Method for the electric power supply of furnaces for melting and / or heating metal materials and corresponding apparatus

Abstract

The invention concerns a method for the electric power supply of furnaces (100) for melting and / or heating metal materials (M) which provides the supply of an input voltage (Ur, Us) and a corresponding input current (Ir, Is) having a predefined frequency (fr, fs); the rectification of said input voltage (Ur, Us) and input current (Ir, Is) into direct current intermediate voltage (Ui) and intermediate current (li), and the conversion of the direct current intermediate voltage (Ui) and intermediate current (li) into a selectively settable alternating supply voltage (Ua) and supply current (la); the supply of said supply voltage (Ua) and supply current (la) to a plurality of electrodes (102, 106, 108) of the furnace (100). The invention also concerns an apparatus (10) for the electric power supply of furnaces (100) for melting and / or heating metal materials (M).

Description

[0001] “METHOD FOR THE ELECTRIC POWER SUPPLY OF FURNACES FOR MELTING AND / OR HEATING METAL MATERIALS AND CORRESPONDING APPARATUS”

[0002] FIELD OF THE INVENTION

[0003] The present invention concerns a method for the electric power supply of furnaces for melting and / or heating metal materials, and a corresponding electric power supply apparatus.

[0004] The present invention can be applied in the iron and steel industry, in the steel production sector, or even in sectors where other metals are worked, in which electric furnaces are present, for example electric arc furnaces, ladles, submerged arc furnaces, melting or refining furnaces, or suchlike.

[0005] BACKGROUND OF THE INVENTION

[0006] Plants for heating and / or melting metal materials, comprising an electric furnace and one or more power supply apparatuses connected to an electric power supply network, are known.

[0007] The electric furnaces of the type in question can be chosen from a group comprising: electric arc furnaces, submerged arc furnaces, ladle furnaces, and in general melting, refining, heating furnaces or suchlike.

[0008] By way of example, the melting cycle of an arc melting furnace provides the following operating steps:

[0009] - charging metal material, usually scrap, into the furnace by means of overhead unloading baskets, or by means of continuous charge conveyor apparatuses fed with scrap and / or direct reduced iron (DRI);

[0010] - generation of the electric arc, during which the electrodes are lowered toward the metal material until triggering the melting electric arc that is generated between the ends of the electrodes and the material to be melted;

[0011] - boring of the layer of metal material using the generated electric arc, during which the melting of the scrap begins;

[0012] - formation of the molten metal bath;

[0013] - refining of the molten material to regulate the bath temperature and carbon content of the steel and / or define a desired composition of the steel by adding chemical components; - tapping of the molten material present in the electric furnace, after possible slagging.

[0014] The step of refining the material can substantially correspond to what happens in the ladle furnace, used in the process downstream of the tapping, to definitively adjust the chemical composition of the steel.

[0015] During the boring steps, the electric arc between the electrodes and the charge of metal material has a very unstable behavior, which progressively improves as the melting progresses. This can cause unexpected and sudden changes in the absorbed power that also negatively affect the electric power supply network, causing for example the so-called flicker phenomenon, with possible damage to the utilities powered by the electric power supply network.

[0016] During boring and melting, in fact, any scrap that has accumulated and not yet melted can collapse near the electrodes, generating short-circuit conditions that correspond to a significant reduction in the active power useful for melting operations and a rapid increase in the current absorbed by the electricity network.

[0017] As the melting progresses, that is, when the arc is suitably shielded by the solid material or by the foamy liquid (slag), the behavior of the electric arc becomes gradually more stable, thus allowing its length to be increased, thus also increasing the thermal power transferred to the material to be melted. The voltage and length of the arc are adjusted according to the melting process, also to prevent excessive wear of the refractory.

[0018] To limit unwanted effects on the power supply network, it is known to make a rapid adjustment of the power supplied to the furnace by means of a continuous adjustment at least of the position of the electrodes and of the parameters of voltage and current supplied to the electrodes.

[0019] In particular, the voltage and current parameters, as well as the position of the electrodes, are appropriately adjusted at each step of the process.

[0020] In the aforementioned plants for heating and / or melting metal materials, the electric furnaces are usually powered by three-phase alternating current, supplied by the public electricity network.

[0021] It is known that in a circuit powered by alternating current not all the electrical energy supplied is transformed into active power to perform useful work, since part of the electrical energy is transformed, due to the inductive and capacitive components of the circuit, into reactive power that does not generate useful work.

[0022] In general, melting and / or heating plants require a high power supply for the furnace; for example, the required power supply can be a few dozen megawatts (MW), in particular comprised between 5 MW and 300 MW depending on the size of the plant and / or furnace.

[0023] As mentioned above, known power supply apparatuses have a disadvantage linked, on the one hand, to the wide variation in instantaneous absorption of power taken from the power supply network, which occurs in particular during the boring due to the movements of the scrap which cause short circuits of the phases and, on the other hand, to the fact that they require an energy consumption that is higher than the one actually required by the electric furnace to which they are applied, because a large part of the energy supplied is not used to generate useful work and is dispersed and wasted.

[0024] WO2024 / 194333 Al discloses a method for regulating the operation of an electric furnace during the melting process which provides a model-based predictive calculation of a target course of at least one operating property up to a time horizon which is based on the desired result of the melting process, and setting the operating property by means of electrode positioning means and / or power supply means in such a way that the operating property is kept on the target course in correspondence with a predetermined future time point.

[0025] CA2578527A1 discloses a method and system for stabilizing energy consumption in multiple loads, or in single three-phase loads, also compensating for any imbalances between the phases. A central controller monitors the variable reactances present in the loads and identifies any power and / or current fluctuations, as well as situations with unbalances. Based on this, it determines the corrective actions to be taken through the other loads or phases, sending control signals to the devices that regulate the variable reactances so as to compensate for these variations.

[0026] US2021 / 231373A1 concerns an electric power supply apparatus for arc furnaces which comprises transformers, rectifiers and converters for transforming input alternating current into direct current and then back into alternating current for the furnace electrodes. In US2021 / 231373A1 , a regulation of the current fed to the electrodes is provided in order to reduce the voltage and therefore the length of the electric arc and reduce the melting time.

[0027] There is therefore the need to perfect a method for the electric power supply of furnaces for melting and / or heating metal materials, in particular in alternating current, that can overcome at least one of the disadvantages of the state of the art.

