Power Quality Impacts of an Electric Arc Furnace and Its Compensation

Transcription

1 Journal of Electrcal Engneerng & Technology, Vol. 1, No. 2, pp. 153~160, Power Qualty Impacts of an Electrc Arc Furnace and Its Compensaton Ahmad Esfandar, Mostafa Parnan and Hossen Mokhtar Abstract - Ths paper presents a new compensatng system, whch conssts of a shunt actve flter and passve components for mtgatng voltage and current dsturbances arsng from an Electrc Arc Furnace (EAF). A novel control strategy s presented for the shunt actve flter. An extended method based on nstantaneous power theory n a rotatng reference frame s developed for extracton of compensatng sgnals. Snce voltages at the pont of common couplng contan low frequency nterharmoncs, conventonal methods cannot be used for dc voltage regulaton. Therefore, a new method s ntroduced for ths purpose. The passve components lmt the fast varatons of load currents and mtgate voltage notchng at the Pont of Common Couplng (PCC). A three-phase electrc arc furnace model s used to show power qualty mprovement through reactve power and harmonc compensaton by a shunt actve flter usng the proposed control method. The system performance s nvestgated by smulaton, whch shows mprovement n power qualty ndces such as flcker severty ndex. Keywords: electrc arc furnace, flcker severty, power qualty, shunt actve flter, voltage flcker 1. Introducton Applcaton of nonlnear loads such as adjustable speed drves, electrc arc furnaces and power converson devces n power systems results n power qualty problems such as harmoncs, nterharmoncs and voltage fluctuatons that force the utltes and consumers to take countermeasure actons. Due to the varety of nonlnear loads and ther problems, dfferent compensaton systems have been used. Passve flters are conventonal solutons to mtgate harmoncs. The lmtaton of passve flters for compensatng complex problems such as nonnteger harmoncs and flcker has made actve flters attractve. Control strategy s the heart of an actve flter whch substantally affects compensaton characterstcs. Sgnal condtonng, compensatng sgnals extracton and command sgnals generaton for the nverter swtches are mportant stages of the control strategy. Frequency and tme doman methods can be used for the detecton of compensatng sgnals. Selecton of the method depends on load characterstcs and compensaton objectves. Shunt actve flters usng tradtonal control methods have successfully been used to compensate for basc power qualty problems such as current harmoncs, reactve power and load mbalance [1],[2]. Some loads such as electrc arc furnaces draw erratc varyng currents whch result n severe power qualty problems. Reactve power of electrc arc furnaces has hghly varyng feature that requres a fast detecton method. Interharmonc components and Dept. of Electrcal Engneerng, Sharf Unversty of Technology, Iran (shesf2000@yahoo.com, parnan@sharf.edu, mokhtar@sharf.edu ) Receved June 17, 2005 ; Accepted February 17, 2006 nonstatonary characterstc of current sgnals make frequency-doman methods n the detecton of reference sgnals neffcent. The reason s that these methods are based on sgnal transformaton and extracton of harmonc components. Another dsadvantage of frequency-doman methods s the transfer of energy of sgnal to sde-lobes due to wndowng. In tme-doman methods, whch are based on sgnal flterng and manpulaton, compensatng sgnals have averaged or nstantaneous forms. Electrc arc furnace compensaton s usually performed by usng passve flters, seres nductor [3], Statc Var Compensators (SVCs) [4]-[7], and Dstrbuton STATc COMpensators (DSTATCOMs) [8]. Passve flters cannot be used for compensatng problems such as varable frequency harmoncs and flcker. Seres nductors may cause reducton of short crcut power and decrease productvty [3]. Although SVCs are effectve n mtgatng flcker and VAR control, but ther performance s lmted due to ther nherent delays [9]. Moreover, they nject large amount of current harmoncs, whch