RU2476034C2 - Power control method of alternating-current electric arc furnace using controlled combined reactor transformer - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method consists in power adjustment as to phases by controlling the arc current of each electrode and control of electric parameters of low-voltage circuit due to introduction to the diagram of indirect voltage control as the second electromagnetic unit of controlled combined reactor transformer and increase in inductive resistance of its secondary windings.

EFFECT: increasing productivity; reducing specific power consumption of the furnace and improving the operating reliability of the control device, and minimising negative action on power line.

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Description

The invention relates to electrical engineering, namely to electrometallurgy and to methods of regulating power in three-electrode AC arc furnaces, and is aimed at increasing their productivity, reducing specific energy consumption, reducing the negative impact on the power supply network and improving the reliability of the control device.

A known method of regulating the power of a three-phase arc electric furnace, which consists in controlling the magnitude of the arc current of each electrode and regulating the reactance of a short network using batteries of static capacitors (BC), connected in parallel with the phase with the highest reactance [Pat. 2275759 Russian Federation, IPC Н05В 7/148 (2006.01), declared. 09/01/2004].

Connecting the BC to the secondary voltage of the furnace transformer requires that the condition of the sinusoidality of the current of the three-electrode arc furnace be met [2]. This causes difficulties when using this method in practice, because the furnace current has a non-sinusoidal character, due to the conditions of arc burning and the influence of random factors on its resistance.

When operating, arc furnaces generate electromagnetic interference. Voltage fluctuations are caused by fluctuations in arc currents. The asymmetry of currents and voltages occurs when the load is distributed unevenly in phases or in case of emergency. Overvoltages on the higher voltage side of the furnace transformer are caused by switching electric furnace transformers with vacuum circuit breakers, and voltage dips are caused by short circuits of the electrodes on the charge. The presence of non-sinusoidality of the shape of the voltage curve, higher harmonics of currents and voltages is explained by the specificity of the furnace load. These interferences have a negative effect on the supply network, reduce productivity and increase the energy consumption of the furnace.

The specified method has some disadvantages. Significant overloads of static capacitor batteries with currents of higher harmonic components and difficult working conditions with frequent switching of capacitors are possible. This causes their premature failure. Reducing the reactance of a short network by turning on the BC can reduce the stability of arc burning. This leads to an increase in current and voltage fluctuations. The considered method does not allow to minimize the depth of the voltage dip occurring on the higher voltage side of the furnace transformer during operational short circuit of the electrodes on the charge.

During the operation of the furnace unit, a dynamic change in the reactance of the short network is observed. This leads to the appearance of an asymmetric mode of operation, which causes a shift in the neutral point of the load relative to the neutral point of the transformer. In this case, the productivity of the furnace is reduced in comparison with the symmetric mode of operation [3]. The sharpest fluctuations in the reactance of a short network occur during the melting period. They are due to the characteristics of the process. The first period is characterized by unstable combustion and arc breaks, as well as short circuits of the electrodes on the charge. The main impact is exerted by significant movements of the electrodes and a change in the current path in the working space of the furnace. As a result of this, the furnace resistance also changes, the magnitude of which is determined to a greater extent by reactance. It can reach 97.7% of the total resistance. On the investigated DSP-100 furnace, the impedance value ranges from 4 to 16 mOhm. Due to the asymmetry of the electrical parameters of the short network, one phase, called the "wild", has the lowest reactance, the largest current and radiation. Due to this, the electrode of this phase quickly melts the solid charge, filling and sinks harder. The location of the electrodes and flexible cables of the short network changes relative to each other, and there is an increase in the asymmetry of the electrical parameters of the short network. According to [4], the phase imbalance is calculated by the expressions

where R _iperc is the bias of the power of the i-th phase of the electric furnace, W;

I is the specified current in the electrodes, A;

X _i - reactance of the low-voltage circuit of the i-th phase of the furnace circuit, Ohm.

In the case of equality of the electrical parameters of the phases of the furnace installation, there is no skew power. Power is transferred from one phase to another due to electromagnetic induction, and its total value in the electric circuit of the arc furnace is zero. In the general case of asymmetries, all three phases participate in energy exchange, and the operating conditions of each of them are different [4].

As the electrode deepens, more and more of the arc radiation is shielded by the charge. The radiation of the arc of the "wild" phase does not pose a serious danger to the lining of the walls and the arch of the furnace and is almost completely spent on melting the charge. It does not compensate for the decrease in radiation of other phases. This explains the decrease in furnace productivity in asymmetric mode. During the recovery period, the change in reactance of the short network is less significant, and the arc radiation is not shielded by the charge. Compared to the melting period, the lining of walls and the arch absorbs a greater amount of energy emitted by the arcs. Its part, located near the "wild" phase, is destroyed faster. This causes furnace downtime required for repairs. Unit performance is reduced. Therefore, from the point of view of minimizing the negative effects of the radiation of the arc of the “wild” phase, the alignment of the electrical parameters of different phases of the short network during the recovery period is of crucial importance. The above confirms the relevance of reducing the power imbalance in phase and minimizing the offset of the neutral point of the load relative to the neutral point of the furnace transformer in all modes of operation of the electric furnace.

