Three-phase Active Power Filter for Power Quality: Compensation and Balancing

Transcription

1 1 Three-phse Active Power Filter for Power Qulity: Compenstion nd Blncing C. G. Binchin, Electronic System Division, LACTEC, R. Demonti, Electronic System Division, LACTEC nd J. S. Omori, Prnense Electic Compny, COPEL Abstrct-- Present pper dels with the implementtion of prllel three-phse ctive power filter used to compenste current hrmonics, to correct the imblnce between voltge nd current generted for cpcitive or inductive lods nd, the implementtion of the control strtegy, to crry through the blncing of currents for three phses, being minimized the current of neutrl. The justifictions for the choice of this topology nd the strtegy of control re presented dopted beyond the mthemticl nlysis, procedures of clcultion, results of simultion nd lbortory time tht, s finl stge of the developing, ws constructed prototype with 10 kw. II. CONVERTER TOPOLOGY The inverter used in this project is voltge fed inverter. Considering the possible topologies for the voltge inverter meet the inverter of three legs nd the inverter of four legs. Fig. 1 illustrtes s the second type. Index Terms Hrmonic distortion, ctive filter, voltge control, inverters, digitl control. A I. INTRODUCTION ctive filter technology consist of mesuring hrmonic currents of one or more phses of the power grid nd generte ll hrmonic currents components in opposition of phse in reltion to those mesures. Active power filters lredy re used with suess for consumers nd suppliers of electric energy in mny countries. In [1] is shown bout use of incorported ctive power filters, for the proper mnufcturers, the equipment tht trditionlly origintes gret hrmonic currents. The ctive power filters cn be prllel or series [, 3, 4]. Filters series re voltge restorers, hve the function to eliminte distortions in the voltges of the electricl system. Prllel ctive filters re current restorers, hve the function to eliminte the hrmonic currents components. The hybrid filters still exist, tht combine the two functions [, 3]. This work dels with solely the prllel filters, which operte s currents sources in derivtion with the electricl system. Its function is to inject in the point of connection of the electricl system with the lod (PAC), currents of compenstion cpble to ttenute or to eliminte hrmonic currents originted by not liner lods. In the described configurtion in this pper the prllel ctive power filter is cpble to inject current in the PAC or to remove current for lod of the cpcitive bnk, since the structure is bidirectionl. This work ws supported by Prnense Electric Compny COPEL, which we would like thnk. Crlos Gbriel Binchin (binchin@lctec.org.br) nd Rogers Demonti (rogers@lctec.org.br) re with LACTEC Instituto de Tecnologi pr o Desenvolvimento. Julio Shigeki Omori is with COPEL (milto:julio.omori@copel.com). Fig. 1. Voltge inverter with four legs In [5] cn be found lot of techniques of modultion for these structures. Inverter of three legs is pproprite for blnced three-phse systems, where the voltges supplied for the inverter nd currents demnded for the lod re blnced. Is possible to use this topology in systems unblnced through the ddition of neutrl conductor to the center of the continuous voltge side ( link in Fig. 1), however this cn cuse problems of control nd stbility, time tht is necessry to crry through the blncing of the voltges over the two cpcitors in link. Moreover, the neutrl currents re drined or injected directly in the cpcitors, wht it requires high vlues of cpcitnces. Another disdvntge of this converter, when used with the neutrl conductor, is the poor use of the voltge in link [6]. The inverters with four legs re not so known, however comes being pointed s the topology most pproprite with respect to pplictions in unblnced systems [7]. The existence of n exclusive leg for the neutrl conductor becomes possible the independent control of the current of neutrl supplied by the inverter. Moreover, the inverter does not require the ddition of conductor to the link (continuous voltge), wht it mens tht the cpcitor will not be connected directly to the neutrl conductor. This becomes possible to reduce the vlue of the cpcitnce nd still it excuses the necessity of controller to crry through the blncing of the voltges. A system of current control with this inverter is more robust of wht its competitor with three legs.