need to be fltered. DSTATCOMs compensate only for reactve power at the fundamental frequency. The nstantaneous reactve power theory based on rotatng reference frame s presented n [10]-[ 14] for threephase four-wre systems for reactve power compensaton and neutral current elmnaton. In ths paper, an extended method s used for extractng the compensatng sgnals to suppress the harmoncs and to correct the power factor. Also, a new dc voltage control loop s presented that can be used n the presence of nonnteger hormoncs and flckery voltages at the pont of actve flter connecton. The proposed actve flter can mprove power qualty of hghly

2 154 Power Qualty Impacts of an Electrc Arc Furnace and Its Compensaton varyng loads n three-phase three-wre power systems wth unbalanced voltages. 2. System Confguraton Fg. 1 shows the proposed compensatng system. It conssts of shunt actve flter and passve components. The shunt actve flter compensates for the load current dsturbances and regulates the dc lnk voltage. The seres nductor and shunt capactor form a passve compensatng devce. These components lmt fast varatons of load currents and mtgate voltage notchng at the PCC. nstantaneous current vector on the α qο coordnates s obtaned by applyng the rotatng transformaton. Then, pqr coordnates s obtaned by rotatng α ο plane by θ 2 about q-axs so that α -axs algns wth the projecton of nstantaneous voltage space vector on α ο plane. The nstantaneous current vector on the pqr coordnates can be obtaned by applyng the rotatng transformaton. The overall transformaton matrx, T, from abc to pqr coordnates s obtaned by multplyng the above three transformaton matrces. More detals can be found n [11],[15]. 3.1 Compensatng Sgnal Detecton T 1 s c V l L se As shown n Fg. 1, at the PCC, n the abc frame one can wrte: C sh T 3 shunt actve flter T 2 electrc arc furnace sa ca la sb + cb = lb sc cc lc (1) Fg. 1 Basc scheme of a system wth an actve flter 3. Control Strategy Instantaneous voltages and currents on the abc coordnates can be transformed nto the quadrature αβο coordnates. Next, as shown n Fg. 2, α qο coordnates set s formed by rotatng αβ plane by θ 1 about ο -axs so that α -axs algns wth the projecton of nstantaneous voltage space vector on αβ plane. Therefore, the r axs v α α axs v α v ο θ 1 ο axs θ 2 v αβ v p axs v β q axs α axs β axs Fg. 2 Relaton between αβο and pqr reference frames Multplyng (1) by the transformaton matrx T [15], new current sgnals n the pqr coordnates are: sp cp lp sq + cq = lq sr cr lr Snce the voltage vector s algned wth p-axs, the fundamental frequency component of phase voltages n the pqr frame s a DC value on the p-axs. Other components supermposed on v p are seen as fluctuatng values. Instantaneous actve current of the load current,. e. lp, ncludes a DC value and an oscllatory component. The latter component s an oscllatory actve power whch should be compensated to acheve a constant actve power,. e.; where ~ lp and lp lp lp lp (2) ~ = + (3) are the DC and fluctuatng components of lp, respectvely. ~ lp can be extracted by passng lp through a low pass flter and subtractng the output from lp [15]. The cutoff frequency of the flter should be low enough to block all dsturbance components. On the other hand, a very low bandwdth slows down the dynamc response of

3 Ahmad Esfandar, Mostafa Parnan and Hossen Mokhtar 155 the devce. The EAF load currents may be expressed as: where ma causng flcker and a ( t) = ma ( t)[ fa + ha ( t)] (4) b ( t) = mb ( t)[ fb + hb ( t)] (5) c ( t) = mc ( t)[ fc + hc ( t)] (6), mb and mc are ampltude modulatng factors f and h are fundamental and harmoncs of currents, respectvely. Due to the low frequency of the flcker term, low pass flterng of EAF currents faces the abovementoned drawback, and deterorates the actve flter performance. An alternatve method to remedy ths problem s shown n the upper part of Fg. 3. Here, peak values of postve sgnals a, b and c are detected and used to defne the ampltude of