Electric power supply apparatus for an electric arc furnace

Abstract

An electric power supply apparatus (10) for an electric furnace (11), comprising at least one base power supply module (20, 20 A, 20B) having a first (21), a second (22) and a third sub-module (23), which are respectively connected at input to a three-phase electric network (13) by means of respective transformers (24, 25, 26) and at output to a three-phase power supply line (32) configured to power one or more electrodes (16) of said electric furnace (11).

Description

[0001] “ELECTRIC POWER SUPPLY APPARATUS FOR AN ELECTRIC ARC FURNACE”

[0002] FIELD OF THE INVENTION

[0003] The present invention concerns an electric power supply apparatus for an electric furnace for steelmaking applications for the production of steel, or for other sectors that process metals or vitreous materials, or other similar or comparable materials. The electric power supply apparatus is, in particular, applicable to electric furnaces that operate with alternating electric currents and voltages.

[0004] BACKGROUND OF THE INVENTION

[0005] As is known, the electric furnaces used to melt metal in steelmaking applications require an efficient electric power supply system that supplies high powers.

[0006] It is also known that a melting process comprises several steps, which generally comprise a step of boring the metal material, a step of melting the material and a step of refining.

[0007] The power and electric energy required by the electric furnace during the melting process vary even significantly from one step to another, therefore it is necessary to suitably adapt the amount of electric energy supplied on each occasion.

[0008] In particular, the power absorbed by the electric furnace during the step of boring the metal material, or even during the melting step, is generally greater than the one required during the refining step and, depending on the type of material that is fed into the furnace, can vary considerably even within the same process step.

[0009] Power supply apparatuses for electric arc furnaces are known that connect to a power supply network, generally three-phase, and convert the electric voltage and current supplied by the power supply network into electric voltage and current suitable to power the electrodes of the electric arc furnace.

[0010] Known apparatuses comprise a rectifier device, which transforms the alternating current supplied by an electric network into direct current, and one or more inverter devices which transform the direct current into alternating current to power the electrodes.

[0011] The amount of electric energy supplied to the electrodes is then adjusted by appropriately commanding the inverter devices.

[0012] Generally, these inverter devices comprise one or more switches that are opened and closed with a high frequency, whereby if it is necessary to dynamically change the amount of energy supplied to the electric furnace, and even if there are a plurality of inverter devices, fluctuations in the electric voltage can be generated that can also have an effect back along the circuit, creating problems for the electric network.

[0013] These inverter devices, in fact, cause the modulation of the current that is performed, generate current harmonics that can be harmful if fed into the electric power supply network.

[0014] There is therefore the need to perfect an electric power supply apparatus for a direct current user device that can overcome at least one of the disadvantages of the state of the art.

[0015] One purpose of the present invention is to provide an apparatus and a method for powering an electric arc furnace which allows to effectively regulate the operation and power of an electric furnace according to requirements.

[0016] Another purpose of the present invention is to provide an apparatus and implement a method which allows to adjust the characteristics of voltage and current supplied to an electric furnace, in particular an arc furnace, in order to guarantee the stability of the electric arc during the various steps of the melting process.

[0017] Another purpose of the present invention is to perfect a balanced electric power supply apparatus which reduces any disturbances on the side of the electric power supply network to a minimum. Another purpose of the present invention is also to perfect an electric power supply apparatus which has a modular construction and can therefore be adapted based on the requirements of the plant, or the characteristics of the electric furnace, to which it has to be applied.

[0018] Another purpose of the present invention is to perfect a method for powering an electric furnace which allows to supply the desired and necessary quantity of electric energy on each occasion and, at the same time, prevents possible disturbances toward the electric power supply network.

[0019] The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

[0020] SUMMARY OF THE INVENTION

[0021] The present invention is set forth and characterized in the independent claim. The dependent claims describe other characteristics of the present invention or variants to the main inventive idea.

[0022] In accordance with the above purposes, there is provided an electric power supply apparatus according to the present invention, suitable to power an electric furnace for steelmaking applications for the production of steel, or for other sectors that process metals or vitreous materials, or other similar or comparable materials.

