JPH10106744A - Operation method of ac arc furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To quickly perform ignition at the arc starting time and restriking at the arc break time by varying a frequency of supply AC electric power by an inverter device, and supplying AC electric power not less than a specific frequency at the ignition operating time. SOLUTION: In an AC arc furnace 1, electric power is supplied to an electrode 2 from a power supply circuit composed of a high voltage power source 4, a switch 5, a circuit breaker 6 and a transformer 7 for a furnace, and an arc is generated between it and an inserted raw material 3, and the raw material 3 melts. An inverter device 8 having a rectifier circuit and an inverter circuit is connected between the electrode 2 and the transformer 7. Preferably, when AC electric power having a high frequency of 1MHz to 2MHz is supplied at the ignition time by the device 8, it is easily ignited, and a time loss is reduced, and a burden on an operator is also reduced. It is though that this is because dielectric breakdown voltage is reduced by ionization the atmosphere between the electrode 2 and the raw material 3. An AC frequency can be changed in the device 8, and electric power of not a high frequency but an ordinary commercial frequency is supplied except at the ignition operating time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of operating an AC arc furnace which can be operated efficiently, and is advantageously applied to, for example, an arc furnace for steelmaking.

[0002]

2. Description of the Related Art Steelmaking arc furnaces are broadly classified into AC arc furnaces using AC as a power source and DC arc furnaces using DC. Among them, the DC arc furnace has a furnace body structure having a movable electrode on the furnace as a negative electrode and a positive electrode on a furnace bottom, and supplies power by a DC converter using a thyristor rectifier. In addition to the arc voltage control by automatic raising and lowering, the arc control characteristics are improved by high-speed control of the arc current by the thyristor rectifier, and it is considered to be more advantageous than the AC arc furnace in that a large flicker reduction effect can be obtained. It is being actively performed.

However, in the DC arc furnace, since it is necessary to provide an electrode on the bottom of the furnace, the life of the bottom refractory including the bottom electrode is short, and a complicated work of replacing the bottom is required. Is high. In addition, there is a problem in operation reliability and maintenance management that there is a concern about a steel leak accident from the furnace bottom electrode (for example, Japanese Patent Publication No. 7-1390). Also, in DC arc furnaces, it is necessary to leave hot heel (pre-charged molten steel) in order to ensure conductivity during ignition,
There is also a problem that the amount of wear of the furnace bottom refractory is large.

Accordingly, recently, AC arc furnaces, which have been considered to have achieved a certain degree of technical maturity by the introduction of technologies such as large capacity and UHP operation, are being reviewed, and the conventional AC arc furnaces have been left. Research and development to solve the various problems are underway.

[0005]

SUMMARY OF THE INVENTION In an arc furnace,
In order to generate an arc between the raw material charged into the furnace and the electrode (upper electrode) disposed above the raw material, the upper electrode was brought close to the raw material in the furnace as an ignition operation, and We are working on applying power. However, depending on the state of the charged raw material (scraps and the like) immediately below the electrodes, there is a problem that it is difficult to fire or that the arc is cut off immediately after being fired, that is, a so-called arc breakage may occur.

That is, in the melting period, the state of the scrap immediately below the electrode changes every moment as it melts, and the arc is likely to be cut due to a change in the power supply load due to the scrapping of the scrap. When an arc break occurs, it is necessary to re-ignite promptly in order to minimize the loss of the operation time, but conventionally, it is not always easy to re-ignite depending on the condition of the raw material. I couldn't say. Particularly, in an AC arc furnace, the polarity of the applied power is inverted every cycle, so that the arc is likely to break at the time of the inversion. Furthermore, in order to maintain a stable arc, it is necessary to operate within an appropriate arc impedance range.However, since arc current control cannot be performed by a thyristor such as a DC arc furnace, the arc impedance is not affected by power supply load fluctuations. It changes directly.

[0007] The present invention advantageously solves the above-mentioned problems, and it is an object of the present invention to quickly perform ignition at the start of an arc and re-ignition when an arc breaks during operation of an AC arc furnace. It is an object of the invention to propose a method of operating an AC arc furnace, which makes possible and thereby achieves an efficient operation.

[0008]

SUMMARY OF THE INVENTION The present invention relates to an AC arc furnace in which an arc is generated between an electrode and a raw material charged in the furnace by AC power to melt the raw material and raise the temperature of the raw material melt. In the operation method of (1), an AC arc furnace having an inverter device is used as power supply equipment connected to the arc furnace, and the frequency of the AC power supplied to the arc furnace is made variable by the inverter device. A method for operating an AC arc furnace, characterized in that AC power is supplied to the arc furnace.

In the present invention, the inverter device is
A device that includes not only a so-called inverter circuit that converts direct current into alternating current, but also a rectifier circuit provided on the input side of the inverter circuit.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventors of the present invention have conducted research and development in order to facilitate an ignition operation in an AC arc furnace. As a result, the frequency of a power supply for supplying power to the arc furnace was changed to a conventional commercial frequency. From 50Hz and 60Hz, it was found that it was effective to increase the frequency. Then, among the high frequencies, particularly, if the frequency is 1 MHz or more, it has been found that particularly excellent ignition characteristics can be obtained, and the present invention has been achieved.