[0028] In particular, one purpose of the present invention is to develop a method, and provide a corresponding apparatus, for the electric power supply of furnaces for melting and / or heating metal materials which increase the efficiency of the melting and / or heating process and reduce the power required thereby.

[0029] Another purpose of the present invention is to develop a method and provide a corresponding apparatus for the electric power supply of a furnace which allow to achieve high efficiency while simultaneously reducing the energy consumption required to a minimum.

[0030] Another purpose of the invention is to develop a method and a corresponding apparatus for the electric power supply of furnaces which allow to maintain high efficiency without the need to provide additional compensation components.

[0031] A purpose of the invention is also to develop a method for the electric power supply of furnaces for melting and / or heating metal materials that allows to reduce the melting time.

[0032] A purpose is also to provide an apparatus for the electric power supply of furnaces for melting and / or heating metal materials that is simple, economical and reliable, reducing any phenomena of disturbances to the electric power supply network, such as the generation of harmonics and flicker.

[0033] 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.

[0034] SUMMARY OF THE INVENTION

[0035] The present invention is set forth and characterized in the independent claims. The dependent claims describe other characteristics of the present invention or variants to the main inventive idea.

[0036] In accordance with the above purposes and to resolve the technical problem described above in a new and original way, also achieving considerable advantages compared to the prior art, a method for the electric power supply of furnaces for melting and / or refining and / or heating metal materials according to the present invention comprises:

[0037] - the supply of an alternating input voltage and corresponding input current by means of electric power supply means, at a predefined mains frequency;

[0038] - the rectification of the alternating input voltage and current with a plurality of rectifiers to obtain a direct current intermediate voltage and corresponding current;

[0039] - the conversion, with a plurality of converters, of the direct current voltage and current into an alternating supply voltage and corresponding current having a supply frequency independent of a mains frequency, wherein the supply voltage and current are selectively settable by means of a control and command unit connected to the converters;

[0040] - the supply of the supply voltage and current to a plurality of electrodes of the furnace.

[0041] In accordance with one aspect of the present invention, the electric power supply method provides that, during the operation of the furnace to carry out a work cycle of the metal material comprising at least one of either a step of melting the metal material or a refining step, the supply voltage and supply current are detected in real time and a phase shift angle present between them is actively determined by means of the control and command unit, and the operation of the converters is commanded by varying the supply frequency (in a range comprised between 25 and 35 Hz, in particular approximately 30 Hz) in order to maintain the value of the cosine of the phase shift angle, corresponding to a power factor, greater than or equal to 0.9 during at least one of the melting or refining step.

[0042] In this way, the power factor can be monitored and determined in an active manner. In other words, the power factor becomes a control parameter, on the basis of which the operation of the converter devices is regulated effectively in order to maintain it at a specific value, and it is not simply a parameter on the basis of which to evaluate the efficiency of the power supply apparatus and / or the furnace a posteriori.

[0043] Maintaining a power factor greater than or equal to 0.9, particularly during the steps of melting and refining the metal material, allows to significantly reduce consumptions compared to any known solutions.

[0044] Regulating the supply frequency to around 25-35 Hz and maintaining the power factor at a value greater than or equal to 0.9 allows to simultaneously obtain a reduction of the losses induced on the conductors caused by the skin effect, and an improvement in the molten bath’s stirring effect, increasing temperature uniformity and system efficiency.

[0045] In accordance with one aspect of the invention, the method provides that regulation devices regulate the operation of the converter devices in order to modify the supply frequency of the supply voltage and current, and vary an active power supplied by the latter so that the power factor is maintained around a value greater than or equal to 0.9, at least during the melting and / or refining step.

[0046] Since the power factor can have a jagged trend, by the expression “around a value” it is understood that this parameter is greater than or equal to 0.9 for the entire interval, or that this parameter is greater than or equal to 0.9 for the entire interval considered, with the exception of possible minimum peaks at certain instants, wherein the overall duration of such instants is less than 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1% with respect to the duration of the interval considered, or that this parameter is greater than or equal to 0.9 considering an average value thereof within the interval considered.

[0047] The power factor can be defined as the ratio between the active power, that is, the power that can be converted entirely into useful work, for which the electric voltage and current are in phase with each other, and the total apparent power, given by the sum of active power and reactive power, which derives from the inductive and capacitive type components that may be present within the circuit, the entity of which varies in relation to the phase shift: that exists between the electric voltage and current.

[0048] The apparent power is the algebraic product of the voltage and current flowing in a circuit at a given instant, considered as an absolute value, and represents the electric power actually flowing. In general, apparent power is greater than active power, and active power coincides with apparent power only when the electric current vector and the electric voltage vector are concordant, which occurs only in an ideal circuit that contains only elements in the form of resistors.

[0049] In general, however, the impedance of a load comprises a real part corresponding to the electric resistance and an imaginary part corresponding to the reactance associated with energy accumulation phenomena that are caused by inductive and capacitive components; the reactance is proportional to the frequency of the voltage / current according to the following relations: in which XL is the inductive reactance, L is the inductance, f is the frequency, Xc is the capacitive reactance, and C is the capacitance.

[0050] Due to energy accumulation phenomena, therefore, in real situations the current is never in phase with the electric voltage, that is, the maximum, minimum, and zero values of the current are not simultaneous with the corresponding values assumed by the voltage in each period: the current is “not in phase”, meaning it is either ahead (less frequently) or behind (in most cases) with respect to the voltage that generated it.

[0051] The phase shifts caused by the inductive and capacitive components occur in opposite directions. Specifically, as the frequency increases, the inductive reactance increases proportionally, while the capacitive reactance decreases. The increase in the difference between inductive and capacitive capacitance in turn causes an increase in the phase shift between voltage and current, with a consequent decrease in the power factor, since a greater portion of the overall power is transformed into apparent power, which is not used to perform useful work.

[0052] When the frequency decreases, on the other hand, the inductive reactance decreases and the capacitive reactance increases, reducing the phase shift between voltage and current and thus decreasing the apparent power component, resulting in a greater portion of the overall power being transformed into useful work.

[0053] Furthermore, the possibility of regulating the frequency to values lower than the mains frequency allows to reduce the losses induced on the conductors, for example caused by the skin effect, improving the flow of current within the copper conductors so that the current passes through a greater portion of the section of the conductors.