[0023] The apparatus comprises at least one base power supply module having a first, a second and a third sub-module and a respective three-phase transformer for each of the sub-modules, which are respectively connected at input to a three-phase electric network by means of respective transformers and at output to a three-phase power supply line configured to power the electrodes of the electric furnace.

[0024] In accordance with the present invention, each sub-module comprises, disposed in succession one to the other, a rectifier device connected to a respective one of the three-phase transformers, which is configured to transform a three-phase alternating electric current and voltage into direct electric current and voltage, a direct current intermediate circuit, or DC-link, configured to store electric energy, and two inverter devices which are each connected to the intermediate circuit and are configured to transform the direct voltage and current into alternating supply voltage and current suitable to supply a different phase of the power supply line, and wherein each of the sub-modules is configured to supply a pair of phases of the power supply line different from the pairs of phases supplied by the other submodules. In this way, the at least one base power supply module has at output two connections for each of the three phases of the power supply line.

[0025] Doing so achieves at least the advantage of balancing the voltages and currents supplied to the electrodes, since at least one component for each phase is supplied by two different sub-modules.

[0026] Providing three input phases and two output phases for each of the base modules, in such a way as to recompose a three-phase current through the cooperation of the three sub-modules, also allows to manage any malfunctions of one of them, since in any case two sub-modules are sufficient to supply the three phases.

[0027] According to some embodiments, the rectifier device comprises a rectifier circuit for each of the phases, all of which are connected to the same direct current intermediate circuit.

[0028] This allows to achieve a common storage of the electric energy supplied overall by the three phases between the rectifier devices and the inverter devices.

[0029] In accordance with one aspect of the present invention, each transformer comprises a transformer primary and a transformer secondary, both having inputs and outputs of the three-phase type, wherein the inputs of the transformer primary are connected to the phases of the electric network and the outputs of the transformer secondary are each connected to one of the rectifier circuits.

[0030] According to some embodiments, the phases in the respective transformer primaries and secondaries of one or more of the transformers are connected partly according to a star configuration and partly according to a delta configuration, in order to standardize the electric currents and voltages respectively inside the, or each, base module.

[0031] In accordance with another aspect of the invention, the apparatus comprises a plurality of base modules connected in parallel to each other between the electric network and the power supply line, wherein each base module receives three three- phase connections at input and supplies two three-phase connections at output.

[0032] The number of base modules can be multiplied according to requirements, for example doubled, tripled, quadrupled so as to obtain a total of six, nine, twelve, eighteen sub-modules, and so on. This modular construction advantageously allows to adapt the power supply apparatus to the requirements of a plant, both in its design phase in order to define the total number of base modules in relation to the required size and productivity needs, and also in the usage phase, making it possible to optimally adjust the electric energy supplied in relation to requirements, for example keeping only some of the base modules active on each occasion.

[0033] According to one aspect of the invention, at least one transformer has different connections between the respective phases of the transformer primary and / or of the transformer secondary with respect to at least another transformer associated with a same base module, or possibly of a different base module, as regards at least one of either a star or delta type connection, or a phase shift between the respective phases.

[0034] According to another aspect of the invention, the phases in the respective transformer primary and transformer secondary of the transformers are connected partly according to a star configuration and partly according to a delta configuration.

[0035] According to another aspect of the invention, in the case of star type connections, at least one connection has the respective phases out of phase by a phase angle comprised between 10° and 20° in one or the other direction with respect to the phases of another connection.

[0036] In general, if there are two or more base modules, the phase connections of the respective transformers can assume configurations that are partly or entirely different from each other, in order to balance and standardize the voltages and currents both in each of the sub-modules, and also overall between one base module and another.

[0037] In accordance with another aspect of the present invention, a method for the electric power supply of an electric furnace, comprises:

[0038] - supplying, by means of a three-phase electric network, an alternating electric current and voltage having a predefined frequency;

[0039] - transforming, with respective transformers, the alternating electric current and voltage into three-phase alternating electric current and voltage suitable to respectively power at least a first, a second and a third sub-module of a base module which is configured to convert the electric energy supplied by the transformers into electric energy suitable to supply a three-phase power supply line;

[0040] - powering one or more electrodes of the electric furnace by means of the phases of the power supply line.

