WO2025004119A1

WO2025004119A1 - Electric power supply apparatus and method for an electric furnace

Info

Publication number: WO2025004119A1
Application number: PCT/IT2024/050129
Authority: WO
Inventors: Antonello MORDEGLIA
Original assignee: Danieli Automation SpA
Current assignee: Danieli Automation SpA
Priority date: 2023-06-30
Filing date: 2024-06-21
Publication date: 2025-01-02
Anticipated expiration: 2025-12-30
Also published as: IT202300013641A1

Abstract

An apparatus (10) for the electric power supply of furnaces (100) for melting and/or heating metal materials provided with electrodes (102) comprises at least one power supply line (201, 20 IL, 20 IM) and at least one connected base module (20, 120, 220) which is configured to convert an alternating mains voltage and current (Ur, Ir) having a mains frequency (ft) into alternating supply voltage and current (Ua, la) with a desired power supply frequency (fa), wherein the at least one base module (20, 120, 220) comprises a transformer (11), a plurality of rectifiers (14) connected to the transformer (11), one or more direct current intermediate circuits (16) configured to store electrical energy, a plurality of inverter devices (15) and a control and command unit (17) configured to control and command the operation of the inverter devices (15) and adjust the power supply frequency (fa). The invention also concerns a method for the electric power supply of furnaces (100) for melting and/or heating metal materials.

Description

“ELECTRIC POWER SUPPLY APPARATUS AND METHOD FOR AN ELECTRIC FURNACE”

FIELD OF THE INVENTION

The present invention concerns an electric power supply apparatus and method for an electric furnace for steelmaking applications for the production of steel, or for other sectors in which metals or glassy materials, or similar or comparable materials, are worked. The electric power supply apparatus is applicable, in particular, to electric furnaces that operate with alternating electric currents and voltages.

BACKGROUND OF THE INVENTION

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

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 refining step.

The power and electrical 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 electrical energy supplied on each occasion.

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 even considerably within the same process step. During the boring step, the electric arc between the electrodes and the metal material has a very unstable behavior, which progressively improves as the melting progresses, since the accumulated and not yet melted scrap can collapse near the electrodes, generating short circuit conditions that correspond to a considerable reduction in the useful active power and a rapid increase in the current absorbed from the electric network. The instability of the arc causes unexpected and sudden changes in the absorbed power that also negatively affect the power supply electric network, causing for example the so-called flicker phenomenon, with possible damage to the electric network and to the connected utilities. 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 gradually becomes more stable, thus allowing its length to be increased, thus also increasing the thermal power transferred to the material to be melted. The tension and length of the arc are adjusted according to the melting process, also to prevent excessive wear of the refractory.

In order 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 voltage and current parameters impressed on the electrodes.

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

There are known power supply apparatuses for electric arc furnaces 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. 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, and the amount of electrical energy supplied to the electrodes is adjusted by appropriately commanding the inverter devices.

These inverter devices comprise one or more switches that are opened and closed with a high frequency, whereby fluctuations in the electric voltage can be generated that can also have an effect back along the circuit, creating problems for the electric network.

Furthermore, these inverter devices, because of the modulation of the current that is performed, generate current harmonics that can be harmful if fed into the electric power supply network.

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.

One purpose of the present invention is to provide an apparatus and a method for the power supply of an electric arc furnace which allow to regulate the operation and power of an electric furnace effectively, according to requirements.

In particular, one purpose of the present invention is to provide an apparatus and perfect a method 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.

Another purpose of the present invention is to provide an apparatus and implement a method which allows to regulate the voltage and current characteristics 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.

It is also a purpose of the invention to perfect an apparatus and a method for the electric power supply of furnaces for melting and/or heating metal materials which allow to reduce the melting time or the metal treatment process in general.

It is also a purpose 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 disturbance phenomena in the electric power supply network such as the generation of harmonics and flicker.

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 needs of the plant or the characteristics of the electric furnace to which it has to be applied.

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. SUMMARY OF THE INVENTION

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.

In accordance with the above purposes, there is provided an electric power supply apparatus according to the present invention, suitable to power a melting and/or heating electric furnace for steelmaking applications for the production of steel, or for other sectors in which metals or glassy materials, or similar or comparable materials, are worked. The apparatus comprises at least one power supply line and at least one base module connected to the power supply line and configured to convert an alternating voltage and current having a predefined mains frequency into alternating supply voltage and current with a desired power supply frequency, wherein the base module comprises:

- a transformer connected to the power supply line and configured to transform the mains voltage and the mains current into an alternating secondary voltage and secondary current, respectively;

- a plurality of rectifiers connected to the transformer and configured to transform the alternating secondary voltage and secondary current into direct electric voltage and current;

- one or more direct current intermediate circuits configured to store electrical energy;

- a plurality of inverter devices connected to the one or more intermediate circuits and configured to convert the direct electric voltage and current into alternating supply voltage and supply current having the desired frequency, and

- a control and command unit connected at least to the inverter devices and configured to control and command the operation of the inverter devices and regulate, during each step of a process of a furnace, the power supply frequency. The power supply apparatus is suitable to be used to power an electric melting or heating furnace chosen from an electric arc furnace provided with electrodes disposed through a covering vault of the furnace, a submerged arc electric furnace, a ladle furnace, a smelter or similar or comparable furnaces, in which it is provided to generate an electric arc between respective electrodes and the metal to be treated. According to some embodiments, the apparatus comprises an adapter device connected between the at least one base module and the respective electrode of a furnace and configured to adapt the parameters of the supply voltage and/or current, supplied by the at least one base module, into adapted electric voltage and current suitable to power the electrodes. According to some embodiments, the adapter device can comprise one or more of either an adapter transformer, a reactance, a filter or a disconnector switch.

According to some embodiments, the power supply line is connected to a three- phase power supply network, having a mains frequency, by means of at least one input transformer device.

According to some embodiments, the three-phase power supply network is high voltage, in the order of 70-600kV, and the input transformer device can be configured to transform the high voltage into medium or low voltage. In particular, as a function of the type and number of transformer devices present between the high voltage network and the power supply line, the latter can be configured to operate at medium voltage, approximately between 1.5 and 35 kV, or at low voltage, approximately between 50V and 1.5 kV.

According to some embodiments, the adapter device is an adapter transformer, comprising a transformer primary connected to the at least one base module or possibly to the plurality of base modules, and a transformer secondary connected to an electrode. This solution is particularly advantageous if the power supply network is medium voltage. In this case, in fact, the power supply apparatus and its components can be configured to operate at medium voltage, and the adapter transformer can be configured to transform the voltage from medium to low.