[0054] Moreover, the use of a low-frequency current to power the electrodes allows to obtain an improvement in the stirring effect within the melting bath, increasing heat exchange, temperature uniformity within the bath and therefore the efficiency of the system. In accordance with one aspect of the present invention, the furnace is an arc furnace and the work cycle comprises at least a first and a second step of loading respective baskets of metal material, each followed by a corresponding first and second step of boring the metal material and a first and second melting step, and a refining step downstream of the last melting step, and the power supply method provides to regulate the frequency of the supply voltage and current so that in each of the melting steps and the refining step the power factor is greater than or equal to 0.9.

[0055] In accordance with one aspect of the present invention, the furnace is an arc furnace in which the work cycle comprises at least a first, a second and a third step of loading respective baskets of metal material, each step being followed by a respective first, second and third step of boring the molten material and a first, second and third melting step, and a refining step downstream of the third melting step, and the power supply method provides to regulate the frequency of the supply voltage and current so that in each melting step and in the refining step the power factor is greater than or equal to 0.9.

[0056] In accordance with one aspect of the present invention, the furnace is an arc furnace in which the work cycle comprises a first step of charging a basket of metal material, followed by a step of boring the metal material, a melting step, during which a continuous charge of metal material is carried out, and a final refining step, and the power supply method provides to regulate the frequency of the supply voltage and current so that for the entire duration of the melting step and the refining step the power factor is greater than or equal to 0.9. In this case, the power factor is maintained at a value greater than or equal to 0.9 substantially for the entire duration of the work cycle downstream of the step of initial boring of the metal material, except for a brief initial transient.

[0057] In accordance with another aspect of the invention, the furnace is a ladle furnace able to carry out a step of refining the metal material in order to regulate its final composition, and the power supply method provides to maintain the power factor at a value greater than or equal to 0.9 substantially for the entire work cycle, except for an initial transient period.

[0058] In accordance with another aspect of the invention, the furnace is a submerged arc furnace and the work cycle comprises at least one melting step in which the metal material present in the furnace is melted and a continuous charge of metal material is carried out simultaneously, and the power supply method provides to maintain the power factor at a value greater than or equal to 0.9 substantially for the entire work cycle, except for an initial transient period.

[0059] According to another aspect of the invention, the method provides to supply a mains voltage and current having said mains frequency and to transform them, by means of a transformer, into the alternating input voltage and current having a selectively settable amplitude value, and whose frequency substantially corresponds to the mains frequency.

[0060] Some embodiments of the present invention also concern an apparatus for the electric power supply of furnaces for melting and / or heating metal materials which comprises:

[0061] - a plurality of rectifiers configured to transform an alternating input voltage and corresponding input current into direct current electric voltage and current;

[0062] - a plurality of converters connected to the rectifiers and configured to convert the direct current voltage and current into alternating supply voltage and supply current having a supply frequency independent of the input frequency, the converters being connected to electrodes of the furnace and to a control and command unit configured to control and command the operation of the converters and regulate the alternating supply voltage and current over time.

[0063] According to one aspect of the invention, the apparatus comprises detection devices configured to detect the supply voltage and supply current during the operation of the furnace, and processing means connected to or provided in the control unit configured to determine a phase shift angle present between the supply voltage and current, wherein the cosine of the phase shift angle corresponds to the power factor and the control and command unit is configured to command the operation of the converters to suitably vary the supply frequency in a range comprised between 25 and 35 Hz in order to maintain the value of the cosine of the phase shift angle greater than or equal to 0.9, as a function of the phase shift angle determined at least during one of either the at least one melting step or the refining step, preferably in each melting and / or refining step.

[0064] In accordance with one aspect of the present invention, the control and command unit is provided with regulation devices configured to regulate, during the operation of the furnace to carry out a work cycle of the metal material comprising at least one of either a step of melting the metal material or a refining step, the supply frequency of the supply voltage and current so that the power factor remains greater than or equal to 0.9 at least during the melting and / or refining step.

[0065] Advantageously, the configuration of the apparatus allows to protect the electric power supply means from any disturbances caused by the melting process (reduction of flicker, harmonics and suchlike), while at the same time guaranteeing the stability of the arc in all steps and keeping consumptions to a minimum.

[0066] In accordance with one aspect of the invention, the apparatus comprises a transformer connected to power supply means supplying an alternating mains voltage and corresponding mains current, with predefined mains frequency, the transformer being configured to transform the mains voltage and corresponding mains current into the alternating input voltage and corresponding input current.

[0067] According to some embodiments, the rectifiers and the converters are disposed according to a modular type configuration. In this case, the power supply apparatus comprises a plurality of conversion modules, each of which contains at least one rectifier and one converter and is capable of supplying power from a minimum of 1 MW to a maximum of 30 MW.

[0068] According to another aspect of the invention, each of the modules comprises a rectifier, an intermediate circuit, and a converter for each phase of a three-phase electricity network.

[0069] DESCRIPTION OF THE DRAWINGS

[0070] 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: - Fig. 1 is a diagram illustrating the variation over time of the electric power supplied to the electrodes of an arc furnace during a work cycle (a) and the corresponding trend of the power factor (b) in accordance with the power supply method according to the present invention;

[0071] - Fig. 2 is a schematic view of an apparatus for the power supply of furnaces for heating and / or melting metal materials, according to the present invention;

[0072] - Figs. 3, 4, and 5 are example diagrams showing the variation over time of the voltage (a) and current (b) supplied to the electrodes of a furnace and a comparison (c) between the power factor of the prior art and the control of the power factor in accordance with the present invention;

[0073] - Fig. 6 is a graph showing the trend of the active power and of the reactive power, and the corresponding power factor in a work cycle of a furnace according to the prior art;

[0074] - Fig. 7 is a graph showing the trend of the active power and of the reactive power, and the corresponding power factor in a work cycle of a furnace according to the present invention.

[0075] We must clarify that the phraseology and terminology used in the present description, as well as the figures in the attached drawings also in relation as to how described, have the sole function of better illustrating and explaining the present invention, their purpose being to provide a non-limiting example of the invention itself, since the scope of protection is defined by the claims.

[0076] 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. DESCRIPTION OF SOME EMBODIMENTS OF THE PRESENT INVENTION

[0077] With reference to the attached drawings, a method for the electric power supply of a furnace 100 for melting and / or heating metal materials and a corresponding apparatus 10 are described. Below we will first describe the apparatus 10 with reference to Fig. 2.

[0078] According to some embodiments, the apparatus 10 can be powered by electrical energy supply means 200. In the present description, by way of a non-limiting example we will refer to a three-phase electricity network 201.