[0041] In accordance with one aspect of the electric power supply method of the present invention, in each of the sub-modules there is provided a rectifying of the respective phase voltages and currents to obtain a common direct electric voltage and current for the three phases, a temporary storage of energy and a separation between the electric network and the electrodes by means of a direct current intermediate circuit, and an inversion of the direct electric voltage and current by means of two inverter devices to obtain respective phase alternating electric voltages and currents, suitable to each supply a different phase of the power supply line, wherein each of the sub-modules supplies a pair of phases of the power supply line different from the pairs of phases supplied by the other sub-modules so that the at least one base module supplies at output two connections for each of the phases.

[0042] DESCRIPTION OF THE DRAWINGS

[0043] These and other aspects, characteristics and advantages of the present invention will become apparent from the following description of some embodiments, given as a non-restrictive example with reference to the attached drawings wherein:

[0044] - fig. 1 is a schematic view of an electric power supply apparatus according to some embodiments described here, having a single base module comprising three submodules and applied to an electric arc furnace; - fig. 2 is a schematic view of the circuit of a sub-module of the electric power supply apparatus of fig. 1 ;

[0045] - fig. 3 is a schematic view of an electric power supply apparatus according to some variants, comprising two base modules and applied to an electric arc furnace;

[0046] - figs. 4a-4f show some schematic examples of the star and / or delta connections between the transformer primaries and the transformer secondaries of the electric power supply apparatus of fig. 3.

[0047] To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can be conveniently combined or incorporated into other embodiments without further clarifications.

[0048] DESCRIPTION OF SOME EMBODIMENTS

[0049] We will now refer in detail to the possible embodiments of the invention, of which one or more examples are shown in the attached drawings, by way of a nonlimiting illustration. The phraseology and terminology used here is also for the purposes of providing non-limiting examples.

[0050] The embodiments described here with reference to the attached drawings concern an electric energy supply apparatus 10 for an electric furnace 11, in particular an alternating-current electric furnace. The electric furnace 11 can be for example an electric arc furnace of the type usable in a plant 15 for steel making applications for melting scrap or other metal material by means of an electric arc ignited by two or more electrodes 16a, 16b, 16c, generally indicated as a whole with number 16.

[0051] These electrodes 16 generally enter the inside of the furnace crucible from the vault, and can normally be moved toward or away from the material to be melted.

[0052] The electric furnace 1 1 in question can also be a submerged arc furnace.

[0053] The power supply apparatus 10 comprises electric energy supply means 12, suitable to supply electric energy having predefined voltage, current and frequency values.

[0054] According to some embodiments, the electric energy supply means 12 comprise a three-phase electric network 13, configured to supply alternating current electric energy. In fig. 1, the letters R, S and T indicate the three phases.

[0055] The power supply apparatus 10 is connected to the electric network 13 and to the electric furnace 11 by means of suitable input and output connection means.

[0056] The input and output connection means generally comprise connection lines, connectors and suchlike.

[0057] The apparatus 10 preferably has a modular structure and comprises at least a first 21, a second 22 and a third sub-module 23, which are configured to each receive at input three electric lines, each corresponding to one alternating current and voltage phase R, S, T, and to supply at output two alternating current and voltage electric lines, each corresponding to one of the aforementioned phases R, S, T.

[0058] According to some embodiments, each sub-module 21 , 22, 23 supplies a pair of phases R-S, S-T, R-T different from that of the other sub-modules 21, 22, 23.

[0059] In this way, while a three-phase electric line 13, 32 is provided both at input and also at output from each module 20, each sub-module 21, 22, 23 helps to supply at output two of the aforementioned phases R, S, T of a three-phase power supply line 32.

[0060] According to one aspect of the invention, the three sub-modules 21, 22, 23 together define a base module 20 of the apparatus 10, which can be multiplied by N times, as a function of requirements or of parameters such as, for example, the power required for the furnace 11, the productivity required, the number of electrodes 16.

