arc furnace model for the study of flicker

8/22/2019 Arc Furnace Model for the Study of Flicker

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2026 IEEE Transactions on Power Delivery, Vol. 9, No . 4, October 1994ARC-FURNACE MODEL FOR THE STUDY OF FLICKERCOMPENSATION IN ELECTRICAL NETWOR KSG. C. Montanaril, M. Logginil, A. Cavallinil, L. Pittil, D. ZaninellizSenior Mem ber Member Non Mem ber Non Mem ber Mem ber

1Istituto di E lettrotecnica Industriale, U niversith di B ologna2 Dipartimento di Elettrotecnica, Politecnico di MilanoItaly

A A . This paper presents an arc-furnace modelconsi sting of non-linear, time varying resistance wheretwo different time-variation laws of arc length areconsidered. One consists of a periodic, sinusoidal law, theother of a band-limited white-noise law. The arc-furnacemodel is implemented by EMTP, referring to actualelectric-plant configurations. Simulations are reportedwhere the values of flicker sensation and short-term flickers ev er it y, P a r e d ete rm in ed ac co rd in g t o U IEspecifications. It results that the model based on sinusoidaltime-variation law can be useful for worst-caseapproximations, while the model using white-noise law isable to fit flicker measurements m ade in electric plantssupplying arc furnaces. Th e models are used t o investigatethe effect o n flicker com pensation of the insertion of seriesinductors at the supply side of the furnace transformer. Itis shown that considerable reduction of Pm is obtained atthe point of common coupling by series inductorinstallation at constant fu rnace active power.Keywords: Arc Furnace, EMTP simulation, Flicker.

except for short periods, while ii) depends on networkmanagement.Solution iii) is more easily available for the arc-furnacecustomer and preferable for the electricity supplier. Often,static-var-systems (SVS) are installed in plants feeding arcfurnaces in order to compensate both voltage fluctuationand voltage distortion. However, this solution can be quiteexpensive, depending on the plant size, and, moreover,fluorescent-light flicker have been addressed to SVS-network interactions [4].On-field experiences, as well as computer simulations,have proven the effectiveness of inserting series inductorsat the supply side of the furnace transformer [5, 61, but indepth investigations on this topic are still lacking.The choice of the most convenient solution for flickercompensation requires availability of accurate arc-furnacemodels. Th is would allow simulating the electric plantsupplying the arc furnace and studying the effects ofcompensation systems on arc and flicker behavior, as wellas on voltage and current distortion factor, power factor,furnace active and non-active Dower. At mesent. electricar c is usually modelled by *voltage ggnerators whichprovide fundamental and harmonic voltages whoseamplitudes are time-modulated to describe arc-length

kc urnac es used for steel production are a main cause variations and , hence, network-voltage fluctuations. Th eof voltage fluctuations in electrical networks, which m ay modulation law can be sinusoidal at frequencies typical ofgive rise to the flicker effect. Voltage fluctuations, due to flicker [6, 71. However, it has been shown in [81 that thisram dom arc-length variations durin g scrap melting, have Solution is not fully satisfactory being a lineartypical frequencies in the range 0.5-25Hz. representation of non-linear phenomena, which is unableFlicker consists of luminosity variations of lamps which to take into account the effect on electric-arc behavior offrequency and intensity. For example, voltage-amplitude insertion of serie s inductors or shunt filters). Therefore, anto get Over the mean human perceptivity threshold [1-31. ar c is describ ed by a non-linear, time-varying resistance.since oltage fluctuations ar e no t limited to electric In this pape r, the effect on flicker compensation of theplants supplying arc furnaces, but ma y affect the HV installation of series inductors at the supply side of thenetwork to a large extent, several MV and LV customers furnace transformer is investigated, using E M T Pcan be disturbed by flicker, so that e~ectricity-supp~y simulation [9]. The electric arc is described by a non-companies must take care of this problem. ~ ~ ~ ~ ~ ~ l l ~ ,he linear resistance and two different time-variation lawssolutions able to reduce flicker are [4 ] (based on sinusoidal and white-noise functions) are

i) to decrease furnace power, considered. Comparisons of the proposed models withii) to increase short-circuit power at the point of measurements made on north Italian plants are reported.common coupling (PCC),As is obvious, solution i) is not economically valid,

l"may affect the human visual system, depending on theirvariations of about 0.3% at frequency 10 Hz are sufficent

changes Of electric-plantimproved model was proposed in L81, where th e

(due to,

iii) to install appar atus for flicker compensation. f SA typical circuit diagram of an electric plant supplying

9 4 WM 0 8 6 - 9 PWRD A paper recommended and approved an ar c furnace is shown in Fig. 1. The furnace isby the IEEE Transmission and Distribution Committee connected to bus 1, the PCC, by means of a HV/MVof the IEEE Power Engineering Society for presentat- tranformer (TI) nd is fed by a MV/LV tranformer (T2).ion at the IEEE/PES 1 9 9 4 Winter Meeting, Ne w York, Th e furnace-side of this transformer usually has adjustableNew York, January 3 0 - February 3 , 1 9 94 . Manuscript voltage in order to vary the furnace power. XLSC s the

inserted for flicker compensation (both XLSc an d X, ar evaried in the simulations; the reference value of the short-circuit power at PCC is 3500 MVA). Xc and arereactance and resistance of the connection line between

submitted June 24, 1 9 9 3 ; made available for PrintingDecember 6, 1 9 9 3 . short-circuit reactance at the p c c , xPhe series reactance

