High-impedance electric arc furnace for mitigating impact on the power grid

1 Introduction

Since the day the electric arc furnace was born, people have begun to study ways to get the maximum arc power. It is well known that the arc power determines its productivity, which in turn is proportional to the voltage and electrode current. For many years in the past, it has been relying on increasing the electrode current to increase the arc power. However, the resulting drawback is that it must be configured with a large cross-section of the secondary current-carrying conductor and the development of expensive, special-sized super-large-diameter silicon electrodes to meet the requirements of large current transmission. In this smelting operation mode, due to the short arc smelting, the electrodes are frequently in contact with the charge, and a short circuit is often generated, which has a serious impact on the power supply grid, causing voltage fluctuations and flicker of the power grid, and generating a large number of higher harmonics; It also results in a very high electrode break rate, often requiring electrodes, which affects both production and steelmaking costs.

Another negative effect of short arc operation is that during electrode penetration, the operating reactance is very high, resulting in lower average power, reduced arc power, and longer smelting time.

If high voltage, low current operation is used, electrode consumption and power consumption can be reduced. However, high voltage and low current operation are adopted. Due to the small reactance value of the main circuit, the short circuit current is too high, the high voltage switch frequently trips, the power factor is too high, and the arc combustion is unstable. In summary, one can draw an important conclusion: if the main circuit of the electric arc furnace is transformed from low impedance to high impedance, that is, a reactor is connected in series with the main circuit, the above drawbacks can be solved. That is to say: the additional reactance can make the arc combustion stable, the electrode current is reduced, the voltage fluctuation is reduced, the harmonic generation is reduced, the secondary voltage is increased, the arc power is increased, the electric efficiency is increased, and the foaming slag is completely surrounded by the covering arc. , thus also increasing the life of the lining. Such an electric arc furnace having a large reactor in the main circuit of the electric arc furnace and having a higher secondary voltage is called a high-impedance electric arc furnace.

2 Theoretical basis of high-impedance electric arc furnace

Since the concept of high-impedance electric arc furnace with additional reactor was first proposed more than ten years ago, it has been widely accepted by steel mills in the practice of electric arc furnace operation, and has been rapidly promoted at home and abroad, and has received obvious economic benefits. . Therefore, high arc voltage, long arc smelting, and low electrode current operation mode are the only way for further development and improvement of existing ultra high power arc furnaces.

The advantages of increasing the secondary voltage of the transformer to increase the arc voltage and arc length, and increasing the total reactance of the furnace to reduce the electrode current and improve the electrical efficiency can be illustrated by equations (1)-(6).

Active power P=Scosφ (1)

Electrode current Ie= (2)

Arc voltage uarc= -Ier (3)

Where: Xop is the operating reactance;

S is the apparent power;

r is the impedance of the short net to the electrode;

Uph is the phase voltage.

Arc length Larc=uarc-35 (4)

Arc power Parc=3Ieuarc (5)

Electrical efficiency η= (6)

It can be clearly seen from equation (2) that in the case of the same power factor, increasing the reactance can reduce the electrode current. It can be seen from equation (3) that increasing the secondary voltage of the transformer and lowering the electrode current can significantly increase the arc voltage. As can be seen from the equation (5), since the arc voltage is multiplied by a large amount, the arc power is greatly increased. It can be clearly seen from equation (6) that the high-impedance electric arc furnace is characterized by high arc voltage and small electrode current, so its electrical efficiency is very high, and generally a large-capacity high-impedance electric arc furnace can reach 0.94 to 0.97.

A reasonable combination of a higher transformer secondary voltage and a larger reactor is necessary to maintain the power factor within the proper range. However, if the power factor is too high, the arc will be unstable during the melting of the charge, and therefore, the power factor value is a prerequisite for determining the stability of the arc.

If the power factor is too high, the composite reactance is too small. Then, after connecting a large reactor in the main circuit of the AC arc furnace, the arc current can be continuously circulated without interruption, and the arc is stably burned. In theory, How to explain? Answer this question below.

Studies of arc physics have indicated that arcing occurs only when a sufficiently high voltage is applied between the two electrodes. After arcing, an alternating voltage of 50 Hz is applied between the two electrodes. In each half cycle, current flows only when the voltage rises to a certain value. When the voltage drops to a certain value, the arc is extinguished, which means that the 50 Hz AC arc has 100 arcing and extinction in 1 s. Since the heat of the arc is the product of the arc voltage and the arc current, the arc current is zero during the time when the arc is delayed, so the arc heat is also zero. This causes the arc to be intermittent and the current is not continuous, as shown in Figure 1.