[0061] The number N can for example be variable between 2 and 40, for example 3, 4, 5, 6, 8, 10, 12, 18, 24, 36, or even other intermediate even or odd numbers.

[0062] Preferably, the number N of the base modules 20 is a multiple of three.

[0063] By way of example, two base modules 20a, 20B are shown in fig. 3, therefore a total of two first sub-modules 21 A, 2 IB, two second sub-modules 22 A, 22B and two third sub-modules 23 A, 23B are present. Obviously, by multiplying the base modules 20 by a number N, there will contextually be N sub-modules 21, 22, 23 for each type.

[0064] According to some embodiments, each sub-module 21, 22, 23 comprises, disposed in succession one to the other, a rectifier device 27, configured to transform the three-phase alternating electric current and voltage into direct electric current and voltage, a direct current intermediate circuit 28, or DC-link, common for all phases, and two inverter devices 29, 30, both connected to the intermediate circuit 28 and configured to transform the direct voltage and current into alternating voltage and current.

[0065] According to some embodiments, the apparatus 10, that is, each base module 20, can comprise an inductor 17, 18 connected between a respective inverter device 29, 30 and the power supply line 32.

[0066] According to some embodiments, each rectifier device 27 comprises a respective rectifier circuit 27R, 27S, 27T for each phase R, S, T.

[0067] The rectifier device 27, and in particular each rectifier circuit 27R, 27S, 27T can be made as a diode bridge or a thyristor bridge, and comprise devices chosen from diodes, Silicon Controlled Rectifier (SCR), Gate Turn-Off (GTO) thyristor, Integrated Gate-Commuted Thyristor (IGCT), Metal-Oxide Semiconductor Controlled Thyristor (MCT), Bipolar Junction Transistor (BJT), Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET), Insulated-Gate Bipolar Transistor (IGBT) or Silicon Carbide (SiC) device.

[0068] According to some embodiments, each rectifier circuit 27R, 27S, 27T is connected to the same direct current intermediate circuit 28.

[0069] This intermediate circuit 28 can comprise one or more capacitors 35, for example a capacitor bank, suitable to store energy and create a separation between the rectifier circuits 27R, 27S, 27T and the inverter devices 29, 30, and therefore also between the electric network 13 and the electrodes 16.

[0070] According to some embodiments, the inverter devices 29, 30 can comprise one or more switches 36 chosen, for example, from a thyristor or a transistor, of the following types: Gate Turn-Off (GTO) thyristor, Integrated Gate-Commuted Thyristor (IGCT), Metal-Oxide Semiconductor Controlled Thyristor (MCT), Bipolar Junction Transistor (BJT), Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET), Insulated-Gate Bipolar Transistor (IGBT), or suchlike. The switches 36 can generally be associated with respective diodes, not shown.

[0071] According to some embodiments, it can also be provided that each inverter device 29, 30 comprises a plurality of inverter circuits, connected in parallel to each other, wherein the inverter circuits of a same inverter device 29, 30 are all connected at output to a same phase of the power supply line 32.

[0072] The operation of the inverter devices 29, 30, and therefore of the switches 36, can be adjusted by a control unit 40 in order to obtain at output a current having desired intensity and frequency.

[0073] The control unit 40 can be configured to manage and command the inverter devices 29, 30 in relation to the specific steps of the melting process performed in the electric furnace 11 , in order to vary one or more parameters of the electric current and voltage supplied to the electrodes 16, for example the intensity and / or frequency.

[0074] In accordance with possible solutions of the present invention, the control unit 40 can comprise, or be connected to, adjustment devices, not shown, configured to adjust the supply electric frequency of the electric voltage and current to be supplied to the electrodes 16, in order to obtain a contextual variation of the reactance value of the power supply circuit of the electrodes 16.

[0075] By way of example, the adjustment devices can comprise a hysteresis modulator or a PWM (Pulse- Width-Modulation) modulator.