0885-8977/94/$04.000 1994 EEE


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furnace electrodes and MVILV transformer, whichgenerally make a significant contribution to the totalimpedance seen by the a rc furnace.The values attributed to the plant parameters for theEMTP simulations presented in this paper are typical ofarc-furnace plants installed in northern Italy. The HV/MVand MV/L V transformers (220121 kV and 21/0.9+0.6 kV)have rated power 95 MVA and 60 MVA, respectively, percent values of short-circuit voltage and losses 12.5% and0.5% (HV/MV), 10% an d 0.5% (MVILV). T he values ofthe lead reactance and resistance are &=3 10-3 Ohm andk = 3 10-4 hm [7, 101.As mentioned above, simulation of the electric arc isrealized by mean s of a non-linear model. Th e arc voltage-current characteristic,v, = Va(Ia) (1)

can be described by the fo llowing relationshipLv, = Vat +-+ I, ( 2 )

whe re V,, I, are ar c voltage and current, V at is thethreshold value to which voltage tends when currentincreases, C and D are constants whose values (C,, D, an dc b , Db) determine the difference between the increasingand decreasing-current parts of the v-i characteristic(eq.(2) is written for I , >O , but can be easily arranged forI,


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0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1,/Is.Fig. 3. Characteristic curves of single-phase arc furnace obtained byEMTP simulation, with non-linear arc resistance. Th e voltage ratioof the furnace transformer is 2U0.6 kV .

The complex nature of these phenomena does not favora physical approach to the study of arc-length variation.Therefore, flicker investigations have been performed onthe basis of deterministic, [6, 81, or stochastic, [13],assumptions fo r the time-variation law.The deterministic approach considers that the arc lengthis subjected to sinusoidal time variations, with frequencysuitably chosen in the range of those typical of flicker (i.e.0.5-25 Hz). This assumption does not clearly represent anormal working condition for furnaces, but has theadvantage of being easily manageable by computersimulation and require short-time simulations due to theperiodicity of the law.Th e stochastic approach is supported by the observationthat the arc-length time-variation can be considered arandom phenomenon. In fact, extensive measurements ofactive and reactive power, voltage and current in plantsfeeding a rc furnaces have shown that voltage fluctuations,as well as reactive power variations, at the PCC and thefurnace bus, behave as a band-limited white noise, withtime-varying amplitude [13]. Therefore, random-variationlaws should be attributed to arc-length. Simulation time sare longer than in the previous case, due to the the factthat the signal is not periodic. On the other hand,standards introduce weighted stochastic indices to evaluateflicker sev erity, such as PsT and PLT, which a re based on10 minutes, or more, of recording time [3].Th e above-proposed non-linear model (eqns. (2)-(5)) isable to fit both deterministic and stochastic approachesproviding that appropriate laws are attr ibuted to a rc length,I=l(t). 'Sinusoidal law for time variation

In order to approach periodic flicker behavior,simulations can be made attributing to arc length asinusoidal law with frequency close to the most sensitivefor flicker perceptivity. For example, the frequency of 10Hz can be chosen, which lies in the center of thesensitivity range, close to the minimum of the flickerperceptivity threshold curve for sinusoidal voltagefluctuations [3].With reference to eqns. (2)-(5), the arc-length time-variation law can be expressed asl(t) = 1, - (D1/2) (l+sinwt) ( 6 )

where D1 is the maximum variation of arc length. Thetime variation of the arc voltage-current characteristic th usbecomes, from (3), (5) and (6)Va(Ia) = k(t) Vao(Ia) ( 7 )

withA+Bl(t) ( B D1/2) (l+sinwt)- 1 - ( 8 )(t) =-A+B10 A + B 1 0

Th e whole procedu re for AT P modellization of the non-linear arc resistance with time-varying sinusoidal law canbe implemented in the TACS section of EMTP, asreported in [8]. Figure 4 shows the voltage and currentwaveforms at the arc-furnace bus, obtained by EMTPsimulation on the basis of the proposed model, withsinusoidal time-variation law at frequency 10 Hz. The arclength varies in a wide range, corresponding to values ofar c threshold voltage 40VI ,, I2 4 0 V and continuousconduction. The furnace transformer has secondaryvoltage of 600 V. Under these conditions, the relativevoltage variation at the PCC, DV/V, is 1.35%.Actually, the UIE flickermeter has an output which isrepresentative of flicker sensation S(t). This quantity isobtained referring the voltage fluctuations to the threshold-perceptivity curve by means of a weighting filter havingminimum attenuation at the frequency of 8.8 Hz which isabout that of maximum eye sensitivity to light emitted byincandescent la mp s [3]. One unit of output corre spon ds tothe visual perceptivity threshold of flicker occurrence.Th e computer simulations based on the proposed model,with deterministic sinusoidal time-variation load, shouldconveniently provide flicker estimates which are directlycomparable with the values given by the UIE flickermeter.For this purpose, the flickermeter can be implemented byEMTP, as shown in [8], so that the 8.8Hz equivalentvoltage variation, DV,,/V, can be obtained. Withreference to the simulation providing Fig. 4, he value1.29% of DV, /V is thus derived (the small differencewith respect to ?he value of DV/V above reported is due tothe choice of the frequency of the sinusoidal law, which isvery close to 8.8 Hz).Figures SA and 5 B show the harmonic analysis of thevoltage at bus 2 (Fig. l) , obtained by simulations withtime-varying (frequency of the sinusoidal law 10 Hz) andconstant arc length. As can be seen com paring the figures,the non -linearity of the model causes, the presence ofcharacteristic and non-characteristic harmonics (Fig. 5B)[14, 151, while interharmonics (i.e. non-multipleharmonics [161) are generated when the sinusoidal time-variation law is considered (fig. 5A). These side-bands ofthe multiple harmonics are always expected in thepresence of voltage modulation due to arc-length timevariation. The interharmonics responsible for the flickereffect are mainly those included in the sidebands of th e

