Jestr Journal of Engneerng Scence and Technology Revew 5 () (01) 5-41 Research Artcle JOURAL OF Engneerng Scence and Technology Revew www.jestr.org Evaluaton and mplementaton of a Shunt Actve Power Flter under balanced Source Voltages P.M.Balasubramanam 1,* and G. Gurusamy Dpt of Electrcal & Electroncs Engneerng,KKT, Combatore, nda. Dean, Bannar Amman nsttute of Technology, Sathy, nda. Receved June 01; Accepted 5 September 01 Abstract The Shunt Actve Power Flter has proved to be a useful devce to elmnate harmonc currents and to compensate reactve power for nonlnear loads. The basc prncple of operaton of a Shunt Actve Power Flter s to nject a sutable non-snusodal current (compensatng current) nto the system at the pont of common couplng. An current control algorthm based on the tme-doman approach for three-phase Shunt Actve Power Flters s analyzed n ths paper. A basc overvew and evaluaton of the performance of exstng algorthms for actve power flters are presented. Accordng to dfferent complcated power qualty ssues and varous compensaton purposes, a current control scheme based on tme doman approach s proposed. Comparng wth exstng algorthms; ths algorthm has shorter response tme delay. Dfferent compensatng current references can thus, be accurately and easly obtaned by adoptng the proposed algorthm. Smulaton results usng MATLAB / Smulnk have proven excellent performance of the proposed system and t s more effectve than the avalable approaches. Keywords: Shunt actve power flter, Synchronous reference frame, nstantaneous reactve power theory, Pont of common couplng. 1. ntroducton Electrc power generated by the utltes s dstrbuted to the consumer n the form of 50 Hz ac voltage. The utltes have a tght control on the desgn and operaton of the equpment used for transmsson and dstrbuton, and can therefore keep frequency and voltage delvered to ther customers wthn close lmts. Unfortunately, ncreasng portons of loads connected to the power system are comprsed of power electronc converters. These loads are nonlnear and nject dstorted currents n the network and consequently generate harmonc voltage waveforms. Wth the prolferaton of nonlnear loads such as dode/thyrstor rectfers, non-snusodal currents degrade power qualty n power transmsson/dstrbuton systems. otably, voltage harmoncs n power systems are becomng a serous problem for both utltes and customers. The dstorton, whether t s produced by a large sngle source or by the cumulatve effect of many small loads, often propagates for mles along dstrbuton feeders. As the use of non-lnear power equpment s spreadng, the degradaton of the power qualty n the utlty networks s ncreasng and s becomng a major problem. Lmtng the voltage dstorton s therefore a concern for both utltes and consumers. * E-mal address: baluanujayen@gmal.com SS: 1791-77 01 Kavala nsttute of Technology. All rghts reserved. The smple block dagram of Fg. 1 llustrates the dstorton problem due to harmonc at low and medum power levels. Here, the utlty s represented by an deal ac voltage source n seres wth lumped mpedance representng lnes and transformers. The voltage waveform at the pont of common couplng s dstorted due to harmonc current generated by the non-lnear load. Ths results n the followng effects on the power system components 1. Malfuncton of harmonc senstve loads. ncreased losses n parallel connected capactor, transformers and motors. mproper operaton of protecton relays and crcut breakers Fg. 1. Harmonc dstorton at PCC
P.M.Balasubramanam and G. Gurusamy/Journal of Engneerng Scence and Technology Revew 5 () (01) 5-41. Actve Flterng Technology The frst attempt to reduce harmoncs wthout the use of conventonal passve flters was made by B. Brd. Ths desgn s based on changng the waveform of the current drawn by the load by njectng a thrd harmonc current, dsplaced n phase, nto the converter tself. Wth ths method however t s mpossble to fully elmnate more than one harmonc. The next attempt was made by Ametan, whch s based on expandng the current njecton method by proposng a technque to elmnate multple harmoncs. Accordng to ths theory, an actve control crcut could be used to precsely shape the njected current. deally, ths current would contan harmonc components of opposng phase, thus the harmoncs would be neutralzed, and only the fundamental component would reman. Despte the promsng theoretcal concept, Ametan was not successful n producng a practcal crcut capable of creatng a precse current. The total harmonc dstorton was reduced, but sngle harmoncs were not completely elmnated. On the other hand, Sasak and Machda theorzed that harmoncs could be elmnated by