Performance Enhancement of DVR for mitigating voltage sag/swell using Vector Control Strategy

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1 9th EcoEnergy and Materals Scence and Engneerng Symosum, hang Ra, Thaland, 58 May Perormance Enhancement o DVR or mtgatng oltage sag/swell usng Vector ontrol Strategy K. Bhumkttch and N. Mthulananthan Abstract Ths aer resents dynamc oltage restorer DVR wth ector control strategy or mtgatng ower qualty n ower dstrbuton systems. Wheneer ower qualty roblems, oltage sag/swell, occur DVR has to automatcally detect and ect oltage comonents. The detecton unt should be ast enough to know the araton o all hase oltages, ncludng magntude and hase, to trgger the DVR to ect arorate oltage comonents wth requred hase angles wthn ery short erod o tme. Here, control strategy adoted or drng the DVR lays and mortant role n ts erormance. Any delay n the rocess or ncorrect ecton would be harmul to senste loads that are ulnerable to oltage sag/swell. Ths aer llustrates ector control strategy based on decoulng control to enhance erormance o DVR durng the rocess o comensaton. It was ound out that oltage sags/swells can be corrected aster wth the roosed control strategy comared to conentonal methods. Keywords Dynamc oltage restorer; ector control algorthm; oltage sag; oltage swell.. INTRODUTION Industral and commercal consumers o electrcal ower are becomng ncreasngly senste to ower qualty roblems. When dsrutons occur n consumers remses due to oor ower qualty, downtme, deects and loss o roducton, and damage to equment can be substantal. Ths would result n nancal losses to consumers as well as utltes. Hence, the study o electrcal ower qualty s becomng a oular research toc. One o the ower qualty roblems that are tycal n all dstrbuton system s oltage sag/swell. The oltage sags/swells are caused by remote aults or swtchng o large loads e.g. motor startng and energze caactor or transormer []. Voltage sag s dened as a short duraton reductons n the rms oltage that can last a ew cycles, wth reducton n oltage rangng rom.9 to..u. o nomnal oltage. Voltage swell, on the other hand, s dened as a short duraton ncreasng n rms oltage wth ncrease n oltage rangng rom. to.8.u. o nomnal oltage []. Through the oltage sags/swells last as lttle as a ew cycles, they can dsrut some senste loads such as adjustable seed dres ASD, rogrammable logc controllers P, and semconductor lants. There are arous solutons to ths roblem, such as desgnng nerter dres or rocess equment to be more tolerant to oltage luctuatons or nstallaton o oltage correcton deces. It has been shown that or customers o large loads, rom a hgh kva to low MVA range, good solutons are nstallaton o custom ower dece such as DVR [3]. Alcatons o DVR are manly or rotectng the senste loads that may be consderably K. Bhumkttch Dr.Pasan Booncham s wth Power and Energy System Research enter, the Deartment o Electrcal Engneerng, Faculty o Engneerng, Rajamangala Unersty o Technology Thanyabur, Klong 6, Thanyabur, Pathumthan, Thaland, Emal: krschonme.b@en.rmutt.ac.th N.Mthulananthan s wth Power and Energy System Grou, School o Inormaton Technology and Electrcal Engneerng, Unersty o Queensland, Australa. Emal: mtulan@tee.uq.edu.au aected by luctuaton n dstrbuton oltage. Fg. shows a sngle lne dagram o ower dstrbuton system wth DVR rotectng a senste load. Assumng a short crcut ault occurs at A, the oltage at A wll be droed to.u. and the oltage at ont B wll also be reduced about.64.u. Wth ths condton or sure any senste load that s ulnerable to oltage sag wll be aected. In order to rotect the senste or crtcal load, DVR s nstalled at ont o common coulng P. The oltage at ths bus wll be remanng at..u. due to the resence o DVR. It means that the man uncton o DVR has to mantan the amltude and hase angle o load oltage. Howeer, uon detectng a ault DVR should react quckly to correct the roblem. A ast control strategy s desrable to ncrease the erormance o DVR. B Fault 5 kv 5 Hz B To Normal oad 5/ kv B kv 5Hz V s B B B B B A % B B 64 % V I DVR load V load rtcal oad V dc % Fg. Smled dagram o DVR rotectng a senste load. The control strategy o DVR can be classed nto two arts namely detecton o oltage sag/swell and ecton o the oltage comonent [4][]. The Oen loo control method as a conentonal one s generally used or drng DVR because o ts smlcty and low cost. Howeer, there are seeral drawbacks such as the deendency on lter dynamcs and sustaned oltage oscllatons n ower system. Snce the resonse tme aganst oltage sags/swell ares accordng to the dynamcs o nductance and caactance o the rle lter. The oen loo wth eedorward control can reduce the trackng tme to deal wth the unexected oltage sag but stll hae the error on

