CN1057658C - Direct electric arc furnace fed by controllable current and method for supplying controllable current to direct electric arc furnace - Google Patents

Abstract

Method for supplying a controlled current to a three-phase direct arc furnace and a furnace supplied with such power for smelting metals and ferrous alloys, comprising means for regulating the arc length; a furnace power supply comprising at least one medium voltage line and a transformer supplying the furnace; elements for regulating the arc current in each phase consisting of a regulating element comprising at least one inductor, a controllable thyristor switch or a saturable reactor connected in parallel with at least part of the inductor, which together with means (SI) measure the arc current intensity and work together with means (GC) which control the furnace arc current by acting on the value of the equivalent series reactance.

Description

Direct electric arc furnace fed by controllable current and method for supplying controllable current to direct electric arc furnace

The invention relates to a three-phase direct arc electric furnace supplied with controllable current. It also relates to a method of supplying a controllable current to a three-phase direct arc electric furnace.

The invention is used in three-phase electric arc furnaces for melting metals, especially iron and its alloys.

Direct electric arc furnaces are currently mainly used for melting and refining steel, and almost exclusively are three-phase electric furnaces.

In the past two decades, the power of a single furnace has been greatly improved, and the power of equipment has increased from 16MW and 20MVA to more than 85MW and 120MVA.

Such a large power brings big problems such as voltage disturbance (fluctuation) to the power supply grid, as well as considerable phase shift due to inductive loads.

To correct the phase difference and reduce voltage fluctuations caused by this inductive load, modern compensation techniques use various power-free compensators operating with controllable diodes.

The regulation principle is shown in Figure 1 and described as follows:

Three inductors and three-phase medium-voltage lines are arranged in parallel, and this line is used as the electric furnace power supply for strong inductive loads; these inductors are powered by thyristors T, and their conduction angles are controlled according to the current detected by the device SI.

This regulation system keeps the total reactive power required by the furnace constant and balanced at zero. Inductors L1 and L2 and multiple sets of power factor correction capacitors CR are connected to the medium voltage power line.

Multiple groups of power factor correction capacitors CR plus appropriate inductors can also achieve the function of filtering the harmonics generated by the electric furnace and compensation system.

With the help of a suitable hydraulic device GI to change the electrode height, try to keep the resistance of the arc constant, so as to adjust the active power of the arc of the electric furnace.

In order to overcome the difficulties and disadvantages of this type of indirect regulation of the current drawn, direct arc furnaces have recently been produced. This type has a single electrode and the return of the current is by the shell of the furnace.

The supply current of the arc is provided by a rectifier device composed of controllable diodes or thyristors. This system has two major drawbacks. On the one hand, it is difficult to obtain the return path of the current, and on the other hand, the rectification system produces strong odd harmonics.

In order to eliminate these serious disadvantages of these two electric arc furnaces, the applicant has designed, tested and carried out the present invention, which has as its object the intended purpose.

The invention provides a three-phase direct arc electric furnace supplied with controllable current, comprising: electrodes; means for adjusting the height of the electrodes so as to control the length of the arc generated; At least one medium voltage line for power supply and a furnace transformer, a part of said main power supply, which connects said medium voltage line and said furnace transformer and includes a series connection between said medium voltage line and said furnace transformer A saturable reactor and a device for measuring the intensity of the current drawn by the arc; characterized by: a device for controlling the reactance of the saturable reactor according to the intensity of the current drawn by the arc.

Method for supplying controllable current to a three-phase direct arc electric furnace for melting metals and, although advantageously, not exclusively for melting base alloys, characterized in that a control device acts directly on the arc current of the electric furnace and makes the equivalent series The overall value of the reactance changes.

According to the present invention, the control mechanism acts directly on the arc current of the electric furnace to determine the operating point and reduce disturbances, which is different from the prior art, which allows the current in the electric furnace to be generated freely, and only uses the hydraulic system to adjust the arc length. control, while the underlying anti-fluctuation control system attempts to regulate the state of the main power supply side.

