WO2009087176A1

WO2009087176A1 - Electronic power supply device for arc furnace powered by alternating current

Info

Publication number: WO2009087176A1
Application number: PCT/EP2009/050140
Authority: WO
Inventors: Philippe Ladoux; Gianluca Postiglione
Original assignee: Institut National Polytechnique De Toulouse
Priority date: 2008-01-08
Filing date: 2009-01-07
Publication date: 2009-07-16
Also published as: FR2926182B1; FR2926182A1

Abstract

The invention relates to a power supply device (2) for an arc furnace (1) powered by an alternating current, and for controlling the active and reactive powers at said arc furnace. The supply device is provided on the side of a high-voltage network comprising at least one phase upstream from a voltage-lowering furnace transformer (5). The supply device includes a transformer (21) including a plurality of secondary windings (23), each secondary winding including a direct alternating-alternating converter Cij producing an output voltage adjustable on each phase from a given input alternating voltage.

Description

ELECTRONIC POWER SUPPLY DEVICE FOR ALTERNATING CURRENT ARC FURNACE

The invention relates to an arc furnace powered by alternating current. More particularly, the invention relates to an electronic power supply device capable of controlling the active and reactive powers at an arc furnace powered by alternating current. Electric arc furnaces are used especially in the steel industry for the production of steel for the melting of scrap metal.

Such arc furnaces operate under voltages of the order of a few hundred volts and currents of the order of a few tens of kiloamperes.

There are two main types of electric arc furnaces, those with direct current and those with alternating current.

In a steel plant using, for example, a conventional three-phase AC arc furnace, as illustrated in FIG. 1, the supply of said furnace is made from a high voltage network UH, of the order of a few tens to a few hundred kilovolts. Generally, a first step-down transformer 8 makes it possible to create a voltage level, called medium voltage UM, of the order of a few kilovolts to a few tens of kilovolts, from which other parts of the steel plant are also supplied with power. The three-phase arc furnace 1 which operates at low voltage UB, that is to say of the order of a few hundred volts, is supplied from the medium voltage using a second step-down transformer, said furnace transformer 5, which comprises a load adjuster adapted to adapt a voltage of a secondary circuit 53 of said transformer oven during operation of the arc furnace 1, depending on the load. Preferably, an inductor 6 is connected in series upstream of a primary circuit 52 of the furnace transformer 5 and makes it possible to limit the short-circuit current. The reciprocating arc furnace 1 comprises a melting vessel 1 1, receiving in an inner part 11 scrap 13, and three electrodes 12 mobile, mainly in height, each connected to one of the phases of the secondary circuit 53 of the transformer. furnace 5. Electric arcs 14 between the electrodes 12 and the scrap 13 cause the melting of the latter.

A control of the electrical power absorbed by the arc furnace 1 is made from mechanical positional regulations of the electrodes 12 which, by a vertical displacement of the electrodes 12, allow the average length of the electric arcs to be approximately adjusted. a load of scrap is made during a cycle of operation of the electric arc furnace 1. The cycle, forty minutes in general, consists of:

- A loading of a first basket of scrap 13 in the tank 1 1 of the furnace 1, - contacting the electrodes 12 with the scrap 13, to create a short-circuit current, then a rise of the electrodes 12 to start bows,

a so-called drilling period during which the electrodes 12 are lowered when the electric arcs 14, by melting of a part of the scrap 13, cause the appearance of a well in said scrap,

a melting period in which all the scrap 13 placed in the vessel 1 1 is gradually melted, a temporary stop of the operation of the arc furnace 1 for charging a second scrap basket 13 in the arc furnace 1, said arc furnace being most often at full load after this second charging,

a restart of the arc furnace 1 with initiation of the electric arcs 14,

- a drilling period,

- a melting period, until the scrap 13 of the second basket is completely melted,

- Finally, a refining step during which the bath temperature is maintained during the supply of additives to adjust the quality of the steel. During this sequence, it is possible to reduce the height of the electrodes to work with reduced arc lengths and thus protect the walls of the tank from radiation.

Significant disturbances are related to the movements of scrap inside the furnace.

On the one hand, the movements of scrap lead to a variation of the arc voltage leading to arc extinguishing or short circuits.

During drilling and melting sequences, when an arc is initiated between an electrode 12 and the scrap 13, said scrap begins to melt. The collapses of the scrap 12 inside the tank 11 cause an increase in the distance between the electrodes 12 and the scrap 13. If the arc length becomes such that the arc voltage is too great, the arc 'off. This phenomenon is accompanied by the cancellation of the currents taken from the network, which causes a sudden variation of the active and reactive powers.

Generally, during the drilling sequences and at the beginning of melting, the operation of the furnace 1 is carried out with short arcs under reduced tension in order to limit the risks of arc extinction. The arc length is then increased during the melting phase to reach a nominal operating point of the furnace. The scrap motions inside the furnace can also lead to a short-circuit between a moving electrode 12 and the scrap 13. During these short-circuiting, the arc current fluctuates and the arc voltage is minimal. The active power absorbed on the network then tends to zero and the reactive power is maximum.