2 A. Control Strtegy Gret prt of technicl literture bout ctive filters dels with solely its ppliction in blnced three-phse systems. However, the number of electric instlltions tht hve neutrl conductor with current phses unblnced is incresing, which re minly cused by the significnt singlephse lod presence [1]. The filter is connected to the electric instlltion through coupling inductors, for which they circulte currents synthesized for the current controllers, chrcterizing the ctive filter s controlled current source. The strtegy to produce the correct control is composed twopiece bsic: system of identifiction of the references nd system of current control (Fig. ). Fig. : Strtegy of control for ctive filter. The function of the references identifiction system is to determine the compenstion currents tht must be injected by the ctive filter in the electricl system. When the ctive filter is used in systems unblnced with neutrl, the compenstor hs still the function to determine the compenstion currents tht become possible the blncing of the phses nd the elimintion of the neutrl current. To get these objectives the strtegy of compenstion bsed on the sinusoidl currents synthesis will be used, blnced nd in phse with the bsic source voltges. This will gurntee current in the PAC with low distortion hrmonic nd high power fctor, beyond minimizing the neutrl current. The synthesis of currents of the filter is crried through with using of proportionl nd integrl controllers (PI) tht they ct in set with modultor of pulse width. Fig. 3 shows the simplified block digrm of system of current control. Rigorously ech one of the proportionl nd integrl regultors receives the nme from controller, but the nme of current controller will be given to the complete system, simplifying the dopted nomenclture. Fig. 3. Current controller of the ctive filter. In the block digrm of Fig. 3 four controllers PI exist which generte references currents for pulse width modultor. This modultor genertes the pulses for inverter firing used for the synthesis of current compenstion. The voltges signls re generted by controllers PI bsing on the errors between mesured nd reference currents. These signls re pplied the switch of the inverter, in this in cse tht, IGBT trnsistors. The system opertes in closed loop looking for to do with tht the currents in the inductors re next possible to reference currents. Controllers PI operte in the intention to minimize the errors between the references nd effectively synthesized currents in the L inductors until Ln (Fig.). In this pper the control of the filter ws implemented digitlly in processor DSP TMS30F81, mnufctured for the Texs Instruments. Fig. 4 shows more detiled strtegy of implemented control in the DSP. The control strtegy must synthesize current (if) in the output of the filter, tht ssures low distortion the current (is) in the PAC, considering tht the lod current (ic) hs distortion of up to 50%. To keep high fctor of power, the voltge signl in the PAC is used s reference. The voltge in cpcitor C of the inverter is monitored to gurntee the system power blncing. Fig. 4. Block digrm to use in voltge nd current controller design B. Mthemticl Anlysis Mthemticl nlysis contemplted ll the procedures for implementtion of the control of the three-phse inverter of four legs operting s ctive filter of hrmonics. The requirements of hrmonics ttenution until 1ª order, reduction of the neutrl current (unblnce between the

3 3 phses) nd reduction of the displcement between voltge nd current in the three phses hd been the min considertions in the design. In the filter, two controllers PI re used: current controller PI of fst response nd controller PI of link, of slower response. Controller PI of link ws clculted in function of the described prmeters below: Vlue of the link cpcitor; Vlue of the rms phse voltge; End of scle of the sensors of link voltge nd lternted voltges; Vlue of voltge desired in link; Anlogicl-digitl converter gin; Considering the bidirectionl inverter for blnced lods, the link voltge it must be bigger tht the vlue of line voltge pek. In such wy, to ssure the synthesis of the current in the filter, the link voltge (V ) must be 3/ greter tht the pek of the mximum rms phse voltge (V ) [8], tht is: 3 V V 3 The link cpcitor is responsible for some specific chrcteristics, such s: to ssist in the blnce of energy in conditions of trnsitory in the lod, to gurntee one high tx of vrition of the output current inverter nd, to mke possible the current circultion of negtive sequence for the inverter. In the cse of the topology dopted for