Patent Publication Number: US-11384986-B2

Title: Open arc condition mitigation based on measurement

Description:
FIELD 
     The present application relates generally to AC and DC electric arc furnaces and, more specifically, to open arc condition mitigation based on measurement for such furnaces. 
     BACKGROUND 
     An electric arc furnace is a device in which material may be heated by means of an electric arc. Electric arc furnaces are used in a variety of applications in a wide range of scales, from a few dozen grams to hundreds of tons. One application for electric arc furnaces is secondary steelmaking. Another application is the smelting of non-ferrous ores. The latter is often a shielded arc smelting application of electric arc furnaces. 
     An Alternating Current (AC) electric arc furnace uses a furnace transformer to deliver power from a power grid to an arc at two or more electrode tips. A Direct Current (DC) electric arc furnace uses a rectifier transformer and a rectifier to deliver power from the power grid to an arc at one or more electrode tips. 
     In the secondary steelmaking application and the shielded arc smelting application, variations in the load experienced by the power grid that supplies electricity to the electric arc furnace give rise to something called “power grid flicker.” Unfortunately, power grid flicker can be shown to cause malfunction in sensitive electronic equipment and lighting. Furthermore, power grid flicker can be shown to disturb other consumers on the same power grid. Even further, excessive power grid flicker can violate an electricity contract entered into by the operator of the electric arc furnace. 
     One contributing factor to stability in the power drawn, from the power grid, by the electric arc furnace is the presence or absence of an arc cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made, by way of example, to the accompanying drawings which show example implementations; and in which: 
         FIG. 1  illustrates a system including an AC electric arc furnace in combination with a variable reactor and an open arc mitigation system including an analyzer and a first control unit, wherein the analyzer receives measurement from a primary side of a furnace transformer in accordance with aspects of the present application; 
         FIG. 2  illustrates the system of  FIG. 1 , wherein the analyzer receives measurement from a secondary side of the furnace transformer in accordance with aspects of the present application; 
         FIG. 3  illustrates the system of  FIG. 1  as applied to a DC electrical arc furnace in accordance with aspects of the present application; 
         FIG. 4  illustrates a steel scrap furnace implementation of the electric arc furnace of  FIG. 1  with an arc cavity; 
         FIG. 5  illustrates the steel scrap furnace implementation of  FIG. 4  in an open arc condition; 
         FIG. 6  illustrates a non-ferrous shielded arc smelting furnace (without foam) implementation of the electric arc furnace of  FIG. 1  with an arc cavity; 
         FIG. 7  illustrates the non-ferrous shielded arc smelting furnace implementation of  FIG. 6  in an open arc condition; 
         FIG. 8  illustrates a non-ferrous shielded arc smelting furnace (with foam) implementation of the electric arc furnace of  FIG. 1  with an arc cavity; 
         FIG. 9  illustrates the non-ferrous shielded arc smelting furnace implementation of  FIG. 8  in an open arc condition; 
         FIG. 10  illustrates steps of an example method of analyzing current and voltage measurements at the analyzer of  FIG. 1 ; 
         FIG. 11  illustrates steps of an example method of analyzing voltage measurements at the analyzer of  FIG. 1 ; 
         FIG. 12  illustrates steps of an example method of operating the first control unit of  FIG. 1 ; and 
         FIG. 13  illustrates steps of another example method of operating the first control unit of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     A system measures parameters of the electricity drawn by an arc furnace and, based on an analysis of the parameters, provides indicators of whether arc coverage has been optimized. Factors related to optimization of arc coverage include electrode position, charge level, slag level and slag behavior. More specifically, such indicators of whether arc coverage has been optimized may be used when determining a position for the electrode such that, to an extent possible, a stable arc cavity is maintained and an open arc condition is avoided. Conveniently, by avoiding open arc conditions, the internal linings of the furnace walls and roof may be protected from excessive temperature and wear. 
