the effects of series inductors for flicker reduction

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The Effects of Series Inductors for Flicker Reductionin Electric Power Systems Supplying Arc FurnacesGian C arlo M ontanari, Mauro Loggini, Luca Pitti

Universith degli Studi di BolognaIstituto di Elettrotecnica Industriale

Viale Risorgimento 2 40136BOLOGNA, Italy

Enrico Tironi, Dario ZaninelliPolitecnico di Milano

Dipartimento di ElettrotecnicaPiazza Leonard0 da Vinci 32 20133Milano, Italy

Abstract - The purpose of this paper is to assess the effectof a series inductor on the reduction of voltage flicker inelectric power systems supplying arc furnaces. The ar cfurnace is simulated as a voltage generator with variableamplitude during the melting process. The design of theinductor hinges on the fact that, when connected into theelectric power system, it should not interfere with theproduction capacity of the plant itself. The use of apowerful simulation program (EMTP)makes it possible tostudy a large number of cases and, therefore, todetermine the appropriate size of the series inductor forthe purpose of reducing voltage flicker. The simultaneouspresence of the series inductor with capacitors andlorfilters, with the purpose to limit harmonic pollution andimprove the power factor, is also discussed.

1. INTRODUCTIONThe arc furnace is a highly disturbing load in electricpower systems. It generates harmonic and inter-harmonic currents and voltages in supply networks, andgives rise to flicker effects for other end-usersconnected to the same feeder.While shunt filters can reduce harmon ic pollution inthe network, voltage flicker is normally offset byvariable capacitor support, i.e., the static varcompensator (SVC). This solution is, however, quiteexpensive, especially for low-power plants.This paper proposes to investigate a means ofcompensation of flicker co nsisting in the connection ofseries inductors.A single-phase electric power system supplying anarc furnace is simulated by the EMTP program [ I ] , inorder to examine the different situations related to theactual furnace operation. The electric power systemconsidered for EMT P simulation is shown in Fig. 1.

It consists of a 60 MVA arc furnace supplied, via a

95 MVA HV/MV transformer, from a transmissionnetwork with symmetrical short-circuit power of 3500MVA (data taken from a typical actual plant). In thefigure, X, is the network short-circuit reactance; RT,,XT, and R X are equivalent transformerresistance and reactance respectively, p is the seriesreactance inserted for the purpose of flickercompensation.The transformers' main technical data aresummarized in Tab.1. Transformer T2 has a variabletransforming ratio, so that rated voltage on thesecondary winding may vary between 600 and 900 V (itis assumed that the percentage of transformer short-circuit reactance remains cons tant with variations in thetransforming ratio).The impedance Zc = R, + jX, in Fig. 1 representsthe connections between transformer T2 and theelectrodes of the arc furnace. Typically, in furnaces ofthis pow er, the impedance Z, varies between 3 andm Q with a ratio XJR, between and 10 [2], [3], [41).In our example, we have: R, = 0 3 mfl, X, = 3 mfl.

2. THE ARC FURNACE MODELThe model of a single-phase furnace, or of one phaseof a three-phase furnace, consists in determining theone-port that, on certain assumptions, simulates the

2-cBUS1 T l BUS2 T2

95 MVA 60 MVA220/21 kV 2110.6 .9 kVARCFURNACE

Fig. I . EMTP schematic diagram of t he electric power systemsupplying the arc furnace.14960-7803-1462-x/93$03.0001993IEEE


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TABLE I.MAINECHNICAL DATA OF TRANSFORMERS T1 AND T2,PRESENT IN THE DIAGRAM IN FIG. 1 XT S THE SHORT-CIRCUIT REACTA NCE AND P, ARE THE SHORT-CIRCUITPOWER LOSSES)

T2 2 U 0 . 6 i 0 . 9 0.5

I 1 1 I I

t 4behaviour of the electric arc. Such a model must, in 0 0.002 0.004 0 006 0 008 0.01 0.012 0.014 0.016 0.018 0.02addition, be com patible with its representation by meansOf the program, which is the One Often Fig. 2. Waveform ,fan: fumct v o j ~ g e .used internationally for simulating electric powersystems.In this paper, the arc furnace is represented by acontrolled-voltage generator set. TABLE I1AMPLITUDE ND PHASE OF THE HARMONIC-VOLTAGE

