Series Active Compensation of Current Harmonics Generated by High Power Rectifiers

Transcription

1 Europea Associatio for the Developmet of Reewale Eergies, Eviromet ad Power Quality (EA4EPQ) Iteratioal oferece o Reewale Eergies ad Power Quality (IREPQ ) Graada (Spai), 3rd to 5th March, Series Active ompesatio of urret Harmoics Geerated y High Power Rectifiers D. Figueroa, L. Morá, G. Joos ad J. Dixo 3 Departmet of Electrical Egieerig Uiversidad de ocepció asilla 6-, orreo 3 - ocepció (hile) Phoe/Fax umer: / , dfigueroa@udec.cl, lmora@udec.cl Departmet of Electrical ad omputer Egieerig McGill Uiversity 348 Uiversity Street, H3AA7, Motreal, Quéec (aada) Phoe/Fax umer: , geza.joos@mcgill.ca 3 Departmet of Electrical Egieerig Potificia Uiversidad atólica de hile icuña Mackea 486, omua Macul, Satiago (hile) Phoe/Fax umer: / , jdixo@ig.puc.cl Astract. This paper presets a series active filter to compesate curret harmoics geerated y high power rectifiers. The advatage of the proposed scheme is that is ale to compesate curret distortio y ijectig a distorted voltage i oly two lies, avoidig resoaces ad reducig compesator s rated capacity. The paper presets the series active filter i terms of priciples of operatio, desig procedure, ad compesatio performace. Simulated results applied to a electro-wiig rectifier demostrate the viaility of the proposed scheme. The proposed desig procedure ad compesatio performace are demostrated experimetally o a ka laoratory prototype. Key words Active power filter, power quality, harmoic compoets, high power rectifiers.. Itroductio Differet compesatio techiques ased i active ad passive filters to elimiate curret harmoics ad to compesate reactive power have ee preseted ad pulished [,,3]. Shut, series ad hyrid active filter topologies have ee discussed ad demostrated to e a effective solutio for o-liear load compesatio. Although active filters are a good alterative for harmoic compesatio, their applicatios i high power medium voltage are still limited due to power semicoductor ratigs, higher cost ad reduced reliaility. So far, with traditioal series active compesatio it has ot ee possile to compesate curret harmoics uless they are used with passive filters coected i parallel [4]. To reduce the effects of o-liear loads the covetioal solutio has ee the utilizatio of secod-order passive filters, sice they ca reduce curret harmoics ad improve the load s power factor. Nevertheless, practical implemetatio rigs alog several well kow disadvatages, such as: Ifluece of the utility impedace i the filterig performace. Sice the harmoic curret ad the fudametal frequecy curret flow through the filter, the ratig of power compoets is high. If the harmoic cotet i the ac lie voltage icreases, the filter is overloaded. The risk of resoaces etwee the filter ad the utility impedaces ca amplify curret harmoics to a dagerous level. Irush currets are high ad ca activate overcurret protectios. I order to overcome these prolems, active power filters have ee proposed ad aalyzed i the techical literature []-[4]. The classical shut active power filter operates as a cotrolled curret source while the series active approach acts as cotrolled voltage source. For these reasos, shut active power filters are more suitale to compesate curret harmoics ad load reactive power, while the series approach compesates for voltage distortio, ualaces, ad regulatio. The pricipal 55 RE&PQJ, ol., No.8, April

2 disadvatages of active compesatio techiques oth shut ad series are the followigs: Higher cost. More difficult ad costly to implemet o a large scale ad at higher powers. For medium voltage applicatio, oth the iverter topology ad its cotrol scheme preset a high degree of complexity. The other well kow topology is the hyrid filter [7], which employs a comiatio of a active ad a passive filter, ad is a good alterative for medium voltage applicatios; however, the compesatio effectiveess depeds o the passive filter tued frequecy. i a electro-wiig applicatio are show. I additio, a ka experimetal prototype was implemeted