Method for operating an electric arc furnace and melting plant having an electric arc furnace operated according to said method

Abstract

In a method for operating an electric arc furnace operated with an alternating voltage, a structure-borne sound signal occurring on a wall of the electric arc furnace is detected, from which structure-borne sound signal a parameter characterizing the flicker properties of the electric arc furnace is calculated. At least one process variable of the electric arc furnace is controlled on the basis of the calculated parameter. An electric arc furnace operated according to the method is used in and for a melting plant.

Claims

1. A method for operating an alternating voltage electric arc furnace, comprising: detecting, by a structure-borne sound sensor arranged on a wall of the electric arc furnace, a structure-borne sound signal arising at the wall; calculating a parameter which predicts a flicker value of the electric arc furnace, the parameter being calculated based on the structure-borne sound signal, wherein the flicker value corresponds to a variation in light intensity from a light source over time caused by fluctuations of voltage in the electric arc furnace; and controlling a process variable for the electric arc furnace by reference to the parameter to avoid flicker.

2. The method as claimed in claim 1, wherein the parameter is a numeric measure related to a Kst value, the Kst value specifying characteristics, including at least one of weight and density, of scrap metal disposed in the electric arc furnace.

3. The method as claimed in claim 2, wherein calculating the parameter comprises: subjecting the structure-borne sound signal to a Fourier transformation; determining amplitudes of the Fourier transform at a plurality of frequencies; and calculating the parameter from the amplitudes.

4. The method as claimed in claim 3, wherein a measure for a low-frequency scrap movement is calculated from amplitudes of the Fourier transform at frequencies f which lie below a fundamental frequency f.sub.0, a measure for a high frequency vibration is calculated from amplitudes of the Fourier transform at frequencies f which lie above the fundamental frequency f.sub.0 and do not include harmonic frequencies mf.sub.0, a measure which characterizes stability of the electric arc is calculated from amplitudes of the Fourier transform at frequencies f which lie at and between the harmonic frequencies mf.sub.0, and the parameter is calculated from the measures.

5. The method as claimed in claim 4, wherein the parameter is determined using: $\begin{array}{cc}\mathrm{SV}=B*E\mathrm{where}& \left(1\right)\\ B=\underset{n=n0}{\overset{n1}{.Math.}}{F}^{-}\left(f_n\right)& \left(2\right)\end{array}$ where B is the measure for a low-frequency scrap movement, where E is the measure for a high frequency vibration, where F is the Fourier transform, where f.sub.n1<f.sub.0 and f.sub.n+1f.sub.n=ff.sub.0 and where $\begin{array}{cc}E=\underset{n=n2}{\overset{n3}{.Math.}}{F}^{}\left(f_n\right)& \left(3\right)\end{array}$ where f.sub.nm*f.sub.0 and f.sub.n2>f.sub.0, 210 and m is a natural number, and $\begin{array}{cc}\mathrm{SSG}=\left(\underset{k=2}{\overset{k_{\max }}{.Math.}}F\left(k{f}_0\right)\right)*\left(\underset{n}{.Math.}F\left(f_n\right)\right)/F^2\left(f_0\right)& \left(4\right)\end{array}$ where SSG is the measure which characterizes stability of the electric arc, where f.sub.nm*f.sub.0 and f.sub.n>f.sub.0, where k and m are natural numbers and the frequencies f.sub.n are those used in equation (3) and k.sub.max10 applies, and
K=*SV+b*SSG(5) where K is the parameter and where a and b are experimentally determined weighting factors.

6. The method as claimed in claim 1, wherein calculating the parameter comprises: subjecting the structure-borne sound signal to a Fourier transformation; determining amplitudes of the Fourier transform at a plurality of frequencies; and calculating the parameter from the amplitudes.

7. The method as claimed in claim 6, wherein a measure for a low-frequency scrap movement is calculated from amplitudes of the Fourier transform at frequencies f which lie below a fundamental frequency f.sub.0, a measure for a high frequency vibration is calculated from amplitudes of the Fourier transform at frequencies f which lie above the fundamental frequency f.sub.0 and do not include harmonic frequencies mf.sub.0, a measure which characterizes stability of the electric arc is calculated from amplitudes of the Fourier transform at frequencies f which lie at and between the harmonic frequencies mf.sub.0, and the parameter is calculated from the measures.

