JP6132181B2 - DC arc melting furnace power control method - Google Patents

Description

  The present invention relates to a power supply control method for a DC arc melting furnace, and more particularly to a power supply control method for a DC arc melting furnace for ensuring operational stability even when an instantaneous voltage drop occurs in a primary power supply system.

  There is an arc melting furnace that generates an arc in a furnace body to melt a metal to be melted. Such arc melting furnaces include an AC arc melting furnace and a DC arc melting furnace. Compared with an AC arc melting furnace that repeats arc extinction and re-ignition when the polarity of AC is reversed, the DC arc melting furnace Has excellent arc stability and continuous operability. Also, DC arc melting furnaces should use solid-state semiconductor rectifiers such as diodes and thyristors even when attempting to maintain stable operation by maintaining the arc current and arc voltage and keeping the input energy to the furnace constant. This makes it possible to finely control the arc and has excellent controllability in continuous operation.

  For example, Patent Document 1 discloses a DC arc melting furnace using a thyristor rectifier and a control method thereof. The AC power source transformed by the transformer is converted to DC by a thyristor rectifier, the negative electrode is connected to the upper electrode via the reactor, and the positive electrode is connected to the furnace bottom electrode under the metal to be melted. As a result, arc discharge occurs between the upper electrode and the metal to be melted in the arc melting furnace to melt the metal to be melted. Here, the current control unit compares the preset set current and the detected current (arc current), controls the firing angle of the thyristor rectifier, and controls to maintain the arc current at the set current. On the other hand, the electrode lifting / lowering control unit compares the set voltage set in advance with the detected voltage, calculates the deviation, and controls the lifting / lowering of the electrode. Since the firing angle of the thyristor rectifier can be changed in a very short time of about several milliseconds in general, the arc control can be made fine.

  By the way, when a voltage drop (hereinafter referred to as “instantaneous drop”) occurs for a short time of about several ms to several hundred ms in a primary power supply system as a commercial power supply due to a lightning strike or the like, The output voltage of the rectifier also decreases, and the arc may be disturbed and extinguished. In such a case, if the field of use is relatively small power capacity, the direct current stored in the power storage device can be converted to alternating current by an inverter, and measures can be taken to compensate for the voltage drop of the primary power supply system. . However, in an arc melting furnace with a large power capacity, it is necessary to increase the scale of the power storage device, which cannot be a practical measure.

  Thus, for example, Patent Document 2 discloses a power source control method for a DC arc melting furnace that maintains an arc even during a momentary drop and enables re-ignition even when the arc is extinguished. . In arc melting furnaces, we focus on the ability to re-ignite the arc as soon as the power is restored if there is sufficient remaining ions on the arc path. At least within this time, the power supply for the arc The device is expected to remain operational in preparation for power recovery. Specifically, in a DC arc melting furnace in which a power source is connected to an electrode via a thyristor rectifier to generate an arc so that a control signal can be continuously supplied to the gate of the thyristor rectifier within a predetermined time after the occurrence of a sag. If so. One method is to use power stored in the power storage device in advance to supply a control signal to the gate of the thyristor rectifier.

Japanese Patent Application Laid-Open No. 08-0698777 Japanese Patent Application Laid-Open No. 07-263139

  In an arc melting furnace, once a sufficient amount of ions remain on the arc path, it is relatively easy to re-ignite the arc once extinguished. However, the state of ions on the arc path is likely to change. In particular, since the arc path itself changes greatly due to the movement of the metal to be melted, the state of ions also changes greatly. If it changes, the complicated control for the re-ignition of an arc will be needed.

  The present invention has been made in view of such a situation, and an object thereof is a DC arc melting furnace for ensuring operational stability even when an instantaneous voltage drop occurs in a primary power supply system. The power supply control method is provided.

  The power control method for a DC arc melting furnace according to the present invention is a direct current that melts a material to be melted by introducing an AC from a primary side power supply system to a rectifier through a saturable reactor and energizing between the bottom electrode A power control method for an arc melting furnace, wherein the power control device for controlling the arc receives a detection signal for detecting a voltage drop from a detection device provided in the primary power supply system, and receives a detection signal from the rectifier. It is characterized by performing a voltage drop control that suppresses a drop in output.