usng the prncple of magnetc flux compensaton. Ths n prncple s the use of current to produce a flux to counteract the flux produced by the harmoncs. Once agan, theoretcally, any number of harmoncs could be drectly elmnated. The current that would be requred to elmnate waveform dstorton caused by harmoncs was calculated mathematcally, but agan, a practcal control crcut was not realzed. Over the last ten to twelve years the remarkable progress n capacty and swtchng performance of devces such as Bpolar Juncton Transstors (BJT), Gate Turn-Off thyrstors (GTO) and nsulated Gate Bpolar Transstors (GBT), has spurred n the study of actve power flters for harmonc compensaton. n addton, advances n topologes and control schemes for statc PWM converters have enabled actve power Flter usng these converters to generate specfed harmonc currents, such as created by non-lnear loads. As actve power flters are powerful tools for the compensaton not only of current harmoncs produced by dstortng loads but also of reactve power and unbalance of nonlnear and fluctuatng loads. They can be smaller, more versatle, better damped, more selectve, and less prone to falure for component drft than ts passve counterpart..1 Shunt Actve Flters The shunt actve flter approach s based on the prncple of njecton of harmonc currents nto the ac system, of the same ampltude but opposte n phase to that of the load harmonc currents. Fg. shows the actve power flter compensaton prncple, whch s controlled n a closed loop manner to actvely shape the source current nto snusod. the flter but the rated voltage s agan lower. Therefore, harmonc mnmzaton can be mplemented wth converters havng a reduced power ratng... Hybrd Actve Flters Hybrd structures were proposed for harmonc compensaton of large rated loads n hgh voltage networks. Hybrd actve flters confguratons, combnes passve and actve flters. These flters mprove the compensaton characterstcs of the passve flters and thus realze a reducton n the ratng of the actve flter. They are partcularly suted n nstallatons where L-C tuned passve flters already exst. n the hybrd seres confguraton, the seres voltage njecton s to be regarded as an solator, ether determnng the harmonc currents to be suppled to the non-lnear load or the harmonc currents that wll be absorbed by the tuned LCflters., seres and shunt. n the frst case, the njected voltage s n seres, and n the second case t s n seres wth the shunt passve flter.. Control Methodology Compensaton of harmones can be accomplshed n tmedoman or frequency doman. Frst approach s based upon "on lne computaton of an nstantaneous error functon, whle the second case uses the prncple of Fourer analyss and perodcty of the dstorted waveform to be corrected. The error-functon n tme-doman could be computed n the followng ways: 1. Extracton of the fundamental component from the dstorted waveform through a notch flter. nstantaneous reactve power compensaton, whch uses an nstantaneous orthogonal power transformaton on both the actual and the fundamental components of voltage and current to produce a power functon. The dfference between these two transformatons s the error.. Synchronous reference frame approach. Many PWM strateges exst (for compensaton n the tme-doman) to generate the gatng sgnals to the swtches and thereby to reconstruct the dstorted current. The shunt APFs are used most wdely used to cancel the current dstorton. The performance of SAPF strctly depends on the features of the mproved algorthms and controllers. However, usually one algorthm s only more approprate to some stuaton but not to all stuaton. An mproved algorthm of SAPF for harmonc elmnaton, power factor correcton, and balancng of nonlnear loads s proposed n ths paper.. Seres Actve Flters n seres actve flter confguraton, a voltage source, s constructed n such a way that when ts voltage s added to the load voltage, the dstorted voltage s canceled, thus resultng of a snusodal voltage at the Pont of Common Couplng (PCC). For harmonc compensaton, both shunt and seres actve flters have smaller ratngs than the apparent power of the load. The shunt actve flter s rated for supply voltage, but a reduced current. n the case of seres dynamc flters, the rated load current passes through Fg.. Basc Prncple of SAPF System. 6