[0079] The mains voltage Ur and the mains current Ir supplied by the network 201 can have a predefined mains frequency fr.

[0080] In accordance with possible solutions, the mains frequency fr has a value chosen between 50 Hz and 60 Hz, that is, based on the frequency of the electricity network of the country in which the furnace 100 is installed.

[0081] According to some embodiments, the apparatus 10 can be configured to power three-phase type loads, in particular three-phase furnaces 100. The furnace 100 of the type in question can be an electric arc furnace (EAF), a ladle furnace (LF), a submerged arc furnace (SAF) and smelters using electrodes 102 to transfer thermal energy to the material to be treated.

[0082] Fig. 2 shows the apparatus 10 connected, by way of example, to an electric arc furnace EAF, to a ladle furnace LF and to a submerged arc furnace SAF. In the event that both an EAF or SAF arc furnace, as well as a ladle furnace LF are present in a steel plant, two apparatuses 10 can be provided, each connected to one of them, or a single apparatus 10 can be provided suitable to appropriately power each of the two furnaces EAF, SAF, LF.

[0083] In the case of a furnace 100 of the electric arc furnace EAF type, it comprises a container 101, or vat, into which metal material M to be melted is introduced.

[0084] The EAF furnace is also provided with a plurality of electrodes 102, in the case shown here, three electrodes 102 indicated with the letters A, B, C, configured to ignite an electric arc through the metal material M and melt it.

[0085] In the case of a ladle furnace LF, it generally comprises a ladle 104 suitable to contain the liquid metal tapped from the EAF furnace, a vault 105 which closes the ladle 104 at the top, and a plurality of electrodes 106, indicated with the letters A, B, C, disposed passing through the vault 105.

[0086] In the case of a submerged arc furnace SAF, for example shown, there are provided a container 107, or vat, into which metal material M to be melted is introduced, and a plurality of electrodes 108, in the example case six electrodes 108 indicated with the letters A-F.

[0087] According to some embodiments of the present invention, the electrodes 102, 106, 108 are installed on movement devices 103 configured to selectively move the electrodes 102 approaching or away from the metal material M, or the metal bath in general.

[0088] The movement devices 103 can be chosen in a group comprising at least one of either a mechanical actuator, an electric actuator, a pneumatic actuator, a hydraulic actuator, an articulated mechanism, a mechanical kinematics, similar and comparable members, or a possible combination thereof.

[0089] In accordance with one possible solution of the present invention, if the number of electrodes 102, 106, 108 is three, or a multiple thereof, each one of them is connected to a respective power supply phase of the apparatus 10. If there are more than three electrodes 102, 106, 108, each power supply phase can be connected to two or more of them.

[0090] According to some embodiments, the apparatus 10 is able to receive energy supplied by the network 201 and transform it into supply voltage and current having certain electrical parameters Ua, la, fa which are suitable to power the furnace 100, and specifically the respective electrodes 102, 106, 108.

[0091] The apparatus 10 according to the present invention comprises a plurality of rectifiers 14 configured to transform an alternating input voltage Us and corresponding input current Is into direct intermediate voltage Ui and intermediate current li.

[0092] According to some embodiments, the input voltage Us substantially corresponds to the mains voltage Ur.

[0093] According to some variants, the apparatus 10 can comprise at least one transformer 11 connected to the network 201 upstream of the rectifiers 14. In accordance with possible solutions, the transformer 11 can comprise a transformer primary 12 magnetically coupled to at least one transformer secondary 13.

[0094] The presence of the transformer 11 allows to reduce the impact of network-side disturbances, that is, reduce the harmonic content and the reactive power exchanged with the network 201.

[0095] In such a case, the mains voltage and current Ur, Ir can be supplied to the transformer primary 12 and the input voltage and current Us, Is can substantially correspond to the voltage and current downstream of the transformer secondary 13.

[0096] The input voltage and current Us, Is have predefined amplitude and frequency fs, which are set by the design characteristics and the transformation ratio of the transformer 11 itself. The transformer 1 1 can be provided with regulation devices, not shown, provided to selectively regulate its electrical transformation ratio in relation to specific needs.

[0097] The input frequency fs can be substantially the same as the mains frequency, or different.

[0098] According to some embodiments, the input frequency fs can be substantially equal to or lower than the mains frequency fr or, in general, a primary frequency fp of the current circulating in the primary 12. The rectifiers 14 can be chosen from a group comprising a diode bridge, a thyristor bridge, or other.

[0099] In accordance with one possible solution, the rectifiers 14 comprise devices chosen, for example, in a group comprising diodes, SCR (Silicon Controlled Rectifier), GTO (Gate Turn-Off Thyristor), IGCT (Integrated Gate-Commutated Thyristor), MCT (Metal-Oxide Semiconductor Controlled Thyristor), BJT (Bipolar Junction Transistor), MOSFET (Metal-Oxide Semiconductor Field- Effect Transistor) and IGBT (Insulated-Gate Bipolar Transistor).

[0100] According to some embodiments, the apparatus 10 comprises a plurality of converters 15 connected to the rectifiers 14 and configured to convert the direct voltage and current into an alternating supply voltage Ua and supply current la for the electrodes 102.

[0101] In accordance with one possible solution, the converters 15 comprise devices chosen, for example, in a group comprising SCR (Silicon Controlled Rectifier), GTO (Gate Turn-Off Thyristor), IGCT (Integrated Gate-Commutated Thyristor), MCT (Metal-Oxide Semiconductor Controlled Thyristor), BJT (Bipolar Junction Transistor), MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) and IGBT (Insulated-Gate Bipolar Transistor).

[0102] In accordance with possible solutions, the rectifiers 14 can be connected to the converters 15 by means of at least one intermediate circuit 16 that works in direct current.

[0103] The intermediate circuit 16 can be configured to generate a separation between the rectifiers 14 and the converters 15 and, therefore, with the power supply means 200 supplying electrical energy connected upstream of the intermediate circuit 16 with respect to the furnace 100. In particular, the rapid power fluctuations resulting from the process are partly filtered by means of the intermediate circuit 16, thus reducing their impact on the side of the power supply means 200.

[0104] The intermediate circuit 16 can also be configured to store direct electrical energy. According to some embodiments, this intermediate circuit 16 is a DC-link and comprises at least one capacitor.

[0105] According to some embodiments, the apparatus 10 comprises a control and command unit 17 configured at least to control the converters 15 in such a way as to selectively set the parameters of the above-mentioned supply voltage Ua and supply current la which are generated by the converters 15 and supplied to the electrodes 102.

