CONVERTER-FED ELECTRIC ARC FURNACE WITH CAPACITOR ASSEMBLY IN THE SECONDARY CIRCUIT

Abstract

An electrical arrangement for an electric arc furnace (1) operated with alternating current has a converter (2) which converts mains voltage (U), having a mains frequency (f), of a supply grid (3) into primary voltage (U) having a furnace frequency (f). A furnace transformer (4) of the electrical arrangement transforms the primary voltage (U) into a secondary voltage (U). The secondary voltage (U) is supplied to a number of electrodes (6) of the electric arc furnace (1). The electrodes (6) are arranged in a furnace vessel (8) of the electric arc furnace (1). They apply electric arcs (10) to a melt material (9) in the furnace vessel (8). The secondary voltage (U) is also supplied to a capacitor assembly (7) which is on the output side of the furnace transformer (4) and to which the furnace transformer (4) is connected on the output side. The converter (2) is controlled by a control device (5) such that a primary voltage (U) output from the converter (2) to the furnace transformer (4) has a furnace frequency (f) which is at least ten times the mains frequency (f) and/or is greater than 1 kHz.

Claims

1. An electrical arrangement for an electric arc furnace operated with alternating current; wherein the electrical arrangement comprises a converter, a furnace transformer, a number of electrodes and a capacitor assembly; wherein the electrodes are arranged in a furnace vessel of the electric arc furnace, such that they apply electric arcs to a melt material in the furnace vessel; wherein the converter has an input side connected to a supply grid having a network frequency (f) and a network voltage (U) and has an output side connected to the furnace transformer; wherein the furnace transformer is configured to step down a primary voltage (U) which is fed to the furnace transformer from the converter into a secondary voltage (U) which is fed to the electrodes; wherein the furnace transformer the input side is connected to the converter and the output side, is connected to the electrodes of the electric arc furnace and to the capacitor assembly; wherein a control device controls the converter such that a primary voltage (U) output from the converter to the furnace transformer has a furnace frequency (f) which is equal to at least ten times the network frequency (f) and/or is greater than 1 kHz; and wherein the furnace frequency (f) is the fundamental frequency at which the electrodes are supplied.

2. The electrical arrangement as claimed in claim 1, further comprising no controlled reactive power compensator is provided, either between the supply grid and the converter, or between the converter and the furnace transformer, or on the output side of the furnace transformer.

3. The electrical arrangement as claimed in claim 1, wherein the furnace transformer is configured as a medium-frequency transformer.

4. The electrical arrangement as claimed in claim 1, further comprising the capacitor assembly comprises capacitors, which are connected in parallel and/or in series with the electrodes of the electric arc furnace.

5. The electrical arrangement as claimed in claim 1, wherein the furnace frequency (f) lies within a resonance range of an electrical oscillating circuit which is constituted by the inductances (L) of the electrodes of the electric arc furnace and the capacitor assembly.

6. An operating method for an electric arc furnace operated with alternating current, comprising: controlling a converter by a control device, such that the converter converts a network voltage (U) at a network frequency (f) from a supply grid into a primary voltage (U) at a furnace frequency (f); wherein the furnace frequency (f) is equal to at least ten times the network frequency (f) and/or is greater than 1 kHz; wherein the primary voltage (U) is fed to a furnace transformer, which steps down the primary voltage (U) into a secondary voltage (U) not exceeding 2 kV; wherein the secondary voltage (U) is fed to a number of electrodes of the electric arc furnace arranged in a furnace vessel of the electric arc furnace, such that the electrodes apply electric arcs to a melt material in the furnace vessel; wherein the secondary voltage (U) is moreover fed to a capacitor assembly arranged on the output side of the furnace transformer; wherein the furnace frequency (f) is the fundamental frequency at which the electrodes are supplied.

7. The operating method as claimed in claim 6, further comprising no controlled reactive power compensation is executed between the supply grid and the converter, nor between the converter and the furnace transformer, nor on the output side of the furnace transformer.

8. The operating method as claimed in claim 6, wherein the furnace frequency (f) is greater than 3 kHz.

9. The operating method as claimed in claim 5, wherein the furnace frequency (f) lies within the resonance range of an electrical oscillating circuit which is constituted by the inductances (L) of the electrodes of the electric arc furnace and the capacitor assembly.