8. The method as claimed in claim 7, wherein the parameter is determined using: $\begin{array}{cc}\mathrm{SV}=B*E\mathrm{where}& \left(1\right)\\ B=\underset{n=n0}{\overset{n1}{.Math.}}{F}^{-}\left(f_n\right)& \left(2\right)\end{array}$ where B is the measure for a low-frequency scrap movement, where E is the measure for a high frequency vibration, where F is the Fourier transform, where f.sub.n1<f.sub.0 and f.sub.n+1f.sub.n=ff.sub.0 and where $\begin{array}{cc}E=\underset{n=n2}{\overset{n3}{.Math.}}{F}^{}\left(f_n\right)& \left(3\right)\end{array}$ where f.sub.nm*f.sub.0 and f.sub.n2>f.sub.0, 210 and m is a natural number, and $\begin{array}{cc}\mathrm{SSG}=\left(\underset{k=2}{\overset{k_{\max }}{.Math.}}F\left(k{f}_0\right)\right)*\left(\underset{n}{.Math.}F\left(f_n\right)\right)/F^2\left(f_0\right)& \left(4\right)\end{array}$ where SSG is the measure which characterizes stability of the electric arc, where f.sub.nm*f.sub.0 and f.sub.n>f.sub.0, where k and m are natural numbers and the frequencies f.sub.n are those used in equation (3) and k.sub.max10 applies, and
K=*SV+b*SSG(5) where K is the parameter and where a and b are experimentally determined weighting factors.

9. A smelting plant with an electric arc furnace, the electric arc furnace comprising: an electrode operated using an alternating current; a wall; a structure-borne sound sensor arranged on the wall, to sense structure-borne sound signals of the electric arc furnace; and a control and analysis device to analyze the structure-borne sound signals, to calculate a parameter which characterizes flicker properties of the electric arc furnace and to control a process variable for the electric arc furnace by reference to the parameter to avoid flicker, wherein the flicker properties include a flicker value which corresponds to a variation in light intensity from a light source over time caused by fluctuations of voltage in the electric arc furnace.

10. The smelting plant of claim 9, wherein a plurality of structure-borne sound sensors are arranged at different locations on the wall, to sense structure-borne sound signals of the electric arc furnace.

11. The smelting plant of claim 9, wherein the control and analysis device is configured to calculate a parameter that predicts the flicker value based on a first value corresponding to a degree of scrap movement, a second value corresponding to a degree of scrap vibration, and a third value corresponding to a content of the scrap.

12. The smelting plant of claim 11, wherein the parameter is calculated based on a product of the first value and the second value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:

(2) FIG. 1 a schematic diagram of the principle of a proposed smelting plant,

(3) FIG. 2 a diagram in which the measured structure-borne sound signal is plotted against time,

(4) FIG. 3 a diagram in which the amplitude of the Fourier transform of the structure-borne sound signal is plotted against frequency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(5) Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

(6) As shown in FIG. 1, a smelting plant proposed by the inventors incorporates an electric arc furnace 2 with, for example, three electrodes 4a-c, which are connected electrically to the furnace transformer of a power supply facility 6. The electrodes 4a-c project down into a furnace vessel 8, which holds the scrap 7 which is to be melted down, on the wall 9 of which are arranged a plurality of structure-borne sound sensors 10. The structure-borne sound signals S detected by the structure-borne sound sensors 10 are communicated to a control and analysis facility 12 for further processing.

(7) In addition to the structure-borne sound signals S, the current flowing in the electrodes 4 and/or the voltage across them are/is measured with the aid of measuring sensors 14a-c, and the corresponding measurement signals M are communicated to the control and analysis facility 12. In this control and analysis facility, control signals C are generated, with which at least one process variable of the electric arc furnace 2 is controlled or regulated.

(8) In the diagram in FIG. 2, an example of a structure-borne sound signal S, measured using a structure-borne sound sensor 10, is plotted against the time t, where in principle the structure-borne sound signals S generated by several structure-borne sound sensors 10 could be combined into one summary signal. From this structure-borne sound signal S, the frequency spectrum is now determined by a Fourier transformation (FFT), this being illustrated in FIG. 3, in which the amplitude of the Fourier transform F is plotted against the frequency f. In this FIG. 3 it can be seen that the magnitude (amplitude) of the Fourier transform F has significant maxima for the frequencies f.sub.0, 2f.sub.0, 3f.sub.0, 4f.sub.0 and 5f.sub.0, the height of which decreases with increasing frequency f. These maxima lie at harmonic frequencies mf.sub.0 of a fundamental frequency f.sub.0, i.e. they correspond to integer multiples of this fundamental frequency f.sub.0, the value of which is double the frequency (operating frequency) of the voltage by which the electrodes are operated. For an operating frequency of 50 Hz, these harmonic frequencies f.sub.0 are at 100, 200, 300, 400 or 500 Hz.

