ANALYSIS OF RIDE THROUGH WITH THE INTEGRATION OF DIRECT DRIVE SYNCHRONOUS WIND GENERATORS IN POWER SYSTEMS

Transcription

1 ANALYSIS OF RIDE THROUGH WITH THE INTEGRATION OF DIRECT DRIE SYNCHRONOUS WIND GENERATORS IN POWER SYSTEMS João Paulo Abeu ieia Macus inicius Alves Nunes Ubiatan Holana Bezea UFPANESC UFPANESC UFPANESC Ané Cavalcante o Nascimento Selênio Rocha Silva Manoel Fimino e Meeios J. UFPANESC UFMG UFRN bogonio@amazon.com.b selenios@cpee.ufmg.b fimino@ca.ufn.b José Tavaes e Oliveia UFRN jtavaes@ct.ufn.b Simone Fonseca Feeia UFPANESC simonefonsecaf@yahoo.com.b Abstact This pape pesents new opeational poceue elate with the potection of win paks in the configuation of vaiable spee win system with iect ive synchonous geneatos with pemanent magnet (PMSG) connecte on the electical gis. This new poceue is popitiate by an aeuate contol of the convetes of the geneato. Also, it is pesente a new moel base on voltage to epesent the PMSG in tansient stability stuies. The esults obtaine emonstate that maintaining the vaiable spee win tubines with iect ive synchonous geneatos uing sevee istubances in an eal electical system is a moe aeuate poposal with espect to enhance the tansient stability magin of the electical powe system. It is clealy obseve, when the simulations esults ae compae with the case whee the conventional fixe spee win systems with suiel cage inuction geneatos without convetes ae maintaine on the gi. The cage inuction geneatos pesent seious poblems elate with the subexcitation ue to the incease in the eman of eactive powe in the seuence of the fault. Keywos New opeational poceue, vaiable spee win tubines, iect ive synchonous win geneatos, potection, tansient stability magin. I. INTRODUCTION The evelopment of win powe geneation has gown consieably uing the last yeas. The use of win geneatos foming goups enominate win fams, opeating togethe with conventional souces of enegy in weak gis has also incease [], [2]. Examples of this combination can be foun in vaious pats of the wol. The configuations of vaiable spee win geneation that employ iect ive synchonous geneatos with pemanent magnet (PMSG) an oto fiel excitation (RESG), pesent noticeable avantages such as the ecouple contol of active an eactive powe of the geneato, the impovement of system efficiency an the fact that the stato feuency of the machine is ecouple fom gi feuency. The stato convete uses a high feuency switching PWM convete to achieve high contol pefomance with low hamonic istotion, [3]. Due to those aspects in wolwie an specifically in Bazil, thee is an ambition to install a lage amount of win powe base on that technology. The inteaction with the gi becomes inceasingly impotant then. This can be unestoo as follows. When all win tubines woul be isconnecte in case of a gi failue, these enewable geneatos will unlike conventional powe plants not be able to suppot the voltage an the feuency of the gi uing an immeiately following the gi failue. This woul cause majo poblems fo the systems stability. It is theefoe wolwie ecognize that to enable lagescale application of win enegy without compomising system stability, the tubines shoul stay connecte to the gi in case of a failue. They shoul simila to conventional powe plants supply active an eactive powe fo feuency an voltage suppot immeiately afte the fault has been cleae, which is nomally within a faction of a secon [4]. With the pespective of integation of moe Win paks in Bazil the Gi National Opeato (ONS) aleay has set euiements fo the behavio of the potection of the tubines. Instea of isconnecting them fom the gi, the tubines shoul be able to follow the chaacteistic shown in Fig.. oltage in pu 0,9 0,85 0,2 0 T ime fault occue 0,5 Lowe va lue of the voltage ban 5 T ime in s Fig. ONS euiements fo win paks behavio uing faults

2 Only when the gi voltage goes below the cuve (in uation o voltage level), the tubine is allowe to isconnect. When the voltage is in the gay aea, the tubine shoul supply eactive powe. In this pape a metho is popose that makes it possible fo win tubines using iect ive synchonous geneatos to stay connecte to the gi uing faults. The popose potection metho also offes the possibility to supply eactive powe uing the voltage ip an immeiately afte voltage ip. Because the geneato an convete stay connecte, the synchonism of opeation emains establishe uing an afte the fault an nomal opeation can be continue immeiately afte the fault has been cleae. II. WIND TURBINE AND GENERATOR MODEL A. Moeling Assumptions The fist stage in the simulation