Evaluation and Implementation of Three Phase Shunt Active Power Filter for Power Quality Improvement

Transcription

1 Internatonal Journal of Electrcal Engneerng. ISSN Volume 5, Number 7 (2012), pp Internatonal Research Publcaton House Evaluaton and Implementaton of Three Phase Shunt Actve Power Flter for Power Qualty Improvement P.M. Balasubramanam 1, G.Gurusamy 2 1 Assstant Professor, Kalagnar Karunandh Insttute of Technology, Combatore,TN,Inda. 2 Dean, Bannar Amman Insttute of Technology, Sathyamangalam, TN,Inda. 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. A current control scheme based on the tmedoman 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. Key Words Shunt actve power flter, Synchronous reference frame, Instantaneous reactve power theory, Pont of common couplng INTRODUCTION 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 [1,2]. 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

2 830 P.M. Balasubramanam and G.Gurusamy currents degrade power qualty n power transmsson/dstrbuton systems [3]. Notably, 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 [4,5,6]. 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. The smple block dagram of Fg.1 llustrates the dstorton problem due to harmonc at low and medum power levels. Fgure 1. Harmonc dstorton at PCC 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[7]. Ths results n the followng effects on the power system components 1. Malfuncton of harmonc senstve loads 2. Increased losses n parallel connected capactor, transformers and motors 3. Improper operaton of protecton relays and crcut breakers 2. Actve Flterng Technology 2.1 Basc Compensaton Prncple 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[8,9,10]. Fg. 2 shows the actve power flter compensaton prncple, whch s controlled n a closed loop manner to actvely shape the source current nto snusodal

3 Evaluaton and Implementaton of Three Phase Shunt Actve Power Flter 831 Fgure 2.Basc Compensaton prncple The nstantaneous source current s represented as Fgure 2. I S (t) = I L (t) I C (t) (1) The Instantaneous source voltage s V S (t) = V m Sn ωt (2) The load current contans the fundamental component and harmonc current components, whch s represented as [3] I L (t)= In sn( nt n ) = I1 sn( t 1 ) In sn( nt n) (3) n1 n2 The nstantaneous load power (P L ) can be computed from the source voltage and load current and the calculaton s gven as P L (t) = I S (t) * V S (t) = 2 V Sn t * Cos V I Snt * m 1 1 m 1 Cost * Sn V Snt * ( I Sn( nt )) m n n n2 = P F (t) + P R (t) + P H (t) (4) Ths load power contans fundamental actve power, reactve power and harmonc power. From Eq. (4), t s found the real fundamental power drawn from the load s 2 P F (t) = V I Sn t * Cos (5) m 1 1 If the actve power flter provdes the total reactve and harmonc power, the source current s (t) wll be n phase wth the utlty voltage and snusodal. The three

4 832 P.M. Balasubramanam and G.Gurusamy phase source currents after compensaton can be expressed as I A * = I m Sn ωt (6) I B * = I m Sn (ωt-120 ) (7) I C * = I m Sn (ωt+120 ) (8) Ths peak value of the reference current Iref s estmated by regulatng the DC-bus capactor voltage of the nverter[11]. Ths only contans the fundamental component of the load current and t s thus free from harmoncs. The njected shunt AF current completely cancels the current harmoncs from the load, resultng n harmonc free lne current [12,13]. 3. Control Strategy and Gan Adjustment 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[14,15]. 2 nk 2l 2 nk 2l ( n) Sn Sn N 3 N 3 I I x 1k 1k 2k 2k k1 (10) Commonly, only the postve-sequence, negatve-sequence, 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 ( ) 2 2l 2 2l n x1 I 11 Sn n Sn n N I N 21 3 (11) I I I I 2 2 l S n n C o s N l C o s n S n N l 2 l S n n C o s N l 2 l C o s n S n N Where, the frst term of equaton (2) corresponds to the postve-sequence component n phase wth the phase voltage, whch s called the actve power component of the postve- sequence fundamental current; the second term of equaton (2) 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 term of equaton (2) corresponds to the negatvesequence component n phase wth the lne voltage, whch s called the actve power component of the negatve-sequence fundamental current; the forth term of equaton (2) corresponds to the negatve-sequence component orthogonal wth the lne voltage, whch s called the reactve power component of the negatve sequence fundamental