[0106] Specifically, the supply voltage Ua and the supply current la can be selectively regulated in relation to the required work powers, in the case of an EAF furnace, for example, in relation to the melting powers involved.

[0107] In addition, some solutions of the present invention provide that the control and command unit 17 is also connected to the movement device 103, to allow the position of the electrodes 102 to be adjusted in relation to the different steps of the melting process. In particular, the electrodes 102 are moved by the movement device 103 to follow the position of the material and thus modify the arc’s length.

[0108] For example, during the melting step, the electric power supplied to the electrodes 102 can be increased compared to the boring step, as shown by way of example in Fig. 1, since the arc is now presumed to be covered and distant from the furnace’s vault, and therefore the risk of damage to the latter is avoided.

[0109] The control and command unit 17 can manage and command, in relation to the specific steps of the process performed in the respective furnace 100, at least the following parameters: supply voltage Ua, supply current la, supply electric frequency fa and possibly also the position of the electrodes 102.

[0110] The high possibility of controlling the different parameters allows to optimize the transfer of energy to the process, while reducing the effects on the network 201 resulting from rapid variations in power on the furnace side.

[0111] Through the electrical topology adopted for the converters 15 it is also possible to protect the network 201 from the disturbances linked to the melting process (flicker reduction, harmonics, power factor, etc.), while guaranteeing the stability of the arc in all the work steps of the furnace 100, both in the case of an EAF or SAF furnace, and of a ladle furnace LF.

[0112] The control and command unit 17 can comprise regulation devices 18.

[0113] In accordance with possible solutions of the present invention, the regulation devices 18 can comprise, purely by way of example, a hysteresis modulator or a PWM (Pulse-Width-Modulation) modulator, or suchlike.

[0114] These types of modulators can be used to command the semiconductor devices of the rectifiers 14 and converters 15: suitably controlled, they generate voltage or current values to be supplied to the furnace 100, in this specific case to the electrodes 102, 106. In particular, the modulator processes these voltage and current values and produces commands for driving at least the rectifiers 14 and the converters 15, so that the voltage and current quantities required by the control are present at the terminals for connection to the electrodes 102, 106. The voltages and currents to be actuated are the result of operations carried out by the control and command unit 17 on the basis of the quantities read from the process and on the basis of the process’ model.

[0115] In particular, by means of the control and command unit 17, the parameters of the supply voltage Ua and supply current la, and their frequency fa, can be modified during the operation of the furnace 200 so as to directly control a power factor PF given by the ratio between the active power and the apparent power which are supplied to the electrodes 102, 106, 108 and correlated to the supply voltage Ua and current la.

[0116] In particular, the control and command unit 17 is configured to regulate at least the operation of the converter devices 15 so as to maintain or return the value of the power factor PF to a value greater than or equal to 0.9, at least during the melting and / or refining steps, preferably both where present.

[0117] The value of the power factor PF can be defined by the following formula: in which the active power PA is given by the scalar ratio between the vectors of the supply voltage Ua and supply current la, the apparent power Ps corresponds to the sum of the active power P and the reactive power PQ and is given by the algebraic product of the voltage and current circulating in a circuit in a given instant, considered in absolute value, and is the phase shift angle between the supply voltage Ua and the supply current la.

[0118] According to the invention, the regulation devices 18 are configured to regulate the supply electric frequency fa of the supply voltage Ua and supply current la during the work process of the furnace 100, so that the power factor PF has a value greater than or at most equal to a predefined value, in particular equal to 0.9 or higher, at least during some steps of the work process, that is, the phase shift angle (|) between supply voltage Ua and current la remains at a value comprised between 0° and 25°, in particular around 20°-25°. The regulation devices 18 are in turn commanded by the control and command unit 17.

[0119] According to some embodiments, the apparatus 10 comprises detection devices 21, 22 configured to detect the supply voltage Ua and the supply current la, respectively, for example ammeters and / or voltmeters, during the operation of the furnace 100, and to transmit the detected data to the control and command unit 17.

[0120] Alternatively or in addition, the apparatus 10 can comprise detection devices 24, 25 able to detect one or both of either the active power and the reactive power, for example a wattmeter and / or a varmeter, which are disposed downstream of the converter devices 15, given by the respective supply voltage and current Ua, la.

[0121] The apparatus 10 also comprises processing means 23, such as a processor, an electronic board or a CPU, which can be connected to, or integrated in, the control and command unit 17, which are configured to process the data detected by the detection devices 21, 22, 24, 25 so as to determine a phase shift angle <j) present between the supply voltage Ua and current la, wherein the cosine of such phase shift angle <j> is indicative of the value of the power factor.

[0122] According to this embodiment, the control and command unit 17 is configured to command the operation of the converters 15 by means of its regulation devices 18 to suitably vary the supply frequency fa as a function of the phase shift angle determined, in order to maintain the value of the cosine of this angle greater than or equal to 0.9.

[0123] According to possible solutions, the rectifiers 14 and the converters 15 are connected according to a modular configuration, defining as a whole a power supply module 19.

[0124] According to some embodiments, the apparatus 10 comprises a plurality of power supply modules 19, each of which contains at least one rectifier 14 and one converter 15 and is capable of supplying power from a minimum of 1 MW to a maximum of 30 MW.

[0125] According to some embodiments, each power supply module 19 also comprises at least one intermediate circuit 16, or DC-link, connected between the at least one rectifier 14 and the at least one converter 15.

[0126] According to possible embodiments, each power supply module 19 comprises at least one rectifier 14, one intermediate circuit 16 and one converter 15 for each phase of the three-phase network 201.

[0127] Preferably, all the power supply modules 19 can have a same size, that is, they can supply the same range of electric power.

[0128] Normally, the preferred sizing ranges of each of these power supply modules 19 vary between 5 and 20 MW.

[0129] In a preferred embodiment, all the power supply modules 19 are of a same size, for example they are all 10 MW, all 20 MW, etc.

[0130] According to some embodiments, each power supply module 19 also comprises a transformer 11.

[0131] In the event the, or each, module 19 has respective rectifiers 14, intermediate circuits 16 and converters 15 for each phase of the network 201, the transformer 11 can comprise a single transformer primary 12 and a plurality of transformer secondaries 13, wherein each transformer secondary 13 is connected to a rectifier 14.