reference currents. Ths value s gven as: k = ( am + bm + cm ) / 3 (7) where m = max( ), for =a,b and c. Three current templates are obtaned by multplyng and the outputs of a sne-wave generator. Then, the resultant vector t s tranformed to the pqr rotatng frame. Thus, tp would be equvalent to lp, and subtractng t from lp results n load actve current dsturbance. snθ s v dc t cosθs v dc lp lq lr tp ~ cp cq cr ca cb cc Fg. 3 Controller block dagram ca, cb, cc Smlarly, nstantaneous reactve current lq has two components. Compensatng for the DC component of the reactve power yelds power factor correcton, whereas compensatng ts oscllatory component together wth the oscllatory actve current leads to harmonc elmnaton and k load balancng. Therefore, an deal compensaton requres reference current space vector n pqr coordnates be chosen as follows: ~ cp = lp (8) cq = lq (9) cr = lr = 0 (10) Fnally, compensatng sgnals n abc frame are obtaned as: ca cp 1 cb = T cq cc cr (11) A hysteress current controller determnes the swtchng pattern of the nverter devces to acheve the requred compensatng current. A control block dagram of the shunt actve flter s shown n Fg DC Bus Voltage Control Output voltage of the actve flter s generated such that reactve power of the source s reduced. Ths causes a flow of nstantaneous power nto the nverter whch charges/dscharges the nverter dc bus capactor. Despte the resultant dc bus voltage fluctuatons, ts average value remans constant n a lossless actve flter. However, the converter losses and actve power exchange causes ths voltage to vary. Regulaton of ths voltage, whch s essental for proper operaton of the actve flter, requres balancng actve power exchange at the fundamental frequency. The nverter s controlled to generate a fundamental frequency current sgnal n phase wth the fundamental frequency voltage at the actve flter termnals to regulate the dc bus voltage. When the actve flter termnal voltages and load currents are polluted wth nonnteger low frequency harmoncs, the current vector component whch s n-phase wth the voltage vector n the rotatng reference frame can not be drectly used to establsh dc voltage control loop. That s because actve power exchange occurs at a frequency range ncludng nonnteger low frequency terms, and low pass flterng can not be used as explaned n [16]. In order to overcome ths problem, the dfference of measured dc voltage ( v dc ) and ts reference value ( v ) s fed to a PI controller. The output of the PI controller s dc

4 156 Power Qualty Impacts of an Electrc Arc Furnace and Its Compensaton consdered as d-axs compensatng current component n the synchronous reference frame dqo. Ths sgnal s transformed to phase currents. Then, the output s used to modfy compensatng sgnals. The developed control method s shown n Fg Electrc Arc Furnace Model In order to smulate the proposed method, a three-phase electrc arc furnace model s employed whch s composed of two man parts. The frst part s based on a dynamc, mult-valued v- characterstc of an electrc arc whch s obtaned by usng a general dynamc arc model n the form of a dfferental equaton. The second part of the model conssts of Chua s chaotc crcut whch represents the arc voltage fluctuatons. Detaled nformaton about the model parameters can be found n [17],[18]. Ths model whch s developed n a sngle-phase form n [17], s extended to a three phase model, and used for the smulatons. 