5 Evaluaton and Implementaton of Three Phase Shunt Actve Power Flter 833 current where Sn 2 s synchronous wth the postve-sequence fundamental n N 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 PLL Desgn The basc confguraton of the PLL system s shown n Fgure 3. The phase voltages U as, U bs, U cs are obtaned from sampled lne-to-lne voltages. These statonary reference frame voltages are then transformed to voltages U de, U qe (n a frame of reference synchronzed to the utlty frequency) usng the 3/2 and e / s transformatons. The angle θ* used n these transformatons s obtaned by ntegratng a frequency command ω*. If the frequency command ω* s dentcal to the utlty frequency, the voltages U de and U qe, appear as dc values dependng on the angle θ*. Fgure 3.Input phase voltage and PLL output In the gven method, a PI regulator s used to obtan that value of θ* (or ω*) whch drves the feedback voltage U de to a commanded value U de *. In other words, the regulator results n a rotatng frame of reference wth respect to whch the transformed voltage U de has the desred dc value U de *[16-19]. The frequency of rotaton of ths reference frame s dentcal to the frequency of the utlty voltage. The Magntude of the controlled quantty U de determnes the phase dfference between the utlty voltages and sn(θ*) or cos(θ*). The method results not only n the utlty frequency ω* but also allows one to lock at an arbtrary phase angle θ* wth respect to the utlty angle θ. The angle Δθ s controlled by the commanded values U de * 3.2 Gan Set Up The control problem reduces to pckng the correct gans for the model for varous operatng condtons. Takng the samplng delay nto account, the plant s a smple lag along wth an ntegratng element H plant 1 st s U s 1 (12) Where T, s the samplng tme. The open-loop transfer functon H 0l wth the

6 834 P.M. Balasubramanam and G.Gurusamy controller then becomes 1 s T pll H o1 K pll st pll 1 1 s T s U s (13) Where K pll, T pll are the gans assocated wth the PI 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 1 c T s (14) 2 T pll T s K 1 1 U T pll s Substtutng (14) nto (13) t can be shown that the factor α and the dampng factor ξ are related by the relatonshp 1 2 By changng α, the system bandwdth and dampng can be controlled. The control dagram of the PLL s shown n Fgure 4. When the reference ude * 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, ths phase dfference wll not affect the valdty of the selected harmoncs detecton. The phase voltage s expressed by per-unt; the base quanttes for per-unt value are the peak value of postve-sequence fundamental phase voltage. Then, three phase voltages can be expressed as 2 2l Sn n,the phase voltage s multpled by N 3 fundamental current (t) Smlarly, the nstantaneous power of harmoncs can be obtaned by multplyng 2 2l current by Sn n N 3 It 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. 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