GENERATORS, USE D TO REPRESENT THE ARC VOLTAGE INeasurements On arc furnaces showed that thevoltage on the electrodes (referred to ground) has atrapezoidal waveform with a small peak correspondingto passage of the curre nt through zero [2], s shown inFig. 2. vat indicates the voltage of the arccorresponding to the highest current values; thisdepends only on the length of the arc and on thecharacteristics of the gas in which the arc burns, andnot on the value of the current absorbed by the furnace[SI. he waveform of Fig. 2ntroduces a simplification,because, under real furnace operating conditions, theamplitude of the two half-periods might be differen t.In the EMTP program, the waveform of Fig. 2 canbe realized by a set of sinusoidal voltage generatorswhose amplitude and pha se are shown in Tab. 11, withreference to an arc length I corresponding to which

The model of the arc furnace must be able toreproduce the conditions that cause flicker. We assumethat the variations in arc length that occu r in the furnaceduring melting are the main causes of flicker {6].How ever, flicker may also occur due to changes in thephysical and chemical characteristics of the gases in themelting area.A variation in the arc voltage corresponds to theselength variations in accordance with the followingequation [ 5 ] :where A and B are constant A is the sum of thevoltage drop on the electrodes and B is the averag e

vat = 100 v.

vat = A BI 1)

FIG. 2harmonicnumber

135791 11315

amplitudeM

126.5038.9817.756.1572.3124.3833.9251.909

phase-87.89-86.75-92.70-1 12.8135.886.2965.2642.00

value of the voltage drop per arc length, usually = 40V and = 10V/cm, respectively) and I is the a rc length.As regards the instantaneous voltage at the electrodesva, it may be described by the equation:

where:A+BZA +BZ,A = k(l/l,J = (3)

The am plitude of the vo ltage generators representingarc voltage based on = I (see Tab. 11) is modulated1497


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by the coefficient k, to take into account the variationsin length l(t) of the arc during the melting process.It is assumed that this variation in arc length l(t) issinusoidal and periodic, with a frequency typically inthe range S i 5 Hz (e.g. 10Hz, close to the m aximumsensitivity of human eye to luminous fluctuations) . Th isis the cond ition that emp hasizes the flicker phenomenonand is, in any case, useful for ascertain ing the steps tobe taken for flicker com pensation. Thus we have:l ( t) = 1 - T 11 +sinPt) (4)

L

where AI is the maximum arc length variation (in thefollowing simulations Q = 20 T rad/s has been taken).Hence (3) becomes:B A 12- 1 +s inRt)k = k ( l ( t ) / l J = 1 - A BZ,

It should be mentioned that this model does not refer tothe arc itself (which would involve defining parametersA and B and more detailed research on the physics ofthe electric arc), but to the voltage on the arc furnaceelectrodes. What we propose is therefore only a modelable to take into account the power absorbed by thefurnace and its fluctuations, with the purpose ofstudying its effects on the electric power systemsupplying the furnace.

3 FLICKER MEASUREMENT DURINGSIMULATIONAs described in [7] and [8], with each voltagefluctuation measured by the flickermeter, whatever itstrend and amplitude, we can associate an "equivalen t"sinusoidal fluctuation at frequency 8.8 Hz Thisequivalent voltage fluctuation (AV,) has an amplitudesuch that it causes the same disturbance, and thereforethe same response from the flickermeter. One cantherefore use, as an indicator of the level of voltageflicker, the param eter AV,/V, that is, the equiva lentvoltage variation evaluated at 8.8 Hz, referring to theRMS voltage. This index is usually expressed as apercentage, and the value of 0.25% corresponds to theflicker perceptibility threshold . The UIE flickemeter hasbeen implemented by EMTP in order to derive thevalues of AV,/V for any modulation frequency of thevoltage generators used for arc furnace modelling. In

the electric power system considered (see Fig. l),AV /V will be calculated at the network feeding busB U ~ I ) , hat is, at the point of common coupling(PCC).