On the other hand, during the collapse of the scrap 12 inside the tank, the short circuits and arc extinguishing succeed one another. The currents absorbed by the furnace are therefore very fluctuating and unbalanced. The variations of the active and reactive powers supported by the network are important and, when they are poorly controlled, cause electrical disturbances, in particular in the form of "flicker", that is to say, low frequency fluctuations of the value. effective voltage, causing "flickering" of lighting on devices connected to the high voltage network UH. In order to reduce the phenomenon of flicker on a power supply network, one solution consists in installing a static compensator for reactive power and imbalance 7 upstream of the arc furnace 1, generally at the level of the medium voltage UM. To have some efficiency, said static compensator reactive power and unbalance 7 must be sized, in apparent power, at least twice the power of the arc furnace 1, which makes it a very heavy device, bulky.

Another disadvantage of existing arc furnaces 1 relates to the position of the electrodes 12 regulators. In fact, these position regulations of the electrodes make it possible to control the operating point of the arc furnace 1 and limit the fluctuations of the active and reactive powers. However, given the masses in motion, of the order of a few tons per electrode, these positional controls have a slow dynamic, with a response time substantially of the order of one second. Therefore, the electric power can not be controlled under any circumstances. the scale of a period of the power grid, which is for example twenty milliseconds in France, and the active power transfer between the network and the furnace is not optimal.

The invention relates to an electronic power supply device for an arc furnace supplied with alternating current at a voltage UM of a supply network comprising at least one phase. The device is placed between the power supply network and a voltage-reducing furnace transformer with a constant ratio between an input voltage of said furnace transformer and a so-called low-voltage voltage UB for supplying the furnace. According to the invention, the electronic power supply device comprises:

at least one transformer comprising, for each phase, at least one primary winding, at least one secondary winding of a first category delivering a voltage V _3J , i = 1 to a number of phases of the network, N secondary windings of a second category delivering voltages V _2Jj1 with j = 1 to N ≥ 1,

a means for converting alternating-alternating voltage Dy, associated with each secondary winding of the second category, capable of delivering an adjustable voltage Vsy equal to or less than

V ₂ ij to output terminals (cy.dy) of said conversion means, and wherein said output terminals (cy.dy) of the at least one conversion means Dy and the at least one first-class secondary winding are connected to one another; series so that Vsy voltages at the terminals of said conversion means and said at least one secondary winding of first category are added to deliver a voltage V ₈ = V ₃ , +

feeding the oven through the furnace transformer. Each reciprocating-alternating conversion means D _rj delivers an output voltage VS _I J as a function of a value cx _u of a control parameter between 0 and 1.

Advantageously, each value α ,, is developed from an error signal between a current value representative of a measured arc current and a current setpoint l _rβf .

Advantageously, the electronic power supply device comprises a control loop of a current rms value representative of a measured arc current and an active power regulation loop which acts as long as the current reference l _ref is less than a predefined maximum value l _max . When the current setpoint developed by a corrector reaches the value l _max , the current rms value is therefore limited to l _max and the reactive power is kept constant.

Preferably, the first category secondary winding (s) taken as a whole are determined so as to define a maximum predefined reactive power Q during the short-circuit phases.

Preferably, the number N of alternative reciprocating conversion means D, _j is determined so that the sum of the output voltages Vs _I1

N secondary windings of first and second category, V ₃ , + ^ V _SlJ ,

is greater than a nominal operating voltage of the oven.

The alternative-to-alternative conversion means D _g comprise elementary-to-alternative elementary converters B | _Jt , with t = 1 to q> 1 associated in parallel and whose output voltage Vsi is determined by the value of α _u received by said alternating-AC elementary converters B _IJt .

Preferably, all second category secondary windings (23) and all alternative-to-alternative conversion means D ₁ are identical. Advantageously, the value α, _j is taken, for each phase i, equal to a value α ,. The invention also relates to an electric arc furnace powered by alternating current under a low voltage network UB which comprises:

a melting tank adapted to receive scrap in an interior part of said tank,

one or more electrodes able to generate electric arcs between said electrodes and scrap placed inside the vessel, the electrode or electrodes being connected to a secondary circuit of a furnace transformer, for converting a UM voltage; a low-voltage power supply network UB supplying the electrodes, characterized in that the oven is powered by the electronic power supply device, said device being placed between the supply network and a primary circuit of the furnace transformer. In one embodiment of a three-phase furnace, the phases of the primary and secondary circuits of the furnace transformer are star-coupled with a neutral output.

In another embodiment of a three-phase furnace, the phases of the primary and secondary circuits of the furnace transformer are coupled in a triangle.

