Hybrid Power Line Conditioner Based on Two Parallel Converters Topology

Transcription

1 Hybrd Power ne Condtoner Based on Two Parallel Converters Topology MaríaIsabel Mlanés Montero Enrque Romero Cadaval Fermín Barrero González Abstract A new Hybrd Power ne Condtoner s proposed n ths paper. It s formed by an actve condtoner n parallel wth a hybrd condtoner composed by an actve flter n seres to one or more passve flters. Ths topology allows the reducton of the nverters ratng consttutng an effectve soluton n hgh power levels where classcal power flters can not be used. The novel topology operates adequately n threephase fourwre systems reducng the harmonc dstorton and/or unbalance and attanng unty dsplacement power factor. Expermental results are ncluded for testng the topology and ts control. Index Terms Actve flter hybrd flter multconverter topology power lne condtoner. I. INTRODUCTION The use of nonlnear loads njectng harmonc and reactve current components n the electrcal power system has suffered a prolferaton n the last years. The reducton or elmnaton of these components can be acheved by usng compensaton equpments nstalled at the pont of common couplng (PCC). Conventonal topologes such as passve flters actve or hybrd condtoners or unversal condtoners have been used for ths purpose [1] [3]. However these solutons can not be used n hgh power applcatons (above kva [4]) due to the nverter ratng. An dea for solvng ths drawback s the separaton of the compensaton among two or more equpments resultng a multconverter condtoner. An actve multconverter topology was frst presented n [] and recently smlar topologes wth dfferent control strateges have been developed [6]. In ths paper a new hybrd multconverter topology whch we call paph (parallel Actve wth parallel Hybrd condtoner) formed by two converters connected n parallel sharng the dc bus s presented. One of the converters called slow because t operates wth low swtchng frequency s n charge of the fundamental and domnant harmonc components n the load current. Ths equpment s helped by the fast condtoner whch operates wth a hgher frequency compensatng the hgher harmonc components. The advantages of ths topology are on the one hand the separaton of the correcton whch attans a reducton n the nverters ratngs makng vable ths equpment n hgh power levels and on the other hand the reducton n the volume losses and prce of the condtoner due to the possblty of usng ferromagnetc core flter nductors for the slow converter []. The sharng strategy between both condtoners and each prncple of operaton are explaned n the followng sectons. Expermental results for a 1.2 kva laboratory prototype are presented. II. SHARING STRATEGY AND PRINCIPE OF OPERATION The hybrd flter topology chosen for the multconverter paph condtoner s AsP (Actve flter n seres to Passve flter) because for loads above 1 MW t consttutes the most economc soluton due to the reducton attaned n the nverter power [4] [7]. If ths hybrd flter s desgned and controlled for behavng as a DHF (Domnant Harmonc Flter) [8] t wll be formed by an actve flter (AF) n seres to one or more parallel passve flters (PF) tuned at the domnant harmonc frequences n the load current that usually have low orders. These components would be manly fltered n a passve manner whle the AF whch wll have a small ratng wll collaborate for solvng the drawbacks of passve flters workng alone. Besdes the hybrd flter could compensate dynamcally the dsplacement power factor. As the harmoncs n the reference current for ths flter are low a low commutaton frequency could be used for the nverter of ths equpment whch s called the slow condtoner. The other parallel actve (pa) condtoner wll be then n charge of the hgher harmonc orders whch usually have lower ampltude so the nverter ratng wll be smaller. The tuned harmonc components left not compensated by the slow equpment for avodng passve flters overload and the commutaton harmoncs of the hybrd condtoner could be also task for ths equpment. As hgh harmonc components wll be ordered to ths condtoner a hgher commutaton frequency would be needed for t actng as the fast equpment. If both flters operate as currentcontrolled sources the snglephase equvalent crcut of the multconverter topology s the one shown n Fg. 1 where AFs and AFf are the currents njected by the nverters of the slow and fast condtoners respectvely. The reference current for the hybrd flter HF ref behavng as a DHF s: = = ( ) (1) lm HF ref AFs ref 1 1d PF Fg.1. Correcton usng a hybrd multconverter paph condtoner.

