WO2017085330A1

WO2017085330A1 - Energy supply system for an electric furnace

Info

Publication number: WO2017085330A1
Application number: PCT/EP2016/078361
Authority: WO
Inventors: Mircea Ionita; Roland König; Martin Köneke
Original assignee: Sms Group Gmbh
Priority date: 2015-11-19
Filing date: 2016-11-21
Publication date: 2017-05-26

Abstract

The invention relates to an energy supply device for an electric furnace, in particular a reduction furnace having at least one electrode. In order to guarantee an effective input of power into the electric furnace during an immersion of the electrodes into the melt or into highly conductive slag, the energy supply device according to the invention has a secondary power regulation device, which is designed to keep constant the power output to the electrode, in particular the effective power, even in the event of an increase in the electrode current, or at least to only allow same to increase in a controlled manner via appropriate decreasing of the electrode voltage.

Description

Energy supply system for an electric furnace

The invention relates to a power supply device for an electric furnace with reduction processes, for. Slag cleaning furnaces, FeCr-AC furnaces or FeNi-Submerged Arc Furnaces SAF. In doing so, the electrodes typically dip into slag that floats on a melt in the furnace. In the prior art, such a power supply device z. B. from WO 2007/048502 A1. This application discloses a control apparatus for an AC reduction furnace. The power supply device has at least one transformer with a primary side for connection to a supplied supply voltage and a secondary side for providing a secondary voltage. In addition to the transformer, the disclosed power supply device comprises two control devices for regulating the power input into the electric arc furnace. A first Regelbzw. Control means controls a height adjustment for the electrodes in the electric arc furnace. A second closed-loop power control device connected between the secondary side and the electrode is designed to correct short fluctuations of electronic parameters. This secondary-side Leistungsregelungseinnchtung has a phase control, with which it is possible to control the rms value of the secondary currents continuously. The phase control is very fast compared to the previously mentioned and also occurring height adjustment of the electrodes. With the aid of the phase-angle control, it is possible to respond more quickly to a change in the electronic parameters of the process and thus to stabilize the furnace output. The said height adjustment of the electrodes within the electric arc furnace takes place against the background that when lowering the Electrode in the melt, the material resistance, z. B. the slag resistance between two electrodes decreases. Thereafter, the electrode current automatically increases for the same power, as far as this is permitted by the corresponding electronic circuit. When the current is limited, the only way to achieve a desired further increase in active power is to allow a further increase in the secondary voltage provided by the furnace transformer.

In some cases, especially when the electrodes of the furnace are immersed in highly conductive slag, e.g. As in cleaning furnaces for copper slag, or in particularly deeply immersed electrodes, however, there is the problem that - if the secondary voltage rises above a predetermined threshold - the power input into the electric furnace can no longer be effective, due to unwanted current flows within the slag between two electrodes , The said threshold value of the secondary voltage depends on the furnace design, the chemical composition of the melt, the charged materials and possibly other parameters. In other words: deep electrodes cause high electrode currents and, as a result, too high reactive power and, as a result, high apparent power. Much of the input power does not arrive in the lower furnace area because it "fizzles out" and virtually dissipates, partly because it heats false slag zones, such as on the surface.

In other words, in the metallurgical processes in which the electrodes are immersed in very conductive slags, the known control system, in which the power input is controlled by tap changer on the transformer and by changing the height position of the electrodes, reaches its limits, especially with low resistances. The controllability is then very difficult. However, the choice of higher voltages leads In addition, the current still "finds additional ways", ie, for example, via beds (coke, Möller, ...) or via slag flows between the electrodes and form unwanted micro-arcs The invention is based on the object, a known This object is achieved according to the invention in that the power control device is designed to keep the power delivered to the electrode, in particular active power, constant even with an increase in the electrode current or only in the case of an increase in the electrode current to rise in a controlled manner by suitable lowering of the electrode voltage.

When immersing the electrodes in the melt, or at least in a floating conductive slag floating on it, an increase in the electrode current at the same power is unavoidable. However, the power control device according to the invention advantageously makes it possible to keep the power introduced into the melt, in particular active power, constant, or at least to increase it only in a controlled manner. This is especially true with unchanged setting of the tap changer on the transformer.

