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WO1993017533A1 - Procede pour mesurer des caracteristiques electriques d'un four a arc a courant alternatif - Google Patents

Procede pour mesurer des caracteristiques electriques d'un four a arc a courant alternatif Download PDF

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Publication number
WO1993017533A1
WO1993017533A1 PCT/FI1993/000062 FI9300062W WO9317533A1 WO 1993017533 A1 WO1993017533 A1 WO 1993017533A1 FI 9300062 W FI9300062 W FI 9300062W WO 9317533 A1 WO9317533 A1 WO 9317533A1
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Prior art keywords
furnace
electrode
power
currents
measuring
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PCT/FI1993/000062
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English (en)
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Lassi Toivonen
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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B7/00—Heating by electric discharge
    • H05B7/02—Details
    • H05B7/144—Power supplies specially adapted for heating by electric discharge; Automatic control of power, e.g. by positioning of electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/25—Process efficiency

Definitions

  • the so-called electrode furnaces can be categorized in (1) resistance furnaces, wherein electrodes are submerged in molten metal, (2) submerged-arc furnaces, wherein
  • electrodes are submerged in a non-melting material, e.g., slag, and which are used for the reduction of metals etc., e.g., ferrochromium furnaces, ferromanganese furnaces, calcium-carbide furnaces, and (3) open-arc furnaces, wherein electrodes are only occasionally in contact with a material to be melted, e.g., scrap melting furnaces.
  • a non-melting material e.g., slag
  • ferrochromium furnaces e.g., ferromanganese furnaces, calcium-carbide furnaces
  • open-arc furnaces e.g., wherein electrodes are only occasionally in contact with a material to be melted, e.g., scrap melting furnaces.
  • this description only deals with a three-phase AC electric-arc furnace, but all the described aspects can be readily extended to cover any desired polyphase electrode furnace.
  • the open-arc and submerged-arc furnaces most commonly employ a so-called knapsack connection in which each line voltage is connected between two working electrodes; the total number of electrodes is three [1] p. 3.
  • the system does not include a neutral power connection.
  • the advantage of the connection arrangement is that the electrode current is ⁇ 3 times the transformer secondary current. Since the electrode currents in a large-scale furnace exceed 100 kA, this represents a major advantage in transformer construction. Besides knapsack connected furnaces, this description deals with other AC electric-arc furnaces as well, including those with a neutral power connection.
  • Erosion of the working electrodes is generally compensated for by adding blocks of self-baking carbon paste to the electrode tops and by slipping the electrode downwards according to its rate of erosion [1], p. 7.
  • the parameters of each working electrode must be adjusted individually. This is because the furnace does not operate in a homogeneous way: charging and tapping operations, for example, result in an uneven distribution of the charge.
  • the adjustment operations include, e.g., manipulating the electrodes in the direction of the longitudinal axis, i.e., up and down, and adjusting the furnace transformer secondary voltages by the use of voltage tap changers.
  • the purpose of the furnace control is to maximize the power factor and the active power delivered into the furnace and to eliminate the overload situations, such as surpassing the maximum current of the electrodes or the transformer secondary, and the apparent-power capacity of the
  • control is to maintain an ideal reaction zone below the electrodes [1], p. 34, which most often means that the same active power is supplied through all electrodes.
  • the inductance of the furnace circuit in relation to the resistance is significant; as a result of this and the asymmetric state of the furnace, the relative active powers supplied by the electrodes may differ considerably from the ratios of electrode currents [1], p. 36...40. Therefore, the
  • the furnace active power in relation to the apparent power i.e., the power factor
  • the furnace active power in relation to the apparent power can be maximized by keeping the arcing phenomenon at its minimum and by maximizing the thermal power produced by resistive dissipation [1], p. 25.
  • the strength of arcing can be assessed, e.g., by measuring the amplitudes of distortion components of the electrode voltages [1], p. 34.
  • the distortion components of arc voltages can also be used for obtaining information about the operational phase of cyclically operating furnaces, e.g., scrap-melting furnaces.
  • the length of working electrodes must be periodically measured.
  • the measurement can be most easily performed after stopping the furnace.
  • many existing furnaces operate continuously and, thus, the length of electrodes can be readily measured only during operation halts.
  • the fewer the opportunities of performing measurements the more useful are the erosion models for prediction of the rate of the consumption of electrodes. For the above reasons, it is generally desirable either to measure the electrode-related distortion voltages or to measure directly the powers dissipated in the arc.
  • the furnace has no power return connection.
  • the voltages per working electrode are measured relative to the neutral connection [1], p. 26.
  • the furnace transformer secondary currents or electrode currents are measured by means of current transformers. If the furnace is in knapsack connection the current transformers of the furnace transformer may be wired in a delta-star arrangement so that the currents at the secondary are together directly proportional to the electrode currents [1], p. 26, 27.
