Application knowledge - ultra high power electric arc furnace

Ultra high power arc furnace mainly USES arc to heat and melt scrap steel.The so-called arc is a form of discharge between the cathode and the anode, and the ions produced by it have the characteristics of low voltage, high current and high energy density.In the arc column, with the increase of internal energy, some molecules will dissociate into atoms, and then ionize when excited, forming plasma.Unlike solids, liquids and gases, plasma belongs to the fourth state of matter.Arc temperature of each part, the cathode point for 3500-4000 ℃;Near the cathode of up to 15000-20000 ℃;The highest on the outside of arc column is about 6000 ℃, the closer the inside the higher temperature.Ac arc is different from dc arc, its voltage and current will change the direction twice in each cycle, that is, the use of 50 hz power supply, its voltage and current in is from zero through the number of ioo times, and change the direction.When the current decreases, the charged particles in the arc column will recombine and the plasma state tends to disappear.If there is an additional inverse voltage and current, the arc will persist until the plasma disappears.Ac arc is easier to extinguish than dc arc, and the main factors that can play the role of stable arc are: large current, short arc, large reactance percentage, high power supply frequency;The less the furnace gas flow, the more stable the arc;The higher the furnace gas pressure, the more stable the arc;Slag can also improve the stability of arc.The less affected the external magnetic field, the more stable the arc.

1, The Arc

The electric arc is the heat source in the furnace of the electric arc furnace.Because of the characteristics of arc itself, it determines the electrical characteristics of arc furnace equipment.Then decided eaf steelmaking each aspect.Arc is a phenomenon of gas discharge.When a certain voltage is applied between the two electrodes, the gas can discharge by itself. At the same time, the gas between the two electrodes is ionized, and a large number of charged particles -- free electrons and positive ions appear.

2. Conditions for stable combustion of electric orphan

According to the nature of the power supply, the arc can be divided into DC arc and AC arc. Most of the arcs used in industrial applications are AC arcs. The voltage and current of an ideal AC arc is a sinusoidal waveform whose voltage and current magnitude and polarity change periodically over time.

In order to ensure the burning of the arc, in addition to a certain number of charged particles between the end faces of the two electrodes, an electric field of a certain strength is required, that is, an arc voltage of a certain size is required. The voltage at which the arc begins to burn is called the arcing voltage or the arcing voltage. When the external voltage exceeds the maximum value, it begins to decrease. When the arc voltage decreases to a certain value, the arc is extinguished, and the voltage at this time is called the arc-extinguishing voltage. Due to the influence of the arc temperature, the arcing voltage is slightly larger than the arc extinguishing voltage. For high-power AC arcs, the arc temperature is not significantly reduced at the moment of extinction, and the arc voltage is approximately equal to the arc-extinguishing voltage.

If the reactance x = o in the external circuit, the arc current and the arc voltage are in phase. When the external voltage u is less than the arcing voltage, the arc is extinguished and the current is zero. It can be seen from the horizontal axis of time that there is a time interval between the power supply voltage before and after the zero point, and the arc is extinguished at this time, which is the source of unstable AC arc combustion. In order to not extinguish the arc, it is required to have a certain inductive in the circuit, that is, a phase difference is maintained between the arc current and the power source voltage.

The polarity of the AC arc changes rapidly during the combustion process. The graphite electrode has a relatively stable cathode spot when it is used as a cathode in the negative half cycle, and it is easy to form a stable arc. On the contrary, when it is in the positive half cycle, the cathode spot is at the high speed of the molten steel due to the steam ejected from the cathode molten steel bath. When it turns, the shape of the arc becomes irregular. In addition, during the melting period, due to the presence of cold steel, the arc continuously jumps from one charge to the other, and the arc voltage and current waveforms are not regularly changed drastically. All of these conditions have led to instability of the arc under operating conditions. The drastic changes in the AC arc during operation cause severe distortion of the arc current and arc voltage ripple, which leads to a series of higher harmonics and reduces the power quality of the grid.

3. Arc in steelmaking furnace

If the steelmaking furnace is an AC high-power arc, the reactance in the external circuit of the arc must be of sufficient magnitude. This is to satisfy the circuit conditions so that the arc current has a certain size.

In a three-phase steelmaking arc furnace, each phase of the arc is subjected to the magnetic field established by the other two-phase arc, and is then moved by the electromagnetic force to the outside of the electrode end near the lining. This is the arc blow. Due to the presence of the external blow, the angle between the arc column and the metal surface is reduced to 45°-75°. If the high-temperature airflow, that is, the arc arc filling, rushes to the furnace wall and throws metal, slag and graphite at high speed, a hot spot is formed on the furnace wall above the slag line near the arc. Because of the highest heat load here, it is also the most severely chemically attacked.

Current flowing through the molten steel creates a magnetic field that causes the molten steel to agitate. In high-power electric furnaces, about 20%-30% of the arc heat is transferred to the molten steel by this stirring action, and the molten steel per minute accounts for about 9% of the total weight.

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