5. The IEC Flckermeter A flckermeter [19], [20] was smulated to evaluate the flcker mtgaton of the compensatng system. The flckermeter s shown by the block dagram of Fg. 4. The frst block dynamcally scales the mans rms voltage down to an nternal reference level on the bass of a one-mnute average. Then block 2 recovers the voltage fluctuatons by squarng the scaled nput voltage to smulate a lamp behavor. The thrd block elmnates the DC component and double mans frequency component of the squarng demodulator output. Ths block s composed of a frst order hgh-pass flter and a sxth order Butterworth low-pass flter. As suggested n [19], the hgh pass flter has a 0.05 Hz cut-off frequency and the low pass flter has a 35 Hz cut-off frequency. The fourth block weghts voltage fluctuatons accordng to the lamp-eye-bran senstvty. The weghtng flter transfer functon s specfed n Appendx A. Block 5 squares the weghted flcker sgnal to smulate the nonlnear eye-bran percepton. Then, block 6 takes a sldng mean of the sgnal to smulate the storage effect n the bran. Ths functon s mplemented by a frst order low-pass flter wth a tme constant equal to 300 ms,. e., a cut-off frequency of 0.53 Hz. The output of block 6 represents Instantaneous Flcker Level (IFL). In the followng secton, IFL wll be used to evaluate how the performance of the compensaton system. The statstcal analyss block conssts of a classfer that uses 64 classes. The block analyzes the IFL, usng tme-at-level statstcs over a fxed tme nterval of 10 mn to yeld the short-term Flcker Severty ndex, P st [19],[21]. The ndex formula s gven n Appendx B. 6. Smulaton Results The system n Fg. 1 was smulated usng Matlab/ Smulnk wth the parameters lsted n Tables 1 and 2. The PI controller parameters are chosen as k p = and k = Table 1 Parameters of the smulated system Parameter Value Short crcut power, MVA 3500 Shunt actve flter nductance, mh Shunt actve flter resstance, Ohm 0.01 Seres nductance, mh 10 DC bus capactance, mf 14 DC bus voltage, kv 12 Shunt capactance, µ F 420 Table 2 Parameters of the transformers Transformer T 1 T 2 T 3 V 1 (kv)/v 2 (kv) 220/21 21/ /21 MVA Resstance, pu Inductance, pu Power Qualty Problems In addton to voltage and current harmonc polluton, an EAF generates other power qualty problems such as mbalance, voltage flcker and voltage notchng whch s nput sgnal nput voltage adapter demodulatng wth squarng multpler db -3 0 Hz Hz 1 squarng multpler sldng mean flter nstantaneous flcker level Fg. 4 Block dagram of the IEC flckermeter statstcal evaluaton flcker severty P st

5 Ahmad Esfandar, Mostafa Parnan and Hossen Mokhtar 157 caused by arc phenomenon. The latter effect s a hgh frequency dsturbance and makes desgnng of dc voltage control loop complcated. Fgs. 5 and 6 show the voltage dsturbances. Fg. 5 Three-phase PCC voltages after passve compensaton 6.2 Compensaton Results At frst, voltage notchng s compensated by a seres nductor and a shunt capactor as ndcated n Fg. 6. Inserson of a seres nductor solely, may cause reducton of short crcut power at the pont of common couplng whch n turn decreases productvty. Whereas the seres nductor together wth an actve flter can be used to mtgate flcker and to mprove power factor. Moreover, t s used to lmt the hghly varyng EAF currents and mprovng actve flter performance as shown n Fg. 7. Fgs. 8 and 9 show three phase load and source currents, respectvely. FFT of the source current sgnal befor and after compensaton s presented n Table 3. Comparson of harmonc content n load and source currents ndcate that n addton to nteger harmoncs, nonnteger harmoncs are consderably suppressed. Fg. 6 Shunt capactor mpact on PCC voltage, phase-a Smlar problems are observed n the load currents that are depcted n Fgs. 7 and 8. Fast Fourer Transform (FFT) of the load current has a contnuous dstrbuton of harmonc content. Ampltudes of some of the notceable harmonc components are mentoned n Table 3. Fg. 9 Source currents The actve flter current contans the varyng part of the load current as depcted n Fg. 10. Load and source nstantaneous reactve powers are shown n Fgs. 11 and 12, respectvely. It s observed that the system s capable of compensatng for hghly varyng reactve power. Fg. 7 Seres nductor effect on load current, phase-a Fg. 10 Shunt actve flter current, phase-a Fg. 8 Three phase load currents The three-phase PCC voltages n Fg. 13 verfy reducton of the voltage flcker and mbalance. Frequency Table 3 Harmonc content of source current H A A H: Harmonc, A1:Harmonc ampltude before compensaton, A2: Harmonc ampltude after compensaton