2 9th EcoEnergy and Materals Scence and Engneerng Symosum, hang Ra, Thaland 58 May. transent state. The double loo control can comensate the transent state error. But, the steady state load oltage may not be comensated to a desred alue owng to oltage dro across the transormer seres medance and the lter. Thereore, t s requred to comensate wth regulaton or the rle lter current o the DVR crcut as an adanced control strategy or drng DVR. The roortonalntegral PI control scheme s commonly used n conentonal control system due to smle algorthm and structure [6] [8]. onsequently, the synchronous PI decoulng control scheme s adoted as control strategy or drng DVR aganst oltage dsturbances. Ths control strategy s dered on the bass o the restoraton o lnearty n control erormance usng control arables such as nductor currents n the rle lter and comensatng oltages ected through transormer []. The coulng terms n crcut equatons exressed n the synchronous reerence rame, whch deterorate the lnearty n control erormance, wll be easly remoed by addng as eedorward terms. Thereore, the lnearty can be retreed so that t s easy to control control arables ndeendently. Ths control scheme has the characterstcs such as ast resonse wthout oltage oscllaton n the transent state and low error n the steady state. The ast resonse n transent state guarantees aster acton aganst the oltage sag/swell and the low error n steady state guarantees enough oltage comensaton aganst oltage sag/swell. Moreoer, snce the currents lowng through the rle lter nductor are used as the sulementary control arables, t s ossble to regulate the nductor current wthn a ermtted lmt. Accordngly, the surge current lowng nto the swtchng ower dece can be reented thoroughly. Thereore, ths aer rooses ector control strategy or mrong dynamc erormance o DVR. The controller desgn s based on statesace model and oltage ector control model. The oltage and current ector control s used to roduce the arorate outut comonents. The outut lter s also used to comute the control arameters and theoretcal consderatons hae been ered by smulaton results. Ths aer s organzed as ollows. The system conguraton o DVR s resented n Secton. Secton 3 descrbes the DVR control strateges aganst oltage sag and swell. In Secton 4, the mathematcal model o DVR usng synchronous reerence rame s resented. The control desgn o DVR s elaborated n Secton 5. Secton 6 shows the smulaton results or both balanced and unbalanced condtons. Fnally, contrbutons and conclusons o the aer are summarzed n Secton 7.. SYSTEM ONFIGUATION OF DVR Fgure shows comonents o DVR and ther tycal connecton. The DVR conssts o multleel oltage source conerter VS [], detecton and reerence generaton unt DRU, ulse wdth modulaton unt PWM, control unt, outut lter, ecton transormer, D lnk and energy storage dece ESD. The measured oltage and currents are nut sgnals to DRU. It ges sgnals to the control unt when the measured quanttes der rom the settngs o the controller. The DRU trggers start o comensaton when the suly oltage comes outsde o normal range. The control unt then generates the oltage reerence. Voltages reerences are nut to the modulaton unt generatng the modulaton sgnal or VS o DVR. The D lnk ects or absorbs the requred ower to comensate the dented oltage sag/swell. Installng an outut lter between the VS and ecton transormer reduces harmonc n the outut o DVR. Thus, the lter conerts the ulsemodulated oltage o the VS nto snusodal oltage. The ltered oltage s ected nto dstrbuton system a seresectng