  • the problems of the traditional measurement method include strong disturbances in voltage measurement [1], p. 26 and the fact that the process is not symmetrical, e.g., as a result of uneven charging, so that the neutral connection does not lie at the real star point of the system.
  • the disturbance problem originates mainly from the electromagnetic flux, caused by electrode and furnace currents and extending through a large loop formed by the measuring conductors.
  • the considerable loop size results from the fact that the objects to be measured, the top portions of working electrodes and, on the other hand, the neutral connection(s), are separated from each other by a distance of several meters.
  • the neutral connection can also be easily broken in the hot environment and replacement of the connection is very difficult if the breakage is inside the furnace shell.
  • a measuring system provided with three neutral connections is a substantial improvement if the voltage measuring leads are routed in the best possible way [1], p. 29.
  • connection(s) at the bottom of the furnace 4.
  • the invention A novel measuring method
  • the presently described novel method requires neither a neutral connection at the bottom of the furnace nor assumptions of, e.g., electrode-related inductances.
  • the method employs simple, direct measurements of either a furnace transformer primary or secondary; thus, the measurements can be carried out without worrying about disturbances. It is possible to perform the measurements separately at the power system fundamental frequency and at each of those harmonic and interharmonic frequencies which carry a significant amount of energy. (Especially the open-arc and submerged-arc furnaces are non-linear, which is why the waveforms of voltages and currents supplying them are significantly distorted; as a result of this, also distortion components carry energy into the furnace.) The results are accurate since, despite a somewhat asymmetrical process, no symmetry assumptions are made and since it is possible to model the furnace
  • the virtual process star point may be determined separately at each frequency on the basis of the above-mentioned quantities, without process-related assumptions, such as symmetry or constant proportions of inductances.
  • the effect of mutual impedances is automatically taken into account.
  • the mutual impedances refer to those impedances, through which a current passes from one electrode to another past the material in molten state at the bottom of a furnace.
  • Another novel feature is that the modeling makes use of the intensive and fast fluctuation of
  • the virtual star point calculated as directed above differ from the real star point; the virtual star points are determined on the basis of electrode-related quantities and take into account also the effect of mutual impedances and, thus, in terms of measurement and control, they are more important than the real star point.
  • the method utilizes those strong and abrupt variations which continuously occur in the power of especially an open-arc and submerged-arc furnace and which result from the fluctuation of furnace impedances and arc voltages. If the furnace is not provided with neutral power connection, the variation of a single electrode-related voltage or power has an inevitable effect on the electric parameters of other electrodes as well. It should be noted that the electrodes of a furnace with even a neutral power
  • the furnace transformer cannot be regarded as a voltage source with zero internal impedance, and the impedance of a neutral power connection cannot be regarded as zero.
  • a change occurring at one electrode has an effect on the voltages and currents supplying the other electrodes.
  • a (virtual) resistive and reactive star point i.e., the electrode-related resistances, reactances and arc voltages or, alternatively, these quantities at frequencies experiencing a significant transfer of power.
  • the electrode currents and the furnace transformer secondary voltages are measured either directly from the secondary or primary or from an
  • the measured signals can be used for calculating either the powers and r.m.s. values or the spectra of powers and signals.
  • the most essential objective is the determination of the electrode-related powers of a furnace.
  • Z E1 ...Z E3 are equivalent impedances representing electrodes 1...3, R E1 ...R E3 and X E1 ...X E3 are the real and imaginary components of these impedances, respectively, and I 1 ...I 3 are the absolute values of the currents of electrodes 1...3.
  • the expressions are valid on the r.m.s. values of the currents and total powers, but also at each frequency separately. They provide the furnace with a linear model for obtaining the electrode-related
  • Expressions (1), (2a) and (2b) include six unknowns, i.e., the real and imaginary components of three complex
  • impedances It is possible to find the estimates for these by assuming that the impedances and/or arc voltages include a quickly varying component, whose magnitude is not an object of interest and which adds to an
  • the obtained solution may contain an error of nearly arbitrary magnitude, due to insufficient input information. Therefore, it is either necessary to make sure that sufficiently significant variations occur between successive measurements or it is necessary to use non-linear filtering for calculating an estimate from successive results obtained, e.g., as solutions of groups of equations. Linear filtering would not attenuate
  • Non-linear filtering can be effected, e.g., by searching the obtained results for the medians of
  • electrode-related quantities in other words, by using a so-called standard median filter, which is described in publications dealing with digital signal processing and which calculates a new median for each new sample, in this case for a new measurement result.
