Aalborg Unverstet Hybrd Actve Flter wth Varable Conductance for Harmonc Resonance Suppresson n Industral Power Systems Lee, Tzung-Ln; Wang, Yen-Chng; L, Jan-Cheng; Zapata, Josep Mara Guerrero Publshed n: I E E E Transactons on Industral Electroncs DOI (lnk to publcaton from Publsher):.9/TIE.4.478 Publcaton date: 5 Document Verson Early verson, also known as pre-prnt Lnk to publcaton from Aalborg Unversty Ctaton for publshed verson (APA): Lee, T-L., Wang, Y-C., L, J-C., & Guerrero, J. M. (5). Hybrd Actve Flter wth Varable Conductance for Harmonc Resonance Suppresson n Industral Power Systems. I E E E Transactons on Industral Electroncs, 6(), 746-756. https://do.org/.9/tie.4.478 General rghts Copyrght and moral rghts for the publcatons made accessble n the publc portal are retaned by the authors and/or other copyrght owners and t s a condton of accessng publcatons that users recognse and abde by the legal requrements assocated wth these rghts.? Users may download and prnt one copy of any publcaton from the publc portal for the purpose of prvate study or research.? You may not further dstrbute the materal or use t for any proft-makng actvty or commercal gan? You may freely dstrbute the URL dentfyng the publcaton n the publc portal? Take down polcy If you beleve that ths document breaches copyrght please contact us at vbn@aub.aau.dk provdng detals, and we wll remove access to the work mmedately and nvestgate your clam. Downloaded from vbn.aau.dk on: december, 8
Hybrd Actve Flter wth Varable Conductance for Harmonc Resonance Suppresson n Industral Power Systems Abstract Unntentonal seres and/or parallel resonances, due to the tuned passve flter and the lne nductance, may result n severe harmonc dstorton n the ndustral power system. Ths paper presents a hybrd actve flter to suppress harmonc resonance and reduce harmonc dstorton as well. The proposed hybrd flter s operated as varable harmonc conductance accordng to the voltage total harmonc dstorton, so harmonc dstorton can be reduced to an acceptable level n response to load change or parameter varaton of power system. Snce the hybrd flter s composed of a seventh-tuned passve flter and an actve flter n seres connecton, both dc voltage and kva ratng of the actve flter are dramatcally decreased compared wth the pure shunt actve flter. In real applcaton, ths feature s very attractve snce the actve power flter wth fully power electroncs s very expensve. A reasonable trade-off between flterng performances and cost s to use the hybrd actve flter. Desgn consderaton are presented and expermental results are provded to valdate effectveness of the proposed method. Furthermore, ths paper dscusses flterng performances on lne mpedance, lne resstance, voltage unbalance and capactve flters. KEYWORDS Hybrd actve flter, harmonc resonance, ndustral power system v s s L L s R s L f C f R f C dc v dc vdc e e e qd e e qd,h e h ω h h f G k p k NOMENCLATURE source voltage source current load current flter current source nductor source resstor flter nductor flter capactor flter resstor dc capactor of the hybrd flter dc voltage of the hybrd flter dc voltage command termnal voltage termnal voltage n the SRF termnal harmonc voltage n the SRF termnal harmonc voltage harmonc frequency n radan harmonc current command fundamental current command current command conductance command proportonal gan of the tunng control ntegral gan of the tunng control K c THD I h E(s) I(s) I (s) proportonal gan of the current controller voltage THD command flter harmonc current ampltude termnal voltage n s-doman flter harmonc n s-doman flter harmonc command n s-doman I. INTRODUCTION Harmonc polluton s becomng ncreasngly serous due to extensve use of nonlnear loads, such as adjustable speed drves, unnterruptble power supply systems, battery chargng system, etc. These equpment usually uses dode or thyrstor rectfers to realze power converson because of lower component cost and less control complexty. However, the rectfers wll contrbute a large amount of harmonc current flowng nto the power system and the resultng harmonc dstorton may gve rse to malfuncton of senstve equpment or nterferng wth communcaton