transormer. In Fg., P reresents Phase og oo and connecton o senste load n on the rght hand sde. sa sb sc P d q θ z a z b z c ESD Detecton and Reerence Generaton rd rq θ r PWM q d ρ control a a b b c T T s a T 3 s b s ca c cb c cc Fg. Schematc dagram o DVR. ca cb cc 3. DVR ONTRO STRATEGY AGAINST VOTAGE SAG/SWE A DVR must be able to react ery ast on derent knds o oltage sag and swell. The amltude o the loadsde oltage must be restored and or most loads large hase jums must be aoded. Esecally, the correct comensaton o snglehase oltage sag/swell s a major ssue. Snce DVR uses ESD and Dlnk, t wll be dscharged durng the comensaton. Hence, the oltage o Dlnk wll be constantly decreasng durng oeraton mode. Thereore, the modulaton ndex must be ket constant and adated durng the ault. There are two basc control strateges, namely amltude comensaton and amltude and hase angle comensaton adoted or VS control. In ths aer, the later s selected to achee wth otmal control [8]. To aod a loss o ower suly, the amltude o the load oltage has to be restored by the DVR. The standard soluton or comensatng oltage sag/swell s to reestablsh the exact nomnal oltage. Thereore, the amltude and hase angle o the oltage hae to be exactly restored. The resultng ector dagram o DVR comensaton or oltage sag/swell as shown n Fg. 3. The comensaton leads to control amltude and hase angle o load oltage at nomnal alue. For ths strategy, the angle θ o the utlty s obtaned through the use o a threehase P [], []. As soon as oltage sag/swell occurs, the threehase P wll be locked and thereore the hase angle can be restored. Deendng on the hase o the new grd oltage, the DVR has to deler hgher oltage amltude than needed n order to restore the correct oltage magntude. The system has to be desgned or a hgher maxmum oltage V DVR wth mnmum energy. The requred DVR oltage or a sag/swell s gen z a z b z c

3 9th EcoEnergy and Materals Scence and Engneerng Symosum, hang Ra, Thaland 58 May. by. Im ϕ V S =V OAD δ I OAD V S,Sag V DVR Re Im ϕ V S =V OAD δ I OAD V S,Sag a omensaton or sag. b omensaton or swell. Fg. 3 DVR comensaton or oltage sag/swell. V DVR ˆ VDVR = VN ε εcosδ 3 Where V N s the nomnal grd oltage, ε s sag deth whch wll be negate or a swell and, δ s the hase deth durng the sag and swell erod. Fg. 4 shows the cures o derent sag/swell and hase jum whch s the angle between load oltage and current durng the ault. In order to comensate or the reduced suly oltages to the desred oltage leel, the comensaton oltage must be ected nto the ower system by DVR. In ths case, the ected comensaton oltage must be comuted delcately so that the suly oltages may be wellcomensated wth resect to the amltude and hase angle o suly oltages. needed oltage [.u.] Swell deth Sag deth wth hase jum angle wthout hase jum angle sag and swell deth [.u.] Fg. 4 Voltage amltude deendng on Sag/Swell Deth. The requred acte ower can be calculated based on the ector dagram. In general, the ower must be equal to the derence between load and acte ower delered by the grd. The acte ower equaton o DVR s gen by. PDVR = 3 Poad PGrd = 3 VNINcos ϕ εcos ϕδ Based on the maxmum caactance can be calculated. The acte ower s lmted by the sze o Dlnk caactor and the lowest ossble Dlnk oltage whch s sucent or a roer restoraton o the load oltage. Based on these assumtons, the ollowng equaton, 3, can be dered or maxmum caactor alue. PDVR max = tmax 3 ˆ V DVR VD mmax n where t max = the maxmum comensaton tme m max = Maxmum modulaton ndex Re n = Transormer rato 4. DVR MATHEMATIA MODE The equalent crcut o DVR s dslayed n Fg. 5, where threehase oltage o the grd s denoted by sx. The threehase oltage and currents o the VS are denoted by nx, and x, resectely. The lter caactor oltages and currents are denoted by cx, and cx, resectely. The oltages and current ected by the DVR are denoted by x, and x, resectely. The Dlnk oltage denoted by dc and the load oltage s denoted by loadx. r dc ESD dc dc [ n R, [ [ [ c Fg. 5 Snglehase equalent crcut o grd wth DVR. [ The control system o the DVR s consttuted by two closedloo controllers connected n cascade: an outer loo that