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High-impedance electric arc furnace power supply circuit

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0 Introduction

High-impedance electric arc furnace is a new type of high efficiency furnaces, it has a series of outstanding advantages: to substantially reduce the power and electrode consumption, can significantly reduce the impact of short-circuit power supply grid and harmonic pollution.

High-impedance electric arc furnace to draw the past 25 years all the new electric arc furnace steel-making technology, coupled with the successful application of foaming slag, making the exchange has been the slow development in the electric arc furnace stability, efficiency and reduce the impact on power grids in both short-circuit can be comparable with the DC electric arc furnace.

In this paper, with saturable reactor reactor and fixed high-impedance electric arc furnace. The former has a high volt-ampere characteristics, so that short-circuit current is very small, primarily electric arc furnace to achieve a constant current characteristic.

1 of the high impedance electric arc furnace power supply

1.1 pairs of power supply reliability

Electric arc furnace thermal processing equipment is, if the half-way power, can cause great loss: to increase power consumption and raw materials, product quality or even the entire furnace steel scrap, the greater the loss of the larger stoves. In accordance with the provisions of the relevant norms, electric arc furnace is the secondary load.

50t capacity of the furnace and electric arc furnace is usually more than two independent high-voltage power supply may be relatively small furnace volume all the way high-voltage power supply.

1.2 Determination of the public power supply point

Electric arc furnace power supply point means a public power system with the power supply point is connected, and then load other users. Of public power supply requirements of the main points to consider the following factors:

1) power supply transformer capacity to be able to adapt to the requirements of electric arc furnace load characteristics;

2) caused by the electric arc furnace load point of the public power supply voltage fluctuations and voltage flicker values, as well as the harmonic current values shall not exceed GBl4549-93 GB of allowable values;

3) caused by the electric arc furnace load voltage public electricity supply points, degree of asymmetry shall not exceed 2%.

Electric arc furnace of the public power supply situation, there are two points, one of which is directly related to electric arc furnace system is connected to the power system; the other is electric arc furnace system, through the company substations and power systems. Electric arc furnace transformer power supply is generally not for the workshop.

When the electric arc furnace by the company bus power substation, in order to prevent the load on the other have a negative impact on quality of supply, the general requirements for the capacity of power transformers for electric transformer capacity of more than 2.5 times. When unable to meet this requirement, or increase the capacity of power supply transformer; or dedicated power supply intermediate transformer, which is required to determine the technical and economic comparison.

When using a dedicated power supply when the middle of the transformer, the transformer capacity of choice, often with electric transformer overload running suit. At this point, the power supply transformer secondary side voltage fluctuations can be unrestricted; when the power supply transformer secondary side with the dynamic reactive power compensation devices, the transformer capacity of the load should be compensated choose.

2 high-impedance electric arc furnace of the main circuit

High-impedance electric arc furnace main circuit and main circuit of the traditional main difference is that the former is the main circuit in a great series reactor. It enables continuous and stable arc burning, reducing the arc current, arc voltage increase in arc power up, power efficiency, harmonic occurred on power supply capacity and reduce the impact of power grid.

High-impedance electric arc furnace as a result of the work of current, so its current carrying conductors in the second, short cross-sectional area of a small network, this is the superiority of high-impedance electric arc furnace.

Reactor Reactor is divided into fixed and two saturable reactor. The disadvantage is that the former does not automatically adjust the value of reactance. When the technology changes, need to change the reactor, it is necessary to bring the electrode, power, and then to change the reactor; and saturable reactor furnace in accordance with the status can automatically change the reactance value, basically reached a constant current electric arc furnace operation.

The following with a different reactor, respectively, the high impedance electric arc furnace for discussion.

2.l reactor with a fixed high impedance electric arc furnace

In the high impedance electric arc furnace, using high-voltage, low current, long arc operation, select the appropriate power factor, and a suitable reactor system in order to achieve stable operation is essential. In most cases, must be used reactor in series with the electric transformer. Reactor with a fixed high impedance electric arc furnace of the main circuit shown in Figure l.

High-impedance electric arc furnace power supply circuit

This high-impedance electric arc furnace design features are as follows:

1) reactor inductance effects and high energy from the dynamic characteristics of arc voltage obtained arc stability conditions, resulting in highly integrated power input;

2) short-circuit current, when the scrap collapse, the electrode, electrode arm and cables on the current small, so the risk of electrode damage to small, less mechanical wear;

3) small fluctuations in the electrode current, and thus interfere with the grid is also small;

4) reactor type tap-coil is often made in accordance with different processes need to change the reactance value;

5) When a reactor in series connected to the circuit, the reactor may be the relative value oftype (1) calculated, XK% = 100 * (QK / Se)% (1)

Where: QK reactor for the rated capacity, Kvar;

Se rated capacity for the electric arc furnace transformer, KVA.