In accordance with this solution, the input transformer device can be of the high voltage/medium voltage type, or medium voltage/medium voltage type in the event that the network segment connected to the transformer primary is already medium voltage, or there is an additional transformer device upstream configured to transform high voltage into medium voltage.

According to some variants, in particular in the event the power supply line itself is low voltage, or the input transformer is of the medium voltage/low voltage type, the adapter device can comprise at least one of either an inductor or a capacitor suitable to allow to obtain a desired reactance value by regulating the power supply frequency, a filter or a disconnector switch.

In this case, the adapter device, in addition to suitably adapting the voltage and current to be supplied to the electrode, allows to eliminate any harmonics belonging to certain unwanted frequency bands, thus contributing to eliminating disturbances. In accordance with one aspect of the present invention, the control and command unit is provided with regulating devices configured to regulate, during each step of a melting cycle of the furnace, the electric power supply frequency of the alternating supply voltage and supply current in such a way that the power supply frequency is lower than or equal to the mains frequency and, in at least one of the steps of the work cycle in the furnace, the power supply frequency is comprised between 40% and 80% of the mains frequency.

Advantageously, the configuration of the apparatus allows to protect the electric power supply means from the disturbances caused by the work process, in particular the melting process in an electric arc or submerged arc furnace (reduction of flicker, harmonics, and suchlike), while guaranteeing the stability of the arc in all the steps.

According to some embodiments, the apparatus comprises a plurality of base modules which are connected in parallel to each other between the three-phase network and the load to be powered, wherein each base module is configured to convert the electrical energy supplied by the electric network and supply at output at least one pair of single-phase alternating currents and voltages having a desired intensity and frequency. According to some embodiments, the or each base module comprises at least two sub-modules, each suitable to supply a single-phase voltage and current.

In particular, the or each base power supply module comprises a number of submodules corresponding to the number of connections, or the number of phases, to be supplied at output. Preferably, in the at least one base module there are provided both a direct current intermediate circuit, or DC link, for each of the phases that the base power supply module supplies at output, and also at least one inverter device connected to each of the DC links. The number of DC links and phases is therefore preferably the same. In accordance with the present invention, all the intermediate circuits of the submodules are short-circuited to each other, that is, they are all at the same electric potential, and this allows to compensate for a set of harmonics and to obtain a common average value, thus reducing the extent of the disturbances in one or more sub-modules. Furthermore, in the event that the direct current intermediate circuit of a submodule has a malfunction, the other sub-module or sub-modules can also compensate for the non-functioning sub-module and possibly supply energy to the inverter device connected to the direct current intermediate circuit which is not working.

This solution, thanks to the redundancy of the rectifier devices and to the short circuit between the different intermediate circuits, allows to power all the desired output phase connections in any case, even if with reduced powers, and to absorb any unbalances between the at least two phases.

In accordance with one aspect of the present invention, the transformer comprises a single transformer primary provided with three-phase inputs which are connected, during use, to the phases of the power supply line, and a plurality of transformer secondaries, each connected with respective three-phase outputs to a respective sub-module. In this solution, the single transformer primary is coupled to all the transformer secondaries. This solution allows to reduce the impact of any disturbances on the network side, that is, to reduce the harmonic content and the reactive power exchanged in the network by the combination of the transformer secondary and the rectifier device. According to some embodiments, the phases in the respective transformer secondaries of a same base power supply module are out of phase with respect to each other, so as to obtain a balance between the currents and/or the respective electric voltages within each base power supply module.

In accordance with another aspect of the invention, the apparatus comprises a plurality of base modules connected in parallel to each other between the power supply line and a connection line, wherein each base module receives three three- phase connections at input and supplies two, three, four, six or more single-phase voltage and current connections at output.

The number of base modules can be multiplied according to requirements. For example, the number of base power supply modules can be comprised between 2 and 60, for example 24, 30, 36, 48 or even intermediate numbers, even or odd. By way of example, each conversion module can be configured to supply power from a minimum of 1 MW to a maximum of 30 MW. Normally, the preferred sizing ranges of each of these base modules vary from 15 to 20 MW, even more preferably around 10 MW.

This modular construction advantageously allows to adapt the power supply apparatus to the needs 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 during use, making it possible to optimally adjust the electrical energy supplied in relation to the requirements, for example by keeping only some of the base modules active on each occasion.

Some embodiments described here also concern a method for the electric power supply of furnaces for melting and/or heating metal materials, comprising:

- supplying an alternating mains voltage and current by means of electric power supply means, at a predefined mains frequency;

- transforming, by means of a transformer, the mains voltage, current and frequency into alternating secondary voltage, current and frequency having a value which can be selectively set, wherein the secondary frequency is substantially the same as the mains frequency;

- rectifying the secondary voltage and current with a plurality of rectifiers to obtain a direct current intermediate voltage and current;

- converting, with a plurality of inverter devices, the intermediate voltage and current into an alternating supply voltage and current with a frequency which can be selectively set by means of a control and command unit connected to the inverter devices;

- adapting and/or filtering the supply voltage and current in order to regulate their parameters and obtain an adapted supply voltage and current; - supplying the adapted supply voltage and current to the furnace’s electrodes.

In accordance with one aspect of the present invention, the electric power supply method provides that, during each step of a work cycle of the furnace, regulating devices of the control and command unit regulate the power supply frequency of the supply voltage and supply current so that, at least for some steps of the work process, the power supply frequency is lower than or equal to the mains frequency and, in at least one step of the work cycle, the power supply frequency is comprised between 40% and 80% of the mains frequency.

According to some embodiments, the method according to the present invention provides that the frequency is lower than the mains frequency at least for 80% of the melting process.

In the case of an electric arc or submerged arc furnace, the frequency can be comprised between 40-80% of the mains frequency at least in flat bath conditions, that is, when refining occurs and the metal material is no longer loaded. In the case of a ladle furnace, the frequency can be comprised between 40-80% of the mains frequency essentially for the entire process.

In accordance with one aspect of the present invention, the electric power supply method provides that, during each step of a work cycle of the furnace, regulating devices of the control and command unit regulate the power supply frequency of the supply voltage and supply current so that the power supply frequency is lower than the mains frequency for at least 80% of the duration of the work cycle.

According to other embodiments, the power supply frequency is lower than the mains frequency for 100% of the overall duration of a work cycle.

The possibility of regulating the frequency to values lower than the mains frequency allows to reduce the losses induced on the conductors, for example due to the skin effect, improving the passage of current inside the copper conductors in such a way that the current passes through a greater proportion of the section of the conductors.

Moreover, the use of a low frequency current to power the electrodes allows to obtain an improvement in the stirring effect inside the melting bath, increasing the heat exchange, the temperature uniformity inside the bath and therefore the efficiency of the system.