The present invention aims to at least indirectly limit the current and maintain the voltage at the lowest possible level, even with large electrode movements. Due to the immersion of the electrodes in the Möller and the slag, no arcs are usually formed; In this respect, the power control device according to the invention is also used at low powers, without arcs being present. The power control is used to control the power input, especially in the Möller and / or the melt. This differs the power control according to the invention of the power control of WO 2007/048502 A1, which serves to keep the current and the voltage at a high voltage level constant with minimal electrode movements. In the WO document the power control is used to stabilize or maintain the arc in the oven.

According to a first embodiment, the power control device comprises a thyristor circuit with a phase control for varying the firing angle of the thyristor. Thus, the electrode current, the electrode voltage as well as the active power input into the furnace can be varied from their original values, as provided on the secondary side of the furnace transformer. The energy supply device according to the invention is suitable for any forms of electric arc furnaces; These can be round, rectangular, oval or shaped differently. By virtue of the power control device according to the invention, which keeps the active electrical power introduced into the lower furnace area constant or only rises in a controlled manner, a limitation of the maximum current is simultaneously achieved, in particular even if the electrodes penetrate deep into the melt or at least into the conductive slag floating on the melt, immerse. Said current limitation, which occurs indirectly in the power control according to the invention, is dependent on parameters of the metallurgical process given in "real time".

However, the invention is not limited to the operation of electric arc furnaces with deep plunging electrodes or arcs, but rather the said power supply or the power control device according to the invention can also be used for furnaces, in whose operation an open arc is formed, ie where the electrode is not immersed in the melt or slag. The description is a total of 8 figures attached, where

Figure 1 shows the energy supply device according to the invention for an electric furnace with lowerable electrodes;

Figures 2-4 the power supply devices with three

single-phase transformers; Figures 5 - 7, the energy supply device according to the invention for

Electric arc furnaces with one each

Three-phase transformer; and

Figure 8 current and voltage waveforms when used

Power control device according to the invention show.

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

FIG. 1 shows an electric furnace 200 in which melt 220, electrically highly conductive slag 230 floating on the melt and melt 240 floating on the melt is located. In the Möller and the slag dip two electrodes 210, which are supplied via a transformer 1 10 with electrical energy. The electrical power provided on the secondary side is adjustable, typically via a tap changer. The secondary side of the transformer 1 10 has two taps, with each of which one of the electrodes is connected via an electrical branch. In one of the branches, the power control device 120 according to the invention is connected. This is formed, the electrical power delivered to the electrodes, in particular, the active power to keep constant even at an increase in the electrode current or at least to increase only controlled by suitable lowering of the electrode voltage. An increase in the electrode current is unavoidable upon immersion of the electrodes 210 in the Möller 240 or in the slag 230, because this reduces the electrical resistance. The electrical resistance depends, in particular, on the depth of immersion Tiefeni, depth 2 of the electrodes in the Möller or the slag and on the electrical conductivity of the Möllers or slag. Figures 2 to 4 show the power supply device according to the invention for an electric furnace 200, which is operated with three single-phase transformers 1 10.

FIGS. 5 to 7 show the operation of the electric furnace 200, each with a three-phase transformer 110.