     According to an aspect of the present disclosure, there is provided a system including an analyzer and a first control unit. The analyzer is adapted to receive a signal representative of an electrical signal measurement of the electrical power provided to an electric arc furnace and analyze the signal to determine, by analyzing the electrical signal measurement, a characteristic electrical parameter. The first control unit is adapted to receive the characteristic electrical parameter, determine, based upon the characteristic parameter, a change in operation for the electric arc furnace and transmit, to a second control unit provided for the electric arc furnace, an indication of the change. 
     According to another aspect of the present disclosure, there is provided a method. The method includes receiving a characteristic electrical parameter related to operation of an electric arc furnace, determining, based upon the characteristic electrical parameter, a change in operation for the electric arc furnace, where the change is related to mitigating an open arc condition and transmitting, to a control unit provided for operation of the electric arc furnace, an indication of the change. 
     According to a further aspect of the present disclosure, there is provided a method of open arc detection. The method includes obtaining an electrical signal measurement, detecting, based upon the electrical signal measurement, an open arc condition, determining, based upon the electrical signal measurement, a change in operation for the electric arc furnace, where the change is related to ending the open arc condition and transmitting, to a control unit associated with operation of the electric arc furnace, an indication of the change. 
     Other aspects and features of the present disclosure will become apparent to those of ordinary skill in the art upon review of the following description of specific implementations of the disclosure in conjunction with the accompanying figures. 
     Traditionally, power grid flicker (or, simply, “flicker”) may be mitigated by installing shunt reactive power compensation equipment. Examples of reactive power compensation equipment include a traditional Static VAR Compensator (SVC) or a more advanced, power-converter-based, Static Synchronous Compensator (STATCOM). Another proven technology for flicker reduction is a Smart Predictive Line Controller (SPLC), which may be connected in series with a fluctuating load. 
     In electric power transmission and distribution, volt-ampere reactive (VAR) is a unit in which reactive power is expressed in an Alternating Current (AC) electric power system. Reactive power exists in an AC circuit when the current and voltage are not in phase. 
     An SVC consists of a shunt-connected harmonic filter bank and a shunt-connected thyristor-controlled reactor. The filter bank and the thyristor-controlled reactor operate in concert to lower voltage flicker, maintain constant supply bus voltage or maintain a constant power factor. The SVC operates by shunt injection of either capacitive reactive power or inductive reactive power, thereby maintaining a constant voltage by maintaining the total reactive power draw (MVAR) of the furnace balanced near zero (i.e., neither inductive nor capacitive). SVCs typically have a half cycle time delay due to thyristor commutation requirements. An example of an early SVC can be seen in U.S. Pat. No. 3,936,727. 
     SVC-based arc furnace controllers dynamically supply reactive power by the controlled summation of constant capacitive MVAR and variable inductive MVAR. The controller compares load reactive power to a reactive power set-point derived from power factor set-point and dynamically controls the summated MVAR to the set-point. As a secondary steelmaking electric arc furnace frequently short circuits and open circuits during the bore-down phase of the furnace electrodes, the furnace reactive power swings vary from zero to 200% of the furnace transformer rating. An SVC is normally sized at 125% to 150% of the furnace rating and typically reduces flicker by approximately 40% to 50%. Some SVCs use a voltage set-point and adjust a shunt reactor to match a supply voltage to the set-point voltage. 
     An SPLC consists of a thyristor controlled reactor connected in series with an electrode of the electric arc furnace. An SPLC functions as a dynamically controlled series reactor that uses predictive software to stabilize the real power or the current on an electric arc furnace. The SPLC reduces flicker by lowering arc current fluctuations on the power systems. When arc current fluctuations are flat-lined, the voltage flicker is reduced. An example of an SPLC can be seen in U.S. Pat. No. 5,991,327 issued Nov. 23, 1999. 