[0132] According to some embodiments, the apparatus 10 can be provided with a plurality of power supply modules 19, connected in parallel to each other, to the network 201 and to the furnace 100.

[0133] The combination of several power supply modules 19 allows to achieve an apparatus 10 scalable in size in relation to the specific size of the furnace 100 to be powered.

[0134] In accordance with one possible solution, the control and command unit 17 is connected to all the power supply modules 19 in order to control at least the respective converters 15 so that each module 19 supplies the same values of supply voltage Ua, supply current la, and supply frequency fa to the electrodes 102. In this way, it is possible to prevent any malfunctions of the entire system.

[0135] According to other variants, it can also be provided that the power supply modules 19 can be controlled in such a way as to supply respective different values of supply voltage Ua, supply current la, and supply frequency fa to each electrode 102, for example to vary the power distribution within the molten bath.

[0136] In accordance with one possible solution, the apparatus 10 can comprise an inductor 20 configured to obtain a certain overall reactance value of the apparatus 100. The inductor 20 can be connected downstream of the converters 15 and be sized so as to obtain a desired total equivalent reactance value, given by the contribution of the inductor 20 and by the reactance introduced by the conductors that connect the apparatus 10 to the electrodes 102, 106, 108 of the furnace 100.

[0137] The operation of the apparatus 10 for the electric power supply of furnaces 100 for melting and / or heating metal materials described heretofore, which corresponds to the method according to the present invention, provides:

[0138] - the supply, by means of electric power supply means 200, 11 , of an alternating input voltage Us and input current Is having a predefined mains frequency fs, substantially corresponding to the mains frequency fr;

[0139] - the rectification of the input voltage Us and input current Is with a plurality of rectifiers 14 to obtain a direct current intermediate voltage Ui and intermediate current li;

[0140] - the conversion, with a plurality of converters 15, of the direct current intermediate voltage Ui and intermediate current li into an alternating supply voltage Ua and supply current la having a supply frequency fa independent of a mains frequency, wherein the supply voltage Ua and current la are selectively settable by means of a control and command unit 17 connected to the converters 15;

[0141] - the supply of the supply voltage Ua and supply current la to a plurality of electrodes 102, 106, 108 of the furnace 100.

[0142] The method provides that, during each step of a work cycle of the furnace 100, comprising at least one of either a step of melting the metal material or a refining step, the supply voltage Ua and the supply current la are detected in real time, and the detected data are used to actively determine, or calculate, the phase shift angle <j) present between them by means of the control and command unit 17 in order to determine a cosine of the phase shift angle > which corresponds to the power factor PF, that is, the ratio of an active power to an apparent power which are supplied to the electrodes and are correlated to the supply voltage Ua and current la.

[0143] The method also provides to regulate the supply frequency fa of the supply voltage Ua and supply current la in a range comprised between 25 and 35 Hz, in particular around 30 Hz, so as to vary the active power supplied by the latter so that the power factor PF remains at around a value greater than or equal to 0.9, at least during the melting and / or refining step.

[0144] In accordance with one aspect of the invention, the method provides to regulate the supply frequency fa by means of regulation devices 18 commanded by the control and command unit 17.

[0145] According to another aspect of the invention, the method provides the supply of a mains voltage Ur and current Ir having the mains frequency fr and their transformation, by means of a transformer 11, into the alternating input voltage Us and current Is having a selectively settable amplitude value, and whose input frequency fs substantially corresponds to the mains frequency fr.

[0146] In the present description, by work cycle we mean the set of work steps provided for a certain furnace 100.

[0147] For example, and as shown in Figs. 1, 3 and 4a and 4b, for an EAF arc furnace the work cycle can comprise at least a step of boring the metal material M, a melting step and possibly a step of refining the molten material.

[0148] In particular, during the boring step the electrodes 102 are brought closer to the discharged solid metal material M, so as to trigger the electric arc and initiate the melting of the metal material M. As the metal material M gradually melts, the electrodes 102 penetrate into the still solid part of the metal material M to progressively melt it. When the electrodes 102 reach a position inside the container 101, the actual melting of the remaining metal material M surrounding the electrodes 102 begins.

[0149] In accordance with possible solutions, the charging, boring and melting steps can be repeated several times before the refining step.

[0150] For example, with reference to Fig. 1, graph (a) shows the trend of the electric power supplied to the electrodes 102 of an EAF arc furnace during a work cycle comprising a first, a second and a third step of loading respective baskets of metal material, each followed by a respective first, second and third step of boring the molten material and a first, second and third melting step, and a refining step downstream of the third melting step.

[0151] On the other hand, graph (b) of Fig. 1 shows the value of the power factor PF during the work cycle in accordance with the power supply method according to the invention.

[0152] The method provides in particular that the regulation devices 18 regulate the converters 15 in such a way that the power factor has a value greater than or equal to 0.9 substantially for the entire duration of each of the three melting steps and of the refining step. During the charging step, that is, when the power supplied is substantially zero, the power factor is also consequently zero, while during the boring steps, that is, in the portions with a lined hatching in graph (b) of Fig. 1, the power factor PF may not be controlled and therefore assume a variable value.

[0153] Fig. 3 refers to a work cycle of an EAF arc furnace with a three-basket charge, thus comprising three successive melting steps, wherein graph (a) shows the change over time in the intensity of the supply voltage of the electrodes, and graph (b) shows the change in the intensity of the supply current.

[0154] Graph (c) of Fig. 3 shows, with a continuous fine line, the corresponding trend over time of the power factor according to the state of the art PF_0, which has a jagged development that for most of the time considered is below the value of 0.9, and in particular lower than 0.8, and is greater than 0.9 only in a few instances, while a dashed and thick line represents the power factor PF obtained by means of the present invention, which is substantially greater than or equal to 0.9 for the entire duration of the work cycle.

[0155] Fig. 4 refers to a work cycle of an EAF arc furnace with a first step of charging a basket of metal material, followed by a step of boring the metal material, and a melting step, during which a continuous charge of metal material is carried out, in particular metal material in the form of direct reduced iron (DRI). In particular, downstream of the first charging step, the material is introduced directly into a substantial liquid bath, which in the sector is also defined as “flat bath operation”. A refining step can be provided at the end of the melting step.

[0156] Graph (a) of Fig. 4 shows the change over time in the intensity of the supply voltage Ua supplied to the electrodes, and graph (b) shows the change in the intensity of the corresponding supply current la.

