Object of the Invention

The present invention relates to a device, an installation, and a method for regulating the electric current in a normally three-phase AC-powered electric arc furnace; particularly the invention relates to a regulator device which regulates the current circulating through the electrodes of the electric arc furnace and allows preventing the circulation of short-circuit currents that may damage the electrodes, as well as the intervention of any element causing the current to be cut off by a protective device which would stop the operation of the furnace in the event of short circuits.

Background of the Invention

Wear on electrodes in a metallurgical electric arc furnace caused by short circuits that occur in the shaft is a well known technical problem in steel plants using electric arc furnaces.

These electric arc furnaces comprise three electrodes, one for each phase of the electric system. To melt aluminum scrap, the heat generated by the striking electric arc between the electrodes and the aluminum scrap in contact with the mass of the furnace is used. Normally, a power transformer with the possibility of modifying the number of active coils, and therefore the possibility of modifying the applied voltage in a stepwise manner, is used to power the furnace. However, this number of positions is very limited, so the voltage variations that can be obtained are reduced and the on-load use thereof is not possible, consequently it is considered that the voltage applied to the furnace is constant during a given process.

This makes it necessary to modify the impedance in order to be able to control the melting current. The impedance can be changed by modifying the position of the electrodes which is achieved by moving the electrodes to or away from the aluminum scrap; by moving the electrodes closer to the material, the arc finds a path with lower impedance and the material melts by means of Joule effect. The impedance gradually changes as the aluminum scrap melts, so since the voltage is constant, the current changes and the electrodes must be moved by means of mechanical elements to keep the current approximately constant within a range of given values, preventing the initiation of the protective devices and cutting off the current to prevent a short circuit when the impedance is practically null. The electrodes are moved with mechanical drives that are very slow in comparison with current variations, making it very difficult to efficiently control the circulating current. This leads to the wear on the electrodes and causes the impact of the electrodes against the aluminum scrap, which in turn generates breakage therein. Broken electrodes must be replaced to keep the furnace in operation.

Although broken electrodes have not involved a significant economic impact to date, the situation has changed with the increase in the prices of graphite, i.e., the material from which electrodes are generally made, so electrode protection has become a priority in the metallurgical industry which is already seeking technical solutions to solve the problem.

Description of the Invention

The present invention proposes a solution to the preceding problems by means of a continuous current regulating device according to claim 1, an electric arc furnace installation according to claim 11 , and a method for regulating the current according to claim 12. Preferred embodiments of the invention are defined in the dependent claims.

A first inventive aspect provides a continuous current regulating device for an electric arc furnace, wherein for each phase the furnace comprises at least one furnace electrode to which a voltage V _h is applied and through which a current circulates , and the furnace is connected to a voltage source generating a voltage V _t , characterized in that the device is configured for being connected in series between the voltage source and a furnace electrode, and comprises:

- switching means configured for being connected in series between the voltage source and the at least one furnace electrode, and wherein the switching means are configured for activating or deactivating the phase of the regulating device to which it is connected;

- a converter connected in parallel to the switching means, such that the current circulates through the converter when the switching means are open, and wherein the converter comprises, at least one inverter, with its AC terminals configured for being connected in series between the voltage source and the at least one furnace electrode, wherein the at least one inverter is configured for converting DC voltage into AC voltage, an energy dissipator connected in parallel to the capacitive means, wherein the energy dissipator comprises an electronic regulator and resistive means connected in series with the electronic regulator;

- voltage measuring means for measuring the voltage V _h , voltage measuring means for measuring the voltage V _t , voltage measuring means for measuring the voltage V _c and current measuring means for measuring the current , wherein the measuring means are configured for emitting measurement signals Sv _h , Sv _t , Sv _c , S, corresponding to the measured voltage and current values;

- at least several control means configured for receiving and processing the measurement signals Sv _h , Sv _t , Sv _c , S, of the measuring means, and for emitting control signals to the converter and to the switching means in response to a time variation of the current which exceeds a current threshold _max as a result of the start of a short circuit induced by the electric arc, wherein the control means emit, a control signal S1 to the switching means to activate the device, a control signal S2 to activate the inverter, and a control signal S3 to activate the energy dissipator, such that the electronic regulator of the energy dissipator regulates the passage of current to the resistive means by controlling the dissipated energy and keeping the DC voltage Vc between the DC terminals of the inverter under control, converting the voltage Vc into a phase- and amplitude-regulated compensation AC voltage V _x , which is in turn subtracted from the voltage V _t of the voltage source, such that the voltage V _h applied to the furnace electrodes is reduced, and accordingly the current does not exceed the threshold _max -

Throughout the present document, it must be understood that the voltage source, the furnace, and the continuous current regulating device are single-phase elements configured for use in single-phase or three-phase systems.