The detailed description of the invention is made with reference to the figures which represent:

FIG. 1, already mentioned, an overall and principle diagram of a supply of an AC arc furnace with a conventional power supply according to the prior art,

FIG. 2 is a block diagram of an AC arc furnace with an electronic power supply device according to the invention, according to a first embodiment,

FIG. 3, a block diagram of one of the elements of the electronic power supply device according to the invention, for a phase, 4, an example of an alternative-AC conversion means of the electronic power supply according to the invention,

FIG. 5, an example of a direct-AC direct converter comprising an AC-to-AC conversion means, FIG. 6a, 6b, an exemplary operating principle of an AC-AC elementary converter comprising a direct converter, based on FIG. four-segment switches,

FIGS. 7a, 7b, two examples of four-segment switches,

FIG. 8, an example of an AC voltage chopper comprising two nested cells with three-segment switches,

FIG. 9, a block diagram of a regulation of the electronic power supply device according to the first embodiment,

10, a representation in a Q-P plane of the active and reactive powers for a phase, according to the first embodiment of the electronic power supply device,

11, an illustration of an active power, for a phase, transmitted to the scrap of an oven with a conventional power supply (1) and an oven with the first embodiment of the electronic power supply device (2). )

Figure 12a, an illustration of a reactive power absorbed by the oven and a conventional power supply,

FIG. 12b, an illustration of a reactive power, for a phase, absorbed by the furnace and the first embodiment of the electronic power supply device,

Figure 13 is an illustration of instantaneous apparent power of a static compensator for an arc furnace with a conventional power supply and an arc furnace with the first embodiment of the electronic power supply device, FIG. 14, an illustration of a flicker index P _st generated by the arc furnace with the first embodiment of the electronic power supply device,

FIG. 15, an overall and principle diagram of an arc furnace according to a second embodiment of the electronic power supply device of the invention,

FIG. 16, a block diagram of a regulation of the second embodiment of the electronic power supply device,

FIG. 17, an illustration of a total active power transmitted to the scrap of a furnace with a conventional power supply (1) and of an oven with the second embodiment of the electronic power supply device (2),

Figure 18a, an illustration of a total reactive power, absorbed by the furnace and a conventional power supply, Figure 18b, an illustration of a total reactive power, absorbed by the furnace and the second embodiment of the electronic power supply device. power,

Figure 19, an illustration of an apparent power of a static compensator for an arc furnace with a conventional power supply and an arc furnace with the second embodiment of the electronic power supply device,

Figure 20 is an illustration of a flicker index P _st generated by the arc furnace with the second embodiment of the electronic power supply device. An AC arc furnace 1, as shown in FIG. 2, comprises:

a melting tank 11,

at least one melting electrode 12.

The arc furnace 1 is fed by a low voltage network UB, for example a network obtained by a step-down furnace transformer. constant-ratio voltage between a medium-voltage network UM and the low-voltage network UB. The medium voltage network UM is, for example a network obtained by lowering in a transformer 8 of a high voltage network UH electrical distribution. The tank 1 1, adapted to receive in an inner portion 11 1 scrap 13 to be melted, is conventionally made of a refractory material.

The arc furnace is advantageously described in the case where it is powered by a three-phase network. The three-phase MV medium voltage network comprises three phases

A, B, C.

In the case of such an arc furnace fed by a three-phase network, three fusion electrodes 12 are movable above the vessel 1 1 and are each connected to one of the three phases of a secondary circuit 53 of the furnace transformer 5 .

In order to adjust the average power used by the arc furnace 1 and to control the arcing 14 between the electrodes 12 and scrap 13 deposited in the tank 11, the heights of the electrodes 12 above the tank 1 1 are adjustable in a known manner. According to the invention, a device 2 for electronic power supply of the arc furnace 1 is inserted in series between the medium voltage source UM and a primary circuit 52 of the furnace transformer 5.

At the output of the electronic power supply device 2, the three-phase network comprises three phases called a, b, c. Preferably, on each phase, an inductor 6 is connected in series in the supply circuit of the primary circuit 52 of the furnace transformer 5 to limit a short circuit current when one of the electrodes 12 comes into contact with the scrap 13 , during a merge operation. In addition, the electronic power supply device 2 of the arc furnace 1 according to the invention receives a setpoint value P _ref , a value determined according to a step of a melting cycle in which the arc furnace operates. 1. The device 2 electronic power supply essentially comprises a three-phase transformer 21, for example a three-phase transformer with columns having three columns.

The three-phase transformer 21 comprises, as illustrated in FIG. 3, for each column 211: a primary winding 22, at least one secondary winding of a first category 24, at least one secondary winding of a second category 23, and on at least secondary second-class windings 23, alternative-conversion means Dy, where the index i is one of the phases a, b or c, and the index j is an integer denoting the number of the conversion means alternative alternative.

Advantageously, the three primary windings 22 are coupled in a triangle.

A supply voltage of the primary winding 22 of a column 21 1 of the three-phase transformer 21, for each phase A, B, C is denoted Vi _x , x = 1 to 3.

The primary and secondary windings of first and second categories 22, 23, 24, in addition to sections of wires adapted to the powers involved, are such that a voltage Vsi across the secondary windings of first and second category 23, 24, mounted in series or at least a desired maximum voltage at the input of the primary circuit 52 of the furnace transformer 5, taking into account the constant active power regulation range required for the furnace.