x

t (IFig. 4.Voltage and curre nt waveforms at the arc-furnace bus (point4 of fig. 1) obtained by arc-furnace simulation made by non-linear,time varying resistance.


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10

10-L102

10

0 100 200 300 400 500 600 700 800 900 I000100

f [HzlFig. 5 . Harmonic analysis of the voltage at bus 2 of Fig. 1, forsinusoidal time-varying arc-length (A) and constant arc-length (B).fundamental frequency, i.e. 50 Hz (on the other hand, theUIE fl ickermeter, [3], prevailingly takes into accountfundamental-voltage fluctuations).Wh ite-ndse time variation

With reference to the proposed model, the timedependence of the arc-length can be expressed as

where lo is the maximum arc length compatible withcontinuous conduction and r(t) is the law of arc-lengthvariation with respect to the reference condition lo. Signalr(t) is a white noise with band in the frequency rangewher e voltage fluctuations produce flicker.The time variation of the arc voltage-currentcharacteristic has again expression (7), where k(t)becomes, for the white-noise time-variation law of eq. (9),( 1 0 )

Even in this case, the procedure for EMTP simulationof variable arc length ca n be implemented in the TACSsection resorting to three blocks, that is, a random-numbergenerator, a pass-band filter with lower and upper cutfrequencies 4 Hz and 14 Hz (according to [13]), and thethird block where eq.(9) provides k(t) as output signal.With respect to the case of periodic time-variation laws(e.g. that sinusoidal previously accomplished),considerably longer simulation times are required for therandom arc-length time law here considered. Moreover,

the flicker sensation, S(t), should be processed in order toobtain the comprehensive quantity also available at theUIE flickermeter output, the so-called short-term flickerseverity, PsT [3]. It consists of a weighted sum ofpercentiles of the cumulative probability distribution ofS(t), having the purpose to provide objective informationon the flicker-severity level independently of the type offlicker, its time law and evolution. PsT is thus defined asPST = ( 0 . 0 3 1 4 Sgg.g% + 0 .0 5 2 5 Sgg% + 0.0657 S97%(11)0.28 Sgo% + 0.08 S 5 0 ~ ) ~ /

where the percentiles 50% , 90% , 97%, 99% and 99.9%of S(t) are considered.PsT estimates are based, according to UI Erecommendations, on 10-minute observations (anotherquantity is also proposed in [3], that is, long-term flickerseverity, P,, referred to two-hour observations), but theresults here reported are relevant to one-minutesimulations for the s ake of computing-time saving.Figures 6A an d 6B show the time behavior of non-activepower (i.e. Q=(S2-P2)0.5 ) and the corresponding flickersensation, S(t), at the PC C obtained by computersimulation based on the white-noise time variation of arclength. The plant parameters are the same as the previoussimulations, pertinent to sinusoidal time law, with arc-threshold voltage, V , , varying in the range 40-240 V,where continuous conduction, as well as wide arc-lengthvariations, are allowed. The value of PsT calculated forone-minute simulation is 1. 6 (according to [3], PsT valuesexceeding 1 ndicate flicker disturbance).roved to reproducequite-well real cases for both icker and voltagedistortion-factor evaluations (considering also theapproximations due to single-phase simulation). Indeed,measurements made on a plant with characteristics veryclose to those used for the simulations provided values ofP approaching 1.6 in the starting melting period. Insimulations longer than one minute, the arc-lengthvariation range could be changed with time, such as todescribe the voltage-fluctuation decrease that generallyoccurs increa sing the quantity of melted metal.Model discussion

The results of these simulations 8

A direct comparison of the two kinds of arc-length time-variation laws presented above can be performed once theresults obtained by the sinusoidal law, that is, DVeq/V,areconverted into PsT. Indeed, PsT calculation is meaninglessfor a deterministic signal and, moreover, shorter timesthan 1 o r 10 minutes are needed to evaluate flicker effcctfor periodic signals. However, for the sake of comparisonit can be observed that the probability distribution of adeterministic, sinusoidal signal is stepwise ([3]), hencefrom (11)pST x (0.5096 S & ) l I 2 ( 1 2 )

where SMm s the maximum value of flicker sensation.Under these premises, the value of PST derived from thesimulations with sinusoidal time-variation law pertinent toFigures 4 , 5 (providing DVeq/V= 1.29%) is 3.7.Therefore, the value of PsT obtained by the white-noisetime-variation law is significantly lower (1.6) than thatderived by the sinusoidal law (3.7), for the same plant andfurnace working conditions.It can be argued that the sinusoidal law provides limitconditions for furnace operation, that is, a worst-caseapproximation, which enables determination of maximumflicker sensation caused at the PCC by a furnace of known