       FIG. 1  illustrates an example of an SPLC in series with one electrode  142  of a multiple electrode AC electric arc furnace (EAF)  140 . Three phase power is provided to the electric arc furnace  140  from a local supply bus  110 . The supply bus  110  is connected to receive power from a utility power supply through transmission line and step down transformer (not shown) or, alternatively, from a local generating station (not shown). The electric arc furnace  140 , being an AC electric arc furnace, often includes multiple electrodes  142  (not individually illustrated), with an individual one of the multiple electrodes or one pair of the multiple electrodes  142  being associated with an individual one of the phases among the three power phases. Arcing ends of the electrodes  142  are positioned in a furnace vessel  144  to, for example, melt a work material, such as scrap metal, and may be mounted such that the position of the electrode  142  within the furnace vessel  144  can be adjusted. The electrodes  142  are connected to a furnace side (secondary windings) of a tapped furnace transformer  108 . 
     A variable reactor is connected, in series with the tapped furnace transformer  108 , between the electric arc furnace  140  and the supply bus  110 . Each of the three phases of the variable series reactor (only one phase of which is illustrated) includes a series combination of a variable reactor  134 , a fixed reactor  135  and a current transformer  136  connecting a respective phase of a supply side (primary windings) of the furnace transformer  108  to a corresponding phase of the supply bus  110 . In the illustrated embodiment, the representative variable reactor  134  includes a reactor  137  connected in parallel with a thyristor switch  139 . Each thyristor switch  139  preferably includes a pair of thyristors, or pairs of thyristor groups, arranged in opposite polarity to each other. The variable series reactor has a control range. The thyristor switch  139  may be considered to be a specific implementation of what may be called a power electronics static switch. 
       FIG. 3  illustrates a DC electric arc furnace  340  and its related connection to the supply bus  110 . The connection to the supply bus  110  includes a rectifier  337  and a DC reactor  344  on a furnace side of a furnace transformer  308 . 
     Operation of the EAF  140  may be considered in view of  FIG. 4 , illustrating the electric arc furnace  140 , in section, being used for processing scrap steel. Within the furnace vessel  144 , during operation, there are several zones of material. At the bottom of the furnace vessel  144 , a molten metal (e.g., steel) layer  402  collects. Above the metal layer  402  are piles of feed  408  (e.g., scrap steel). In one manner of adding scrap steel to the furnace vessel  144 , the roof of the furnace vessel  144  is moved aside to allow a bucket of scrap steel to be dumped into the furnace vessel  144 . 
     The feed  408  in the electric arc furnace  140  of  FIG. 4  may be iron or steel material distinct from scrap steel. For example, the feed may be Direct Reduced Iron (DRI), Hot Briquetted Iron (HBI) or molten iron from a blast furnace. 
     In one manner of adding feed to the steel furnace, certain iron or steel material may be fed into the furnace vessel  144  through a plurality of apertures  412 . 
     Responsive to arcs from the electrode  142 , a volume of foamy slag  406  forms around the tip of the electrode  142 . The height and distribution of the piles of feed  408  may be measured by a plurality of level measurement units  414 . Example devices for use as the level measurement units  414  exist and may use such technology as RADAR. 
     Responsive to an arc being repeatedly generated at the end of the electrode  142 , an “arc cavity”  410  may be understood to form. There is a mutually beneficial relationship that forms within the arc cavity  410 . Responsive to the arc being repeatedly generated at the end of the electrode  142 , an ionized plasma column is formed. It turns out that an ionized plasma column is beneficial to the generation of the next arc. The ionized plasma column may be considered to be hot. Indeed, the ionized plasma column may be, for example, maintained at 5000 degrees Kelvin. Conveniently, the heat of the plasma column may be considered to assist in the maintenance of the ionization of the plasma column. Furthermore, a hot plasma column allows for the possibility of relatively long arcs. The heat of a long arc is preferred over the heat of shorter arcs because of lower furnace power loss. Accordingly, an operator of the EAF  140  is interested in adjusting the position of the electrode  142  to allow for long arcs. 
       FIG. 5  illustrates the steel scrap furnace implementation of  FIG. 4  in an open arc condition. The open arc condition may result responsive to something causing an absence of the arc cavity  410 . In  FIG. 5 , for example, the absence of the arc cavity  410  may be caused by a change in the foaminess of the foamy slag  406 . In the open arc condition, the internal linings of the furnace walls and roof are in danger of experiencing excessive temperature and wear. 