Likewise, throughout the present document, reference will be made interchangeably to dissipated energy or power, with dissipated power being the amount of energy dissipated per unit of time.

Voltage source shall be understood as any means capable of supplying a sinusoidal voltage suitable for powering an electric arc furnace; in a particular embodiment, three voltage sources are grouped to form a three-phase voltage source. In the present document, the terms electrode, furnace electrode, and electrode of the furnace must be understood as equivalent; it must also be understood that a three-phase furnace comprises at least three electrodes.

It must be understood that the term DC bus refers to the set of elements connected to the DC terminals of the inverter.

The AC voltage generated by the DC regulating device will also be referred to as compensation voltage or V _x , whereas the voltage between the DC terminals of the inverter will also be referred to as DC bus voltage or Vc.

To regulate the current circulating through the electric furnace electrodes, the device generates a phase- and amplitude-controlled sinusoidal voltage in series between the furnace feed voltage source and the electric arc furnace itself. This sinusoidal voltage, or compensation voltage, is subtracted from the voltage supplied by the voltage source, such that for a given impedance of the furnace, the circulating current is modified by means of changing the compensation voltage continuously.

The compensation voltage generated by the device is produced between the AC terminals of the inverter which is connected, on its DC side, in parallel to capacitive means that are charged when current passes through the inverter, and to an energy dissipator with resistive means and an electronic regulator, wherein the function of the energy dissipator is to convert the energy generated in excess into thermal energy which dissipates into the environment.

By applying the compensation voltage, the AC current circulating through the inverter causes the absorption of an energy striking the capacitive means, raising the voltage Vc. This energy must be dissipated to maintain the voltage Vc and protect the device, particularly the capacitive means, and the electronic regulator and resistive means are used for that function. As the voltage increases, the regulator connects more resistors, so the total ohmic value of the resistor of the resistive means decreases and the dissipated energy increases.

The electronic regulator and the resistive elements allow controlling the voltage of the DC bus, Vc. The inverter converts this voltage Vc into an AC voltage referred to as phase- and amplitude-controlled compensation voltage Vx.

In a particular embodiment, the energy dissipator is used for channeling the excess energy to other elements in which it can be utilized.

The energy dissipator, the inverter, and the switching means are activated by control means which, in response to voltage and current signals in the furnace and in the voltage source, are capable of generating control signals to regulate the amplitude and phase of the compensation voltage.

Advantageously, the current regulating device can be installed in a pre-existing electric arc furnace system or in a new installation and allows preventing the problems caused by short circuits occurring in the shaft of the furnace during operation. Particularly, the invention allows controlling the currents circulating through the furnace electrodes both to prevent the short- circuit currents from damaging the electrodes and to prevent the protective elements from short circuits frequently stopping the operation of the furnace. Even more advantageously, in a particular embodiment three current regulating devices, one for each phase, are connected in series between three voltage sources and the electrodes of a three-phase furnace.

Generally, the present invention achieves more stable circulating currents that are kept centered with the furnace operation setpoint value, achieving more stable melting processes of shorter duration and improving production process efficiency.

Controlling the current also allows solving the problem of mechanically moving the electrodes in order to change the impedance of the furnace; continuously controlling the current prevents having to move the electrodes all of a sudden, so the impacts of the electrodes against the aluminum scrap will be lower, reducing the breakage of the electrodes or completely preventing damage at the ends of the electrodes.

During normal furnace operation, the switching means allow the passage of current through their branch, such that the current does not circulate through the parallel branch of the converter, so the elements of the converter hardly consume any energy or experience any degradation. This translates into a longer service life of the elements, thus resulting in a very highly reliable device.

In a particular embodiment, the at least one inverter comprises at least one single-phase inverter bridge with semiconductor devices.

Advantageously, an inverter bridge allows transforming a DC current into a sinusoidal current, particularly an inverter bridge formed by semiconductor devices or elements allows obtaining an efficient and compact inverter without any mechanical elements that are susceptible to failures.

In a particular embodiment, the at least one inverter comprises a bridge with four semiconductor devices, preferably insulated-gate bipolar transistors (IGBTs) with four diodes connected in anti-parallel.