10. The operating method as claimed in claim 5, wherein the capacitor assembly comprises a plurality of capacitors, which are connected in parallel and/or in series with the electrodes of the electric arc furnace.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In the drawings, in a schematic representation:

[0036] FIG. 1 shows an electrical arrangement and an electric arc furnace,

[0037] FIG. 2 shows a frequency diagram,

[0038] FIG. 3 shows a potential capacitor assembly, and

[0039] FIG. 4 shows a further potential capacitor assembly.

DESCRIPTION OF AN EMBODIMENT

[0040] According to FIG. 1, an electrical arrangement for an electric arc furnace 1 comprises a converter 2. The converter 2 is connected on the input side to a supply grid 3, and on the output side to a furnace transformer 4. The converter 2 converts a network voltage U of the supply grid 3 into a primary voltage U. The supply grid 3 is generally a three-phase AC voltage grid. The network voltage U is generally greater than 10 kV. For example, it can assume a value of 30 or 33 kV. The converter 2 can be configured, for example, as an intermediate circuit converter. A potential configuration of the converter 2 can in particular be executed in the manner described in DE 10 2008 049 610 A1, Applicant: SIEMENS AKTIENGESELLSCHAFT, Title: Stromversorgungsanlage fr einen Drehstrom-Lichtbogenofen mit Zwischenkreisumrichter zwischen Netzanschluss and Ofentransformator, and from the corresponding Canadian Patent CA2738729C and U.S. Pat. No. 8,933,378 B2, incorporated herein by reference. However, other configurations of the converter 2 are possible.

[0041] The network voltage U has a network frequency f of customarily 50 or 60 Hz. The primary voltage U is smaller than the network voltage U. In many cases, however, the primary voltage U is only slightly lower than the network voltage U. For example, it can lie within the range of 70 to 100% of the network voltage U. However, the primary voltage U assumes a furnace frequency f, which can be selected independently of the network frequency f. In particular, the furnace frequency f can be substantially greater than the network frequency f, for example at least ten times the value thereof. The furnace frequency f can be even greater than 1 kHz, in particular greater than 3 kHz. Preferred ranges for the furnace frequency f lie between 5 and 15 kHz. However, even higher frequencies, of up to 100 kHz, are also possible. For the primary voltage U, a corresponding target value U* for the electric arc furnace 1 can be determined by a control device 5. The target value f* for the furnace frequency f can be fixedly specified, can be stipulated to the control device 5 from the exterior, or determined by the control device 5. On the basis of the target values U*, f*, the control device 5 can execute a corresponding actuation of the converter 2. It is thus possible to employ the primary voltage U as a control variable for the electric arc furnace 1.

[0042] On the grounds of the high furnace frequency f, the furnace transformer 4 is preferably configured as a medium-frequency transformer.

[0043] The primary voltage U is fed to the input side of the furnace transformer 4. The furnace transformer 4 transforms the primary voltage U into a secondary voltage U. The secondary voltage U is lower than the primary voltage U. The secondary voltage U generally lies in the range of several hundred volts, or sometimes slightly more. In specific cases, values somewhat in excess of 1 kV, but not exceeding a maximum of 2 kV, are possible. Independently of the specific value of the secondary voltage U, however, the secondary voltage U also assumes the furnace frequency f.

[0044] On the output side, the furnace transformer 4 is connected to a number of electrodes 6 of the electric arc furnace 1 and a capacitor assembly 7. In the rare instance of a single-phase AC electric arc furnace, either two electrodes 6, or a single electrode 6 and a ground anode are provided. In the customary case of a three-phase AC electric arc furnace, the number is at least three, and in general is exactly three. Independently of the number of electrodes 6, however, the electric arc furnace 1 is operated by alternating current. The electrodes 6 are arranged in a furnace vessel 8 of the electric arc furnace 1. The electrodes 6 apply arcs 10 to a melt material 9 which is situated in the furnace vessel 8. The secondary voltage U is fed to the electrodes 6 and to the capacitor assembly 7.

[0045] In any event, the supply of electricity to the electrodes 6 proceeds exclusively via the converter 2 and the furnace transformer 4. Accordingly, no other voltages or currents, potentially at different frequencies, are applied to the electrodes 6. Even if, in a specific case, any further additional application is delivered to the electrodes 6, this additional application will constitute only a small fraction (substantially below 10%, but in general even a maximum of 1%) of the electrical energy delivered to the electrodes 6. Accordingly, in this case, the lion's share of electrical energy will again be fed to the electrodes 6 via the converter 2 and the furnace transformer 4.