P.M.Balasubramanam and G. Gurusamy/Journal of Engneerng Scence and Technology Revew 5 () (01) 5-41 4. Smulnk Model of The Overall System The smulaton of power electronc systems provdes advantages n the desgn process by allowng varous optons. To valdate the feasblty of the proposed approach, a vrtual mplementaton of the SAPF s done usng Smulnk. The man components are the reference compensaton current detector, a phase loop lock, a current controller, and dc voltage controller. The reference compensaton currents are determned and are gven as nput to a current controller to produce sgnals of the PWM nverter. n addton, snce the capactor voltage at the nverter termnal must be mantaned at a constant level, the loss caused by swtchng and capactor voltage varatons s suppled by the source. Smulatons show that the compensaton current calculator yelds neglgble tme delay n steady state for the SAPF operaton. 5. Mathematcal Modellng of the Proposed Method n the three-phase three-wre system, the nstantaneous load currents of phase a, b, c ( a, b, and c ) can be dsassembled nto postve-sequence and negatve-sequence components accordng to the symmetrcal wegh law, whch was proposed by Fortes cue separately nk l nk l ( n) Sn π π Sn π π = + + + + (1) x 1k 1k k k k = 1 Commonly, only the postve-sequence, negatvesequence, actve power, and reactve power of the fundamental current are cared, and t s not necessary to decompose the harmonc. Then, the fundamental current component s expressed as follows π π x1() n = Sn n Sn n + + + + = Sn n Cos + Cos n Sn + Sn n Cos + Cos n Sn where, the frst tem of () corresponds to the postvesequence component n phase wth the phase voltage, whch s called the actve power component of the postvesequence fundamental current; the second tem of () corresponds to the postve-sequence component orthogonal wth the lne voltage, whch s called the reactve power component of the postve-sequence fundamental current; the thrd tem of () corresponds to the negatve-sequence component n phase wth the lne voltage, whch s called the actve power component of the negatve-sequence fundamental current; the forth tem of () corresponds to the negatve-sequence component orthogonal wth the lne voltage, whch s called the reactve power component of the negatve sequence fundamental current. Fg. 4 shows the block dagram of the proposed currentdetecton algorthm, where Sn π n s synchronous wth the postve-sequence fundamental voltage of phase a, whch determnes the calculaton precson of actve and () reactve power components. The low-pass flter used determnes the performance of the system. Accordng to dfferent compensaton purposes, the segregator wll obtan dfferent components expedently, whch s superor to the algorthm based on the nstantaneous reactve power theory. 6. Gan Selecton Wth the above confguraton, the control problem reduces to pckng the correct gans for the model of Fg. 4.4 for varous operatng condtons. Takng the samplng delay nto account, the plant s a smple lag along wth an ntegratng element H plant 1 U = 1 s + s s T where T, s the samplng tme. The open-loop transfer functon H 0l wth the controller then becomes H 01 T U ( ) 1+ s pll 1 = K pll st 1 s s pll + T s where K pll, T pll are the gans assocated wth the P regulator. Ths s a standard control problem very smlar to a current controlled speed loop of a drve system where the ntegral term n the plant mmcs the mechancal nerta and the lag element emulates the current control loop. Several methods can be used to select the gans based on the desred performance crtera. Here, the method of symmetrcal optmum was used to calculate the regulator gans. Accordng to ths method, the regulator gans K pll and T pll are selected such that the ampltude and the phase plot of H 01 are symmetrcal about the crossover frequency ω c, whch s at the geometrc mean of the two corner frequences of H 0l. Gven a normalzng factor α the frequency ω c, K pll, T pll are related as followng ω c T pll K = 1 pll ( αt s) α T s ( 1α )( 1 ( UT s) ) = = Substtutng (4) nto (5) t can be shown that the factor α and the dampng factor ξ are related by the relatonshp α 1 ξ = By changng α, the system bandwdth and dampng can be controlled. A three-phase PLL system was developed whch s sutable for tme doman analyss under dstorted utlty condtons and t was tuned the control effects such as loss of gan, lne harmoncs, and frequency dsturbances. The PLL was completely mplemented n software wthout the use of * any hardware flters. When the reference u de s set to zero, the θ * calculated s synchronous wth the postve-sequence * component of fundamental voltage. When u de s not set to zero, a fxed phase dfference s between the θ * and the postve-sequence component of fundamental voltage, whch make the control of the dsplacement factor easy. Moreover, () (4) (5) 7