Advantageously, an inverter formed by IGBTs allows transforming a DC voltage into a sinusoidal voltage without harmonic components which damage the voltage sources, the furnace, or the elements of the device.

In a particular embodiment, the switching means comprise a switch configured for deactivating the device and a static bypass switch configured for instantaneously activating or deactivating on-load voltage regulation, wherein the switch and the static bypass switch are connected to one another in parallel.

Advantageously, the switch allows activating or deactivating the current regulating device without disconnecting it physically from the installation, for example to perform maintenance tasks; the static bypass switch allows instantaneously activating or deactivating the operation of the regulating device while the furnace is in operation. During normal operation, the switch will usually be open and the static bypass switch will usually be closed.

In a particular embodiment, the switch is one from the following list: an automatic switch, a load switch, a motor-driven no-load isolator.

Advantageously, the switch can be operated manually by an operator or can be operated by means of an external control signal, whether on-load or under no-load.

In a particular embodiment, the static bypass switch comprises thyristors, IGCTs, GTOs, and/or IGBTs.

Advantageously, the static bypass switch comprises a specific type of semiconductor device depending on the desired response speed.

In a particular embodiment, the static bypass switch comprises an auxiliary switching circuit configured for cutting off the on-load current. Advantageously, the auxiliary circuit comprising thyristors, capacitors, and inductors, allows canceling the current circulating through the semiconductor devices of the static bypass switch as a result of the current generated by the discharge of the capacitors.

In a particular embodiment, the electronic regulator comprises IGBT-type semiconductor devices.

Advantageously, the response speed of the IGBT allows performing a very quick control which successfully maintains the DC voltage of the DC bus in response to control signals activating the electronic regulator.

In a particular embodiment, the resistive means comprise a plurality of resistors with different electrical resistance values, the control means being configured for emitting a control signal S4 for activating or deactivating a control of the electronic regulator selectively connecting at least one resistor of the plurality of resistors.

Advantageously, the resistive means allow regulating the amplitude of the DC bus voltage by means of changing the total resistive load connected in series with the electronic regulator.

In a particular embodiment, the control means act on at least one phase of the device.

In a particular embodiment, the device comprises respective secondary control means for activating or deactivating the elements of the switching means, of the converter, and/or of the energy dissipator, wherein the secondary control means are configured to begin operation in response to the signals S1, S2, and/or S3 of the control means, signals S1, S2, and/or S3 emitted in turn by the control means in response to the measurement signals Sv _h , Sv _t , Sv _c , Si.

Advantageously, the control means only emit signals for activating or deactivating the controlled elements of the device, particularly the switching means, the converter, and the energy dissipator, and the secondary control means generate the signals required for controlling each of the constituent elements of the switching means, of the converter, and of the energy dissipator. In a particular embodiment, the secondary control means are capable of emitting trigger pulses to the semiconductor devices of the switching means, of the converter, and of the energy dissipator. Advantageously, the control means receive signals, process and emit control signals for each of the phases. In a particular embodiment, the control means emit control signals for three phases simultaneously, particularly control signals for activating three phases of the switching means. In the manner, in the event that a short circuit current emerges in one phase, the device can quickly react before the short circuit spreads to the remaining phases.

In a second inventive aspect, the invention provides a three-phase electric arc furnace installation, comprising a three-phase voltage source with at least three voltage sources, a three-phase electric arc furnace with at least three furnace electrodes, and at least one continuous current regulating device according to the first inventive aspect for each of the three phases.

In a particular embodiment, the control means are common for the three phases of the installation.

In a third inventive aspect, the invention provides a method for continuously regulating the current of an electric arc furnace with a current regulating device according to the first inventive aspect, comprising the following steps:

- providing a regulating device for each phase connected in series between the voltage sources and the furnace electrodes; for each phase, when the furnace is in operation, the voltage measuring means measure the voltages V _h , V _t , \/ _c and the current measuring means measure the current l _h , and continuously emit measurement signals Sv _h , Sv _t , Sv _c , S, to the control means; for each phase, if the time variation of the current l _h increases above a threshold value I _max , the control means:

• emit a signal S1 to open the switching means and activate the device;

• emit a control signal S3 to the energy dissipator depending on the measurement signals Svh, Svt, Svc, S,;

• for each phase, the control means emit a control signal S2 to the inverter, such that for each phase, the electronic regulator of the energy dissipator controls the passage of current to the resistive means by controlling the dissipated energy and keeping a DC voltage Vc between the DC terminals of the inverter under control, converting the voltage Vc into a phase- and amplitude-regulated AC compensation voltage \/ _x , which is in turn subtracted from the voltage V _t of the voltage source, such that the voltage V _h applied to the furnace electrodes is reduced, and accordingly, the current l _h does not exceed the threshold I _max . Advantageously, the method for continuously regulating current allows preventing short-circuit currents from going through the furnace electrodes, so triggering of the protection does not constantly stop the process of the furnace and deterioration of the electrodes caused by impacts resulting from the control performed by abruptly moving the electrodes is prevented.