controls the oltage across the lter caactor t and an nner controller that controls the current x through the lter reactor x t. The basc assumton s that the ected oltage s equal to the oltage across the caactors o the VS outut lter.e. the ecton transormer s consdered deal wth a :n turn rato, x t = n x t and x t = n Sx t, and analogously or the other two hases. Under these assumtons, alyng the Krchho s current law K and Krchho s oltage law KV to the lter, the ollowng derental equatons or the three hases can be wrtten d x t = x t x t = x t x t 6 dt d nx t x t R x x t = 7 dt where x can be relaced wth hases a, b and c. By alyng larke s transormaton, 6 and 7 can be wrtten n the αβcoordnate system as 8 and 9, resectely. d t = t t R t 8 dt αβ αβ αβ αβ n d t = t t 9 dt αβ αβ αβ whch, by usng αβ to transormaton wth a P synchronzed wth the grd oltage ector, becomes and. d t = t t j t dt ω

4 9th EcoEnergy and Materals Scence and Engneerng Symosum, hang Ra, Thaland 58 May. d t t t R t dt jω t = n wth the chosen P, the oltage ector s algned wth the drecton o the daxs durng steady state. The grd oltage comonent n the ddrecton s equal to ts rmsalue when usng owernarant transormaton and the qcomonent o the grd oltage s equal to zero. Thus, the dcomonent o the current ector n steady state arallel to the grd oltage ector becomes the acte current comonent dcurrent and the qcomonent o the current ector becomes the reacte current comonent qcurrent. Fg. 6 shows the hyscal model o DVR wth lter. It can be notced that the coulng terms n the hyscal model o DVR, the lnearty may be deterorated resultng n an unsatsactory erormance. Thereore, t s necessary to retree the lnearty o controller so that the control erormance, such as ast resonse and low steady state error, should be enhanced. In concluson, a ector controller addng the coulng terms as eedorward terms s adoted or drng the DVR.,d V n,d,d,d s R V n,q ω ω R s,q,q,q s ω ω s Fg. 6 Physcal model o DVR wth lter. 5. ONTRO DESIGN OF DVR The control technque that has been mlemented to control the ected oltage by the DVR s oltage ector control wth decoulng aroach. To mroe the transent resonse o the DVR and control the ected acte and reacte ower searately, ths control has been mlemented n a way smlar to the control n seed dres. The ablty o oltage sag/swell comensator to eectely rotect senste loads deends on the tme requred by the detecton strategy and on qualty o the ected oltage. The detecton scheme consdered n ths work uses an algorthm whch extracts amltude, hase and angle normaton rom the utlty oltages. The angle θ o the utlty s obtaned through the use o a threehase Phaseocked oo P. The normaton extracted rom the P s used n an algorthm or detecton and reerence generaton, as llustrated n Fg. 7. As t can be seen, the synchronous rame [4], d and q are used as nuts or lowass lters to generate oltage reerences n the synchronous rame. Ths normaton, along wth the angle generated by the P, s used n the reconstructon o the oltage reerences n a statonary reerence rame. By comarng these oltage reerences wth the nstantaneous oltage at the utlty grd t s to obtan the command sgnal or the PWM modulator. V,d V,q a b c α abc αβ β θ P q d ow ass lter Detecton d q Reerence Generaton αβ abc a b c Fg. 7 Detecton and reerence generaton scheme or oltage sag and swell comensaton. From a block dagram o the detecton and reerence generaton scheme as dected n Fg. 7, the grd oltages,, are measured and transormed to a statonary a b c reerence rame αβ. A P s exloted to calculate the transormaton angle θ, whch s requred to transorm the grd oltage rom the statonary reerence rame to the synchronous reerence rame. Then the grd oltage s subtracted rom the reerence o the load oltage load to calculate the reerence o the ected oltage. The am o the controller s to kee the load oltage constant. Thus, the DVR should ect the oltage such that = load s where load s the reerence oltage demanded by the load. The mssng oltage s ected through the ecton transormer. The nuts to the controller are the grd oltages, grd currents, the VS currents and the caactor oltages o the