Internatonal Journal of Electrcal Engneerng. ISSN 0974-2158 Volume 5, Number 2 (2012), pp. 153-166 Internatonal Research Publcaton House http://www.rphouse.com A Novel Reference Current Generaton Algorthm for Harmonc and Reactve Power Compensaton n Non Ideal Three-phase Systems John George, Jose T.L. and Jeevamma Jacob Department of Electrcal Engneerng, Natonal Insttute of Technology Calcut, NIT Campus P. O., Calcut-673601, Kerala, Inda. E-mal: johngeorge@ntc.ac.n, tlj@ntc.ac.n, jeeva@ntc.ac.n Abstract Modern power system networks use actve power flters for compensaton of harmonc currents generated by non lnear loads. In ths paper a new for generatng reference currents for harmonc and reactve power compensaton s. The has the advantage that t generates the reference currents rrespectve of the supply voltage and load condtons. So the source has to delver only fundamental postve sequence actve component of the load current even when the source voltage s dstorted or unbalanced. Ths also ensures that the source currents after compensaton are balanced and at unty power factor. Smulaton results and the comparatve evaluaton for dfferent load and supply voltage condtons presented n ths paper confrm the valdty and practcablty of the. Keywords: Actve power flters,, power qualty, reactve power compensaton. Introducton The modern dstrbuton systems are polluted due to the extensve use of power electronc devces such as unnterrupted power supples, motor drves etc. These non lnear loads draw non snusodal current from the source and causes voltage dstorton
154 John George et al at the pont of common couplng (PCC). Other devces that are connected to the PCC are affected by the voltage dstorton and hence sutable harmonc compensaton s to be provded to mantan power qualty. Nowadays actve power flters (APFs) are used together wth the non lnear loads for the compensaton of harmoncs and reactve power. In four wre dstrbuton systems, unbalanced loadng, harmoncs and huge reactve power requrement are common and hence sutable compensaton have to be provded. Akag et.al n 1984 the frst compensator whch can compensate for the harmoncs and reactve power. They assumed a balanced three phase system wth snusodal voltages and the theory by Akag s well known as the nstantaneous reactve power theory [1, 2]. Several modfcatons were added to the theory ncorporatng three phase four wre systems and zero sequence components [3 6]. In [7] snusodal source current strategy was ntroduced consderng the unbalanced and dstorted system voltages. The unty power factor compensaton strategy for APFs n [8] corrects the lne currents so that the source currents ft to the voltage wave form n phase and shape. The APF control strategy employng decomposton of currents usng the rotatng coordnate transformaton nto actve and reactve components s presented n [9] and [10]. Instantaneous actve and reactve current component ( ) presented n [10], also called as synchronous reference frame uses the mans voltages for dervng the rotatng frame for the transformaton and hence results n poor compensaton under non deal source voltages. Several researchers have contrbuted dfferent s for ncorporatng non deal mans voltage condtons [11-21] n APF control strateges. Other approaches lke optmzaton [17], neural network etc. are also ntroduced n the control of APFs. The APF control strategy generates proper reference currents for harmoncs and reactve power compensaton. It has two control loops vz. the current control loop and the voltage control loop. The current control loop generates proper swtchng sgnals for the flter converter so that t tracks accurately the reference currents for compensaton. The dc bus voltage of the converter s controlled by the voltage control loop and s mantaned at the reference dc voltage. In ths paper a new for generatng current reference for compensatng unbalanced loadng, reactve power and harmoncs n three-phase four-wre system s presented. Fgure 1 shows the schematc representaton of three-phase four-wre APF connected to the four wre system feedng a non lnear load. The splt capactor converter topology for four-wre APF s consdered here and hysteress band controller s used for the current control. The presented performs well even when the mans voltages are unbalanced and/or dstorted. Dgtal smulaton results presented n ths paper confrm the valdty and practcablty of the.