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0

37 37.5 38 38.5 39 39.5 40 40.5 41 41.5 42t [SIFig. 6. Time behavior of non-active power (A) and thecorresponding per-unit flicker sensation (B) at the PCC obtained bycomputer simulation based on the w hite-noise time variation of arclength.

characteristics (similar results are expected by modulationwith rectangular law). This approximation seems moreeffective than the criterion used in [12, 131, where DV/Vis evaluated referring to the limit cases of not-feededfurnace and short-circuit conditions. Moreover, theproposed model allows to realize investigations on theeffects of flicker-compensation systems in worst-caseconditions, and provides the resulting flicker evaluations interms of S(t) or PsT, according to UIE instrumentation.When the white-noise time-variation law is assumed forthe ar c length, real working conditions can be approachedand a sort of average P es timated for the studied plant .This analysis can be useful to evaluate flicker-compensation strategies in the design stage of electricplants supplying ar c furnaces or in existing plants wheremeasurements show the need to resort to flicker-compensation systems.

FLICKER COMPEN SATION BY SERIESINDUCTORSTh e calculations reported up to this point are relevant tosecondary voltage of the MV/LV transformer of 600V,absence of the series inductor (i.e. Xp=O in Fig. 1) an dshort-circuit ratio (SCR) at PCC equal to 58 (the short-cir cui t, ratio is defined as the ratio of the sho rt-circuit

power to the mean apparent power required by the load).In order to envisage the effects of the installation ofseries inductors in the plant feeding the furnace, thepresence of a series reactance, X,, at the supply-side ofthe MV/LV furnace transformer can be considered, asshown by Fig. 1 (the equivalent reactor resistance isneglected). H owev er, insertion of series inductors gives

rise to decrease of furnace power so that actions arer e q u i r e d to avoid significant reduc t ions of furnaceproductivity. Mainly, the transformer turns ratio of th eMV/LV transformer can be changed, taking profit of theadjustable secondary voltage (varying from 600V to 9OOV,step 60V, in our simulations). This causes arc-lengthvariations, too.Two design criteria are here compared in order to lookin detail at the behavior of series inductors. One consistsof keeping constant the power absorbed by the systemseries reactance-transformer-furnace with short-circuitedelectrodes (so that the SCR at PCC does not significantlychange with inductor insertion). The other, which betterconforms with the above requisites of furnace efficency, isto keep constant the mean active power absorbed by thefurnace. In both cases, simulations with several values ofseries reactance have been realized, cha nging the furnace-side voltage of the M V/L V transformer in accordance tothe design criteria. As a consequence of the assumed arc-furnace model, an increase of furnace-side voltage, due toinsertion of series inductor, causes arc lengthening.According to on-field observations, [17], the maximumarc-length variations, D1, have been assumed independentof arc length in the range of values used for thesimulation s (i.e. correspon ding to continuous conduction).Hence, longer ar cs provide smaller variation of relativelength D1/1. Both sinusoidal and white-noise laws for arc-length time variation have been considered in thesimulations (the former with frequency 10Hz).Tables 1 an d 2 report the values of the series reactance,X,, inserted in the plant of the characteristics abovedescribed, together with the secondary voltage of thefurnace transformer, the equivalent voltage variation,DV,,/V, and the correspondin g PsT, for the two designcriteria. The ratios of the series reactance to the totalreactance, X,, as seen upstream the arc-furnace electrodes,are also given. The sinusoidal law of arc-length timevariation is assumed. In Figure 7 the graph of voltagevariation vs ratio Xp/Xt, relevant to the data of Table 2, isdrawn.The simulation results show that by both design criteriathe insertion of series inductors at the supply side of thefurnace transformer can significantly reduce voltagefluctuations, and, therefore, flicker, at PCC. Takingadvantage of the maximum voltage adjustment allowable atthe secondary side of the furnace transformer, the voltagevariation at PCC decreases to 80% an d 60% with respectto the values determined in the absence of series reactance.Hence, significant compensation possibilities are providedby the design criterion of constant mean power absorbedby the furnace (Table 2), but also the other criterion,where the short-circuit power at bus 2 of Fig. 1 ispractically kept constant, provides non-negligible flickercompensation. In the former case, however, higher series-reactance values are required for the same transformervoltage.The dependence of the series-reactance compensationeffect on the short-circuit ratio is depicted in Figure 8,referred to the criterion of constant furnace power andsinusoidal modulation. The surface shows that increasingSCR (SCR=58 corresponds to short-circuit power, Ssc, of3500 MVA ), the PsT decreases for any value of seriesreactance, as expected. In fact, a simple relationship,derived under the approximate assumption that varying theshort-circuit power, the furnace current does notconsiderably change (being XLsc very small with respectto the total reactance of the line feeding the furnace) pointsout the inverse relationship between voltage variation (orPsT) and short-circuit power:


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1

I I I0 0.1 0.2 0.3 0.4 0.5 0.6

-. ...,,... .... ._ i . . ....... .......... .


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2032Table 1. SBries-reactance values (Xp), ratios betweenseries reactance and total feeding-line reactance(xp/xt), secondary voltages of the furnace transformer(vz), voltage vari ation (D Veq/V) and short-term flickerseverity at the PCC (PsT) for design realized atconstant power absorbed by the system reactance-transfor mer-furn ace with short-circuited electrodes.Sinusoid al time-variati on law of arc length.