All the features and/or method steps described in this specification (including the claims, description, and drawings) can be combined in any combination, with the exception of combinations of such mutually exclusive features.

Description of the Drawings

These and other features and advantages of the invention will become more apparent from the following detailed description of a preferred embodiment, given only by way of non-limiting illustrative example in reference to the attached figures

Figures 1a-1b show two embodiments of the current regulating device connected in series with the voltage generator and the furnace, for a single-phase system and for a three-phase system.

Figure 2 shows the single-phase circuit connected in series with a furnace electrode.

Figure 3 shows the converter of the device.

Figure 4 shows the device with its control circuit and its operative connections.

Figure 5 shows examples of waveforms for several relevant magnitudes.

Detailed Description of the Invention

Figure 1 shows a set of three current regulating devices (1) connected in series between a three-phase source and a three-phase furnace. The three-phase source that is shown is equivalent to a set of three single-phase AC voltage sources (9); generally the three-phase source comprises a three-phase transformer, not shown in the figures, to adapt the supply voltage of the grid to the working voltage of the furnace. Additionally, the three-phase source comprises a series of protective devices, not shown in the figures, which are common in installations of this type, particularly switches configured for cutting off the current from a short- circuit current, for example, a magnetothermal switch. In addition to other elements which allow melting the metal, such as the shaft, the three-phase furnace comprises at least three furnace electrodes (8), these electrodes are usually manufactured in graphite, and comprise a mechanical drive which allows changing the distance between the end of the electrodes (8) and the material being melted in the shaft.

The regulating device (1) is configured for being connected in series between a voltage source (9) of a three-phase source and a furnace electrode (8) to make the assembly thereof on the installation of a pre-existing furnace easier and to facilitate both control response and current regulation. Connection in series is performed through connection terminals (1.1) of the device. The current must be regulated individually for each phase given that, generally, a short circuit will not occur simultaneously in the three furnace electrodes (8). Therefore, three devices (1) which can work independently will have to be installed in each three-phase installation, and as shown in Figure 2 each of the phases in turn comprises switching means (2) and a converter (3) which is the element performing current regulation perse.

The switching means (2) and the converter (3) are connected to one another in parallel for activating or deactivating each phase of the device (1) separately; i.e., the change in the state of the switching means (2) allows the selective circulation of current through the branch of the converter (3) and the start of current regulation.

The switching means (2) can have different configurations depending on the installation characteristics and needs, specifically depending on the power of the furnace installation and the response speed required. Preferably, the switching means (2) will act by activating the three phases.

In a particular embodiment, the switching means (2) comprise a switch (2.1), or a main switch, for disconnecting the device to perform maintenance operations, although not necessarily with the capacity to cut off the on-load current; in this embodiment, the switch (2.1) is an automatic switch-type element. In another embodiment, the switch (2.1) is a no-load isolator or a load switch; these elements are capable of cutting off the on-load current, but their response time makes them rather unsuitable for the type of regulation required in the present invention.

Therefore, to complement the switch (2.1), a static bypass switch (2.2) connected in parallel with the switch (2.1) is required; the static bypass switch (2.2) is configured for cutting off the current instantaneously when the start of a short circuit is detected, so it must be an element capable of working with high powers and with a quick response speed. In one embodiment, the static bypass switch (2.2) comprises a series of semiconductor devices capable of cutting off the on-load current when they receive an opening signal S1 through the processing means. In some embodiments, the static bypass switch (2.2) is formed by thyristors, IGCTs, GTOs, IGBTs, or a combination thereof. If the chosen semiconductor devices or their configuration alone do not allow cutting off the circulating current within the time required by the application, the static bypass switch (2.2) includes an auxiliary switching circuit, or a secondary control means, formed by auxiliary inductors, capacitors, and thyristors; the previously charged capacitor discharges when the auxiliary thyristors are triggered, providing a current which cancels the current circulating through the semiconductor devices of the static bypass switch (2.2) which causes the blocking thereof at its zero-crossing.