       FIG. 6  illustrates the non-ferrous shielded arc smelting furnace  140 , in section, being used in an application that does not, generally, lead to foamy slag. Within the furnace vessel  144 , during operation, there are several zones of material. At the bottom of the furnace vessel  144 , a molten metal layer  602  (e.g., ferro-nickel) collects. Above the metal layer  602  is a slag layer  604 . Sitting on top of the slag layer  604  are piles of feed  608 . The feed  608  is fed into the furnace vessel  144  through a plurality of apertures  612 . 
     The height and distribution of the piles of feed  608  may be measured by a plurality of level measurement units  614 . 
     Responsive to arcs from the electrode  142 , the feed  608  may be converted to the slag  604  and the metal  602 . In contrast with the application illustrated in  FIG. 4 , the slag  604  is not foamy. Also responsive to arcs being repeatedly generated at the end of the electrode  142 , an arc cavity  610  may be understood to form. 
       FIG. 7  illustrates the non-ferrous shielded arc smelting furnace of  FIG. 6  in an open arc condition. In  FIG. 7 , the absence of the arc cavity  610  may be caused by a shifting of the feed  608 . 
       FIG. 8  illustrates the electric arc furnace  140 , in section, being used in a non-ferrous shielded arc smelting application with foamy slag. Within the furnace vessel  144 , during operation, there are several zones of material. At the bottom of the furnace vessel  144 , a molten metal layer  802  collects. Above the metal layer  802  is a slag layer  804 . Sitting on top of the slag layer  804  are piles of feed  808 . The feed  808  is fed into the furnace vessel  144  through a plurality of apertures  812 . 
     The height and distribution of the piles of feed  808  may be measured by a plurality of level measurement units  814 . 
     Responsive to arcs from the electrode  142 , the feed  808  may be converted to the slag  804  and the metal  802 . In common with the application illustrated in  FIG. 4 , the slag  804  is foamy, forming a foamy slag layer  806 . Also responsive to arcs being repeatedly generated at the end of the electrode  142 , an arc cavity  810  may be understood to form. 
       FIG. 9  illustrates the non-ferrous shielded arc smelting furnace implementation of  FIG. 8  in an open arc condition. In  FIG. 9  an absence of the arc cavity  810  may be caused by a change in the foaminess of the foamy slag  806 . 
     It is notable that a plasma column that is hot is understood to be associated with a power draw that is much more stable than the power draw present in an open arc condition. Accordingly, an operator of the EAF  140  is interested in maintaining the arc cavity  410 ,  610 ,  810  and, by doing so, the operator of the EAF  140  may be seen to be avoiding an open arc condition. 
     The arc cavity  410 ,  610 ,  810  is also beneficial because, when the arc cavity  410 ,  610 ,  810  is present, the roof of the furnace vessel  144  and the upper sidewalls of the furnace vessel  144  are shielded from the arc generated by the electrode  142 , thereby prolonging the expected lifetime of the furnace vessel  144 . In the application illustrated in  FIG. 4 , the shielding is accomplished by a combination of the feed  408  and the foamy slag  406 . In the application illustrated in  FIG. 6 , the shielding is accomplished by the feed  608 . In the application illustrated in  FIG. 8 , the shielding is accomplished by the foamy slag layer  806 . 
     It may be seen, therefore, that there is a balance to be struck between raising the electrode  142  to achieve a long arc in the arc cavity  410 ,  610 ,  810  and avoiding the open arc condition, which condition may be seen to be more likely as the electrode  142  is raised. 
     At relatively high power level, which may be defined, for example, as greater than 60 Mega Watts, electrical resistance may be seen to increase responsive to the raising of the electrode  142 . A stable power measurement and a stable resistance measurement may be understood to be indicative of the electrode  142  being well positioned within the material that optimally surrounds the end of the electrode  142 . That material may be, in some applications, foamy slag, and may be, in other applications, granular feed banks. 