Thus, in the present invention, a new inverter device is provided in the power supply equipment connected to the electrodes, and the frequency of the AC power supplied to the arc furnace is changed by the inverter device so that the frequency becomes 1 MHz or more during the ignition operation. AC power is supplied to the arc furnace. This makes it possible to easily ignite regardless of the state of the raw material immediately below the electrode, reducing the time loss at the start of ignition and re-ignition during operation, and also reduces the work load on the operator. You do it. As a result, the operating efficiency is improved and the power is saved.

The reason why the ignition is facilitated by supplying alternating current power having a high frequency of 1 MHz or more to the arc furnace according to the present invention is explained by the fact that the atmosphere between the upper electrode and the charge is ionized. This is probably because the dielectric breakdown voltage decreased.

Further, since the frequency of the AC power supplied to the arc furnace is variable by the inverter device, the AC power having a normal commercial frequency or another frequency is supplied to the arc furnace during the operation except for the ignition operation. You only need to supply them. In other words, since high-frequency power is applied only at the time of ignition, the skin temperature caused by the high frequency locally increases the surface temperature of the electrode and suppresses preferential wear from the peripheral surface as much as possible.

FIG. 1 shows an example of a main part of a power supply equipment of an AC arc furnace to which the present invention is applied. In the figure, 1 is an AC arc furnace,
Reference numeral 2 denotes an electrode disposed above the AC arc furnace. An arc is generated between the electrode 2 and the raw material 3 charged in the AC arc furnace 1 so as to melt the raw material 3 and increase the temperature. ing. The power supply circuit connected to the electrode 2 includes a high-voltage power supply 4,
The apparatus includes a switch 5, a circuit breaker 6, and a furnace transformer 7 whose voltage can be changed by tap switching. An inverter device 8 is connected between the furnace transformer 6 and the electrode 2. The electrode automatic control device and the like are not shown.

In the prior art, electric power having a commercial frequency such as 50 Hz or 60 Hz is supplied in order not to perform frequency control of AC power for causing an arc as in the present invention. Firing of an arc,
As described above, there are cases where re-ignition is difficult, which causes problems such as a loss of working time and a loss of power efficiency. On the other hand, an AC arc furnace having an inverter device in the power supply circuit as shown in FIG. 1 is used. At the time of ignition, the inverter device has a high frequency of 1 MHz or more, more preferably 2 MHz or more. By supplying AC power of 5 MHz or less from the surface, it is possible to easily perform an arc ignition operation. Such frequency control can be easily achieved by using an inverter device. Further, the inverter device can change the AC frequency, and in the present invention, there is no need to apply high-frequency power except at the time of ignition.

The inverter device used in the present invention includes a rectifier circuit and an inverter circuit. The inverter circuit can be configured using, for example, a GTO element, an SI thyristor, and the like. In the rectifier circuit, a conventionally known rectifier such as a thyristor may be used.

[0017]

EXAMPLE A steelmaking operation of ordinary steel was carried out using an AC arc furnace of 100 t, 80 MVA (three upper electrodes). At this time, 66 kV, 60 Hz was transformed to 700 V by the furnace transformer. In the conventional operation method in which the arc was started at the frequency of 60 Hz and the re-ignition was performed when the arc cut out, the arc was not generated immediately after the power was turned on. In addition, since the arc was cut during the temperature raising operation, the arc was not generated immediately even when re-ignition was manually performed by the operator. For that reason, the TAP-TO-TAP time was 59 minutes, and the power consumption was 395 kWh / t.

Next, according to the present invention, an inverter device is provided on the output side of the furnace transformer, and a high frequency of 1 MHz is applied by the inverter device at the time of ignition and re-ignition. The ignition and re-ignition work were completed immediately. After ignition and re-ignition, melting and operation at a frequency of 60 Hz resulted in TAP-TO-TAP time
The time was reduced to 57 minutes, and the power consumption was reduced to 380 kWh / t.
In addition, 3MHz high frequency is ignited by the inverter device,
When applied at the time of restriking, the TAP-TO-TAP time was 57 minutes and the power consumption was 380 kWh / t, as in the case where a high frequency of 1 MHz was applied.

[0019]

As described above, according to the present invention, in an operation method of an AC arc furnace, an AC arc furnace having an inverter device is used for power supply equipment connected to the arc furnace, and the AC power supplied to the arc furnace by the inverter device is used. Since the frequency is variable and alternating current power of high frequency of 1 MHz or more is supplied to the above-mentioned arc furnace at the time of the ignition operation, it is possible to easily ignite regardless of the condition of the raw material immediately below the electrode. , The time loss due to the start of ignition and the re-ignition are reduced, and the work load on the operator is also reduced. Further, since no high frequency is applied except at the time of ignition, the adverse effect that the electrode is worn by the skin effect due to the high frequency is also avoided as much as possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a main part of a power supply equipment of an AC arc furnace to which the present invention is applied.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 AC arc furnace 2 Electrode 3 Raw material 4 High voltage power supply 5 Switch 6 Circuit breaker 7 Furnace transformer 8 Inverter device

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Nakato Sanka 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. 1-chome (without address) Inside Kawasaki Steel Corporation Mizushima Works

Claims (1)

[Claims] An arc arc is generated between an electrode and a raw material charged in the furnace by AC power to melt the raw material and raise the temperature of the raw material melt. An AC arc furnace having an inverter device is used as the power supply equipment to be connected. The frequency of the AC power supplied to the arc furnace is varied by the inverter device, and the AC power having a high frequency of 1 MHz or more during the ignition operation is supplied to the arc furnace. A method for operating an AC arc furnace characterized by supplying.