2 where 1d component and s the actve postvesequence fundamental lm PF s the lmted tuned current allowed by the condtoner for avodng passve flters overload calculated as: lm f I PF I PF FPhPF lm = (2) PF PF lm lm I PFh f I PF > I PF PFhPF I PF beng I PF the RMS value of a tuned harmonc component n the load current and the maxmum I lm PFh PF RMS value of each tuned harmonc current n the passve flter/s. The reference current for the fast condtoner wll be the one necessary so that collaboratng wth the slow equpment attans the desred source current S ref whch depends on the compensaton objectves. Ths current wll be calculated as: = (3) AFf ref S ref AFs meas where AFs meas s the measured hybrd flter current. Wth these references the actve element of the slow condtoner secures the fundamental reactve power compensaton unbalance elmnaton caused n the fundamental and tuned components the actve tunnng of passve flters n case of mstunng passve flters overload allevaton and elmnaton of seres and parallel resonance among the passve flters and the source mpedance. On the other hand f a PHC (Perfect Harmonc Cancellaton) global control strategy [9] s selected for the multconverter equpment tryng that the source current wll be n phase wth the postvesequence fundamental component of the voltage at the PCC the actve equpment of the fast condtoner wll be n charge of the nontunng harmoncs elmnaton the unbalance n those components the rest of the tuned harmoncs not compensated by the hybrd equpment and the harmoncs due to the operaton of the slow condtoner. It should be taken nto account that under normal condtons that means snusodal and balanced voltage and balanced currents wth nomnal fundamental reactve power demand (the value used as reference for the passve flters desgn) the actve flter of the slow condtoner has not to operate and the actve flter of the fast condtoner has not to be n charge of the tuned harmoncs. These facts are summarzed n Table I. Under normal operaton condtons the power ratng of the actve flters are very small however f they are wanted to operate also under the abnormal condtons descrbed n Table I the ratng wll ncrease. III. SEECTED TOPOOGY AND CONTRO STAGE A. Descrpton of the topology The expermental prototype tested n laboratory (Fg. 2) s used for descrbng the hybrd multconverter topology. The hybrd flter s formed by one actve branch and one passve branch per phase what consttute a partcular case of the proposed topology whch allows the reducton of passve elements. TABE I FUNCTIONS OF THE ACTIVE EQUIPMENTS OF EACH CONDITIONER Normal operaton condtons Abnormal operaton condtons AsP (slow condtoner) NO FUNCTION Changes n Q 1 Unbalance n the fundamental and tuned harmoncs u PCChPF PF Mstunng PF Seres resonance Parallel resonance pa (fast condtoner) Flterng of harmoncs h h and h 1 Commutaton harmoncs of the slow condtoner lm PF PF The actve part of the hybrd multconverter condtoner s formed by a neutralpontedclamped voltagesource nverter wth sx branches. The three fast condtoner branches are connected to each phase of the utlty by a flter nductor whle the slow condtoner branches are connected n seres to the passve flters mpedances. The nductor of the passve flter wll be desgned so that t can operate as a flter nductor n the actve equpment attanng another reducton n the elements of the condtoner. A controlled rectfer has been selected as nonlnear load so the domnant harmoncs are the th and 7th orders. As a 1 passve branch has been selected for the hybrd condtoner t s possble to tune the passve flter to the lower hgher or an ntermedate order. After studyng these optons fnally the tunng to the lower harmonc ( th order) has been selected because: the flter mpedance at the hgher domnant harmonc wll be nductve elmnatng the possbltes of resonance near ths frequency as the ampltude of the harmoncs n the load current decrease as the harmonc order ncrease the voltage that the actve flter has to generate to make the 7 th order harmonc component to crculate across the hybrd flter wll be reduced besdes the passve flter wll offer a hgher mpedance at hgher and nondomnant harmonc