Device and method for reducing network reactions when an electric arc furnace is in operation

Abstract

Disclosed is a device and method for reducing network feedback during operation of an electric-arc furnace (10). The electric-arc furnace (10) has three lines (7) with one electrode (4) each. A sensor (15) for measuring the currently flowing current and a sensor (16) for measuring the currently applied voltage are provided in each line. A control and regulating unit (30) calculates an electrical actual value (E.sub.ist1, E.sub.ist2, E.sub.ist3). A semiconductor tap changer (20) is assigned to the furnace transformer (6) such that, by suitably selecting three target winding taps (T.sub.SOLL1, T.sub.SOLL2, T.sub.SOLL3) of a primary side (P) of the furnace transformer (6), a target current (I.sub.S1, I.sub.S2, I.sub.S3) is adjustable in each line (7).

Claims

1. A method for reducing network feedback during operation of an electric-arc furnace, the method comprising the steps of: measuring a current and a voltage for each of the phase conductors of a secondary side of a furnace transformer are carried out in each line; calculating a currently active electrical actual value for each line; calculating target phase voltages for each line in such a manner that, with a given electrical value, a target current can be adjusted for each line; selecting a winding tap to be adjusted and on the primary side of the furnace transformer corresponding to the required target phase voltage; carrying out a power regulation of the furnace transformer to the winding taps of the primary side to be adjusted by a semiconductor tap changer separately for all lines of the electric-arc furnace and then switching the semiconductor tap changer to the appropriate target position; controlling power independently of adjustment of the target current with a cycle time for adjusting the winding taps by the semiconductor tap changer on the primary side of the furnace transformer in the range of 10 milliseconds.

2. The method according to claim 1 wherein the winding tap to be adjusted results from a difference between a currently active winding tap on the primary side of the furnace transformer and the difference of the winding taps of the primary side of the furnace transformer.

3. The method according to claim 2 wherein a central position or an upper limit of the currently active winding tap of the transformer stage is taken into account by a superordinate process control system.

4. The method according to claim 1, wherein the electrical actual value is an impedance or an admittance.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) These and other features and advantages of the various disclosed embodiments set forth here will be more fully understood with reference to the following description and the drawings, throughout which the same reference characters designate the same elements, and in which:

(2) FIG. 1 is a schematic view of a system for smelting metal by an electric-arc furnace;

(3) FIG. 2 is a schematic diagram of the integration of the regulation of an electric-arc furnace in the initial phase of the smelting process into the overall regulation of the electric-arc furnace; and

(4) FIG. 3 is a flowchart of an electric-arc furnace regulation for reducing network feedback during operation of an electric-arc furnace.

SPECIFIC DESCRIPTION OF THE INVENTION

(5) Although the following description of the invention is made with regard to impedance as electrical value, this is not intended as a limitation of the invention.

(6) FIG. 1 shows a schematic presentation of a system 1 for smelting metal by an electric-arc furnace 10. The electric-arc furnace 10 is composed of a crucible 11 in which steel scrap is smelted and a melt 3 is produced. The crucible 11 can additionally be provided with a lid that is not illustrated. An outer wall surface 12 and lid of the crucible 11 are provided with a water cooling system. In dependence on the operating mode of the electric-arc furnace 10, the furnace has one or three electrodes 4. Three electrodes 4 are used in an alternating current electric-arc furnace 10. The following description illustrates the principle of the invention as exemplified by an alternating current electric-arc furnace. An unillustrated refractory material lines an inner wall surface 13 of the electric-arc furnace 10.

(7) The electrodes 4 are arranged on a support arm that is not illustrated, and can be inserted into the crucible 11 as required. Each of the electrodes 4 is equipped with a phase conductor 5 connected with a secondary side 6S of a furnace transformer 6. Each phase conductor 5, the respective electrode 4, and the respective electric arc formed thereby thus form a phase or a line 7 of the alternating current circuit. A primary side 6P of the furnace transformer 6 is supplied with the required high voltage from a power supply network 9. An on-load tap changer 20 that is a semiconductor tap changer is connected with the primary side 6P of the furnace transformer 6.