DESCRIPTION OF THE DRAWINGS

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 schematic view of an electric power supply apparatus according to the invention in accordance with a first embodiment;

- fig. 2 is a schematic view of an electric power supply apparatus according to the invention in accordance with a second embodiment;

- fig. 3 is a schematic view of an electric power supply apparatus according to the invention, having a base module comprising two sub-modules and applied to an electric arc furnace;

- fig. 4 is a schematic view of the simplified circuit of two sub-modules;

- fig. 5 is a schematic view of an electric power supply apparatus according to a variant of the present invention, having a base module comprising three submodules and applied to an electric arc furnace; - fig. 6 is a schematic view of an electric power supply apparatus according to another variant, in which each base module comprises six sub-modules, applied to a submerged arc furnace;

- figs. 7 and 8-8b are diagrams showing the variation over time of the electric parameters applied to the electrodes of an arc furnace during a work cycle of a melting furnace, in accordance with some embodiments of the present invention;

- fig. 9 is a diagram showing the variation over time of the electric parameters applied to the electrodes of a ladle furnace during a work cycle, in accordance with some embodiments of the present invention; - fig. 10 is a graph of the power trend in a work cycle of an arc furnace;

- fig. 11 is a diagram showing the power consumption trend of a ladle furnace as a function of the frequency variation.

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

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 non- limiting illustration. The phraseology and terminology used here is also for the purposes of providing non-limiting examples.

With reference to figs. 1 and 2, some embodiments of the present invention concern an apparatus 10 for the electric power supply of furnaces 100 for melting and/or heating metal materials. According to some embodiments, the apparatus 10 comprises a power supply line 201 connected to electrical energy supply means 200, in particular of the three- phase type.

In figs. 1-5, the letters R, S, T indicate the three phases of a three-phase electric voltage/current. According to embodiments described with reference to fig. 1, the power supply line 20 IL is of the low voltage (LV) type, whereby the apparatus 10 can be configured to operate entirely at low voltage, indicatively between 50V and 1.5 kV. According to this embodiment, upstream of the power supply line 201L a reducer transformer 202 can be provided configured to lower the electric voltage supplied by a main network 203 which can be high or medium voltage, obtaining a low voltage mains voltage Ur.

According to a variant, described with reference to fig. 2, the power supply line 20 IM is of the medium voltage (MV) type, whereby the apparatus 10 can be configured to operate at medium voltage, indicatively between 1.5 and 35 kV. According to this embodiment, upstream of the power supply line 20 IM a reducer transformer 204 can be provided configured to lower the electric voltage supplied by the main network 203 which can be high or medium voltage, obtaining a medium voltage mains voltage Ur.

Hereafter, unless specified, reference number 201 will generically indicate both low and medium voltage power supply lines 20 IL, 20 IM.

The mains voltage Ur and the mains current Ir supplied by the power supply line 201 can have a predefined mains frequency fr, for example a value chosen between 50Hz and 60Hz, that is, according to the frequency of the electric network of the country in which the furnace 100 is installed.

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 generally be a melting, refining or heating furnace or suchlike, of the kind suitable to be used in a steel plant for producing steel, or in metalworking plants. Preferably, the invention is applicable to electric arc furnaces (EAFs), ladle furnaces (LFs), submerged arc furnaces (SAFs) and smelters which use electrodes 102 to transfer thermal energy to the material to be treated. Figs. 1 and 2 show the apparatus 10 connected, by way of example, to an electric arc furnace EAF and to a ladle furnace LF. In the event that both arc furnaces EAF as well as ladle furnaces 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 power each of the two furnaces EAF, LF in a suitable manner. 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.

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

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 102 indicated with the letters A, B, C disposed passing through the vault 105.

In the case of a submerged arc furnace SAF, for example shown in fig. 6, there is provided a container 107, or vat, into which metal material M to be melted is introduced, and a plurality of electrodes 102, in the example case six electrodes 102 indicated with the letters A-F. The following description will mainly refer, by way of example, to the EAF furnace, the ladle furnace LF and the submerged arc furnace SAF.

According to some embodiments of the present invention, the electrodes 102 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.

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. In accordance with a possible solution of the present invention, if there are three electrodes 102, each one of them is connected to a respective phase voltage and three-phase current of the apparatus 10.

If there are more than three electrodes 102, each power supply phase can be connected to two or more of them. According to some embodiments, the apparatus 10 is able to receive energy supplied by the power supply line 201 and transform it into supply voltage and current having certain electric parameters Ua*, la*, fa suitable to power the electrodes 102 of the furnace 100.

According to some embodiments, the apparatus 10 comprises at least one base module 20 configured to convert an alternating voltage and current having the mains frequency fr into alternating voltage and current with a desired power supply frequency fa.

According to some embodiments, the apparatus 10 can comprise a plurality of base modules 20 connected in parallel to each other, between the power supply line 201 and the furnace 100, that is, the electrodes 102.

Each base module 20 comprises a transformer 11 connected to the power supply line 201 and configured to transform a primary alternating electric voltage Up and current Ip into a secondary alternating electric voltage Us and current Is.

In accordance with possible solutions, the transformer 11 can comprise a transformer primary 12 magnetically coupled to at least one transformer secondary 13.

This solution allows to reduce the impact of network-side disturbances, that is, reduce the harmonic content and reactive power exchanged with the main network 203.

The secondary electrical energy supplied by the transformer 11 has a secondary voltage Us, a secondary current Is, and a secondary frequency fs, which are predefined and set by the design characteristics of the transformer 11 itself.

According to some embodiments, the secondary frequency fs can be substantially equal to or lower than the mains frequency fr or, in general, the primary frequency fp of the current circulating in the primary 12.

The secondary voltage and current Us, Is can be correlated, respectively, to the mains voltage and current Ur, Ir or, in general, to the primary voltage and current Up, Ip of the primary 12, by the transformation ratio of the transformer 11 itself.

The transformer 11 can be provided with regulating devices, not shown, provided to selectively regulate its electrical transformation ratio in relation to specific requirements.

The apparatus 10 according to the present invention also comprises, for each base module 20, a plurality of rectifiers 14 connected to the transformer 11 and configured to transform the alternating secondary voltages and currents Us, Is into direct intermediate voltage and current Ui, li.

According to some embodiments, the apparatus 10 comprises, for each base module 20, a plurality of inverter devices 15 connected to the rectifiers 14 and configured to convert the direct intermediate voltages and currents Ui, li into an alternating supply voltage Ua and current la.

In accordance with possible solutions, the rectifiers 14 can be connected to the inverter devices 15 by means of at least one intermediate circuit 16, or DC-link, which works in direct current.