systems n the vcnty of the harmonc sources. Normally, tuned passve flters are deployed at the secondary sde of the dstrbuton transformer to provde low mpedance for domnant harmonc current and correct power factor for nductve loads [], []. However, due to parameter varatons of passve flters, unntentonal seres and/or parallel resonances may occur between the passve flter and lne nductance. The functonalty of the passve flter may deterorate and excessve harmonc amplfcaton may result [], [4]. Thus, extra calbratng work must be consumed to mantan the flterng capablty. Varous actve flterng approaches have been presented to address the harmonc ssues n the power system [5], [6], [7]. The actve flter ntended for compensatng harmonc current of nonlnear loads s the most popular one, but t may not be effectve for suppressng harmonc resonances [8]. Bhattacharya et al. proposed a hybrd seres actve flter to solate harmoncs between the power system and the harmonc source [9]. A so-called actve-nductance hybrd flter was presented to mprove the performance of the passve flter []. Fujta et al. proposed a hybrd shunt actve flter wth flter-current detectng method to suppress the ffth harmonc resonance between the power system and a capactor bank []. A hybrd flter n seres wth a capactor bank by a couplng transformer was proposed to suppress the harmonc resonance as well as to compensate harmonc current [], []. However, ths method needs extra matchng transformers or tuned passve flters to guarantee flterng functonalty. Recently, a transformerless hybrd actve flter was presented to compensate harmonc current and/or fundamental reactve
current [4], [5], [6], [7], [8]. Desgn consderaton of the hybrd flter for current compensaton has been extensvely studed. Hybrd actve flter wth dampng conductance was proposed to suppress harmonc voltage propagaton n dstrbuton power system [9]. Nevertheless, ths method dd not consder the resonance between the passve flter and the lne nductance. The fxed conductance may deterorate the dampng performances. An ant-resonance hybrd flter for delta-connected capactor bank of power-factor-correcton applcatons was presented []. Ths crcut was lmted to three sngle-phase nverters and the flterng performance was not consdered. In addton, the hybrd actve flter was proposed for the unfed power qualty condtoner to address PQ ssues n the power dstrbuton system []. Several case studes of the hybrd actve flter consderng optmal voltage or current dstorton were conducted n []. In prevous work, the authors have presented a transformerless hybrd actve flter to suppress harmonc resonances n the ndustral power system n [], [4]. The hybrd flter s constructed by a seventh-tuned passve flter and an actve flter n seres connecton. It operates as a varable conductance at harmonc frequences accordng to the voltage total harmonc dstorton (THD), so that harmonc dstorton can be reduced to an acceptable level n response to load change and power system varaton. Snce the seres capactor s responsble for sustanng the fundamental component of the grd voltage, the actve flter s able to operate wth a very low dc bus voltage, compared wth the pure shunt actve flter [4], [9]. Hence, both the rated kva capacty and the swtchng rpples are reduced accordngly. Moreover, the proposed harmonc conductance s able to avod overcurrent of the passve flter n case of mstunng parameters. These features wll beneft practcal applcatons. In ths work, we further present desgnng consderaton of the hybrd flter. A prototype crcut of the hybrd flter based on V/kVA system has been establshed to verfy theoretc analyss, ncludng steady-state behavor, transent response and stablty analyss. The flterng performance of the hybrd flter s dscussed consderng X/R rato and magnfed varatons of lne mpedance. We also focus on flterng deteroraton due to lne resstance, voltage unbalance and capactve flters n the power system. In many cases, actve power flter s desgned to compensate harmonc current produced by a specfc nonlnear load, n such a way that t needs to measure the load current to be compensated [4], [5]. In ths paper, the actve flter s desgned as a harmonc conductance