Figure 1 AC arc simplified waveform diagram (in the absence of series reactors)

When the string in the circuit has sufficient reactance, the current lags the voltage by an angle φ. When the applied voltage is zero, the arc current continues to circulate by means of the energy stored in the reactance. When the arc current is close to zero, the voltage in the negative half cycle is already high, and the arcing voltage value has been reached (see Figure 2), so that the arc is ignited, that is, the current in the negative half cycle is connected in time without interruption. Therefore, as long as there is sufficient reactance value in the main circuit string, the arc can be continuously burned, and the current flows continuously without interruption. This is the theoretical basis for continuous arc combustion in high-impedance arc furnaces.

Figure 2 AC arc simplified waveform diagram with series reactor

3 reactor in high-impedance electric arc furnace

The only difference between a high-impedance electric arc furnace and a conventional electric arc furnace is that a series reactor is added, so the reactor must be described. From the economic point of view (layout and footprint), the primary side series reactor of the electric furnace transformer is the most reasonable. From the structural form, it can be made into a hollow or iron core reactor. The former can be made into three independent single-phase coils, which are bulky and have a strong magnetic field around it. Therefore, it requires a lot of physical space, usually placed in a separate room or substation; The main magnetic flux of the main magnetic flux can be made into a compact three-phase device, which is suitable for being placed in the transformer room. In order to suppress the overvoltage, the transformer should be placed close to the transformer. The reactor has an adjustment switch for adjusting the reactance value. Local operation or remote operation.

At present, iron core reactors are commonly used in the design of high-impedance electric arc furnaces at home and abroad. The connection of the reactor in the high-impedance electric arc furnace is shown in Fig. 3. The structure and characteristics of the core reactor are briefly described below.

Figure 3 Reactor connection diagram of high-impedance electric arc furnace

The iron core reactor is basically the same as the transformer. Except that the reactor has only one coil per phase, the main difference in structure is on the core. The magnetic column of the iron core reactor is formed by stacking a plurality of iron cakes, and the iron cakes are separated by insulating plates (such as epoxy glass cloth sheets, marble and other insulating materials) to form a gap.

The magnetic circuit of the core reactor has a gap and a coil is placed on the outside. Since the magnetic material has a much higher magnetic permeability than air, the inductance value L of the core reactor is also much larger than that of the hollow reactor. However, the inductance L of the reactor decreases as the saturation of the core increases, and one of the functions of the high-impedance arc furnace reactor is to limit the value of the short-circuit current, which does not allow the inductance L to decrease as the current increases, so When designing the reactor, the reactor is not allowed to work in a saturated region, that is, when the furnace is short-circuited, the reactance of the reactor is not allowed to decrease. To ensure this, a lower flux density value must be chosen at design time. Usually B ≤ 0.75 ~ 0.85T is selected, which cannot be higher than this value.

Since the non-magnetic spacer is placed in the steel magnetic conductor, the magnetic resistance Rm of the entire magnetic circuit will be the sum of the magnetic resistance Rs and the air gap magnetic resistance Ra of the steel magnetic conductor, namely:

Rm=Rs+Ra (7)

Since the magnetic resistance Ra of the air gap has a considerable value and is constant, the change in the magnetic resistance Rs of the steel magnetic conductor has little effect on the total magnetic resistance Rm of the entire magnetic circuit.

The reactor body is immersed in the fuel tank, and the system is usually cooled by forced oil circulation water, and the cooling effect is good. This product is made reinforced because the electric arc furnace is often overloaded by 20%. In the melting period, the working short circuit often occurs, and the working short-circuit current exceeds twice the rated current. Therefore, when selecting the cross section of the reactor coil wire, it should be selected according to the rated current of the transformer twice.

The total reactance value of the high-impedance electric arc furnace and the secondary maximum voltage value of the transformer are the key parameters for designing the electric arc furnace. If you choose not to, you can't take advantage of the high-impedance electric arc furnace. Table 1 lists the secondary maximum voltage values ​​and total reactance values ​​of several high-impedance electric arc furnaces whose total reactance is much higher than the general level. These furnaces have a reactance value in the range of 3.0 to 3.5 mΩ. After the reactor is connected to the primary side of the transformer, the total reactance value is 4 to 6 mΩ, which is about 50% higher than the original.