orders (h > 7) reducng the voltage provded by the actve flter to avod the dervaton of these harmoncs through the hybrd branch. B. Control Stage The condtoner has to be able to operate adequately for every frng angle of the rectfer between α = º 6º (contnuous operatng range ndependently of the load) and for every type of resstve load wth power lower than the bass power of the system. In a more precse way the global compensaton objectves are: dsplacement power factor correcton for every frng angle n the controlled rectfer harmonc reducton n the source current fulfllng the lmts establshed by the standard IEEE 19 [1] and unbalance elmnaton n the source current. FP

3 Fg.2. Threephase fourwre source wth nonlnear load and shunt hybrd multconverter condtoner. Control Strateges If control strateges based on the load current detecton are employed as t was prevously proposed n secton II n threephase four wre systems three load currents ( a b and c ) sx compensatng currents ( HFa HFb HFc AFa AFb and AFc ) three phasetoneutral PCC voltages ( u PCCa u PCCb and u PCCc ) and the dc bus voltage U dc need to be sensed what mples thrteen measurements for controllng the multconverter condtoner. An mprovement s proposed so that the control strategy and trackng technque wll be based on the source current detecton n order to mnmze the number of sensors. The calculus of the reference currents for the hybrd flter proposed n (1) has to be changed because the load currents are not measured. An adaptve control whch estmates these varables from the measure of the source currents wll be used resultng the reference current for the slow converter: = ( 1 1 ) (4) lm lm AFs ref d 7 where the supper ndex means that hs varable s not measured but estmated. If the domnant harmonc load currents are desred to be absorbed by the hybrd flter t means that those components have to be null n the source current. Usng PI controllers as t s shown n Fg. 3 t s possble to obtan an estmaton of the load current components. Two synchronous reference frames one postvesequence and one negatvesequence and one 1phase ASRF [11] for extractng the zerosequence component are used for estmatng the hth domnant harmonc component n the load current. In Fg 3 the block dagram of a hth Harmonc oad Current Extractor (hce) s dsplayed. Besdes for avodng the th passve flter overload equaton (2) s partcularzed as: h lm f I I lm = () h lm h lm I f I > IPF I beng I the RMS value of the tuned harmonc component n the estmated load current calculated from ( a ) ( b ) ( c ) I = = (6) 3 3 and I h lm FP the maxmum RMS value of each tuned harmonc current n the th passve flter. Fnally two blocks lke the one n Fg. 4 one for the th harmonc and another for the 7 th order wll be needed. 1 Fg.3. h th Harmonc oad Current Extractor (hce) usng control strateges based on the measure of the source current. CE Eq. () Fg.4. Harmonc load current extracton lmted to the maxmum th passve flter. 1 1

4 For the fundamental harmonc t s necessary to add an addtonal block for the dc bus voltage control. For gettng ths voltage to be constant and near ts reference the hybrd flter has to absorb fundamental postvesequence actve power from the utlty. So n the reference source current a new term 1 d ( Udc) has to be ncluded resultng fnally: FAs ref 1 = ( 1 1 d ) 1 d ( Udc). (7) Ths term s obtaned from the output of a PI controller whose nput s the error between the reference dc bus voltage and ts measure. The PI controller has been desgned wth an enable sgnal for choosng f the slow condtoner s n charge of the dc bus control or ths functon s left to the fast condtoner. The control block for the fundamental component s shown n Fg.. The reference source current usng a Snusodal Source Current strategy operatng as PHC but based on the measurement of the source current n dq coordnates can be calculated as: S ref ps1 p 1( Udc) Sdref = K upcc1 d = u 2 2 PCC1d upcc1 d u PCC1q Sqref u PCC1q u PCC1q where the term p 1( Udc) s the postvesequence fundamental actve power absorbed from the utlty for controllng the dc bus voltage. Ths term s obtaned from the output of a PI controller whose nput s