to suppress harmonc resonance and harmonc dstorton as well by usng local voltage measurement. Notce that t does not requre current nformaton of the nonlnear loads. Thus ths approach can be sutable as well n power dstrbuton networks n whch the loads can be dstrbuted along a feeder [9]. In addton, compensatng fundamental reactve power due to unbalanced load s possble, but t s out of scope of ths research [5], [6]. II. OPERATION PRINCIPLE Fg. (a) shows a smplfed crcut dagram consdered n ths paper, where L s represented the lne nductance plus the e abc ω vsa v sb vsc abc to qd e v dc Ls v dc Rs Harmonc loop e e qd sa sb sc ec e b c b La ea Lb Lc a C f Passve Flter R f L f (a) Crcut dagram of the HAFU. e e qd,h HPF PI e q,f = ω e d,f ω Fundamental loop qd e to abc qd e to abc e abc e h e h Fg. G h f HAFU (b) Control block dagram of the HAFU. Kc v Nonlnear Load Lnear Load v dc PWM HAFU Fg.. The proposed hybrd actve flter unt (HAFU) n the ndustral power system and ts assocated control. leakage nductance of the transformer. The hybrd actve flter unt (HAFU) s constructed by a seventh-tuned passve flter and a three-phase voltage source nverter n seres connecton. The passve flter L f C f s ntended for compensatng harmonc current and reactve power. The nverter s desgned to suppress harmonc resonances and mprove the flterng performances of the passve flter. Fg. (b) shows the overall control block dagram of the HAFU, ncludng harmonc loop, fundamental loop, current regulator, and conductance control. Detaled prncple wll be presented as follows. A. Harmonc loop In order to suppress harmonc resonances, the HAFU s proposed to operate as varable conductance at harmonc frequences as gven, h = G e h () where h represents the harmonc current command. The conductance command G s a varable gan to provde dampng for all harmonc frequences. Harmonc voltage component e h s obtaned by usng the so-called synchronous reference frame (SRF) transformaton [9], where a phase-locked loop (PLL) s realzed to determne the fundamental frequency of the power system [7]. In the SRF, the fundamental component becomes a dc value and other harmonc components are stll ac values. Therefore, harmonc voltage componente e qd,h can be extracted from e e qd by usng hgh-pass flters (HPFs). After transferrng back to three-phase system, the harmonc current command h s obtaned by multplyng e h and the conductance command G as shown n ().
B. Fundamental loop In ths paper, the q axs s algned to a-phase voltage. Snce the passve flter shows capactve at the fundamental frequency, the passve flter draws fundamental leadng current from the grd, whch s located on the d axs. The proposed nverter produces slght fundamental voltage on the d axs, whch s n phase wth the fundamental leadng current. Therefore, the control of dc bus voltage s able to be accomplshed by exchangng real power wth the grd. Thus the current command e d,f s obtaned by a proportonal-ntegral (PI) controller. The fundamental current command f n the three-phase system s generated after applyng the nverse SRF transformaton. () shows harmonc voltage drop on the passve flter due to compensatng current of the HAFU [9], where I h represents the maxmum harmonc current of the actve flter and voltage drop on flter resstance R f s neglected. As can be seen, large flter capactor results n reducton of requred dc voltage. On the other hand, the flter capactor determnes reactve power compensaton of the passve flter at the fundamental frequency. Thus the dc voltagevdc can be determned based on ths compromse. Note that the compensatng current should be lmted to ensure that the hybrd flter operates wthout undergong saturaton. I (s) I(s) Fg.. v dc > h K c e st Current Computatonal Controller Delay +jω h L f I h. () jω h C f e st PWM E(s) Closed-loop model of the current control. C. Current regulator sl f+ sc f +R f Passve Flter The current command s conssted of h and f. Based on the current command and the measured current, the voltage command v can be derved by usng a proportonal controller as follows, I(s) v = K c ( ) () where K c s a proportonal gan. Accordng to the voltage commandv, the space vector PWM