  • a so-called standard median filter which is described in publications dealing with digital signal processing and which calculates a new median for each new sample, in this case for a new measurement result.
  • Another type of filter can also be used, e.g., a so-called hybrid median filter [3], which is a combination of a linear digital filter and a median filter. The most important advantage over a standard median filter is a lesser computational loading.
  • the above-described method can be specified by taking into account the non-linearity of a furnace, i.e., by splitting the power loss of a furnace on the one hand in a component dissipated in impedances and, on the other hand, in a component dissipated in the electric arc, see [1], fig. 20, p. 23. Since the voltage of a high-current electric arc is more or less independent of the current, see [1], equations (34) and (35), the following expression is obtained for the total power:
  • the electrode-related voltages relative to virtual star points can also be calculated in a way other than
  • the expressions include six unknowns,
  • the method can also be used for estimating the
  • An electric-arc furnace is a non-linear and highly
  • a furnace quickly produces varying energy distributions resulting primarily from arc voltage and impedance fluctuations.
  • the power transfers into an electric furnace primarily just at the fundamental frequency and at the lowest harmonic
  • the electrode-related quantities can only be measured at these frequencies.
  • the consistency of results obtained at a given frequency can be estimated by means of generally applied measuring techniques, e.g., by analyzing the distribution of results and by measuring the coherence function.
  • the measurements carried out as directed above need not be performed in the immediate vicinity of the furnace. They can take place in the secondary or primary of the furnace transformer. This description does not take into account dissipations in the bus bars between the furnace transformer and the furnace, the furnace connection (e.g., straight or knapsack connection), transformer losses, the wiring arrangement of the transformer, the state of transformer tap changers or other such factors. However, the dissipations of the wiring and the transformer can be measured or calculated at a high accuracy and, thus, can be taken into account in signal processing. Similarly, the effect of furnace connection and furnace transformer wiring arrangement can also be taken into account, if necessary. If measurements are carried out in the primary of the furnace transformer it is further necessary to know the positions of the transformer tap changers in order to be able to reduce the measurement results into the
  • phase voltages, currents and powers can be split in symmetrical
  • the time required by the novel method for collecting the measurement data may be too long in view of the furnace control.
  • the performance may be significantly improved by using "hybrid"
  • the accompanying drawing illustrates a block diagram of a wiring system for carrying out the method of the
  • currents are measured by means of current transformers and the secondary signals of current transformers are sampled using a sufficient time and amplitude resolution.
  • the voltages are measured as line voltages, e.g., using resistive dividers (dividers not shown in drawing) and the divider output signals are sampled with resolution
  • each frequency requires 12 filters, the result produced thereby being, e.g., a median of 100 or 1000 successive impedance values.
  • the impedances and measured currents can be used for
  • the filtering can be, e.g., an integrating operation (or, in terms of sampled data, rather a summing operation), the duration of which is equal to that of a system voltage cycle or its
  • the r.m.s. values are first calculated by low-pass filtering the squares of relevant time functions in a manner that a component oscillating at a frequency twice that of the power system fundamental frequency and, possibly, components having even higher frequencies are attenuated, followed by square rooting the filtered samples.
  • I 1 (t) is obtained by squaring the samples of i ⁇ (t), then by low-pass filtering the thus obtained i 2 1 (t) (or actually samples thereof), and by finally square rooting the filtered samples.
  • the reactive power can be split in the "pure" reactive power and distortion power [4].
  • the pure reactive power refers to the type of reactive power produced as a correlation between the voltage and 90 degrees phase shifted current.
  • the distortion power appears when the waveforms of voltage and current differ from each other. The following applies generally to apparent power, active, reactive and distortion power:
  • the pure reactive power can be calculated either in a time or frequency domain, i.e., using the spectra of voltage and current as input data in the latter case.
  • the pure reactive power is obtained by using the same correlation and filtering operations as those used for obtaining the active power, but by replacing the current with a signal that is produced by shifting the phase of current by 90 degrees.
  • the phase shift can be performed in the time domain (simultaneously at all frequencies) by means of a Hubert transformer, the practical realization of which has been described in text books and publications dealing with digital signal processing and which is preferably calculated as a
  • the voltage samples must be delayed according to the delay caused by the Hubert
  • the total power could also be measured using phase voltages and currents as input data; after all, the star point can be selected arbitrarily in a three-phase system without neutral power connection when measuring the total power.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Discharge Heating (AREA)