pocess is to moel iniviual system components with an appopiate egee of complexity. Paticulaly in this poject, moeling of win geneation units was pefome. Seveal win tubine moels have been popose in the existing liteatue [5], an some ae capable of simulating the aeoynamic an the mechanical fluctuations of the tubines that pouce nonconstant toues, such as win shea an towe shaow. A numbe of altenative configuations exist epening on the moe of opeation of the tubine an thei iffeent ynamic esponses, e.g. fixe an vaiable spee moe of opeation, as in [6]. A ynamic win tubine an geneato moel fo application in stability stuies is evelope hee consieing that it is a complex electomechanical system, whee the mechanical powe of the tubine is tansfee though the shaft to the electical geneato an, then, convete to active electic powe. B. Inuction Geneato Moel The moel fo simulation of giconnecte inuction geneato hee consists of:. A 3 oe ynamic inuction geneato moel with oto tansients, [7],[]; 2. A moel of the win tubine an geneato shaft system whee the flexible shaft chaacteistics contibute to stability wosening, [6], [8]; 3. A moel of the tubine, coesponing to the oto aeoynamics, [2]. The asynchonous machine euations ae eive fom Pak s euations [9], [0]. The stato electical tansients ae neglecte as being much faste compae with the oto ones. These euations ae given in peunit (p.u.) system with espect to the synchonous efeence fame. The opeation moe of the asynchonous machine is etemine by the signs of the electomagnetic toue an slip, that is, negative toue an slip coespon to geneato opeation. The algebaic stato euations of the cage geneatos ae: s = R s i s ω s ψ s () s = R s i s ω s ψ s (2) The iffeential euations escibing the ynamics of the oto winings ae: = R i s ω s ψ + ψ /t (3) = R i s ω s ψ + ψ /t (4) The electomagnetic toue euation in p.u. is: T E = ψ i ψ i (5) C. Moel of the Win Tubine an Geneato Shaft System In seveal stability stuies of electic powe systems with giconnecte win tubines pefome so fa, only a lumpe element epesentation of the inetia of the otating system has been consiee, [0]. Howeve, win tubine shafts ae elatively flexible, theefoe it is impotant to incopoate the shaft in the moel in powe system tansient stuies, [6], [8]. Then all the otating shaft systems ae ivie into sections an moe accuate esults ae obtaine by inceasing the numbe of masses, which ae use to epesent the physical chaacteistics of these otating systems, i. e., inceasing the egees of feeom. In case of win tubines, this coespons to the win oto itself, which is uite massive, an the much lighte electical geneato. Since the shaft (plus geabox) seves as mechanical connection between these otating masses is elatively flexible, it is impotant to inclue its epesentation. The main chaacteistic of the two masses an shaft moel can be escibe as given in (6) to (8). ωt H t = ωs t ω 2HG = ω t [ Tt K ( ωt ω ) D ( ωt ω )] s[ K (ωt ω ) TE + D (ωt ω ) ] 2 γ = (ω t t ω ) H t an H G ae the inetia constants of the tubine an geneato espectively, ω t an ω G the angula spee of the tubine an geneato espectively (a/s), k the stiffness of the shaft (pu toue/a. ele.), an γ the shaft twist (eg). Since the tubine shaft is elatively flexible, the value of the initial shaft twist given by, γ o =, [6], [8], cannot be K neglecte an thee will be an accumulate nonnegligible amount of potential enegy in that, which is given by: O E 2 T t ( ( ω ) 2 to ω po = K o ) 2 T (0) E t po = 2 K Whee ω to an ω Go ae the initial angula spee of the tubine an geneato in (a/s). (6) (7) (8) (9)

3 D. Synchonous Geneatos Synchonous iect ive win geneatos ten to be vey lage ue to the highe numbe of poles, esigne fo low spee. It can eithe be on synchonous geneatos with oto fiel excitation o with pemanent magnet. The DC excitation of the fiel wining can be povie by pemanent magnets. Pemanent magnet machines ae chaacteize as having lage ai gaps, which euce flux linkage, even in machines with many electomagnetic poles, [3]. The pactical conseuence of this aspect is the possibility to manufactue low otational spee geneatos with elatively small sizes with espect to ate powe that can be achieve. In this case the electical geneato opeates at low otation spees, being iectly couple to the win tubine shaft with no nee fo geabox, as illustate in Fig. 2. PMSG s have usually a highe efficiency an ae moe compact than synchonous geneatos with oto fiel excitation. Howeve, they ae still consieable moe expensive an euie moe avance ectifies because they on t allow fo eactive