The intermediate circuit 16 can be configured to generate a separation between the rectifiers 14 and the inverter devices 15 and, therefore, with the power supply means 200 of 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.

The intermediate circuit 16 can also be configured to store electrical energy continuously. According to some embodiments, the intermediate circuit 16 comprises at least one capacitor.

According to some embodiments, the base module 20 comprises respective independent rectifiers 14, intermediate circuits 16 and inverters 15 for two or more sub-modules 21 , each associated with a phase R, S, T of the power supply line 201.

According to some embodiments, each sub-module 21 comprises a plurality of inverter devices 15 connected in parallel with respect to each other to the intermediate circuit 16, all of which power a same output phase, for example a phase R, S, T of a connection line 24.

For example, between 2 and 8 inverter devices 15 can be provided, preferably between four and six. According to some embodiments, the transformer 11 is preferably physically separated and distanced away from the respective sub-modules 21 , and can also be built in a different building, or in a different plant site.

For example, the apparatus 10 can comprise a regulating unit Gl, comprising rectifiers 14, inverters 15, intermediate circuits 16 and an adapter device 19, which is disposed in proximity to the furnace 100, and a transformer unit G2, comprising the transformers 11, which is separated and distanced away from the regulating unit Gl, even by a few tens of meters, or more.

According to possible variants, also in relation to the spaces available, the two units Gl and G2 can also be built in a same building. In figs. 3-6, the transformer secondaries 13 have been indicated, by way of example, with the letters R, S or T depending on the phase respectively supplied by the sub-module 21 with which they are associated.

In accordance with this solution, the transformer 11 can comprise a single transformer primary 12 provided with three-phase inputs connected to the power supply line 201 and a plurality of transformer secondaries 13, one for each phase R, S T, wherein each transformer secondary 13 is connected to a rectifier 14.

According to some embodiments, the transformer device 11 comprises a single transformer primary 12 which is coupled to all the transformer secondaries 13.

According to some embodiments, the phases R, S, T of the transformer primary 12 and of the transformer secondary 13 can be connected according to a star or delta configuration.

According to preferred embodiments, the phases R, S, T of the transformer primary 12 and of the transformer secondary 13 are connected in a delta configuration.

According to some embodiments, the phases in the respective transformer secondaries 13 of the same base power supply module 20, 120, 220 are out of phase with respect to each other so as to obtain a balance between the currents and/or the respective electrical voltages within each base power supply module 20, 120, 220.

According to possible embodiments, the transformer secondaries 26 associated with a same base module 20, 120, 220 all have different connections between the phases from each other.

According to some embodiments, all the intermediate circuits 16 of the base module 20 are short-circuited to each other.

In other words, between the respective intermediate circuits 16 belonging to the same base module 20 there are respective electrical connections 22, 23 with impedance substantially equal to zero, or at least negligible, such that the intermediate circuits 16 of the base module 20 are all substantially at the same electric potential.

According to possible variants, the intermediate circuits 16 are separated from each other, that is, there is no electrical connection between them.

According to some embodiments, each base module 20 is configured to supply at output at least one pair of single-phase alternating electric currents and voltages having the desired intensity and frequency.

Each phase supplied by the base module 20 can be connected to an electrode 102 of the electric furnace 100, possibly by means of a connection line 24.

According to some embodiments, the apparatus 10 comprises a control and command unit 17 configured at least to control the inverter devices 15 in such a way as to selectively set the parameters of the supply voltage Ua and supply current la generated by the inverters 15 and supplied to the electrodes 102.

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

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 so as to follow the position of the material and thus change the arc length.

During the melting step, in fact, the electric power supplied to the electrodes 102 can be increased compared to the boring step, since the arc is now presumed to be covered and distant from the vault of the furnace, and therefore the risk of damage to the latter is avoided.

The references of the supply voltage Ua and supply current la can be changed through the control and command unit 17 in order to increase the active power. In this step, the arc is more stable since it is protected by the scrap or slag.

In addition, during the refining step, the process is much more stable and also requires less power.

In this way, the control and command unit 17 can manage and command, in relation to the specific steps of the process, at least the following parameters: supply voltage Ua, supply current la, electric power supply frequency fa and position of the electrodes 102. 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.

Through the electrical topology adopted for the inverters 15 it is also possible to protect the network 203 from disturbances caused by 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 furnace and a ladle furnace LF or submerged arc furnace SAF. According to one aspect of the invention, the apparatus 10 comprises at least one adapter device 19 connected between the at least one base module 20 and the electrodes 102, and configured to adapt the parameters of the supply electric voltage and/or current Ua, la supplied by the at least one base module into adapted electric current and voltage Ua*, la* suitable to power the electrodes 102.

According to some embodiments, the adapter device 19 can comprise one or more of either an adapter transformer 25, a reactance 26, a filter 27 or a disconnector switch 28.

According to some embodiments, in the case of a low voltage power supply line 201L, as shown in fig. 1 for example, the adapter device 19 can be chosen from a reactance 26, such as an inductor or a capacitor, a filter or a disconnector switch, since the supply voltage Ua at output from the at least one base module 20 is already at low voltage and therefore suitable to be fed to the electrodes 102 without the need to be lowered further. In accordance with one possible solution, the reactance 26 can be or comprise an inductor sized in such a way as to obtain a certain equivalent reactance, given by the contribution of the inductor and by the reactance introduced by the conductors that connect the apparatus 10 to the electrodes 102, for example the conductors of the connection line 24. By modifying the power supply frequency fa with respect to the mains frequency fr it is possible, with the inductor remaining the same, to change the reactance value that the component assumes in the circuit and thus reach the desired total equivalent reactance value.

According to some embodiments, in the case of a medium voltage power supply line 20 IM, as shown in fig. 2 for example, the adapter device 19 is or comprises an adapter transformer 25, comprising a transformer primary 29 connected to the at least one base module 20 or to the plurality of base modules 20, and a transformer secondary 30 connected to the electrodes 102.

In this case, the adapter transformer 25 is configured at least to lower the medium voltage MV supply voltage Ua to a low voltage LV adapted supply voltage Ua*. The power supply frequency fa upstream and downstream of the adapter transformer 25 can be substantially the same.

The transformer primary 29 and the transformer secondary 30 can comprise a number of inputs and outputs corresponding, respectively, to the number of base modules 20 or to the current/voltage phases supplied thereby, on one side, and of electrodes 102 on the other side, or there can also be provided a plurality of adapter transformers 25 connected in parallel with each other or in cascade. In the case of a plurality of adapter transformers 25, these can preferably have partly star type connections and partly delta type connections, and even more preferably be out of phase with respect to each other or with respect to a common reference.

