US3483300A - Electric arc furnaces - Google Patents

Description

Dec. 9, 1969 L. M GEE ELECTRIC ARC FURNACES .5 Sheets-Sheet 1 Filed May 24, 1968 Dec. 9, 1969 Mc 3,483,300

ELECTRIC ARC FURNACES Filed May 24, 1968 5 Sheets-$heet 2 Dec. 9, 1969 MCG EE 3,483,300

ELECTRIC ARC FURNACES Filed May 24, 1968 5 Sheets-Sheet 5 WLJQ Nam L. M GEE ELECTRIC ARC FURNACES Dec. 9, 1969 .5 SheetsSheet 4 Filed May 24, 1968 H United States Patent M US. Cl. 13--9 4 Claims ABSTRACT OF THE DISCLOSURE A three-phase electric arc furnace having three electrodes each energized from a common supply transformer by an arrangement of flexible and rigid conduct-ors. There are at least two rigid conductors connected in parallel for each electrode, those for the two outer electrodes lying in respective vertical planes and those for the inner electrode lying in a horizontal plane, this arrangement presenting a particularly low inductance.

This invention relates to three-phase electric arc furnaces and in particular to the electrical connections between the supply transformer and the electrode holders.

In a conventional furnace of this kind, the three electrodes extend vertically downwards into the furnace shell, each being held in this position by a respective electrode holder. Normal practice is for the three holders to be located so that the electrode axes are at the apices of an equilateral triangle. Each holder is carried adjacent one end of a respective horizontally extending electrode arm. The three arms extend above the furnace shell and are generally parallel, often diverging at a small angle to one another towards the furnace shell. Accordingly when viewed from above there are two outer electrode arms carrying outer electrode holders and a centre electrode arm carrying the centre electrode holder. Each arm is normally carried by a respective vertical mast by which it and thus the associated electrode can be displaced vertically to adjust the arc length. The three masts are located between the furnace shell and the supply transformer which is electrically connected to the electrodes by an arrangement of flexible conductors and rigid conductors, normally bus-tubes. The flexible conduct'ors are necessary to cater for the relative movement between the electrode arms and the supply transformer. These conductors are connected to the rigid conductors which extend above the arms to the electrode holders which are electrically connected to the electrodes.

Such a furnace may be considered as belonging to one of three categories depending respectively on whether it has a power rating of below mva., of between 10 mva. and mva., or above 20 mva. In a furnace of any rating it is important that the power dissipation in the three phases should be equal and hence that the impedances of the electrical connections between the supply transformer and electrode holders should be equal for each phase so as to give phase-balance. With furnaces rated at above 10 mva. phase-balancing becomes increasingly difficult. It is equally important that the inductance of the secondary connections (i.e. those between the transformer secondary terminals and the electrode holders) should be low as otherwise the furnace will operate at a low uneconomical power factor. However, achieving the desired value of inductance of the secondary connections in larger furnaces rated at above 20 mva. presents considerable problems. This is because of the increased length and wider spacing of the secondary conductors for the increased size of furnace and also because of the higher current carried by the sec- 3,483,300 Patented Dec. 9, 1969 ondary system to provide the higher power input. Hence for furnaces rated at above 10 mva. phase-balancing is a serious problem while for furnaces rated at above 20 mva. both phase-balancing and the simultaneous attainment of low inductance are serious problems. The latter problem is naturally more serious if the supply frequency is 60 Hertz as for instance in North America, instead of 50 Hertz normally used in this country.

Attempts have been made in the past to reduce the inductance of the secondary connections while simultaneously balancing the inductance of each phase so as to obtain phase-balancing. For instance, the secondary delta connections of the supply transformer have been transferred from the transformer to adjacent the electrode holders, necessitating the provision of an extra mast and arm. Saturable supplementary reactors have been proposed for automatically varying the inductance of the secondary connections to maintain phase-balance. A fixed reactor has been fitted to the centre secondary conductors of at least one are furnace. However, besides requiring expensive and complicated constructions, these attempts have not succeeded in providing phase-balancing simultaneously with a reduction in inductance.

