Research Journal of Appled Scences, Engneerng and echnology 3(): 246-252, 20 ISSN: 2040-7467 Maxwell Scentfc Organzaton, 20 Submtted: July 26, 20 Accepted: September 09, 20 Publshed: November 25, 20 oltage Qualty Enhancement and Fault Current Lmtng wth Z-Source based Seres Actve Flter F. Gharedagh, H. Jamal, M. Deys and A. Khall Dashtestan Branch, Islamc Azad Unversty, Borazjan, Iran Abstract: In ths study, seres actve flter or dynamc voltage restorer applcaton s proposed for reducton of downstream fault current n addton to voltage qualty enhancement. Recently, the applcaton of Z-source nverter s proposed n order to optmze DR operaton. hs nverter makes DR to operate approprately when the energy storage devce s voltage level severely falls. Here, the Z-source nverter based DR s proposed to compensate voltage dsturbance at the PCC and to reduce the fault current n downstream of DR. By calculatng nstantaneous current magntude n synchronous frame, control system recognzes f the fault exsts or not, and determnes whether DR should compensate voltage dsturbance or try to reduce the fault current. he proposed system s smulated under voltage sag and swell and short crcut condtons. he smulaton results show that the system operates correctly under voltage sag and short crcut condtons. Key words: Dynamc voltage restorer, fault current, nverter, z-source voltage dsturbance INRODUCION Dynamc oltage Restorer (DR) s one of the power electroncs devces whch are connected to dstrbuton system n seres to protect senstve loads aganst the voltage changes. Fg () shows a knd of DR crcut. As shown n fg., DR s composed of an nverter, an energy storage element, LC flter, and a transformer. In many references, oltage source nverter (SI) s used due to the approprate output voltage wth low harmoncs level. he man defect of SI s ts reducng characterstcs whose maxmum output voltage s lmted by DC lnk voltage. hs means that the compensaton ablty of DR decreases when the DC lnk voltage falls due to energy reducton n energy storage element (Sng et al., 2004), (lathgamuwa et al., 2006). A dc-dc boost converter applcaton between nverter and the energy storage element s proposed (Sng et al., 2004). Usng boost converter leads to an ncrease n sze, prce and ntegraton of system. In (Sng et al., 2004) t s proposed to apply a Z-source nverter (ZSI) nstead of SI. Z- source nverter s a new converter wth some specal advantages, presented recently as a substtuton to conventonal converters (Peng, 2003). In (Sng et al., 2004) n-phase compensaton of voltage sag s studed and executed usng Z-source nverter based DR. Durng fault occurrence n downstream of DR, the voltage falls at the Pont of Common Connecton (PCC). DR mght try to compensate the voltage fall whch enlarges the short crcut current, whle ths current flow through the DR causes damage n DR. In conventonal DRs, passve methods are used to bypass the DR by applyng parallel swtches to avod mentoned damages durng the short crcut (Woodley et al., 999, 2000). Applyng actve methods s proposed to reduce the fault current n seres grd connected devces (Lee et al., 2004; Cho et al., 2005). L et al. (2006) and Axente et al. (2006) DR s appled to reduce the short crcut current usng such methods. In ths study, the short crcut current reducton s studed and smulated applyng Z-source nverter based DR. Accordng to the voltage boost ablty of Z-source nverter, the DR can nject the requred voltage to reduce the fault current even at severe DC lnk voltage reducton. In contnuous, the Z-source nverter s descrbed and then the DR operaton under two voltage sag and fault occurrence s explaned. At the next part, t s expressed how fault s recognzed. Fnally, the smulaton results of system are analyzed and studed. Z-source nverter: Fgure 2 shows the structure of Z-source nverter (ZSI). ZSI uses an LC mpedance grd to couple power source to nverter crcut and prepares the possblty of voltage boost by short crcutng the nverter legs. As shown n Fg. 2, Z-source nverter s mpedance grd s composed of two parted L and L 2 nductors and C and C 2 capactors arranged n X shaped confguraton. Usual nverters have sx actve swtchng vectors and two zero swtchng ones where, ZSI has another zero vector Correspondng Author: F. Gharedagh, Dashtestan Branch, Islamc Azad Unversty, Borazjan, Iran 246
