The 33rd Annual Conference of the IEEE Industrial Electronics Society (IECON)Nov. 5-8, 2007, Taipei, Taiwan

Effect of Electrode Regulation System on ArcParameters in Furnace OperationDr.W.Z.Gandhare

Principal, Govt. College of Engineering,Aurangabad (M.S.), India

wz_gandhare@yahoo.co.in

Abstract-The paper presents the investigations carried out fordetermining arc parameters in the operation of electric arcfurnaces. Electric arc furnace is a non linear load causing randomfluctuations in current. The measurements for quantities like arcparameters, power balance distribution in all phases; concept ofwild phase is illustrated. Power quality is affected by the erraticoperation of arc causing disturbances in plant operation.Histograms provide the useful information about regulationsystem behavior. The refractory wear is a major issue in furnaceoperation The arc voltage and power distribution has influenceon refractive index. The records on furnace measurement areanalyzed.

Key words: Wild Phase, Refractory Symmetry, ImpedanceHysteresis, Arc stability

I. INTRODUCTION

Widespread steel production by means of electric power hasbeen rising. Despite the advantages of arc furnaces ascompared to blast furnaces, this type of load produces voltagedisturbances which can adversely affect other loads in theelectrical network. The arc furnace is a highly nonlinear loadwhich produces current harmonics. This current harmonicswill give rise to high voltage harmonics depending on theimpedance of the electric network. When network resonanceconditions appear, the voltage harmonics became very high,causing malfunction or damaging the electric equipmentconnected to it. The electrodes are modeled as a seriesconnected resistance and inductance. The electric arc ismodeled as a nonlinear current-voltage characteristic whichrepresents the voltage drop for a given arc

In any arc furnace, the arc length varies with the time,basically because of electrode movement and also because ofmovement of scrap. The arc melting process is a verycomplicated process. It converts the electrical energy intothermal energy. The electric arc is used to melt the rawmaterials held by the furnace. The random movement of themelting material results in heavy current fluctuations duringthe arc melting process. During the refining period, the scrapmetal is at a molten form and hence fluctuations are small.The furnace operation depends upon arc voltage, arc current

D.D.LulekarLecturer in Electrical Engineering

Govt. Polytechnic Khamgaon (M.S.) Indiaddlulekar@yahoo.co.in

and arc length, which is determined by the position of theelectrodes. By examining the actual V-I characteristic of thearc furnace, the arc melting process is divided into threeperiods [1]. The furnace measurements are carried out on twofurnaces and the analysis is carried out in detail. The arcparameters and overall balance in current, voltage and powerare calculated.

I. THE ELECTRODE POSITION CONTROL

The electrode control system keeps the electrodes in eachphase at the right distance from the scrap or melt, so that thepower of the arc is optimized for production and the requisitemetallurgical processes as well as for economical wall life.Electrode control requirements for the melt-down period arequite different from those for refining periods. The control ofone electrode must not interfere with the other electrodes andmust not cause any system stability. The electrode controlsystem must fulfill the following requirements.

1. Selected power must be kept constant during themeltdown period.2. Different up and down electrode speeds3. Maximum electrode up speed as soon as current riseswhen scrap contacted to avoid tripping of circuit breaker.4. Current interruptions should be avoided by using highquality control system with good dynamic response. Thecontrol system must behave differently when melting downscrap than when melt is liquid.5. The operating points i.e. the impedance values per phasemust be selected automatically for each secondary voltagestep of the furnace transformer during operation.6. manual control of electrodes

Arc Voltage Control and Power distributionAn arc is a gas discharge between two electrodes which are

supplied by a voltage source. In an arc furnace the arc burnsunder atmospheric pressure between the end of the graphiteelectrode and the steel melt. The arc is sustained by analternating current. A voltage of at least 40 volt and a current

1-4244-0783-4/07/$20.00 C 2007 IEEE 921

of 4 kA are required to maintain ionization and produce theheat of the arc. A typical arc is unstable and non-linear and itscurrent and voltage are in phase as they pass through zero.This instability and non-linearity are greatest when meltingdown cold scrap. The delayed and erratic process of strikingthe arc and resulting gaps in the current are conspicuous. Asmelting down progresses, the striking becomes more stable,but the current can still contain low frequency fluctuations.The temperature and heat of an arc are high with a liquid steelbath, and the thermal conduction is low, so that the arccharacteristic begins to approach the linear behavior of anordinary resistance. The arc voltage is almost rectangular. Therectangular arc voltage waveform is not entirely adopted bythe supply current since inductance in the circuit gives rise toinductive reactance which increases with frequency andtherefore resists harmonics. Although the current waveform isalmost sinusoidal, the noticeable harmonic content causes anincrease in reactive power and with it a higher reactance [2].The magnetic fields of the adjacent phases exert horizontalelectrodynamic forces on the arc, with the result that arc whichhas virtually no inertia , is deflected sideways away from thevertical, towards the furnace wall. The way the arc fluctuates isespecially is important during melting down. Largemovements of the arc roots on the electrodes causeconsiderable unacceptable changes in voltage and current inmain supplies which have insufficient power and short circuitcapacity. Hydrodynamic forces can act upon the arc cathode(melt) even in the absence of an external magnetic field.'Pinch effect'-necking of the arc on the cathode produces arecoil force P on the cathode in the direction of the arc axis.