outut lter. The roosed controller s a dscrete controller and uses a samlng tme o. Hence the samlng requency s equals to /. The ollowng assumtons are made to dere the controller: the grd current s constant, ndeendent to aratons n currents and oltages o the outut lter; the caactor oltage and the nductor current change lnearly durng one samle erod; and 3 the aerage alues o the caactor oltage and the nductor current are each equal to the hal sum o the real alue and the reerence alue at samle k. To dere the oltage ector controller to be mlemented n a dgtal controller, t s necessary to dscretze Eqs. and. ths s done by ntegratng the equaton oer one samle erod and then ddng by, thus obtanng k k = k, k k, k jω k, k k, k = n k k 3 4, k k R k jω, k k, Where k k denoted the aerage alue o t between samle k and samle k. Under the

5 9th EcoEnergy and Materals Scence and Engneerng Symosum, hang Ra, Thaland 58 May. assumtons are made n order to ormulate the oltage controller [4]. The roortonal oltage controller cam now be ormulated as 5 and 6. ω k = k j k k k k T s ω u k k k k = n R k k 5 6 In order to remoe statc errors due to non lneartes, such as nose n the measurements and nondeal comonents, an ntegral art has to be ntroduced n the control system. The PIcontroller can be ormulated as k = k k k k Δ k 7 u k = u k k k k Δ u k 8 Where Δ k and Δ u k are the ntegral terms or the d and the qcomonents, resectely, whch can be wrtten as k k k k k Δ =Δ 9 Δ u k =Δ u k k k k where the roortonal and ntegral gan o the current and oltage controllers can be wrtten as Voltage controller gans: k = ; k = k T T R urrent controller gans: k = ; k = k T where T s the ntegrator tme constant Fg. 8 shows the whole control block dagram or calculatng the oltage reerences o nerter. Snce the rle lter current dynamcs o 7 must be aster than the comensaton oltage dynamcs o 8, the controller gan k, k o 7 or aster current dynamcs must be determned so that the controller has the tme constant o msec. In addton, the controller gan k, k o 8 s determned so that the controller has the tme constant o 5 msec. V,d V,q PI V,d,d ω ω,q PI V,q,d,d,q ω ω PI,q PI V,d V,q V n,d V n,q s,d,d s R ω ω R s,d,q,q s ω ω s V,d V,q 6. SIMUATION RESUTS To llustrate a tycal resonse o DVR wth the roosed control strategy, a smle 5 Hz ower dstrbuton system wth a senste load as shown n Fg. s consdered. The system data and arameters are gen n Aendx. The erormance o DVR wth ector control strategy s shown n Fg. 9 or balanced oltage sag due to a three hase ault that was ntated at. sec. and lasted or.5 sec as resented n the suly oltage grah. The load oltage and the ected oltage by DVR are also shown n Fg. 9. As can be seen rom the gure, the roosed control strategy s able to dre the DVR to ect the arorate three hase oltage comonent wth correct hase to remoe the suly oltage anomales due to three hase ault. It was obsered that durng the normal oeraton the DVR s not unctonng. It quckly ects necessary oltage comonents to smoothen the load oltage uon detectng oltage sag. Smlar erormance s obsered or an unbalanced oltage sag case as well. Fgure shows the erormance o DVR control or unbalanced oltage sag created by double lne ault n the system. As dected n suly oltage the ault was ntated at. sec and t was cleared at.5 sec. As shown n Fg., DVR wth the roosed control strategy s quck n ectng the requred unbalanced oltage comonent or correctng the load oltage and kees t at nomnal alue. In order to see the erormance o roosed DVR control or oltage swells, a balanced oltage swell was smulated by connectng three hase caactor banks n the system at about. sec. The duraton o the oltage swell was.5 sec. Fg. shows the suly oltage, ected oltage comonent by DVR and the load oltage. As can be seen rom load oltage, the DVR s quck to resond to correct the oltage swell by ectng negate three hase oltage comonents. Fg. shows the erormance o DVR control or unbalanced oltage sag and swell created by sngle lne to ground ault n the system. As dected n suly oltage the ault was ntated at. sec and t was cleared at.5 sec. As shown n Fg., the DVR wth ector control s