In one embodiment, such as that of FIG. 3, the column 211 of the transformer 21 comprises, on the one hand, a secondary winding of first category 24 delivering a voltage V _3J and secondly N secondary coils of second category 23 identical each delivering a voltage V _2I1 , j = 1 to N, N> 1. This is only an example embodiment and secondary windings of second category may be different. The placing in series of the secondary windings 23, 24 leads to an upward operation of the transformer 21

Each AC-AC conversion means has two input terminals ay, by, connected to a secondary second category winding 23, and two output terminals Cy, dy. To simplify FIG. 3, only the first alternative-to-alternative conversion means Dn, the second alternative-to-alternative conversion means Dj ₂ and the N-th alternative-to-alternative conversion means DJN have been represented.

An output voltage between the output terminals Cy, dy of each AC-AC conversion means is noted by Vsy. The output terminals of the AC-AC conversion means

Dy are connected in series and connected with the secondary winding of first category 24. More precisely, the terminal of is connected with the terminal c-β, the terminal _d12 is connected with the terminal Cj ₃ , and so on. The terminal djN is connected with a terminal Cj of the secondary winding of a first category 24.

The AC-AC conversion means receive αy values developed from a power control loop so that a voltage Vs at the output of the electronic power supply device 2 for a phase i is equal to to the sum of the tensions:

FIG. 4 schematically represents an alternative-to-AC conversion means Dy of the electronic power supply device 2. The alternating-alternating conversion means Dy comprises between the input terminals a and bj an input filter 25 intended to eliminate a AC component of a current at a given switching frequency. The input filter 25 is a low pass filter which comprises, for example, essentially a capacitor C _e and an inductance L _e . The inductance L _e may be, for example, a leakage inductance of the secondary winding of a second category 23. The alternating-alternating conversion means D, _j comprises, between output terminals e, _{] t} f, _j of input filter, a direct-to-AC direct converter C _y . Said AC direct-AC converter is a device delivering an output AC voltage whose effective value is adjustable from a given input AC voltage. Each direct converter C _j preferably comprises, as illustrated in FIG. 5, a number q of alternating-AC elementary converters Bi _j t where the index t is an integer denoting the number of the elementary-to-alternative elementary converter between 1 and q, q> 1, in order to distribute the total current. The elementary converters 3 _yt are connected in parallel, between the terminals e _u and f, _j , and comprise an inductance Lq of current smoothing output.

In a preferred embodiment of the AC-B alternative elementary converters, said converters are of the AC voltage chopper type. The principle of operation of an AC voltage chopper is shown in Figures 6a, 6b. The AC voltage chopper comprises a switching cell 26 comprising controllable switches 27 and making it possible to obtain, on the scale of a switching period, an average voltage less than or equal to the input voltage without excessive heat dissipation. thanks to said switches operating in blocked / passing mode.

Advantageously, the commands of the elementary converters

2π can be interleaved and shifted by -, which reduces the ripple of a total output current. In an exemplary embodiment of an AC voltage chopper, the chopper includes bidirectional current and voltage switches, called four-segment switches, so as to obtain operation in the four quadrants of the voltage-current plane. By way of example, FIGS. 7a and 7b illustrate examples of four-segment switches comprising an assembly of diodes D and transistors T of the Insulated Gate Bipolar Transistor (IGBT) type or of the Integrated Gate Commutated Thyristor (IGCT) type.

In another exemplary embodiment of an AC voltage chopper, as illustrated in FIG. 8, the AC voltage chopper comprises a nesting in opposition of two switching cells comprising unidirectional voltage and bidirectional current switches, called three-segment switches. A three-segment switch is formed by the inverse paralleling of a diode D1 and a transistor T1, (respectively a diode D2, D3, or D4 and a transistor T2, T3 or T4), of the IGBT or IGCT type. This configuration also makes it possible to operate in the four quadrants of the voltage-current plane. This example makes it possible to guarantee, during the changes of state of the switches, so-called natural commutations, thanks to commutations disjoined by a dead time sufficient to avoid the simultaneous conduction of the two transistors.

The value OCy (FIGS. 3, 4, 5) represents an adjustable duty cycle which determines the output voltage VSJ by acting on the q elementary converters AC-Byt, and whose value is strictly between 0 and 1, which is ie for ocij = 0, V _Sij = 0, ai] = 1, Vsy = V _2ij . Thus, according to the value of αy, the total output voltage Vsi is adjustable between a minimum value Vsi _m j _n (for OCy = O) and a maximum value

Vsi max (POUr OCy = I), where:

V 'Si mm - V 3;

A ( ³ ) c ^{ι y} Sι ma \ '' il ^ /, ^γ read 7 = 1

Advantageously, voltage V ₂ i _j is determined based on the withstand voltage by semiconductor used to make the AC-AC converters elementary B _yt.

Advantageously, the minimum possible voltage V _Slmm = V ₃₁ is chosen so as to limit a reactive power during the short circuit phases Q _CC i on the arc furnace 1, such that:

V ² Q \ _cι = ^ - (4)

^Λ ccι where X _0C i represents a total reactance of short circuit.