P= 1 0-7i2In rc/ rca N

where rc=-mean radius of the arc column and rca= radius ofthe cathode hot spot. The arc displacement increases with theextent of the necking and square of the current. In this way thehigh current arcs produce a desirable stirring effect in the

electrode convection-

radiation-

Arc columnRadiation Ra

Arc Plasma Radiation Rf

7%

3%

Fig. 1. Typical Values of Energy Balance for an Arc

melt.. The proportions depend on the arc length and slagthickness.

II. EQUIVALENT CIRCUITThe simplified example of a single phase arc furnace with abottom electrode is shown in figure 2. This case is identical tothe symmetrical three phase unit. The transformer voltage Vphcomprises the arc voltage, the voltage drop VR over the lossresistance RR of the circuit, the induced voltage VI due to selfinductance L.

Va

Fig 2. Equivalent Circuit of Arc Furnace

Vph = VR + VL + VABecause the arc voltage is non-linear, the current also containsharmonics. The apparent operational reactance is not constantbut depends on the harmonic content of the current. Theharmonic content of the current and the operational reactanceboth increase with rising arc voltage. Increased operationalreactance can be observed mainly at the start of scrap meltingpoint when it reduces the power applied to the melt. Currentharmonics also increases current displacement effects in theelectrodes, so that loss resistance increases by several percent.The equivalent circuit can be simplified by replacing the nonlinear arc by arc resistance RA. The operational reactance mustthen be assumed to be 10-15% higher than the short circuitreactance. The equivalent circuit of three phase arc furnaceconsists of three phase with inductance, loss resistance and arcvoltage source connected in series meeting at a free star point.The equivalent circuit of the three phase arc furnace can belinearized.

Electrical symmetry is important in the operation of arcfurnace. If the resistances, reactances and conductor voltagesare balanced, then the furnace currents will be equal too. Inpower calculations, we have to consider the reactances oftransformer, conductor section, flexible cable, tubularconductor and electrode. The reactances in all the phases mustbe symmetrical for balanced arc furnace operation. Themaximum arc power occurs at a lower current than overallmaximum power. The radiation is greatest with long arcs.

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In regular operation single controller, impedance regulation,current regulation, constant control dynamics and degressivecontrol dynamics are the features. The control aspects includeguiding electrodes 1, 2, 3, mean current and minimum current.The auxiliary functions of electrode control are automaticadjustment for manual operation, automatic calculation of finalamplification, plotting programs, software transient recorderand visualization interface. A typical value for digitalelectrode control response time is 17 msec. The A/Dconversion and smoothing may take 4 msec, and 1 msecrequired for D/A conversion and so.

TABLE IOPERATING PARAMETERS FOR ARC

Parameters December 2006 April2003

Arc Power 26.83 28.01

Arc Voltage 231 227

Arc Resistance (milliohms) 5.96 5.52Average arc lengths ( mm) 228 223Electrical Efficiency of arc 92 91.3

Arc Stability Index 1.4 2.3

Arc radiatiiiiion intensity (MW- 2.0 .21meter)Arc radiation index 6.8 7.0

Table I shows the arc parameters for studied furnace. Arclength is very important parameter. All parameters aim toachieve optimization of arc power and better heat conditionsrequired for operation. That is why melting is preferablycarried out at a low power factor using short arcs [3].

The furnace current I = VL f'13 Z

Total apparent power = S = AI3 VL I

Reactive Power Q = 3 12 XActive Power = PR + PAArc Power losses = PA = 3 I RA

(1)(2)(3)(4)(5)

III. PERFORMANCE ANALYSIS

Manual commands

Fig. 3. Digital Electrode Control

Figure 3 shows block diagram of digital electrode control.The various operation modes are as regular operation, manualoperation and programmed operation.

The performance of electric arc furnace is determined fromcircle diagrams. The performance can be calculated providedthe electrical parameters like arc length, efficiency, refractiveindex, etc are measured. For two furnaces the measurementsare carried out and the various graphs are plotted.From table I, the arc stability index is higher than desired

value of 1.25. Electrode position is to be controlledaccordingly.