The control and command unit 17 can comprise regulating devices 18. In accordance with possible solutions of the present invention, the regulating devices 18 can comprise, purely by way of example, a hysteresis modulator or a PWM (Pulse- Width-Modulation) modulator or suchlike.

These types of modulators can be used to command the semiconductor devices of the rectifiers 14 and inverters 15: suitably controlled, they generate voltage or current values to be supplied to the furnace 100, in this case to the electrodes 102.

In particular, the modulator processes these voltage and current values and produces commands for driving at least the rectifiers 14 and the inverters 15, so that the voltage and current quantities required by the control are present at the terminals that connect to the electrodes 102. 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.

According to the invention, the regulating devices 18 are configured to regulate, during each step of a melting cycle of the furnace 100, the electric power supply frequency fa of the supply voltage Ua and supply current la.

The regulating devices 18 are commanded by the control and command unit 17.

In particular, the regulating devices 18 are commanded by the control and command unit 17 in such a way that the power supply frequency fa is lower than or equal to the mains frequency fr at least for 80% of the total duration of a work cycle.

According to some embodiments, in at least one step of the work cycle the power supply frequency fa is comprised between 0.5% and 200% of the mains frequency fr. According to some embodiments, the power supply frequency fa is always lower than or equal to the mains frequency fr, right from the start of the work cycle and, moreover, at least in one of the steps of the work cycle in the furnace 100, the power supply frequency fa is lower than the mains frequency fr of the power supply means 200 of electrical energy, in particular comprised between 40% and 80% of the mains frequency fr.

According to some embodiments, in general, in at least one step of the work cycle in the furnace 100, the power supply frequency fa can be lower than the mains frequency fr. In accordance with one possible solution, the control and command unit 17 is connected to all the base modules 20 in order to control at least the respective inverters 15 so that each module 20 supplies the same values of supply voltage Ua, supply current la, and power supply frequency fa to the electrodes 102. In this way, it is possible to prevent malfunctions of the entire system. According to other variants, it can also be provided that the base modules 20 can be controlled in such a way as to supply respective different values of supply voltage Ua, supply current la, and power supply frequency fa to each electrode 102, for example in order to vary the power distribution within the metal bath.

Fig. 3 is used to describe an example of an apparatus 100 provided with a plurality of base modules 20, each comprising two sub-modules 21 , indicated with the letters A and B only for the purpose of facilitating their identification.

In this case, the transformer device 11 comprises a transformer primary 12 and two transformer secondaries 13 indicated with the letters R and S.

In this case, the phases of the two transformer secondaries 13 can be connected with a delta configuration and have a respective positive and negative phase shift angle a symmetrical with respect to a common reference, for example comprised between 15° and 25°.

Each sub-module 21 comprises, disposed in succession to each other, a rectifier device 14, an intermediate circuit 16, or DC-link, common for all phases, and at least one inverter device 15 connected to the intermediate circuit 16.

The intermediate circuits 16 of the two sub-modules 21 are connected to each other by means of electrical connections 22, 23 having negligible impedance.

In the example of fig. 3, the apparatus 100 is configured to operate at medium voltage and comprises an adapter transformer 25 connected between the base modules 20 and the electrodes 102.

According to possible embodiments, which can be combined with the other embodiments described here, there can also be an inductor 33 between the base modules 20 and the adapter transformer 25, having the function of filtering the output voltage and current.

According to possible variants, the apparatus 10 according to the conformation of fig. 3 could be configured to operate at low voltage; in this case, the adapter transformer 25 could even not be present.

According to some embodiments, for example described with reference to fig. 4, each rectifier device 14 can comprise a respective rectifier circuit 14R, 14S, 14T for each phase.

The rectifiers 14 can be chosen from a group comprising a diode bridge, a thyristor bridge, or other.

In accordance with one possible solution, the rectifiers 14 comprise devices, for example chosen in a group comprising diodes, SCRs (Silicon Controlled Rectifiers), GTOs (Gate Turn-Off thyristors), IGCTs (Integrated Gate- Commutated Thyristors), MCTs (Metal-Oxide Semiconductor Controlled Thyristors), BJTs (Bipolar Junction Transistors), MOSFETs (Metal-Oxide Semiconductor Field-Effect Transistors) and IGBTs (Insulated-Gate Bipolar Transistors).

Each sub-module 21 is provided with its own direct current intermediate circuit 16. The rectifier device, that is, each rectifier circuit 14R, 14S, 14T, connects to the intermediate circuit 16 on one side, and one or more inverter devices 15 connect on the other side.

This intermediate circuit 16 can comprise one or more capacitors 31, for example a capacitor bank, suitable to store energy and create a separation between the rectifier device 14 and the one or more inverter devices 15, and therefore also between the electric network 201 and the electrodes 102, that is, the connection line 24.

According to some embodiments, the inverter devices 15 can comprise one or more switches 32 chosen, for example, from a thyristor or a transistor of the following types: Gate Turn-Off thyristor (GTO), 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 32 can generally be associated with respective diodes, not shown. Fig. 5 shows a second example embodiment of an apparatus 10 comprising a base module 120 with three sub-modules 21 each comprising a rectifier device 14, a direct current intermediate circuit 16 and five inverter devices 15. In this case, the base module 120 is configured to supply at output three different phases R, S, T of current and voltage. The three intermediate circuits 16 are all connected to each other by means of the short-circuit connections 22, 23.

In the example case of fig. 5, the transformer secondary 13S has a zero phase shift angle, while the remaining transformer secondaries 13R, 13T are offset by a respective phase shift angle a, positive and negative, respectively.

In the example embodiment of fig. 5, the apparatus 10 can be configured to operate at low voltage and the adapter device 19 can be a filter 27, although according to possible variants the latter can be replaced with a reactance 26 or a disconnector switch 28.

Variants can also be provided in which the modules 120 are configured to operate at medium voltage; in this case, the adapter device 19 can comprise an adapter transformer 25.

Fig. 6 shows another embodiment of an apparatus 10, in this case connected by way of example to an electric furnace 100 of the submerged arc SAF type which, in the example case, comprises six electrodes 102A-102F, although their number may be different. The apparatus 10 according to this embodiment comprises one or more base modules 220, each provided with six sub-modules 21, the intermediate circuits 16 of which are connected to each other by means of short circuit connections 22, 23. In this example, there are five inverter devices 15, but as mentioned above, this number may also be lower or higher. In this case, the six sub-modules 21 of the base module 220 are connected in pairs to the phases R, S, T of the connection line 24, and each electrode 102 is connected to one of said phases R, S, T. In the example case, each phase R, S, T is connected to two different electrodes 102. In this embodiment, the transformer primary 11 is coupled to six transformer secondaries 13.