7 Evaluaton and Implementaton of Three Phase Shunt Actve Power Flter 835 twce the fundamental frequency or by a sldng-wndow wth N/2 samples. Then, multplyng by 2, the followng equaton can be obtaned: Cos Cos 4l 3 (15) B x1 I I A 4 1 I Sn 11 Sn 11 I 21 l x 21 3 (16) Then, defne x11 x 21 x1 xk A 11 A A 2 2l Cos n N 3 2 2l Cos n N 3 2 2l Cos n N 3 2 2l n N 3 21 x1 B B 2 2l Sn n 11 N 3 2 2l Sn n N 3 2 2l Sn n x1 N 3 2 2l Snk n N 3 21 B A Cos x1 Bxk (17) (18) (19) (20) * * ( n) ( n) ( n) by subtractng from the load current. If the lne cx x x11 x11 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 postvesequence fundamental current can be obtaned by assumng px11 A 0 n (17), and 11 * the current reference can be obtaned as ( n) ( n) ( n) by subtractng cx x px11 px11 from the load current. Smlarly, by settng to zero, the reactve power B11 component of the postve-sequence fundamental current can be obtaned. The negatve-sequence component of the fundamental current can be obtaned by (18). If the APF s used to compensate the selected order harmoncs, the compensatng reference can be obtaned by (20). In fact, the actve power component and reactve power component of harmoncs do not need to be dvded, so the factors 2 2l Cosk n and 2 2l Snk n can be replaced wth 2 nk Sn and 2 nk Cos, N 3 N 3 separately. Then, the programmng can be greatly smplfed. Based on the detecton methods, dfferent compensaton ams can be acheved by usng specfc combnatons. N N 4. Model of the current Control Scheme The man components of the current detecton algorthm nclude a Phase Loop Lock, a sne wave generator and the separator. The current detecton algorthm s mplemented accordng to the proposed strategy to determne the reference compensatng current. Fg. 4 depcts the Smulnk model of the current control algorthm. It 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 IRPT. Besdes, the algorthm proposed could

8 836 P.M. Balasubramanam and G.Gurusamy 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 IRPT and DFT. Fgure 4. Smulaton dagram of the current control algorthm 5. 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. In case 1, the source voltages are snusodal and balanced wth a magntude of 230 V and a frequency of ω=100π and the source supples an mbalanced nonlnear load Smulaton Results for Snusodal, Balanced Source Voltages The balanced and snusodal three phase voltages consdered are, V a =230 sn (ωt) V b =230 sn (ωt-120 o ) V c =250 sn (ωt+120 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.5, Fg 6, Fg 7, exhbt the source voltage, lne current, reference compensaton current and source current after compensaton for the three phases respectvely. Smulaton results of compensaton current generated by the controller are shown n Fg. 8 Fg 9 depcts the source voltage, load current and the source current after compensaton.

9 Evaluaton and Implementaton of Three Phase Shunt Actve Power Flter 837 Fgure 5. Source current for Phase A after compensaton Fgure 6. Source current for Phase B after compensaton Fgure 7. Source current for Phase C after compensaton

10 838 P.M. Balasubramanam and G.Gurusamy Fgure 8.Compensaton Current Fgure 8.Source Current After compensaton Fgure 9. THD Plot

11 Evaluaton and Implementaton of Three Phase Shunt Actve Power Flter Analyss of Smulaton Results The smulaton results are avalable for balanced source voltage 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 fgure 10 for the three revewed avalable methods namely Generalzed Instantaneous Reactve Power Theory based methods, Synchronous Reference Frame method and the Synchronous Current Detecton methods. From the results of Fg. 10 t s 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 IRPT. Fgure 10.THD Comparson 7. 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 Instantaneous 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 IRPT. 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-b-c 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 compensaton currents, t s found that the source currents become deal and reman n phase wth the postve-sequence fundamental source voltages. Therefore, the utlty source power factor at the postve sequence fundamental

