EP1915889B1 - Electronic circuit and method for electric power supply to an alternative current electric furnace - Google Patents

Abstract

The invention relates to an electronic circuit and a method for feeding power to at least one electrode of an alternating-current electric-arc furnace, particularly for melting metal. Known circuits of this type typically comprise a series connection with a transformer for providing a supply voltage for the electric-arc furnace from a power grid (1) and a AC power controller (8) connected between the transformer (6) and the electrode (11) for regulating the current through the electrode (11). According to the invention, a further development for such electronic circuits is proposed, which development has a simple design, is inexpensive and prevents overload of the AC power controller (8) even in operating modes of the electric-arc furnaces at high electrode currents. This further development provides to bypass the AC power controller with a bypass switch (9) that is opened or closed with the help of a controller as a function of the amount of current flowing through the electrode (11).

Description

The invention relates to an electronic circuit and a method for feeding at least one electrode of an AC electric furnace, in particular for melting metal with energy.

The invention is applicable to electric furnaces for the production of non-ferrous metals, iron alloys, process slags, steel and slag cleaning. The electric ovens can be designed as electric reduction ovens, as electric low-shaft ovens or as electric arc furnaces.

Such an electronic circuit for feeding an AC electric furnace is known from German Offenlegungsschrift DE 2 034 874 known. The disclosed there electronic circuit is connected between a power grid and the at least one electrode of the electric furnace. It comprises a series circuit consisting of an on / off switch for the electric furnace, a transformer for providing a supply voltage for the electric furnace from the power grid and an AC controller connected between the transformer and the electrode for controlling the current through the electrode.

An alternating current controller typically consists of two antiparallel-connected thyristors and realizes the current control in the form of a phase control. The thyristors, which realize the power part of the current controller, are typically designed for the entire working range of the electric furnace, that is to say a very large current range. Especially at High-performance ovens, which are operated with high supply voltages, are usually required due to the high thyristor blocking voltages very expensive series of thyristors. However, high blocking-voltage thyristors typically can not switch large currents; For switching large currents, as they can certainly occur in certain operating conditions, especially in a resistance operation, the electric furnace, therefore, many individual thyristors or whole AC power controller must be connected in parallel. This is the only way to achieve the high electrode currents required for at least individual operating states. In order to ensure a reliable operation of the electric furnace in all operating conditions, especially at high electrode currents, traditionally expensive and expensive converter circuits are therefore required.

Based on this prior art, the present invention seeks to develop a known electronic circuit and a method for feeding electrical energy into an AC electric furnace constructively simple and inexpensive to the effect that operation of the electric furnace in all operating conditions, especially at high Electrode currents, easily possible.

This object is solved by the subject matter of patent claim 1. Accordingly, an electronic circuit according to the invention for feeding an AC electric furnace is characterized by a current measuring device for measuring the amount of current flowing through the electrode, a bypass circuit breaker connected in parallel with the AC plate, and a control device for opening or closing the bypass Circuit breaker according to the amount of current flowing through the electrode.

The mentioned characteristic features are very simple and thus inexpensive to implement. In their claimed configuration, they advantageously enable bridging of the AC power actuator in the event of imminent danger Overload, that is during operating conditions of the electric furnace, which require a particularly large electrode current. Advantageously, these operating conditions, such as a resistance operation with immersed electrodes and no arc content, no special control of the electrode current through the AC power controller; its function is then dispensable and then, as claimed, bridged. In other operating states of the electric furnace, for example during a resistance operation with an arc component, the bypass switch is opened according to the invention, with the result that the electrode current is then passed through the AC power controller and can be controlled by this. Typically, the amount of current through the electrode during operation with arc is less than during a resistance operation without arc.

Due to the realized by the bridging disconnect switch according to the invention limitation of the current through the AC controller this can advantageously be much smaller dimensions and made easier and cheaper, without this being a limitation for the operation of the electric furnace connected.

The provision of additional circuit breakers immediately in front of and behind the AC power controller, but still between the terminals of the bypass circuit breaker, offers the advantage that when the bypass isolator is closed, that is, when the AC power is bypassed, this, for example, for maintenance of the electronic circuit can be removed without the electrode current and thus the operation of the electric furnace should be interrupted.