o a DSP digital cotrol, ad used to compesate a six pulse thyristor phasecotrolled rectifier. Simulated ad experimetal waveforms cofirm the feasiility, verify the desig procedure ad aalysis preseted i this paper.. Priciple of Operatio urret harmoics are compesated y ijectig i series the same voltage harmoics as appear at the rectifier termials, ut phase-shifted i 8, as show i Fig.. v S ( t) v F ( t) v ( t) is ( t) L This paper presets a curret harmoic compesatio techique ased o two series active power filter, as show i Fig.. a c a c ca a _ load ca _ load c _ load Load Fig.. Sigle-phase equivalet circuit ad associated voltage ad curret waveforms of the proposed series active filter coected to a six pulse thyristor cotrolled rectifier (α= ). urret Referece Geerator oltage Referece urret otrol Uit D oltage otrol Uit Gatig Sigal Geerator (ostat Switchig Frequecy) Fig.. Power circuit topology of the proposed series active filter icludig the cotrol scheme lock diagram. The pricipal advatage of the proposed filter scheme is that requires oly two sigle-phase PWM voltage-source iverters that share the same dc lik capacitor, ad are coected i series with the power distriutio system through couplig trasformers. Assumig that the load is ot grouded, oly two iverters ca e used to compesate a three-phase load. I order to geerate siusoidal ac system currets, the voltage at the load termials must e distorted. It is importat to ote, that y chagig the voltage waveform, especially for rectifiers type of loads, the rectifier operatio ad performace is ot affected. Other operatig characteristics iclude the followig: I figure, v F (t) represets the voltage waveform ijected y the series active power filter ad v L (t) the voltage at the load termials. The voltage ijected y the series active power filter is equal to the load voltage mius the fudametal compoet, phase-shifted y 8. Figure 3 shows the sigle phase equivalet circuit for harmoic compoets ad illustrates that curret harmoic compesatio is achieved if F-k is equal to Load-k. If these two voltages are equal, they cacel each other, forcig I k to e equal to zero. urret harmoic compesatio is effective if the rectifier ehaves as voltage source (with a capacitor coected i the dc us). I case a reactor is coected to the rectifier dc us, it ehaves as a curret source ad the compesatio effectiveess remais the same, ut the rectifier dc voltage presets a larger distortio factor. j k X s I k F _ k load _ k Simple cotrol scheme; oly oe siusoidal sigal is required as a cotrol referece. The curret compesatio is doe i time domai, resultig i a fast dyamic respose. Load power factor ca e corrected y forcig the load voltage to e i phase with the lie curret. The power circuit topology reduces the filter compoets part cout, icreasig reliaility ad reducig the total cost. The aalysis preseted i this paper demostrates the feasiility of the proposed compesatio techique ad its advatages i the compesatio of large power rectifiers coected to a dedicated power distriutio us. The paper descries the priciples of operatio, cotrol circuit desig ad implemetatio. Simulated steady state ad trasiet results for the compesatio of a MA phase cotrolled rectifier used Fig. 3. Sigle-phase equivalet circuit of the series active filter for harmoic compoet (X S system equivalet impedace, F_k series active filter voltage, load_k rectifier iput voltage harmoic compoet, k order of the harmoic). 3. ompesatio Performace The compesatio performace of the proposed series active filter scheme depeds o the load characteristics, ad is etter if the rectifier ehaves as a voltage source. Static rectifiers ehave as voltage-sources if they have a dc capacitor coected at the dc us. I case they have a iductive load, rectifiers ehave as curret sources. I case of curret sources types of loads, the proposed series 56 RE&PQJ, ol., No.8, April