6 158 Power Qualty Impacts of an Electrc Arc Furnace and Its Compensaton spectrum analyss of the PCC voltage before and after compensaton shows the value of 10 Hz component (a nonnteger harmonc) befor and after compensaton s %0.14 and %0.06, respectvely. one can see that the proposed voltage control loop offers effcent performance under unbalanced and flckery PCC voltages. Fg. 11 Load nstantaneous reactve power Fg. 15 DC bus capactor voltage 7. Concluson Fg. 12 Source nstantaneous reactve power Fg. 13 Three phase PCC voltages after compensaton IFL of the voltage sgnal before and after compensaton s depcted n Fg. 14. The short-term flcker severty ndex, P st s calculated as descrbed n secton 5. Ths ndex s changed from to due to the compensator operaton. Maxmum permssble value of P level s 1 [19]. st In ths paper, a combnaton of a shunt actve flter and passve components s proposed for mprovng power qualty of a system supplyng an EAF. The compensator mtgates PCC voltage and load current dsturbances, and compensates for reactve power, harmoncs, nterharmoncs, and mbalance. In the proposed scheme, current compensatng sgnals are detected based on generatng phase current templates and mappng the current vector onto a rotatng reference frame formed by the nstantaneous voltages. Alos, snce voltages at the pont of common couplng contan low frequency nterharmoncs, conventonal methods wll not be effcent for dc voltage regulaton. Therefore, a new method s ntroduced for ths purpose. Smulaton results on a three-phase EAF system model are presented to verfy the control strategy and to assess the performance of the compensatng system. 8.1 Appendx A 8. Apendces Transfer functon of the weghtng flter s gven as: wth ( 1+ s / ω2 ) ( 1+ s / ω )( 1 s / ω ) kω1s F w ( s) 2 2 s + 2λs + ω1 3 + = (12) 4 k = , λ = 2π , ω 1 = 2π , ω 2 = 2π , ω 3 = 2π , ω 4 = 2π Fg. 14 The IFL of the voltage sgnal The other part of the control strategy s the dc voltage regulaton. Based on the capactor voltage shown n Fg. 15, 8.2 Appendx B Accordng to IEC specfcatons [19],[21], flcker severty ndex s caculated from:

7 Ahmad Esfandar, Mostafa Parnan and Hossen Mokhtar 159 Pst = ( P P1 s P3 s 1/ P 10s P50s ) + (13) Here the percentles P 0. 1, P 1 s, P 3 s, P 10 s, P 50 s are the flcker levels exceeded for 0.1%, 1%, 3%, 10%, and 50% of the tme durng the observaton perod. The suffx s n the formula ndcates that the smoothed values should be used. These values are obtaned usng the followng equatons: ( P30 + P50 80 )/ 3 ( P6 + P8 + P10 + P13 17 )/ 5 ( P2.2 + P3 4 )/ 3 ( P + P )/ 3 P50 s + P P10 s + P P3 s + P P1 s P1.5 = (14) = (15) = (16) = (17) References [1] W. Dxon, J.J. Garca, L. Moran, Control System for Three-Phase Actve Power Flter whch Smultaneously Compensates Power Factor and Unbalanced Loads, IEEE Trans. Ind. electron., vol. 42, pp , Dec [2] B. Sngh, K. Al-Haddad, A. Chandra, A New Control Approach to Three-Phase Actve Flter for Harmoncs and Reactve Power Compensaton, IEEE Trans. Power Syst., vol. 13, pp , Feb [3] G.C. Montanar, M. Loggn, L. Ptt, E. Tron, and D. Zannell, The Effects of Seres Inductors for Flcker Reducton n Electrc Power Systems Supplyng Arc Furnaces, n Proc. IEEE IAS Annual Meetng, pp , [4] L. Gyugy and A.A. Otto, Statc Shunt Compensaton for Voltage Flcker Reducton and Power Factor Correcton, n Proc. Amercan Power Conf., pp , [5] I. Hosono, M. Yano, M. Takeda, S. Yuya, and S. Sueda, Suppresson and Measurement of Arc Furnace Flcker wth a Large Statc Var Compensator, IEEE Trans. Power App. Syst., vol. PAS-98, pp , Nov./Dec [6] A. Wolf and M. Thamodharan, Reactve Power Reducton n Three-Phase Electrc Arc Furnace, IEEE Trans. Ind. Electron., vol. 47, pp , Aug [7] C. Surapong, C.Y. Yu, D. Thukaram, D. Npon, and K. Damrong, Mnmzaton of the Effects of Harmoncs and Voltage Dp Caused by Electrc Arc Furnace, n Proc. IEEE PES Wnter Meetng, pp , [8] J.R. Clouston and J.H. Gurney, Feld Demonstraton of a Dstrbuton Statc Compensator Used to Mtgate