Arrangement for Controlling the Electrodes of an Arc Furnace

1. Field of the Invention

The invention relates to an arrangement for controlling the electrode height in an arc furnace fed from a transformer. In particular, the invention relates to an arrangement in which an impedance controller controls the height of the electrode or, in 3-phase arc furnaces, of each of the electrodes above the material being melted in response to an impedance signal obtained from one measuring element furnishing a quantity proportional to the electrode voltage measured between the secondary terminal of the transformer of the respective electrode and the furnace jacket, and another measuring element furnishing a quantity proportional to the electrode current of the same electrode.

2. The Prior Art

In known arrangements of this kind shown in German Auslegeschrift No. 1,183,185 and German Auslegeschrift No. 1,163,996, a control deviation for the impedance is obtained directly from the measured quantities proportional to the electrode voltage between the transformer terminal and the furnace jacket, and the electrode current.

It is known in addition, as shown in U.S. Pat. No. 2,367,979 and German Auslegeschrift No. 2,238,006, to vary the desired impedance value as a function of the supply voltage of the arc furnace by means of a function generator influencing the quantity proportional to the electrode voltage.

Strictly speaking, the known arrangement according to German Auslegeschrift No. 2,238,006, supra, operates in principle not with impedance control but with a linearized admittance control.

It is an object of the invention to obtain the electrode control by means of a nonlinear impedance control and to enter into the controller unequivocal action criteria for the short-circuit case and the placing of a leading electrode on the material to be melted during the start-up of the furnace.

According to the invention, this problem is solved by the provision that the control deviation is formed as the difference between a settable desired impedance value and the actual value obtained from the measurement quantities by means of a divider. The divider is followed by a lowpass filter and a first circuit component shunted across it for setting a signal that drives the controller in full in the short-circuit case. The sum of the measurement quantities is formed in a summing element, and another circuit element connected to the summing element shuts off the output of the impedance controller at the instant the sum reaches zero.

The invention will be described in greater detail in connection with the drawing in which the only FIGURE is a schematic circuit block illustrating one embodiment.

The electrodes 1, 2, 3 of a 3-phase arc furnace 4 are connected, via a stepwise variable transformer 5, to a 3-phase network that supplies sufficient electrical energy to generate melting heat in the material contained in the furnace. Each electrode of the arc furnace is equipped with a control arrangement to control its positioning, and since the control arrangements are identical, only the control arrangement for electrode 1 is shown in detail in the drawing.

Mechanical positioning of the electrode 1 is accomplished by means of a positioning system 6, which may be an electrohydraulic positioning system, for example, to control the spacing between the electrode 1 and the material to be melted. This distance corresponds approximately to the length of the arc and can be adjusted by changing the height of the electrode support arm. The positioning system 6 is connected to the output of a proportional-action impedance controller 7, the gain K of which can preferably be varied as a function of the step position of the furnace transformer 5.

A first measuring element 13 consisting of a current transformer and a rectifier furnishes a signal I.sub.1 proportional to the current flowing through the electrode, and another measuring element 14, which consists of a voltage transformer and a rectifier, furnishes a signal U.sub.1 proportional to the furnace voltage between the corresponding secondary terminal of the furnace transformer and the neutral point formed by the furnace shell. The measurement quantities proportional to the electrode current and the electrode voltage are fed through lowpass filters 11 and 12, respectively, to a divider 10 that furnishes the actual impedance value 1. The output of the divider is connected via a first-order lowpass filter 9, the time constant of which is relatively large, to a comparison point 8, in which the control deviation of the input variable for the impedance controller 7 is formed from a settable desired impedance value Z* and an actual value z.

The actual impedance value Z is also present at a switching element 15, which has an on-off characteristic and is shunted across the lowpass filter 9. If a short circuit of the electrode 1 to the bath is imminent, the switching element delivers a signal voltage that causes the impedance controller 7 to be driven fully in one direction. This happens as soon as the actual impedance value Z reaches a predetermined value z.sub.k, which is slightly above the value of the short-circuit impedance, so that the respective electrode 1 is raised at maximum velocity.

The sum of the two measurement variables I.sub.1 and U.sub.1 that have been smoothed in the lowpass filters 11 and 12 is formed continuously in a summing stage 16. The output of the impedance controller 7 is shut off by an on-off switch 17 connected to the summing stage if the sum has the value zero, and the output remains at zero potential, as indicated in the drawing by the double-throw switch 7a. As a result the electrode 1, 2, or 3 that was moving downward during the start-up of the furnace is stopped at the instant when it is the first one to reach material to be melted. At this moment, the sum of the measurement variables I.sub.1 and U.sub.1 is zero because the voltage breaks down and no current flows yet as long as only one electrode has settled.

An overcurrent control, which includes a selection element 18, a comparison point 19, and an overcurrent controller 20, engages the impedance controller 7 by way of substitution and reduces the wear on the furnace breaker, the thermal and dynamic stress of the electrodes and the influence of network reaction. The selection element 18 determines which of the currents I.sub.1, I.sub.2 or I.sub.3 of the 3-phase current is the largest. The purpose of the selection element is to determine overcurrents which occur particularly in the melting-down operation due to increases of the network voltage or to short circuits in the adjacent phases. The highest value current is compared at the comparison point 19 with the permissible current I.sub.max and the difference is fed to the overcurrent controller 20 with proportional-integral action. If the selected maximum current value exceeds the permissible value, the positive difference drives the output of the controller to its positive limit and thereby leads the variable limit device 7b, which is customarily part of the impedance controller 7, to its upper limit value. As a result, the positioning signal fed to the positioning element 6 assumes its adjustable maximum value. If the selected maximum current does not reach the permissible current, then all output signals of the impedance controller larger than the lower limit of the limiter 7b, which is set to the minimum positioning signal, address the positioning element 6. The integration constant of the overcurrent controller 20 is chosen small so that the overcurrent control will respond rapidly. The overcurrent control, which is common to all three phases, acts on the two other impedance controllers (not shown) via the connection 21 and 22.

A further limiting device 24 and 25 limits the desired impedance value z*. The limiter 25 prevents the setting of desired values smaller than the short-circuit impedance z.sub.k. This prevents the electrode control from repeatedly running the installation into a short circuit. In order to prevent the impedance control and the overcurrent control from bucking each other in the event of an overcurrent, the desired impedance value is raised by the limiting device 24 to a limit value z.sub.G which is determined in an amplifier 23 from the ratio of the electrode voltage U.sub.1 to the permissible electrode current I.sub.max.

The arrangement for controlling electrodes as described here can be used directly as a resistance control if the electrode voltage is replaced by the true arc voltage, which is not accessible by direct measurement but can be determined indirectly.