s employed to synthesze the requred output voltage of the nverter. Fg. shows the model of the current control. Computatonal delay of dgtal sgnal processng s equal to one samplng delay T and PWM delay approxmates to half samplng delay T. Hence, the proportonal gan K c can be smply evaluated from both open-loop and closed-loop gans for sutable stablty margn and current trackng capablty. Frequency-doman analyss of current control wll be gven n the expermental secton. e a,h e b,h e c,h e a e b e c Fg.. LPF LPF SQRT SQRT Conductance control block dagram. D. Conductance control THD THD Fg. shows the proposed conductance control. The harmonc conductance command G s determned accordng to the voltage THD at the HAFU nstallaton pont. The voltage THD s approxmately calculated by the control shown n Fg.. Here, two low-pass flters (LPFs) wth cut-off frequency f LP =Hz are realzed to flter out rpple components [8], [9]. The error between the allowable THD and the measured THD s then fed nto a PI controller to obtan the harmonc conductance command G. The allowable dstorton could be referred to the harmonc lmt n IEEE std. 59-99 []. Note that PI parameters need to be tuned for requred response and stablty. For example, the proportonal gan can be tuned for transent behavor and the ntegral gan s responsble for suppressng the steady-state error. The bandwdth should be lower than one-tenth of the cut-off frequency of the current loop to assure stable operaton. Ths way the HAFU s able to dynamcally adjust G to mantan harmonc dstorton at an allowable level. III. ANALYSIS OF FILTERING PERFORMANCE The flterng performance of the HAFU has been addressed n [4] by developng equvalent crcut models, n whch both harmonc mpedance and harmonc amplfcaton are consdered. The frequency characterstc of the passve flter s changed by the proposed harmonc conductance n order to avod unntentonal resonances. In ths secton, we wll concentrate on the dampng performance wth varaton of lne mpedance L s, lne resstance R s and THD. Voltage unbalance and flter capactors n the power system are also consdered. A. L s on dampng performances Fg. 4 shows voltage THD for varous values of L s. Ffth harmonc voltage s severely amplfed at L s =.mh(.%) as shown n Fg. 4(a). Ths resonance s allevated f L s s not equal to.%. However, voltage dstorton s stll sgnfcant due to harmonc voltage drop on L s. After the HAFU s started, Fg. 4(b) shows voltage dstorton s mantaned at % by ncreasng G as shown n Fg. 4(c). It s worth notng that the HAFU s operated at ant-resonance mode,.e. G =, f L s s less than.% for NL. Ths means that the voltage dstorton s less than %. At that tme, a lower THD command s needed to further reduce the current dstorton of s. PI G
4 8 THD 6 4 NL NL 4.5.5 THD.5.5 HAFU OFF HAFU ON..4.6.8...4.6 L s (a) Voltage THD at e when the HAFU s off.....4.5.6.7.8.9..5 R s (a) Voltage THD at e..5 NL NL.5.5 THD.5 G.5.5.5..4.6.8...4.6 L s (b) Voltage THD at e when the HAFU s on.....4.5.6.7.8.9. R s (b) G when the HAFU s on. Fg. 5. Voltage THD(%) and the requred G (pu) wth varyng lne resstance R s(pu). G.5.5.5 NL NL B. R s on dampng performances In the low-voltage system, X/R rato becomes lower and lne resstance on dampng performances must be taken nto consderaton. Fg. 5(a) shows voltage dstorton wth varyng R s for NL. Snce ncreasng R s could help n reducng voltage dstorton, the requred conductance to mantan voltage dstorton at % s accordngly reduced as shown n Fg. 5(b). From ths observaton, the HAFU could provde effectve dampng capablty even though R s s as large as %..5..4.6.8...4.6 L s (c) G when the HAFU s on. Fg. 4. Voltage THD(%) and the requred G (pu) wth varyng lne mpedance L s(pu). C. Determnaton of THD Accordng to IEEE std. 59-99 [], voltage THD s lmted to 5% and ndvdual dstorton should be below 4%. Thus THD s set n the range of % and 5%. If v s,h and R s are neglected, voltage THD at E, due to harmonc current load I h, can be expressed as follows, THD = X pu h (h I h,pu ). (4) 4