(8) A control and regulating unit 30 co-acts with the semiconductor tap changer 20 to switch winding taps T.sub.S1 . . . T.sub.SN of the furnace transformer 6 on the primary side 6P in such a manner that corresponding phase voltages U.sub.ist12, U.sub.ist23, and U.sub.ist31 are present, so that lines 7 are supplied with a corresponding target current I.sub.S1, I.sub.S2, and I.sub.S3. As a result, a predetermined impedance Z.sub.SOLL1, Z.sub.SOLL2, and Z.sub.SOLL3 will prevail in the lines 7. The primary side 6P of the furnace transformer 6 has the plurality of winding taps T.sub.S1 . . . T.sub.SN that are switched by semiconductor switching components S.sub.1 . . . S.sub.N of the semiconductor tap changer 20. The control and regulating unit 30 receives input from a respective current sensor 15 and a respective voltage sensor 16 on each of the lines 7 of the electric-arc furnace 10. From the input data, the control and regulating unit 30 determines the switching sequence of the semiconductor tap changer 20 such that it switches to the corresponding target position S.sub.SOLL1, S.sub.SOLL2, and S.sub.SOLL3 and thus effects the switching of the winding tap T.sub.SOLL1, T.sub.SOLL2, and T.sub.SOLL3 that is to be adjusted and that is on the primary side 6P of the furnace transformer, so that the current is adjusted in the lines 7 or in a specific line 7.

(9) Strong fluctuations of the current or of the voltage occur during the initial phase of the smelting process in the electric-arc furnace 10. The current fluctuations can be significantly reduced by the fast semiconductor tap changer 10 according to the invention.

(10) FIG. 2 is a schematic diagram of the integration of a regulation of an electric-arc furnace 10 in the initial phase of a smelting process into the overall regulation 22 of the electric-arc furnace 10. The overall regulation of the electric-arc furnace 10 is ultimately realized via the semiconductor tap changer 20. A superordinate process control system 24 works at a frequency in the range of 1 second. Flicker regulation 28 works at a frequency in the range of 10 milliseconds. The frequency for each of the regulations corresponds to the repetition rate of the corresponding regulations. As a result of the measurements, it is possible by the semiconductor tap changer 20 to switch over to the appropriate winding tap T.sub.S1 . . . T.sub.SN on a primary side 6P of the furnace transformer 6 for carrying out the required regulation of the electric-arc furnace 10 in such a manner that current fluctuations are minimized.

(11) FIG. 3 a flowchart of an electric-arc furnace 10 regulation for obviating or reducing network feedback during operation of the electric-arc furnace 10. In the first step 31, a current measurement and a voltage measurement are carried out for each of the phase conductors 5 that lead from a secondary side 6S of the furnace transformer 6 to the electrodes 4. The current measurement and the voltage measurement are thus carried out in each line 7.

(12) In a second step 32, a currently active impedance Z.sub.ist1, Z.sub.ist2, and Z.sub.ist3 is calculated for each line 7. In a subsequent step 33, three phase voltages U.sub.ist12, U.sub.ist23, and U.sub.ist31 are calculated in such a manner that, with given present impedances Z.sub.ist1, Z.sub.ist2, Z.sub.ist3 it is possible to adjust a target current I.sub.S1, I.sub.S2, I.sub.S3 in each line 7. According to a fourth step 34, a difference in the winding taps ?T.sub.S1, ?T.sub.S2, and ?T.sub.S3 of a primary side 6P of the furnace transformer 6 are selected in such a manner that fluctuations of the measured currents and voltages can only be taken into account outside a defined range of fluctuation. In a final step 35, the result for each line 7 is a winding tap T.sub.SOLL1, T.sub.SOLL2, T.sub.SOLL3 that is to be adjusted and that is on the primary side 6P of the furnace transformer 6. In order to effect a reduction of the flicker in each line, the winding tap T.sub.SOLL1, T.sub.SOLL2, T.sub.SOLL3 is calculated from a difference between a currently active stage T.sub.A1, T.sub.A2, and T.sub.A3 on the primary side 6P of the furnace transformer 6 and the difference of the winding taps ?T.sub.S1, ?T.sub.S2, and ?T.sub.S3 on the primary side 6P of the furnace transformer 6. The semiconductor tap changer 20 enables a quick adjustment of the required winding tap T.sub.SOLL1, T.sub.SOLL2, or T.sub.SOLL3, thereby spanning several winding taps. The semiconductor tap changer 20 makes it possible to carry out the readjustment of the winding taps T.sub.SOLL1, T.sub.SOLL2, or T.sub.SOLL3 that are to be adjusted and that are on the primary side 6P of the furnace transformer 6.

(13) Only by using the semiconductor tap changer 20 is it possible to achieve a cycle time in the range of 10 milliseconds for adjusting the required winding tap T.sub.SOLL1, T.sub.SOLL2, and T.sub.SOLL3 on the primary side 6P of the furnace transformer 8.

(14) The invention was described with reference to two embodiments. Those skilled in the art will appreciate that changes and modifications of the invention can be made without departing from the scope of protection of the following claims.