3 active power filter scheme also compesates curret harmoics, ut the dc voltage is more distorted. Simulated results otaied for oth cases are show i figures 4 ad 5. Figure 4 shows simulated curret ad voltage waveforms of a pulse voltage-source rectifier usig the proposed series active power filter. Figure 4-a shows the compesated rectifier ac iput curret waveform (THD = 4.8 %) demostratig the compesatio effectiveess of the proposed method. Figure 4- shows the rectifier dc voltage with a distortio factor equals to 4.5 %. The low distortio factor of the dc voltage idicates that the proposed filter scheme does ot affect the rectifier operatig coditio. urret [pu] (a) () Mag(% of Fudametal) - (c) Harmoic order (d) Fig. 4. Simulated waveforms of a pulse voltage-source rectifier with the proposed series active power filter scheme ( MA, 3.8 k). (a) Simulated ac iput curret waveform (THD = 4.8 %). () Simulated dc us voltage (DF = 4.5 %). (c) oltage ijected y the series active power filter ad associated frequecy spectrum (THD = 6.7 %). (d) Simulated lie to lie voltage at the rectifier termials. urret [pu] (a) Mag(% of Fudametal).5.5 () (c) Harmoic order (d) Fig. 5. Simulated waveforms of a pulse curret-source rectifier with the proposed series active power filter scheme ( MA, 3.8 k). (a) Simulated ac iput curret waveform (THD = 4,5 %). () Simulated dc us voltage (DF = 5,3 %). (c) oltage ijected y the series active power filter ad associated frequecy spectrum. (THD = 8, %) (d) Simulated lie to lie voltage at the rectifier termials. Simulated results illustrate the compesatio performace of the proposed filter scheme. Power factor ca also e compesated y forcig the ac source voltage to e i phase with the respective lie curret. Sice the proposed series scheme ijects voltages i oly two phases the reactive power compesatio rage is limited. I case of thyristor-cotrolled rectifiers, power factor ca e kept equal to uity dow to a phase-shift agle of Filter Desig Procedure The series active power filter is composed of two sigle phase full ridge iverters, a commo dc capacitor, a LL output filter ad two couplig trasformers. The cotroller has two cotrol loops, oe to keep the dc voltage costat, ad the secod oe to geerate the 57 RE&PQJ, ol., No.8, April

4 iverter output voltage required to compesate lie curret harmoics. A. D apacitor. The dc capacitor value is otaied usig (). With this value, the capacitor is ale to store eough eergy to supply the istataeous power required y the ac system durig half a cycle without resultig i dc voltage variatios of more tha 5 %. D sm I T Lm () ( ) Dref sm is the ac system phase to eutral peak value, I Lm is the rms load curret, T is the ac system period, Dref is the dc voltage referece value ad D is the miimum voltage allow i the dc us. B. LL output filter. The output filter elimiates the ripple voltage geerated y the sigle phase PWM full ridge iverter. Its desig is ased o the maximum high frequecy voltage harmoic amplitude allowed at the couplig trasformer termials. The filter must ot atteuate the low frequecy voltage harmoics required to elimiate the load curret harmoic. F is equal to 5 %, with respect to the ase impedace, ad ca e calculated usig (). S F = x ω E F S ω E D = x ω Z.5 =.5 () Where S ad E are the rated load apparet power ad rated phase to eutral voltage. Tale I. LL Passive Filter alues. Real alue p.u. value L,4 mh,3 L F,4 mh,3 F 8 uf,893 D uf,67 The desig procedure for the curret ad voltage loops is ased o the respective time respose requiremets. The trasiet respose of the active power filter is determied y the curret cotrol loop (iteral loop). It must e fast eough to force the curret to follow the referece waveform as closely as possile []. The respose time of the dc voltage cotrol loop does ot eed to e fast, due to the slow dyamics of the electrolytic capacitor, ad is selected to e at least times slower tha that of the curret cotrol loop []. I this way, oth cotrol loops ca e decoupled ad e desiged as two idepedet systems. 