Arc Furnace Electric Operation Parameters
The arc furnace equivalent circuit is very simple:
X is the circuit reactance, V is the voltage without load, Va is the arc voltage and I is the current. The arc is resistive, so current has the same phase of the arc voltage. The arc voltage is defined by the arc length. The main correlations among the parameters (in a single phase circuit ) are:
Va = V x cos fi
I = V x sen fi / X
X = V x sen fi / I
cos fi = cos(asen fi )
P = Va x I = V x I x cos fi = V2 x cos fi x sen fi / X
In order to calculate the electric furnace efficiency it can be included the circuit resistance losses.
A small complication is the fact that the circuit reactance varies according to the operation stage and cos phi. Because of that, for calculation of the furnace operational parameters is usually necessary to start with a certain cos fi and, for each stage of operation to correct sinusoidal reactance, in accord with the corresponding operational factor (that varies between 1,05 and 1,45 approximately).
Arc furnace productivity and energy consumption
To produce one ton of steel it is necessary an amount of energy that depends on the type of metallic charge, the final product, final temperature, type and amount of slag and others factors. If furnace is badly operated, with long off and refining times and frequent shell openings, the energy consumption per ton grows a little. However, in general, for steel production in normal condition, it is normal that furnace consumes nearly 550 kWh per ton produced. Part of this energy is usually produce by chemical means, throughout oxygen injection, but the main energy portion is supply by the electric arcs. As an example, we can consider a furnace that works consuming 30 Nm3 of oxygen per ton of liquid steel produced. In this case, the energy given by carbon combustion will be nearly 30 Nm3/t x 3,5 kWh/Nm3 = 105 kWh/t. Moreover, to complete the production of one steel ton will be necessary 550 kWh/t – 105 kWh/t = 445kWh/t that will be supplied by the electric arc. The furnace production per hour will be proportional to its active power (kW) and inversely proportional to the electric specific energy consumption (kWh/t). In the former example, if active power were 44.500 kW, the productivity would be 44.500 kW/445kWh/t = 100t/h. If the furnace had a capacity of 100 tons, the working furnace time (power on) would be 100 t/h/100t = 1 hour. If the power off time were 0,25 h, the total time to produce 100 tons (tap to tap time) would be 1,25 hours, and the actual productivity would be 100 t/1,25 h = 80 t/h and the production in a 24 hours period would be 24h x 80 t/h = 1920 tons.
The specific energy consumption depends too much on the type of charge. Furnaces that use charge of pre-reduced material, besides the necessity to melt scrap until the pouring temperature (1620 - 1735 C) need to give enough energy to reduce the levels of iron oxides that can vary between 5 - 10 %. The energy needed to reduce one ton of iron oxide is too high and, for this reason, the specific energy consumption working with pre-reduced material charges can be between 50 to 100 kWh/t higher comparing to the charges of 100% of scrap. Although, furnaces operating with a high percentage of pre-reduced material can currently reach very high levels of productivity because of the excellent foamy slag that allows the use of high levels of power per ton and smaller downtimes.
Electrodes consumption reduction
The electrode consumption is an operational indicator that heavily depends on the electric operational parameters. Until the 80’s, specific electrodes consumption of melting furnaces were between 4 to 6 kilos of graphite per ton of steel produced. In the late 90’s, the consumptions were close to 1.5 kg/t.
The reduction of electrodes consumption was mainly achieve through the increase of the voltage/current ratio. Furnace transformers had been modify in order to operate with higher voltages. To keep the low values of cos phi that are need to stabilize the arc during scrap melting were installed reactors in a series connection. In the case of furnaces that operate with continuous charge of pre-reduced material, or in general, when furnaces operate with good foamy slag, the reduction of consumption can be achieved by operating with high voltages and at the same time with high cos phi, dispensing the reactors.
* (See on this site, the paper "Electrodes consumption equation" and "Some mistakes over the arc furnace operation")
Arc furnaces productivity improvement through the increasing of transformers secondary voltages
The advantage of operation with higher voltages was know in the beginning of the 80’s. Unfortunately, until the end of this decade, few furnaces had been modify. In Brazil, we had opportunity to do a project to improve the performance of two furnaces, through the increasing of voltages and powers, at Siderúrgica Barra Mansa, in 1988, modifying two EAF transformers from 24 MVA – 415 V to 30 MVA – 830 V. That job were followed in the 90’s by a number of re-powering that we implemented in many Brazilian and others countries furnaces.
Higher voltages allow operation with higher powers without the necessity of secondary circuit arc furnaces modification (bars, flexible cables, arms, electrode holders, and electrodes). Investment can be pay in few months with the electrodes consumption reduction. To stabilize the arc during scrap melting it is necessary to increase the circuit reactance, installing series reactors.