During the normal operation of the furnace (8), the switch (2.1) is open and the static bypass switch (2.2) is closed, such that the current circulates only through the branch of the switching means (2) until the static bypass switch (2.2) receives the opening signal S1 for opening the branch or branches corresponding to the switching means (2) and causing the current to go through the branch or branches of the converter (3). The switch (2.1) is configured to operate when there is a need to deactivate the device (1), for example to perform maintenance operations in the installation.

Figure 2 shows the converter (3) connected in parallel with the switching means (2), the converter (3) formed by an inverter (3.1), and an energy dissipator (4). The function of the inverter (3.1) is to change the DC bus voltage kept constant by the energy dissipator (4) from DC to AC.

The inverter (3.1) is shown in Figure 3 as a generic inverter with AC terminals (3.1.1) and DC terminals (3.1.2), capable of transforming a DC voltage Vc into an AC voltage V _x . The elements of the DC side, also referred to as DC bus, are connected to the DC terminals (3.1.2) and comprise capacitive means (3.2) which, in the embodiment shown, is a single capacitor, and connected in parallel to the capacitive means (3.2), a set of electronic regulator (4.1) and resistive means (4.2), connected in series with the electronic regulator (4.1).

In a preferred embodiment, the inverter (3.1) is formed by a bridge with four IGBTs. Semiconductor devices of another type can be used, but IGBTs allows a quick response and have the additional advantage of generating a sinusoidal signal that does not introduce harmonic components, which prevents the circulation of harmonic currents and the need to include expensive equipment to correct these problems. To be able to perform the inversion, the IGBTs must receive trigger pulses generated by an auxiliary control circuit which is activated in response to a control signal S2 of the control means (7).

The electronic regulator (4.1) of the embodiment shown in Figure 3 is a chopper activated by means of signal S3, and comprises several IGBTs which allow connecting one or more resistors, therefore changing the ohmic value connected to the bus and regulating the current circulating through the resistive means (4.2) and the dissipated energy.

Moreover, when current passes through the resistive means (4.2), consumption of power, which dissipates into the environment, occurs. In a preferred embodiment, the resistive means (4.2) comprise a plurality of resistors which can be connected and disconnected selectively, and with different resistor setpoint values and different wiring diagrams, for example in series or in parallel, such that the resulting value of the resistor can be modified as deemed necessary; for this purpose, the control means (7) emit a control signal or series of control signals, S4, which allow modifying the total value of the electrical resistor. As a result, the power consumed depends on the total value of the connected electrical resistor.

The DC voltage Vc is converted into an AC compensation voltage V _x after going through the inverter (3.1), the AC terminals (3.1.1) of which are connected in series between the voltage source (9) and the furnace electrode (8), such that the compensation voltage V _x is subtracted from the voltage of the voltage source (9), V _h . The result is that, for a given impedance in the furnace electrode (8) Z _h and a constant feed voltage V _h , the circulating current in the furnace electrode (8) is reduced as a result of the compensation voltage V _x :

In that sense, although the voltage of the source (9) is kept constant and the impedance Z _h of the electrode (8) is not modified by moving the electrodes (8), current l _h does not increase above the maximum value allowed by the installation as a result of the start of a short circuit.

This regulation requires adapting the voltage value to the variation of the circulating current. This is achieved by means of the constant monitoring of the main variables of the installation, V _h , V _t , V _c e l _h , corresponding to the voltage applied to the furnace electrode (8), to the voltage of the voltage source (9), the voltage of the capacitive means (3.2) and the current circulating through the furnace electrode (8); as shown in Figure 4, the monitoring is carried out by means of respective voltage measuring means (5.1, 5.2, 5.3) and current measuring means (6) continuously emitting measurement signals Sv _h , Sv _t , Sv _c , S, to the control means (7). Therefore, in the event of a sudden increase in current l _h which anticipates a short circuit, the control means (7) start the control actions necessary to produce a compensation voltage V _x according to the variation of the current l _h .

The control means (7) process the signals of each phase independently, but physically they can be integrated in one and the same device common to the three phases; in any case, the control signals are emitted individually for each phase. Advantageously, the control means (7) of each phase are operatively connected to one another.

In one embodiment, the control means (7) are integrated in a single physical device with a channel for each phase. In one embodiment, the device implementing the control means (7) is a computer, a PLC, a system with microprocessors, etc.