X represents the seres mpedance of both L s and leakage nductance of transformer. In ths secton, we wll consder three cases n TABLE I to llustrate how to determne voltage THD, where only ffth and seventh harmoncs are consdered. In the frst one, ffth harmonc s domnant, so THD lower than.x pu s a suffcent condton to confrm wth the harmonc lmt. If ffth and seventh harmoncs have the same dstorton, THD =.X pu s acceptable. When seventh harmonc becomes crtcal, THD =.X pu works n the thrd case. Therefore, the frst case s the crtcal one to determne the requred THD. Note that THD should be reduced to enhance flterng capablty n case of low system mpedance....5.5 TABLE I CALCULATION OF THD. I 5,pu I 7,pu THD Case.4..X pu Case.5.5.X pu Case..4.X pu (a) Harmonc mpedance. D. Capactve flters In power electronc equpment, low-pass flters or EMI flters are usually nstalled at the grd sde of the nverter to allevate swtchng rpples nto the power system. Snce these flters present capactve characterstcs, harmonc resonances may unntentonally occur [], [], [], [4], [5]. Ths scenaro becomes much more sgnfcant n the so-called mcrogrd system because a large number of output flters nstalled by the nverter-based dstrbuted generators may partcpate n resonances [6]. Fg. 6 shows harmonc mpedance and source current amplfcaton for dfferent capactors C e nstalled at PCC. As can be seen, C e shfts the resonant frequency and nduce another hgh-frequency resonance, whch may result n serous harmoncs. Smulaton results n Fg. 7 show amplfcaton of E h and s,h can be effectvely suppressed by the proposed hybrd flter. Note that flterng capablty s dependent on the bandwdth of the HAFU...5..5 (b) Source current amplfcaton. E. Voltage unbalance Voltage unbalance n low-voltage system s usually sgnfcant due to hgh lne mpedance and uneven dstrbuton of sngle-phase loads [7]. Large unbalance may cause secondorder harmoncs n executng SRF control of the HAFU. In ths sense we need to add a band-rejected flter tuned at the second order harmonc frequency n Fg. to reduce ths unwanted component. We also can use second-order generalzed ntegrator (SOGI)-based methods to separate negatve-sequence component [8] n the proposed control. It s worth nothng that unbalanced voltage or unbalanced current s possble to be compensated by the proposed HAFU. In ths case, the HAFU has to generate fundamental negatve-sequence voltage. Ths ssue s open for further research. IV. EXPERIMENTAL VERIFICATION A power stage setup was bult and tested as shown n Fg.8. TABLE II gves expermental parameters based on the per Fg. 6. Harmonc amplfcaton consderng dfferent passve flter capactors C e(.5,.,.5,. pu). unt system n TABLE III. VD 5 and VD 7 represent ffth and seventh voltage dstorton n the laboratory. The flter capactor C F s desgned to compensate nductve load and the flter nductor L F s chosen so that the LC flter s resonant at seventh order harmonc frequency. The dc lnk capactor s based on the allowed voltage rpple (5%). The control of the hybrd flter was mplemented by the evaluaton platform of TMSF85 chp [9] to perform the phase-locked loop, the synchronous reference frame transformaton, lowpass flters, PI controllers, current regulator, A/D converson and PWM unts. Note that the OFF state of the HAFU corresponds to turnng on three upper swtches and turnng off three lower swtches, whch means the three phases of the nverter are short-crcuted. At ths moment the HAFU works as a pure passve flter. 5
TABLE II EXPERIMENTAL PARAMETERS -5 E / Ih Compare P.U. Power system V(L-L), 6 Hz, VD 5 =.7%, VD 7 =.5% Transformer /7 V, kva, mpedance 5% Resstve load kw(%) Nonlnear load NL =.8kW(8%), NL =.8kW(8%) Passve flter L f =.mh(7.8%), C f = 5µF(7%) Q f = Swtchng frequency khz Samplng frequency khz Current control k c=5v/a DC voltage control k p=a/v, k =A/(V s), vdc =5V Tunng control k p=a/v, k =5A/(V s), THD =.% f HP =Hz, f LP =Hz Magntude (db) - -5 - -5 -.5..5. Voltage kva Impedance Conductance TABLE III BASE VALUE V kva 4.84 Ω.7Ω -5 4 Frequency (Hz) (a) Harmonc mpedance. Is / Ih Compare P.U. TABLE IV HARMONIC DISTORTION FOR NL =.8KW. - - (a) Voltage dstorton of e. THD HD 5 HD 7 HD HD HAFU OFF.9%.7%.%.5%.4% HAFU ON.%.6%.9%.5%.4% (b) Current dstorton of s. THD HD 5 HD 7 HD HD HAFU OFF 8.5% 8.%.7%.%.% HAFU ON 4.8%.7%.8%.4%.4% (c) Current dstorton of L. THD HD 5 HD 7 HD HD HAFU OFF 9.