II 0 I O I 0.60 I 1.29 I 3.68 1111 0.63 I 0.12 1 0.66 I 1.21 I 3.45 1111 1.12 I 0.22 I 0.72 I 1.15 I 3.28 111 1.50 1 0.29 I 0.78 I 1.10 I 3.14 11(1 1.79 I 0.35 I 0.84 I 1.06 I 3.03 1111 2.04 I 0.40 I 0.90 I 1.02 I 2.91 11I1 I I I 1 11

Table 2. Series-r eactance .values (Xp), ratios b etweenseries reactance and total feeding-line reactance(Xp/Xt), secondary voltages of the furnace transformer(VZ), volt age vari atio n (DVe,/V) and short-t erm flic kerseverity at the PCC (PsT) for design realized atconstant mean power absorbed by the furnace. Sinusoidaltime-vari ation law of arc length.

I I I Iii 0 I O11 0.98 1 0.18 I 0.66 I 1.12 I 3.20 1111 1.73 I 0.30 I 0.72 I 1.02 1 2.91 1111 2.37 I 0.41 I 0.78 I 0.93 1 2.66 11)I 2.86 I 0.46 1 0.84 I 0.87 I 2.48 11(1 3.29 I 0.52 1 0.90 I 0.79 I 2.26 11I I I I I 11

SERIES INDU TORSAND VO TAGE-CURRENDrsToRTIoNArc furnaces are well-known harmonic voltage andcurrent sources, owing to their non-linear characteristic.Multiple harmonics are injected in the feeding plant, dueto waveform distortion with respect to the sinusoidalframe, besides non-multiple harmonics, or interharmonics,[16], which are caused by arc-length time variation [ 5 , 7,lo]. Figure 5A, relevant to sinusoidal modulation, andFigure 11 , pertinent to white-noise time-variation law,show that voltage and current in plants supplying arcfurnaces have almost continuous frequency spectrum (theharmonic analysis of Fig. 1 is relevant to the entiresimulation of one minute).However, calculation of distortion factor shouldconveniently separate the effect of interharmonicsgeneration, which is computed by flicker measurements,by the waveform deviation from sinusoidal shape, which iswell taken into account by the usual expression for totalharmonic distortion (THD) calculation, recommended byI E E E 519 [14], that is

THD = 100%A 1

where Af is the amplitude of multiple harmonic voltagesor currents, AI is the amplitude at the fundamentalfrequency (50 or 60Hz), N is normally lower than 50.It was recognized that insertion of protection reactancesin electrical plants supplying static power convertersgenerally causes reduction of voltage distortion at thesupply bus [18, 191. In the case of arc furnaces, theopposite occurs for both design criteria (as shown byTables 3, 4). Insertion of series reactances enhances arclength (due to feeding voltage adjustement), so that thecurrent T H D (TH DI) increases. Consequently, the voltageT HD (THD,) at bus 2, and then at the PCC, increases a sthe series-reactance value rises. However, only a slightincrease is detected for the constant furnace-powercriterion, which is likely the sought condition, for evidenteconomical advantages (Table 3). It is interesting toobserve that current THD does not depend on designcriterion, but only on the arc-length variation range,determined by the value of furnace voltage (the samevalues of THDi are, in fact, obtained for V2=900V inTables 3 an d 4).On the other hand, insertion of capacitors and/or shuntfilters can compensate for non-active power due to reactiveand distortion powers [7], while they do not providenoticeable conttibution to flicker reduction. Use of filtersis promoted to avoid dangerous and uncontrolledresonances that might occur when capacitor banks areused, and, in addition, to contribute to distortioncompensation [ 2 0 ] .However, even the use of filters shouldbe carefully regarded due to the almost-continuousharmonic spectra of voltage and current.

CONCLUSIONSThe description of flicker behavior in electrical plantssupplying arc furnaces by the models proposed in thispaper seems quite satisfactory. The non-linear arc modelprovides voltage and current waveforms as well as arccharacteristics which are similar to those observed inactual plantsBy means of suitable time-variation laws attributed toarc length, quite accurate evaluations have been made onthe effects of the insertion of series inductors at the supplyside of the furnace transformer, showing that the short-term flicker severity, that is, the voltage variations cause

of flicker, can be significantly reduced. Clearly, a limitvalue for inductor size is conditioned by arc stability, thatis, the range of continuous conduction.104

I1100 IB 100 200 300 400 500

f a?>Fig. 11. Harmonic analysis of the arc-furnace current at the supply-side of the furnace transformer, for white-noise law of arc-lengthtime variation. Xp=3.29 Ohm, transformer secondary voltage 900V (bandwith for the m easurement data 1 Hz).( 14


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2033Therefore, the use of series inductors, associate withcapacitors an dlor filters for non-activ e powe rcompensation, seems to be, in certain plant conditions andafter technical-economical evaluations, an alternativesolution to in stallation of static var systems.Improvements of modelling accuracy would be achievedworking on three-phase simulations, where imbalances ofthe arc-furnace operations as well as appropriate furnacecharacteristic curves can be taken into account and properfilter-effectiveness investigation realized.

