Australan Journal of Basc and Appled Scences, 5(5): 287-295, 20 ISSN 99-878 Dual Functonal Z-Source Based Dynamc Voltage Restorer to Voltage Qualty Improvement and Fault Current Lmtng M. Najaf, M. Hoseynpoor, R. Ebrahm Bushehr Branch, Islamc Azad Unversty, Bushehr, Iran. Abstract: Dynamc Voltage Restorer (DVR) s a powerful custom power devce for short duraton voltage compensaton, whch s connected n seres wth the load. Due to the fact that t s connected n seres wth the dstrbuton lne, DVR would suffer from downstream faults. o lmt the flow of large fault currents and protect DVR tself as well, a fault current lmtng functon s proposed n the DVR control strategy. Fault current lmtng functon of DVR wll be actvated by the protecton system and then DVR wll start njectng a seres voltage to the lne n such a way as to lmt fault current to an approprate level. Recently, the applcaton of Z-source nverter s proposed n order to optmze DVR operaton. hs nverter makes DVR to operate approprately when the energy storage devce's voltage level severely falls. In ths paper, the Z-source nverter based DVR s proposed to compensate voltage dsturbance at the PCC and to reduce the fault current n downstream of DVR. 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 word: Dynamc voltage Restorer, Z-Source Inverter, Voltage dsturbance, fault Current INRODUCION Dynamc voltage restorer (DVR) 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 DVR crcut. As shown n fg., DVR s composed of an nverter, an energy storage element, LC flter, and a transformer. In many references, Voltage source nverter (VSI) s used due to the approprate output voltage wth low harmoncs level. he man defect of VSI s ts reducng characterstcs whose maxmum output voltage s lmted by DC lnk voltage. hs means that the compensaton ablty of DVR decreases when the DC lnk voltage falls due to energy reducton n energy storage element (Sng et al, 2004),(Vlathgamuwa 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 VSI. 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 DVR. Durng fault occurrence n downstream of DVR, the voltage falls at the pont of common connecton (PCC). DVR mght try to compensate the voltage fall whch enlarges the short crcut current, whle ths current flow through the DVR causes damage n DVR. In conventonal DVRs, passve methods are used to bypass the DVR by applyng parallel swtches to avod mentoned damages durng the short crcut (Woodley et al, 999; Woodley et al, 2000). Applyng actve methods s proposed to reduce the fault current n seres grd connected devces (Lee et al, 2004),(cho et al, 2005). In (L et al, 2006),(Axente et al, 2006) DVR s appled to reduce the short crcut current usng such methods. In ths paper, the short crcut current reducton s studed and smulated applyng Z-source nverter based DVR. Accordng to the voltage boost ablty of Z-source nverter, the DVR 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 DVR 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. Correspondng Author: M. Najaf, Bushehr Branch, Islamc Azad Unversty, Bushehr, Iran. 287
Aust. J. Basc & Appl. Sc., 5(5): 287-295, 20 Fg. : Z-source based structure of DVR. Fg. 2: Voltage ype Z-Source Inverter. Fg. 3: Equvalent crcut of ZSI. MAERIALS AND MEHODS he proposed system s conssted of four man parts: Z-source nverter, DVR operaton to compensate the voltage dsturbance, DVR operaton to reduce the fault current, detecton and recovery from downstream fault. heses parts have dscussed and fnally the smulaton results have been ntroduced. A. Z-source Inverter: Fg. 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 called short crcut vector used to boost the nput voltage. Fg. 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 288
Aust. J. Basc & Appl. Sc., 5(5): 287-295, 20 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: KVL equatons: v v v L C L2 C2 V V 0 () KCL equatons: I C L I C2 L2 (2) 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 almost constant voltages of capactors. he voltage and current equatons of crcut are as follow: KVL equatons: vl Vo VC2 vl2 Vo VC v V V V C C2 KCL equatons: I C L2 I C2 L o (3) (4) he average value of nductors voltages should be zero n each cycle: v ( ) v ( ) 0 o L o L V ( V V ) 0 o C o C2 v ( ) v ( ) 0 o L2 o L2 V ( V V ) 0 o C2 o C (5) (6) And so the capactors voltages would be as follow: V V V V C C C2 o o (7) v durng s: v V V o t o o (8) 289