% 7.% 4.%.8%.6% HAFU ON 9.% 7.7%.4%.5%.5% (d) Current dstorton of. THD HD 5 HD 7 HD HD HAFU OFF 7% 5% 9.%.7%.% HAFU ON % 8.9% 6.7%.7%.% TABLE V HARMONIC DISTORTION FOR NL =.8KW. (a) Voltage dstorton of e. THD HD 5 HD 7 HD HD HAFU OFF 4.6% 4.5%.%.6%.4% HAFU ON.%.6%.8%.5%.% (b) Current dstorton of s. THD HD 5 HD 7 HD HD HAFU OFF 7% 6%.%.6%.% HAFU ON 4.%.%.%.4%.4% (c) Current dstorton of L. THD HD 5 HD 7 HD HD HAFU OFF % % 6.5%.6%.% HAFU ON 4% % 5.5%.7%.% (d) Current dstorton of. THD HD 5 HD 7 HD HD HAFU OFF 4% 4% % 4.5%.7% HAFU ON % 7% 9.% 5.6%.% Magntude (db) - -4-5 -6 4 Frequency (Hz) (b) Source current amplfcaton. Fg. 7. Dampng performances for dfferent passve flter capactors C e(.5,.,.5,. pu) wth G =. pu. V 6Hz VD 5 =.7% VD 7 =.5% Fg. 8. /7 V kva 5% Expermental setup. A. Three-phase load s Ls L e mh.5. 5µF.mH.5 kw. NL =.8 kw NL =.8 kw v dc mf Fg. 9 and Fg. show the grd voltage e, the source current s, the flter current, and the load current L for NL =.8kW and NL =.8kW, respectvely. When the HAFU s n the OFF state, the HAFU becomes a passve flter. Snce the resonant frequency between the passve flter L f C f and lne nductance L s s close to ffth harmonc frequency, ffth 6
e s L e s L (a) The HAFU s off. (b) The HAFU s on. Fg. 9. Lne voltage e, source current s, load current L, and flter current n case of NL ntated. X axs: 5 ms/dv. e s L harmonc dstorton on e, s, f are sgnfcantly amplfed as shown n Fg. 9(a) and Fg. (a). As can be seen, the passve flter loses ts flterng functonalty and even causes excessve harmonc current n s or harmonc voltage on e. It s worth notng that the resonant frequency could be shfted toward the lower frequency due to exstng of the leakage nductance of the transformer. After the start of the HAFU, the harmonc dstorton s clearly mproved as shown n Fg. 9(b) and Fg. (b). THD of e s reduced to.% wth G =.97 pu for NL and G =.5 pu for NL, respectvely. THD of s s also mproved below 5% n both cases. TABLE IV and TABLE V summarze THD data of e, s,, L measured by a power qualty analyzer (HIOKI 96). Hgh-order harmoncs (> ) are not ncluded here due to nsgnfcance. Seventh harmonc voltage dstorton s ncreased after the HAFU s started. Ths s because the HAFU emulates conductance for all harmonc frequences. Ths feature can be used to avod the overloadng of the passve flter at the tuned (seventh) frequency. We also observe that ffth harmonc component of load current L s slghtly ncreased. Ths may result from mprovement of ffth voltage dstorton on e. The detaled results ndcate that the proposed HAFU s able to suppress harmonc resonances as well as reduce harmonc dstorton. More mportantly, the HAFU only consumes 47 VA, whch s approxmately 4.7% of the system ratng or 6.7% of NL. Obvously, the requred kva ratng of the flter s sgnfcantly reduced, n comparson wth the use of a pure shunt actve power flter. Fg. shows the transent waveforms of G, THD of e, v dc as the nonlnear load s changed by a stepped ncrease from NL to NL at T. Large nonlnear current wll result n large voltage dstorton on e. Thanks to the proposed tunng control, the conductance command G s ncreased to.5 pu to draw more harmonc current shown n Fg. (b) n order to mantan voltage THD at %. Fg. (a) also demonstrates v dc s well controlled to 5V to ensure proper operaton of the actve flter. (a) The HAFU s off. e s L (b) The HAFU s on. Fg.. Lne voltage e, source current s, load current L, and flter current n case of NL ntated. X axs: 5 ms/dv. B. Comparson wth current-compensatng method Addtonally, tme-doman smulatons have been carred out to compare flterng performances between currentcompensatng and voltage-dampng hybrd actve flters. In the current-compensatng case, the load current s measured and harmonc components are extracted by usng synchronous reference frame transformatons. In Fg., source current THDs and ndvdual harmonc dstortons are gven for both lght and heavy nonlnear load condtons. As shown, both methods are able to reduce source current dstorton and ther flterng performances are smlar. Further, by usng the proposed approach, a small voltage THD value can provde even better flterng results, e.g. THD <.8%. In [5], currentcompensatng hybrd actve flter has been presented. Expermental results from[5] show that source current dstorton can be reduced from 4.% to 4.%, whch s smlar to the results of the proposed voltage-dampng hybrd flter as gven n TABLE V. 7