[0157] Graph (c) of Fig. 4 shows, with a thin continuous line, the corresponding trend over time of the power factor according to the state of the art PF_0, which has a jagged development that for most of the time considered is below the value of 0.8, while a dashed and thick line shows the power factor PF obtained by means of the present invention, which is maintained at a value of 0.9 or greater substantially for the entire duration of the work cycle downstream of the step of charging and initial boring of the metal material, with the exception of a brief initial transient, or at least in the continuous charge and flat bath condition. Fig. 5 refers to a work cycle of an EAF arc furnace with a two-basket charge, thus comprising two successive melting steps, wherein graph (a) shows the change over time in the intensity of the supply voltage of the electrodes, and graph (b) shows the change in the intensity of the supply current.

[0158] Graph (c) of Fig. 3 shows, with a continuous fine line, the corresponding trend over time of the power factor according to the state of the art PF_0, which has a jagged development that for most of the time considered is below the value of 0.8, while a dashed and thick line represents the power factor PF obtained by means of the present invention, which is substantially greater than or equal to 0.9 for the entire duration of the work cycle.

[0159] According to another aspect of the invention, the furnace 100 can be a ladle furnace LF in which a step of refining the metal material is performed in order to obtain a metal having a specific composition. In this case, since the step of charging material in the form of scrap or suchlike is not present or necessary, but there is only the possible addition of the materials required to achieve the desired composition, the method according to the invention can provide to maintain the power factor at a value greater than or equal to 0.9 substantially for the entire work cycle, apart from an initial transient period.

[0160] According to another aspect of the invention, the furnace 100 can be a submerged arc furnace SAF, suitable to melt the metal material. The operation of the submerged arc furnace SAF can be assimilated to that of an electric arc furnace EAF with continuous charge, described with reference to Fig. 4. Also in this case, the power supply method provides to maintain the power factor at a value greater than or equal to 0.9 substantially for the entire work cycle, apart from an initial transient period, or at least in the continuous charge and flat bath condition.

[0161] According to another aspect of the invention, the method provides to detect the supply voltage Ua and the supply current la, or respective values corresponding to the active, reactive or apparent power supplied to the electrodes, during the operation of the respective furnace 100, and on the basis of the detected data determine the phase shift angle present between them by means of the processing means 23 connected to or integrated in the control and command unit 17. The method also provides to command the operation at least of the converters 15 and suitably vary the supply frequency fa as a function of the phase shift angle (|> determined, in order to maintain the value of the cosine of such phase shift angle greater than or equal to 0.9.

[0162] The detection and regulation can be done in real time by implementing a feedback loop control.

[0163] Fig. 6 shows the trend of the active power PA, the reactive power PQ and the power factor according to the state of the art PF_0 in a work cycle of an electric furnace 100. Fig. 7 shows the trend of the active power PA, the reactive power PQ and the power factor PF according to the invention for the same work cycle as Fig. 6. As can be observed, thanks to the method according to the invention, the value of the active power PA increases while the value of the reactive power PQ due to the presence of the inductive and capacitive loads of the circuit, and therefore correlated to the voltage and current frequency, decreases, therefore the power factor PF is greater than or equal to 0.9, at least considering its average value.

[0164] According to another aspect of the invention, the method provides to carry out an adjustment of the supply frequency fa in relation to certain instants of the work cycle, which are identified according to the project or defined on the basis of a history of the operation of the respective furnace 100.

[0165] Once one or more work points of the furnace 100 have been determined in terms at least of power, voltage, current and frequency, which correspond to certain instants of the work cycle, the method can provide that the control and command unit 17 tries to follow these work points through the continuously performed regulation of the supply frequency fa.

[0166] The work points can be determined by an operator, or they can also be determined automatically by the control and command unit 17, for example on the basis of a mathematical model of the furnace 100 and / or of a given melting and / or heating process, or even calculated on the basis of data received at input in relation to type of material to be melted, final product to be obtained, characteristics of the furnace 100, hourly productivity required, or other factors.

[0167] It is clear that modifications and / or additions of parts or steps may be made to the apparatus 10 and to the method as described heretofore, without thereby departing from the field and scope of the present invention, as defined by the claims.

[0168] It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art will be able to achieve other equivalent forms of method and apparatus 10 for the electric power supply of furnaces for melting and / or heating metal materials, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

[0169] In the following claims, the sole purpose of the references in brackets is to facilitate their reading and they must not be considered as restrictive factors with regard to the field of protection defined by the claims.

Claims

CLAIMS1. Method for the electric power supply of a furnace (100) for melting and / or heating metal materials (M) which provides:- the supply, by means of electric power supply means (200, 11), of an alternating input voltage (Ur, Us), and corresponding input current (Ir, Is), having a predefined input frequency (fr, fs);- the rectification of said alternating input voltage (Ur, Us) and current (Ir, Is) with a plurality of rectifiers (14) to obtain a direct current intermediate voltage (Ui) and intermediate current (li);- the conversion, with a plurality of converters (15), of said direct current intermediate voltage (Ui) and intermediate current (li) into an alternating supply voltage (Ua) and corresponding supply current (la) having a supply frequency (fa) independent of said input frequency (fr, fs) and selectively settable by means of a control and command unit (17) connected to said converters (15);- the supply of said supply voltage (Ua) and supply current (la) to a plurality of electrodes (102, 106) of the furnace (100); said method being characterized in that, during the operation of the furnace (100) to carry out a work cycle of the metal material comprising at least one of either a step of melting the metal material or a refining step, said method provides to detect said supply voltage (Ua) and supply current (la) in real time, actively determine a phase shift angle (<j>) present between them by means of said control and command unit (17) and command the operation of said converters (15) by varying the supply frequency (fa) in a range comprised between 25 and 35 Hz, in particular approximately 30 Hz, in order to maintain the value of the cosine of said phase shift angle (<J>), corresponding to a power factor, greater than or equal to 0.9 during at least one of said melting and refining steps.

2. Method as in claim 1, wherein to maintain said power factor at a value greater than or equal to 0.9, regulation devices (18) of said control and command unit (17) regulate the operation of said converter devices (15) in order to modify said supply frequency (fa) and vary an active power supplied by said supply voltage and current (Ua, la).

3. Method as in claim 1 or 2, characterized in that it provides to detect said supply voltage (Ua) and supply current (la) in real time, actively determine saidphase shift angle ((|)) and command the operation of said converters (15) by varying the supply frequency (fa) in order to maintain said power factor greater than or equal to 0.9 during each of said melting step and refining step.