[0076] According to some embodiments, the two inverter devices 29, 30 of each submodule 21, 22, 23 supply at output respective alternating electric currents and voltages for supplying a different phase R, S, T of the three-phase power supply line 32.

[0077] The three-phase power supply line 32 can in turn be connected to the electric furnace 11 to be powered. In particular, in the event three electrodes 16 are provided, each phase R, S, T can be connected to a respective electrode 16a, 16b, 16c.

[0078] In the event the number of electrodes 16 is lower, or higher, only some of the three phases R, S, T can be connected, or two or more phases can be connected to two or more electrodes 16.

[0079] The apparatus 10 also comprises a respective transformer 24, 25, 26 for each of the sub-modules 21, 22, 23, configured to transform the three-phase primary alternating voltage and current supplied by the electric network 13 into three-phase secondary alternating voltage and current having desired characteristics suitable to power the respective sub-module 21, 22, 23, and consequently the electric furnace 11.

[0080] In fig. 3, the transformers 24, 25, 26 respectively associated with one or the other base module 20a, 20B have also been indicated with the respective letter A, B in addition to the reference number, simply to facilitate their identification. Each transformer 24, 25, 26 has respective outputs and inputs of the three-phase type and comprises a transformer primary 33 connected, during use, to the electric network 13, and a transformer secondary 34 connected to the rectifier device 27.

[0081] In particular, each output of the transformer secondary 34 is connected to a respective rectifier circuit 27R, 27S, 27T. According to some embodiments, the phases R, S, T of the transformer primary 33 and transformer secondary 34 can be connected according to a star or delta configuration.

[0082] Preferably, at least one transformer 24, 25, 26 has the connections between the respective phases R, S, T of the transformer primary 33 and / or transformer secondary 34 of a different type compared to at least one other transformer 24, 25, 26.

[0083] By “connections of a different type” we mean both the case in which the phases R, S, T of at least one of either the transformer primary 33 or the transformer secondary 34 of one of the transformers 24, 25, 26 are connected with a star connection, while the phases R, S, T of the other are connected with a delta connection, and also the case in which both have a connection of the same type, for example delta, but there is a phase shift of a certain angle between the respective phases R, S, T. According to possible embodiments, the transformers 24, 25, 26 associated with a same base module 20 all have different connections from each other.

[0084] In the event that several base modules 20 are provided, the connections between the input and output phases R, S, T of the respective transformers 24, 25, 26 can all be different from each other, or can be repeated according to a desired order.

[0085] The different conformations of the phase connections in the transformers 24, 25, 26 of one base module 20 or between several base modules 20 are preferably chosen in such a way as to be overall different from each other, at least according to the type of connection or a phase angle between the phases, in order to obtain a balance of both the current and the voltage within the single base module 20, as well as overall within the apparatus 10.

[0086] Fig. 4 shows, by way of example, some conformations of the connections that can be assumed by the respective transformers 24A, 25A, 26A, 24B, 25B, 26B of the apparatus 10 of fig. 3. Although they are shown in a given order, it is clear that this order can be modified or that additional configurations can be provided.

[0087] Fig. 4a shows, by way of example, a transformer 24 A in which the phases of the transformer primary 33 and of the transformer secondary 34 are connected according to the star conformation.

[0088] Figs. 4b and 4c show, by way of example, two transformers indicated with references 25 A, 26 A, in which the phases of the transformer primary 33 are connected with a star connection, while the phases of the transformer secondary 34 are connected with a delta connection. In this example, the phases of the transformer primaries 33 are reciprocally in-phase with each other, while the phases of the transformer secondaries 33 are respectively out of phase by a phase angle 2a.

[0089] Figs. 4d-4f show, by way of example, three transformers indicated with references 24B, 25B, 26B, in which the phases of the transformer primary 33 and of the transformer secondary 34 are both connected with a delta configuration. In this example, the phases of the transformer secondaries 34 are reciprocally in phase with each other, while the phases of the transformer primaries 33 of two transformers 25B, 26B are out of phase, respectively, by a phase angle a in one or the other direction with respect to the phases of transformer 24B.