v dc T 5V Current Dstorton and current THD n Lght Load condton Current dstorton (%) 6.5 G.97 % % THD 5 4 THD*=.5 THD*=.6 THD*=.8 THD*=. Current Type (a) Waveforms of v dc, Voltage THD, G. X axs: ms/dv; Y axs: v dc (V), G (. pu/dv), THD(.5 %/dv) THD 5TH 7TH TH TH Indvdual Harmonc Order (a) Current dstorton n lght load condton. Current Dstorton and current THD n Heavy Load condton Current dstorton (%) 6 5 4 THD*=.5 THD*=.6 THD*=.8 THD*=. Current Type (b) Current waveforms. Fg.. Transent response when the nonlnear load s ncreased at T. THD 5TH 7TH TH TH Indvdual Harmonc Order (b) Current dstorton n heavy load condton. C. Sngle-phase load In addton, flterng experment consderng sngle-phase nonlnear load s conducted. The setup of three-phase dode rectfer s changed to sngle-phase one by addng a smooth dc capactor 56 µf. Snce the nonlnear load s connected between a- and b-phase, large thrd-order harmonc current s generated between them. As shown n Fg., harmonc current s amplfed between the source current s and the flter current. After the HAFU s started, harmonc resonance s suppressed and current dstorton s reduced as ndcated n Fg. 4. Test results are summarzed n TABLE VI. Voltage dstorton ofes reduced from 4.6% to.% wth conductance command G =.5 pu. Snce the passve flter s tuned at the seventh-order harmonc frequency, the proposed hybrd flter s not able to suppress thrd-order harmonc dstorton effectvely for sngle-phase nonlnear load. In ths case, the passve flter mght be tuned at ffth-order harmonc frequency to mprove flterng performance for thrd-order harmonc. D. Stablty analyss The open-loop gan of the current control can be obtaned accordng to Fg.. As shown n Fg. 5(a), the resonant peak s due to the passve flter. In ths paper, the proportonal gan K c s chosen so that the bandwdth s approxmately 97Hz wth phase margn 8 degree. Closed-loop gan n Fg. 5(b) also demonstrates that current trackng performance s acceptable for low-order harmonc frequences. However, Fg.. Flterng comparson between current-compensatng and voltagedampng hybrd actve flters. TABLE VI HARMONIC DISTORTION FOR SINGLE-PHASE NONLINEAR LOAD. (a) Voltage dstorton of e. e ab e bc e ca HAFU OFF 6.% 4.%.4% HAFU ON 4.%.6%.8% (b) Current dstorton of s. sa sb sc HAFU OFF 9% 8%.8% HAFU ON % 6%.5% (c) Current dstorton of L. La Lb Lc HAFU OFF % %.% HAFU ON % %.% (d) Current dstorton of. a b c HAFU OFF % 5% 6.4% HAFU ON % % 4.7% the current control can be further mproved by usng the socalled resonant controller [4], [4]. V. CONCLUSION Ths paper presents a hybrd actve flter to suppress harmonc resonances n ndustral power systems. The proposed 8
(a) Termnal voltage. (a) Termnal voltage. (b) Source current. (b) Source current. (c) Flter current. (c) Flter current. (d) Load current. (d) Load current. Fg.. The HAFU s off for sngle-phase nonlnear load. Fg. 4. The HAFU s on for sngle-phase nonlnear load. 9
M (db) P (deg) M (db) P (deg) Fg. 5. 4 9 9 8 7 6 5 5 5 5 9 9 8 7 6 4 Frequency (Hz) (a) Open-loop gan. 4 Frequency (Hz) (b) Closed-loop gan. Frequency doman analyss of current control. hybrd flter s composed of a seventh harmonc-tuned passve flter and an actve flter n seres connecton at the secondary sde of the dstrbuton transformer. Wth the actve flter part operatng as varable harmonc conductance, the flterng performances of the passve flter can be sgnfcantly mproved. Accordngly, the harmonc resonances can be avoded and the harmonc dstorton can be mantaned nsde an acceptable level n case of load changes and varatons of lne mpedance of the power system. Expermental results verfy the effectveness of the proposed method. Extended dscussons are summarzed as follows: Large lne nductance and large nonlnear load may result n severe voltage dstorton. The conductance s ncreased to mantan dstorton to an acceptable level. Lne resstance may help reduce voltage dstorton. 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