A Novel Reference Current Generaton Algorthm 155 Fg. 1. Schematc representaton of three-phase four-wre APF connected to the four wre system feedng a non lnear load. A revew of the basc control strategy s gven n secton II as t s used as frame work for the control. Secton III descrbes the control. Smulaton results and comparatve evaluaton of the new wth control strategy are presented n secton IV and the effectveness of the new s establshed. The paper s concluded n secton V. Instantaneous Actve and Reactve Current Component Method The nstantaneous actve and reactve current component ( ) for APF s amed at compensatng the harmoncs and unbalance [9], [10], [20]. The control s based on a synchronous reference frame ( dq0 frame) derved from the mans voltage. The compensatng currents are obtaned from the nstantaneous actve and reactve current components n the dq0 frame. The reference currents for compensaton are derved as follows. The set of nstantaneous phase voltages ( ua, ub, u c) and nstantaneous currents ( a, b, c) at PCC for three phase four wre system s frst transformed to the statonary reference frame ( αβ 0 frame) usng the Clarke transformaton by (1). u ο ua ο a uα = C u ; b α = C (1) b u β u c β c where 1 1 1 2 2 2 2 1 1 C = 1 3 2 2 3 3 0 2 2 For load current L, ts components n the dq0 frame are obtaned usng the Park transformaton usng (2) where angle θ s the reference angle wth respect to the αβ 0 frame.
156 John George et al as Lο 1 0 0 Lο = 0 cosθ snθ Ld Lα 0 snθ cosθ Lβ uβ = uα 1, θ tan Lq As the zero sequence component s nvarant by transformaton, (2) can be wrtten Ld cosθ snθ L α = Lq snθ cosθ Lβ By geometrcal relatons [10], (3) can be rewrtten as Ld 1 uα uβ L α = 2 2 u u Lq u + u β α Lβ α β The actve current Ld and reactve current Lq n the dq0 frame are then decomposed nto average and oscllatory components. = +, = + (5) Ld Ld Ld Lq Lq Lq In, the reference currents are generated for compensatng Ld, Lq and zero sequence component Lο of load current so that the source delvers only the Ld component [9], [20]. And the reference source current n the αβο frame s gven by; 0 1 sαβο ref = Ld u (6) 2 2 α uα + uβ u β The reference currents n the abc coordnates can be calculated by usng the nverse transformaton of (1). (2) (3) (4) Proposed Method In the, the control algorthm ams at makng the compensated mans current purely snusodal and balanced, rrespectve of the mans voltage condton and load characterstcs. Ths s acheved by compensatng the load current for harmoncs and unbalance. Thus the mans current after compensaton contan only the fundamental postve sequence component of the load current. The ensures reactve power compensaton also so that the source currents after compensaton are at unty power factor,.e.; the compensated source currents wll be n phase wth the postve sequence voltage. Satsfyng the above requrements ths ncludes a novel control algorthm for generatng the reference currents for a shunt actve flter, assumng three-phase four-wre system. Ths algorthm focuses on extractng the postve sequence load current component whch forms the reference for compensated source current. The unqueness of the new s that the load currents are transferred to a synchronously rotatng frame locked to the postve sequence voltage. In ths paper, ths frame s denoted as dq0 + frame. The frame s extracted from the mans voltage
A Novel Reference Current Generaton Algorthm 157 usng a postve sequence voltage detector (PSVD) employng phase locked loop (PLL) [22]. The transformaton angle θ + s obtaned from the PSVD for transformng the currents to the dq0 + frame. In ths frame, the postve sequence current components appear as dc components. These dc components are fltered n the dq0 + frame and transformed back to abc coordnates to form the source current reference. (a) Postve sequence voltage detector + The PSVD s used to derve the transformaton angle θ wth respect to the postve sequence component for transformng the load current from abc frame to dqο + frame. A smple, fast and robust three-phase PLL based PSVD s employed for + generatng θ. The basc confguraton of PLL adopted here s shown n Fg. 2 [22]. The angle θ + s obtaned by ntegratng a frequency command ω. If ths frequency ω matches wth the utlty frequency, the voltages u d and u q appear as dc values when θ + s synchronzed wth the postve sequence voltage. Fg. 2. Control block dagram of a PLL based PSVD It should be noted that the voltage nputs to the PSVD may be unbalanced and/or dstorted. So the performance of PLL wll be deterorated, snce varaton of utlty angle wth respect to tme s not unform. So n order to make the PLL mmune to such dsturbances the utlty voltage nputs are pre-fltered usng a fourth order Butterworth flter so that θ + perfectly locks to the postve sequence voltage component. The flter ntroduces a total phase lag of 2π radans. Ths wll not affect the performance of PSVD snce no quanttatve nformaton other than frequency and phase are extracted from the voltage nputs. Snce the power system s a stff one, t s assumed that any substantal changes that cannot be accommodated nherently by the PLL wll not occur. Fgure 3 shows the transent behavor of PSVD. It s seen that the PSVD locks perfectly wth the postve sequence voltage.
