US2758144A - Compensation means in three-phase electric arc furnace - Google Patents

Description

L. DREYFUS Aug. 7, 1956 COMPENSATION MEANS IN THREE-PHASE ELECTRIC ARC FURNACE Filed NOV. 16, 1953 United States Patent COMPENSATION MEANS IN THR-EE-PHAS ELECTRIC ARC FURNACE Ludwig Dreyfus, Vasteras, Sweden, assignor to Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, a Swedish corporation Application November 16, 1953, Serial No..392,372

Claims priority, application Sweden December 15,1952

4 Claims. (CI. 13-12) In electric three-phase arc furnaces there are as a rule one middle electrode and two outer electrodes arranged symmetrically to the former whereat each electrode is fed through a supply line in the shape of a bundle of wires. The terminals of the furnace transformer are arranged in alignment at one side of the furnace whereat that bundle of wires which feeds the one electrode is 13/- ing along a great part of its length between the two bundles of wires which feed the other electrodes. The different lengths of the bundles of wires and above all the mutual inductances between the bundles of wires cause an unsymmetrical current distribution whereat the current in the middle bundle of wires, or referred to in the following description as middle phase becomes considerably greater than that within the outer bundles of Wires or outer phases, resulting in a corresponding diiference between the power in the three phases. Cfr. Handbuch der technischen Elektrochemie, part IV, Dr. Walter, Grundlagen der elektrischen 'Ofenheizung, 1950, page 250.

The greatest disadvantage of that asymmetry for the operation of the furnace is that the heat radiation from the arc of the middle phase becomes considerably larger than the radiation from the arcs of the outer phases with the consequence that the lining of the furnace is overheated in the vicinity of the middle are. A remedy thereagainst has been searched in an electrode control system, which holds constant the impedance for every phase. By phase-impedance thereat is meant the proportion between the phase voltage and the phase current, the phase-voltage being measured between the mantle of the furnace and the secondary terminals of the transformer. For a certain secondary voltage and 'load it is possible to set purely empirically the three relays .of the regulator for holding constant three (different) impedances in such a manner, that equal currents are obtained in all electrodes. But this implies that the middle phase has been set for greater phase impedance, :i. e. greater length of the arc than the outer phases, but it'has become evident, even at the same current but longer length of the arc in the middle phase, that the "part of the lining of the furnace lying nearest this electrode is worn away too quickly. To this'unfavorable result very likely the fact contributes, that the neutral iconductor connected to the mantle of the furnace andttrace'd in great distance from the electrode zfeeders probably does not bring about the phase voltages in the impedance relays which are expected. It will vbe seenfthat' unsymmetrical phase voltages would be obtained even though the circuit would be entirely symmetrical.

The present invention has for its object to :provide means for-compensating both asymmetries-caused by the feeders of the electrodes and asymmetries of the neutral-point voltages of the impedance relays caused by the location of the neutral conductor.

For the explanation of the invention reference is made to the accompanying drawings in which the furnace equipment is'shown in perspective view, :for thesake of clarity. The distribution of the currents and voltages according to the calculations is shown in Figs. 1 and 2 while the invention is illustrated schematically in Fig. 2. Fig. 3 and Fig. 4 are detail diagrammatic views showing modifications of the phase connections.

Referring to the drawing, 1, 2 and 3 indicate the electrodes, 4, 5 and 6 their associated feeders, 7 the secondary side of the furnace transformer, the wiring .0011- nection of which .is to be considered as one example only of any possible wiring connection. Indicated by '8 is a portion of the bottom of the furnace, and by -9 the mantle thereof. Inserted in the feeder to the middle electrode is a reactance element 10 which consists, in the example shown, of a laminated iron core with air gap arranged about :the feeder 5. This core is provided with a conductor 11 in series with the connection lead 12 bet-ween the mantle -9 ;of the furnace and the neutral point 13 of the voltage coils ,in the relays of the regulator 14 for the electrode control shown diagrammatically on the drawing.