P.M.Balasubramanam and G. Gurusamy/Journal of Engneerng Scence and Technology Revew 5 () (01) 5-41 ths phase dfference wll not affect the valdty of the selected harmoncs detecton. The correspondng smulaton dagram of the PLL s shown n Fg. 5. The phase voltage s expressed by per-unt; the base quanttes for per-unt value are the peak value of postvesequence fundamental phase voltage. Then, three phase voltages can be expressed as, respectvely. The nstantaneous power of the fundamental current can be obtaned by multplyng current by phase voltage Sn n x1 ( n)* Sn n 1 4π 4lπ = Cos 1 Cos n 4π 4lπ + Sn Sn n 4π 4lπ + Cos 1 Cos n where the frst tem of the equaton corresponds to the nstantaneous actve power component of the postvesequence fundamental (the sum of three phases s constant, whch contrbutes to the total power delvered from source to load). The second tem of the equaton corresponds to the nstantaneous reactve power component of the postvesequence fundamental (the sum of three phases s zero, whch crculates between the phases and can be compensated by a compensator wthout an energy storage element). The thrd tem corresponds to the nstantaneous actve power component of the negatve-sequence fundamental (the three-phase sum of the prevous part of ths tem s zero, whch crculates between the phases and can also be compensated by a compensator wthout an energystorage element). The three-phase sum of the rest (ncludng the posteror part of the thrd and fourth tems, whch s 4π equal to Cos n ϕ and the same for each phase) s not zero, and ts frequency s twce the fundamental, whch can be compensated by a compensator wth an energy-storage element). Therefore, the negatvesequence fundamental currents do not contrbute to the power delvered to the load. Smlarly, the nstantaneous power of harmoncs can be obtaned by multplyng current by xk ( n) = 1 nπ nπ Cos ( k 1) Cos ( k 1) 1k + + + + 1k 1k nπ nπ + Cos ( k 1) + Cos ( k 1) k + + k k t can be seen that ether the negatve-sequence or postve-sequence component has one part of whch the three-phase sum up to zero, whch can be compensated by a compensator wthout energy storage, and the rest can be compensated by a compensator wth energy storage. (6) (7) Fg.. Smulaton dagram of the current control algorthm. n (7) and (8), the lowest frequency component s twce the fundamental frequency; the dc component can be obtaned by a low-pass flter wth a cutoff frequency lower than twce the Fundamental frequency or by a sldngwndow wth / samples. Then, multplyng by, the followng equaton can be obtaned: 1 ( 4 ) B = Cos Cos l x + + π (8) Multplyng (1) Cos n the followng equaton can be obtaned ( n)* Cos n = x 1 4 4l π π Sn + Sn n + 4π 4lπ + Sn n Sn + + + + h} Usng the same method, the followng equatons can also be obtaned ϕ 1 ( ϕ 4 ) A = Sn Sn l x + + π (10) Here, defne ϕ = A B Sn Cos ϕ ϕ ϕ Sn Cos A B where A and B (9) () are the peak values of the reactve power component and actve power component of postvesequence fundamental current, respectvely. A and B are the peak values of the reactve power component and actve power component of negatve-sequence fundamental current of phase a, respectvely. Accordng to phase b Sn ϕ π, and c, the peak values are ( ) Cos ( ϕ π ) and ( ) Sn π Cos( ϕ + π ) ϕ +,. Respectvely, smlarly, multplyng Snk n and Cosk n, respectvely, and gong through the low-pass flter, A and xk B can be xk obtaned: 8
P.M.Balasubramanam and G. Gurusamy/Journal of Engneerng Scence and Technology Revew 5 () (01) 5-41 ( 1) π ( 1) Axk Sn Sn 1k ϕ k k l k lπ = + + + + ϕ (1) 1k ( 1) π ( 1) Bxk Cos Cos 1k ϕ k k l k lπ = + + + + ϕ (1) 1k Then, defne Sn n l l Cos π n π Sn π n π = + x A B l l Cos π n π Sn π n π = + + + x A B l l Cos π n π Sn π n π = + x1 Ax1 Bx1 l l Cos π n π Snk π n π = + xk Ax1 Bxk (14) (15) (16) (17) Equatons () (16) compose the separator shown n Fg. 4, by whch varous results can be acheved accordng to dfferent compensaton purposes. f the SAPF s used to compensate harmoncs and the negatve-sequence component of the fundamental current, the postve-sequence component of the fundamental current can be obtaned x from (14), and the current reference can be obtaned as * * cx x x ( n) = ( n) ( n) by subtractng from the load x current. f the lne current after compensaton s expected to be a symmetrcal three-phase fundamental current, and the power factor s 1, the actve power component of the postve-sequence fundamental current can be obtaned px A = n (4.14), and the current reference by assumng 0 * can be obtaned as cx x px from the load current. Smlarly, by settng px ( n) = ( n) ( n) by subtractng B to zero, the reactve power component of the postve-sequence fundamental current can be obtaned. The negatve-sequence component of the fundamental current can be obtaned by (15). f the APF s used to compensate the