JP2828478B2 - Power supply for DC arc furnace - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a power supply device for a so-called DC arc furnace which melts or heats a metal by the arc heat of a DC arc.

(Prior Art) The configuration of a power supply device of a DC arc furnace is typically represented by that shown in FIG.

In FIG. 1, the voltage of a three-phase AC power supply 1 is reduced by a converter transformer 2 to an AC voltage proportional to a target DC voltage, and further converted to a DC voltage by a polyphase rectifier 3 comprising a thyristor converter. Is done. The DC anode side of the rectifier 3 is connected to the bottom electrode 4 of the arc furnace 8 via the large current bus 7. The DC cathode side of the rectifier 3 is connected to a movable graphite electrode 6 via a DC reactor 5 and a large current bus 7.

Then, a DC arc A is generated between the tip of the graphite electrode 6 and the metal to be melted or the metal to be heated M in contact with the furnace bottom side electrode 4, and the metal is melted or heated by utilizing the arc heat. Things.

In such a DC arc furnace 8, in order to effectively transfer the thermal energy generated by the DC arc A to the metal to be melted or the metal to be heated M, the rectifying element constituting the rectifying device 3 is, for example, a thyristor (SCR). The length of the DC arc A is controlled by controlling the DC voltage and current by controlling the arc phase angle and controlling the position of the movable graphite electrode 6. The firing phase angle control by the thyristor has a high-speed response within several milliseconds in response to a physical state change of the metal to be melted, and the graphite electrode 6 has several tens to several A relatively slow position control is performed by a response of 100 ms.

In the DC arc furnace 8 described above, the physical form of the metal to be melted or the metal to be heated M is changed at random and at high speed as the melting and heating progresses. Of course,
In response to this physical change, the length of the generated DC arc A also tends to vary. On the other hand, since the response of the position control of the graphite electrode 6 is relatively slow as described above, it is impossible to eliminate the high-speed fluctuation of the arc length. Therefore, the arc length has to be changed at high speed with the progress of melting and heating.

On the other hand, the practical specifications of the DC arc furnace 8 are a voltage of several hundred volts and a current of the order of tens of thousands of amps, but in the case of a large-capacity arc of this magnitude, it is assumed that the arc length is almost proportional to the arc voltage. Can be. Therefore, when the arc length fluctuates at a high speed as described above, especially when the arc length is going to be longer than a certain stable state, unless the arc voltage is increased accordingly, the arc current decreases or the arc disappears. This results in a stable state, which impairs stabilization of the arc furnace operation.

In other words, when the arc length fluctuates at a high speed, a power supply device that can ensure a large so-called voltage thrust is required in order to prevent interruption of the arc.

Therefore, in actual operation, the DC voltage
As shown in the current characteristic curve, the DC output voltage of the rectifier 3
E _d is also average arc current I _d1 arc length from the intersection of (direct current rated value) and the average arc voltage E _d1 (DC voltage rated value) is changed to be longer at a high speed, the arc The length is set with a sufficient voltage margin so that a sufficient voltage can be supplied to the length.

That is, in FIG. 6, the voltage E _{d1s obtained} by adding the voltage margin ΔV to the average arc voltage E _d1 is equal to I _{d that} can be supplied by the power supply device.
= Maximum voltage when _Id1 . Therefore, according to this characteristic curve, even if the arc length fluctuates from the length corresponding to the voltage E _d1 to the length corresponding to E _d1s , the DC current (arc current) I _d
It is apparent that DC arc A can be continued without interruption while maintaining I _d1 .

Note that in Figure 6, VI _s is the maximum DC voltage-current characteristic curve of a power supply can output, the curve is of the case where the control angle of the thyristor rectifier 3 alpha is the minimum value. VI ₁ is a target setting curve of the DC voltage / current output from the power supply, and the control angle α is a set value (= α ₁ ).
This is the case. Furthermore, E _d0 represents the DC no-load voltage,
₀ is the equivalent DC no-load voltage and the value when the control angle α is the minimum, and E ₀₁ is the equivalent DC no-load voltage and the control angle α
Indicates the value when the set value (= α ₁ ).

Here, the DC voltage _Ed is given by the following general formula.

E _d = E _d0 · cos α−e _T −e _R (1) As is apparent from the equation (1), the DC voltage E _d is cos
It can be controlled by changing α. In the equation (1), α is the control angle of the thyristor as described above, e _T is the voltage drop due to the reactance of the AC power supply 1 and the like, and e _R is the voltage drop due to the resistance.