     Unfortunately, the depth of the foam layer  406 ,  806  and the feed  408 ,  608 ,  808  can be inconsistent. Accordingly, even when the position of the electrode  142  is maintained, a reduction of the depth of the foam layer  406 ,  806  or the feed  408 ,  608  may cause an open arc condition. It follows that a reduction in the depth of the foam layer  406 ,  806  may result in more frequent open arc conditions. Operation in an open arc condition may be shown to be associated with a higher resistance than the resistance measured during operation with the arc cavity  410 ,  610 ,  810 . Furthermore, operation in an open arc condition may be shown to make arc re-ignition more difficult. Operation in an open arc condition may be shown to result in higher fluctuation in furnace power draw than the fluctuation in furnace power draw measured during operation with the electrode  142  in the arc cavity  410 ,  610 ,  810 . 
     Insufficient arc coverage may occur based upon a variety of factors. One factor is the resistance of the slag. That is, due to the composition of the slag, the electrical resistivity of the slag may be lower or higher than expected. Another factor related to the composition of the slag relates to the extent to which the slag layer  804  forms the foam layer  806 . It turns out that the carbon content of the slag in the slag layer  804  relates directly to the extent to which the slag layer  804  forms the foam layer  806 . Another factor leading to insufficient arc coverage is insufficient volume of slag in the slag layer  804 . That is, a desired depth and volume in the foam layer  806  may not be achievable given a lower than desired depth and volume in the underlying slag layer  804 . For the steel scrap furnace implementation of  FIG. 4 , the quality of the scrap metal  402 , the carbon injection, the temperature and the lime injection will impact the depth and volume of the foam layer  406 . 
     In an aspect of the present application, the SPLC of  FIG. 1  is augmented with an open arc condition mitigation system  150 . The open arc condition mitigation system  150  includes an analyzer  102  connected to the SPLC in a manner that allows for the collection of electrical parameters characterizing the electricity drawn by the EAF  140 . The analyzer  102  provides output to a first control unit  104 . In turn, the first control unit  104  provides output to a second control unit  106  and a feed control unit  120 . 
     The analyzer  102 , the first control unit  104 , the second control unit  106  and the feed control  120  are shown as separate elements in  FIG. 1 . However, it should be understood that these elements may be implemented in hardware as a single unit or as multiple units. 
     In overview, the analyzer  102  obtains measurements of each phase of the power being drawn by the EAF  140  and analyzes the measurements. In one instance, the analyzer  102  obtains voltage measurements via a voltage transformer  122 . In another instance, the analyzer  102  obtains current measurements via a current transformer  136 . The analyzer  102  passes data to the first control unit  104 . The first control unit  104  determines, for each phase, the extent to which various operating parameters should be changed and instructs the second control unit  106  to carry out the changes. The second control unit  106 , acting upon the instructions from the first control unit  104 , adjusts operating parameters of the EAF  140  and the variable reactor  134 . 
       FIG. 2  illustrates the system of  FIG. 1 , wherein the analyzer  102  receives measurements from a secondary side of the furnace transformer  108  in accordance with aspects of the present application. In particular, the measurements are obtained from the voltage transformer  122  and the current transformer  136  positioned between the furnace transformer  108  and the EAF  140 . 
     In operation in view of  FIG. 10 , the analyzer  102  receives (step  1002 ) measurements of current and/or voltage from each phase. In one example, the analyzer  102  processes (step  1004 ) the measurements of the current and/or voltage to extract a plurality of harmonics of the current and/or voltage waveforms of the three phases. These harmonics, or a subset thereof, are then analyzed. The subset of harmonics may, for example, comprise just the lower order harmonics. 
     The analysis may, for example, involve determining (step  1006 ), for a selected time period, a particular harmonic characteristic parameter. More specifically, in one example, the analysis may be focused on a 3 rd  harmonic parameter, a 5 th  harmonic parameter, a total harmonic distortion (THD) parameter or a combination of these. The analyzer  102  may then output (step  1008 ) the determined harmonic characteristic parameter and return to receive (step  1002 ) further measurements. 