In each of FIGS. 2 to 7, a supply voltage U is provided for operating the energy supply device and the electric furnace. The supply voltage is an input to a furnace switch 140 for switching on and off the electric furnace. A tap changer 130 and the transformer 1 10 typically form a structural unit. Regardless of whether three single-phase transformers or in each case only a three-phase transformer is / is provided, the electrodes on the secondary side of the transformer 1 10 may be interconnected, for example in a Knappsackschaltung (Figures 2 and 5), a star or a delta connection. The electrodes can also simply be connected to the taps of the phases on the secondary side of the transformer. In any case, three power control devices 120 designed according to the invention are connected between the secondary side of the transformer and the electrodes, one for two phases. The operation of the power control device according to the invention is illustrated graphically in FIG. Figure 8 is a graph plotted on the left ordinate with the secondary voltage of the furnace transformer and the thyristor voltage. On the right-hand ordinate the ignition angle for driving the thyristors, the electrode current and the active power are plotted. On the X-axis, the electrical resistance, in particular slag and melt resistance is shown, which decreases further from left to right, ie with increasing immersion depth. This drop in the ohmic resistance, in this case for example from 7 mOhm to 3 mOhm, inevitably leads to an increase in the electrode current, here by way of example from 29 kA to 42 kA. This phenomenon occurs at a constant voltage tap on the tap changer 130 of the transformer, which tap here is at 222 V and is independent of the immersion depth. By way of example, the power control device according to the invention effects that the electrical active power P introduced into the furnace remains essentially constant, in FIG. 8 at +/- 6.0 MW. This is effected by the power control device according to the invention in that they are the thyristors with increasing depth of immersion with an increasingly larger ignition angle, in Fig. 8, for example, accelerated from 36 ° to 84 °, whereby the output voltage decreases with increasing immersion depth of the electrodes, according to Figure 8 of initially 217 V at 7 mOhm to 169 V at 3 mOhm. As a result of the said lowering of the thyristor output voltage, ie essentially of the electrode voltage at the same time as unavoidable current increase, the entry of the active power into the furnace can be kept constant as desired.

LIST OF REFERENCE NUMBERS

100 power supply device

1 10 transformer

120 performance control

130 tap-changer, integrated in each transformer 140 Oven switchgear

200 electric oven

210 electrode

220 melt

230 slag

240 coins

U supply voltage

Claims

claims:

1 . Energy supply device (100) for an electric furnace (200) with

at least one electrode (210) comprising:

at least one transformer (110) having a primary side for connection to a supplied supply voltage (U) and a secondary side for providing a secondary voltage,

at least one power control device (120) connected between the secondary side and the electrode (210) for controlling the electrical power delivered to the electrode, in particular active power; characterized,

in that the power control device (120) is designed to keep the power delivered to the electrode, in particular active power, constant even when the electrode current increases or only to increase it in a controlled manner by suitably lowering the electrode voltage.

2. Energy supply device (100) according to claim 1,

characterized,

in that the power control device (120) has a thyristor circuit with a phase control control for lowering the

preferably effective electrode voltage.

3. Energy supply device (100) according to one of the preceding claims,

characterized,

that three single-phase transformers (1 10-1, 1 10-2, 1 10-3) are provided, each having a primary and a secondary side, which together form a three-phase device;

that three electrodes (210) are provided on the three Secondary sides are connected in a Knappsackschaltung, and that each of the three electrodes is associated with a power control device (120).

4. Power supply device (100) according to claim 2,

characterized,

that three single-phase transformers each having a primary and a secondary side are provided, which together form a three-phase current device;

that six electrodes (210) are provided, each two

Electrodes are connected to two taps on each secondary side; and that one of each of the three single-phase transformers

Power control device (120) is connected between the secondary side and one of the two electrodes.

5. Power supply device (100) according to claim 2,

characterized,

that the transformer (1 10) is designed as a three-phase transformer having a primary side and a three-phase secondary side;

that three electrodes (210) are provided, which are connected to the three-phase

Secondary side are connected in a triangular circuit; and

in that three power control devices (120) are provided, wherein in each of the three branches of the delta circuit one of the

Power control devices (120) is arranged.

6. energy supply device (100) according to claim 2,

characterized,

that the transformer (1 10) is designed as a three-phase transformer having a primary side and a three-phase secondary side with two taps per phase; that six electrodes (210) are provided, wherein one of the electrodes is connected to each of the taps; and

in that three power control devices (120) are provided, wherein in each of the three phases on the secondary side one of the

Power control devices (120) between the secondary side and one of the electrodes (210) is connected.

7. Energy supply device (100) according to one of the preceding claims,

marked by

a preferably each of the transformers (1 10) associated tap changer (140) for adjusting a secondary voltage.

8. Method for operating a secondary side

Power control device (120) of a power supply device (100) according to one of the preceding claims, characterized by the following steps:

Lowering the effective electrode voltage as the electrode current increases.