the inverter (4 legs), the currents of zero sequence re mnipulted by the fourth rm, mking tht the link voltge will be influenced only from the hrmonics nd components of negtive sequence, especilly in the bsic frequency (bigger power). It is considered tht the voltges of the three-phse power grid re blnced nd tht the fluctution of voltge (V ) is lesser the totl vlue of link voltge (V <<V ), where V is pek of the voltge ripple. The prcel most significnt of this ripple is proportionl to the oscilltion of power in the inverter, which hd to currents of negtive sequence: P neg (1) ~ [ v v v ]*[ il il il ] P () n bn cn neg bneg cneg From () it is possible to show tht the power of negtive sequence hs oscilltory chrcteristic in nd cn be written s: P neg 3 V I neg _ pk cos( t + φ) (3) where: I neg_pk is the pek vlue of negtive sequence current, which if desires tht to the ctive filter support. Therefore, the fluctution of energy pek-pek ( is: E 1 pp 3V I neg _ pk 1 E pp ) provoked by the lod ( + V ) ( V ) CV V (4) C V C V In such wy, the minimum cpcitor to get vlue for V C, is: min 3 V I neg _ 4V V pk (5) Coupling inductors, between the inverter nd the power grid, must operte in big bnd of frequencies, wht it must be considered in the design for definition of the constructive chrcteristics of the inductor nd the core mteril. The specifiction of the inductnce vlue is mde to stisfy criteri nd specific functions, such s: to limit the current ripple in the output of the converter, being operted s first-clss filter psslow, without provoking drop of voltge exggerted on its terminls. Additionlly the inductor does not hve to limit the tx of vrition of the converter current, wht it would limit the performnce of the ctive filter for compenstion of hrmonic currents of higher order. It must hve commitment between the link voltge nd the vlue of the inductor. High vlues of voltge mke possible to get high di/dt to cncel the hrmonics completely, however this lso cuses vlues rised for the current ripple. A chosen time the voltge, must be proceeded to the clcultion of the coupling inductors. Diverse ppers s [9, 10, 11], estblish criteri for the clcultion. Thus ording [1], the tx of rising (di/dt) of the current generted for the ctive filter must be bigger of wht the tx of hrmonic currents rising of the lod. Considering the irchhoff s Voltges Lw, for one of the phses of the system (Fig. nd 3), it results: di 3V L + V 0 dt Soon, time tht if desires tht the inductnce vlue llows tht the tx of vrition of the current of the ctive filter (di/dt), either bigger of wht the tx of vrition of hrmonic currents of the lod, becomes: L V di dt 3V (6) (7) As much for the project of the current controller, how much for the one of voltge, the system is considered ording Fig. 4. As it does not hve power plnt in cpcitor C, the system must observe nd keep constnt this voltge. It is necessry lso tht the output current of the filter synthesizes the references defined for the compenstion strtegy. Thus the system cn be configured s two subsystems of control: one for the voltge in C nd nother one for output hrmonic currents. It is noticed tht the controller of the voltge of cpcitor C is responsible for modulting the vlue of the current reference of the ctive filter, to keep fixes this voltge. In such wy, the trnsfer function of the physicl system of the voltge control cn be represented by the cpcitive function 1, while the trnsfer function of the system of sc control of the output current is given by the function

4 4 1 [13]. Additionlly the gins of the voltge sensors sl conv nd current must be considered. Finlly, depending on the technique of dopted modultion the gin of converter PWM is defined. In the cse of the used three-phse inverter, modulting set PWM - converting it is understood, of the point of view of control, s voltge gin equl to: V (8) 50 PWM Fig. 5 illustrtes the used bsic project for the design of the current controller. It is observed tht the current in the output of the ctive filter is converted in voltge nd scled through the current sensor (block si ) nd the conditioning bord. To follow then it is converted by A/D converter (block DSP ) into p.u. (per unit) scle. stedy stte the converter does not hve to bsorb nor to deliver ctive power. Tht is the clculted ctive power in side AC of the converter must be the sme one of tht one clculted of side. In such wy the gins of the PI cn be clculted by similr expressions to those used for the current control. III. PROJECT PROCEDURES In this section the bsic criteri for the dimensioning of the components of the prllel ctive filter