[0090] The phase angle a can be comprised between 10° and 20°, for example. This allows to better balance the energy, so as to reduce any disturbances on the electric network 13 end.

[0091] Some embodiments described here also concern a plant 15 comprising a power supply apparatus 10 according to the invention and an electric furnace 11 provided with one or more electrodes 16 each connected to a phase R, S, T of the power supply line 32.

[0092] According to other embodiments, there is also provided a method for the electric power supply of an electric furnace 11 comprising:

[0093] - supplying, by means of a three-phase electric network 13, an alternating electric current and voltage having a predefined frequency;

[0094] - transforming the alternating electric current and voltage with respective transformers 24, 25, 26 into three-phase alternating electric current and voltage suitable to power, respectively, at least a first 21, a second 22 and a third submodule 23 of a base module 20 which is configured to convert the electric energy supplied by the transformers 24, 25, 26 into electric energy suitable to supply a three-phase power supply line 32;

[0095] - powering one or more electrodes 16, 16 A, 16B, 16C of the electric furnace 11 by means of the phases R, S, T of the power supply line 32.

[0096] According to one aspect of the present invention, in each of the sub-modules 21, 22, 23 there is provided a rectifying of the respective phase voltages and currents R, S, T to obtain a common direct electric voltage and current for the three phases R, S, T, a temporary storage of energy and a separation between the electric network 13 and the electrodes 16, 16a, 16b, 16c by means of a direct current intermediate circuit 38, and an inversion of the direct electric voltage and current by means of two inverter devices 29, 30 to obtain respective phase alternating electric voltages and currents, suitable to each supply a different phase R, S, T of the power supply line 32.

[0097] In accordance with another aspect of the method according to the invention, each of the sub-modules 21, 22, 23 supplies a pair of phases R-S, S-T, T-R different from the pairs of phases supplied by the other sub-modules 21, 22, 23.

[0098] In this way, the at least one base module 20 supplies at output two connections for each of the phases R, S, T.

[0099] In accordance with some embodiments, the method according to the invention provides to control at least the operation of the inverter devices 29, 30 in relation to the type of electric furnace 11 to be powered, or to the characteristics of the melting process performed therein, for example the type of material to be melted, the powers required, the number of electrodes 16 used, or the different process steps.

[0100] According to some embodiments, the method provides to adjust the inverter devices 29, 30 by means of a PWM adjustment or a hysteresis adjustment.

[0101] According to other embodiments, in the event that there are several base modules 20, the method according to the invention can also provide to verify, on each occasion, the requirements of the plant 15 or the characteristics of the electric furnace 11 or the material to be melted, and determine the characteristics of the voltage and current to be supplied to the electrodes 16 and activate, on each occasion, all and only the base modules 20 necessary and sufficient to supply a certain voltage and current.

[0102] It is clear that modifications and / or additions of parts or steps may be made to the power supply apparatus 10 and the method as described heretofore, without departing from the field and scope of the present invention, as defined by the claims.

[0103] In the following claims, the sole purpose of the references in brackets is to facilitate reading and they must not be considered as restrictive factors with regard to the field of protection defined in the specific claims.

Claims

CLAIMS1. Electric power supply apparatus (10) for an electric furnace (11), comprising at least one base power supply module (20, 20A, 20B) having a first (21), a second (22) and a third sub-module (23), which are respectively connected at input to a three-phase electric network (13) by means of respective transformers (24, 25, 26) and at output to a three-phase power supply line (32) configured to power one or more electrodes (16) of said electric furnace (11), characterized in that each of said sub-modules (21, 22, 23) comprises, disposed in succession one to the other, a rectifier device (27) connected to one said respective three-phase transformer (24, 25, 26), configured to transform a three-phase alternating electric current and voltage into direct electric current and voltage, a direct current intermediate circuit (28), or DC-link, configured to store electric energy, and two inverter devices (29, 30) which are each connected to said intermediate circuit (28) and are configured to transform the direct voltage and current into alternating supply voltage and current suitable to supply a different phase (R, S, T) of said power supply line (32), and wherein each of said sub-modules (21, 22, 23) is configured to supply a pair of phases (R-S, S-T, T-R) of said power supply line (32) different from the pairs of phases supplied by the other said sub-modules (21, 22, 23), so that said at least one base power supply module (20) has at output two connections for each of the three phases (R, S, T) of said power supply line (32).