4. THE EFFECTS OF THE SERIES INDUCTORON VOLTAGE FLICKERLet us consider the case in which the series inductoris not present in the single-phase electric power systempresented in Fig. 1 (that is, = 0). It is assumed that,in these conditions, furnace transformer T2 stransforming ratio is equal to 21/0.6 V. In this case,the power and current absorbed by the transformer-furnace system are the rating ones for vat = 220 V. Inorder to simulate the worst flicker conditions, vat is

varied in the range:4OV aat240 Vwhere 240 V corresponds to the continuous conductionlimit and 40 V corresponds to an operating point veryclose to a short-circuit (the sum of the vo ltage drop onthe electrodes and that in the melting bath is equal toabout 40 V). The variation law applied to vat istherefore:vat = 240 - 100 1 + sin a).In these conditions, at BUS1 of Fig. 1 the flickereffect provides:A VJV = 0.75%.

When an inductor is inserted in series to the MV inesupplying the furnace (reactance 5 in Fig. l), thefurnace power - and therefore its production capacity -must not be reduced. Hence a variation in transformerT2 s transforming ratio is needed. In the plant con-cerned in this research, transformer T2 has a secondaryvoltage that may vary between 6 and 900 V in 60-Vsteps.The following two criteria are examined for insertionof the inductor in the power system:1. Constant short-circuit powe r, S,, of the reactance-transformer-furnace system.2. Constant energy absorbed by the arc furnace duringthe melting process.The use of a powerful simulation program enables alarge number of cases to be studied, thus making itpossible to determine the a ppropr iate size (in relation tothe size of the system) of the series inductor that canlimit voltage flick er.

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The values of the series reactance inserted in theplant and the relevant values of the secondary voltageof transformer 2, in accordance with the firstcriterion, ar e given in Tab . 111. Tab. IV shows thevalues of AV,N at BUSl (the point of commoncoupling) obtained from simulations with EMTP for thedifferent series reactance values reported in Tab. 111.Tab. IV also explains the ratio 4 between the seriesreactance and the total reactance of the power system(evaluated at the same voltage levels).According to the second criterion, the values of theseries reactance inserted in the plant, and the relevantvalues of the secondary voltages are shown in Tab. V.Tab. VI gives the consequent AV N values, measuredat BUSl and resulting from the E h imulations.The voltage flicker index AV,N calculated as afunction of the ratio X /X,s shown in Fig.s 3 and 4,with respect to the af6rementioned criteria 1 and 2,respectively.As can be seen from Tab.s IV and VI, and fromFig.s 3 and 4, insertion of the series inductor into thearc-furnace supply system greatly reduces the voltageflicker present at the point of common coupling.Moreov er, the following comments can be draw n.- The first criterion of keeping constant the short-circuit power Sf f the series reactance-transformer-furnace system shows that it is possible to obtain aflicker reduction even when the short-circuit ratio(defined as SCR = S J S where S, is the short-circuit power of the transmission network at thefeeder bus - BUSl in Fig. 1) remains constant. Thesecond criterion, on the other hand, leaves theenergy absorbed by the furnace in the meltingprocess unchanged, and this is also the criterionused in [3] to study the effect of the SVCs.- The two design criteria are compared in Fig. 5 ,which shows the points corresponding to the sam esecondary-voltage value of the transformer (ai andbi, with i = 1,2, ...6 respectively). It is emphasizedthat the second criterion, i.e. constant energyabsorbed by the furnace, allows more significantflicker reduction.The influence of the short-circuit power S of thetransmission network supp lying the system is shown

in Fig. 6, where AVq/V is given as a function ofthe ratio Xp/X, for different values of the short-circuit ratio (SCR). The three-dimensional Fig. 7shows the variations of AV,N both with SCR andfit ig. 7 indicates that, given the same

-

/xt, flicker is reduced as SCR increases,

TABLE I11S E E OF SERIES REACTANCE ACCORDING TO THE FIRSTCRITERION.