In FIG. 6, if the voltage margin ΔV is sufficiently large to obtain arc stability, the control angle α may be set large in the above equation (1).

On the other hand, the power factor during operation of the rectifier 3 is given by the following equation (2).

In the equation (2), u is a commutation overlap angle. It is apparent from the above equation (2) that when the control angle α is increased in order to obtain arc stability, the power factor of the rectifier 3 decreases. Usually, the power receiving point power factor of a factory having this kind of load is required to be a certain value or more, and when the power factor is reduced as described above, a capacitor facility or the like for improving the power factor is newly added. Will be required.

(Problems to be Solved by the Invention) As described above, in the conventional power supply device for a DC arc furnace, the control angle α is constantly increased in order to obtain arc stability, and the maximum output voltage that can be generated by the rectifier 3 is obtained. By outputting a DC voltage that is sufficiently lower than that, the voltage margin ΔV is increased to secure the voltage thrust and to prepare for a change in the arc length. However, increasing the voltage boost increases the reactive power taken from the AC power supply 1 and results in a decrease in the power factor of the rectifier 3 during a steady operation, requiring a large-capacity capacitor facility or the like for improving the power factor. The financial burden was extremely high.

The present invention has been proposed to solve the above problems, and its purpose is to reduce the reactive power consumed by the power supply device while securing a sufficient voltage margin to obtain arc stability. Another object of the present invention is to provide a power supply device for a DC arc furnace in which a power factor is prevented from lowering during a steady operation and the economical burden of a power factor improving facility or the like is reduced.

(Means for Solving the Problems) To achieve the above object, a first invention is a DC arc furnace that supplies DC power to a DC arc furnace by a power converter connected to an AC power supply and operable as a rectifier. In the power supply device for use, the power conversion device, a main conversion device including a thyristor conversion device of a low voltage and a large current rating connected in parallel and a thyristor conversion device of a high voltage and a small current rating,
Auxiliary conversion device consisting of GTO conversion device etc.
When the arc current flowing in the DC arc furnace is large, the main converter and the auxiliary converter are operated to share the arc current between the two converters, and the arc current is below the level at which the auxiliary converter can flow. At times, the main converter is stopped and only the auxiliary converter is operated, so that the DC output voltage of the power supply is increased to a voltage that can be output by the auxiliary converter.

Further, the second invention connects the main conversion device such as a thyristor conversion device and the auxiliary conversion device such as a diode rectifier and a thyristor conversion device in series to convert the DC voltage that can be output from the main conversion device into the auxiliary conversion device. The DC voltage that can be output by the device is larger than the DC voltage that can be output, and a fluctuation of the DC output voltage of the power supply device when the arc current flowing in the DC arc furnace fluctuates is secured on the main converter side. .

(Operation) According to the first aspect, when the arc length is increased due to a physical change of the metal to be melted or the metal to be heated, the DC current (arc current) supplied to the arc furnace by the power supply device decreases. When the DC current falls below the level that can be passed by the auxiliary converter, the main converter is stopped and only the auxiliary converter with a high voltage and small current rating is operated to ensure sufficient voltage thrust and arc. The arc can be stably continued without reducing the current. Here, the main converter composed of a thyristor converter and the like can be operated with a small control angle in a steady state and does not cause a decrease in power factor. Since the reactive power taken from the power supply is small, the reactive power of the power supply device as a whole is small.

According to the second aspect of the present invention, the surge of the voltage is ensured on the side of the main converter including the thyristor converter, and even if the reactive power consumed by the main converter is large, the diode rectifier, the GTO converter, etc. Since the reactive power taken by the auxiliary converter consisting of is very small or can be canceled with the reactive power of the main converter, the reactive power of the power supply device as a whole is reduced.

(Embodiment) Hereinafter, an embodiment of each invention will be described with reference to the drawings. First,
FIG. 1 shows a first embodiment of the first invention. In FIG. 1A, a three-phase AC power supply 1 includes a transformer for a conversion device.
An auxiliary thyristor converter 3a as a rectifier and a main thyristor converter 3b as a rectifier having a larger capacity than the auxiliary thyristor converter 3a are connected to output sides thereof. . Here, the thyristor constituting each of the converters 3a and 3b is generally a general reverse blocking three-terminal thyristor called SCR,
(Gate turn-off) It is distinguished from a thyristor.

The DC anode side of each of these thyristor converters 3a, 3b is commonly connected and connected to the bottom electrode 4 of the DC arc furnace 8, and the DC cathode side of each thyristor converter 3a, 3b is connected to a DC reactor 5a. Arc furnace 8 through 5b
Is connected to the graphite electrode 6.