According to some embodiments, it can be provided that the phases of the transformer secondaries 13 are offset as in the example of fig. 3, or differentiated phase shift angles can also be provided. For example, the transformer secondaries 13 A, 13C can be offset by a first phase shift angle al with respect to the transformer secondary 13B, while the transformer secondaries 13D, 13F can be offset by a second phase shift angle a2, different from the first angle al, with respect to the transformer secondary 13E. It is clear, however, that other combinations of connections and/or respective phase shift angles are possible.

In the example embodiment of fig. 6, the apparatus 10 can be configured to operate at low voltage and the adapter device 19 can be a disconnector switch 28, although according to possible variants the latter can be replaced with a reactance 26 or a filter 27.

Variants can also be provided in which the modules 220 are configured to operate at medium voltage; in this case, the adapter device 19 can comprise an adapter transformer 25.

According to one aspect of the invention, the apparatus 10 can comprise N base modules 20, 120, 220 connected in parallel to each other between the network 201 and the electric furnace 100. The number N of base modules can be chosen according to requirements or parameters such as, for example, the power required for the furnace 100, the productivity required, the total number of electrodes 102 to be powered. The number N can for example vary between 2 and 60, for example 3, 4, 5, 6, 8, 10, 12, 18, 24, 36, 48 or even other intermediate even or odd numbers.

Preferably, the number N of base modules 20, 120, 220 is such that the total number of the sub-modules 21 is a multiple of three, so as to evenly power the three phases R, S, T of the connection line 24. According to possible variants, in the case of a plurality of base modules 20, 120, 220, it can be provided that at least one transformer 11 has the connections between the respective phases R, S, T of the transformer primary 12 and/or of the transformer secondary 13 of a type that is different from at least one other transformer device 11.

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 12 or the transformer secondary 13 of one of the transformer devices 11 are connected in a star configuration, while the phases R, S, T of another are connected in a delta configuration, 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. This in order to obtain a balance of both the current and the voltage within the single base module 20, and also overall within the apparatus 10.

Some embodiments described here also concern a plant 50 comprising a power supply apparatus 10 according to the invention and an electric furnace 100 provided with two or more electrodes 102, each connected to a phase R, S, T of at least one base module 20, 120, 220 by means of at least one adapter device 19. The operation of the apparatus 10 for the electric power supply of furnaces 100 for melting and/or heating metal materials provided with electrodes 102 described heretofore, which corresponds to the method according to the present invention, provides to:

- supply an alternating mains voltage and current Ur, fr of a three-phase power supply line 201, 201L, 201M which have a predefined mains frequency fr to at least one base module 20, 120, 220;

- transform, by means of a transformer 11 , the mains voltage and current Ur, fr into alternating secondary voltage and current Us, Ir which can be selectively set;

- rectify the secondary voltage and current Us, Is with a plurality of rectifiers 14 to obtain a direct current intermediate voltage and current Ui, li;

- convert, with a plurality of inverter devices 15, the direct current intermediate voltage and current Ui, li into an alternating supply voltage Ua and supply current la which can be selectively set and have a desired power supply frequency fa by means of a control and command unit 17 connected to the inverter devices 15; - adapt and/or filter the supply voltage and current Ua, la downstream of the inverter devices 15 in order to adjust their parameters and obtain an adapted supply voltage and current U*, I* to be fed to the electrodes 102.

According to some embodiments, the method provides to use base modules 20, 120, 220 comprising at least two sub-modules 21, each suitable to power a single phase R, S, T of a three-phase network, wherein in each of the sub-modules 21 there is provided a rectification of the respective phase voltages and currents, a temporary storage of energy and a separation between the power supply line 201 and the electrodes 102 by means of a direct current intermediate circuit 16, and an inversion of the direct electric voltage and current Ui, li by means of at least one inverter device 15 in order to obtain respective single-phase alternating supply voltage and current.

In accordance with another aspect of the invention, the method provides that all the direct current intermediate circuits 16 of the at least one base module 20, 120, 220 are short-circuited to each other in such a way as to bring them to a same electric potential.

In this way, the inverter devices 15 belonging to each sub-module 21 of the same base module 20, 120, 220 operate on common direct voltages and currents, which can be considered as an average value between the voltages and currents supplied in each sub-module 21 by a respective rectifier device 14.

According to another aspect of the invention, the method provides that, during each step of a work cycle of the furnace 100, regulating devices 18 of the control and command unit 17 adjust the power supply frequency fa of the supply voltage Ua and supply current la so that the power supply frequency fa is lower than or equal to the mains frequency fr at least for 80% of the duration of a work cycle and, at least in one step of the work cycle in the furnace 100, it is lower than the mains frequency fr, preferably comprised between 40% and 80% of the mains frequency fr.

According to preferred embodiments, the power supply frequency fa is lower than or equal to the mains frequency fr at least for 90% of the overall duration of a work cycle.

According to further embodiments, the power supply frequency fa is lower than or equal to the mains frequency fr at least for 95% of the overall duration of a work cycle.

According to some embodiments, the power supply frequency fa is lower than the mains frequency fr at least for 90% of the overall duration of a work cycle, preferably at least for 95% of the overall duration. According to some embodiments, the method provides that, in at least one step of the work cycle, the power supply frequency is comprised between 10% and 80% of the mains frequency.

According to some embodiments, the method provides that, in at least one step of the work cycle, the power supply frequency is adjusted to a value comprised in the range of 1 to 48 Hz.

According to some embodiments, the method provides that, in at least one step of the work cycle, the power supply frequency fa is comprised between 45% and 75% of the mains frequency fr. According to further embodiments, the method provides that, in at least one step of the work cycle, the power supply frequency fa is adjusted to a frequency equal to about half of the mains frequency fr.

According to some embodiments, the method provides that, in at least one step of the work cycle, the power supply frequency fa is comprised between 101% and 200% of the mains frequency fr.

The possibility of adjusting the power supply frequency fa to values higher than the mains frequency fr allows to increase the stability of the electric arc, reducing the time for melting the metal material.

According to some embodiments, the power supply frequency fa is kept above the mains frequency fr in the conditions of instability of the absorbed power, that is, in the conditions in which rapid oscillations of the supply power of the electric furnace occur, such as in the boring steps, in such a way as to counteract such oscillations and improve the melting process.

According to some embodiments, the method provides that, in at least one step of the work cycle, the power supply frequency fa is adjusted to a value comprised in the range of 55 to 120 Hz.