6) series reactor and transformers, are running at heavy load, therefore, its thermal stability and higher mechanical strength;

7) of the existing electric arc furnace transformer and a little shorter to improve the system can be of high impedance.

Transformer secondary side is equivalent to the total reactance of the system operation of electric arc furnace process can be expressed by the following relationship (in this case assuming that there is a strong equilibrium of the three power grid):

High-impedance electric arc furnace power supply circuit

Where: U for the transformer secondary voltage;

Uarc for the arc voltage;

I for the electrode current;

X is equivalent to the transformer secondary side reactance of the overall system;

R is equivalent to the transformer secondary side resistance of the overall system;

For the active power P;

for the phase angle φ.

By Type (5) is clear-cho: For the same power and power factor, increase the reactor can reduce the electrode current.

In order to illustrate the different systems the total reactance of the impact of electric arc furnace operation, Table 1 gives the same 3 companiescapacity (90MVA), different reactor running instance of the electric arc furnace.

High-impedance electric arc furnace power supply circuit

A typical example of the traditional electric arc furnace-based sauce, while the example of B is designed as a high electrical resistance and low current operation, arc power and the same instance of A, the electrode only current 50kA, examples of A as 65kA. Examples of c is in full accordance with the design of high-impedance electric arc furnace, the secondary voltage up to 1100V, the total reactor system for 8.2mΩ, electrode current of 50kA, loss of power is very small, only 2. . 3MW, power efficiency is very high. Small short-circuit current, and only 93kA, short-circuit current in multiples of only 1.86 times. The advantage is to reduce the impact of power grid, so that a more stable arc.

This 3 characteristics of the furnace load (active power, power factor, arc power), respectively, in Figure 2 and Figure 3 below.

High-impedance electric arc furnace power supply circuit

2.2 saturable reactor with the high impedance electric arc furnace

Saturable reactor is a magnetic field at the same time there is a constant and alternating magnetic field under the reactor to work. Saturable reactor reactor because of the constant changes in the magnetic field changes in the characteristics of being widely used in a variety of power adjustment device. Use of saturable reactor to limit the fall characteristics of short-circuit current, electric arc furnace in a vacuum there have been examples of successful applications, for this purpose and the use of the reactor, sometimes called current compensation reactor.

When the normal work of a high impedance electric arc furnace, the main circuit of the current rating, when the saturable reactor magnetized by the largest role in the characteristic curve of the operating point in Figure 4 of a point below, saturable reactor smaller voltage drop. When the work of the furnace in case of short circuit, the flow through reactor to increase the AC current, and DC current is unchanged, when the operating point to the same curve on the 1 o'clock), can be seen from Figure 4, when saturable reactor voltage drop a lot, thus limiting the short-circuit current. Saturable reactor that is the magnetization with the main circuit automatically required to change the voltage.

High-impedance electric arc furnace power supply circuit

Saturable reactor with a high-impedance electric arc furnace main circuit as shown in Figure 5. Saturable reactor is the use of ferromagnetic materials, nonlinear magnetization curve of the work. Each phase saturable reactor can be regarded as the single-phase two-winding transformer, which turn with the load line NL (electric transformer) in series, known as load windings; another turn with the NL line NC electrical isolation, and pass in order to direct current ( IDC), known as the control winding.

Saturable reactor safety by controlling the winding turns, core saturation adjustment, as long as the load winding turns over control of security and low winding (equivalent to Figure 4 of a point), then load windings have very low voltage drop or even negligible. If the load current ILMAX ≥ IDC * NC / NL, core saturation will be reduced, and any incremental load current will have a large magnetic flux change, the results generated in the load winding larger voltage drop (equivalent to Figure 4 The b-point). This is the resulting volt-ampere characteristics of fall-type theory.

Controlled by changing the current IDC, on the possibility of O to the maximum allowable by the scope of current control load current. When the load current tends to exceed ILMAX, the saturable reactor will have a larger voltage drop will limit the current value of ILMAX. By choosing the control current, saturable reactor or to the entire current control mode or as a peak current limiter work.

2.3 application

Ferriere Nord, Italy steel 80t DANARC the exchange of electric arc furnace is the use of saturable reactor to control the high-impedance electric arc furnace. The main data of the electric arc furnace are as follows:

Furnace shell diameter 5300mm;

Electrode diameter of 600mm;

Electrode diameter 1150mm;

Furnace Transformer 55MVA +20%;

Maximum active power 43MW;

The largest sub-voltage 985V;

Saturable reactor capacity of 76MVA;

Saturable reactor excitation system 0.4MVA.