The inventive provision of the bypass switch, the electronic circuit is adapted to different operating conditions of the electric furnace, as they arise due to metallurgical requirements, very simple and inexpensive.

The above object is further achieved by a claimed method for feeding electrical energy into an AC electric furnace or in the electrode thereof. The advantages of this method correspond to the advantages mentioned above with respect to the claimed electronic circuit.

Advantageous embodiments of both the electronic circuit as well as the method are the subject of the dependent claims.

Figur 1

den erfindungsgemäßen elektronischen Schaltkreis;

Figur 2

ein typisches Spannungs-Strom-Leistungsdiagramm U-I-P-Diagramm für einen Elektroreduktionsofen;

Figur 3

einen Querschnitt durch die Elektrode und Schmelze in einem Elektroofen sowie ein zugehöriges elektrisches Ersatzschaltbild für diesen Abschnitt des Elektrodenstromes; und

Figur 4

das Diagramm nach Figur 2 mit zusätzlich eingezeichneten unterschiedlichen Betriebsbereichen des Elektroofens und eingezeichnetem Stromschwellenwert

zeigt.The description is a total of 4 figures attached, wherein

FIG. 1

the electronic circuit according to the invention;

FIG. 2

a typical voltage-current-power diagram UIP-diagram for an electroreduction furnace;

FIG. 3

a cross section through the electrode and melt in an electric furnace and an associated electrical equivalent circuit diagram for this portion of the electrode current; and

FIG. 4

the diagram after FIG. 2 with additionally marked different operating ranges of the electric furnace and drawn current threshold

shows.

The invention will now be described in detail in the form of embodiments with reference to said figures.

Typically, electric furnaces with three or six electrodes are used to melt steel. For ovens with six electrodes, the electrodes become 11 interconnected for energy input into the furnace vessel 12 in pairs. In electric ovens with 3 electrodes 11, the electrodes are usually connected in a knapsack circuit to reduce the reactance of the high current line. As an alternative to the Knappsackschaltung but also a star connection of the electrodes is possible.

FIG. 1 shows the electronic circuit according to the invention for feeding electrical energy into an electric furnace. FIG. 1 shows a single-phase representation; corresponding circuits could be provided for further phases.

The power supply for the electric furnace usually takes place from a medium-voltage network 1. Between the medium-voltage network and the electrode 11, the electronic circuit comprises a furnace transformer 6, which with its primary side the medium-voltage network 1, hereinafter also called power network, and facing with its secondary side of the electrode 11. Between the power grid 1 and the primary side of the furnace transformer 6, the electronic circuit comprises a first series circuit comprising a voltage measuring device 2, a furnace circuit breaker 3 for switching on or off the electric furnace, a current measuring device 4, optionally a star / delta switch for selectively switching the primary winding of the furnace transformer in a star or delta connection and an overvoltage protection 13. The star / delta switch allows a shift of the rated voltage range of the furnace transformer 6 by, for example, a factor of 1.73 upwards or downwards.

Between the secondary side of the furnace transformer 6 and the electrode 11, the electronic circuit essentially comprises a second series circuit consisting of a first circuit breaker 10a, an alternating current controller 8 and a second circuit breaker 10b. The circuit breakers 10a and 10b allow for closed high-current circuit breaker 9 an electrical separation or removal of the AC power controller 8; for example For maintenance, without the furnace operation, in particular the resistance operation with immersed electrodes and without arc portion should be interrupted for it. The AC power controller 8 allows control of the electrode current in the form of a phase control.

According to the invention, the electronic circuit has been supplemented by a bypass switch 9, which is connected in parallel with the alternating current controller 8 and optionally also in parallel with the first and second disconnectors 10a and 10b, which is controlled by a control device 14. This controls the bypass switch 9 in accordance with the measured by the current measuring device 4 amount of current flowing through the electrode 11 current. The controller 14 may be implemented in the form of a stored program controller, a process control system, or other computerized system.

After the construction of the electronic circuit, the operation of the electric furnace in cooperation with the electronic circuit according to the invention will now be described in detail.