According to the present invention it is possible to compensate the asymmetry of the phases only by properly adjusting the increase of the self-reactance of the middle phase, in spite of .the fact that said asymmetry substantially is due to the mutual inductances of all of the three phases. A calculation shows that in ,order to attain symmetry :it .is necessary to insert in the middle phase -a reactance of the amount wherein n and x3 are thereactance of the single-phase current circuits, comprising the middle electrode and one of the outer electrodes, .and \the associated ifeeders, while ;y is ,themutual inductance between said circuits. The asymmetry of the currents has often been explored both experimentally and theoretically -(latest in the above mentioned Handbuch, part IV, page 245-266). But hitherto 'the asymmetry ihas always been considered as unevitableand moreover \the formulae of Walter-deliver an unsymmetrical system :even the reactance ,xzd is added to the reactance of the middle *phase. (This vcircumstance shows that the problem is not as simple as the formula disclosed 'in the present invention might indicate but that new considerations have been necessary for the understanding :that a:solution at all is .possible.

The main feature of the invention is that the reactance of the middle phase is increased with respect to the -reactances of the outer phases. The .most favourable increase related to the low voltage side lies about the value whereat x1 and x3, resp. are the reactances of the single phase circuits established according to Fig. 1 by the middle electrode 2 and one of the outer electrodes for 3 including their connection leads 4 and 6 resp. Therefore, upon single phase supply of the electrodes 1 and 2 which are short-circuited at their i lower ends, from the transformer voltage E21 .21 current is obtained. The dilference between the reactances x1 and x2, as a rule, is so small that they may be replaced by their average value i-gwa Furthermore, by y is indicated the mutual inductance between the two single-phase circuits mentioned above. Thus, if the single-phase current 1 is fed through the electrodes l and 2, the voltage 113 is induced in the open circuit comprising the electrodes 2 and 3 and their feeders 5 and 6. If the single-phase current 3 is fed through the electrodes 2 and 3, in the open circuit 4+5 the induction voltage Jay will arise.

The same reference characters may now be used in order to describe the distribution of the current and voltage upon three-phase supply. Here the ohmic resistances r1, 1'2, and 1'3 are also taken into consideration, in which resistances the resistances of the arcs are included as the main part. Furthermore, for the arc currents the Y-wiring condition is valid:

Thanks to these circumstances the current scheme according to Fig. 1 may be considered as a superimposition of the single-phase current J1 within the outer phase for the electrode 1 and the middle phase for the electrode 2, and of the single-phase current Is within the outer phase for the electrode 3 and the middle phase for the electrode 2. Hence follow the terminal voltages E21 and E23 The Equations 3 and 4 relate to the uncompensated unsymmetrical system. This system now is compensated by inserting in the middle phase the reactance xa by which the compensating voltage is added to the other voltages.

Thereby the above equations are transformed to The increase of reactance of xza of the middle phase may be obtained in various manners. If the compensation of the asymmetry in existing equipments is concerned it is probably the simplest way to surround the bundle of wires of the middle phase by a laminated yoke 10 having a suitable air gap. If the transformer phases are Y-connected on the low voltage side, the increase of reactance also may be carried out on the primary side, possibly even by using the reactances present on this side viz. by decreasing the reactances of the outer phases as shown in Fig. 3 in such a manner that the desired difference x2e is brought about. Finally it is possible to insert on the primary side negative (capacitive) reactances (i. e. series condensers connected over the transformers) within the outer phases as shown in 4 in order to reduce their resulting reactances with respect to the reactance of the middle phase.

In order to obtain, upon the control of the arcs for constant power, the same power in all the three phases. the neutral point to which the voltage coils of the power regulator are connected should coincide-if possible--- with the neutral point of the transformer voltages. Usu ally the relay voltages of the regulator are connected to the mantle of the furnace and since the regulator equipment usually is arranged at great distance from the furnace, the coils receive unsymmetrical phase voltages. Neither can this asymmetry be removed by such a com pcnsation which is made according to the above in order to equalize the power in the feeders of the electrodes. Thanks to the invention this asymmetry of the voltages of the regulator relays can be compensated entirely by the introduction of a compensation voltage into the connection lead between the mantle of the furnace and the voltage coils of the relays, this voltage being derived from half the value of the reactance inserted in the feeder of the middle electrode. With the referenc charactrs used in the above calculations, the said sation voltage is For the understanding of this condition, the neutral conductor in Fig. 2 is considered as the return conductor for the currents J1, J2 and Is, in the feeders to the electrodes, which is permitted since J1+]2-1-]3:0. In accordance therewith the three phase system is interpreted as three single phase circuits ll-tl, 2-@ and 3-4), the self reactances of which are no, x2e, .rao rcwherein E1, E2 and E3 at secondary Y-connection are the phase voltages of the transformer and at A-connection the (ideal) neutral-point voltages. Upon symmetrical If therefore the secondary voltages E21, E13, E23 constitute a symmetrical voltage system, the same is true according to (11) for the phase voltages and according to (10) for the phase currents.