     Further particularly, in one example, the extracted 5 th  harmonics of each phase may be compared to each other to determine which phase has the greatest 5 th  harmonic. Once the phase having the greatest 5 th  harmonic has been determined, the analyzer  102  may then output (step  1008 ), to the first control unit  104 , the magnitude of the greatest harmonic, the magnitude of the corresponding fundamental and also a value representative of the largest 5 th  harmonic divided by the corresponding fundamental harmonic. 
     The same process may be repeated for the 3 rd  harmonic and for the THD. 
     Additionally, dependent upon configuration, the analyzer  102  may output (step  1008 ) a 5 th  harmonic percentage, a 3 rd  harmonic percentage or a THD percentage. Notably, for each harmonic, the analyzer  102  may employ an average value of all plurality of samples obtained in one second. 
     In sum, based on configuration, the analyzer  102  outputs (step  1008 ), to the first control unit  104 , an indication of a selected harmonic parameter. 
     In view of  FIG. 11 , the analyzer  102  may also receive (step  1102 ) measurements of voltage from each phase. The analyzer  102  may extract (step  1104 ) instantaneous voltage flicker samples and average (step  1106 ) voltage flicker samples in a time period for each phase. Based on the flicker samples, the analyzer  102  may determine (step  1108 ) a flicker characteristic parameter to associate with each phase. The analyzer  102  may determine (step  1108 ), for example, which phase has a flicker characteristic parameter that meets a predetermined criterion. More particularly, the greatest flicker characteristic parameter among the flicker characteristic parameters for the three phases may be of interest. The analyzer  102  may then output (step  1110 ) an indication of the flicker characteristic parameter that meets the predetermined criterion and return to receive (step  1102 ) further measurements. 
       FIG. 12  illustrates steps of an example method of operating the first control unit  104 . For one example, the first control unit  104  may, based on data received (step  1202 ) from the analyzer  102 , determine (step  1204 ) a current set point offset. The first control unit  104  may then transmit (step  1206 ) the current set point offset (say, expressed in kilo Amps) to the second control unit  106  and return to receive (step  1202 ) further indications. 
     For another example, the first control unit  104  may, based on data received (step  1202 ) from the analyzer  102 , determine (step  1204 ) a voltage set point offset. The first control unit  104  may then transmit (step  1206 ) the voltage set point offset to the second control unit  106  and return to receive (step  1202 ) further indications. 
     In each example of set point offset determination, the set point offset (current or voltage or both) is intended to mitigate changes in the arc cavity  410 ,  610 ,  810 . Of particular concern is changes that are indicative of an open arc condition. The changes in the arc cavity  410 ,  610 ,  810  may, for one example, be related to changes in the quality of the foamy slag  406 ,  806 . The changes in the arc cavity  410 ,  610 ,  810  may, for another example, be related to changes in the structure of the feed  408 ,  608 ,  808 . The first control unit  104  may, based on data received from the analyzer  102 , determine whether the data is indicative of an undesirable amount flicker and/or poor harmonics. The first control unit  104  may responsively generate a signal representative of bad foamy slag. Indeed, as the foam layer  406 ,  806  is either bad or not, the signal representative of the bad foamy slag may be a one-bit flag (a “Bad Foamy Slag” flag). In another aspect of the present application, the first control unit  104  may generate a signal representative of an open arc condition. Indeed, as the arc is either open or contained in the arc cavity  410 ,  610 ,  810 , the signal representative of the open arc condition may be a one-bit flag (an “Open Arc Condition” flag). 
     The first control unit  104  may determine a value known as Flicker Error, which may be representative of a deviation of measured flicker from a flicker detection threshold. Similarly, the first control unit  104  may determine a value known as Harmonic Error, which may be representative of a deviation of measured harmonic value from a harmonic detection threshold. 
     The first control unit  104  may include a foamy slag override enable module (not shown). This module may be arranged to take the Flicker Error, Harmonic Error and the Open Arc Condition Flag to calculate a voltage set point offset and current set point offset (step  1206 ). 