Abstract

Cette invention concerne un procédé pour mesurer les caractéristiques électriques des électrodes d'un four à arc à courant alternatif. Le procédé est basé sur l'observation de la corrélation mutuelle entre l'intensité des courants des électrodes et la puissance totale fournie au four, en utilisant les fluctuations naturelles et rapides de l'intensité des courants et de la puissance fournie au four et sur des observations estimatives. L'information requise pour l'estimation est recueillie en réalisant une pluralité de mesures successives d'intensité et de tension à des instants proches et à différentes distributions de l'énergie d'un four. Les résultats des mesures sont utilisés pour obtenir par calcul des estimations des tensions, des impédances et des puissances des électrodes. Il est en outre possible de scinder les puissances des électrodes et les tensions des électrodes d'une part en puissance et en tension de l'arc électrique et d'autre part en perte de puissance et de tension provoquée par la conduction résistive. L'effet des impédances mutuelles est automatiquement pris en compte dans l'estimation. Lorsque cela est souhaité, l'estimation peut être réalisée indépendamment à différentes fréquences, c'est-à-dire à la fréquence fondamentale du réseau fournisseur de puissance et aux fréquences harmoniques et interharmoniques qui occasionnent un transfert significatif d'énergie. Le procédé est utile pour des mesures sur un four alimenté par un réseau, aussi bien avec que sans connexion neutre. Les mesures peuvent être effectuées sur le primaire ou sur le secondaire du transformateur du four.
PCT/FI1993/000062 1992-02-25 1993-02-23 Procede pour mesurer des caracteristiques electriques d'un four a arc a courant alternatif Ceased WO1993017533A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FI920820A FI100740B (fi) 1992-02-25 1992-02-25 Menetelmä vaihtosähköelektrodiuunin sähkösuureiden mittaamiseksi
FI920820 1992-02-25

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WO1993017533A1 true WO1993017533A1 (fr) 1993-09-02

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PCT/FI1993/000062 Ceased WO1993017533A1 (fr) 1992-02-25 1993-02-23 Procede pour mesurer des caracteristiques electriques d'un four a arc a courant alternatif

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AU (1) AU4672793A (fr)
FI (1) FI100740B (fr)
WO (1) WO1993017533A1 (fr)
ZA (1) ZA931283B (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0668711A1 (fr) * 1994-02-22 1995-08-23 Wladimir Danilov Méthodes et dispositifs pour la mesure et le contrôle des débits massiques et des grandeurs correlées
ES2172433A1 (es) * 2000-10-19 2002-09-16 Ferroatlantica Sl Equipo multifrecuencia detector del estado de los electrodos en los hornos de arco electrico.

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4580272A (en) * 1983-06-03 1986-04-01 Outokumpu Oy Method for controlling and balancing the power in an electric furnace

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4580272A (en) * 1983-06-03 1986-04-01 Outokumpu Oy Method for controlling and balancing the power in an electric furnace

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0668711A1 (fr) * 1994-02-22 1995-08-23 Wladimir Danilov Méthodes et dispositifs pour la mesure et le contrôle des débits massiques et des grandeurs correlées
ES2172433A1 (es) * 2000-10-19 2002-09-16 Ferroatlantica Sl Equipo multifrecuencia detector del estado de los electrodos en los hornos de arco electrico.

Also Published As

Publication number Publication date
FI920820A7 (fi) 1993-08-26
AU4672793A (en) 1993-09-13
FI920820A0 (fi) 1992-02-25
ZA931283B (en) 1993-11-11
FI100740B (fi) 1998-02-13

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