FIG. 2 shows a typical voltage-current-power diagram UIP-diagram for a 6-electrode electroreduction furnace. In this diagram, effective power lines 100 as a function of the secondary current, plotted on the ordinate axis, and the secondary voltages, plotted on the abscissa axis, are shown. The family of curves 200 identifies the furnace resistance. The short-circuit impedance of the electric furnace is symbolized here by the characteristic 300. These characteristics in the diagram are only valid for a constant thyristor firing angle. At a larger or smaller ignition angle, the characteristics shift over the abscissa.

The characteristic curves 4a and 4b show the maximum permissible current through the electrode as a function of the secondary voltage for the primary-side star connection of the transformer windings 4a and for the primary-side triangular circuit the transformers of the transformer windings 4b. Characteristic curve 500 illustrates the maximum rated current of the AC converter 8 according to the invention, that is to say the current threshold value.

1. a) Widerstandsbetrieb mit eingetauchten Elektroden und ohne Lichtbogenanteil;
2. b) Widerstandbetrieb mit nur geringem Lichtbogenanteil; und
3. c) Betrieb mit hohem Lichtbogenanteil.

Depending on the process, feedstocks and products, an electric furnace can typically be divided into essentially the following metallurgical operating states:

1. a) resistance operation with immersed electrodes and without arc content;
2. b) resistance operation with only a small arc fraction; and
3. c) operation with high arc content.

These three operating states are explained in more detail below:

Resistance operation with immersed electrodes and without arc content.

The energy required for the process is generated by resistance heating of the slag. The electrodes 11 are clearly submerged in the slag, the immersion depth is dependent inter alia on the electrode diameter; However, it is usually about 200 mm. In this mode of operation, electrical current is passed through the slag, converting electrical energy into Joule heat due to the electrical resistance of the slag, which promotes a metallurgical endothermic reaction, for example, reduction and melting. The resistance operation with immersed electrodes and without arc portion is characterized by high electrode currents and relatively low secondary voltages, which are well below 1000 V.

In this mode, there is no special requirement for the control due to the immersed electrodes. The electric furnace can therefore also be operated conventionally, that is without current control. Therefore, it is recommended during this operation, the bypass switch. 9 close and so to bridge the AC power 8. In this way, the power semiconductors, typically thyristors, in the AC power controller 8 are protected from excessive currents.

Resistance operation with only low arc content

The majority of the energy required for this operation of the electric furnace is generated by resistance heating of the slag. Electric current is passed through the slag, converting the electric energy into Joule heat through the resistance of the slag. The Joule heat thereby promotes a metallurgical endothermic reaction, for example reduction and melting. An additional smaller energy input can be effected by an arc occurring in the lower region of the electrodes or below them. This is possible with only minimally immersed electrodes or at an electrode position directly above the slag bath. For this mode of operation usually relatively large currents and comparatively low voltages are required; please refer FIG. 4 , Area b). However, the voltages in this mode are significantly higher than with immersed electrodes. Specifically, the secondary voltages are typically in the range of about 1000 V in furnaces of about 30 - 50 MW capacity.

Resistance operation with high arc content

In this mode, a larger proportion of the energy input takes place by arcing. The arcs transmit their radiant heat directly to the Möller and slag layer of the furnace. In principle, a distinction is made between a driving style with an open arc and a driving mode with a covered arc.

In the open arc mode of operation, the arc strikes Möller Mö or slag S without the use of side heat radiation; see figure 3, where N represents the area of the uncovered arc. In FIG. 3 Also shown is an electrical equivalent circuit diagram for the electrical path through the electrode 11, the arc L, the slag S and the molten metal 15. In an idealized view, the ohmic resistance of the electrode 11 and the molten metal 15 can be assumed to be zero. There then remain for the electrode current an ohmic resistance R _L due to the arc L and an ohmic resistance R _S through the slag S.

When driving with covered arc of the edge region of the electrode 11 is partially covered by Moeller Mö; see in FIG. 3 the right edge of the electrode. In addition to the arc energy, an approximately equal or smaller proportion of the introduced energy is entered via the resistance heating in the electrodes. For the described operation mode with high arc fraction usually low currents at high voltages are necessary; please refer FIG. 4 Area c).