     Upon receipt of the current set point offset, the second control unit  106  may control the variable reactor  134  to regulate the current to the revised current setpoint. 
     Upon receipt of the voltage set point offset, the second control unit  106  may use the voltage set point offset to determine a new position for the electrode  142 . The second control unit  106  may then control the electrode  142  to move to the new position. 
     The second control unit  106  may be further adapted to control, based on values received from the first control unit  104 , the firing angle of the thyristor switch  139 . 
     As discussed hereinbefore, one aspect of the operation of the EAF  140  is the feeding of new material into the furnace vessel  144  through the plurality of apertures  412 ,  612 ,  812 . 
     In one aspect of the present application, the analysis performed at the analyzer  102  in combination with the determinations, made at the first control unit  104 , with regard to whether there is an open arc condition, may be used to adjust a rate at which new material is fed into the furnace vessel  144 . Further data, indicative of the height and distribution of the piles of feed  408 ,  608 ,  808  within the furnace vessel  144 , may also be useful when adjusting the rate at which new material is fed into the furnace vessel  144 . 
       FIG. 13  illustrates steps of another example method of operating the first control unit  104 . In this example, the first control unit  104  may receive (step  1302 ) parameter data from the analyzer  102  and feed level data from the plurality of level measurement units  414 ,  614 ,  814 . Based on the received data, the first control unit  104  may determine (step  1304 ) a change in the existing feed rate. The first control unit  104  may then transmit (step  1306 ) the change in feed rate to the feed control unit  120  and return to receive ( 1002 ) further indications. 
     Broadly speaking, it has been discussed hereinbefore that the analyzer  102  may receive a signal representative of a measurement related to operation of the electric arc furnace  140  and analyze the signal to determine a characteristic parameter. Based upon the characteristic parameter, the first control unit  104  may act to communicate to the second control unit  106  a change in the manner in which the electric arc furnace  140  is operating. Current set point offset and voltage set point offset have been discussed, as well as feed rate. It should be clear that other adjustable factors related to the manner in which the electric arc furnace  140  is operating may also be changed. Examples of adjustable factors include power set point offset, position of the electrode  142 , an angle of tilt for the furnace vessel  144  and speed of rotation of one or more cooling fans. The electric arc furnace  140  may have an associated additive system for adding, to the furnace vessel  144 , various substances that can change the nature of the contents (metal layer  402 ,  602 ,  802 ; slag layer  604 ,  804 ; foam layer  406 ,  806 ; feed  408 ,  608 ,  808 ) of the furnace vessel  144 . The substances may, for example, include lime, carbon and coal. 
     In one example, carbon may be added to a scrap steel bucket used to store the feed  408  before the feed  408  is introduced to the furnace of  FIG. 4 . In another example, coal may be added to a rotary kiln feeding the smelting furnace of  FIG. 6 . In further examples, carbon may be added via sidewall lances together with natural gas and oxygen or via a hopper and feed pipe through apertures  412 ,  612 ,  812  on the furnace roof. 
     Although the analyzer  102  has been described, to this point, as receiving an electrical signal representative of a measurement related to the operation of the electric arc furnace  140 , it is contemplated that the analyzer  102  may be configured to receive indications of non-electrical measurements related to the operation of the EAF  140 . Such non-electrical measurements may be representative of vibrations and/or sounds in and/or around the EAF  140 . 
     Aspects of the present application are directed toward mitigating an open arc condition. Indeed, the term “mitigating” in the present application is meant to reference both the act of taking steps to prevent the open arc condition as well as the act of taking steps, once in the open arc condition, to adjust the operation of the electric arc furnace to end the open arc condition and return to operation in the presence of the arc cavity  410 ,  610 ,  810 . 
     The above-described implementations of the present application are intended to be examples only. Alterations, modifications and variations may be effected to the particular implementations by those skilled in the art without departing from the scope of the application, which is defined by the claims appended hereto.