Series reactancea)Series reactancea)0.6351.1251SO51.7952.045

0.6351.1251SO51.7952.045

Secondary nominalvoltage0.660.720.780.840.90

Secondary nominalvoltage0.660.720.780.840.90

TABLE IVAV,/V, AT THE POINT OF COMM ON COUPLING (BUS]) ,WlTH SIZE OF THE SERIES REACTANCE ACCORDING TOTHE FIRST CRITERION. X IS THE RATIO OF TH E SERIESREACTANCE TO THE TOTAL R EACTANCE OF THE POWERSYSTEM EVALUTE D AT THE SAME VOLTAGE LEVELS).

Seriesreactance0.752

0. 24 0.6810.6170.6351.125 0.2201SO5 0.294 0.5671.795 0.351 0.5252.045 0.399 0.480

TABLE VSIZE OF SERIES REACTANCE ACCORDING TO THE SECONDCRITERION.

Series reactanceQ)

0.9791.7252.3712.8553.285

Secondary norminalvoltage0.660.720.780.840.90

as is known From technical liter ature on the subject.Insertion of the series inductor might limit the range1499


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TABLE VIAV,/V AT TH E POINT OF COM MON COUPLING (BUSI),WlTH SIZE OF THE SERIES REACTANCE ACCORDING TOTHE SECOND CRITERION. X IS THE RATIO OF SERIESR E A C T A N C E T O T H E T O T A L R E A C T A N C E O F T H E P O W E RSYSTEM EVALUATED AT THE SAME VOLTAGE LEVELS).

0.1790.3020.4140.462

of variation of the transformer's secondary voltage, thusaffecting arc furnace power regulation. However,regulation of power in the arc furnace at high currentsis mainly carried out varying the position of theelectrodes.

0.6400.5620 4690.416

Series reactancefJ)00.9791.7252.3712.8553.285

0 450 0 0 5 0 1 0 1 5 0 .2 025 0 3 035 0 4XP/Xt

Fig. 3. Effect of the series reactance on voltage flickerreduction with constant short-circuit power Sf of thereactance-transformer-fumace ystem. The short-circuitratio at BUSl is 58.

0 8

Fig. 4. Effect of the series reactance X,, on voltage flickerreduction with constant energy absorbed by the arc furnace.The short-circuit ratio at B USl is 58 for Xp = 0

5. EFFECT OF INSERTION OF SERIESINDUCTORS AND FILTERS ON HARMONICDISTORTIONThe insertion of series inductors into the electricpower systems supplying arc furnaces likely interactswith furnace harmonic generation, which consists of- 1 I

0 0 1 0 2 0 3 0.4 0 .5 0 6

0 4 -

0.35

XP/XtFig. 5 . Comparison between the equivalent voltage variations at8.8 Hz AV IV ) as a function of X,,lX, with the twocriteria for inserting X,,: a) keeping constant the fumaceshort-circuit power SF; b) keeping constant the energy ab-

sorbed by the furnace. The tw o curves show the points cor-responding to the same transforming ratio of the fumacetransfonner (a l , bl , e tc .) .

s

1 . 1 1

0 90 8

5 0 7: 0 6

0 5

0 4

3

Fig. 6. Effect of series reactance X,, on voltage flicker reductionfor different short-circuit ratio values (SCR).1500


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Fig. 7. Equivalent voltage variation nt 8 .8 Hz (AV ' ) as afunction of the SCR and the ratio XplX,.charac teristic and non-characteristic harmonics, besidesnon-multiple harm onics (or inter-harmonics [9]). Inter-harmonics ar e caused by the arc length time-variationintroduced in order to reproduce the conditions that giverise to flicker, as can be mathematically deduced from(2) and 5) . As an example, Fig. 8 shows the spectralanalysis of the line current and voltage at BUS2 (seeFig. 1) in the presence of a series inductor with =2.045 Q and secondary transformer voltage equal to900 V (the frequency step of the bar representation ofFig. 8 is 10 Hz).In these conditions, definition of distortion factormight be a debatable topic. According to IEEE 519,current and voltage total harmonic distortion arecalculated as [lo]:

where h is an integer, Ah is the amplitude o f . heharmonic of order h, A , is the amplitude of thefundamental harm onic and N is usually lower than 50.Therefo re, only multiple harmonics are considered. Inthe case of arc furnaces producing flicker, bothfundamental and mu ltiple harm onics are modulated seeFig. 8), so that several inter-harmonics are present,with an almost continuous harmonic spectrum.However, distortion factor is intended to evaluate thedeviation from sinusoidality of a single period of the

