JP2526554B2 - Dissolution control method for AC arc furnace - Google Patents

Description

TECHNICAL FIELD The present invention relates to a melting control method for an AC arc furnace, and more particularly to prevention of uneven melting.

[Prior Art] In a three-phase AC arc furnace, the electrical characteristics of the furnace are determined by the impedance of the copper tube feeding the furnace, the flexible cable, the secondary conductor of the transformer such as the electrode. An imbalance is unavoidable in the arrangement of the secondary side conductor, and it is very difficult to make the impedances of the respective phases exactly the same, and it is almost impossible. Therefore, the arcs generated at the electrodes of each phase are not the same, and a phase in which a large amount of power is input (hereinafter referred to as a turbulent phase) and a phase in which a small amount of power is input (hereinafter referred to as a stationary phase) are generated, resulting in uneven melting. I'm running.

In the turbulent phase, the scrap quickly melts, which is disadvantageous such as heat loss to the furnace wall or damage to the furnace wall, but in the stationary phase, melting is delayed, which causes a decrease in productivity.

The uneven melting in the furnace is caused not only by the uneven power supply but also by the uneven oxygen concentration in the furnace, the condition of the scrap, the position of the dust collection port, etc. It is also a problem in terms of sex.

In order to solve this inequality of melting, conventionally, the current flowing through each of the three phases is individually changed to equalize the power of each phase, or the tap voltage of the transformer is individually adjusted for each phase. A method of changing and adjusting is used.

[Problems to be solved by the invention]

In the conventional method of adjusting the imbalance of melting by changing each line current, the range in which the line current can be varied cannot exceed the maximum current of the furnace transformer. To reduce the total input power, resulting in lower productivity.
In addition, the arc of the electrode with reduced current becomes unstable. Moreover, the utilization rate of the transformer for the furnace is poor, and the adjustment range is narrowed. Further, there is a problem that an unbalanced current flows in each phase.

Further, even in the method of changing the voltage for each phase, the variable range of the voltage cannot exceed the maximum voltage of the furnace transformer, so the input power is reduced because the voltage is selected to be reduced,
There was a problem that the utilization rate of the transformer for the furnace was poor and the adjustment range was narrow. In addition, the tap switching mechanism of the transformer for the furnace becomes complicated, resulting in a decrease in reliability, which is disadvantageous for maintenance. Also in this case, there is also a problem that an unbalanced current is supplied from the viewpoint of the power supply.

The present invention has been made to solve the above problems, and an object of the present invention is to propose a melting control method for an AC arc furnace that can correct uneven melting while making the current flowing through each phase uniform. It is a thing.

[Means for solving problems]

The melting control method for an AC arc furnace according to the present invention has a specific self-inductance and mutual inductance when the arc length of an electrode is changed in response to a change in the melting rate of a three-phase AC arc furnace to promote melting. Adjustable variable reactors provided individually for each phase of the power supply circuit keep the current flowing from each phase of the power supply circuit to each electrode constant while adjusting the variable reactor according to the amount of input power required for the electrodes of each phase. adjust.

In addition, in order to eliminate the phase having slow or fast melting characteristics by providing a fixed reactor on one or more of each phase of the power supply circuit, the melting rate is averaged to cope with changes in the melting rate of the three-phase AC arc furnace. When the arc length of the electrodes is changed to promote melting, adjustment is provided individually for each phase of the power supply circuit so that the current flowing from each phase of the power supply circuit having a specific self-inductance and mutual inductance is constant. With the possible variable reactor, the variable reactor is adjusted according to the amount of input electric power required for the electrodes of each phase.
Further, the power factor cos θ of the power supply circuit is set to 0.9 or less.

[Action]

In the present invention, when the arc length of the electrode is changed in response to the change in the melting rate of the three-phase AC arc furnace to change the arc resistance and promote the melting, by adjusting the variable reactor provided in each phase, The input power of each phase is adjusted while maintaining the balance of the current flowing in each phase. That is, the arc length of each phase is changed according to the charging ratio and the amount of electric power required for each phase according to the nonuniformity of melting. When the arc length of each phase is changed, the input electric energy of each phase is adjusted by changing the inductance of the variable reactor of each phase.
Impedance can be balanced and the dissolution can be controlled to be uniform. In addition, when there is a fast-dissolving phase or a slow-dissolving phase by the fixed reactors provided in each of the three-phase alternating current phases, adjustment is performed so that an average uniform dissolution can be performed. Furthermore, the power supply circuit can be operated in a stable arc state by setting the power factor cos θ to 0.9 or less.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which Ei (where i = 1, 2, 3) is a power supply voltage of each phase in a Y-connected three-phase AC power supply, and Li is a furnace circuit. Phase self-inductance, Mij is mutual inductance between furnace circuit phases, R
i is the total resistance of each phase, and the total resistance Ri of each phase consists of the resistance Rfi of each phase of the furnace circuit and the arc resistance Rai.