The power supply frequency fa can be adjusted dynamically during the work cycle, manually by an operator, or automatically in relation to instructions and procedures performed by the control and command unit 17. In the present description, by work cycle we mean the set of work steps provided for a certain furnace 100.

For example, and as shown in figs. 7 and 8, 8a(A) and 8b(B), for an EAF 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.

In particular, during the boring step the electrodes 102 are brought closer to the discharged solid metal material M, in order to trigger the electric arc and initiate the melting of the metal material M. As the metal material M 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.

In accordance with one possible solution (fig. 7), the boring step and the melting step can be repeated several times before the refining step, and between them there is provided a step of loading further metal material M into the electric furnace 100.

For example, with reference to fig. 3, it is provided to load the metal material M, perforate the metal charge with the electrodes 102 and melt it. This operating sequence can be repeated three times with each loading of the metal material M.

In accordance with the solutions shown in figs. 8, 8a(A) and 8b(B), a substantially continuous loading is provided, which starts before the boring step and continues until the furnace is completely filled and during the step of melting the metal material.

According to this embodiment, the power supply frequency fa is lower than or equal to the mains frequency fr for the entire duration, that is, 100%, of the work cycle.

According to some embodiments, the method can provide that the power supply frequency fa is decreasing with respect to the progress in time of the work cycle of the furnace 100.

The power supply frequency fa can be decreasing starting from a preset value, such as the value of the mains frequency fr or of the primary frequency fp on the primary 12 of the transformer 11 , preferably it is decreasing starting from the value of the mains frequency fr .

The power supply frequency fa can be decreasing in time in a continuous manner, for example decreasing linearly or exponentially or suchlike, as shown by the dash-dot line in fig. 4(B).

The power supply frequency fa can be decreasing in time in a discontinuous manner, for example with a stepwise trend, as shown by the dash-double dot line in fig. 4(B). The power supply frequency fa can therefore assume a plurality of values fl that are lower than the mains frequency fr.

The method can also provide that the power supply frequency fa is substantially constant at least during the time corresponding to each work step of the furnace 100. The method can provide that the power supply frequency fa, at the end of the work cycle in the furnace 100, reaches a value lower than at least 20%, preferably at least 40%, of the mains frequency fr, even more preferably it is substantially halved with respect to the mains frequency fr.

The method can provide that the power supply frequency fa assumes, at least in one or more steps of the work cycle of the furnace 100, a value substantially comprised between 30 and 40 Hz.

For example, with reference to an EAF furnace, the power supply frequency fa can be substantially equal to the mains frequency fr in the boring step and can be comprised between 0.45 and 0.55 times the mains frequency fr in the refining step. The method can provide that, in an EAF furnace, the power supply frequency fa is substantially equal to the mains frequency fr during the boring step and decreasing in the subsequent work steps, until it assumes a value fl , for example substantially equivalent to half the value of the mains frequency fr (fig. 3).

According to a further example and as shown in fig. 4(A), the method can provide that, in an EAF furnace, the power supply frequency fa is substantially equal to the mains frequency fr during the step of boring and the step of melting the charge, and decreasing with a stepwise trend in the subsequent work steps.

According to other embodiments, not shown, it can also be provided that the power supply frequency fa is lower than the mains frequency fr in all work steps. According to some variants, the method provides that, in at least one step of the work cycle, the power supply frequency fa is higher than the mains frequency fr, for example comprised between 101% and 200% of the mains frequency fr.

According to some embodiments, the method provides that, in at least one step of the work cycle, the power supply frequency is adjusted to a value comprised in the range of 51 to 100 Hz, or 61 to 120 Hz, depending on value of the mains frequency.

According to some embodiments, the power supply frequency fa is adjusted so as to be higher than the mains frequency fr at least in situations where there are rapid oscillations of the power absorbed by the EAF furnace, for example in correspondence with the loading of metal material.

For example, in fig. 10 there is a graph that at the top shows the trend of the electric power absorbed by the charge during a work cycle and at the bottom shows how the power supply frequency fa is adjusted with respect to the mains frequency fr.

The parts highlighted with closed lines indicate situations in which the oscillations and rapid variations in power occur: as can be seen, in correspondence with these situations the power supply frequency fa is higher than the mains frequency fr, while for the remaining duration of the work cycle it is lower than or equal to the mains frequency fr.

With the present invention, therefore, once the work points of the furnace 100 have been determined at least in terms of power, voltage, current and frequency, the method can provide that the control and command unit 17 tries to follow these work points also by continuously adjusting the power supply frequency fa.

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.

With the present invention it is therefore possible, by adjusting the frequency during the different steps of the process, to optimize the electric parameters in each step. As a further example, for example described with reference to fig. 9, in a ladle furnace LF, the work cycle comprises at least one step of refining the molten metal material M.

According to possible embodiments, the method can provide that in the ladle furnace LF the power supply frequency fa remains constant for the entire duration of the work cycle, or that the power supply frequency fa decreases over time, linearly, stepwise, exponentially, or according to other mathematical curves, and possibly also with a combination thereof.

In any case, in the ladle furnace LF the power supply frequency fa remains preferably lower than the mains frequency fr for the entire duration of the work cycle.

For example, according to the embodiment described with reference to fig. 9, the method can provide that, in the ladle furnace LF, the power supply frequency fa is constant for the entire work cycle and assumes a value lower than the mains frequency fr, preferably at a value comprised between 0.4 and 0.6 times the mains frequency fr.

Preferably, the power supply frequency fa in the ladle furnace LF is substantially equal to half the value of the mains frequency fr, until the end of the refining step.

Advantageously and as shown in fig. 11, in the example case of an LF furnace, with the same temperature gradient obtained, the present invention allows to reduce the power consumption required by the furnace 100: for example, with the same other work conditions, in an LF furnace at a work frequency of 40Hz it is possible to achieve a reduction of the power consumed of substantially 12% compared to the power required at a frequency of 50Hz.

As a further advantage and by way of example, in a work cycle at a work frequency of 40Hz, the power factor can be increased, all other conditions being equal, from 0.90 to 0.96. As a further advantage, thanks to an increase in the arc power achievable by reducing the work frequency, the melting time can be reduced.

By way of example, in an EAF furnace, the reduction in melting time can be approximately 20% at a frequency of 25Hz and 35% at a frequency of 10Hz. As a further example, in an LF furnace, at a work frequency of 40Hz the power-on time can be reduced on average by about 20-22 minutes.

Advantageously, it is also possible to reduce the consumption of the electrodes 102, 106: for example, at a work frequency of 40Hz, the consumption of the electrodes 102, 106 can be reduced by approximately 10%.