The task of the invention is to increase productivity and reduce the specific energy consumption of an electric arc furnace, minimize the negative impact on the supply network and increase the reliability of the control device.

The supply voltage of the electric furnace is regulated using a transformer unit. In most cases, an indirect regulation scheme consisting of two electromagnetic units. As the first, an adjusting autotransformer is used, and as the second, a step-down transformer [5].

To regulate the power of the electric furnace, it is proposed to regulate the arc current of each of the electrodes and align the electric parameters of the short network in phases by changing the reactance of the short network by including a combined controlled reactor transformer in the indirect voltage control circuit as the second electromagnetic unit and increasing its secondary inductance windings corresponding to the phases with the smallest reactance of the short network. Power regulation of an arc three-electrode AC electric furnace is carried out in all modes of its operation. SURT has some design differences from the power transformer and can operate in transformer, reactor and reactor-transformer combined mode [6]. In this method, it is used in a reactor-transformer mode.

In the case of a decrease in reactance and an increase in the current of the "wild" phase, to equalize the power in phases, the inductive resistance of the secondary winding of the SURT corresponding to this phase is increased. An example of including an SURT in an indirect furnace voltage (EA) voltage control circuit with an autotransformer is shown in Figure 1. The electrical circuit for replacing the secondary circuit of the furnace transformer of an electric arc furnace with a short network configuration "asymmetrical triangle on electrodes" will take the form shown in Figure 2. On figure 2 the following notation: jX _PTA , jX _PTC , jX _PTB - reactance of the secondary windings of CURT; jωL _KCn - reactance of the n-th cable of the short network; jωL _B is the reactance of the furnace bath, taking into account the resistance of the electrode; Z _D - nonlinear resistance of the electric arc. With an increase in the reactance of one phase and a decrease in its value in two other tales, in order to equalize the power in phases, the inductance of the secondary windings of the SURT corresponding to the phases with the lowest reactance of the short network is increased.

The proposed method is applicable to electric furnaces, the voltage regulation of which is carried out according to indirect control schemes, and the short network has one of the following configurations: "asymmetrical triangle on the electrodes", "triangle on the electrodes", "triangle on the electrodes with an additional phase rack", "triangle on movable shoes. "

Using the method of regulating the power of an AC electric arc furnace using SURT allows to minimize the phase imbalance and the offset of the neutral load point relative to the neutral point of the furnace transformer. The radiation of the arc of the "wild" phase and the overheating of the lining of the walls and the arch of the furnace near it during the recovery period are reduced. The stability of arc burning increases, and fluctuations in currents and voltages decrease. In the event of an operational short circuit of the electrodes on the charge, an increase in the reactance of the operating windings of the SURT minimizes the depth of the voltage dip that occurs on the primary voltage side of the furnace transformer. The power introduced into the furnace increases, the duration of the melting cycle and the duration of periods of downtime of the furnace, designed to carry out repair work to restore the lining of the walls and arch near the "wild" phase, decrease.

The technical result is to increase productivity, reduce the specific energy consumption of the furnace, increase the reliability of the control device and minimize the negative impact on the supply network. The advantage of the method is to minimize the negative impact on the supply network, and also that, in comparison with the BC, the combined controlled reactor-transformer is less sensitive to the effects of higher harmonic components of the current of the electric furnace, has a long service life and high reliability during operation.

Information sources

1. Pat. 2275759 Russian Federation, IPC H05B 7/148 (2006.01). A method for controlling power in phases of a three-electrode AC electric furnace [Text] / Shpiganovich AN, Zakharov KD, Zatsepin EP, Bosch V.I .; Applicant and patent holder Lipetsk State Technical University. - No. 2004126602/09; declared 09/01/2004. - 5 p.: Ill.

2. Mineev, R.V. Improving the efficiency of power supply of electric furnaces [Text] / R.V. Mineev, A.P. Mikheev, Yu.L. Ryzhnev. - M .: Energoatomizdat, 1986. - 206 p.

3. Althausen, A.P. Electro thermal equipment. Handbook [Text] / A.P. Altthausen. - M .: Energy, 1980 .-- 416 p.

4. Dantsis, Ya.B. Methods of electrotechnical calculations of ore-thermal furnaces [Text] / Ya.B. Dancis. - L.: Energy, 1973. - 184 p.

5. Anshin, V.Sh. Transformers for industrial electric furnaces [Text] / V.Sh. Anshin, A.G. Krayz, V.G. Meykson. - M .: Energoizdat, 1982. - 296 p.

6. Zabudsky, E.I. Combined adjustable electromagnetic reactors [Text] / EI Zabudsky. - M .: Energoatomizdat, 2003 .-- 436 p.

Claims (1)

A method for controlling the power of an electric furnace, in which the arc current of each of the electrodes is adjusted and the electrical parameters of the short network are aligned in phases, characterized in that for equalizing the powers in phases by changing the reactance of the short network, the indirect voltage control circuit includes combined controlled reactor-transformer and increase the inductive resistance of its secondary windings corresponding to phases with the smallest p short circuit resistance.

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