[0046] No further reactive power compensation is required, additionally to the capacitor assembly 7. Preferably, no controlled reactive power compensator is thus provided, either between the supply grid 3 and the converter 2, or between the converter 2 and the furnace transformer 4, or on the output side of the furnace transformer 4. This applies in particular to a TCR (thyristor controlled reactor). In many cases, additionally, no uncontrolled filter circuit is provided. However, if an uncontrolled filter circuit is provided, this is preferably arranged up-circuit of the converter 2, and is tuned to a harmonic of the network frequency f.

[0047] As a result of the inductances L of the electrodes 6 and the capacitance C of the capacitor assembly 7, the electrodes 6 of the electric arc furnace 1 and the capacitor assembly 7 constitute an oscillating electrical circuit. The oscillating electrical circuit has a resonance frequency fR, which is given by the following relationship:

[00001]$\begin{array}{cc}\mathrm{fR}=\frac{1}{2.Math..Math.\sqrt{\mathrm{LC}}}& \left(1\right)\end{array}$

[0048] The furnace frequency f, as mentioned above, is substantially greater than the network frequency f. According to the representation shown in FIG. 2, the furnace frequency f can lie within the resonance range of the oscillating electrical circuit. In particular, the following relationship can apply:

f/fR<(2)

is a value of at least 0.3. In general, lies within a range of 0.5 to 0.9. In particular, can be of the order of 0.7. is a value not exceeding 3.3. In general, lies between 1.1 and 2.0. In particular, can be of the order of 1.4. Moreover, (at least to an accuracy of approximately 10%) can be the reciprocal of .

[0049] The specific configuration of the capacitor assembly 7 can be determined as required. For example, the capacitor assembly 7, as per the representation in FIG. 3, can comprise capacitors 11 which are arranged in the conductors to the electrodes 6 themselves, and are thus connected in series with the electrodes 6 of the electrode arc furnace 1. Alternatively, the capacitor assembly 7, according to the representation in FIG. 4, can comprise capacitors 12, 13, by means of which the conductors to the electrodes 6 are connected to one another and/or to a common neutral point. The neutral point can be grounded, but must not necessarily be so. In this case, the capacitors 12, 13 are connected in parallel with the electrodes 6 of the electric arc furnace 1. Combinations of arrangements of this type are also possible.

[0050] The present invention has numerous advantages. In particular, as a result of the high furnace frequency f, the stability of the arc can be ensured over an extended operating range. The probability of internal flashover associated with uncontrolled voltage surges is also minimized. Moreover, the electrical components employed, with respect to both their size and their nominal electrical ratings (for capacitance and inductance) can be of reduced dimensioning. The arrangement of the capacitor assembly 7 on the secondary side of the furnace transformer 4 permits the power factor to be set to values of 0.95 and higher, ideally to values of 0.98 and higher. The high furnace frequency f additional permits the target value U* for the primary voltage U to be tracked very rapidlywithin the range of millisecondsand permits the correction of the primary voltage U within this time range. Accordingly, a tap changer is no longer required on the primary side of the furnace transformer 4. Moreover, the primary voltage U can be varied. Flicker is reduced. The size of the furnace transformer can also be reduced. Energy consumption can be reduced. Maintenance costs are also reduced.

[0051] Although the invention has been illustrated and described in greater detail with reference to the preferred exemplary embodiment, the invention is not limited by the examples disclosed, and further variations can be inferred by a person skilled in the art, without departing from the scope of protection of the invention.

LIST OF REFERENCE SYMBOLS

[0052] 1 Electric arc furnace [0053] 2 Converter [0054] 3 Supply grid [0055] 4 Furnace transformer [0056] 5 Control device [0057] 6 Electrodes [0058] 7 Capacitor assembly [0059] 8 Furnace vessel [0060] 9 Melt material [0061] 10 Arc [0062] 11-13 Capacitors [0063] C Capacitance [0064] f Network frequency [0065] f Furnace frequency [0066] fR Resonance frequency [0067] f* Target value for furnace frequency [0068] L Inductance [0069] U Network voltage [0070] U Primary voltage [0071] U Secondary voltage [0072] U* Target value for primary voltage [0073] , Values