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Tap changerPosition

Ui/U1N

Characteristic number

Arc voltage, Power distribution and the influence onrefractory symmetry

To determine the refractory wear the values of arc voltage (=arc length) , arc power (= radiation intensity) are used. Ofcourse the optimum of refractory symmetry is equal values ofarc voltage and power in all three phases. In practice, equalvalues of arc voltage and power in all three phases are notfound at the same currents. The relative refractory index is acompromise and includes the influence of both value supplyHeavy wear of the refractory lining near the wild phase iscaused by excessive arc length. Changing the arc length in onephase causes only minute changes in current, because the starpoint is free. If the arc is shortened too much then anotherphase can become wild. Refractory index is the product of arcvoltage and arc power given by Schwabe [3].

The refractory wear is not influenced by distribution ofarc voltage and power also the factors like capacity of fumeextraction, direction of oxygen and carbon blowing, kind ofscrap , direction of DRI feeding etc., geometry of furnacevessel, alignment of electrodes. The following parameters arecalculated for each phase. Arc voltage in volt, arc power inMW, relative refractory index in %, power factor

Ei ib

=t i i.Ars S"blility Ind-le

Fig 3. Operating Current, Active Power and Tap Voltage as Function of ArcStability Index

The arc stability index value is related to the electrodemovement. If arc stability index is higher, there is localizedheating and hunting decreases. If stability index is low,hunting is more and electrode movement is continuous rapidfluctuations. The desired refractive index is 1.25. The valuesfor arc furnaces 1 and 2 are 1.4 and 1.18 respectively in recentmeasurements. The analysis provides the desired setting of tap

voltage and tap number. For 1.25 index value, the operatingcurrent is decided and required tap can be used.

Fig 4. Occurrence Variation with Impedance

The value of impedance 7 to 7.5 is occurring 21 times forindividual phases as well as cumulative Similarly themagnitude 6.5 to 7.0 is also occurring 21 times as cumulativevalue. This clearly shows the variation of impedance is 6.5 -7.5 ohms. The working band of impedance is however 6 to 8.The extreme values are 5 and 13.

Fig.5. Impedance Variation with Operating Current

Impedance hysteresis is due to sluggishness response ofelectrical and mechanical system. Impedance hysteresis is thedeviation in actual impedance and calculated as difference inset impedance and actual measured impedance. Hysteresisvariance = (Impedance hysteresis/set impedance)*100. Theupward tendency shows that actual currents are less than setpoint.

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_____________________________ I_________.__o___._,____-___-_____________r_______r_r_L____?_____

LIII

I I

ii 1.I I

ii J. .

if. . . I I......................................

4.1

A,,. st.,billtv iAdk

IV. CONCLUSION

1 Arc Resistance around 6 ohms for furnace is slightly lessthan 7 which is desired value for conditions where inputconsists of 50% Sponge iron.

2. Arc lengths are sufficient in existing slag level. In phase-2 the arc lengths are normally lesser than other two being adead phase during arcing with both other phases. Same is thecase for arc stability.

3. A close operating band for Impedance leading preciseelectrode control is observed.

4. Operating reactance for phase 2 may be set fordeviations by gain control. Thermal balancing can be done forall phases by adjustment of current by electrode regulation.

5. Arc burns more stable when arc resistance is around 7milliohms. A good foaming slag will be helpful in maintainingcovered arc thereby stable burning. This will also reducevoltage flicker and refractory wear cuts. During longer arcs,arc stability needs to be always considered. Flaring increaseswhen arc length is increased. For furnace the value of arcresistance is 5.96. From above value, the arc seems to bestable.

6. At preset tap voltage, increase in arc length will reducecurrents resulting in increase in operating power factor.

7. Increased arc lengths will reduce electrode consumptionand phase losses.

8. Increased arc length will increase arc flaring radiationintensity affecting the life of intensity.

9. Arc stability will be reduced due to increased arc length;arc power will be deteriorated causing overall disturbance

ACKNOWLEDGMENTWe sincerely express our thanks to Nitin Ghodake, AGM of

Lloyd steel industries Ltd. Wardha for discussions andsupport. We are grateful to the management and maintenancepersonnel for kind assistance during the measurements.

REFERENCES[1] C Sharmeela, G.Uma, M.R.Mohan and K.Kartikeyan"Voltage Flicker

Analysis and Mitigation -Case study in AC Electric Arc Furnace usingPSCAD/EMTDC, "2004 International conference on power technology-Powercon 2004,Singapore 21 -24 November, pp.707-71

[2] Klaus Timm, Hamburg, basic Principals of electric furnaces Edited byE. Plockinger and 0. Etterich, John Wiley and Sons, Ltd, 1985, pp.127-160.

[3] G. Manchur, C.C. Erven, "Development of a model for predictingflicker rom electric arc furnaces," IEEE Transactions Power delivery,vol.7, no. 1, January 1992, pp.416-426.

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