158 John George et al Fg.3. Output of PLL based PSVD. (b) Reference Current generaton The control algorthm focuses on generatng the reference for compensated source current whch s ensured to be balanced and snusodal and consequently the reference for harmoncs, unbalanced loadng and reactve power compensaton s derved. Fgure 4 shows the complete confguraton of the strategy. Wth the help of the PSVD, sne and cosne templates are derved and the current space vectors n abc frame are transformed to the synchronous postve sequence reference frame dqο + usng the Parks transformaton. The transformaton angle θ + s a functon of tme and d θ + s a constant, provded the frequency remans constant. Any transent dt change n frequency wll be adjusted by the PSVD and therefore the control strategy developed s nherently stable n generatng the reference. Assumng three-phase fourwre system, the control algorthm s formulated as follows. The three phase load currents La, Lb, Lc are frst transformed to the αβ0 coordnates as L α, Lβ, L ο usng (1). Then these currents are then transformed to the dq0 + frame usng the Parks transformaton as, 1 0 0 Lο Lο + + Ld + = 0 cosθ snθ Lα (7) + + Lq+ 0 snθ cosθ Lβ Fg. 4. Proposed Control Scheme
A Novel Reference Current Generaton Algorthm 159 The transformaton angle θ + s derved usng the PSVD. The zero sequence component Lο s nvarant under transformaton and t s to be compensated fully. The load current components n the dqο + frame, and Ld + Lq+ are decomposed nto average and oscllatory components as, = Ld + + Ld Ld (8) = Lq + + Lq Lq (9) The dc quanttes Ld and Lq represent the fundamental postve sequence actve and reactve components respectvely of the load current. The oscllatory components Ld and Lq represent all hgher order harmonc components ncludng the frst harmonc negatve sequence current, as these components undergo a frequency shft n the spectra durng transformaton. For compensatng unbalance, harmoncs and reactve power, the zero sequence component ( Lο ) and the oscllatory components ( Ld, Lq ) of the load current are to be compensated. So the reference for source current s generated frst and hence flter current reference s obtaned ndrectly as shown n Fg. 4. The source current reference n the αβ 0 coordnates can be computed as, Lο 1 0 0 0 + = 0 cosθ snθ + + Lα Ld (10) 0 sn cos 0 + θ + θ + Lβ Hence the postve sequence source current reference sabc ref s obtaned as sa ref Lο T sb ref C + = Lα sc ref + Lβ So the flter current reference ref Cabc Ca ref La sa ref = Cb ref Lb sb ref Cc ref Lc sc ref can be obtaned as From (10) and (11), t s clear that the reference source current contans only the postve sequence actve current component. All the other load current components are compensated by the APF and therefore the source current after compensaton becomes snusodal, balanced and at unty power factor. (11) (12)
160 John George et al Smulaton Results and Comparatve Evaluaton The control algorthm for actve power flter s mplemented usng MATLAB-Smulnk to demonstrate the valdty. Also a comparatve evaluaton s done wth the control to prove the effectveness of the new algorthm. The smulaton s performed assumng three-phase four-wre system where dfferent source voltage and load condtons are consdered. The parameters selected for smulaton are shown n Table I. Table I : Smulaton Parameters Source 3 phase,100 V ph-ph, 50 Hz Source Parameters Rs=0.1Ω, Ls=0.1 H Flter resstance and nductance Rf=0.01Ω, Lf=5mH Capactor C1, C2 4000 mcrofarad Intal Capactor Voltage 125 V Reference dc voltage 250 V Hysteress band (current control loop) 0.2 Voltage Control loop Kp=0.4, K=15.4 Load Brdge rectfer on each phase wth R L load Smulaton s performed for four dfferent condtons and the performance of and the s compared n each case. The THD values shown along wth the waveforms are the maxmum values obtaned n each case. The values obtaned are detaled n the respectve tables. Case I: Balanced Source Voltage and Balanced Load Current In case I balanced condton s selected for smulaton. The results are shown n Fg. 5. It can be seen that both the s perform well and the THD values are wthn the lmts of power qualty. The values are summarzed n Table II. From Table II t can be read that the has better performance compared to wth respect to THD.
