JPH0654715B2 - Arc current control method for DC arc furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is to charge a raw material scrap into a furnace while applying a DC voltage to an electrode to generate an arc between the electrode and the raw material. The present invention relates to an arc current control method in a so-called DC arc furnace for steelmaking, in which a raw material is melted by using arc heat.

[Conventional technology]

As shown in FIG. 4, the DC arc furnace has a power supply configuration in which a high-voltage three-phase AC power supply is stepped down to several hundreds of volts by a rectifier transformer 1, and a rectifier 2 using a thyristor is used to obtain a direct current and a DC reactor 7 and a water-cooled cable. DC voltage is applied to the graphite electrode 3 via 8 to generate a DC arc 5 between the tip of the electrode 3 and the scrap or molten steel 4 in the furnace 9, and the arc energy melts the scrap and raises the temperature of the molten steel. Is to do. Reference numeral 6 in the figure denotes a furnace load switch.

With such a circuit configuration, if the DC arc 5 is regarded as a general load, a similar power supply configuration is often found in general industrial applications such as an electrolytic rectifier system, and is used as a method for controlling current. A method is generally used in which a direct current is measured directly or equivalently on the alternating current side and the phase of the thyristor of the rectifier 2 is controlled by a difference from a preset value, and the control response delay is several times. 10 ms ~
Designed in seconds.

Further, a so-called pulse shift circuit for forcibly shifting the ignition pulse phase of the thyristor to a phase at which the current becomes zero when an excessive current flows is provided for the purpose of protecting the equipment.

By the way, in an arc furnace, a phenomenon of so-called raw material scrap burn-off occurs during the melting process, and the electrode 3
The raw material scrap may touch to cause a short circuit. Especially, in the DC arc furnace, the melting shape becomes a pillow shape in the lower part of the furnace, so at some stage the scrap falls off in layers in a “shelf”. Due to this, the short circuit occurs frequently.

This short-circuited state has various durations ranging from a momentary (ms or less) transient to a long time (10 ms or more), and a short-circuit current flows at each occurrence.

[Problems to be Solved by the Invention]

In general industrial systems other than the arc furnace, the short-circuit phenomenon occurs only as a trouble of the equipment, so when this current flows, it is considered as equipment failure by the above-mentioned pulse shift circuit, the method of opening the breaker, etc. It is also possible to stop the system. However, in the case of an arc furnace, the short-circuit current appears as one phenomenon in the normal operation process as described above, and therefore how to handle this short-circuit current is an important point.

This is because if the system is stopped every time a short circuit occurs, the loss time until the arc is generated again becomes large, and the production efficiency is greatly reduced.

Of course, forming a system with a device rating that can continuously withstand this short-circuit current is one of the measures,
The rating itself is dramatically increased, which is a disadvantage from an economical point of view. As shown in FIG. 4, the reactor 7 is provided on the DC side to slow the rise of the short-circuit current, and control is performed during this period. However, since the reactor 7 itself has a resistance component, the reactance is too large. Can't get

The object of the present invention is to eliminate the inconvenience of the conventional example, to keep the rating of the device as small as possible, and to minimize the reactance of the DC reactor, and to suppress the short-circuit current determined by the circuit constant to a low level. An object of the present invention is to provide an arc current control method for a direct current arc furnace.

[Means for Solving the Problems]

In order to achieve the above object, the present invention provides a current detection means in a circuit for converting an AC voltage stepped down by a transformer into a direct current by a rectifier and applying it to an electrode of a DC arc furnace. The current detection level is multistage, and the magnitude of the output current of the rectifier is increased according to the detection level when the detection level is high, and decreased when the detection level is low, respectively, and the output current of the rectifier is limited to a level as low as possible. The main point is to do.

[Action]

According to the present invention, the overcurrent is detected by the current detection means at the time of a short circuit in the furnace, but the contact state between the electrode and the scrap can be detected in several ways by setting the overcurrent detection levels in multiple stages. . Then, by controlling the phase of the rectifier thyristor according to the overcurrent detection level, the rectifier output is greatly reduced when the contact state is strong, and when it is small, the rectifier output is similarly reduced to a constant level. Therefore, the short-circuit current generated at the time of short-circuiting in the arc furnace can be controlled to a level as low as possible.

〔Example〕

 Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block circuit diagram showing an embodiment of an arc current control method for a DC arc furnace according to the present invention. The present invention is the fourth
A current detector 10 was attached to the power supply circuit of the DC arc furnace shown in the figure as current detection means. This current detector 10 receives an output from a DC current transformer 11 provided on the DC output side of the rectifier 2, or converts an AC output from the secondary winding side of the transformer 1 into a DC by a converter 12. It may be converted and received.

