The ultra-high power electric arc furnace mainly uses an electric arc to heat and melt the scrap steel. By arc is meant a form of discharge between a cathode and an anode that produces ions characterized by low voltage, large current, and high energy density. In the arc column, as the internal energy increases, a part of the molecules will be dissociated into atoms, and then excited and ionized to form a plasma. Plasma is different from solids, liquids, and gases, and it belongs to the fourth state of matter. The temperature of each part of the arc is 3500-4000 °C at the cathode point; up to 15000-20000 °C near the cathode; 6000 °C at the outer side of the arc column, the higher the temperature is near the inner side. The alternating current arc is different from the direct current arc. The voltage and current change in two directions in each cycle, that is, the power supply of 50 hz is used. The voltage and current pass through the zero point in is is ioo times, and the direction is changed. . When the current is reduced, the charged particles in the arc column will recombine and the plasma state will tend to disappear. Before this plasma state disappears, if there is an additional reverse voltage and current, the arc will continue to exist. The AC arc is easier to extinguish than the DC arc. The main factors that can stabilize the arc are: high current, short arc, large reactance percentage, high power frequency; less furnace flow, the more stable the arc; the more the furnace pressure High, the more stable the arc; the presence of slag can also improve the stability of the arc; the less the arc affected by the external magnetic field, the more stable the arc.
1, the arc
The arc is the heat source in the furnace of the electric arc furnace, which determines the thermal conditions in the furnace. Due to the characteristics of the arc itself, it determines the electrical characteristics of the electric arc furnace equipment. In turn, various aspects of electric arc furnace steelmaking are determined.
Arc is a phenomenon of gas discharge. When a certain voltage is applied between the two electrodes, the gas can discharge itself, and the gas between the two electrodes is ionized, and a large number of charged particles appearing - free electrons and positive ions, conductive channels appear between the electrodes, and the current density reaches several ka/cm2. The gas temperature is several thousand degrees Celsius.
2. Conditions for stable operation of electric orphans
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 electric 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, which is the outer arc blowing. 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 into the molten steel by this stirring action, and the molten steel per minute accounts for about 9% of the total weight.