  According to this invention, even when a voltage sag occurs in the primary power supply system, it is possible to perform voltage drop control that detects this in the primary power supply system and quickly suppresses arc extinction. The operation stability can be ensured without greatly changing the balance between the arc current and the arc voltage.

  In the above invention, the voltage drop control may be control for maintaining the arc voltage of the arc. According to this invention, even when a sag occurs in the primary power supply system, the arc voltage is maintained to control the arc extinguishing and the balance between the arc current and the arc voltage is greatly changed. Without making it happen, operational stability can be secured.

  Furthermore, in the above-described invention, the voltage drop control may be control for reducing the impedance of the saturable reactor. According to this invention, even when a voltage sag occurs in the primary power supply system, the impedance of the arc can be reduced by controlling the impedance of the saturable reactor that can be controlled with relatively low power so as to maintain the arc voltage. Operation stability can be secured without suppressing arc extinction and without greatly changing the balance between arc current and arc voltage.

  In the invention described above, the rectifier may be a thyristor rectifier, and the voltage drop control may be control of a gate signal of the thyristor rectifier. According to this invention, even when a voltage sag occurs in the primary side power supply system, arc extinction is suppressed by controlling the gate signal of the thyristor rectifier that can be controlled with relatively low power so as to maintain the arc voltage. Therefore, the operation stability can be ensured without significantly reducing the balance between the arc current and the arc voltage.

  In the above-described invention, the power supply control device includes an emergency control power supply of a system different from the primary power supply system, and the voltage drop control is performed by connecting the emergency control power supply to the power supply control device. A control signal for a saturated reactor or the thyristor rectifier may be sent. According to this invention, even when a voltage sag occurs in the primary power supply system, operational stability can be ensured without losing control of the power supply control device.

It is a block diagram which shows the power supply control method by this invention. It is a flowchart which shows the power supply control method by this invention. It is a figure which shows the relationship between the electric current and voltage in the power supply control method by this invention.

  A power control method for an arc melting furnace according to one embodiment of the present invention will be described with reference to FIGS.

As shown in FIG. 1, the DC arc melting furnace 1 is a vacuum melting furnace (VAR) for melting a metal 8 to be melted using a DC arc. The rod-like upper electrode 4 is suspended and supported by the vertical drive mechanism 3 and is movable up and down. With this vertical drive mechanism 3, the lower tip of the upper electrode 4 is inserted into the furnace body 2 from the upper part thereof, and the distance from the metal 8 to be melted introduced into the furnace body 2 can be adjusted. A furnace bottom electrode 6 is provided at the bottom of the furnace body 2, and a voltage is applied between the furnace body 2 and the upper electrode 4. By adjusting this voltage and the distance between the upper electrode 4 and the metal 8 to be melted, an arc current I flows while generating an arc, and the metal 8 to be melted is melted by this arc heat. Note that the top of the molten metal 8 is metal pool 8 'is formed by the arc heat, the distance between the upper electrode 4 and the molten metal 8 may vary, in particular, the arc voltage V _F is changed, as appropriate, It is preferable to send a control signal from an automatic control device (not shown) to the vertical drive mechanism 3 to adjust the position of the lower tip of the upper electrode 4.

  The power source of the DC arc melting furnace 1 is supplied from a primary power supply system 10 as a commercial power source, and the output of the AC is adjusted using a saturable reactor 30 and is guided to a rectifier 32 to obtain a DC to be used. The positive electrode and the negative electrode are connected to the upper electrode 4 and the furnace bottom electrode 6 of the DC arc melting furnace 1, respectively.

  The saturable reactor 30 works as reactance, but can change its impedance upon receiving a control signal (control current) S from a power supply control board 21 of the power supply control device 20 described later. That is, the alternating current from the primary side power supply system 10 is controlled in correspondence with the control signal S of relatively low power and sent to the rectifier 32.