1 0b

E

f WFig. 8. Spectral analysis of the line current a) and of the voltageb) at BUS2 n Fig. 1 with = 2.045 ll and secondary

transformer voltage q u a l to 900 V frequency step of thebar representation ie 10 Hz).reference waveform (usually that at power supplyfrequency), while inter-harmonics are connected withvoltage amplitude variation and, hence, flicker.Therefo re, eq. (6) seems still appropr iate for d istortionestimate, since inter-harmonics are involved in flickermeasurements.Tab.s VI1 and VI11 give the percentage values of theTHD (defined by eq (6) with N = 25) in respect of thecurrent absorbed by the fu rnac e and of the voltage inBUS2, for different values of series reactance insertedat the supply side of the furn ace transformer (Fig. 1).It can be noted that the current distortion rises as theseries reactance value increases. In fact, increasing thefurnace transform er's transforming ratio in accordancewith the criteria previously given, we are working withever longer arcs that amplify the non-linearity of the

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load. It is precisely the range of variations in arc lengthconnected with transformer secondary voltage thatdetermines the degree of current distortion. This issupported comparing Tab.s VI1 and VIII, where we seethat, having fixed the secondary voltage, the currentTHD no longer depends o n the design criterion used.The voltage distortion at BUS2, and then at the point ofcommon coupling, remains almost con stant as the valueof series reactance rises, for the design criterion ofconstant energy absorbed by the furnace, w hile slightlyincreases for the other criterion (Tab. V II).Summarizing, insertion of series inductors, togetherwith capacitors or filters, can bring the electric plant tocomply with standard limits on both flick:er anddistortion factor. Filters can conveniently replacecapacitors for power factor correction and distortioncompensation. Indeed, the use of filters, even in electricplants with relatively small voltage distortion, has theaim to correct power factor avoiding the dangerousresonance situations which can easily occour whencompensation is made by capacitor banks. For thispurpose, filters can be tuned to low-order harmonics,e.g. with tuning frequency lower than 150 Hr . Theireffectiveness for distortion compensation can not beproperly evaluated by single-phase simulations, due tothe different kinds of harm onic orders and com ponentsinvolved.Referring to the plant till now considered, with= 2.045 il and secondary voltage of the furnace

TABLE VI1TOTALDISTORTION ACCORDING TO EQ. 6) (OF THECURRENT ABSORBED BY TH E FURNACE AND OF TH EVOLTAGE IN B U S , WITH SERIES REACTANCE DESIGNEDTO KEEP CONSTANT THE SHORT-CIRCUIT POWER OF TH EREACTANCE-TRANSFORMER-FURNACE SYSTEM.

0.6351.125

1.795

Secondarynominal voltage

orv)0.60.660.720.780.840.9

CURRENTTHD96)

4.484.795.055.265.455.61

VOLTAGET H D( 'I1.932.062.1.52.2 42.332.38

TABLE VI11TOTALDISTORTION ACCORDING TO EQ. 6) OF THECURRENT ABSORBED BY THE FURNACE AND OF THEVOLTAGE IN BUS2, WITH SERIES REACTANCE DESIGNEDTO KEEP CONSTANT THE ENERGY ABSORBED BY THEFURNACE.

00.9791.7252.3712.8553.285

Secondarynominal voltagekv)

0.60.660.720.780.840.9

CURRENTT H D96)

4.484.795.045.265.455.61

VOLTAGETHD96)

1.931.911.901.891.881.88

transformer 900 V, a filter tuned to the third-harmonic(i.e. R F = 0.125 il L F = 4.99 mH, CF = 226 pF,[12], [13], [14]) is able to bring the power factor tovalues higher than 0.95. It is interesting to observe thatthe filter does not give significant contribution to flickerreduction, since AV,/V is 0.48 in the absence offilter and 0.50 in the presence of filter.