In this embodiment, as shown in FIG.
DC no-load voltage E _d0M by the main thyristor converter 3b,
The relationship between the DC no-load voltage E _d0A by the auxiliary thyristor converter 3a is E _d0M <E _d0A , and the DC current rating I _dM by the main thyristor converter 3b and the DC current rating by the auxiliary thyristor converter 3a. the relationship between the I _dA has become a I _dM> I _dA. That is, in this embodiment, the main thyristor converter 3b has a low voltage and large current rating, and the auxiliary thyristor converter 3a has a high voltage and small current rating.

In FIG. 1 (b), similarly to the above, I _d1 is a DC current rated value as a power supply device and has a relationship of I _d1 = I _dM + I _dA , and E _d1 is also a DC voltage rated value, VI ₁ Is a target setting curve of the DC voltage / current output from the power supply device, a is a maximum DC voltage / current characteristic curve that the auxiliary thyristor converter 3a can output, b is a maximum DC voltage / current that the main thyristor converter 3b can output The respective characteristic curves are shown.

In such a configuration, in the steady state, both conversion devices
3a and 3b share the arc current (DC output current of the power supply device as a whole) at an arbitrary ratio. In addition, both conversion devices 3
The DC output voltages of a and 3b are almost equal. In this state, when the physical form of the metal to be melted or the metal to be heated M changes, and the arc length increases and the voltage of the DC arc furnace 8 increases, the DC current decreases. Then, when this DC current is reduced to a predetermined level and the main thyristor converter 3b is stopped, the DC output voltage can be increased to the capability of the auxiliary thyristor converter 3a having a high voltage and a small current rating. Thereby, the arc current can be suppressed from being attenuated and the arc break can be suppressed.

In this state, the graphite electrode 6 is lowered to control the arc length to the original length, and the main thyristor converter 3b is again operated. That is, in this embodiment, since the control angle α of the main thyristor converter 3b in the steady state can be reduced, the main thyristor converter 3b
The reactive power taken from the power supply can be reduced. On the other hand, although the control angle α of the auxiliary thyristor converter 3a is large, the reactive power is relatively small because the converter 3a is originally small in capacity, and as a result, the reactive power can be reduced as a whole of the power supply device. Further, it is possible to prevent the power factor from decreasing in the steady state.

For example, the effect of reducing the reactive power in this embodiment is quantitatively estimated as follows.

(B) Conventional device (see FIGS. 5 and 6) (B) This embodiment Becomes Therefore, the relationship between the reactive power _QΣ in the present embodiment and the reactive power Q in the conventional device is _QΣ <Q, and the effect of reducing the reactive power is apparent.

Next, a second embodiment of the first invention is shown in FIG. In this embodiment, the auxiliary thyristor converter 3a in FIG. 1 (a) is replaced by a GTO converter 3c comprising a GTO thyristor as shown in FIG. 2 (a). Is the same as However, since the GTO converter 3c can be controlled so as to take the leading reactive power, the delay reactive power taken by the main thyristor converter 3b is offset, and the reactive power can be reduced as compared with the first embodiment.

About this embodiment, as in the first embodiment, the reactive power _QΣ (= Q ₁ + Q ₂ ) of the entire converter is roughly calculated as follows.

Therefore, also in this embodiment, the relationship with the reactive power Q in the conventional device is _QΣ <Q, and the reactive power can be reduced.

Next, FIG. 3 shows a first embodiment of the second invention. Whereas the first and second embodiments of the first invention are parallel connection of two sets of converters 3a, 3b or 3c, 3d, the present embodiment has two sets of converters connected in series. It is.

That is, in FIG. 3 (a), 3d is a diode rectifier as an auxiliary converter connected to the converter transformer 2a, and the DC output side of the diode rectifier 3d is connected to the converter transformer 2a. Are connected in series to the DC output side of the main thyristor converter 3b connected to the main thyristor converter 3b. In this figure, reference numeral 5 denotes a DC reactor, and other components are the same as those in FIGS. 1 (a) and 2 (a).

In this embodiment, the total dc output voltage can be controlled from zero by operating the main thyristor converter 3b in the forward / inverse conversion region. In addition, in order to secure the voltage thrust on the main thyristor converter 3b side, the reactive power taken by the diode rectifier 3d becomes extremely small even when the operation is performed with the control angle α set to a large value. Can be reduced.