  The rectifier 32 includes a semiconductor rectifier for converting alternating current sent from the saturable reactor 30 into direct current, for example, a diode or a thyristor. Further, the converted pulsating flow is smoothed and output by a smoothing circuit as appropriate. In the case where a thyristor is used for the rectifier, AC / DC conversion is performed while controlling the output by applying a relatively small power control signal (control current) S ′ from the power supply control circuit 22 described later to the gate terminal. You can

The power supply control device 20 is a power supply control circuit 22 that operates by receiving power supply from the primary power supply system 10. The power supply control device 20 is downstream of the rectifier 32 and includes an ammeter 34 and a voltmeter 36 including a shunt (not shown). controlling the arc of the DC arc furnace 1 the furnace body 2 while receiving the measured values of arc current I and arc voltage V _F of. That is, in accordance with a predetermined control program, based on the measurement values of arc current I and arc voltage V _F, sends a control signal S to the saturable reactors 30, to control the AC output from the primary power supply system 10. When a thyristor is used for the rectifier of the rectifier 32, the gate current of the thyristor is controlled by the control signal S ′, and the output from the rectifier 32 is controlled.

  Furthermore, the power supply control device 20 includes a voltage sag compensation circuit 24 that operates together with the power supply control circuit 22. In addition, the voltage drop detection device 12 measures voltage fluctuations in the primary power supply system 10, and when the voltage drop is detected, the emergency control power supply 26 is connected to the power supply control device 20, and the detection signal indicating the voltage drop is controlled by the power supply. Transmit to circuit 22. When receiving the signal, the power supply control circuit 22 operates the voltage sag compensation circuit 24. Here, even during a power sag, the power supply control circuit 22 can receive power supply from the primary-side power supply system 10 to send a control signal S (or S ′), which is generally low power, Since the power supply from the emergency control power supply 26 is also received, it will not go down. Note that the voltage drop detection device 12 is a voltage relay that can detect a voltage drop in a short period of about several milliseconds to several tens of milliseconds and can output a detection signal immediately. Further, a relay switch for connecting the emergency control power supply 26 to the power supply control device 20 may be provided in the voltage drop detection device 12 or may be provided in the power supply control device 20.

  Next, a power supply control method for the DC arc melting furnace 1 will be described.

As shown in FIG. 1, the melted metal 8 is put into the furnace body 2, and a predetermined voltage is applied between the upper electrode 4 and the furnace bottom electrode 6 by the power supply control device 20. At the same time, when the distance between the upper electrode 4 and the melted metal 8 is reduced by the vertical drive mechanism 3, an arc is generated and an arc current I flows. The melted metal 8 is melted by this arc heat, and a metal pool 8 'is formed. When the distance between the upper electrode 4 and the molten metal 8 is stabilized, as shown in FIG. 3, to maintain a constant relationship as shown in curve 101 between the arc current I and the arc voltage V _F. That is, when the arc voltage V _F is constant, the arc current I arc when lowered below the curve 101 is extinguished. Furthermore, in also the arc current I is constant, the arc voltage V _F is the arc when lowered below the curve 101 is extinguished. Note that input power to the DC arc furnace 1 can be determined by the arc current I and arc voltage V _F which is respectively measured by the ammeter 34 and voltmeter 36.

Here, the circuit impedance Z _F of when considering an equivalent circuit of the DC arc furnace 1 includes an arc resistance,
I = V _F / Z _F (Formula 1)
It is represented by Further, when the rectifier 32 does not include a power transformer, between the primary power supply V and the arc voltage V _F is the use of the impedance Z _L of the saturable reactor, according to the resistance ratio,
V _F = {Z _F / (Z _L + Z _F )} × V = k × V (Formula 2)
The relationship holds. Note that the coefficient k = Z _F / (Z _L + Z _F ).

That is, in Equation 1, the current time is small arc current I measured by the meter 34, when the constant primary power supply V, the control signal of the power supply control circuit 22 so as to reduce the impedance Z _L of the saturable reactor 30 By applying (control current) S, the arc current I can be increased.

When a thyristor is used for the rectifier of the rectifier 32, the power supply control circuit 22 can control the output from the thyristor while sending a control signal (control current) S ′ to the gate of the thyristor. At this time, the arc current I and the arc voltage V _F to be measured, respectively, in the ammeter 34 and the voltmeter 36, for example, can be feedback controlled so that on the curve 101 of FIG.