In the case of furnaces with outputs above 30-50 MW, the voltages are usually above 1000 V. There are high demands on the electrode current control due to the non-linear and stochastic behavior of the arcs with a tendency to instability. In the operating mode c), the entire required electrode current is guided and regulated via the alternating current controller 8. The high-current circuit breaker 9 is opened here.

The transition between modes b) and c) is fluent. In principle, only when increasing the power by increasing the secondary voltage of the transformer 6, with increasing proportion of the arc L at the energy input, see FIG. 3 , and when the current threshold value 300 for the electrode current falls below the bypass switch 9 is opened and the first and second disconnect switches 10a, 10b are closed. In this way, the AC controller is then switched on and serves to optimize the energy input. Conversely, the AC power 8 must with a reduction in the energy input through the arc, with a reduction in the secondary voltage and with increasing electrode current, that is, in principle, when the current threshold value is exceeded by the electrode current, again in time to be taken out of the electrical circuit. Basically, the current threshold 300 for opening the bypass switch 9 is identical to the current threshold for closing the bypass switch. However, different current threshold values, for example combined in a hysteresis, are also conceivable for the two processes.

FIG. 4 shows analogously to FIG. 2 an example of the dimensioning of the electronic circuit according to the invention for energy input into an electric furnace with 6 electrodes for a FeNi process with 129 MVA. Just like in FIG. 2 Here, too, the characteristic curve 300 indicates the maximum current through the alternating current controller 8 and thus the current threshold value for the switching over of the bypass isolating switch 9. The alternating current controller 8 is closed at electrode currents above this threshold value, whereby the alternating current controller is then electrically relieved. This has the advantage that the AC power controller 8 overall and in particular its power semiconductor can be dimensioned considerably smaller, whereby a simple and inexpensive solution is possible.

Even if the electric furnaces, in particular electric reduction ovens for the operating modes b) and c) are designed, they can, however, be operated in the region of a starting operation and a partial load operation with a closed bypass switch 9, that is to say with a bridged alternating current controller 8.

Claims (8)

1. Electronic circuit for the supply of electrical energy to a least one electrode (11) of an alternating current electric furnace, particularly for smelting metal, comprising a serial connection of: a transformer (6) for providing a supply voltage for the electric furnace from a power mains (1); and an alternating current controller (8) for regulating the current through the electrode (11); characterised by a current measuring device (4) for measuring the amount of the current flowing through the electrode; a bridging-over isolating switch (9) connected in parallel with the alternating current controller (8); and a control device (14) for opening or closing the bridging-over isolating switch (9) depending on the amount of current flowing through the electrode (11).
2. Electronic circuit according to claim 1, characterised in that a first isolating switch (10b) is connected between the transformer (6) and the alternating current controller (8) and a second isolating switch (10b) is connected between the alternating current controller (8) and the electrode (11).
3. Electronic circuit according to claim 2, characterised in that the bridging-over isolating switch (9) is so connected that it bridges over the serial connection of the first isolating switch (10a), the alternating current controller (8) and the second isolating switch (10b).
4. Method of supplying electrical energy to at least one electrode (11) of an alternating current electric furnace, particularly for smelting metal, comprising the following steps: providing a supply voltage for the electric furnace from a power mains (1); and regulating the current through the electrode (11) with the help of an alternating current controller (8); characterised by measuring the amount of the current flowing through the electrode and bridging over the alternating current controller, by a short-circuit connection, depending on the amount of the measure current.
5. Method according to claim 4, characterised in that the alternating current controller (8) is bridged over when the amount of current through the electrode (11) exceeds a predetermined current threshold value.
6. Method according to claim 5, characterised in that the electric furnace in start-up operation, keeping-warm operation or resistance operation is operated without an arc component when the amount of the current lies above the predetermined current threshold value.
7. Method according to claim 7, characterised in that the electric furnace is operated in a resistance operation with an arc component when the amount of current lies below the predetermined current threshold value.
8. Method according to any one of claims 4 to 7, characterised in that the alternating current controller (8) can when it is bridged over, be removed during operation of the electric furnace, for example for maintenance purposes, from an electronic circuit for supply the electrode

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