As shown in Figure 4, the control means (7) receive the measurement signals Sv _h , Sv _t , Sv _c , S, from the measuring means (5.1 , 5.2, 5.3, 6) continuously; the control means (7) process the signals also continuously, and if the time variation of the measurement signals, particularly the current measurement signal Si, anticipates a short circuit, the control means (7), depending on the other measurement signals, determine what the compensation voltage V _x should be and emit the corresponding control signals: a control signal S1 for activating the switching means (2) so that the branch of the converter (3) is activated, a control signal S2 for activating the semiconductor devices of the inverter (3.1), and a control signal to the energy dissipator (4) for activating power dissipation.

In the embodiment in which the resistive means (4.2) are a plurality of resistors which can be selectively connected, the control means (7) additionally emit a control signal or series of control signals S4 which allow varying the total value of the resistor of the resistive means (4.2).

In one embodiment, the device comprises secondary control means (not depicted); these secondary control means operate as a distributed control for the switching means (2), the converter (3), and the energy dissipator (4), to which they are operatively connected. In this embodiment, the secondary control means are activated by means of signals S1, S2, and/or S3 emitted by the control means (7), and once activated, the secondary control means emit the trigger signals to activate or deactivate each of the semiconductor devices of the switching means (2), the converter (3), and the energy dissipator (4).

Furthermore, in another embodiment, the device comprises secondary control means (not depicted) for activating or deactivating the elements of the resistive means (4.2); these secondary control means, activated by means of signal S4, operate in the same way as the secondary control means of the switching means (2), the converter (3), and the energy dissipator (4).

Operation of the continuous current regulating device

At the start of the process and during normal operation, the switch (2.1) is open and the static bypass switch (2.2) is closed, allowing the passage of current from the voltage source (9) to the furnace (8)

Upon detection of a time variation of the current l _h which exceeds a current threshold I _hmax that allows anticipating a short-circuit current, and before the current reaches the value which would correspond to a short circuit, the control means (7) emit a control signal S1 to the switching means (2) to activate the device (1), and accordingly, the static bypass switch (2.2) opens.

At the same time the converter (3) starts up and generates, through the inverter (3.1), the compensation voltage V _x , calculated from the values of the voltage V _h , voltage V _t , voltage V _c , and current l _h . When the compensation voltage V _x is generated, the AC current circulating through the inverter causes the absorption of energy from the AC current side to the DC current side of the inverter (3.1) and the capacitive means (3.2) absorb energy and are charged, raising the voltage Vc between the terminals of the capacitive means (3.2). This energy must be dissipated to maintain the voltage Vc and protect the device, and the electronic regulator (4.1) and the resistive means (4.2) are used for that purpose. As the voltage increase, the electronic regulator (4.1) connects more resistors of the resistive means (4.2), so the total ohmic value of the resistor of the resistive means (4.2) decreases and the power absorbed and the energy dissipated increases.

The voltage V _h , resulting from subtracting V _x from the voltage of the voltage source V _t , is applied to the furnace (8), and will be such that the current resulting from the quotient between said voltage V _h and the impedance of the furnace is kept below the threshold value I _hmax . Once the short circuit ceases, or when the current l _h drops below the threshold value I _hmax , the voltage V _x is canceled and the static bypass switch (2.2) closes, returning to the initial state.

Figure 5 shows an example of the time evolution of four relevant magnitudes of the device for one and the same period of time: V _t , l _h , V _x , V _h . The relationship between voltages and currents which has been described above can be verified from this graph.

The first graph shows the signal of the supply voltage or of the voltage source (9), V _t , which is a sinusoidal voltage with a constant amplitude and phase.

The second graph shows the signal of the current l _h circulating through the furnace (8); the occurrence of a current peak at a particular point in time, which may anticipate a short circuit, can be seen in this graph.

The third graph shows the signal of the compensation voltage V _x which is generated in the device (1) in response to the current peak; the graph shows how the amplitude thereof increases practically in the moment in which the current peak of l _h starts and how the amplitude thereof changes until it is again canceled.

Lastly, the fourth graph shows the signal of the voltage V _h applied to the furnace (8) resulting from the combination of the signal of V _t and the signal of V _x . There can be seen in the central area of the graph, i.e. , when the current peak which may anticipate a short circuit is produced, the result of subtracting the compensation voltage V _x from the supply voltage or the voltage source (9), V _t , such that the voltage V _h reaching the furnace (the electrodes) is lower, and accordingly also the current L circulating through said electrodes (8) of the furnace which does not exceed the threshold I _hmax .