A Novel Reference Current Generaton Algorthm 161 Fg. 5. Case I: Balanced Source Voltage and Balanced Load Current Table II : Results Summary of Case I Phase Source Voltage Load Current Source Current A Mag 81.11V 81.19V 4.682A 4.682A 5.417A 4.583A THD 0.08% 0.02% 21.78% 22.26% 3.06% 0.98% B Mag 81.11V 81.19V 4.682A 4.682A 5.419A 4.584A THD 0.08% 0.02% 21.79% 22.22% 3.06% 1.05% C Mag 81.11V 81.19V 4.682A 4.682A 5.418A 4.584A THD 0.08% 0.02% 21.79% 22.25% 3.11% 0.97% Case II: Unbalanced Source Voltage and Balanced non lnear load. In ths case an unbalanced source voltage s appled by njectng 0.1pu fundamental negatve sequence along wth the fundamental postve sequence voltage. The THD of source voltage remans almost same as n case I. In ths case the APF has to compensate for harmoncs and unbalance n the load currents. In Fg. 6, t s seen that the lmts the THD where strategy cannot satsfy the standard. Moreover source current s also balanced usng the new. Table III gves the values n detal.
162 John George et al Fg. 6. Case II: Unbalanced Source Voltage and Balanced non lnear load. Table III : Results Summary of Case II Phase Source Voltage Load Current Source Current A Mag 88.25V 88.34V 5.090A 5.096A 5.661A 4.636A THD 0.10% 0.03% 21.72% 22.12% 6.56% 1.79% B Mag 81.52V 81.62V 4.706A 4.706A 5.431A 4.572A THD 0.09% 0.02% 21.80% 22.26% 4.90% 1.15% C Mag 74.17V 74.22V 4.281A 4.280A 5.189A 4.688A THD 0.09% 0.02% 21.87% 22.24% 6.00% 1.32% Case III: Unbalanced and dstorted voltage; balanced non lnear load. In case III, an addtonal postve sequence ffth harmonc voltage s njected to the unbalanced voltage appled n case II. In ths condton the reference current generated tself s dstorted n the whch results n poor compensaton. At the same tme the works well n ths condton also, because here, the reference currents are generated by usng a decoupled dq0 + reference frame usng the PSVD. Hence the reference currents are purely snusodal. Ths s an added advantage of ths. The results are shown n Fg. 7 and Table IV.
A Novel Reference Current Generaton Algorthm 163 Fg. 7. Case III: Unbalanced and dstorted voltage; balanced non lnear load. Table IV : Results Summary of Case III Phase Source Voltage Load Current Source Current A Mag 88.24V 88.34V 5.162A 5.165A 5.682A 4.706A THD 18.46% 18.49% 25.19% 25.43% 17.78% 2.66% B Mag 81.54V 81.61V 4.528A 4.518A 5.288A 4.626A THD 19.95% 20.01% 33.54% 34.40% 15.99% 1.68% C Mag 74..17V 74.21V 4.349A 4.351A 5.138A 4.738% THD 21.93% 22.00% 13.68% 13.81% 15.41% 0.63% Case IV: Balanced Source Voltage and Unbalanced non lnear load. Smulaton results for case IV are presented n Fg. 8. and the values are tabulated n Table V. In ths case an unbalanced non lnear load s appled n both the s. The load nductance s kept constant and load resstance n phase B s reduced. In ths case the unbalance load currents and the harmoncs have to be compensated. Here also, the shows better compensaton.
164 John George et al Fg. 8. Case IV: Balanced Source Voltage and Unbalanced non lnear load Table V : Results Summary of Case IV Phase Source Voltage Load Current Source Current A Mag 81.05V 81.13V 4.679A 4.679A 6.031A 5.232A THD 0.09% 0.03% 21.77% 22.21% 2.94% 1.23% B Mag 81.05V 81.13V 6.556A 6.556A 6.05A 5.168A THD 0.27% 0.09% 28.75% 29.16% 9.46% 4.04% C Mag 81.05V 81.13V 4.679A 4.674A 6.028A 5.176% THD 0.08% 0.02% 21.77% 22.26% 2.85% 1.11% Concluson A new reference current generaton algorthm for harmonc and reactve power compensaton usng shunt actve power flter s n ths paper. The ensures that the source current after compensaton s balanced and snusodal at unty power factor. The unqueness of ths s that t s mmune to any
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