5. Experimetal Results A ka laoratory prototype usig IGBT switches was implemeted ad tested for compesatig a six pulse phase-cotrolled rectifier. Each iverter was operated at a 3.5 khz switchig frequecy. The cotrol strategy was implemeted i a DSP TMS34x. Steady state ad trasiet results are show i Figs. 6, 7 ad 8. Steady state results demostrate the feasiility of the proposed compesatio scheme. Figure 8 shows the curret ad voltage waveforms of the six pulse rectifier without compesatio. Figure 8 shows steady state waveforms with the compesatio scheme. The THD of the system curret is reduced from 7.4 % to 6.9 % y applyig the compesatio method. Filter iductaces L F ad L atteuate curret harmoics geerated y the PWM iverter. Total filter iductace is otaied from (3). D LT = LF + L 8 i f rp _ max The ratio etwee L F ad L is defied y a costat r, ad depeds o the high frequecy curret harmoic that flows through the couplig trasformer (4) [8], [9]. i i O iv ( ω ) = ( ω ) + r ( ( L ω ) x) Tale I shows the parameter values of the proposed LL filter i pu ad asolute values, for a pulse rectifier rated at MA ad 3.8 k. (3) (4) Fig. 6. Rectifier iput curret ad voltage experimetal waveforms without compesatio. (a) Rectifier iput lie to eutral voltage a ( /div). () Rectifier iput curret I (5 A/div). The rated ka of the series active filter is % of the load rated power. I this case, the series active power filter compesates for curret harmoics ad reactive power, up to a.87 load power factor RE&PQJ, ol., No.8, April

5 Refereces Fig. 7. Steady state rectifier iput curret ad iput voltage experimetal waveforms with compesatio. (a) Rectifier iput lie to lie voltage a ( /div). () Rectifier iput curret I (5 A/div). Fig. 8. Experimetal trasiet results. Rectifier iput lie to lie voltage ad iput lie waveforms with compesatio. (a) Rectifier iput lie to lie voltage a ( /div). () Rectifier iput curret I (5 A/div). 6. oclusios A series active compesatio scheme to elimiate curret harmoics geerated y high power static rectifiers was preseted ad aalyzed. The proposed active compesatio techique is ased i a series active filter that employs two sigle-phase iverters with a simple cotrol scheme. Experimetal results otaied o a laoratory DSP ased cotrolled prototype were preseted, validatig the simulated waveforms ad cofirmig the feasiility of the proposed compesatio techique. [] Hideaki Fujita, Takahiro Yamasaki, ad Hirofumi Akagi, A Hyrid Active Filter for Dampig of Harmoic Resoace i Idustrial Power Systems, IEEE Tras. o Power Electroics, vol. 5, No., March. [] Rué Izuza ad Hirofumi Akagi, A 6.6-k Trasformerless Shut Hyrid Active Filter for Istallatio o a Power Distriutio System, IEEE Tras. o Power Electroics, vol., No. 4, July 5. [3] Zhiguo Pa, Fag Z. Peg, ad Suili Wag, Power Factor orrectio Usig a Series Active Filter, IEEE Tras. o Power Electroic, vol., No., Jauary 5. [4] Araham D. le Roux, Hedrik du T. Mouto, ad Hirofumi Akagi, Digital otrol of a Itegrated Series Active Filter ad Diode Rectifier with oltage regulatio, IEEE Tras. o Id. Appl., vol. 39, N 6, Nov./Dec. 3. [5] Luis A. Morá, Jua W. Dixo, Rogel R. Wallace, A Three-Phase Active Power Filter Operatig with Fixed Switchig Frequecy for Reactive Power ad urret Harmoic ompesatio, IEEE Tras. o Id. Electro, vol. 4, N 4, August 995. [6] ijay B. Bhavaraju ad Prasad N. Ejeti, A Active lie oditioer to Balace oltages i a Three-Phase System, IEEE Tras. o Id. Appl., vol. 3, N, March./April 996. [7] Sagsu Kim ad Prasad N. Ejeti, A New Hyrid Active Power Filter (APF) Topology, IEEE Trasactios o Power Electroics, vol. 7, o., Jauary. [8] M. Liserre, F. Blaajerg ad S. Hase, Desig ad otrol of a LL-filter ased Three phase Active Rectifier Idustry Applicatios oferece,. Thirty-Sixth IAS Aual Meetig. oferece Record of the IEEE. ol., pp Septemer/Octoer. [9] Y. Lag, D. Xu ad S. Hogfei Ma, A Novel Desig of LL Type Utility Iterface for Three- Phase oltage Source Rectifier Power Electroics Specialists oferece, 5. PES 5. IEEE 36th. pp Jue 5. [] D. Figueroa, L. Morá, P. Rumiot, J. Dixo, A series Active Power Filter for urret Harmoic ompesatio, i IEEE PES of. Record, Jue 8, Greece. Ackowledgemets The authors would like to ackowledge the fiacial support otaied from FONDEYT through the 837 project RE&PQJ, ol., No.8, April