the error between the reference dc bus voltage and ts measure. Fnally the control block dagram of the hybrd multconverter condtoner s shown n Fg. 6. One can notce that seven measurements are only needed for the control strateges (three source currents the dc bus voltage and three PCC voltages). For obtanng the angle of the postvesequence fundamental component of the PCC voltages an Autoadjustable Synchronous Reference Frame (ASRF) [11] has been used. Trackng Technque The trackng technque determnes the duty cycle for the condtoner tryng to elmnate the error between the reference and the source current e n a commutaton or samplng perod T C. In a general manner the duty cycle for each branch applyng a deadbeat technque [12] s calculated from (8) AF D = e uaf (9) T where u AF s the voltage n the ac sde of the actve flter. In the slow condtoner the hybrd flter current s measured so e wll be determned from the dfference between the reference and the measured current. The voltage u AF can be calculated as: uaf = upcc HFmeas ZC (1) where ZCPF s the mpedance of the passve flter wthout nductance. C Fg.. Fundamental compensatng load current extracton (1CE) usng control strateges based n the measure of the source current wth the addton of a dc bus control loop. In Fg. 7 t s shown the block dagram of the deadbeat technque for the slow condtoner n whch T Cs s the commutaton perod. As the fast condtoner current s not measured a strategy whch allows the estmaton of the error between the reference and the measured currents s needed. If the hybrd flter s turned on from the fast converter pont of vew the set formed by the load plus the slow condtoner behaves as a new load wthout 1 st th and 7 th orders components to correct. In a samplng perod Ts t can be consdered that the new load current s approxmately constant so: ( ) = = (11) S AFf HF S AFf what means that the error can be calculated as the dfference between the reference and the measured source currents changng the sgn. In Fg. 8 the deadbeat technque for the fast condtoner s dsplayed. The trackng technque precse the measurement of three addtonal sgnals the hybrd flter currents resultng fnally ten varables necessary for the control stage of the novel topology. IV. EXPERIMENTA RESUTS The novel topology and control are tested on a 1.2 kva laboratory prototype. In Fg. 2 the electrc scheme of the expermental system s shown. A threephase fourwre system has been proposed (wth the am of studyng unbalance due to zerosequence components) Hz and nomnal bass parameters U 3 B = 1 3 V and S φ B = 12 VA from whch the bass values of current I B = 4 A and mpedance Z B = 2 Ω can be obtaned. The nonlnear load s formed by a threephase controlled rectfer wth resstve load R = 4823 Ω selected so that the maxmum power demanded (S max = 117 VA) s lower than the bass power of the system The values of the passve flter parameters are ndcated n Table II. As the nductor of the passve flter has to operate as a flter nductor n the actve equpment the value of has to be between the desgn lmts of AFs takng nto account the double crteron: rpple and control. In Table III the parameters used n the expermental condtoner are ndcated. The values of the capactances C 1 and C 2 are calculated assgnng a maxmum rpple n the voltage of 3%.

5 U dcref S (abc) abc P dq S Sd Sq U dcmeas S (dq) PI p 1(Udc) PF p S1 p 1(Udc) Eq. Sref (dq) P 1 (8) dq abc Sref (abc) u PCC (abc) u PCC1 (dq) 1 U dcref U dcmeas 1CE ( 1 1d 1d(Udc) )(abc) CE (abc) I PF lm Eq. () lm (abc) HFref (abc) 7 7 (abc) 7CE I PF 7lm Eq. () lm 7 (abc) Fg.6. Block dagram of the hybrd multconverter condtoner control strateges TABE II PASSIVE FITER PARAMETERS C QF PF 7 Z Z 2.9 mh 19.3 µ F Ω 22.6 Ω TABE III PARAMETERS OF THE EXPERIMENTA CONDITIONER U dc C1 = C2 AFs = PF AFf 9 V 19.1 mf 21 mh 29.4 mh RPF C s1 C s T cs Fg.7. Duty cycles determnaton for the hybrd condtoner (slow) T AF S Fg.8. Duty cycles determnaton for the actve condtoner (fast) The control algorthms are mplemented usng a real tme control system DS114 (dspace) wth PCI target composed by a processor Power PC63e/2MHz and the Texas Instruments DSP TMS32F24. Ths