Lvi is a variable reactor added to each phase,
The reactance value of each phase can be changed individually by changing the variable reactor Lvi individually. Eb is the ground potential of the molten metal in the furnace.

First, the principle of determining the value of the variable reactor Lvi put in each phase in the furnace circuit of the AC arc furnace configured as described above will be described.

In the circuit shown in FIG. 1, the current flowing in each phase is Ii
(I = 1 to 3), the following simultaneous equations hold.

However, ω = 2πf, f is the power supply frequency. Here, if the phase order is positive and I ₁ and E ₁ are the criteria, Can be expressed as

 From the above equations (1) and (2), the following equation is obtained.

From equation (3), −b = R ₁ + jω (Lv ₁ + L ₁ ) + jωM ₁₂ a ² + jωM ₃₁ a
(4) a ² −b = jωM ₁₂ + R ₂ a ² + jω (Lv ₂ + L ₂ ) a ² + jωM
₂₃ a (5) a−b = jωM ₃₁ + jωM ₂₃ a ² + R ₃ a + jω (Lv ₃ + L ₃ )
a (6) From equations (4) and (5), (1-a ² ) = R ₁ + jω (Lv ₁ + L ₁ −M ₁₂ ) + {− R ₂ + j
ω (M ₁₂ −Lv ₂ −L ₂ )} a ² + jω (M ₃₁ + M ₂₃ ) a (7) The impedance includes the resistance R and the inductance L, and the average of each phase is the resistance R. Taking the resistance and the inductance L, and taking the average inductance of each phase, Can be expressed as

By substituting the equation (8) into the equation (7) and expanding it, the variable reactor Lv ₂ is obtained by the following equation.

On the other hand, the total input power P input to the furnace circuit shown in FIG. 1 is P = (R ₁ + R ₂ + R ₃ ) I ² (10), and the power distribution to each electrode is R ₁ : R ₂ : It becomes the ratio of R ₃ . And since the electrode currents are the same, the arc voltage becomes Ra ₁ · I, Ra ₂ · I, Ra ₃ · I in each phase, and the electrode with slow melting increases the arc resistance, that is, lengthens the arc length to melt. Facilitate.

In addition, the circuit power factor cos θ is E when the voltage applied to the load of each phase is E This circuit power factor cosθ is set to cosθ <0.9 so that the arc does not become unstable.

Now, assuming that the tap voltage of the transformer is E ′ and the power factor cos θ that takes into account the required input power P and arc stability is determined, the current I of each phase, the average inductance L of each phase and the total resistance R ₁ + R ₂ + R ₃ can be determined by the following formula.

Also, the power input to each phase is the ratio of R ₁ I ² : R ₂ I ² : R ₃ I ² , so by setting the necessary input ratio to each phase according to melting, R ₁ , The values of R ₂ and R ₃ are determined. By using these values, the values of the variable reactors Lv ₁ , Lv ₂ , and Lv ₃ added to each phase can be determined by the equation (9). In other words, the unevenness of melting progress is detected by observing the inside of the furnace and the heat load level of the furnace wall, setting the required input ratio to each phase, and adjusting the variable reactor of each phase to make the melting uniform. To control.

In addition, the inductance component in the equation (9) for calculating the variable reactors Lv ₁ , Lv ₂ , and Lv ₃ Each value of can be measured and calculated by the following method.

At the end of melting or refining when the arc is stable, the variable reactors Lv ₁ , Lv ₂ and Lv ₃ are cut off and the electrode position is adjusted so that the current in each phase is the same, and the power input to each phase is adjusted.
Measure Pi. Based on this measured input power Pi, the resistance Ri of each phase is obtained based on Pi = Ri · I ² (i = 1 to 3) (13), and for example the case of the first phase is taken from this resistance Ri. It is possible to actually measure and calculate.

FIG. 2 shows the configuration of an AC arc furnace that controls melting according to the above principle. In the figure, 1 is a power source, 2 is a switch for a furnace connected to the power source 1, 3 is a transformer for a furnace, and 4 is a transformer for a furnace. It is a variable reactor that is individually connected to each phase on the secondary side of the container 3, and the variable reactor 4 is provided with a tap for varying the induction. Reference numeral 5 is a voltage detection circuit that individually detects the voltage of each secondary side phase, 6 is a current detection circuit that individually detects the current of each phase, 7 is an electrode, and 8 is a furnace body.
Reference numeral 9 is an electrode lifting device attached to each phase electrode 7, 10 is a furnace condition sensor attached to the furnace body 8, 11 is an electrode based on information from the voltage detection circuit 5, current detection circuit 6 and furnace condition sensor 10. It is a control device for adjusting the lifting device 9, the variable reactance 4 and the furnace transformer 3, and the control device 11 includes a power amount calculation circuit 12, a charging ratio setting circuit as shown in FIG.
13, arc length calculation circuit 14 and inductance calculation circuit 15
It has.