Res. J. Appl. Sc. Eng. echnol., 3(): 246-252, 20 Fg. : Z-source based structure of DR Fg. 2: oltage type z-source nverter called short crcut vector used to boost the nput voltage. Fgure 3 shows the equvalent ZSI crcut where the nverter and load are modeled as swtches and current source respectvely. he operaton of voltage type ZSI can be analyzed n two modes. Frst s the tme durng whch the S swtch s on ( 0 ) and the second s the tme durng whch t s off ( ). Durng 0 the nductors are beng charged by capactors and consequently the capactors are beng dscharged. he currents of nductors are ncreasng lnearly due to ther voltages whch are equal wth the constant voltages of capactors. he voltage and current equatons are as follow: Fg. 3: Equvalent crcut of ZSI KL equatons: = L C LI = C2 KCL equatons: = I C L = I C2 L2 () (2) almost constant voltages of capactors. he voltage and current equatons of crcut are as follow: KL equatons: vl C2 vl2 C v = C C2 0 KCL equatons: (3) Swtch S s off durng. Now, nductors are beng dscharged and capactors are beng charged. he currents of nductors decrease lnearly accordng to negatve and = I c L2 = I c2 L (4) 247
he average value of nductors voltages should be zero n each cycle: Res. J. Appl. Sc. Eng. echnol., 3(): 246-252, 20 v ( ) + ( ) 0 L 0 L v + ( ) 0 C 0 C2 v ( ) + ( ) 0 L2 0 L2 + ( ) 0 C2 0 C2 (5) (6) Fg. 4: oltage sag or swell n PCC And so the capactors voltages would be as follow: C C cc2 0 0 = = = v durng s: 0 v = = t= + 0 0 Consderng = + 0 leads to: = = B 2 0 0 0 (7) (9) In the above relaton, B s called boost coeffcent whch s: B = 2 0 (0) Accordng to the above relaton, the short crcut tme rato D s s defned as follow: D s he output phase voltage peak s as follow: ac 2 0 = MB () (2) where the M s the modulaton coeffcent of nverter. can be expressed n terms of c : = D s c (3) DR operaton to compensate the voltage dsturbance: oltage sag s the most common problem n power qualty feld whch s manly created by fault occurrence n grd. Fgure 4 shows two samples of fault locatons whch cause voltage sag n PCC (DR connecton pont). Here, DR has to nject proper seres voltage to grd n order to restore the load voltage level to ts desred level. he compensaton should be executed n Pre-sag mode (optmum power qualty), n-phase (mnmum voltage ampltude) and wth mnmum energy consumpton, due to the load senstvty and system lmtatons (Meyer et al., 2006). Fgure 5 shows the proposed control crcut of DR under voltage dsturbance compensaton condtons. he voltage njected by DR s determned by feed backng the source voltage and load. Capactor voltage ( c ) s feed backed to control. s calculated usng relaton (3) and D s s determned by comparng wth -ref and applyng the nductor current feed back (I L ). DR operaton to reduce the fault current: Fgure 6 shows the fault occurrence n downstream of DR. hs fault causes severe current flow through DR whch mght damage DR. hs current also causes voltage fall at PCC and t would be worst f DR tres to compensate. In order to solve the problem, DR should be controlled n a way that t reduces the fault current by njectng approprate voltage to grd. Here, DR s operated as vrtual mpedance (L et al., 2006). Fgure 7 shows the phasor dagram of voltages. Lne s the voltage fall at the length of upstream and downstream feeders mpedances and fault mpedances (Z S +Z Lne ) and Inj and S are the voltage njected by DR and the voltage of man feeder, respectvely: s = Inj + Lne (4) he fault currents wth and wthout lmtaton are as follow: I Faut t = Z Z s s+ Lne I Fault s Lmted = Zs + ZLne + z0 (5) (6) If the fault current s reduced to a constant value, Lne wll fall n the dotted crcle depend on the voltage njected by DR. Fgure 7 obvously shows that the mnmum njectng voltage would be requred f the njected voltage ( Inj ) and sde of the DR njecton transformer) as n Fg. 6. When 248
Res. J. Appl. Sc. Eng. echnol., 3(): 246-252, 20 Fg. 5: Proposed control system to compensate voltage dsturbances Fg. 6: Downstream fault n presence of DR Fg. 7: DR njected voltages phasor dagram durng downstream fault Lne are n phase. In order to acheve ths, the R/X rato of vrtual mpedance (Z 0 ) should be n proporton wth the feeder mpedance (Z S +Z Lne ). he exstence of vrtual ohmc component causes actve power consumpton and lmts n voltage reducton of energy storage element. So, DR should operate as a pure vrtual nductance to mnmze the energy consumpton, where the Inj should have 90ºdegrees phase dfference wth fault current. Detecton and recovery from downstream fault: For fast