Table 3. Series-reactance values ( Xp), ratios betweenseries reactance and total feeding-line reactance(xp/Xt), secondary voltages of the furnace transformer(Vz), per cent current and voltage total harmonicdistortion at bus 2 of Fig. 1 (THDI% and THqr%,respectively) for design realized at constant powerabsorbed by the system reactance-transformer-furnacewith short-circuited electrodes. Upper limit ofsummation for THD calculation N=ZO. Sinusoidal time-variation law of arc length.I1 I I I I III1 xp I XP/Xt I v2 I THDI I THDV IIII (Ohm) I 1 (kV) 1 % I % II

I IItI1 0 I O

I I I

I 0.60 I 3.99 I 1.61 1111 0.63 I 0.12 I 0.66 I 4.34 I 1.74 1111 1.12 I 0.22 I 0.72 I 4.72 I 1.85 1111 1.50 I 0.29 I 0.78 I 5.07 I 1.98 1111 1.79 I 0.35 I 0.84 I 5.41 I 2.10 1111 2.04 I 0.40 I 0.90 I 5.72 I 2.21 11I' I I I I 11Table 4. Series-reactance values (Xp), ratios betweenseries reactance and total feeding-line reactance(Xp/Xt), secondary voltages of the furnace transformer(V2), per cent current and voltage total harmonicdistor tion at bus 2 of Fig. 1 (THD I% and THDv%,respectively) for design realized at constant mean powerabsorbed by the furnace. Upper limit of summation forTHD calculation N=20. Sinusoidal time-variation law ofarc length.

i i P i xp/xt i v2 i T H D ~ i THDV iiII (") I I (kv) I % I % III I I I IIII1 0 I o I 0.60 I 3.99 I 1.61 1111 0.98 I 0.18 I 0.66 I 4.34 I 1.62 )I11 1.73 I 0.30 I 0.72 1 4.71 I 1.64 (111 2.37 1 0.41 I 0.78 I 5.05 I 1.68 11

11 2.86 I 0.46 I 0.84 I 5.39 I 1.72 1111 3.29 I 0.52 I 0.90 I 5.72 I 1.74 1I1 I I I , 11

FERENCESJ.J. K oende rink, A.J. Van Doo rn, "Visib ility ofunpredictably flickering lights", Journal uf theOptical Soc. ofAmer ica , Vol. 64, n. 11, November1974.J.J. Ko enderink , A.J. Van Doorn , "De tectability ofpower fluctuation of temporal visual noise", Vision&, Vol. 18, pp. 191-195, Pergamon Press, 1978.UIE Disturbances WG, Flicker measurements andevaluation, 1992.

[4] B. Bhargava, "Arc furnace masurements and control",IEEE Trans. on Power Del., Vol. 8, n. 1, pp. 400-409, January 1993.[5] L. Bisiach, L. Campestrini, C. Malaguti, "Technicaland operational experiences for mitigatinginterferences from high-capacity arc furnaces", Int .Con5 on, Large H igh-Voltage El. Svs., CI G RE' ,Paris, France, September 1992.[6] M. Loggini, G.C. Montanari, L. Pitti, E. Tironi, D.Zanmelli, "The effect of series inductors for flickerreduction in electric power systems supplying arcfurnaces", EE EIM Ann. Meeting, Toronto, Canada,October 1993.[7] W.S. Vilcheck, D.A. Gonzalez, "Measurements andsimulation-combined for state-of-the-art harmonicanalysis" IEEEIIAS Ann. Meeting, pp. 1530-1534,Pittsburgh, USA, October 1988.[SI A. Cavallini, G.C. Montanari, L. Pitti, D. Zaninelli,"ATP simulation for arc-furnace flickerinvestigation", to be p ublished in ETEP, 1993.[9] ATP Rule Book, Leuven EMTP Center, July 1987.[lo] S.R. Mendis, D.A., Gonzalez, "Harmonic andtransient overvoltage analvses in arc furnace powersystems", JEEE Trzns. on ' Ind. Appl., Vol. 28; n.2,pp. 336-342, April 1992.R. Sasdelli, G.C. Montanari, "The compensablepower: its definition for electrical systems innonsinusoidal conditions", IEEE Trans. on Instr. andMeas . , 1993.L. Di Stasi, Electric furnaces (in Italian), Patron ed.,Padov a, Italy, 1976."G. Manch ur, C.C. Erven, "Developm ent of a modelfor predicting flicker from electric arc furnaces",IEEE T uns. on Power Del., Vol. 7, n.1, pp. 416-426, January 1992.IEEE Publ . 519, IEEE recommended practices andreauirements for harmonic contr01 in electric vowersyitems , 99i.[15] IEC T C 33 (Secretariat) 148, Guide for a. c. harmonicfilters for industrial avvlications ,O hober 1992.[16] A.E. Emanuel, J.A.-Orr, D. Cyganski, "Review ofharmonics fundamentals and proposals for a standardterminology", 3rd ICHPS, pp. 1-7, Nashville, USA,September 1988.L. Campestrini, L. Lagostena, G. Sani, A. Bellon, R.Manara, E. Nazarri, "Flicker control in high powerarc furnaces and cumulative flicker analysis in HVnetworks" , Int. Conf on Electricin, Distribution,Liege, Belgium, April 1991.G.C. Montanari, M . Loggini, "Voltage-distortioncompensation in electrical plants supplying staticpower converters", IEEE Trans. on Ind. A&. , Vol.23, n. 1, pp. 181-188, February 1987.G.C. Montanari, M. Loggini, "Filters and protectionreactance for distortion compensation in low-voltageplants", IEEE IAS Annual Meeting, pp. 1488-1494,Pittsburgh, USA, October 1988.D.A. Gonz alez, J.C. M cCall, "D esign of filters toreduce harmonic distortion in industrial Dowersystems", IEEE Trans. on Ind. Appl. , Vol. 23,*n. 3,pp . 504-511, June 1987.