On the other hand, present three-phase electric arc furnaces are usually connected to a compensation system that works independently and in parallel with the electric furnace. According to the three electric arcs of the electric furnace of the present invention, according to a concept of its solution, each electric arc is applied by a first Inductor _L1 limits the first base current to supply power. The second current is superimposed on the first current by the second inductor _L2 , by means of the thyristor T, according to the transfer function that analyzes the value of the first basic current and/or its initial slope or trend taking into account the operating state of the arc operation and adjustment.

According to another solution, in addition to analyzing its value and/or initial slope, it is also necessary to analyze the state of the electricity in question throughout the installation, in particular at the transformer on-load tap changer.

According to yet another aspect of the concept of the solution, a saturable reactor RS may be used instead of the inductors _L1 and _L2 and the thyristor T as appropriate.

According to the invention, the power factor correction capacitor is also used as a filter for absorbing the harmonics generated by the electric furnace to the grid, arranged in parallel on the medium voltage busbar in a completely similar manner to the prior art, but with much smaller capacitance values many.

Referring to the drawings provided as non-limiting examples, Figure 1 represents the prior art.

Figure 2 illustrates the invention and makes the difference between the invention and the prior art apparent. Figures 3 and 4 represent various scenarios of solution concept.

Now let's look at the prior art and content of the invention in detail.

The present invention is different from the prior art in the part between the medium voltage line and the electric furnace transformer.

In Fig. 1, the inductor _L1 has the function of making the operating point of the electric furnace more flexible and optimized by choosing its value according to the available delivered power, arc length and radiation index.

The selection of the inductor _L1 is achieved by determining the operating point, which must weigh the following opposite requirements: not only to ensure the appropriate conversion power and high enough arc current for the technological requirements of the smelting process, but also to limit it to The peak current when the electrodes are short-circuited.

The selection of the inductor _L1 indirectly affects the arc heat radiation and it should vary between the minimum value required considering the production efficiency and the maximum value due to the limitation of refractory lining wear and compliance with the relevant safety limits.

Thus, it is important to compensate for the inductive reactive power drawn by the electric furnace.

The necessary capacitive reactive compensation power is obtained by connecting a fixed power factor correction capacitor bank CR (usually a neutral point insulated or grounded star connection) in parallel to the medium voltage line, and using a fixed inductor _L2 and A controlled thyristor switch T (inductor controlled by a thyristor) results in a variable inductance. Inductor _L2 is a delta connection.

Inductor LF is also used as a filter and is placed in series with thyristor T and capacitor CR.

The need for variable compensation cannot be justified by requiring that the average power factor obtained at the source point be higher than specified by the utility without compensating extremely quickly for the peaks of reactive power drawn that cause disturbances in voltage fluctuations in the grid. .

Capacitor bank CR and inductor L ₂ are selected according to the maximum reactive power required by the electric furnace (equal to the short-circuit power of the furnace), and the reactive power should be used due to the incomplete compensation device (CR+L ₂ +T) The reason for compensation is to correct with a coefficient greater than 1.

Inductor LF also acts as a filter and is arranged in series with thyristor T and capacitor CR.

The measurement of the reactive current drawn by each supply phase of the electric furnace is carried out phase by phase with the device SI, which generates the arc current control system feedback signal.

At all times, the capacitive current drawn by capacitor bank CR must balance the inductive current drawn by the furnace and inductor _L2 controlled by thyristor T.

The second adjustment device marked GI in the figure is related to the circuit configuration for controlling the arc resistance.

For example, suitable hydraulic servo GI arrangements are known to move the electrodes vertically to keep the furnace impedance constant.

Mechanical regulation has a significantly slower time constant than the above-mentioned electrical regulation types and is therefore less effective against electrical disturbances.

Turning now to the present invention shown briefly in FIG. 2, it will be found that inductor _L1 performs the same function as a similar element contained in the prior art shown in FIG.

A variable inductor is obtained with a fixed inductor _L2 and a thyristor operated valve T, and is installed in parallel with inductor _L1 , thus providing a variable inductance in series with the furnace power supply.

The device SI measures the intensity of the current drawn by the arc and sends a signal to drive the control system of the thyristor T.

In this way it is possible to keep the current drawn by the furnace constant within wide limits, thus obtaining a controllable current supply.

The change in arc impedance is compensated by the opposite change in impedance of an equivalent inductor arranged in series, which consists of _L1 and _L2 connected in parallel.