Voltage Flcker, n Proc. IEEE PES Wnter Meetng, pp , [9] L. Gyugy, Dynamc Compensaton of AC Transmsson Lnes by Sold-State Synchronous Voltage Sources, IEEE Trans. Power Delv, vol. 9, pp , Apr [10] H. Km, H. Akag, The Instantaneous Power Theory Based on Mappng Matrces n Three- Phase Four- Wre Systems, n Proc. Power Converson Conf., pp , [11] H. Km, H. Akag, The Instantaneous Power Theory on the Rotatng p-q-r Reference Frames, n Proc. IEEE Int. Conf. on Power Electroncs and Drve Systems, pp , [12] F. Z. Peng, G. W. Ott, D. J. Adams, Harmonc and Reactve Power Compensaton Based on the Generalzed Instantaneous Reactve Power Theory for Three-Phase Four-Wre Systems, IEEE Trans. Power Electron., vol. 13, pp , Nov [13] F. Z. Peng, J. La, Generalzed Instantaneous Reactve Power Theory for Three-Phase Power Systems, IEEE Trans. Instrument. and Meas., vol. 45, pp , Feb [14] H. Akag, S. Ogasawara, and H. Km, The Theory of Instantaneous Power n There-Phase Four-Wre Systems: A Comprehensve Approach, n Proc. IEEE IAS Annual Meetng, pp , [15] A. Esfandar, M. Parnan, and H. Mokhtar, A New Control Strategy of Shunt Actve Flters for Power Qualty Improvement of Hghly and Randomly Varyng Loads n Proc. Int. Symposum on Industral Electroncs, pp , [16] A. Esfandar, M. Parnan, H. Mokhtar, Shunt Actve Flter Control based on Instantaneous Power Theory on a Rotatng Reference Frame n 3-Phase System, presented at the 11th Int. Power Electroncs and Moton Control Conf., Rga, Latva, [17] O. Ozgun, A. Abur, Flcker Study Usng a Novel Arc Furnace Model, IEEE Trans. PowerDelv., vol. 17, pp , Oct [18] E. O Nell-Carrllo, G.T. Heydt, E.J. Kostelch, S.S. Venkata, and A. Sundaran, Nonlnear Determnstc Modelng of Hghly Varyng Loads, IEEE Trans. Power Delv., vol. 14, pp , Apr [19] Internatonal Electrotechncal Commsson, IEC , Testng and measurement technques Secton 15: Flckermeter Functonal and desgn specfcatons, [20] A. Hernández, J.G. Mayordomo, R. Asens, and L.F. Betes, A New Frequency Doman Approach for Flcker Evaluaton of Arc Furnaces, IEEE Trans. Power Delv., vol. 18, pp , Apr

8 160 Power Qualty Impacts of an Electrc Arc Furnace and Its Compensaton [21] Internatonal Electrotechncal Commsson, IEC 868, Flckermeter- Part 0: Evaluaton of Flcker Severty, [22] T. Keppler, N. Watson, and J. Arrllaga, Computaton of the Short-Term Flcker Severty Index, IEEE Trans. Power Delv., vol. 15, pp , Oct Ahmad Esfandar He was born n Sarband, Iran n He receved the B.Sc. and M.Sc. degrees n electrcal engneerng from Sharf Unversty of Technology, Tehran, Iran, n 1996 and 1998, respectvely. Hs nterests nclude applcatons of power electroncs, and power qualty. Hossen Mokhtar He was born n Tehran, Iran. He receved ths B.Sc. degree n electrcal engneerng from Tehran Unversty, Tehran, Iran n He worked as a consultant engneer for Electrc Power Research Center (EPRC) n Tehran n dspatchng projects. In 1994, He receved hs M.A.Sc. degree from Unversty of New Brunswck, Fredercton, N.B., Canada. He obtaned hs Ph.D. degree n electrcal engneerng from the Unversty of Toronto n He s currently an assocate professor n the Electrcal Engneerng Department of Sharf Unversty of Technology. Hs research nterests ncludes power qualty and power electroncs. Mostafa Parnan He obtaned hs B.Sc. and M.Sc. degrees n Electrcal Power Engneerng from Tehran Polytechnc and Sharf Unversty of Technology (SUT), n 1987 and 1990 respectvely; and Ph.D. n Electrcal Engneerng from the Unversty of Toronto n Snce then, he has been wth the Electrcal Engneerng Dept. of SUT as an assstant professor. He s currently a vstng scholar at Rensselaer Polytechnc Insttute, USA. In the past, he has worked wth Ghods Nroo Consultng Engneers Co., Electrc Power Research Center, and Nroo Research Insttute. He has also been a member of IEEE Task Force on Slow Transents, as well as several natonal commttees n hs feld. Hs areas of nterest are power system control and dynamcs, reactve power control, and applcatons of power electroncs n power systems.