Aust. J. Basc & Appl. Sc., 5(5): 287-295, 20 Consderng o V V BV o 2 o o leads to: In the above relaton, B s called boost coeffcent whch s: B 2 o Accordng to the above relaton, the short crcut tme rato D s s defned as follow: (9) (0) D S o () he output phase voltage peak s as follow: V ac Vo MB 2 (2) where the M s the modulaton coeffcent of nverter. V can be expressed n terms of V c : V D S V C (3) B. DVR Operaton o Compensate he Voltage Dsturbance: Voltage sag s the most common problem n power qualty feld whch s manly created by fault occurrence n grd. Fg. 4 shows two samples of fault locatons whch cause voltage sag n PCC (DVR connecton pont). Fg. 4: Voltage sag or swell n PCC. Here, DVR 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). Fg. 5 shows the proposed control crcut of DVR under voltage dsturbance compensaton condtons. he voltage njected by DVR s determned by feed backng the source voltage and load. Capactor voltage (V c ) s feed backed to control V. V s calculated usng relaton (3) and D s s determned by comparng V wth V -ref and applyng the nductor current feed back (I L ). 290
Aust. J. Basc & Appl. Sc., 5(5): 287-295, 20 Fg. 5: Proposed control system to compensate voltage dsturbances. C. DVR Operaton o Reduce he Fault Current: Fg. 6 shows the fault occurrence n downstream of DVR. hs fault causes severe current flow through DVR whch mght damage DVR. hs current also causes voltage fall at PCC and t would be worst f DVR tres to compensate. In order to solve the problem, DVR should be controlled n a way that t reduces the fault current by njectng approprate voltage to grd. Here, DVR s operated as vrtual mpedance (L et al, 2006). Fg. 7 shows the phasor dagram of voltages. V Lne s the voltage fall at the length of upstream and downstream feeders mpedances and fault mpedances (Z S +Z Lne ) and V Inj and V S are the voltage njected by DVR and the voltage of man feeder respectvely: V V V S Inj Lne (4) he fault currents wth and wthout lmtaton are as follow: I I Fault Z S Fault Lmted V S Z Lne VS Z Z Z S Lne o (5) (6) Fg. 6: Downstream fault n presence of DVR. Fg. 7: DVR njected voltages phasor dagram durng downstream fault 29
Aust. J. Basc & Appl. Sc., 5(5): 287-295, 20 If the fault current s reduced to a constant value, V Lne wll fall n the dotted crcle depend on the voltage njected by DVR. Fg. 7 obvously shows that the mnmum njectng voltage would be requred f the njected voltage (V Inj ) and V 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, DVR should operate as a pure vrtual nductance to mnmze the energy consumpton, where the V Inj should have 90 degrees phase dfference wth fault current. D. Detecton And Recovery From Downstream Fault: For fast downstream fault detecton and therefore fast DVR reacton to the fault current, nstantaneous current magntude s calculated n synchronous frame. Once the current magntude exceeds a preset threshold, the DVR 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 sde of the DVR njecton transformer) as n Fg. 6. When the fault s cleared, the load voltage wll ncrease. Once s restored to a preset level, the fault current lmtng functon of DVR can be termnated. Furthermore, to ensure DVR 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 DVR 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). able I: Grd Parameters Source Phase-Phase rms Voltage (V S ) Source Inductance (L S ) Source Resstance (R S ) Load Inductance (L Load ) Load Resstance (R Load ) Lne Inductance (L Lne ) nverter Inductance (L F ) Inverter Capactance (C F ) Inverter Resstance (R F ) Swtchng Frequency (f) 380v 50Hz 0.2 mh 0 Ω 4 mh 8 Ω mh 2 mh 60 μf 0 μ khz RESULS AND DISCUSSION he proposed system s smulated by MALAB/SIMULINK to study the correctness of ts operaton. A Z-source nverter based DVR located on a 0.38kv 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 table ().the smulaton, n the man feeder, a 30% voltage swell has started at t=0. sec and lasts for t=0.2 sec, a 45% voltage sag has started at t=0.3 sec and lasts for t=0.4 sec and fnally n downstream of DVR, a three phase fault to ground has occurred between t=0.5 sec to t=0.6 sec. Fg. 8 shows the man feeder voltage, njected voltage by DVR and load sde voltage respectvely. It s shown n ths fgure that voltage sag and swell n man feeder have been compensated by DVR and the load sde voltage has the nomnal value. Furthermore, by occurrence of fault n downstream of DVR, the PCC voltage has been compensated by DVR accurately. Concluson: In dstrbuton networks to mprove voltage qualty, dynamc voltage restorer s one of most usable devces. Correct operaton of DVR sgnfcantly depends on ts DC lnk voltage. By reducng DC lnk voltage, DVR 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 paper ZSI based DVR s proposed to compensate voltage dsturbance n PCC and reduce the short crcut current at the downstream of DVR. By calculatng nstantaneous current magntude n synchronous frame, control system recognzes f the fault exsts or not, and determnes whether DVR 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. 292
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