spectively and the mutual reactances of which is indicated as:

y12=y21 between the circuit 1--@ and the circuit 2-41 3 23:3 32 between the circuit 2-9 and the circuit ys1=y13 between the circuit 3-@ and the circuit l-t) Thus we obtain for an uncompensated system the following voltages between the neutral conductor and the transformer terminals 2l 3' j( 30-ys1)]j tQM Z/IZ) In the first approximation the difference between yin and yza may be neglected and likewise the difference between x10, x20 and x30 respectively. If furthermore the length of the feeders to the electrodes is great with respect to their reciprocal distance,

with sufiicient approximation.

In the uncompensated system we therefore obtain very unsymmetrical neutral point voltages, namely:

In order to compensate the power system according to the Equation (5) the compensation voltage flame is added in the feeder 2, which means that the right hand side of the Equation 15b is enlarged by the term iJzx2d. However, if no other measures are taken in the system for the impedance relays further a displacement of the neutral point will take place, namely This displacement has to be compensated additionally by an equal but opposed compensation voltage 801-: in the neutral conductor which proves the correctness of the Equation 12.

If the reactance element consists of an iron core arranged about the feeder 5 of the middle electrode 2, said core is provided with a current loop 11, embracing half the reactor field and being series-connected in the lead between the mantle of the furnace and the neutral point of the voltage coils of the relays. Should the reactance element be a reactor the compensation voltage may be taken by voltage division.

I claim as my invention:

1. Compensation means in three-phase electric arc furnace equipment comprising a furnace transformer, a furnace having three equal electrodes arranged as a middle electrode and two outer electrodes, three feeders for connecting said electrodes to their associated phases on the transformer and reactance means in the power supply system of the furnace, for increasing the reactance of the middle phase with respect to the reactances of the outer phases, wherein said reactance means has an amount x2e which has the approximate value related to the low voltage side of the furnace transformer, wherein an and x3 are the reactances of the singlephase circuits established by the middle electrode and one of the outer electrodes including their connection leads, and y is the mutual inductance between said single-phase circuits.

2. Compensation means in three-phase electric arc furnace equipment comprising a furnace transformer, a furnace having a middle electrode and two outer electrodes, and feeders for connecting said electrodes to their associated phases on the transformer, wherein, in the feeder for the middle electrode, is inserted a reactance means consisting of a laminated iron core with air gap surrounding said feeder for increasing the reactance of the middle phase with respect to the reactances of the outer phases, said reactance having an amount xza which has the approximate value related to the low voltage side of the furnace transformer, wherein an and x3 are the reactances of the singlephase circuits established by the middle electrode and one of the outer electrodes including their connection leads, and y is the mutual inductance between said singlephase circuits.

3. Compensation means according to claim 1 having a regulator for the control of the electrodes comprising relays with voltage coils characterized in that, in a connection lead between the neutral point of the furnace and the neutral point of the voltage coils of the relays in said regulator, there is applied an additional voltage derived from half the value of the reactance added in the middle phase of the furnace.

4. Compensation means according to claim 3 comprising an iron core surrounding the feeder of the middle electrode and provided with a current loop embracing half the magnetic field of said iron core and being seriesconnected with the connection lead between the neutral point of the furnace and the neutral point of the voltage coils of the relays in said electrode control regulator.

References Cited in the file of this patent UNITED STATES PATENTS 1,122,555 Troye et al. Dec. 29, 1914 1,242,971 Peters Oct. 16, 1917 1,279,928 Snyder Sept. 24, 1918 1,299,664 Berry Apr. 8, 1919 1,370,016 Greaves et al. Mar. 1, 1921 1,691,365 Woodson Nov. 13, 1928 1,839,148 Greene Dec. 28, 1931 2,419,988 Davis May 6, 1947 OTHER REFERENCES Handbuch der technischen Elektrochemie, part IV, Dr. Walter Grundlagen der elektrischen Ofenheizung," 1950, page 250.

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