According to the present invention, in a three-phase electric arc furnace of the kind previously described, that is to say including three electrodes each held by a respective electrode holder, and three generally parallel, horizontally extending, vertically displaceable electrode arms each carrying one of the holders adjacent one end, at least two rigid electrode supply conductors connected in parallel, extend horizontally from each of the two outer electrode holders above the arms to respective flexible conductors connected to the supply transformer and are located with those from each holder lying in a respective one of a pair of substantially vertical, substantially parallel, spaced apart planes, while at least two rigid electrode supply conductors connected in parallel, extend horizontally from the centre electrode holder above the arms to respective flexible conductors connected to the supply transformer, are located substantially symmetrically between the two'vertical planes, and lie in a respective substantially horizontal plane. Such a furnace which therefore includes six or more substantially parallel electrode supply conductors, normally bus-tubes, has the major advantage over conventional furnaces that with suitable separations between the conductors, it has a particularly low inductance. Moreover conductor separations can be chosen in which besides presenting alow inductance, the inductance of the total secondary connections leading to the three electrode holders are equal thus giving phase-balance. Accordingly the problems previously discussed may be largely overcome in a simple cheap manner.

Naturally the flexible conductors themselves have a certain amount of inductance. Conventional practice is to group the flexible conductors of each phase in a bundle. The usual effect of this on the conductors per phase is that the bundle of flexible conductors for the centre electrode holder, being located between the other two bundles, has a lower inductance than the outer phases thus contributing to phase unbalance. However, utilising a construction in accordance with the present invention, this effect is no longer necessarily undesirable as by a suitable choice of the separation between the rigid conductors, e.g. bus-tubes, which lie in the substantially horizontal plane or planes, the inductance of these conductors can be made less than, equal to or greater than that of the rigid conductors extending to the outer electrode holders. Hence if the difference between the inductance of the centre and outer rigid conductors is made equal and opposite to the difference between the inductance of the centre and outer flexible conductors, the overall effect is that the total inductances of the phases are equal (it is of course understood that the inductances of the electrodes and the transformer are symmetrical). This is particularly desirable in that it is achieved using the conventional arrangement of flexible conductors. As an alternative, however, the separation between the rigid conductors can be so chosen that these conductors have equal inductance. This entails the use of an arrangement of flexible conductors also having equal inductance if phase-balance is to be obtained.

Although satisfactory results are achieved using only six bus-tubes or other rigid conductors, if more than six are employed and they are bus-tubes, it is preferable that there should be an even number so that each pair can provide a go and return flow path for cooling water. For instance there may be two sets of outer rigid conductors, each set comprising eight conductors lying in a respective substantially vertical plane, and eight centre conductors, four of which lie in one substantially horizontal plane and the other four of which lie in a second substantially horizontal plane immediately above. There is thus a total of two vertical planes and two horizontal planes. In other arrangements also in accordance with the invention th number may be increased yet further.

A reduction in the inductance of the secondary connections can also be achieved by completing the delta connection of the supply transformer at the junction of the flexible and rigid conductors. This entails six bundles of flexible conductors connecting the six transformer terminals to the rigid conductors. The flexible conductors secured to the outer transformer terminals are connected between the outer and inner phase rigid conductors while the two inner bundles of flexible conductors are connected between the inner transformer terminals and the outer phase rigid conductors.

By way of example, a three-phase electric arc furnace having a power rating of 36 mva. and an arc current of 42 ka., in accordance with the invention will now be described with reference to the accompanying drawings in which:

FIGURE 1 is a side elevation of the furnace;

FIGURE 2 is a schematic illustration showing the electrical connections;

FIGURE 2a: is a schematic illustration showing an alternative arrangement of the electrical connections.

FIGURE 3 is a more detailed side elevation partly cut away showing the rigid electrode supply bus-tubes;

FIGURE 4 is a plan view corresponding to FIGURE 3;

FIGURE 5 is an outline section taken along the line V-V of FIGURE 3;

FIGURE 6 is an outline section taken along the line VI-VI of FIGURE 3; and

FIGURE 7 ,is a graph showing the effect on the inductance of varying the separation of the centre-phase bustubes.