platform has 4 multplexed nputs A/D 16 bts (2 µ s samplng tme) and 4 nputs A/D 12 bts (8 ns samplng tme). Many tests have been conducted by smulaton confrmng the proper performance of the topology and ts control n threephase fourwre systems wth harmonc dstorton and unbalance n the PCC voltages and fourwre loads. However due to the hardware lmtaton n the number of nputs expermental tests have been smplfed usng a threewre load (see Fg. 2) so zerosequence components currents are not demanded. As the control platform has only 8 nputs they have been used for measurng the three source currents ( Sa Sb and Sc ) two of the three hybrd currents ( HFa and HFb ) obtanng the other one by subtracton supposng these currents to form a threephase system wthout zerosequence components two of the three lnetolne PCC voltages ( u PCCab and u PCCcb ) from whch the three phasetoneutral voltages are calculated and the dc bus voltage U. dc The control strateges and trackng technque employed

6 are the detaled n the prevous secton. For the swtchng sgnals generaton of the multconverter nverter two symmetrc PWM has been employed. These are generated by the slave DSP of the DS114 platform from the duty cycles calculated n the trackng technques. The commutaton frequency of the slow and fast condtoners has been fxed to 4kHz and 2 khz respectvely. The expermental results when the fast condtoner s turned on frstly so t s n charge of the dc bus control are shown n Fg. 9 for dfferent frng angle values. In Fg. 9(a) there s not fundamental reactve power so the fundamental hybrd current component s null appearng only the th and 7 th compensatng currents whch fully elmnate these components n the load current. The actve condtoner demands fundamental current for the dc bus control and reduces the hgh orders components n the load current. However n Fg. 9(b) the frng angle of the rectfer s not zero so fundamental reactve power s demanded by the load. Ths term of power s delvered by the slow condtoner so the hybrd flter current contans fundamental component n ths stuaton. In Fg. 1 smlar experments have been conducted but turnng on the slow condtoner frst so the dc bus control s task of ths equpment. Ths s the reason why fundamental component appear n the hybrd condtoner although no fundamental reactve power s demanded by the load (see Fg. 1(a)). For testng the operaton of the condtoner under unbalanced PCC voltage condtons the phase c conductor s connected to the neutral conductor of the utlty causng nversesequence and zerosequence. (a) (a) (b) Fg.1. Operaton of the multconverter condtoner. From up to down: Waveforms of S AF HF and frequency spectra of these currents n the same order. (a) α = º; (b) α = 4º. components ( U PCC1 / U PCC1 = U PCC1 / U PCC1 = %). As the zerosequence component of the PCC voltage does not take part n the control algorthms and zerosequence power s not demanded by the load because t has only three wres the exstence of ths component does not affect to the control stage and the experment can be conducted n spte of the lmted number of nputs of the expermental platform. The load currents for each phase are shown n Fg. 11(a) whle the three source currents are presented usng the same axs n Fg. 11(b) so that balanced and snusodal source currents can be apprecated. It has been calculated by smulaton usng the load proposed n the expermental tests the rato maxmum apparent power of the nverter n the least favourable frng angle stuaton related to the load power. For a conventonal actve condtoner ths rato s over 9%. Wth a hybrd multconverter topology the fast condtoner nverter rato s below 16% and approxmately 2% for the slow condtoner nverter. These results valdate the usefulness of the topology for hgh power levels where classcal power flters can not be used due to the nverter ratng. The reducton n volume losses and prce attaned wth ths topology due to the use of ferromagnetc core nductors n the slow converter are demonstrated n []. (b) Fg.9. Operaton of the multconverter condtoner. From up to down: Waveforms of S AF HF and frequency spectra of these currents n the same order. (a) α = º; (b) α = 4º.