re exposed. The conditions where the ctive filter is used nd tht type of lod the sme is cpble to compenste re described. The system is three-phse with neutrl conductor. The voltge of phse V 17 V nd 60 Hz frequency. The nonliner lods produce currents with up to 50% of hrmonic distortion nd unblnce of negtive sequence. The lods present power bout 10 kva, wht it results in current of the order of 6 A. Voltge of link is defined in greement (1) in 500 V, with V 17 V + 5%. The cpcitor of link is clculted ording (5), in 3,6 mf, being I neg_pk 50 A nd V 10 V. Considering derivtive of current s 50 ka/s nd voltge in 500 V, is gotten, using (7), vlue of 1,9 mh. Fig. 5. Digrm of blocks of the plnt with the current controller. Considering this design, the trnsfer functions without (G ) nd with (G ) controller PI, re express, respectively for: G G ( s) i 1 sl s s (9) ( s) G ( s) PI( s) (10) PWM conv Soon it is possible to clculte the proportionl gin ( p ) nd integrl gin ( i ) of regultors PI through [14]: i p p tn f f ( mf ) T (11) (1) where f is the frequency of the brodbnd in Hz nd mf is the phse mrgin stipulted for the system in closed loop with regultor PI. T is the period of smpling of the digitl system. In generl, the bnd-pss of the current controller of n ctive filter cnnot be low to disble the synthesis of hrmonic currents necessry, nor high to become the unstble system or to exceed the criteri of Nyquist stbility. How much to the phse mrgin vlues between 70º nd 85º hd been kept. Considering tht the dynmics of the voltge of link of the inverter is sufficiently slow, it is reltively esy to get controller PI who stisfies the conditions of dynmics nd stbility. The clcultion of the gin of the converter seen for the voltge controller is mde by the blnce of power of the converter. Operting s prllel ctive filter in condition of IV. SIMULATION RESULTS Diverse simultions hd been crried through to test the control strtegy nd to verify the sttic nd dynmic conditions of the prmeters of the filter. To follow the results of simultion re presented where some problemtic situtions re considered tht cn our with the electric power grid nd its lods, lso the presence of distorted voltge for the fifth hrmonic in the input. The lod is composed for three-phse rectifier without cpcitor, single-phse rectifier with cpcitor in the phse A, nd n electricl resistnce in phse B, representing n unblnced sitution for phse currents. The switching frequency is 1 khz, the output inductors re of mh nd the link voltge is 500 V. Fig. 6. Simulted circuit showing set of dverse situtions of the lod nd the power grid.

5 5 Fig. 7. Lods currents (superior) nd source currents with voltge in the phse A (inferior). It is observed tht, exctly for this extreme sitution tht combines hrmonic distortion of currents, unblnce nd distortion of the input voltge; the output currents re blnced nd with reduced hrmonic content. Fig. 10: Voltge of the phse A, the currents of the lod in the phses A, B nd in neutrl one (10mV/A). V. EXPERIMENTAL RESULTS To follow the results of the opertion in lbortory of the ctive filter for compenstion of six lods tht compose sufficiently severe sitution of genertion of hrmonics nd unblnced currents meet. Totl lod is 11, kva nd the digrm of connections is presented in Fig. 8. This loding vlue got the filter to next to the limits to current protections in the legs of the inverter. The design power of this filter is of 10 kva Fig. 11: Voltge of the phse A, the currents of the source in the phses A, B nd in the neutrl one (10mV/A) with the filter in opertion. Fig. 8: Digrm of lods used for one of the lbortory tests. Nonliner nd unblnced lod 11, VA. The mesurements hd been crried through with power qulity nlyzer nd n oscilloscope of 4 chnnels. Fig. 1: Voltge of the phse A, lod current, current of compenstion (filter) nd current compensted for source. TABLE I SUMMARY OF EXPERIMENTAL RESULTS Fig. 9: Voltge of the phse A, the currents of the lod in the phses A, B, C (10mV/A). Phses A lod A grid B lod B grid C lod C grid N lod N grid S (kva) 3,7 3,0 3,1 3,0 4,4 3,0 PF 0,59 0,99 0,77 0,99 0,87 0,99 Irms (A) 8,4 3, 4,1 3,5 33,8 3,4 16,, Ipek (A) 55,4 34,6 43,3 40, 6,6 35, 7,4 7,6 DHT (%) 50,1 7, 7,8 7,4 38, 7,8