For example, if the arc tends to disappear, reduce the inductance to increase the current.

Conversely, if the electrodes are short-circuited by the scrap being smelted, the inductance of the inductor becomes maximum in order to limit the resulting voltage drop in the grid. That is, there is a tendency to correct the causes of disturbances in the grid rather than their consequences with static variable compensators as in the prior art.

The automatic control of the equivalent series inductance, dependent on the arc current, thus forms an innovative aspect of the configuration shown.

The illustrated control can also work in conjunction with the arc length hydraulic adjustment GI.

Although both types of control are designed to maintain a constant impedance on the furnace side, they have some differences. The adjustment device GI acting on the arc length to affect the electrode position can only change the resistance part of the impedance, while the electrical adjustment of the equivalent series inductance (L ₁ ·L ₂ in parallel) directly changes the reactance part by acting on the arc current, and also affects the equivalent Resistors work.

Again, the time constants are quite different, since one case involves mechanical actions while the other involves only electrical actions.

The adjustment of the equivalent series reactance carried out phase by phase can also correct the inherent impedance imbalance in the secondary circuit configuration of the electric furnace (from the output of the electric furnace transformer to the arc), and the current in the three phases can be kept constant, thus overcoming So-called "cold phase" (cold phase) and "wild phase" (wild phase) problems.

The inductors controlled by thyristors T, both in the prior art and in the technique proposed here, are actuated according to the signals from the monitor SI, which are processed by a control device GC.

In the technique proposed here, the control device GC can also receive signals reflecting other electrical changes in various parts of the circuit. Thus, for example, it can receive a measurement signal from a medium voltage line by means of a transformer TV, and also, for example, an electrode position signal obtained by controlling a GC, which can receive signals from other sources, such as a transformer on-load changeover switch Positional or other set signal.

Also, the control means GC prevents saturation of the inductor by eliminating the direct current component of the current through the inductor.

The power factor correction capacitor CR is connected to the medium voltage line, and it has the function of correcting the reactive power factor of the power absorbed by the electric furnace to the given limit of the power supply department.

As can be seen in Figures 1, 2, the construction according to the invention provides the same parts as those already included in the usual construction. But these partial functions are exploited in different ways.

The different application of the parts with different selected values brings about several important structural savings.

The comparison of the two solutions can be carried out under the condition that the effective power supplied to the electric furnace is equal and the fluctuation disturbance generated in the grid is equal.

Inductor L ₁₁ , despite its relatively high reactance, is normally injected only part of the furnace operating current in the present invention. In terms of its power, and thus its price, and considering the safety factor of short-circuit overload, it is about 30-40% of what is required in the composition of Fig. 1 .

The power factor correction capacitor CR is also reduced a lot. In fact, in the case of Fig. 1, as we have already noted, the capacitive reactive power is chosen at a value greater than the short-circuit power at the electrodes. In the case of Fig. 2, only a part of the reactive power factor absorbed by the electric furnace at the operating point needs to be corrected, and this reactive power is reduced by about 70-80% as a result.

The value of the inductor L ₂ controlled by the thyristor switch T can theoretically be taken as zero, which allows the range of maximum adjustment of the series reactance of the electric furnace.

Compared with the example of Fig. 1, it is reduced by about 80-90% due to technological reasons associated with the melting process and the example of the thyristor T, if precautions are taken into account.

According to the invention, the thyristor T itself is selected at low voltage and not too high current, the selected value is reduced by a factor of about 40-50%. This value is obtained by calculating the product of the maximum voltage applied to the thyristor T multiplied by the maximum current passing through it.

In addition to the savings obtained in the construction of the various parts, an improvement in operating costs must also be taken into account, which is mainly due to the reduction of the electrical variation of the arc.

In fact, the improved current stability and equalization of the three phase currents results in higher process efficiency, less wear of electrodes and refractory linings, and less electrokinetic stress in the event of a short circuit.

Since the concept of the present invention is based on the automatic control of the reactance connected in series in the electric furnace side circuit, several variants for realizing this control in different ways can be proposed.

Figure 3a is the first such variation, which eliminates the inductor _L2 but retains the thyristor switch T; as we have seen before, the inclusion of the inductor _L2 is not substantial due to the technical reason.