7 (a) (b) Fg.1. Operaton of the multconverter condtoner under unbalanced PCC voltages. From up to down: Waveforms of the currents for phases a b and c and frequency spectra of these currents n the same order. (a) ; (b) S. V. CONCUSIONS A novel hybrd multconverter topology composed by a hybrd condtoner n parallel wth an actve one s proposed n ths paper. There s a slow converter workng wth low swtchng frequency whch s n charge of the fundamental and domnant harmonc components n the load current. The fast condtoner operatng wth a hgher frequency compensates the hgher harmonc components. The nverter of the condtoner s a neutralpontedclamped VSI wth sx branches. The hybrd flter s formed by one actve branch and one passve branch per phase tuned to the th order. The nductance of the passve flter acts smultaneously as the flter nductance of the actve part reducng the number of passve elements n the condtoner. Ths topology results especally proper for hgh power loads wth a spectrum content wth many harmonc components over the lmts of total and ndvdual harmonc dstorton allowed by the standard IEEE 19. The man advantages of the topology are the decrease attaned n the ratng of the nverters and the reducton n volume losses and prce due to the use of ferromagnetc core nductors n the slow converter. Control strateges and trackng technques for the global condtoner wthout the measurement of the load current have been developed needng only ten measurements for the control of the two converters. The operaton of the equpment n threephase fourwre systems wth harmonc dstorton and unbalance has been tested n smulaton. Some expermental results wth a 1.2 kva laboratory prototype are presented under dfferent load condtons and under unbalanced voltages. REFERENCES [1] B. Sngh K. AlHaddad A. Chandra "A rewew of Actve Flters for Power Qualty Improvement" IEEE Transactons on Industral Electroncs vol. 46 no.. Oct [2] S. Km P.N. Enjet "A New Hybrd Actve Power Flter (APF) Topology" IEEE Transactons on Power Electroncs vol. 17 no. 1 pp Jan. 22. [3] F. Barrero S. Martínez F. Yeves F. Mur P.M. Martínez "Unversal and reconfgurable to UPS actve power flter for lne condtonng" IEEE Transactons on Power Delvery vol. 18 no. 1 pp Jan. 23. [4] S. Bhattacharya P.T. Cheng D. Dvan "Hybrd Solutons for Improvng Passve Flter Performance n Hgh Power Applcatons" IEEE Transactons on Industry Applcatons vol. 33 no. 3 pp May/June [] E. Romero Cadaval F. Barrero González and M. I. Mlanés Montero Actve Power ne Condtoner Based on Two Parallel Converters Topology presented at the 4 th Internatonal Workshop on Compatblty n Power Electroncs (CPE 2) Gdyna Poland 2. [6]. Asmnoae C. ascu F. Blaabjerg and I. Boldea Performance Improvement of Shunt Actve Power Flter Wth Dual Parallel Topology IEEE Trans. Power Electroncs vol. 22 no. 1 pp Jan. 27. [7] P.T. Cheng S. Bhattacharya D. Dvan "Control of SquareWave Inverters n HghPower Hybrd Actve Flter Systems." IEEE Transactons on Industry Applcatons vol. 34 no. 3 pp May/June [8] P.T. Cheng S. Bhattacharya D. Dvan " Operatons of the Domnant Harmonc Actve Flter (DHAF) Under Realstc Utlty Condtons" IEEE Transactons on Industry Applcatons vol. 37 no. 4 pp July/Aug. 21. [9] M.R. Rafe H. A. Tolyat R. Ghaz T. Gopalarathanam "An Optmal and Flexble Control Strategy for Actve Flterng and Power Factor Correcton Under NonSnusodal ne Voltages" IEEE Transactons on Power Delvery vol. 16 no. 2 pp Apr. 21. [1] IEEE Standard IEEE Recommended Practces and Requrements for Harmonc Control n Electrcal Power Systems [11] M. I. MlanésMontero E. RomeroCadaval A. Rcode Marcos V. MñambresMarcos F. BarreroGonzález Novel Method for Synchronzaton to Dsturbed Threephase and Snglephase Systems accepted for beng presented at the IEEE Internatonal Symposum on Industral Electroncs (ISIE 27) Vgo Span 27. [12] E. Romero Cadaval M. I. Mlanés Montero F. Barrero González A Modfed Swtchng Sgnnal Generaton Technque to Mnmze the RMS Trackng Error n Actve Flters IEEE Transactons on Power Electroncs vol. 2 pp Sept. 2.