4. Method as in one or the other of the previous claims, characterized in that, in order to determine said phase shift angle ((|>), it provides to detect at least one of either the active power or the reactive power which are given by said supply voltage and current (Ua, la) by means of one of either a wattmeter and / or a varmeter (24, 25) downstream of said converter devices (15).

5. Method as in any claim hereinbefore, characterized in that said furnace (100) is an electric arc furnace (EAF) and said work cycle comprises at least a first and a second step of loading respective baskets of metal material, each followed by a corresponding first and second step of boring the metal material and a first and second melting step, and a refining step downstream of the last melting step and in that said method provides to regulate the supply frequency (fa) of said supply voltage (Ua) and current (la) so that in each of said melting steps and said refining step said power factor is greater than or equal to 0.9.

6. Method as in any previous claim from 1 to 4, characterized in that said furnace (100) is an electric arc furnace (EAF) and said work cycle comprises at least one step of loading a basket of metal material, followed by a step of boring the metal material, a melting step, during which a continuous loading of metal material is carried out, and a final refining step, and in that said method provides to regulate the frequency (fa) of said supply voltage (Ua) and current (la) so that for the entire duration of said melting step and said refining step said power factor is greater than or equal to 0.9.

7. Method as in any previous claim from 1 to 4, characterized in that said furnace (100) is a ladle furnace (LF) and said work cycle comprises at least one step of refining the molten material and in that said method provides to regulate the frequency (fa) of said supply voltage (Ua) and current (la) so that for the entire duration of said refining step said power factor is greater than or equal to 0.9.

8. Method as in claim 6 or 7, characterized in that except for an initial transient it provides to operate at a steady state with a power factor greater than or equal to 0.9 for the entire duration of the work cycle.

9. Method as in any claim hereinbefore, characterized in that it provides to carryout a regulation of the supply frequency (fa) in relation to certain instants of the work cycle, which are identified according to the project or defined on the basis of a history of the operation of the respective furnace (100), wherein said method provides to determine one or more work points of the furnace ( 100) in terms of at least supply power, voltage, current, and frequency (Ua, la, fa) and to continuously regulate said supply frequency (fa) by means of said control and command unit (17) in order to follow said work points.

10. Apparatus (10) for the electric power supply of furnaces (100) for melting and / or heating metal materials (M) which comprises:- a plurality of rectifiers (14) configured to transform an alternating input voltage (Ur, Us) and corresponding input current (Us, Is) having an input frequency (fr, fs) into direct current electric voltage and current (Ui, li);- a plurality of converters (15) connected to said rectifiers (14) and configured to convert said direct current voltage and current (Ui, li) into alternating supply voltage (Ua) and supply current (la) having a supply frequency (fa) independent of said input frequency (fr, fs), said converters (15) being connected to electrodes (102, 106) of said furnace (100); and- a control and command unit (17) configured to control and command the operation of said converters (15) and regulate said supply voltage (Ua) and current (la) over time, characterized in that it comprises detection devices (21, 22, 24, 25) configured to detect said supply voltage (Ua) and supply current (la) during the operation of said furnace (100) to carry out a work cycle of the metal material comprising at least one of either a step of melting the metal material or a refining step, and processing means (23) connected to or provided in said control and command unit (17) configured to determine a phase shift angle () present between said supply voltage and current (Ua, la), wherein the cosine of the phase shift anglecorresponds to a power factor and said control and command unit (17) is configured to command the operation of said converters ( 15) to suitably vary said supply frequency (fa) in a range comprised between 25 and 35 Hz, in particular approximately 30 Hz, in order to maintain the value of the cosine of said phase shift anglegreater than or equal to 0.9 during at least one of said melting step and refining step.

11. Apparatus as in claim 10, characterized in that said control and command unit (17) is provided with regulation devices (18) configured to regulate, during the operation of the furnace (100) to carry out said work cycle, the electric supply frequency (fa) of said supply voltage (Ua) and supply current (la) in order to modify said supply frequency (fa) and vary an active power supplied by said supply voltage and current (Ua, la) so that said power factor, is maintained around a value greater than or equal to 0.9 at least during said at least one melting and / or refining step.

12. Apparatus (10) as in claim 10 or 11, characterized in that it comprises a transformer (11) connected to power supply means (200) of an alternating mains voltage (Ur) and mains current (Ir), having a predefined mains frequency (fr), said transformer (11) being configured to transform said alternating mains voltage (Ur) and mains current (Ir) into said input voltage (Us) and input current (Is), respectively.

13. Apparatus (10) as in any claim from 10 to 12, characterized in that it comprises a plurality of power supply modules (19) each containing at least one rectifier (14) and one converter (15) wherein said plurality of power supply modules (19) are connected in parallel to each other to said power supply means (200) and said furnace (100) and in that said control and command unit (17) is connected to all the power supply modules (19) in order to control at least the respective converters (15) so that each power supply module (19) supplies the same values of supply voltage (Ua), current (la) and electric frequency (fa) to said electrodes (102, 106, 108).

14. Apparatus (10) as in any claim from 10 to 13, characterized in that it comprises a plurality of power supply modules (19) each containing at least one rectifier (14) and one converter (15) wherein said plurality of power supply modules (19) are connected in parallel to each other to said power supply means (200) and said furnace (100) and in that said control and command unit (17) is connected to all the power supply modules (19) to control at least the respective converters (15) so as to supply respective different values of supply voltage (Ua), supply current (la), and supply frequency (fa) to each electrode (102, 106, 108) in order to vary the power distribution within the furnace (100).

15. Apparatus (10) as in claim 14, characterized in that each power supplymodule (19) comprises respective rectifiers (14), intermediate circuits (16) and converters (15) for each phase of a three-phase network (201), and a transformer (11) comprising a single transformer primary (12) and a plurality of transformer secondaries (13), wherein each transformer secondary (13) is connected to a rectifier (14).

16. Apparatus (10) as in any claim from 10 to 15, characterized in that detection devices (21, 22, 24, 25) comprise at least one of either ammeters and / or voltmeters, for example.

17. Apparatus (10) as in any claim from 10 to 16, characterized in that said detection devices (21, 22, 24, 25) comprise devices able to detect one or both of either the active power and the reactive power given by the respective supply voltage and current (Ua, la,), chosen from a wattmeter and / or a varmeter (24, 25) disposed downstream of said converter devices (15).

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