downstream fault detecton and therefore fast DR reacton to the fault current, nstantaneous current magntude s calculated n synchronous frame. Once the current magntude exceeds a preset threshold, the DR would nject a seres voltage so that t would act as a vrtual nductor to lmt the fault current and to restore the PCC voltage. Recovery from a downstream fault can be done by sensng the load voltage (voltage at downstream able : Grd parameters Components alues Source phase-phase Rms oltage ( S ) 380v - 50Hz Source nductance (L S ) 0.2 mh Source resstance (R S ) 0 S Load nductance (L Load ) 4 mh Load resstance (R Load ) 8 S Lne nductance (L Lne ) mh Inverter nductance (L F ) 2 mh Inverter capactance (C F ) 60 :F Inverter resstance (R F ) 0 S Swtchng frequency (f) khz the fault s cleared, the load voltage wll ncrease. Once s restored to a preset level, the fault current lmtng functon of DR can be termnated. Furthermore, to ensure DR compensaton s not turned off by any spurous transent dstorton or measurement noses, low pass flter lke characterstc s added to ths recovery detecton. hs s done by turnng off the DR when (where s a threshold voltage) s sustaned for a specfc perod of tme and no other downstream fault s detected durng ths perod. (In ths paper, 20 ms s used for symmetrcal faults, and 50 ms s used for unsymmetrcal faults to elmnate the effects of oscllatory load voltage magntude due to the unbalanced effects). Smulaton: He proposed system s smulated by MALAB/SIMULINK to study the correctness of ts operaton. A Z-source nverter based DR located on a 0.38 kv lne s smulated to compensate the voltage sag and swell of man feeder to provde nomnal voltage of downstream loads and also to reduce short crcut current of downstream fault and compensate voltage of man feeder. he smulaton parameters are presented n able. he smulaton, n the man feeder, a 30% voltage swell has started at t. sec and lasts for t.2 sec, a 45% voltage sag has started at t.3 sec and lasts for t.4 sec and fnally n downstream of DR, a three 249
Res. J. Appl. Sc. Eng. echnol., 3(): 246-252, 20 (a) (b) Fg. 8: (a) Man feeder sde voltage, (b) Injected voltage by DR, (c) Load sde voltage (c) phase fault to ground has occurred between t.5 sec to t.6 sec. Fgure 8 shows the man feeder voltage, njected voltage by DR and load sde voltage respectvely. It s shown n ths fgure that voltage sag and swell n man feeder have been compensated by DR and the load sde voltage has the nomnal value. Furthermore, by occurrence of fault n downstream of DR, the PCC voltage has been compensated by DR accurately. Fgure 9 shows the fault current wthout and wth DR operaton. It s clear n Fg. 9a that the fault current s obvously reduced n compare wth Fg. 9b. 250
Res. J. Appl. Sc. Eng. echnol., 3(): 246-252, 20 (a) Fg. 9: (a) Downstream lne fault current, (b) Lmted downstream current by DR (b) CONCLUSION In dstrbuton networks to mprove voltage qualty, dynamc voltage restorer s one of most usable devces. Correct operaton of DR sgnfcantly depends on ts DC lnk voltage. By reducng DC lnk voltage, DR faces wth defect. Recently, Z-source nverter applcaton s recommended to solve ths problem. ZSI wth voltage boost ablty can solve ths problem. In ths study ZSI based DR s proposed to compensate voltage dsturbance n PCC and reduce the short crcut current at the downstream of DR. By calculatng nstantaneous current magntude n synchronous frame, control system recognzes f the fault exsts or not, and determnes whether DR should compensate voltage dsturbance or try to reduce the fault current. he proposed system s smulated under voltage sag and swell and short crcut condtons. he results show that the proposed system operates correctly. B D s o L C I L I C M ac lnj lne S Z S Zlne Boost coeffcent he short crcut tme rato Source voltage Input voltage of nverter Inductor voltage Capactor oltage Inductor current Capactor current he modulaton coeffcent of nverter he output phase voltage peak he voltage fall at the length of upstream and downstream feeders mpedances he voltage njected by DR and he voltage of man feeder Source Impedance Lne Impedance REFERENCES 0 NOMENCLAURE me durng whch the S swtch s on he tme durng whch the S swtch s off 25 Axente,., M. Basu, M.F. Conlon and K. Gaughan, 2006. Protecton of DR aganst short crcut faults at the load sde. Inter. Conf. Power Electr. Mach. drves, pp: 627-63.
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