powe o voltage contol. Gi C 2 Gi sie convete Stato sie convete C PMSG Fig. 2 Synchonous Geneatos with pemanent magnet One obvious change when eplacing the electical excitation by a pemanent magnet is the elimination of the coppe losses. On the othe han, the magnet chaacteistic is assume constant with the time. So, the euations which escibe this machine ae simila the ones which escibe the conventional synchonous machine in tems of the flux linkage [7], as follows: λ = si + λω t () λ = si + + λω t (2) 0 = k + /t (3) k 0 = i k + λ k /t (4) k ω efes to the oto spee of the geneato; λ an λ (flux linkages in axis an axis espectively); λ k an λ k (ampe wining flux linkages in axis an axis espectively) an ae given by: λ k m i m m i k λ = L i + L (5) λ = L i + L (6) m = L i + L i + L i (7) λ k m kk k kk k m m = L i + L i (8) This synchonous machine moel is able to epesent oto an stato tansient coectly. In stability stuies howeve, tansient phenomena of the electical netwok ae usually not consieable [7]. Applying the pinciple of neglecting stato tansient to the synchonous machine moel euces stato euations to algebaic euations: = i λ ω (9) s = s i + λ ω (20) Howeve it is possible to eefine the euations an epesent them by a voltage behin the tansient eactance moel as conventional synchonous machine euations. Fom the euations efine peviously whee the ampe winings ae consiee, the pemanent magnet synchonous machine can be epesente by a moel as follows: E ω = E E + ( X X ) I t To ωs (2) E ω = E + ( X X ) I t To ωs (22) In this moel the pemanent magnet synchonous geneato is epesente by the subtansient emfs " E an " E behin the subtansient eactances " X an " X. The iffeential euations (2) an (22) inclue the influence of the ampe winings. The sceening effect of the oto boy eycuents in axis ae neglecte so that E = 0. The fiel flux an the fiel voltage efee to the amatue E E f ae assume constant, so that = 0. In this case: t ω E = E f + ( X X ) I (23) ωs In eveloping the geneato moel the spee changes ae ω accounte by a facto in font of evey eactance. ω s The electomagnetic toue of the PMSG machine is given by [5]: T E ( λ i λ i ) 3 P = (24) 2 2 III. MODELS FOR THE CONERTERS Win geneatos base on synchonous machines ae connecte to the electical gi though static convetes as illustate peviously in Fig. 2. In these cases it is euie to moel both convetes, that is, the convete connecte to the electical gi an the one connecte to the stato of the geneato, which ae both selfcommutate PWM convetes. A. Convete Connecte to the Synchonous Geneato The convete connecte to the stato sie can contol DCvoltage of the intemeiate DCcicuit an ACvoltage, being necessay only to efine an aeuate oientation fo the vectos epesenting the stato voltage an flux linkage along the axis an axis espectively.

4 In this way, as the convete is connecte to the machine stato, the cuent in the axis, namely the stato I cuent, may contol the DCvoltage. ACvoltage may be contolle by the axis stato I cuent, accoing to the contol loops as pesente in Fig. 3 fo the convete connecte to the stato of the synchonous geneatos [4]. Fo woune oto synchonous machines having AR s, eactive powe may be contolle instea of ACvoltage. I Ucc, REF + G, REF + Ucc G + Pm I, REF I, REF + Fig. 3 Dual Static Convete Connecte to the Stato of the Geneato As befoe, the contolle is euippe with fast cuent contolles. The cuent efeence values ae hee efine by voltage egulatos as inicate in Fig. 3. Assuming an ieal PWM moulation, the linetoline ACvoltage an the DCvoltage can be elate to each othe as follows [4]: P m I Pm G Ucc = (25) The pulsewith moulation inex vaiable of the PWM convete. P m is the contol B. Convete Connecte to the Electical Gi Fig. 4 is a simplifie epesentation fo the synchonous machine connecte to the electical gi though a static convete. In this epesentation, c is the convete voltage. In steay state conitions the geneato an convete output powes ae eual, which esults also eual cuents J = J2. E I jx G J U CC C J 2 C jx S I S S Fig. 4 Simplifie Repesentation of the Convete Connecte to the Electical Gi On the othe han, fo a istubing conition occuing in the electical system, an unbalance between the output powes of the convete an the synchonous geneato will appeas which will povoke a fluctuation on the voltage Ucc, ue to the cuent change in the DC link. In this way, the convete connecte to the electical gi must have an inepenent contol of the DCvoltage uing istubance events. As the convete connecte to the electical gi also has the function of contolling