Referring first to FIGURES 1, 2, 3 and 4, the furnace includes a supply transformer 1 having three primary or input terminals 2 and six secondary or output terminal 3, the latter of which are labelled individually a a b b c 0 for reasons which will become apparent later. Each of the three transformer secondary windings is connected between a respective pair of the secondary terminals 3. Six sets of flexible conductors 4, each set comprising a bundle of eight conductors, electrically connect each secondary terminal 3 to a respective pair of electrode supply rigid bus-tubes as shown in FIGURE 2. This arrangement of flexible conductorshas low inductance because the two bundles of each phase hang close together and the mutual inductances between them are 180 apart and therefore fully subtractive. The alternative arrangement of FIGURE 2a on the other hand has a higher inductance because the subtractive mutual inductances between the phases are 120 apart and therefore only half-subtractive. The reference letters a a [1 b n cate corresp g points i?! lb? P figurfis- The furnace includes six bus-tubes 6-11 in all of which only three indicated at 6, 7 and 8 are visible in FIGURE 1. The :bus- tubes 6 and 7 are supported on and electrically insulated from a horizontal electrode arm 12 which itself is carried by a vertically displaceable mast 15. The two bus- tubes 6 and 7 are connected electrically so as to be in parallel by a cross-member 27. An electrode holder 18 is carried adjacent the end of the arm 12 and holds a cylindrical electrode 21. Likewise the bus- tubes 8 and 9 are supported on and electrically insulated from an identical horizontal electrode arm 13 which itself is supported by an identical vertically displaceable mast 16. These two :bus-tubes are connected to an electrode holder 19 which holds an electrode 22 and are connected together electrically by a cross-member 28. Likewise, the two remaining bus-tubes 10 and 11 are supported on and electrically insulated from an identical horizontal electrode arm 14 carried by a third vertically displaceable mast 17 and are connected by means of an identical electrode holder 20 to the third electrode 23. A cross-member 29 connects the bus-tubes 10 and 11 together. As is clear from FIGURE 3, the three electrode arms 12-14 are generally parallel, converging at a small angle towards the electrodes 21-23. The latter are located with their axes at the apices of an equilateral triangle and extend downwardly into a furnace shell 24 (see FIGURE 1). The charge to :be melted is placed in this and the heat of the three arcs which occur between the electrodes 21-23 and the metal surface melts the charge in the usual way. During this process, the three masts 15, 16 and 17 are adjusted vertically so as to adjust the corresponding arc lengths and thus maintain optimum operating conditions. At the end of the process, the shell 24 is tipped on racks 25 and the metal thus poured out.

As previously explained, the present invention is concerned with the disposition of the six rigid bus-tubes and reference should now also be made to FIGURES 5 and 6. All of the bus-tubes have an external diameter of 12.7 cm. (5 inches) and an internal diameter of 9.5 cm. (3.75 inches). The length of the'four longest bus-tubes, that is to say those connected to the outer electrode holders 18 and 20, is 8.5 metres. The three electrode arms have an external diameter of 40.7 cm. (16 inches) and an internal diameter of 35.6 cm. (14 inches). As shown in FIGURES 3 and 4, the pair of bus-tubes connected to each electrode holder is supported above, although not directly above, the respective electrode arm by brackets 26 made of stainless steel. These brackets are electrically insulated from the bus-tubes and are designed to withstand the maximum electromagnetic separation force that can occur between adjacent bus-tubes.

Reference to FIGURE 5 illustrates the disposition of the bus-tubes 6-11 over the greater part of their length and also how they are disposed in relation to the three electrode arms 12-14, which are shown at the same height. It can be seen from this figure that the two bus- tubes 6 and 7 lie in a vertical plane while the two :bus-tubes 10 and 11 also lie in a vertical plane spaced apart from the first plane. The two bus- tubes 8 and 9 lie in a horizontal plane and are symmetrically placed with respect to the other four bus-tubes. All of the bus-tubes are parallel to one another and extend horizontally. The spacings indicated in FIGURE 5 are set forth in the table given below.

Cms. (a) (54 inches) 137.0 (b) (36 inches) 91.5 (c) (11 inches) 28.0 (d) (42 inches) 106.8 (e) (24 inches) 61.0 (f) (45 inches) 114.0

FIGURE 6 illustrates the disposition of the six bustubes adjacent the connections to the flexible conductors 4. Th p c g g is 28.0 cm. (11 i ch The inductance per foot of the bus-tubes may be calculated using the normal mathematical formulae. With the dispositions indicated, the average inductance per metre of all the bus-tubes is 0.335 ,uH (0.102 ,uH/fOOt). The inductance of the centre bus- tubes 8 and 9 is higher than this being 0.374 tH/metre (0.114 H/foot). As previously indicated, it is preferable on furnaces designed with flexible conductor bundles, such as in FIGURE 2a, having low inductance in the centre phase bundle that the separations should be such that the inductance of the centre bus- tubes 8 and 9 is higher than that of the outer bus- tubes 6, 7, 10 and 11 so as to counteract the eifect of the flexible conductors 4 and thus give balance.