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BiograDhiesGian Carlo Montanari (M'86, SM'91)was born in Bologna, Italy, on November8, 1955. He received his Doctor'sDegree in Electrical En ineering in 1979from the University of 0 1 0 a. In 1983,he joined the University opBologna asresearcher and has become rofessor ofElectrical. technology in 19 i6 . He hasworked since 1979 in the field of agmgand endurance of solid insulatingmaterials and systems.He is also engaged in the fields of harmonic compensationin electrical power s stems, power electronics andstatistics. He is an IEE6 Senior member and member ofIEC 15B a nd IEC TC 33.Andrea Cavallini was born in Mirandola,Italy, on December 21, 1963.received his Doctor's De ree 7Electrical En ineering in 19 90 fr om theUniversity ofgBologna. At present he isPh.D. student at the Institute of IndustrialElectrotechnic o f. the University ofBologna/Italy. His interest fields arepower s stems harmonics, reliability ofelectricar systems and pow er electronics.

i (M'86) was born invrosseto, Ita-y, on August 29, 1938. Hereceived his Doctor's Degree inElectronic En ineering from theUnivers i ty of Bo k g a . In 1970, he joinedthe same University as an assistantprofessor of Electrical technolo . Atpresent, he is professor of Ingstriale!ectrical a plica tion s and wor ks in thefield of Karmonic compensation inelectrical power systems and powerelectronics. He is an IEEE member andmember of IEC TC 33.Lu i * was born in Arezzo, Ita1 , nO c ~ ~ ? l 5 , 1963. He rece iv e2 .h isDoctor's De ee in ElectricalEn 'neering in E 9 2 from the Universityof Sologna . He is present1 cooperatingwith the University of Boibgna, and isprivate consultant in electrical plantdes ign, working in Areqo, v ia NazarioSauro 32. His interest fields are powers stems harmonics, reliability ofeyectrical systems and pow er elec tronics.Dario Zaninelli (M'88) was born inRomano di Lombardia on April 3, 1959.He rec eiv ed. the. Ph.D. de ree inEJectrical En neerin at the Pofitecnicodi Milano in B 8 9 a nf b e e m e r e se ar ch erat the Electrical Engineering De artmentof the Pol itecnico d Milano on 1690. Hisareas of research include power systemharmonics and power system analysis .He is an IEEE member.


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2035Discussion

S. Bhattacharya and W. Wong (ABB Transmission TechnologyInstitute): We would like to congratulate the authors on theirpaper. The paper presents an arc furnace model. The authorsuse this model to study the effects on voltage flicker by adding aseries inductor at the supply side of a fumace transformer. Thepaper presents two different time-v ariation laws that are used bythe model. It also mentions the modulation with rectangular lawand it provides similar results. Could the authors provide moreinformation on the modulation with rectangular law and con-trast its effect on the flicker sensation with respect to thewhite-noise time variation?In the text, the authors comment that the simulation resultsare difficult to compare with the on-field measurements. How-ever in the conclusion section, the authors mention that thewaveforms and the arc characteristics obtained from the modelare similar to those observed in actual plants. Would the authorselaborate on the techniques (that overcome the comparisondifficulties) they have used to compare the simulation resultswith the on-field measured waveforms? Would the authors alsoshare a few on-field measured waveforms?Inserting a series inductor between the PCC point andMV/LV transformer reduces the voltage flicker. However, thevoltage variation at the arc furnace increases. The authorssuggest that the secondary voltage of the M V/LV transformercan be adjusted to compensate for the voltage variation. Weforesee that without an on-load tap-changer the voltage regula-tion will be po or. For instan ce, if the secondary tap is set for thefull-load operatio n, it will create overvoltage durin g the light-loadcondition. Do the authors assume that an on-load tap-changer isemployed to adjust the secondary voltage? If so, would theycomment on the freque ncy of operatio n of the tap -changer andthe resulting wear-tear?To obtain an optimum performance and economy, an arcfurnace requires a stable and steady voltage supply. Highersteady voltage provides shorter meltdown times, reduces energycost and extends electr ode life. Many installations effectively usestatic var compensator (SVC) to reduce flicker, to improvepower factor and arc furnac e efficiencies. The authors comm enton the effect of the series inductor concept on the power factorcompensation and the arc furnace efficiencies will be appreci-ated.

Manuscript received February 24, 1994.

G . C. Montanari, M. Loggini, A. Cavallini, L. Pitti, and D.Zaninelli: We thank the discussers for their congratulations andthe stimulating questions. Before answering to each point raisedby the discussers, we would like to point out that the proposedarc furnace m odel is a step forward in the investigation of flickerand distortion compensation in electrical plants supplying arcfurnaces. The authors are well aware that the model does notallow a perfect simulation of furna ce behavior during the wholemelting cycle, but the co mpariso n of previous models and exper-imental results lead to consider the proposed model a satisfac-tory compromise between the need to approach the problem ofpower qua1ity.h plants supplying arc furnaces and the difficultyto know the actual furnace working conditions at any time. Forthis reason, the model resorts to eqns. (6)-(9) where the evidentsimplification of steady power absorbed by the furnace (beyondthe voltage fluctuations responsible by flicker) is made. More-over, simulations are made resorting to the maximum rangeallowable by the condition of continuous conduction for arclength. However, the implemented model is flexible and, com-patible with TACS size, more complex laws for furnace power

and voltage variations can be considered, depending on theamount of data available for the studied plant.But let us answer in detail each qu estion of the discussers.1-Both sinuso idal and rec tangular laws for arc-length modula-tion are determin istic laws, and shou ld be regard ed as worst-caseapprox imations for flicker estimation. The d ifference in PsTvalues obtained by the two aws mainly depend s on the behaviorof demo dulation and weighting filters of f lickermete r [3]. In fact,on the basis of [3]the following, approxim ate expressions canbededucted , which relate the equivalen t voltage variation, DVe,/Vto the flicker sensation S(t):