the powe facto of the win geneation, they must have a contol loop fo contolling eactive powe [3]. The contol loops fo this convete ae epesente schematically in Fig. 5. The gi sie convete is contolle using a synchonous efeence fame. The active pat of the complex cuent becomes I an the eactive pat is I. The ouput powe of the convete may be witten as: S P + iq = ( I ji ) = (26) The convete teminal voltage, S S C, is efine as: 3 C = UCCPm (27) 2 2 The oto spee contol is caie out by two contolles, seies connecte, as illustate in Fig. 5. Reactive powe is contolle iectly by the eactive cuent I. If the convete is to opeate with unity powe facto, the efeence fo the eactive cuent must be set eual to zeo. The output signals in the convete moel ae the pulsewith moulation along the axis an axis, as pesente in Fig. 5. S, REF ω, REF I, REF + + ω I Pm + + I, REF + Pm S Q S, REF Q S Fig. 5 Contol Loops fo the Convete Connecte to the Electical Gi Fo investigating tansient stability phenomena, a simplifie moel can be eive une the assumption that the DCvoltage is constant; this assumption can be justifie by the lage capacitance in the DCcicuit an by the DCvoltage contolles. With this assumption, only the gisie convete nees to be epesente [4]. In this pape, the contol loop pefoming the coupling between the win geneato an the electical netwok is a contolle. Othe contol schemes can be suggest as well [3]. I I. PROTECTION Nomally, iect ive synchonous geneatos ae uickly isconnecte fom the system when a fault is etecte by the powe electonic convete. Potection schemes usually employe in this situation ae liste below:. Ovecuent; 2. Une an ovevoltage; 3. Ovespee. Howeve, it is possible to keep iect ive synchonous geneatos connecte uing a heavy istubance (ie though efault), povie that the contolles of the PWM convete have ability to uickly act on oe to contol

5 spee. With the gisie convete, eactive cuent suppot can be povie uing voltage ips fo lifting the voltage at the geneato teminals in case of faults nea to the connection point.. THE SAMPLE POWER SYSTEM The electical netwok use coespons to a eal powe system belonging to the COSERN utility, that opeates in the notheast egion of Bazil, in the state of Rio Gane o Note. A loa flow solution was caie out to obtain a typical steay state opeating conition fo this netwok. Fo this system, a win pak is planne to be installe at bus 2, as illustate in Fig. 6. The win pak to be connecte is consiee in this stuy as a ynamic euivalent, this way, an euivalent win geneato of 20 MW an 690 is consiee. The win pak is to be connecte to the istibution electical gi by 0.69 k/3.8 k tansfomes. A capacito bank is also planne to be installe in oe to enhance the eactive powe supply fo the win geneation when suiel cage inuction geneatos ae to be simulate. of the iect ive synchonous geneatos with the fixe spee win technology base on suiel cage inuction geneato. The main objective of this type of simulation is to emonstate that the action of the contol loops implemente with the static convetes is effective an avantageous in maintaining the tansient stability of the win geneatos. The use synchonous geneatos ae 2 MW ate powe. The win pak geneation at bus 2 was simulate using the full capacity of the pak. The shot cicuit was locate nea bus 2, whee the win pak is installe, stating at t= s, an lasting fo 00 ms. The istibution line linking busses 2 an 6 was not isconnecte, an this way the win pak was maintaine connecte to the gi, what is an oiginal poposal, as commente peviously. In Figue 7 it is clealy obseve that the ynamic stability limits of the cage geneato without convete contol is exceee. It is ue to the fact that when a voltage ip eaches a win geneato (suiel cage machine), the geneate active powe falls, while the mechanical powe oes not change; so the win geneato acceleates. The oscillation chaacteistics of the oto spee an voltage ae ue two masses moel Inuction Geneato.03 Spee Roto [p.u] Fig. 6 AçuRN Electical Netwok The geneation system is then fome by two euivalent geneation plants, one of them being an euivalent synchonous geneato to epesent the inteconnecte electical system (bus ), fo the which the euivalent automatic voltage egulato use is an IEEE Type moel. The euivalent pimay machine of the synchonous geneato an its spee goveno ae fist oe moels with popotional integal feuency contol [7], [2]. The win pak, will be simulate with iffeent win geneato technologies, namely: suiel cage inuction geneato; iect ive synchonous geneato with pemanent magnet. This system is pesente