Table 1 Some furnaces with higher voltage and additional reactors

130

75

110

140

120

115

115

80

118

66

1070

1000

960

960

950

925

901

900

884

850

6.3

5.4

4.6

3.6

4.2

4.8

4.5

5.7

4.4

5.0

4 Analysis of the causes of voltage fluctuations in high-impedance arc furnace

From the perspective of electric arc furnace steelmaking, working short-circuit is unavoidable, and it often occurs when arcing or collapse occurs during the melting period. The short-circuit duration of the working moment that occurs when the arc is arced generally does not exceed 1 to 2 s, which is not harmful to the power grid because the short-circuit duration is short. The duration of the short circuit when the charge collapses depends on the moving speed of the electrode and the depth of collapse. The depth of the collapse is different, and the difference is very different. However, in the actual steelmaking process, the collapse depth of more than 250 mm is rarely encountered. The arc length of a high-impedance electric arc furnace is generally greater than 250 mm. The example cited in this paper has an arc length of 350 mm, so that it does not cause an electrode short circuit when the charge collapses. When the electrode is short-circuited with the charge, the reactive power is the largest, and the grid voltage fluctuation caused by it is also the largest, because the grid voltage fluctuation is caused by the change of reactive power, which can be explained by the following formula.

Δu= ×100% (8)

Where: ΔQ is the amount of reactive power change (Mvar) of the electric arc furnace;

The SDR is the power supply point (35kV) and the short circuit capacity (MVA) of the power supply system.

In summary, the high-impedance electric arc furnace operates at a higher power factor (above 0.82) and does not produce voltage flicker because the electrode and the charge are kept at a long distance, which causes a small percentage change in the arc length. Then, the secondary current and the primary voltage change rate (Δu/u) are also small, and therefore, the voltage flicker is reduced. For the above reasons, the high-impedance arc furnace absolutely does not need to install a static reactive power dynamic compensation device (SVC device). According to Demag and Dan Aoli, their high-impedance electric arc furnaces operate around the world and are not equipped with SVC units.

Another important reason for the voltage fluctuation caused by electric arc furnaces has been little known in the past. In recent years, it has been tested on many large arc furnaces that the voltage fluctuation of the AC arc furnace is largely due to the mechanical resonance of the electrode lifting system. Caused by it. Practice has proved that in the high-voltage side series reactor of the AC arc furnace transformer, the high-impedance, long-arc operation mode, limiting the current change rate, can effectively reduce the mechanical resonance, because the mechanical resonance is caused by the current change.

Figure 4 shows the effect of mechanical resonance on voltage fluctuations. It can be seen from Fig. 4 that the AC high-impedance arc furnace has no mechanical resonance, and its voltage fluctuation amplitude is only about 5V.

Figure 4 Effect of mechanical resonance on voltage fluctuations

In summary, the AC high-impedance electric arc furnace fundamentally eliminates the voltage fluctuation and voltage flicker of the power supply grid caused by mechanical resonance, which is much superior to the SVC device.

5 High-impedance arc furnace to suppress voltage fluctuations

As mentioned above, voltage fluctuations are caused by fluctuations in reactive power. The following is an example of comparison of reactive power fluctuations between foreign high-impedance electric arc furnaces and ordinary electric arc furnaces.

DANIELI of Italy proposed the range of reactive power fluctuations of high-impedance electric arc furnaces and ordinary electric arc furnaces for comparison [4]. Figure 5 shows the fluctuation range of reactive power versus current for a high-impedance arc furnace. In the figure, Q is reactive power and I is electrode current. As can be seen from Fig. 5, when the electrode current changes to ΔI = 30 kA, the reactive power change is caused by ΔQ ≤ 50 Mvar, and the active power change is ΔP = 7 MW.

(a) Basic circuit diagram

(b) Power/current parameters

Figure 5 Basic circuit and power curve of high-impedance electric arc furnace

The range of fluctuations in reactive power versus current for a conventional electric arc furnace having the same conditions is shown in Fig. 6. It can be seen from Fig. 6 that when the electrode current also changes by 30kA, the reactive power fluctuation value reaches 60Mvar or more, and the active power change is ΔP=13MW. It can be seen from the above two sets of data that the high-impedance arc furnace has lower reactive power fluctuation value and active power fluctuation value than the ordinary electric arc furnace under the same conditions, and thus the voltage of the high-impedance electric arc furnace The fluctuations are also reduced.

