compared with such signals predetermined for given arc exposures, the derivation being made by high-pass filtering the current waveform. The cut-off frequency may be between 50 and 150 Hz. The signal may be a DC level proportional to the root mean square of the high-frequency content, and the given arc exposures may be a submerged arc and an exposed arc. Essentially, this invention is based on the recognition that arcs are more stable when immersed in a slag; the slag reduces the arc mobility, i.e. the tendency to flare out in different directions from beneath the electrode and causes the arc to restrike earlier in each mains cycle after each current zero. This in turn is reflected in the variation in the shape and amplitude of the arc current wave form. This invention possesses the advantages that the measurements made are purely electrical (which is more reliable in the arc furnace environment than say acoustic monitoring), and no additional equipment for maintenance is required around the furnace. Further each of the three arcs can be monitored independently, and while it is recognised that the three arc currents and their respective harmonics are not totally independent, it has been ascertained that the degree of harmonic content in each phase reflects the relative degree of arc exposure/submersion in that phase. In order that the invention may be fully understood, one embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which:- Figures 1(a) and 1(b) illustrate the frequency analysis of arc current waveforms applicable to an exposed arc and a submerged arc, respectively and Figure 2 is a block diagram of the apparatus for performing the method of the invention. Referring to Figure 1 the broad band frequency spectra of arc currents derived from existing current transformers in the furnace instrumentation are determined with exposed arcs - identified by visual and acoustic observations - and with submerged arcs. Both spectra illustrate the marked presence of third harmonics, and others to a lesser extent, which are indicative of the non-sinusoidal nature of the arc current waveform and the hesitancy of the arc to re-strike at current zero. As can be seen the significant difference between Figs 1(a) and 1(b) is the larger area beneath the exposed arc spectrum in the former case. Considering now the apparatus shown in Figure 2 the current signals, whose frequency spectra is typified in Figs 1(a) and/or (b), are applied to a high pass filter 1 to isolate the 50 Hz mains component and other low frequency components, amplified in an amplifier 2 and then converted in a circuit 3 to an equivalent r.m.s. dc signal. Likewise the 'unmodified' input signal is applied direct to a circuit 4 which again provides an equivalent r.m.s. dc signal, the two d.c. signals then being ratiod in a divider 5, the output from which is thus a signal representing the fractional harmonic content of the current. This output signal is applied on the one hand to a 'driver' circuit 6 which operates a bar graph display 7 by which the fractional harmonic content is illuminated at the appropriate level and on the other hand to a comparator 8 to which a preset level signal is applied which, if exceeded by the d.c. signal input, illuminates an 'arc exposed' lamp 9. Like signals from the other phases are similarly applied to the comparator and the channel or channels which illuminates the lamp is/are readily identifiable on the display 7. A 'hysteresis set' or damping signal is applied to the comparator to minimise spurious illumination of the lamp. In operation illumination of the lamp may typically initiate carbon spraying on the melt to foam the slag and submerge the arc again; alternatively if carbon spraying is inappropriate a reduction in the voltage tap will achieve arc submersion.

Claims: A method of monitoring the degree of exposure of an electric arc within a furnace having a molten charge wherein a signal representative of the high-frequency content of the arc current is derived and compared with such signals predetermined for given arc exposures characterised in that the derivation is made by high-pass filtering the current waveform. A method as claimed in claim 1 in characterised in that the cut-off frequency is between 50Hz and 150Hz. A method as claimed in any preceding claim characterised in that the signal is a DC level proportional to the root mean square of the high-frequency content. A method claimed in any preceding claim characterised in that the given arc exposures are submerged and fully exposed.

Inventors: MONTGOMERY, ROBERT WALTER
Applicants: BRITISH STEEL PLC
Application year: 1988
Main classification: H05B7
IPC codes: H05B7