Advantageously, in order to have a sufficient voltage margin to prevent the loss of current control and consequently lead to arc extinction, the number N of series-converted reciprocating conversion means is selected so that

where V _n is a nominal operating voltage of the arc furnace.

Preferably, the number N of reciprocating AC conversion means connected in series is determined so that

Thus, the adjustable output voltage V _Sl , for each phase i, keeps the active power P constant over a wide range of fluctuation of an arc voltage. When the arc voltage decreases too important, the active power P is no longer regulated and the reactive power Q is maintained at the value β ' _c . _α .

An apparent power of the furnace S _c u on a phase i is determined by

where Ui represents an output current for a phase i.

An apparent power S _convi of all the elementary converters Bφ on a phase i is determined by:

S _comι = (V ₈ , - V ₃₁ ) I ₈ , (8)

Using the two previous relationships and placing in a

V ² condition where Q _ca = - -, the analytic expression of the apparent power

S _C onvi of the set of elementary converters is given by the following relation:

Thus, the set of elementary converters is dimensioned for an apparent power lower than that of the furnace.

Preferably, in order to guarantee a better control of the current I _If in transient state, the range of adjustment of the output voltage Vsi can be increased so that the apparent power of all the elementary converters S _CO nvi is substantially equal to half the apparent load power, either

^ _o ,,,,, = 0.5S _c . ,,, (10)

In a first embodiment of the electronic power supply device 2 for an arc furnace 1, as illustrated in FIG. 2, said electronic power supply device is inserted in series on the medium voltage network U _M , with a connection of a neutral point on the medium voltage side U _M and tank side 1 1. Preferably, the electronic power supply device 2 according to this first embodiment can be installed on new sites.

In this first mode, the phases of the primary circuits 52 and secondary 53 of the furnace transformer 5 are star-coupled with the neutral output. The vessel 1 1 of the furnace 1 comprises a bottom electrode 1 12. The primary circuit 52 of the furnace transformer 5 is connected via the neutral n to the electronic power supply device 2. The secondary circuit 53 of the furnace transformer 5 is connected by the neutral n 'to the hearth electrode 1 12.

This first embodiment of the arc furnace makes it possible to obtain an independence of the line currents, which makes it possible to impose constant power operation on each phase i. This operation considerably reduces the impact of the fluctuations of the arc voltage and the unbalanced regime of the arc furnace 1 on the performance of the feed device of said furnace.

A schematic diagram of a regulation of the electronic power supply device is shown in FIG. 9.

Said regulation comprises, for each phase i, two nested control loops.

A first loop, called fast loop 3aj, allows the control of an effective value of an arc current lj _βff . A second loop, called slow loop 4aι, regulates the active power P ₁ .

Each direct converter Cy of each alternative-to-alternative conversion means Dy and each phase i is controlled by the value αy generated by the fast loop.

The fast loop 3aι comprises a corrector 31 a * which generates the value αy from a current error signal ει between a current setpoint I _rβ f elaborated by the slow loop 4aι and the value of the arc current measured lj _Θff . The corrector 31 ai has a limiter which limits the value αy between a predefined minimum value

and a predefined maximum value

Œijmaχ - 1.

Preferably, for each stage i, all the secondary windings of second group 23 and all AC-AC converters elementary By _t are similar and all αy values are taken equal to a control value α ,.

The slow 4AJ loop includes a corrector 41 have that develops the reference current I _ref from an active power error signal ε _p between P _ref power setpoint and the measured active power P].

The corrector 41 ai has a limiter which limits the value l _rΘf between a predefined minimum value U _n = 0 and a predefined maximum value l _max . The active power control loop 4aj acts as long as the current setpoint l _ref is lower than the maximum preset value l _max . When the current setpoint developed by the corrector reaches the value Imax, the current rms value is therefore limited to l _max and the reactive power is kept constant.

The power setpoint is defined as a function of the melting cycle of the furnace 1 and makes it possible to impose constant active power operation P except for the short-circuit phases. During the short-circuit phases, the current setpoint l _rβf is limited, which makes it possible to obtain a constant reactive power operation Q.

In an exemplary embodiment, the correctors 31 ai, 41 a _t are correctors of the Proportional Integral type Pl. The fast loop 3aι comprises a module 34ai for calculating an effective value of an arc current lj _Θff for a phase i, which receives an input representative of the output current Isr of a column of the three-phase transformer 21 via, for example, a current transformer 32aj. The slow loop 4a; comprises a module 44aj for calculating an active power value Pj for a phase i, which receives:

an input representative of the output current Isi via a current transformer 32aj. an input representative of the output voltage Vsi between the terminals

Cu secondary winding second category 23 and dj secondary winding first category 24 via a voltage transformer 42aj.

The terminals dj of the first category secondary winding 24 of each phase i are interconnected and connected to the neutral n.