6 6 The tble indictes the pprent power (S), power fctor (PF), current (rms), pek current nd the current distortion for ech one of the conductors of phse nd neutrl, considering only the lod nd lter the grid (PAC) with the ctive filter operting. Notice tht kva of the lod diminishes in function of the improvement in the power fctor (0,99). The current distortion with filter power on, is less thn 8%. As much the efficient vlues, how much the pek re blnced with the compenstion. The current of neutrl sufficiently is ttenuted by the opertion of the filter. Notice tht the power nd consequently, the currents, re distributed between the three phses. Notice tht the distortion is too reduced, being below of 15%, exctly considering distortions of up to 76% (Phse A). Errors in the scles of the sensors, s well s low the digitl resolution must be considered. VI. CONCLUSIONS This pper presented the design, simultion nd prcticl implementtion of n ctive filter for compenstion of hrmonics currents, with blncing of the phses nd compenstion of rective power. An observed importnt point during the tests in lbortory is tht the result of the compenstion is directly proportionl to the opertionl conditions of the lod, time tht the filter designed for one is determined nominl condition. Thus the study of the type of lod to be compensted it is bsic for the correct dimensioning of this filter nd its optimiztion. It is possible to design filter for ll chrcteristic of lod, but if it will be possible design for only one, the results will be more expressive. VII. REFERENCES [1]. Referen Second Interntionl Workshop on "Power Definitions nd Mesurements under Nonsinusoidl Conditions". Stres-Itly, Sept []. W. V. Lyon, Rective power nd unblnced circuits, Electricl World, Vol. 75 (5), pp , 190. [3]. Budenu C.I. "The Different Options nd Conceptions Regrding Active Power in Non-sinusoidl Systems". Rum. Nt. Inst., Publ no.4, 197. [4]. Fryse S. "Wirk-, Blind-, Scheinleistung in Elektrische Stromkreisen in nicht sinusformigen Verluf von Strom und Spnnung". ETZ, Bd. 53, 193. [5]. F. P. Mrfão, S.M. Deckmnn, A. J. Pomilio, nd R. Q. Mchdo. Selective disturbnce compenstion nd comprisons of ctive filtering strtegies. IEEE Interntionl Conference on Hrmonics nd Qulity of Power (ICHQP), 00. ISBN [6]. IEEE Working Group on Nonsinusoidl Situtions, IEEE Tutoril Course on Nonsinusoidl Situtions: Effects on Performnce of Meters, 90EH037-7-PWR, [7]. M. Depenbrock, Quntities of multi-terminl circuit determined on the bsis of irchhof's lws, Fourth Interntionl Workshop on Power Definitions nd Mesurements under Non-sinusoidl Conditions, pp. 9-36, Miln-Itly, [8]. Gnnett, R. A. Sozio, J. C. Boroyevich, D. Brdley Appliction of synchronous nd sttionry frme controllers for unblnced nd nonliner lod compenstion in 4-leg inverters. Applied Power Electronics Conference nd Exposition, 00. APEC 00. Seventeenth Annul IEEE. Vol., pp Loction: Dlls, TX, USA [9]. Srinivs Ponnluri, Axel Brickwedde, Generlized System Design of Active Filters Power Electronics Specilists Conference, 001. PE. 001 IEEE 3 nd Annul. Vol. 3, pp Vncouver, BC, Cnd. [10]. Luis A. Morán, Jun W. Dixon, Rogel R. Wllce, A Three-Phse Active Power Filter Operting with Fixed Switching Frequency for Rective Power nd Current Hrmonic Compenstion, IEEE Trnsctions on Industril Electronics, Vol. 4, No [11]. Richrd Zhng, High Performnce Power Converter Systems for Nonliner nd Unblnced Lod/Source, Tese de Doutordo, Virgini Polytechnic Institute, [1]. Thierry Thoms, évork Hddd, Gez Joós, Alin Jfri, Design nd Performnce of Active Power Filters, IEEE Industry Applictions Mgzine, [13]. E. Clrke. Circuits Anlysis of A-C Power Systems, volume I - Symmetricl nd Relted Components. John Wiley nd Sons, Inc., New York - USA, [14]. Normn S. Nise. Control systems engineering. Benjmin-Cummings Publishing Co. Redwood City, CA, USA. ISBN: pp. power supplies. VIII. BIOGRAPHIES Crlos Gbriel Binchin ws born in Plmeir D Oeste, Brsil, on August 9, 197. He grduted from the UNESP Universidde Estdul Pulist, Gurtinguetá, nd hd Mster of Science t the UNICAMP. His employment experience included the LACTEC Institute for Development Technology (where works nowdys) nd SUNNYVALE Import. nd Mintennce. His specil fields of interest include power electronics for power qulity, distribution energy (medium voltge), public street lighting nd Rogers Demonti ws born in Brusque, Brsil, on My 7, He grduted from the UF Universidde Federl de Snt Ctrin, where he hd Mster of Science nd Doctor of Science. He works in LACTEC - Institute for Development Technology. His specil fields of interest include ctive hrmonic filtering, solr energy processing nd electronic energy metering. Júlio Shigeki Omori, ws born in São Mteus do Sul, Brsil, on Jnury 9, He grduted from CEFET-PR Centro Federl de Tecnologi. He works in COPEL Compnhi Prnense de Energi since 00, nd CEFET- PR s techer. His specil fields of interest include development of power systems, plnning nd power qulity.