Fig. 3b is another variant, which proposes that the regulation is not continuous, but graded. A number of center taps are branched from the inductor _L1 , which are controlled by thyristor switches. This scheme can make the control simpler, but does not allow fine adjustment of the furnace side impedance, and thus does not allow fine adjustment of the phase current.

The scheme of Fig. 4 proposes to replace L ₁ , L ₂ and T with a saturable reactor RS.

The saturable reactor is excited by a suitable constant direct current supplied by the control circuit GC. Its characteristic is that it has a low reactance value at a small current lower than the rated current _IN of the electric furnace, and a high reactance value at a high current.

This has the effect of considerably limiting overcurrent and thus voltage fluctuations.

The advantage of this solution is that no complex control system GC is required. In fact, when the DC current corresponding to the operating point of the furnace has been set, the saturable reactor automatically limits the overcurrent.

The control circuit GC fixes the excitation current of the saturated reactor according to the operating point of the electric furnace.

To obtain this function, the regulator GC is connected with the electrode height regulator GI and the on-load voltage tap changer of the electric furnace transformer.

According to one solution, the regulator GC not only refers to the signal of the regulator GI, but also analyzes the state of the electricity contained in the various locations within the device.

Claims (17)

1. controllable current is supplied with the method for the three-phase direct-arc electric furnace that is used for smelting metal, being comprised:

By a main power source to described electric furnace supplying electric current, described main power source comprises a power supply network, line ball at least one, a furnace transformer, in each connects in mutually in line ball and the transformer portion and comprise one be series at described in the adjusting arc current device of saturable reactor between line ball and the described furnace transformer, it is characterized in that:

According to the intensity of electric arc current drawn, utilize a control device to change the reactance of described saturable reactor.

2. the described method of claim 1, when described adjusting arc current device by inductor (L ₁) when forming, the whole numerical value of equivalent series reactance is by settling one to be parallel to the first inductor (L ₁) the second inductor (L by thyristor (T) control ₂) change.

3. method as claimed in claim 2, wherein the numerical value of equivalent series reactance integral body is by thyristor (T) the bypass first inductor (L ₁) change.

4. method as claimed in claim 2, wherein the whole numerical value of equivalent series reactance will be to the first inductor (L with thyristor (T) and centre tap ₁) a part connect or disconnect and changing step by step.

5. the method for claim 1, the reactance of described adjusting arc current device changes by the direct current polarization electric current is worked.

6. the method for claim 1, wherein the inherent characteristic of the saturable reactor that encouraged of the Constant Direct Current electric current that is provided by control device makes the reactance of reactor change and change according to the electric furnace electric current.

7. the described method of above-mentioned any one claim, wherein control device will be analyzed the state of some other relevant electric weight (voltage changer, setting signal etc.) at least.

8. the method for claim 1, wherein control device will analyze and consider that transformer has the state that carries tap changer.

9. the method for claim 1, wherein control device is also analyzed the initial slope (trend) of arc current.

10. the method for claim 1, the wherein also state of the hydraulic system of analysis and Control electrode position (GI) of control device.

11. the method for claim 1, wherein control device is determined electric imbalance compensation between each phase.

12. the method for claim 1, wherein control device prevents the saturated of inductor with eliminating by the DC component in its electric current.

13. be provided with the three-phase direct-arc electric furnace of controllable current, comprise:

Electrode;

Be used to regulate electrode height so that the device of the arc length that control is produced;

A main power source, it comprises a power supply network, for electric furnace power supply at least one in line ball and a furnace transformer, a part of described main power source, it connects described middle line ball and described furnace transformer and comprises a saturable reactor that is series between described middle line ball and the described furnace transformer and the device of measuring the current strength of being drawn by electric arc; It is characterized in that:

Intensity according to the electric arc current drawn is controlled the device of described saturable reactor reactance.

14. electric furnace as claimed in claim 13, wherein saturable reactor is handled by control device (GC).

15. as any described electric furnace in claim 13 and 14, wherein control device (GC) is handled electric quantity signal (S1, TV, setting signal etc.).

16. electric furnace as claimed in claim 13, wherein control device (GC) is received transformer tap changer under carrying.

17. electric furnace as claimed in claim 13, wherein control device (GC) processing machine amount signal (GI).

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