FIG. 10 illustrates, for a phase, the operating zones of an arc furnace and presents, for the same values of arc voltage, the active power P and the reactive power Q, calculated at the primary circuit 52 of the furnace transformer 5 for an oven with a conventional feed (curve 1), an oven with a conventional feed and a 10% increase in the voltage at the primary circuit 52 of the furnace transformer 5 (curve 3) and the furnace with the feed device 2 power electronics according to the invention operating at constant active or reactive power (curve 2). Curve 1 in FIG. 10 represents the values of the active power P and the reactive power Q obtained during operation of the oven with a conventional power supply.

For this conventional power supply, the operating points are distributed in a circular arc. The variations in the arc voltage cause strong fluctuations in the active power P and the reactive power Q. FIG. 10 shows that the value of the reactive power Q is very important, especially during a short circuit, where it reaches the double the active power P which can be transmitted to the arc furnace 1.

When the supply voltage of the arc furnace with a conventional power supply is increased (curve 3), the operation are offset in the plane QP and are distributed in an arc larger than that of the curve 1. This operation has the same disadvantages as an oven with a conventional power supply, with an even more degraded power factor. In the case of the furnace with the electronic power supply device 2 (curve 2), each direct converter Cy adjusts the rms value of the voltage to the primary circuit 52 of the furnace transformer 5 so as to impose an active power operation P constant. During the short-circuit phases, each direct converter Cy operates at constant reactive power Q, the value αy is reduced in order to limit the current. The three-phase transformer 21 allows each direct converter Cy to extend the constant power operation over a wider range of variation of the arc voltage.

Thus, for the furnace with the proposed electronic power supply device 2, the constant active power operation P makes it possible to increase the electrical energy transmitted to the scrap 13 and the constant reactive power operation Q allows, during the phases of short circuit, to reduce the fluctuations of said reactive power Q, which decreases the risk of flicker appearing. Simulations carried out for an operating cycle of the arc furnace 1 with this first embodiment of the electronic power supply device 2 made it possible to evaluate a gain made on the electrical energy transmitted to the furnace 1 as well as the level of flicker on the power network. The simulation is performed for an arc furnace of total power S _n0m = 75 MVA fed from a 220 kV high-voltage grid whose short-circuit power is 6 GVA.

A comparison between the active power P, for a phase, transmitted to the steel bath, during an end-of-fusion sequence, an oven with a conventional feed (curve 1) and the oven with the device 2 of electronic power supply according to the invention (curve 2) is illustrated in FIG. 11. The variations of the active power P of the furnace with the electronic power supply device 2 are greatly reduced compared with those of the furnace with a conventional power supply, which thus makes it possible to increase the transferred active power without penalty. in the bath.

Similarly, FIGS. 12a and 12b illustrate a comparison between the total reactive power Q calculated at the primary circuit 52 of the furnace transformer 5, during an end-of-fusion sequence, of an oven with a conventional power supply (FIG. 12a). , and the oven with the electronic power supply device 2 according to the invention (FIG. 12b). The reactive power variations of the furnace with the electronic power supply device 2 according to the invention are greatly reduced compared to a conventional furnace.

Although the active and reactive power variations are reduced, it is necessary to introduce a static compensator of reactive power and unbalance 7 upstream of the electronic power supply device 2 in order to reduce the phenomenon of flicker. However, said static compensation of reactive power and unbalance is advantageously much smaller dimensioning than that of a compensator associated with an oven with a conventional power supply. For example, FIG. 13 illustrates an instantaneous apparent power S _c omp at the static reactive power and unbalance compensator 7 for an oven with a conventional power supply (curve 1) and the oven with the power supply device 2 (curve 2). In the case of the invention, the apparent power S _c omp fluctuates much less and represents only 25% of the total power S _n om of the oven instead of about 160% at certain times.

A value of a flicker index Pst generated by the furnace with the electronic power supply device according to the invention associated with the static compensator of reactive power and imbalance 7 is calculated at the point of connection to the high voltage network (PCC) and shown in Figure 14.

In this case, the flicker index P _st gg does not exceed a normative value equal to 1.

In order to compare the furnace with a conventional power supply and the furnace with the electronic power supply device 2 according to the first embodiment, the performances of these are summarized for a furnace with a total power of 75 MVA. the table below.

The oven with the electronic power supply device 2 according to the invention and the static compensator 7 makes it possible to eliminate the flicker phenomenon at the point of connection P _cc while increasing the electrical energy transmitted to the scrap by substantially 8%. In addition, the dimensioning of the electronics used in the furnace with the electronic power supply device 2 according to the invention and the static compensator 7 is much smaller than that of a static compensator associated with an oven with a conventional power supply. .

In a second embodiment of the electronic power supply device 2 for an arc furnace 1, as illustrated in FIG. 15, the electronic power supply device 2 is inserted in series. on the medium voltage source UM, without connection of a neutral point on the medium voltage side and on the furnace side.

This second embodiment of the device 2 electronic power supply is particularly suitable with a furnace installed on an existing site.