2. Apparatus (10) as in claim 1, characterized in that said rectifier device (27) comprises a rectifier circuit (27R, 27S, 27T) for each of said phases (R, S, T), all of which are connected to the same direct current intermediate circuit (28).

3. Apparatus (10) as in claim 2, characterized in that each transformer (24, 25, 26) has respective inputs and outputs of the three-phase type and comprises a transformer primary (33) connected with respective inputs to the phases (R, S, T) of said electric network (13) and a transformer secondary (34) connected with respective outputs to each of said rectifier circuits (27R, 27 S, 27T).

4. Apparatus (10) as in claim 3, characterized in that at least one said transformer (24, 25, 26) has different connections between the respective said phases (R, S, T) of the transformer primary (33) and / or of the transformer secondary (34) with respect to at least one other said transformer (24, 25, 26).

5. Apparatus (10) as in claim 4, characterized in that said connections aredifferent as regards at least one of either a star or delta type connection or a phase shift between the respective phases (R, S, T).

6. Apparatus (10) as in claim 4 or 5, characterized in that said phases (R, S, T) in the respective transformer primary (33) and transformer secondary (34) of said transformers (24, 25, 26) are connected partly in a star configuration and partly in a delta configuration.

7. Apparatus (10) as in claim 4 or 5 or 6, characterized in that in the case of startype connections, at least one connection has the respective phases (R, S, T) out of phase by a phase angle comprised between 10° and 20° in one or the other direction with respect to the phases (R, S, T) of another connection.

8. Apparatus (10) as in any claim hereinbefore, characterized in that it comprises N base modules (20, 20A, 20B) connected in parallel to each other between said electric network (13) and said power supply line (32), where N is a number comprised between 2 and 40.

9. Apparatus (10) as in any claim hereinbefore, characterized in that it comprises a control unit (40) configured to manage and command at least said inverter devices (29, 30) in relation to the specific steps of a melting process performed in the electric furnace (11) in order to vary one or more parameters of the electric current and voltage supplied on each occasion to said power supply line (32).

10. Plant (50) for melting a metal material comprising a power supply apparatus(10) as in any claim from 1 to 9 and an electric furnace (11) provided with two or more electrodes (16, 16A, 16B, 16C), each connected to a phase (R, S, T) of said power supply line (32).11 . Method for the electric power supply of an electric furnace (11), comprising:- supplying, by means of a three-phase electric network (13), an alternating electric current and voltage having a predefined frequency, transforming it, with respective transformers (24, 25, 26), into three-phase alternating electric current and voltage suitable to respectively power at least a first (21), a second (22) and a third submodule (23) of a base module (20) which is configured to convert the electric energy supplied by said transformers (24, 25, 26) into electric energy suitable to supply a three-phase power supply line (32);- powering one or more electrodes (16, 16A, 16B, 16C) of said electric furnace(1 1) by means of the phases (R, S, T) of said power supply line (32),characterized in that in each of said sub-modules (21, 22, 23) there is provided a rectifying of the respective phase voltages and currents (R, S, T) to obtain a common direct electric voltage and current for the three phases (R, S, T), a temporary storage of energy and a separation between said electric network (13) and said electrodes (16, 16A, 16B, 16C) by means of a direct current intermediate circuit (38), and an inversion of the direct electric voltage and current by means of two inverter devices (29, 30) to obtain respective phase alternating electric voltages and currents, suitable to each supply a different phase (R, S, T), wherein each of said sub-modules (21, 22, 23) supplies a pair of phases (R-S, S-T, T-R) of said power supply line (32) different from the pairs of phases supplied by the others of said sub-modules (21, 22, 23) so that said at least one base module (20) supplies at output two connections for each of said phases (R, S, T).

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