6. CONCLUSIONSThe investigations on the effects of series inductor

insertion into plants supp lying furnaces, with the objectof reducing voltage flicker, has been realized by meansof EMTP simulation. The arc furnace has beendescribed by a voltage generator with a waveformsimilar to that typically measured at the terminals of anarc furnace and an amplitude that varies periodicallyduring the production process.Insertion in the electric power system of a seriesinductor has to be accompanied by variation in thesecondary voltage of the furnace transformer, in ord ernot to affect the production capacity of the furnace.Design of the series inductor was calculated inaccordance with two different criteria: that of keepingconstant the short-circuit power of the inductor-transformer-furnace system, or that of keeping constantthe energy absorbed by the furnace. With both criteria,a remarkable reduction in flicker du e to the presence ofthe series inductor has been noted. Therefore, thisseems to represent an economical, technically-simple

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means of dealing with the problem of voltage flicker inelectric power systems supplying arc furnaces, whichmight be alternative, or used in addition, to the static-var compensator.Moreover, insertion of series inductors is able toreduce the voltage distortion at the MV side of the HV-MV ransformer and, the refore, at the point of comm oncoupling. Hence, combination of se ries inductances(eventually designed into the transformer) andcapacitors and/or filters can provide power factorcorrection and distortion compensation, besides flicker1 mitation.- define more accurate models for the simulation ofarc furnaces;- extend the study to three-phase circuits.

Further research on this subject should seek to:

REFERENCESATP:Alternative Transient Program Rule B o o k ,Leuven EMTP Center, Leuven (Belgium), July1987.W.S. ilcheck, D.A. Gonzales, "Measurementsand simulations combined for state-of-the-artharmonic analysis", Industry Applications SocietyAnnual Meeting, Pittsburgh (PA), USA, October1988.L. C ampestrini, L. L agostena, G. Sani, A. Bellon,R. Manara, E. Nazarri, "Flicker control in high

power arc furnaces and cumulative flicker analysisin HV networks", Proc. of 11th Internationalconference on Electricity Distribution, Likge(Belgium) 22-26 April, 1991.S.R. Mendis, D.A. Gonzales, "Harmonic andtransient overvoltage analysis in arc furnace powersystems", IEEE Transactions on IndustryApplications, vol. 28, N. 2, MarchiApril 1992.

L. Di Stasi, Electric Furnaces (in Italian),Patron Ed., Bologna (Italy) 1976.L. Bisiach, L. Campestrini, C. Malaguti,"Technical and operational experiences formitigating interferences form high-capacity arcfurnaces", International Conference on Large HighVoltage Electric Systems (CIGR k), Par is (France)September 1992.UIE Disturbances Study Committee, VIEFlickermeter, inctiona l anddesign specif cations ,Bulletin UIE, 1983.IEC Pu blication 868, Flickermeter, inctional anddesign specifcations , 1986.E. Tironi, D. Zaninelli, " Interharmonics inelectrical plants" (in Italian), L'Energia Elettrica,vol. 64, N.3, March 1987.

[101 IEEE 5 19, IEEE Guide for h a m n i c control andreactive power compensation of static powerconverter , IEEE, 198 1 .[1 11 B. Bhargava, "Arc furnace flicker measurementsand control", IEEE Transactions on PowerDelivery, vol. 8 , N. January 1993.[12] J. Arrillaga, D.A. Bradley, P.S.Bodger, Powersystem harmonics ,John W iley Sons, 1985.[131 G.C. Montanari, M. Loggini, "Voltagedistortioncompensation in electrical plants supplying staticpower converters", IEEE Transactionson IndustryApplications, vol. 23, N . 1 January/February1987.[14) C.K. Duffey, R.P. Stratford, "Update of harmonic

standard IEEE-5 19: IEEE recommendedrequirements for harmonic control in electricpower systems", IEEE Transactwns on IndustryApplications, vol. 25, N. , Novembermecember1989.

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