In this second embodiment, the phases of the primary circuits 52 and secondary 53 of the furnace transformer 5 are coupled in a triangle. A schematic diagram of a regulation of the electronic power supply device 2 is shown in FIG. 16. The regulation of the electronic power supply device 2 makes it possible to control an effective value of a direct component of the currents in order to impose a constant total active power operation.

Said regulation comprises two nested regulation loops. A first loop, called fast loop 3b, allows the control of an effective value of the direct component of the arc currents Ide _ff - A second loop, called the slow loop 4b, regulates the total active power P.

Each direct converter C _n is controlled by a value α _u generated by the fast loop 3b. Preferably, all the secondary windings of second category 23 and all the elementary-reciprocating converters B _ut are similar and all the values a, _t are taken equal to a single control value α.

The fast loop 3b comprises a corrector 31b which generates a value α from an error signal δι between a set current reference _{ref ref} of the slow loop 4b and the rms value of the direct component of the arc currents. Ide _ff - The corrector 31b includes a limiter which limits the value α between a predefined minimum value

and a predefined maximum value -

The slow loop 4b includes a corrector _{41b which generates} the current setpoint l _rβf from an error signal δp between the power setpoint P _rβf and the measured active power P.

The corrector _41b includes a limiter which limits the value l _rθf between a predefined minimum value I _m i _n = 0 and a predefined maximum value l _max . Said setpoint is defined according to the melting cycle of the arc furnace and makes it possible to impose a constant active power operation P constant.

Advantageously, the total active power constant P operation makes it possible to increase the total electrical energy transmitted to the scrap metal. In an exemplary embodiment, the correctors are correctors of the Proportional Integral type Pl.

In an exemplary embodiment, the fast loop 3b comprises a module 34b for calculating the rms value of the direct component of the arc currents Idem which receives: an input representative of an output current Isb for the phase b via for example, a current transformer 32b,

an input representative of an output current ls _c for phase c via, for example, a current transformer 33b.

In an exemplary embodiment, the slow loop 4b includes a module 44b for calculating the total active power value P and the total reactive power Q, which receives:

an input representative of an output current Is _b for phase b via, for example, a current transformer 32b,

an input representative of an output current ls _c for phase c via, for example, a current transformer 33b, an input representative of an output voltage between the terminals a and _c of two columns of the transformer via, for example, a voltage transformer 42b,

an input representative of an output voltage between the terminals a _to i and abi of two columns of the transformer via, for example, a voltage transformer 42b.

The slow loop 4b comprises, at the output of the module 44b, a module 45b which compares the total measured active power P and the total measured reactive power Q and which retains the maximum of the two values. The terminals di, db, d3 of the first category secondary winding

24 of each phase a, b, c are connected to each other and form a neutral point.

When the total reactive power Q exceeds the total active power P, a total constant reactive power operation Q limits the reactive power.

This operation makes it possible to improve the power factor of the installation and to considerably reduce the total reactive power Q of the installation during a short circuit.

Simulations carried out for an operating cycle of an arc furnace 1 with the electronic power supply device according to this second embodiment made it possible to evaluate a gain made on the electrical energy transmitted to the oven and a level of flicker on the power network. The simulation is performed for an arc furnace 1 of total power S _n _m = 75 MVA, fed from a 220 kV network with a short-circuit power of 6 GVA.

A comparison between the total active power P transmitted to the steel bath, at the end of the melting sequence, of an oven with a conventional feed (curve 1) and of the furnace with the second embodiment of the device of electronic power supply according to the invention (curve 2) is illustrated in FIG. The active power P of the furnace according to the invention is greatly reduced compared to an oven with a conventional feed, which makes it possible to increase the average value of the power transferred to the bath.

Similarly, FIGS. 18a and 18b illustrate a comparison between the total reactive power Q calculated at the primary circuit 52 of the furnace transformer 5, during a fusion sequence, of a furnace with a conventional power supply (FIG. 18a) and of FIG. oven with the second embodiment of the electronic power supply device according to the invention (Figure 18b). The variations in reactive power Q of the oven according to the invention are greatly reduced compared to an oven with a conventional feed.

Although the active and reactive power variations are reduced, it is necessary to introduce a static compensator of reactive power and imbalance 7 upstream of the electronic supply in order to reduce the flicker. However, this compensator is advantageously dimensioned much smaller than that of a compensator associated with an oven with a conventional power supply, but substantially larger than that associated with an oven with the first embodiment of the electronic power supply device. . For example, FIG. 19 illustrates the apparent power S _COmp of the static compensator for reactive power and unbalance and for a conventional power supply and the second embodiment of the electronic power supply device proposed by the invention.

The value of a flicker index Pst generated by the furnace with the second embodiment of the electronic power supply device according to the invention associated with its static compensator of reactive power and unbalance 7 is calculated at the connection point P _cc to the high voltage network and shown in Figure 20. As for the first embodiment of the power supply device, the The value of the flicker index is such that the phenomenon of flicker disturbs the network.

The performance of a conventional furnace and furnace with the second embodiment of the electronic power supply device is summarized for a 75 MVA total power furnace in the table below.