selected order harmoncs, the compensatng reference can be obtaned by (17). n fact, the actve power component and reactve power component of harmoncs do not need to be dvded, π lπ so the factors Cosk n and Snk n nkπ nkπ can be replaced wth Sn and Cos separately. Then, the programmng can be greatly smplfed. Based on the detecton methods, dfferent compensaton ams can be acheved by usng specfc combnatons. separator. The current detecton algorthm s mplemented accordng to the proposed strategy to determne the reference compensatng current. Fg. depcts the Smulnk model of the current control algorthm. t can be seen from the above analyss that the delay resultng from the proposed algorthm s less than half of the man cycle, whch s half of that of DFT and the same as that of the algorthm based on RPT. Besdes, the algorthm proposed could detect the postve/negatve-sequence fundamental current, actve/reactve power component of postve-sequence fundamental current, and selectve harmoncs expedently, whch s more flexble than the algorthm based on RPT and DFT. 8. Smulaton setup Purpose of the smulaton s to show the usefulness of the proposed SAPF control strategy. Two test cases are taken nto consderaton wth dfferent source voltages and load condtons. n case 1, the source voltages are snusodal and balanced wth a magntude of 0 V and a frequency of ω=100π and the source supples an mbalanced nonlnear load. n case, mbalanced / dstorted source voltages supply an mbalanced nonlnear load n parallel wth an mbalanced load. 8.1. Smulaton Results for Snusodal, Balanced Source Voltages The balanced and snusodal three phase voltages consdered are, V a =0 sn (ωt), V b =0 sn (ωt-10 o ), V c =50 sn (ωt+10 o ). The load used s a brdge rectfer whch acts as a nonlnear mbalanced load.the smulaton results have been plotted separately for a clear study. Fg.4, Fg 5, Fg 6, exhbt the source voltage, lne current, reference compensaton current and source current after compensaton for the three phases respectvely. Fg 6 depcts the source voltage, load current and the source current after compensaton. Smulaton results of compensaton current generated by the controller are shown n Fg. 7 Fg. 4. After compensaton phase A. 7. Smulnk Model of the mproved Algorthm The man components of the current detecton algorthm nclude a Phase Loop Lock, a sne wave generator and the 9
P.M.Balasubramanam and G. Gurusamy/Journal of Engneerng Scence and Technology Revew 5 () (01) 5-41 8.Analyss of Smulaton Results Fg 5. After compensaton phase B. The smulaton results are avalable for two cases consdered. From Fg. 8 t s clear that the SAPF njects harmonc currents nto the lne thereby makng the nput supply snusodal. The comparson of THD s gven n Table 1 for the three revewed and avalable methods namely Generalzed nstantaneous Reactve Power Theory based methods, Synchronous Reference Frame method and the Synchronous Current Detecton methods. From the results Fg. 9 t observed that for balanced source voltages the THD for the proposed method s less than the avalable methods and also the delay resultng from the proposed algorthm s less than half of the man cycle, whch s half of that of DFT and the same as that of the algorthm based on RPT. Fg. 6. After compensaton phase C. Fg. 9. THD Plot Table 1. The comparson of THD. Reference compensaton current calculaton % THD of source current balanced supply voltage GRPT method 1.87 SRF method.60 SCD Method 0.98 Proposed method 0.59 Fg. 7. Source voltage, load current and source current after compensaton. Fg. 8. Compensaton current. 9.Concluson Ths paper has outlned the mathematcal modelng and desgn of the reference compensaton current controllers for shunt actve power flters based on tme doman approach n detal. The smulaton results of the proposed method are compared wth that of the avalable results of Generalzed nstantaneous Reactve Power Theory based method, Synchronous Reference Frame method and the Synchronous Current Detecton methods. From the results t can be concluded that the delay resultng from the proposed algorthm s less than half of the man cycle, whch s half of that of DFT and the same as that of the algorthm based on RPT. From the analyss and smulaton t s found that the algorthm presented n ths thess has the advantages of flexblty, accuracy and easy mplementaton. Snce the reference compensaton currents are determned n the a-bc reference frame, there s no reference frame transformaton s requred. Therefore, t results n less complexty n realzng the control crcut of SAPF and stll mantans good flter performance. After SAPF njects the 40
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