(9) From the Fourier transform F it is now possible to calculate a measure of the scrap relocation SV from the product formed from a measure B of a low-frequency scrap movement and a measure E of a high-frequency scrap vibration
SV=B.Math.E(1) using the following relationships: with

(10) $\begin{array}{cc}B=\underset{n=n0}{\overset{n1}{.Math.}}{F}^{-}\left(f_n\right)\mathrm{where}{f}_{n1}<f_0\mathrm{and}{f}_{n+1}-f_n=f{f}_0& \left(2\right)\end{array}$

(11) A suitable value for f.sub.n0 is, for example, 1 Hz. As the upper limit f.sub.n1, a suitable value is one which lies well below the fundamental frequency f.sub.0, preferably below the operating frequency f.sub.0/2 and in the example has a value of 30 Hz, where the amplitudes of the Fourier transform F at frequencies of f.sub.n are raised to the power of and summed, these frequencies being spaced at f=1 Hz, corresponding to a frequency resolution which can typically be achieved by a fast Fourier transform. The parameter is such that: 110, where it has been found that a suitable value is =1.

(12) $\begin{array}{cc}E=\underset{n=n2}{\overset{n3}{.Math.}}{F}^{}\left(f_n\right)& \left(3\right)\end{array}$ where f.sub.nm*f.sub.0 and f.sub.n2>f.sub.0 with 210 and m is a natural number.

(13) For the purpose of calculating the measure E for the vibration, the amplitudes of the Fourier transform F at intermediate frequencies f.sub.nm*f.sub.0 are raised to the power of and summed, these frequencies being spaced far enough from the harmonic frequencies that the latter make no contribution to the magnitudes of the amplitudes used.

(14) Since the electric arc burns unstably and with a fluctuating root for cold heavy scrap, the spectrum of the structure-borne sound which is produced, i.e. the Fourier transform F, has not only greatly raised amplitudes at the higher harmonic frequencies mf.sub.0 but also a large number of maxima at intermediate frequencies lying between the harmonic frequencies mf.sub.0. A measure SSG for the heavy scrap content can therefore be advantageously determined using the following relationship.

(15) $\begin{array}{cc}\mathrm{SSG}=\left(\underset{k=2}{\overset{k_{\max }}{.Math.}}F\left(k{f}_0\right)\right)*\left(\underset{i}{.Math.}F\left(f_i\right)\right)/F^2\left(f_0\right)& \left(4\right)\end{array}$ where f.sub.nm*f.sub.0 and f.sub.n>f.sub.0, where k and m are natural numbers and fn are the frequencies used in equation (3) and k.sub.max10 applies.

(16) From the measure SV for the scrap relocation, and the measure SSG for the heavy scrap content, it is now possible to determine a parameter K by applying the weightings a and b in the relationship
K=*SV+b*SSG(5) where the weights a, b are determined experimentally from the correlation of the value calculated in this way and the flicker actually measured, and are adjusted in such a way that the dynamic parameter K determined in this way is comparable with a Kst value. In this way it is thus possible, during the initial melting down process, to calculate a parameter K, which is correlated with the Kst value, which specifies the current scrap movement and the current scrap status. This calculated parameter K does not correspond exactly to the Kst value as defined in the IEC standard cited above, but does correctly reflect the progress and trend and can thus be used optimally for flicker prediction and for setting the regulation system to avoid flicker.

(17) Taking into account further data, in particular the progress of the current and voltage, the wall temperatures and/or the specific energy injected, it is now possible to create a higher level complete regulation system for the purpose of process management, with which a rapid and status-oriented reaction to the dynamic changes taking place in the electric arc furnace is possible. Such a control or regulation system, as applicable, will preferably work on the voltage stage of the furnace transformer, the impedance or current set-points for the electrode regulation system, the additional reactances and the issuing of set-point values for the burners and lances. As a basis for this, it is possible to use the values in a permanently stored operating diagram, which the regulation system changes dynamically within prescribed limits. The measurement of the dynamic changes in the scrap during the melting down process in the electric arc furnace is combined with a higher-level, modular regulation system, for example based on fuzzy-logic, for process energy management of the electric arc furnace, which prescribes the electrical working point and the set-point values for the burner and lance systems. This permits dynamic status-oriented intervention in the smelting process. By using a complete regulation concept based on linguistic fuzzified rules and further analytical balancing, using among other things an analytical model of the load distribution, the smelting process can be adjusted so that the flicker does not exceed prescribed limiting values.

(18) The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase at least one of A, B and C as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).