In the past, the operation with higher voltages and specific powers were not possible because of the strong refractory erosion that it caused. In the end of the 70’s, the water-cooled walls and shells permitted the powers and voltages increase. The main impulse to operate with high voltages and powers was given by the development of the operation with foamy slag that protect the walls from the attack of the arc, which occurred in the end of the 80’s. Nowadays are used voltages of 900 V and active powers of 45MW in furnaces of just 40 tons. Presently, the EAF´s of high steel capacity (100 ton to 250 ton) operate with voltages from 900 V to 1300 V and transformers of 100 to 170 MVA. However, for this range of power it is preferably to increase the currents up to 75 kA, to operate with lower arc length and lower heat radiation to the walls, as a way to achieve Power On times of about 30 minutes, without excessive heating of the furnace walls.
Flicker caused by arc furnaces
The variations of arc length cause voltage fluctuation that propagates to the supplying circuit reaching the low voltage users. Unfortunately, the EAF fluctuation frequency coincides with the frequency of light fluctuation detected by human eyes (1 to 15 Hz, approximately), causing a certain discomfort to residential consumers. Nowadays, the level of flicker is measured by instruments regulated accordingly to UIE (International Union for Electro-Heat) studies done since 1980, and countersigned by IEC 868 standard. The unity of measurement is the Pst and, accordingly to the initial recommendations, a Pst value equal to or higher than 1pu would cause an unacceptable disturb. The value of Pst would be measured in the High Voltage, in the called point of common coupling PCC with others consumers. However, real measurements showed that between high and low voltage occurs a natural flicker attenuation that can reach 50 % in some cases. Because of that, in order to a residential customer suffers a similar effect to 1pu, the value of Pst caused by the furnace over the HV must reach much higher values than 1pu (2pu, in case of 50 % attenuation).
The flicker levels are proportional to the relation between the furnace power and the short circuit power at the point of common coupling with the others consumers. For this reason, the most direct way to diminish the flicker level is to increase the PCC short circuit power or to reduce the furnace power. The first usually cannot be done or requires too expensive investments and the second reduces furnace productivity.
Nowadays, to reduce the flicker, some companies recommend shunt compensators (SVC) that through fixed capacitor banks and reactances controlled by semiconductors put on the net quantities of reactive energy of opposite sign to the variations of furnace reactive demand. These devices, in general, are expensive and reduce the flicker in approximately 40 %.
Electro-dynamic forces caused by arms and electrodes currents
The currents that circulate through the furnace conductors cause variable magnetic fields, creating forces that can even break the electrodes. In addition, the low frequency fluctuations of the current cause vibrations in the furnace arms and columns. These forces are proportional to the squares of the current peaks and inversely proportional to the distances between electrodes. For this reason, in some cases, after the reduction of the primitive diameters, sometimes done in order to reduce refractory erosion, occurred an increment of arms vibrations and electrodes breakage. On the other hand, as the maximum current depends on the voltage and is inverse to reactance, in certain cases when voltage was increased, without the adequate reactance increase, occurred similar failures.
Phase rotation and electrodes squeeze
Maybe this is the most known effect of furnace electric parameters. In general, companies that supply electrodes and technical assistance know that the rotation of phases must be counter-clockwise in order that the electrodes torque tight them. A clockwise phases sequence causes the electrodes release and eventually fall of columns.
The problem is that many times it is confounded the feeding phases sequence (R,S,T) with the electrodes physical phases sequence (electrodes 1,2 and 3), which is what really matters, reaching to wrong conclusions.
Refractory Erosion
The erosion caused by the arc over the refractories was object of deep studies in the 60’s and 70’s, when there were neither cooling water walls nor foamy slags to protect walls. W. Schwabe defined the expression that allows evaluating the degree of erosion caused by the arc over the furnace walls. Nowadays, the care with the refractories have minor importance, but anyway, the level of refractory erosion continued to be a good way to define the probable refractory damage, in the cases the foamy slag is not proper or in the periods of end of melting when the scrap iron is already melted and still there is no foamy slag.
The Schwabe indicator of refractory erosion can be stated as follows:
RI = Va x Pa / b²
Where Va is the arc voltage, Pa is the arc power and b is the distance from the electrode face to the wall. The factors that increase refractory erosion are the same that decrease electrodes consumption.
Transformers and reactors specification
For determined capacity of furnace and expected production levels are established the electrical operation parameter: average active power, secondary voltage, cos phi and current. Once known these parameters it is possible to specify the transformer. The reactance that the circuit must have to confirm the prescribed values for these parameters is calculated and compared with the reactance existent on the circuit, in order to define the reactance of the complementary reactor.
Once defined the transformer and reactor main characteristics it is necessary to attempt to the aspects of the transformer electrical project: tap changer type, voltages levels between the higher and the lower, primary voltage more adequate, voltage class, cooling, transformer constructive type, tests, protections, accessories.