FIG. 3 (b) shows the DC voltage / voltage in this embodiment.
In the figure, b ′ is the maximum DC voltage / current characteristic curve that can be output by the main thyristor converter 3b,
d is the maximum DC voltage that can be output by the diode rectifier 3d.
Each shows a current characteristic curve. E _d1M is the rated DC output voltage of the main thyristor converter 3b, E _d1A is the rated DC output voltage of the diode rectifier 3d, and has a relationship of E _d1 <(E _d1M + E _d1A ).

The effect of reducing the reactive power in this embodiment is as follows.

Becomes Therefore, the relationship with the reactive power Q in the conventional device is _QΣ <Q, and the reactive power can be reduced as well.

FIG. 4 shows a second embodiment of the second invention. In this embodiment, as shown in FIG. 4 (a), a main thyristor converter 3b and a GTO converter 3c are connected in series. The other components are the same as those shown in FIG.

Here, the main thyristor converter 3b takes the delayed reactive power, the GTO converter 3c takes the leading reactive power, and the values of both reactive powers can be set to the same value. Can be made almost zero. That is, even if the voltage is sufficiently increased, the reactive power does not increase.

FIG. 4 (b) shows the DC voltage / voltage in this embodiment.
In the figure, b ″ represents the maximum DC voltage / current characteristic curve that can be output by the main thyristor converter 3b,
c ′ indicates the maximum DC voltage / current characteristic curve that can be output by the GTO converter 3c, respectively.

(Effects of the Invention) As described above, according to the first or second aspect of the present invention, a sufficient voltage margin is secured on the auxiliary conversion device or the main conversion device side to stably continue the arc without interruption. Can be.

In addition, the power supply device can reduce the reactive power taken from the AC power supply to prevent the power factor from decreasing during normal operation, and reduce the economical burden by eliminating the need for power factor improvement equipment such as a large-capacity capacitor. There are effects such as being able to do.

[Brief description of the drawings]

FIG. 1 (a) is a schematic configuration diagram showing a first embodiment of the first invention, FIG. 1 (b) is a DC voltage / current characteristic diagram thereof, and FIG.
FIG. 3A is a schematic configuration diagram showing a second embodiment of the first invention, FIG. 3B is a DC voltage-current characteristic diagram thereof, and FIG. 3A is a first embodiment of the second invention. FIG. 4B is a schematic configuration diagram showing an embodiment, FIG. 4B is a DC voltage / current characteristic diagram thereof, and FIG. 4A is a schematic configuration diagram showing a second embodiment of the second invention. (B) is a DC voltage / current characteristic diagram, FIG. 5 is a schematic configuration diagram showing a conventional technique, and FIG. 6 is a DC voltage / current characteristic diagram. 1 ... three-phase AC power supply 2a, 2b ... transformer for converter 3a ... auxiliary thyristor converter 3b ... main thyristor converter 3c ... GTO converter 3d ... diode rectifier 4 ... furnace bottom side Electrodes 5,5a, 5b DC reactor 6 Graphite electrode 7 High current bus 8 DC arc furnace A DC arc M Metal to be melted or heated

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor: Attack Motoyoshi 1-1-1, Tanabe-Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fuji Electric Co., Ltd. No. 1 Inside Fuji Electric Co., Ltd. (56) References JP-A-56-157270 (JP, A) JP-A 54-137728 (JP, U) JP-A 45-22179 (JP, Y1) (58) Field (Int.Cl. ⁶ , DB name) H02M 7/00-7/40 H05B ^7/ 00-7/22

Claims (2)

(57) [Claims] 1. A DC arc furnace power supply for supplying DC power to a DC arc furnace by a power converter connected to an AC power supply and operable as a rectifier, wherein the power converter is connected in parallel to a low-voltage power supply. It is composed of a main converter having a large current rating and an auxiliary converter having a high voltage / small current rating.When the arc current flowing through the DC arc furnace is large, the main converter and the auxiliary converter are operated to reduce the arc current. When the arc current falls below the level at which the auxiliary converter can flow, the main converter is stopped and only the auxiliary converter is operated, and the DC output voltage of the power supply is reduced by the auxiliary converter. A power supply for a DC arc furnace, wherein the power is increased to a voltage that can be output from a converter. 2. A power supply device for a DC arc furnace that supplies DC power to a DC arc furnace by a power converter connected to an AC power supply and operable as a rectifier, wherein the power converter is connected in series. A DC voltage that can be output by the main conversion device is higher than a DC voltage that can be output by the auxiliary conversion device, and a power supply device when the arc current flowing through the DC arc furnace fluctuates. A power converter for a DC arc furnace, wherein a fluctuation of the DC output voltage is secured on the main converter side.