(a) Basic circuit diagram

(b) Power/current parameters

Figure 6 Basic circuit and power curve of ordinary electric arc furnace

Figure 7 shows the input power fluctuation log curve [3] for a common electric arc furnace (Figure 7a) and a high-impedance electric arc furnace (Figure 7b) proposed by U.S. Union Carbide Corporation. It can be seen from Fig. 7(b) that after the reactance is increased, the input power is increased and the arc power is stabilized. Obviously, for a conventional electric arc furnace with low reactance, since there is no reactor, the steelmaker chooses to operate at a high power factor. During the melting period of the charge, the voltage position is at the highest level, and the active power is about 20MW. ~50MW; and the high-impedance arc furnace with series reactor corresponding to this, can obtain a smaller electrode current when the power factor is appropriate, and as a result, the active power is about 62MW when operating at the highest voltage position. At 70MW, the fluctuations are significantly reduced. This mode of operation has high active power, good arc continuity, and small impact on the grid, thus suppressing voltage fluctuations.

(a) Ordinary electric arc furnace

(b) High-impedance electric arc furnace

Figure 7 Power fluctuation record curve of steelmaking electric arc furnace

6 Operational advantages of high-impedance electric arc furnace

In 1992, Mannesmann Demag in Germany converted a 60t (40MVA) ultra-high power electric arc furnace into a high-impedance ultra-high-power electric arc furnace. Table 2 lists the comparison of operating parameters before and after the transformation [2].

Table 2 Comparison of parameters between common electric arc furnace and high-impedance electric arc furnace

It can be seen from Table 2 that after changing to the high-impedance operation mode, the same transformer has a full capacity to reach 40 MVA. This is because the secondary current is lowered and the secondary voltage is increased. On the contrary, in the low-impedance operation, since the secondary current is too large (40.3kA), the cross-section of the secondary winding of the transformer is limited, and the secondary voltage is too low, so it can only reach 30MVA.

After changing to a high-impedance electric arc furnace, many indicators have been improved: the electrical efficiency has increased from 90.3% to 94.7%, which can save 20kW·h/t of electricity, ie 4.5% of electricity saving; the arc power is increased from 22.6MW to 31.1MW; The length is increased from 150mm to 350mm; the electrode consumption is reduced from 2.6kg/t to 1.9kg/t, which is reduced by 28%; the smelting time is reduced from 72min to 50min, shortened by 30%; productivity is greatly improved. Among these indicators, the electrode consumption index is the best, because the main production cost of electric arc furnace steelmaking is the cost of electric energy consumption and graphite electrode consumption. The electrode consumption is divided into two parts:

1) end consumption, which is proportional to the square of the electrode current and proportional to the energization time;

2) Side consumption, which depends on the intermittent time from tapping to power transmission and the oxygen content in the furnace exhaust.

In summary, in order to reduce the production cost, it is most important to use a low electrode current and to shorten the intermittent time from the tapping to the power transmission. Assuming that the intermittent time of the two furnaces is the same, the side consumption of the electrodes is the same, and in order to reduce the electrode consumption, it is reduced to a low end consumption. due to

Wt=fTIp×Wtot (9)

Wt=KIe2(10)

Where: Wtot is the total electrode consumption;

Wt is the end consumption of the electrode;

fTIp is the ratio coefficient between end consumption and total consumption, fTIp=0.5 for AC arc furnace;

Ie is the electrode current;

K is a constant.

According to the formulas (9) and (10), if the high-impedance electric arc furnace is compared with the conventional low-impedance electric arc furnace in terms of electrode end consumption, the derivation is omitted, and the formula (11) can be obtained.

= =fTIp× (11)

Where: Wtlr is the electrode end consumption of the low-resistance furnace;

Wthr is the electrode end of the high-impedance furnace;

Ielr is the electrode current of the low-impedance furnace;

Iehr is the electrode current of a high-impedance furnace. The comparison of the electrode end consumption indicators of the 60t common electric arc furnace of Mannesmann Demag and the modified high-impedance electric arc furnace is now

=0.5× =0.28=28%

It shows that the electrode consumption of the high-impedance electric arc furnace is reduced by about 28% compared with the traditional low-impedance electric arc furnace, which is completely consistent with the data obtained from the field measurement.

7 Conclusion

The high-impedance electric arc furnace has been popularized and applied at home and abroad. Its technical and economic indicators are comparable to DC arc furnaces. In addition, it has the advantages of simple and reliable structure. Therefore, it is undoubtedly an inevitable trend in the development of traditional AC arc furnaces.

High-impedance arcs can significantly reduce voltage flicker, and under the same conditions, the level of flicker is only 68% of that of conventional AC arc furnaces [4]. The results of the operation prove that the high-impedance electric arc furnace does not need to be equipped with an SVC device.