The detection output of the current detector 10 is introduced into the multi-stage comparator 13, the output corresponding to each detection level in this comparator 13 is introduced into the pulse shift phase function oscillator 14, and further the pulse emission for firing the thyristor of the rectifier 2 The device 15 is controlled by the pulse shift phase function oscillator 14.

As described above, when the electrode 3 and scrap contact with each other,
A short circuit occurs and an overcurrent is generated in the power supply circuit, which is detected by the current detector 10 and the detection level is determined by the multistage comparator 13.

FIG. 2 is a diagram showing the current change at the time of this short circuit phenomenon.
dc jumps from the rating of the rectifier 2 when a short circuit occurs, but when this overcurrent is divided into contact resistances R ₁ , R ₂ and R ₃ due to the difference in the contact state due to the electrode 3 and scrap, the overcurrent detection levels L ₁ and L ₂ , L ₃ indicates L ₁ = R ₁ and the state where the electrode 3 and the scrap are in relatively light contact with each other, and L ₃ =
R ₃ shows a state where the contacts are fairly strong.

By the way, it is well known that the output voltage of the thyristor rectifier can be controlled by the control phase angle. The relation between typical double star connection and Gretz connection is shown below.

Double star connection Grets connection However, Ed: forward current output voltage u: commutation overlapping angle α: control phase angle These voltage output waveforms greatly change depending on the magnitude of the control phase angle α as shown in FIG.

The DC current Id has a value obtained by dividing the voltage Ed by the circuit impedance (R). However, since the circuit includes an inductance component, the current waveform has a smoothed form from the voltage waveform, and its average value (average DC The current is largely controlled by) α.

When the value of α becomes larger than the value shown in FIG. 3, the voltage Ed value approaches zero, and the value of the current Id also decreases accordingly.

The present invention relates to L ₁ , L detected by the multi-stage comparator 13
_2, none of the L ₃ levels to lower the current Idc to zero current near the value (Idc · ps).

In the Gretz connection type rectifier 2, Therefore, if Idc · ps has the same value, α must be large when R is large and α must be small when R is small.

Therefore, if R ₁ = α ₁ , R ₂ = α ₂ , R ₃ = α ₃ , the phase angle of the pulse shift in the pulse shift phase function oscillator 14 becomes α ₁ > α ₂ > α ₃ and the overcurrent detection level Is large, the phase angle α ₃ is large, and when the level is small, the phase angle α ₁ is small. As an example, L ₁ =
_{130%, L 2 = 150%} , if _{L 3} = 170%, the phase angle 100 ° when _{L 1,} 120 ° when _{L 2,} to 130 ° when _{L 3.}

〔The invention's effect〕

As described above, the arc current control method of the DC arc furnace of the present invention, in the circuit for converting the AC power source stepped down by the transformer into DC by the rectifier and applying it to the electrodes, the short-circuit current generated at the time of short-circuiting in the furnace is always It is possible to limit it to a level as low as possible, which makes it possible to economically design the equipment by making the rating as small as possible and minimizing the reactance, and also contributing to the stability of the arc.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing an embodiment of an arc current control method for a DC arc furnace of the present invention, FIG. 2 is a current waveform diagram at the time of a short circuit phenomenon, and FIG. 3 is a voltage by phase angle control of a thyristor rectifier, FIG. 4 is a circuit diagram showing a power supply configuration of a DC arc furnace, which is a waveform diagram of radio waves. 1 ... Rectifier transformer, 2 ... Rectifier 3 ... Electrode, 4 ... Scrap or molten steel 5 ... DC arc, 6 ... Furnace load switch 7 ... DC reactor, 8 ... Water cooling cable 9 ... Furnace, 10 ... current detector 11 ... DC current transformer, 12 ... converter 13 ... multi-stage comparator 14 ... pulse shift phase function oscillator 15 ... pulse oscillator

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Toshimichi Maki, 1-2, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Shoichi Takahashi, 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside the Steel Pipe Co., Ltd. (72) Inventor Kinzo Okazaki 1-1, Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Fuji Electric Co., Ltd. (56) References

Claims (1)

[Claims] 1. A circuit for converting an AC voltage stepped down by a transformer to a DC by a rectifier and applying it to an electrode of a DC arc furnace is provided with a current detection means, and the overcurrent detection level detected by the current detection means is multistage. According to the detection level, the output current of the rectifier is increased when the detection level is high, and decreased when the detection level is low, respectively, and the output current of the rectifier is limited to a level as low as possible. Arc current control method for DC arc furnace.