  By the way, the power supply control method of the DC arc melting furnace 1 when the voltage of the primary side power supply system 10 falls due to a lightning strike or the like will be described with reference to FIG.

  The voltage drop detection device 12 measures the voltage fluctuation of the primary side power supply system 10 and waits for detection of the voltage drop (ST1). When the voltage drop is detected, the emergency control power supply 26 is connected to the power supply control device 20 (ST2), and a detection signal indicating the voltage drop is transmitted to the power supply control circuit 22 (ST3).

  On the other hand, in the power supply control device 20, the power supply control circuit 22 waits for reception of a detection signal indicating a voltage drop from the voltage drop detection device 12 (ST4). When receiving the detection signal, the power supply control circuit 22 operates the voltage sag compensation circuit 24 (ST5). At this time, the power supply control device 20 is supplied with control power from the emergency control power supply 26 in parallel with power supplied from the primary power supply system 10 via a transformer and a rectifier (not shown). As a result, the power supply control device 20 does not go down even at the time of a momentary drop.

Subsequently, the power supply control device 20 causes the power supply control circuit 22 to operate so as to suppress a decrease in the output of the rectifying device 32 based on the operation of the voltage sag compensation circuit 24. Here, the control signal (control) is supplied to the saturable reactor 30. (Current) S is sent (ST6). Saturable reactor 30 which receives the reduced impedance Z _L, i.e., by increasing the value of the coefficient k shown in equation 2, reduction of the arc voltage V _F also decreases the voltage V of the primary power supply system 10 Suppress.

Here, as shown in FIG. 3 movement, the arc voltage V _F and the arc current I, if you have a stable operation at point A, Shun the low occurs, arc voltage V _F and the arc current I to point B Attempting to extinguish the arc. On the other hand, in the control of ST1~ST4 described above, to increase the V _F lowering the impedance Z _L of the saturable reactor 30, while trying to move to the point A1, even moved to a point B1 by an instantaneous drop, the curve Since it is above 101, the arc is not extinguished.

  When a thyristor is used for the rectifier of the rectifier 32, the power supply control circuit 22 sends a control signal (control current) S 'to the gate of the thyristor (ST4). By maintaining or increasing the gate current of the thyristor constant, at least the arc current I is maintained, and arc extinction is suppressed.

According to the above, even when caused to sag in the primary power supply system 10 to suppress the extinguishing of the arc, the arc current I and without significantly changing the balance of the arc voltage V _F, thus operational stability Can be secured.

  The exemplary embodiments according to the present invention have been described so far, but the present invention is not limited thereto. Those skilled in the art will recognize a variety of alternative embodiments and modifications without departing from the scope of the appended claims.

DESCRIPTION OF SYMBOLS 1 Arc melting furnace 4 Upper electrode 6 Furnace bottom electrode 8 Metal to be melted 10 Primary side power supply system 12 Voltage drop detection device 20 Power supply control device 22 Power supply control circuit 24 Voltage drop compensation circuit 30 Saturable reactor 32 Rectifier 34 Ammeter 36 Voltage Total

Claims (4)

A power supply control method for a DC arc melting furnace in which an alternating current is guided from a primary side power supply system to a rectifier through a saturable reactor, an electric current is generated between the bottom electrode and an arc is generated to melt a material to be melted,
The power supply control device for controlling the arc receives a detection signal for detecting a voltage drop from a detection device provided in the primary side power supply system, sends a control signal to the saturable reactor, and the saturable A method of controlling a power source for a DC arc melting furnace , wherein voltage drop control is performed to suppress arc extinguishing by lowering the impedance of a reactor and raising the arc voltage of the arc . The power supply control device includes an emergency control power supply that is different from the primary power supply system, and the voltage drop control connects the emergency control power supply to the power supply control device and sends a control signal for the saturable reactor. The power supply control method according to claim 1, wherein:   The power supply control method according to claim 1, wherein the rectifier is a thyristor rectifier, and the voltage drop control is control of a gate signal of the thyristor rectifier. The power supply control device includes an emergency control power supply of a system different from the primary side power supply system, and the voltage drop control connects the emergency control power supply to the power supply control device and sends a control signal of the thyristor rectifier The power supply control method according to claim 3 .

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