S(t) =(100(DVeq/V)/0.25)2and

S(t) =(lOO(DV,,/V)/O.20)*for sinusoidal and rectangular laws, respectively. Hence, theperceptivity limit (S=1) is given by DV,/V =0.3 and 0.2 forsinusoidal and rectangular laws, respectively. Moreov er, eq. (12)holds for both laws, since in both cases the probability distribu-tion of th e signal S(t) is stepwise. In conclusion, the rectangularmod ulation provides slightly higher values of S(t) and PsT thanthe sinusoidal one, so that both laws can be considered worstcases with respect to the white-noise time variation law, whichshould more closely simulate the actual arc-length variations.2- As mention ed in the paper, quantitative com parisons of theproposed model with on-field measurements cannot be easilymade, due to the single-phase simulation and the difficulties toknow in any tim e the exact fumac e working conditions. This ledto the use of a sto chastic mod el based on white-noise modula-tion. By this way, values of PsT close to the average measuredduring the starting melting periods of an arc furnace wereobtained. Clearly, accuracy can increase resorting to three-phasesimulations (which is the last achievement of the research) andto more accurate reproduction of the actual working conditionsof the arc furnace. As regards the voltage and current wave-forms, in steady conditions the model can well reproduce thetypical waveforms of an elec tric arc, often displayed in literatur e.Considering a plant which supplies an arc furnace, there are somany differen t op eratin g conditions, involving randow laws, tha tan accu rate reproduction of actual waveforms can be seldomobtained by simulation based on white-noise law. However, theproposed model provides voltage and current waveforms whichresemb le those observed in plants during periods of ar c opera-tion, as it is also confirmed by the values of THD.As anexample, Figs. C1 and C2 report the voltage and current wave-forms measured and sim ulated at the bus 2 of th e plant, whenthe series reactance (X,/X, =0.3) is inserted. S imulation con-siders white-noise law with time variation close to tha t d etectedby the measure. Amounts of plots of active and non-activepower could also be provided (see Fig. 6of the paper), but theirapparent fitting the simulations does not give anymore contribu-tion to prove the model validity.3- Plants where the solution of series inductors for flickerreduction is employed are not uncommon in North-Italy. On thebasis of on-field me asurem ents, the ob servations of the discusserseem app ropria te. The capability of secondary-voltage regulationof the MV/LV transformer limits the size of the series inductor,since insertion of th e seri es indu ctor forces to increase thesecondary voltage (if the furnace active power must be keptconstant),but reduces consequently, he tap-regulation range.Both simulation and experimental data show that increasing thearc voltage (i.e., lengthening the arc), arc stability decrease, butthis can be properly taken into account by electrode control.Hence, th e freque ncy of secondary-voltage adjustments shouldnot significantly vary with respect to operation without series


11/11

2036inductor. Indeed, it must be clarified that the series inductor iskept steadily inserted in the plant, so that the increase of therate of commutation of tap changer is related only to theoperating conditions of the furnace. In the pape r, this .topic wasnot dealt with in depth, since the purpose was to show how theseries inductor works in th e plant.4- nsertion of series inductor affects power factor, PF , due tothe contrasting effects of increased line inductance (that lowersPF ) and changed average working point of the furnace (th e samefurnace power is obtained by lower I/I, values and, hence,higher power factor, as shown by Fig. 3of the paper). On thewhole, power factor variations due to series-inductor insertionare relatively small, so that connection of filter or capacitorbanks is needed. Three-phase simulations would be appropriateto investigate the influence of filters on THD, PF and PsT. Th elast development of the authors research, leading to three-phaseplant simulation, show that filters reduce voltage THD (in theabsence of significant resonance amplification, i.e., for well-de-signed filters), compensate for PF, but do not improve PST.Fo rexample, PsT vanes from 0.85 to 0.88 after filter insertion (tun edto 3rd harmonic), for a plant with xp/x , =0.4, while voltageTHD varies from 1.78 to 0.83. In general, it can be argued thatthe insertion of filters and series reactance can constitute aninteresting solution, alternative an d cheape r than SVC, in plantswhere problems of PST and THD are not too dramatic, that is,the standard limits for these quantities are exceeded for alimited amount.

-21 -- - -1 o(yu no1 -00;s - - a i 2 - - o m --- nL3Time (sec)Fig. C1. Voltage and current waveforms measured at the PCC of the plant(B =voltage,A =current).

/ / i

-*I- ~,. ~ . .L -~.. - ~In RIW o.ni R O IS uoz UIIZ u03Ti m (sec)

Fig. C2. Voltage and current waveforms obtained by simulation with white-noise law at the PCC of the plant (B =voltage,A =current).Manuscript received April 11,1994.

arc furnace model for the study of flicker

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