The second embodiment of the electronic power supply device of the furnace 1 with the static compensator of reactive power and imbalance 7 makes it possible to eliminate the flicker at the connection point P _cc to the high-voltage network while increasing the electrical energy transmitted. scrap.

In addition, the sizing of the electronic portion used in the second embodiment of the electronic power supply device of the furnace is much smaller than that of a reactive power compensator associated with an arc furnace with a conventional power supply.

Whatever the embodiment of the power electronic power supply device 2 for an arc furnace 1, the addition of said electronic power supply device makes it possible to increase the active power transmitted to the arc furnace, substantially on the order of 10% over one operating cycle, and to reduce the reactive power absorbed. Said electronic power supply device makes it possible to eliminate a load adjuster on furnace transformer 5, required on a furnace with a conventional power supply, and to reduce the design power of the static compensator. The increase in active power transmitted over one operating cycle makes it possible to increase the productivity of the kiln in almost the same proportions.

Decreases in the active and reactive power variations allow a reduction of the sizing power of the static compensator in a ratio of at least three, thereby reducing the cost of the static compensator.

The removal of the load regulator finally reduces maintenance at the furnace transformer.

Claims

CLAIMS - Electronic power supply device (2) for an arc furnace (1), said device being supplied with alternating current under a voltage UM from a power supply network comprising at least one phase, said device being placed between the power supply network and an oven transformer (5) voltage step-down with a constant ratio between an input voltage of said oven transformer and a voltage, called low voltage UB, supplying the oven, and comprising:

- at least one transformer (21) itself comprising, for each phase: o at least one primary winding (22), o at least one secondary winding of a first category (24) delivering a voltage V3 ₁ , i = 1 to a number of phases of the network, o N secondary windings of a second category (23) delivering voltages V _21J , with j = 1 to N ≥ 1,

- an alternating-alternating voltage conversion means D _ti , associated with each second category secondary winding (23), capable of delivering an adjustable voltage V _SI J less than or equal to N^ on output terminals (c, _j , d| _j ) of said conversion means, and in which said output terminals (C| _j , d| _j ) of the conversion means D, _j and the at least one secondary winding of the first category (24) are connected in series so that the voltages Vs _y at the terminals of said conversion means and said at least one secondary winding of the first category are added to deliver a voltage V _s = V _3l + ∑V _SlJ for supplying the oven through the

7=1 furnace transformer (5). 2- Electronic power supply device (2) according to claim 1 in which each alternating-alternating conversion means Dy delivers an output voltage Vsy as a function of a value oij _j of a control parameter. 3- Electronic power supply device (2) according to claim 2 in which each value αy is developed from an error signal between a current value representative of a measured arc current and a setpoint in current l _ref .

4- Electronic power supply device (2) according to one of the preceding claims comprising a control loop of an effective current value representative of a measured arc current and a regulation loop of a power active when the current setpoint l _ref is less than a predefined maximum value Im _3x , or an active reactive power when the current setpoint l _ref reaches the predefined maximum value l _max .

5- Electronic power supply device (2) according to one of the preceding claims in which the secondary winding(s) of the first category (24) taken as a whole are determined so as to define a maximum predefined reactive power Q during the short-circuit phases.

6- Electronic power supply device (2) according to one of the preceding claims in which the number N of AC conversion means Dy is determined so that the sum of the output voltages Vsy of the first and second secondary windings category (23, 24), V _3j + ∑V _SiJ , is greater than y=i a nominal operating voltage of the oven.

7- Electronic power supply device (2) according to one of the preceding claims in which the alternating-alternating conversion means Dy comprise converters elementary alternating-alternating units By _t , with t = 1 to q ≥ 1 associated in parallel and whose output voltage Vsi is determined by the value of ccy received by said elementary alternating-alternating converters

8- Electronic power supply device (2) according to one of the preceding claims in which all the second category secondary windings (23) and all the alternating-alternating conversion means Dy are identical.

9- Electronic power supply device (2) according to claim 8 in which the value ccy is taken, for each phase i, equal to a value OCJ.

10-Power arc electric furnace (1) powered by alternating current under a low voltage network UB comprising:

- a melting tank (1 1) capable of receiving scrap metal (13) in an interior part (1 11) of said tank,

- one or more electrodes (12) capable of generating electric arcs (14) between said electrodes and the scrap metal (13) placed inside the tank, the electrode(s) (12) being connected to a secondary circuit (53 ) of an oven transformer (5), for converting a voltage UM of a power supply network into low voltage UB for supplying the electrodes, characterized in that the oven is powered by a device

(2) electronic power supply according to one of claims 1 to 9, said device being placed between the power supply network and a primary circuit (52) of the oven transformer (5).

1 1- Three-phase oven according to claim 10 in which the phases of the primary (52) and secondary (53) circuits of the oven transformer (5) are star-coupled with an output neutral. -Three-phase oven according to claim 10 in which the phases of the primary (52) and secondary (53) circuits of the oven transformer (5) are triangle coupled.