Power factor correction - Capacitors banks - Static compensator
Depending on the point of connection of the steel plant and the governmental regulation in course, can be necessary to keep levels of cos phi higher than 0,85, 0,92, 0,95 or even 0,98, in periods of month measurements or in periods of hour measurements. Due to the fact that the furnace, at least during the melting period, operates with cos phi lower than the allowed limits, it is necessary to compensate the reactive power to elevate the cos phi at the connection point with the energy supplier company.
The easiest and cheapest way to compensate the reactive power it is the installation of fixed capacitors banks. The calculation of the capacitors power rate it is very simple if the operational cos phi is known. When a capacitors bank is going to be installed it must be verified the resonance parallel frequency of the capacitors with the supplying power system, including the step-down transformer. And if there is a resonance close to one of the harmonic frequencies created by the furnace (2nd, 3rd, 4th and 5th) must be modified the characteristics of the bank in order to displace the resonance frequency. After defining the effective power and the bank reactance, must be calculated the steady state voltages and the voltage increase caused by the harmonics generated by the furnace to allow to define the nominal voltage and the nominal power of the bank.
Harmonics and harmonic filters
The main perturbation caused by the arc furnace are the voltage fluctuations (flicker) of lower frequency than the industrial. However, the arc furnace also generates harmonic currents (frequencies multiples of the fundamental) that by their turn cause distortions of the voltage wave of the power supply. The arc furnace generates a range of harmonic frequencies very large, being as the higher amplitudes the 3rd, the 2nd, the 5th and the 4th, in this order. Nevertheless, the average values of the harmonic currents generated by furnaces are relatively low, if compared with those generated by solid-state converters. In practice, some few current semi-cycles of the current show very high percentage of distortion, but in average, in the case of arc furnace, it is not more than 5% for the main harmonics.
The necessity of harmonic filters depends basically on the necessity of attending rules more or less exigent in what refers to voltage distortion. In a practical point of view, it is possible in most part of the cases, to install capacitors banks without filters, since it was assume the precaution to move the parallel frequency out from the main harmonic frequencies. The necessity to install harmonic filters it is imperative when there are static compensators (SVC), because they pursue solid-state components, controlled by the variation of the firing angle, causing high levels of harmonics.
Anyway, in many projects is chosen to install harmonic filters in the substation to guarantee that will not occur too high voltage and current amplifications on the capacitors bank. A good solution can be the installation of filters of third harmonic (tuned in nearly 2,9 times the fundamental). The filters project must consider the steady state overvoltage caused by the inductors over the capacitors and, also the overvoltage caused by the harmonics, the overvoltages of the capacitors energization and the voltage surges caused by the furnace transformers inrush currents. In the case of the 2nd harmonic filters, this last type of overvoltage is critical and obligates to overestimate the capacitors bank, making the project more expensive.
Overvoltages
The arc furnace feeding circuit is quite similar to any other industrial supplying circuit. A particularity of this circuit is the great number of operations of the furnace transformer switch (usually connected in 13,8 kV, 23 kV or 33 kV, but also, in certain cases, in 46 kV, 69 kV or even 120 kV) who can reach 200 per day. Another characteristic is the existence of high power capacitors banks. To maneuver the furnace are used vacuum circuit breakers. The main protections against the switching surges are the surge suppressors, better known as lightning arresters, since they are the same devices used to protect against atmospheric discharges. These suppressors, since nearly 1980, became to be designe with metallic oxides, like the zinc oxide. In the case of a transformer installed in 24 kV, the level of insulation must resists surges of even 150 kV and the lightning arresters, for this level of voltage, usually operate with nearly 60 kV. In the primary of the furnace transformers, it is recommend to install the lightning arresters not only between phases and the ground, but also between phases. In the case of the vacuum circuit breakers there is a phenomenon known as multiple re-ignitions that could cause overvoltages of high frequency which could damage the transformer, especially when there exist surge suppression capacitors in the primary of the transformer and power factor correction capacitors in the main substation. To protect against this unlikely kind of overvoltage, a vacuum circuit breakers manufacturer recommends the RC protection circuits, connected between phase and earth. Another type of overvoltage very common is the called “restrike”, that occurs on the turning off of the capacitors bank when the switch are not proper or damaged. These over-voltages are of high energy and usually cause failures in the lightning arresters.
Protections against overcurrents
The adjustment procedures for protections against overcurrent in arc furnaces must follow some differentiate criteria. During the normal furnace operation can occur short-circuits between electrodes and the scrap that can cause currents twice higher than the nominal during few seconds. If the overcurrent relays were adjusted in the conventional way, acting quickly in the occurrence of smaller currents than the ones of the normal electrode short-circuits, it will often happen shutdowns that will prejudice furnace normal operation. To avoid this, the relays must be adjust to operate with delay. It would be also desired that the relays actuate instantaneously in case of the occurrence of a short-circuit at the exit bus of the transformer, and, certainly, must act instantaneously in the case of a short-circuit in the medium voltage bus.