High-impedance electric arc furnace power supply circuit

3 reactor of the over-voltage protection measures

The operation of vacuum circuit breaker over-voltage is due to the presence of circuit inductance, capacitance, such as energy storage components, a switching operation in the instantaneous release of energy in the electromagnetic oscillation circuit and overvoltage occurs. Inductive load in the electric circuit, vacuum circuit breakers breaking operation will have a serious form of high-frequency Sasser. Have been measured over the maximum value of approximately 4.5 times the peak power. High-impedance electric arc furnace transformer to the original series a big reactor, the inductor value is very large, resulting in breaking of the over-voltage is very high, has been in operation at the scene of the high impedance electric arc furnace indeed proved this point, therefore, must take special effective over-voltage protection.

Commonly used over-voltage protection measures to protect and RC arrester protection. The former there are several different options, but the best option as shown in Figure 7.

High-impedance electric arc furnace power supply circuit

High-impedance electric arc furnace power supply circuit

Such dual-type over-voltage protection RC's run results show that the breaking to eliminate over-voltage oscillation, R1C1 main phase overvoltage protection, R2C2 mainly to over-voltage protection. Used to absorb the energy storage phase circuit R1C1 should be chosen value O. More appropriate to the 1μF capacitor. According to "Electrical engineering Handbook" referred to in the third high-voltage switchgear, for frequent switching operation of the vacuum circuit breaker electric arc furnace transformer, over-voltage protection device R1C1 election C1 = Ol ~ O. 2uF, R1 = 100Ω.

RC combination of the C2 device in order to eliminate the access is relatively over-voltage, while three sets of conventional RC absorption device-to-ground current is too large and the resistance R1 to burn defects. Xi'an High Voltage Apparatus Research Institute of Jinzhou Power Capacitors and cooperation in the development of the modular plant RC overvoltage protection, and have through the identification and mass production, the use of the past few years who have never step circuit device over-voltage accident occurred.

On the second option, with zinc oxide arrester overvoltage cut-off operation and there are different programs, the best combination is the three-phase zinc oxide arrester, is shown in Figure 8.

High-impedance electric arc furnace power supply circuit

It could inhibit the breaking caused by vacuum circuit breakers and relative phase over-voltage operation, to protect the transformers and vacuum circuit breaker to prevent the phase and relative to the purpose of flashover, three-phase modular MOA asked to achieve phase and relative to At the same time, protection, and therefore a modular three-phase surge arresters can be replaced by ordinary type arrester 4. Voltage of 35kV, the use of Y0.1W ~ 41/127 × 41/140 type.

Vacuum circuit breakers cut off the electric transformer, is usually the case in the no-load operation (except motion protection device). Experience has shown that the vacuum circuit breakers cut off the transformer at no-load, over-voltage generated by the highest, must be taken to strengthen the type of over-voltage suppression measures. Therefore, high-impedance electric arc furnace equipment, the adoption of the absorber RC (RC) and double lightning arrester protective measures is needed. Its working principle is over-voltage capacitors slow the first wave, with peak over-voltage surge arresters limit. Because the latter is a discharge gap and varistor zinc oxide non-linear form of the series. Generate over-voltage, the discharge gap by the breakdown, over-voltage increase in the zinc oxide nonlinear resistance, the resistance rapidly decreases, the current flow increased rapidly, thus limiting the over-voltage.

Vacuum circuit breaker with reactor type and the length between the connection with the over-voltage value as well. If the vacuum circuit breakers and reactor cable connection between, as the cable itself and the larger the inductance distributed capacitance, then connect the cable length and the reactor had to bear a direct function of voltage - connect the cable length and over-voltage multiplier into inverse proportion relationship, that is, the longer the cable, the reactor under the lower over-voltage multiplier. When connecting cables when the length is less than 6m in the reactor must repeat the installation of the original RC and the absorption of zinc oxide surge arresters.

4 Conclusion

High-impedance electric arc furnace is to rely on the basic principle of the transformer secondary voltage to increase to increase the arc power, relying on the series reactor to stabilize the arc and short circuit current limit and rely on electricity to increase efficiency to reduce power consumption and increase productivity.

Saturable reactor with a high-impedance electric arc furnace capable of automatically regulating the anti-values, can be done is basically an ideal constant current electric arc furnace.

In the design of high-impedance electric arc furnace power supply circuit, the series reactor as a result of the relatively large inductance value, resulting in breaking the vacuum circuit breaker over-voltage is increased, therefore, must take strong measures to over-voltage protection. That should be taken to modular MOA and four pole RC absorber double protective measures.

High-impedance electric arc furnace in the reactor must be installed in the transformer room, and close to the transformer installed marry.

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