A method of controlling melting by the AC arc furnace configured as described above will be described based on the flow chart shown in FIG.

First, the input power amount P and the power factor cos θ are determined and the control device 11
(Step 40) and set the specified voltage and current to each electrode 7.
To dissolve it. The voltage and current are detected by the voltage detection circuit 5 and the current detection circuit 6 and input to the power amount calculation circuit 12 of the control device 11 to detect the amount of power supplied to each electrode 7 (step 41).

Next, along with the progress of melting, the furnace condition sensor 10 detects the non-uniformity of the progress of melting (step 42), and according to the non-uniformity of progress, sets the necessary ratio of charging to each phase in the charging ratio setting circuit 13. Yes (step 43). The arc length calculation circuit 14 calculates and determines the arc length of each phase under the condition that the current I is constant based on the input ratio of each phase and the input power amount of each phase calculated by the electric energy calculation circuit 12 (step 44). The electrode elevating device 9 is driven so that the arc length is reached (step 45). On the other hand, based on the arc length of each phase calculated by the arc length calculation circuit 14, the inductance of the variable reactor 4 of each phase is calculated by the inductance calculation circuit 15 (step 46), and the tap of the variable reactor 4 of each phase is switched (step 47) Then, control is performed so that the dissolution becomes uniform by advancing the dissolution (step 48).

In the above-mentioned embodiment, when it is clear that the fast-dissolving phase or the slow-dissolving phase is always decided, the fixed reactor 4a is assumed by assuming a fixed capacity as a base in advance according to each phase as shown in FIG. , 4b have been added to enable uniform uniform melting, and then the areas where the melting becomes uneven due to the way the scrap is inserted and the way oxygen is blown for each charge in the furnace. You may control by the variable reactor 4 provided in the phase.

〔The invention's effect〕

As described above, the present invention adjusts the input power of each phase while maintaining the balance of the currents flowing in each phase by adjusting the variable reactors provided in each phase of the three-phase alternating current. It is possible to adjust the input power balance of each phase more broadly without flowing an equilibrium current and to perform uniform melting.

In addition, the current of each phase is the same and only the arc voltage changes, so it is effective to have a long arc effect such as when aiming to prevent unmelted residue, and the estimation of that effect is easy, unlike when adjusting the current. Becomes

Furthermore, by increasing / decreasing the total reactance of the three phases, it becomes possible to select both high reactance necessary for forming a long arc and low reactance effective in forming a short arc. It has the effect of setting the heat balance according to the situation.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG.
FIG. 3 is a block diagram of the AC arc furnace of the embodiment shown in the figure, FIG. 3 is a block diagram of the controller of the above embodiment, FIG. 4 is a flow chart showing the operation of the above embodiment, and FIG. 5 is another embodiment. 3 is a circuit diagram showing a reactor of FIG. 3 ... Transformer for reactor, 4 ... Variable reactor, 5 ... Voltage detection circuit, 6 ... Current detection circuit, 7 ... Electrode, 9 ... Electrode lifting device,
10 ... Reactor condition sensor, 11 ... Control device.

Continuation of the front page (56) References JP-A-53-43254 (JP, A) Actual opening 48-61441 (JP, U) Actual opening 62-47097 (JP, U) Actual opening 57-52096 (JP , U) Actual Development Sho 59-163895 (JP, U) Japanese Patent Sho 48-11538 (JP, B1) Japanese Official Sho 55-17314 (JP, B2) Actual Sho 44-7210 (JP, Y1)

Claims (4)

(57) [Claims] 1. When facilitating melting by changing the arc length of an electrode in response to a change in melting rate of a three-phase AC arc furnace,
Adjustable variable reactors provided individually for each phase of the power supply circuit that have specific self-inductance and mutual inductance make the required currents for each phase electrode while keeping the current flowing from each phase of the power supply circuit to each electrode constant. A melting control method for an AC arc furnace, which comprises adjusting the variable reactor according to the amount of electric power. 2. The melting control method for an AC arc furnace according to claim 1, wherein the power factor cos θ of the power supply circuit is set to 0.9 or less. 3. A change in melting rate of a three-phase AC arc furnace in which a fixed reactor is provided in one or more phases of each of the power feeding circuits and the melting rates are averaged so as to eliminate phases having slow or fast melting characteristics. When the arc length of the electrodes is changed correspondingly to promote melting, the current flowing from each phase of the power supply circuit, which has a specific self-inductance and mutual inductance, to each electrode is made constant so that the current flows to each electrode individually. A melting control method for an AC arc furnace, characterized in that the adjustable reactor provided is adjusted to adjust the variable reactor according to the amount of electric power input to the electrodes of each phase. 4. The melting control method for an AC arc furnace according to claim 3, wherein the power factor cos θ of the power supply circuit is set to 0.9 or less.