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

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. 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

1. Apparatus (10) for the electric power supply of furnaces (100) for melting and/or heating metal materials provided with electrodes (102), comprising at least one power supply line (201, 20 IL, 20 IM) and at least one connected base module (20, 120, 220) which is configured to convert an alternating mains voltage and current (Ur, Ir) having a mains frequency (fr) into alternating supply voltage and current (Ua, la) with a desired power supply frequency (fa), wherein said at least one base module (20, 120, 220) comprises:

- a transformer (11) connected to said power supply line (201, 20 IL, 20 IM) and configured to transform said alternating mains voltage and current (Ur, Ir) into alternating secondary voltage and current (Us, Is);

- a plurality of rectifiers (14) connected to said transformer (11) and configured to transform said alternating secondary voltage and current (Us, Is) into direct current intermediate electric voltage and current (Ui, li);

- one or more direct current intermediate circuits (16) configured to store electrical energy;

- a plurality of inverter devices (15) connected to said one or more direct current intermediate circuits (16) and configured to convert said intermediate electric voltage and current (Ui, li) into alternating supply voltage and current (Ua, la) having said desired power supply frequency (fa);

- a control and command unit (17) configured to control and command the operation of said inverter devices (15) and regulate, during each step of a process of said furnace (100), said power supply frequency (fa), characterized in that said apparatus (10) comprises an adapter device (19) connected to said at least one base module (20, 120, 220) and connectable to said electrodes (102) and configured to adapt the parameters of said supply voltage and current (Ua, la) into adapted electric voltage and current (Ua*, la*) for said electrodes (102).

2. Apparatus (10) as in claim 1 , characterized in that said power supply line (201 , 201 L) is configured to operate at low voltage (LV) and said adapter device (19) comprises one or more of either a reactance (26), a filter (27) or a disconnector switch (28).

3. Apparatus (10) as in claim 1, characterized in that it comprises a reducer transformer device (202) connected upstream of said power supply line (201, 20 IL) and configured to lower an electric voltage supplied by a high or medium voltage main network (203) into a low voltage mains voltage (Ur).

4. Apparatus (10) as in claim 1 , characterized in that said power supply line (201 , 20 IM) is configured to operate at medium voltage (MV) and said adapter device (19) comprises at least one adapter transformer (25) provided with a transformer primary (29) connected to said at least one base module (20) and a transformer secondary (30) connectable, during use, to said electrodes (102).

5. Apparatus (10) as in claim 4, characterized in that it comprises a reducer transformer device (202) connected upstream of said power supply line (201, 20 IM) and configured to lower an electric voltage supplied by a high or medium voltage main network (203) into a medium voltage mains voltage (Ur).

6. Apparatus as in any claim hereinbefore, characterized in that said control and command unit (17) is provided with regulating devices (18) configured to regulate, during each step of a work cycle of said furnace (100), said power supply frequency (fa) in such a way that it is lower than or equal to said mains frequency (fr) and, in at least one of the steps of said work cycle, said power supply frequency (fa) is comprised between 40% and 80% of said mains frequency (fr).

7. Apparatus as in any claim hereinbefore, characterized in that said at least one base module (20, 120, 220) comprises at least two sub-modules (21, 21A-21F), each comprising at least one rectifier (14), an intermediate circuit (16) and an inverter device (15) and each configured to supply at output a single-phase voltage and current of a multi-phase connection line (24).

8. Apparatus as in claim 7, characterized in that all the intermediate circuits (16) of the at least one base power supply module (20, 120, 220) are short-circuited to each other by means of short-circuit connections (22, 23).

9. Apparatus (10) as in claim 7 or 8, characterized in that said transformer (11) comprises a single transformer primary (12) provided with three-phase inputs which are connected, during use, to the phases (R, S, T) of said power supply line (201, 201L, 201M), coupled to a plurality of transformer secondaries (13), one for each of said sub-modules (21, 21 A-21F), and in that the phases of at least two of said transformer secondaries (13) of said at least one base module (20, 120, 220) are out of phase with respect to each other.

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 (100) provided with two or more electrodes (102) connected to respective phases (R, S, T) of supply voltage and current (Ua, la) which are supplied by said at least one base module (20, 120, 220) by means of at least one adapter device (19). 11. Method for powering an electric furnace (11) having two or more electrodes

(102), comprising:

- supplying an alternating mains voltage and current (Ur, Ir) of a three-phase power supply line (201, 20 IL, 20 IM) having a predefined mains frequency (fr) to at least one base module (20, 120, 220); - transforming, by means of a transformer (11), said mains voltage and current (Ur,

Ir) into alternating secondary voltage and current (Us, Ir) which can be selectively set;

- rectifying said secondary voltage and current (Us, Is) with a plurality of rectifiers

(14) to obtain a direct current intermediate voltage and current (Ui, li); - converting, with a plurality of inverter devices (15), said direct current intermediate voltage and current (Ui, li) into an alternating supply voltage (Ua) and supply current (la) which can be selectively set and have a desired power supply frequency (fa) by means of a control and command unit (17) connected to said inverter devices (15); - adapting and/or filtering said supply voltage and current (Ua, la) downstream of said inverter devices (15) in order to regulate their parameters and obtain an adapted supply voltage and current (U*, I*) and supply them to said electrodes (102).

12. Method as in claim 11, characterized in that it provides that, during each step of a work cycle of said furnace (100), regulating devices (18) of said control and command unit (17) regulate said inverter devices (15) in such a way that at least for some steps of the work process said power supply frequency (fa) is lower than or equal to said mains frequency (fr) and, in at least one step of said work cycle, said power supply frequency (fa) is comprised between 40% and 80% of said mains frequency (fr).

13. Method as in claim 12, characterized in that it provides to regulate said inverter devices (15) in such a way that said power supply frequency (fa) is lower than said mains frequency (fr) at least for 80% of the work process, preferably at least for 90% and even more preferably at least for 95%.

14. Method as in any claim from 11 to 13, characterized in that it provides to supply an alternating mains voltage and current (Ur, Ir) in medium voltage (MV) and to adapt said supply voltage and current (Ua, la) downstream of said inverter devices (15) by means of at least one adapter transformer (25) connected with a transformer primary (29) to said inverter devices (15) and with a transformer secondary (30) to said electrodes (102), in order to obtain an adapted supply voltage and current (Ua*, la*) in low voltage (LV).

15. Method as in any claim from 11 to 13, characterized in that it provides to supply an alternating mains voltage and current (Ur, Ir) in low voltage (LV) and to adapt said supply voltage and current (Ua, la) downstream of said inverter devices (15) by means of an adapter device (19) connected between said inverter devices (15) and said electrodes (102), said adapter device (19) comprising one or more of either a reactance (26), a filter (27) or a disconnector switch (28).