12 840 P.M. Balasubramanam and G.Gurusamy frequency s acheved and the harmonc currents are well controlled. The Total Harmonc Dstorton (THD) study reveals that the proposed method has a source current THD less than the avalable methods. System Data Load Parameters R= 50Ω, L= 100mH, Rs = 0.5Ω, L= 1e-4 H Flter Secton DC sde Capactor = 800μF, Flter Inductance =3mH V Ref =800V Acknowledgement The authors are grateful to S.Sangeetha and Master Support. B.S.Anujayen for valuable REFERENCES [1] H. Akag, Y. Kanazawa, and A. Nabae, Generalzed theory of the nstantaneous reactve power n three-phase crcuts, n Proc. IEEJ Int. Power Electron. Conf., Tokyo, Japan, pp ,1983. [2] H. Akag and A. Nabae, Instantaneous reactve power compensators comprsng swtchng devces wthout energy storage components, IEEE Trans. Ind. Appl., vol. 20, no. 2, pp , Mar./Apr [3] A. A. Grgs, W. B. Chang, and E. B. Makram, A dgtal recursve measurement scheme for on-lne trackng of power system harmoncs, IEEE Trans. Power Del., vol. 3, pp , Jul [4] H. Akag, New trends n actve flters for power condtonng, IEEE Trans. Ind. Appl., vol. 32, no. 3, pp , May/Jun [5] G.Chen,Y. Chen and K.M. Smedley, Three-phase four-leg actve power qualty condtoner wthout references calculaton, n Procd. IEEE APEC '04, vol.1, pp , [6] Vadrajacharya Kumar, P. Agarwal and H.O.Gupta, A Smple Control Strategy For Unfed Power Qualty Condtoner Usng Current Source Inverter, n Procd. IPEC2007, pp [7] K. Vswanathan, D. Srnvasan, and R. Orugant, Desgn and analyss of SISO fuzzy logc controller for power electronc converters, n Proc. IEEE Int. Conf. Fuzzy Syst., 2004, vol. 3, Jul , pp [8] S. K. Jan, P. Agrawal, and H. O. Gupta, Fuzzy logc controlled shunt actve power flter for power qualty mprovement, Proc. Inst. Elect Eng., Electr. Power Appl., vol. 149, no. 5, 2002.

13 Evaluaton and Implementaton of Three Phase Shunt Actve Power Flter 841 [9] J. M. Correa, F. A. Farret, J. R. Gomes, and M. G. Smoes, "Smulaton of Fuel Cell Stacks Usng a Computer Controlled Power Rectfer wth the Purposes of Actual Hgh Power Injecton Applcatons", IEEE Trans. Ind. Applcat., Vol. 39, No. 4, pp , [10] N. J. Bershad, M. Bermudez, and J.Y.Tourneret, An affne combnaton of two LMS adaptve flters-transent mean-square analyss, IEEE Trans. Sgnal Precessng, vol. 56, pp , May [11] Gao Yng, Xe Shengl, A varable step sze LMS adaptve flterng algorthm and ts analyses, Acta Electronca Snca, vol. 29, pp ,August [12] G. Escobar, P. Mattavell, and A. Stankovc, An adaptve control for UPS to compensate unbalance and harmonc dstorton usng a combned capactor/load current sensng, IEEE Trans. Ind. Electron, vol. 54, pp , Aprl [13] B. M. Han, B. Y. Bae, and S. J. Ovaska, Reference sgnal generator for actve power flters usng mproved adaptve predctve flter, IEEE Trans. Industral Electroncs, vol. 52, pp , [14] Hongyu L, Fang Zhuo, Zhaoan Wang, et al. A Novel Tme-Doman Current- Detecton Algorthm for Shunt Actve Power Flters, IEEE Trans. Power System, 17(2), pp: , [15] Lu Hu, Lu Guoha, and Shen Yue, A novel real-tme harmonc detecton method usng fast lftng wavelet transform, Journal of Jangsu Unversty(Natural Scence Edton), vol. 30, pp , May [16] M. EI-Habrouk and M. K. Darwsh, Desgn and mplementaton of a modfed Fourer analyss harmonc current computaton technque for power actve flters usng DSP s,, Proc. Inst. Elect. Eng. Elect. Power Appl., vol. 148, no. 1, pp , Jan [17] V. Kaura and V. Blasko, Operaton of a phase locked loop system under dstorted utlty condtons, IEEE Trans. Ind. Appl., vol. 33, no. 3, pp.58 63, May/Jun [18] da Slva, Sergo A. Olvera; Tomzak, Edgar; Novochadlo, Rhodolfo;Antono, Ernane; Coelho, Alves, PLL Structures for Utlty Connected Systems under Dstorted Utlty Condtons, IEEE Industral Electroncs,IECON 2006, pp , Nov [19] Se-Kyo Chung, A phase trackng system for three phase utlty nterface nverters, IEEE Trans. Power Electron., vol 15, pp , May

14 842 P.M. Balasubramanam and G.Gurusamy