The effect of increasing the separation of the two centre bus- tubes 8 and 9 while maintaining them spaced symmetrically between the outer bus-tubes is shown in FIGURE 7 in which I represents the inductance in ,aH/metre and d represents the separation of the bustubes 8 and 9 in centimeters. The outer bus-tubes and the electrode arms are located as shown in FIGURES 5 and 6. The curve m gives the relationship for the centre phase conductors, the curve p that for the outer phase conductors and the curve n the average. It will be understood that the separation of the two centre bustubes 8 and 9 is chosen to make the inductance unbalance equal and opposite to that of the flexible conductors 4.

The effect of increasing the separation of the outer phase bus-tubes, 6 from 7 and 10 from 11, is to decrease their additive mutual inductance but without at the same time greatly decreasing their subtractive mutual inductance with the bus-tubes of the centre phase. The balanced position where all phases are equal is substantially as illustrated in FIGURE 7, at the point of intersection of the three curves. The bus- tubes 6 and 10 and also the bus-tubes 7 and 11 should be close together but a limitation on their minimum separation is caused by the heating effect of the current flowing in them on the centre electrode arm 13, on any occasion when any of these bus-tubes are at the same elevation.

It has already been indicated that the average inductance per metre of the illustrated arrangement is 0.335 ,uI-I. In comparison an equivalent conventional arrangement consisting of six bus-tubes all located in the same horizontal plane has an average inductance of 0.485 aI-I/metre and moreover the centre bus-tubes are lower in inductance than the outer bus-tubes and hence with the similar effect presented by the flexible conductors, there is a considerable degree of unbalance between the phases. Another equivalent conventional arrangement consists of three bus-tubes having their axes at the apices of an equilateral triangle. The size of the triangle is the smallest that the given electrode arm spacing permits. Although the inductances of these bus-tubes are substantially equal, the inductance per metre is high being of the order of 0.508 pH.

I claim:

1. A three-phase electric arc furnace energized from a supply transformer and including three electrodes each held by a respective electrode holder, three generally parallel, horizontally extending, vertically displaceable electrode arms each carrying one of said holders adjacent one end, rigid electrode supply conductors extending horizontally from each of said holders above said arms, and flexible conductors connecting said rigid conductors to said supply transformer, each outer holder of said electrode holders having at least two of said rigid conductors connected in parallel and lying in a respective one of a pair of substantially vertical, substantially parallel, spaced apart planes, and from centre holder of said electrode holders at least two of said rigid conductors connected in parallel, located substantially symmetrically between said vertical planes, and lying in a respective substantially horizontal plane.

2. A furnace according to claim 1 in which said supply transformer has three electrically separate secondary windings each terminating in a respective pair of six secondary terminals, and said flexible conductors connect each terminal of each of said pairs to those of said rigid conductors connected to different electrodes.

3. A furnace according to claim 1 in which the supply transformer has three secondary terminals connected to the rigid electrode supply conductors by the flexible conductors as shown in FIGURE 2a of the accompanying drawings.

4. A three-phase electric arc furnace, comprising a supply transformer, a vertically extending inner electrode, two vertically extending outer electrodes located on either side of said inner electrode, three electrode holders each holding one of said electrodes, three generally parallel, horizontally extending, vertically displaceable electrode arms each carrying one of said holders adjacent one end, three pairs of rigid electrode supply conductors, each pair of which extends horizontally from one of said holders above said arms, and flexible electrode supply conductors connecting said rigid electrode supply conductors to said supply transformer, said rigid conductors being disposed with those pairs extending from holders of said outer electrodes being located in respective substantially vertical, substantially parallel, spaced apart planes and with that pair extending from holder of said centre electrode being located substantially symmetrically between said vertical planes in a substantially horizontal plane.

References Cited UNITED STATES PATENTS Stewart 13l4 BERNARD A. GILI-IEANY, Primary Examiner H. B. GILSON, Assistant Examiner US. Cl. X.R. 1310, 14

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