quck n ectng the requred unbalanced oltage comonents or correctng the load oltage and kee the three hase oltages at nomnal alues. In all the cases, the aerage tme taken by the DVR along wth the controller to resond or arous dsturbances s less than 5 μ seconds. Howeer, or other conentonal control strateges o DVR, the aerage resonse tme s ound to be more than μ seconds. Table shows the erormance o the roosed control comared wth the conersonal control strategy, both n terms o resonse tme and smoothness o the wae orm durng oltage sag/swell. Table : Perormance o the control strategy. ontrol strategy Aerage Aerage THD Resonse tme onersonal > μs 4.3 % Proosed ector 5 μs.47 % Fg.8 Proosed control strategy o DVR.,q

6 9th EcoEnergy and Materals Scence and Engneerng Symosum, hang Ra, Thaland 58 May. Suly oltage Injecton oltage oad oltage Suly oltage Injecton oltage oad oltage Suly oltage Injecton oltage oad oltage Suly oltage Injecton oltage Tme [s] Fg. 9. Resonse o MVDVR wth ector control or balanced oltage sag Tme [s] Fg.. Resonse o MVDVR wth ector control or balanced oltage sag Tme [sec] Fg.. Resonse o MVDVR wth ector control or balanced oltage swell ONUSION A dynamc control strategy has been roosed n ths aer or DVR control or mtgatng oltage sag/swell. The roosed control strategy known as ector control strategy s wdely used n Dres. It s mlemented n the synchronous rame wth two loos, an nner loo whch controls the current and an out loo whch controls the oltage. The erormance o DVR wth ector control has been demonstrated through smulaton n a smle dstrbuton system wth senste load or both balance and unbalanced stuatons. The roosed control strategy roos to be erormed satsactory, both n terms o resonse tme and smoothness o the ected oltage comonents. The resonse tme o the DVR wth the roosed control strategy was less than hal comared to the same o conentonal counterarts. 8. REFERENES [] Ghosh, A. and edwch, G. Power Qualty Enhancement usng ustom Power Deces. Kluwer Academc Publshers, Unted States,. [] IEEE Recommended Practce or Montorng Electrc Power Qualty, IEEE Std. 59, 995. [3] Hngoran, N. G. Introducng ustom Power. IEEE Sectrum 36,. 448, 995. [4] D. M. Vlathgamuwa, A.A.D.R. Perera, and S.S.ho, Perormance mroement o the dynamc oltage restorer wth closedloo load oltage and currentmode control, IEEE Trans. On Power Electroncs, ol.7, n.5, , Set.. [5] I. EtxeberraOtadu, U. Vscarret, S. Bacha, M. aballero, and R. Reyero, Ealuaton o derent strateges or seres oltage sag comensaton, on. Rec IEEE PES, [6] A. Ghosh, and A. Josh, A new algorthm or the generaton o reerence oltage o a DVR usng the method o nstantaneous sysmmetrcal comonents, IEEE Power Engneerng Reew, ol., no.,. 6365, Jan.. [7] u, J. W., ho,s. S., & hen, S. Desgn o ste dynamc oltage regulator or ower qualty enhancement, IEEE Transactons on Power Delery, ol. 8, no.4, ,. [8] Nelsen, J.G.; Blaabjerg, F.; Mohan,; N., ontrol strateges or dynamc oltage restorer comensatng oltage sags wth hase jum, Aled Power Electroncs onerence and Exoston,. APE. Sxteenth Annual IEEE, ol., , 48 March [9] M. J. Newman, D.G. Holmes, J.G.Nelsen, and F. Blaabjerg, A dynamc oltage restorer DVR wth selecte harmonc comensaton at medum oltage leel, on. Rec. IEEE IAS 3,. 835, 3 [] Awad,H., Sensson,J., & Bollen, M. H. Statc seres comensator or oltage ds mtgaton, IEEE Bologna PowerTech on, Bologna, 3. [] S. M. Sla,.N. Arruda and B. de J. ardoso Flho, Wde bandwdth sngle and threehase P structure or utlty connected systemd, 9 th Euroean conerence on ower electroncs and alcatons. EPE. Graz, Austra. August. [] G. Hseh, and J.. Hung, Phaselooked oo technques a surey, IEEE Trans. On Industral Electroncs, ol. 43, no. 6, [3] Ghosh,A.,Jndal,A.K., & Josh,A. Inerter control usng outut eedback or ower comensatng deces, TENON 3. onerence on onergent Technologes or AsaPacc Regon,.48 5, 3. [4] Vlathgamuwa,D. M., Wjekoon,H. M., and ho,s. S., Interlne dynamc oltage restorer: A noel and economcal aroach or multlne ower qualty comensaton, IEEE Trans. Vol.4, no.6, , 4. oad oltage Tme [sec] Fg.. Resonse o MVDVR wth ector control or unbalanced oltage swell.