in Fig. 6, whee the aows epesent loa busses. The system ynamic behavio is simulate using oiginal computing pogams that wee witten in MATLAB TM [2]. I. RESULTS The simulation stuies wee pefome consieing a new opeational pactice that ecommens to keep the win geneatos in opeation even uing the fault peio. The esults to be pesente ae to compae the ynamic behavio Fig. 7 Spee Behavio With the iect ive, the voltage ecoves almost instantaneous afte eclosue, which is assue by the vey fast action of the contolles. The iect ive meets the euie powe balance, which maintain stability an ecove the teminal voltage as can see in Fig.8. Fom the Fig.8, it can be obseve that the pefomance of the iect ive synchonous geneatos pesents small values in tems of steaystate eo, oveshoot, an time euie to each a new steay state. It is ue to the fact that the ynamic behavio of the synchonous geneatos is compensate by the action of the gi sie convetes. The popose opeational poceue of maintaining the iect ive synchonous geneatos in opeation uing the fault peio, also offes the possibility to supply eactive powe uing the voltage ip in oe to facilitate voltage estoation, as shown in Fig.9. The positive influence in the eactive powe behavio of the euivalent synchonous geneato in bus oiginating fom contol system of the gisie of the iect ive, is clealy obseve in Fig.0 afte the fault, whee the eactive

6 powe of the euivalent synchonous geneato at bus etuns to the pevious value befoe the fault. On the othe han, the use of fixe spee win systems base on the cage inuction geneato inceases the eman fo eactive powe necessay to maintain the ynamic voltage stability at the teminals of the win geneatos. oltage [p.u] Inuction Geneato Reactive Powe of the Euivalent Synchonous Geneato[p.u] Inuction Geneato Fig. 0 Reactive Powe of the Euivalent Synchonous Geneato Reactive Powe [p.u] Fig. 8 oltage Behavio Inuction Geneato Fig. 9 Reactive Powe Behavio The aitional eman is met by the eactive powe eseves caie by the ynamic euivalent synchonous geneato in bus, as can be viewe in Fig 0, in the case with cage geneato. In this case the powe system is subject to a suen tansient of eactive powe following the system contingency. This is a limiting fact of the fixe win spee systems configuation, that contibutes to the ecease in tansient stability magin of the powe system, stessing the netwok ue to incease in the eactive powe losses in the lines, theeby causing a heavy eactive powe eman on the gi that can lea to a low unacceptable voltage pofile in a significant pat of the system an then conucting to a voltage collapse of the netwok ue to euction of the eseves of eactive powe. II. CONCLUSIONS With espect to the electical netwok if one isconnects the win geneatos, when a sevee fault occus, what is yet a wiely aopte poceue, may cause seious amage elate with the feuency an voltages of the powe system. Othewise, the simulate esults pesente in this pape have consiee that the win geneatos ae kept unning uing the fault peio (ie though efault) an afte eclosue. It is possible with the contol of the gi sie convetes. The esults have emonstate that the conseuence of this new appoach is positive in the sense of maintaining tansient voltage an angle stability, once a vaiable spee win geneato technology, as iect ive synchonous geneato is use. It was also emonstate that the aoption of vaiable spee win geneatos, base on PMSG with static convetes, like those pesente in this pape, may contibute to incease the contollability of the electical powe system, withstaning sevee istubances fo a longe time without loosing stability. ACKNOWLEDGEMENT The authos wish to thank the Nucleus of Enegy, System an Communication, of the Feeal Univesity of Paá, in Bazil, fo suppoting this eseach, an COSERN the Electical Utility of Rio Gane o Note fo the pemission to use the Açu Netwok paametes. REFERENCES [] N. Jenkins et al., Dynamic Moeling of Doubly Fe Inuction Geneato Win Tubines, IEEE Tans. on Powe Systems, vol. 8 (2), May [2] M.. A. Nunes, J. A. Peças Lopes, U. H. Bezea an H. H. Zün, Influence of the aiable Spee Win Geneatos in Tansient Stability Magin of the Conventional Geneatos Integate in Electical Gis, IEEE Tans. on Enegy Convesion, ol. 9 (4), pp , Dec

7 [3] L. H. Hansen et al., Conceptual suvey of Geneatos an Powe Electonics fo Win Tubines, Risø National Laboatoy, Roskile, RisøR205 (EN), Denmak, Decembe 200. [4] J. Moen, S.W.H. Haan, Riethough of win tubines with oublyfe inuction geneato uing a voltage ip, IEEE Tans. on Enegy Convesion, ol. 20 (2), pp , June [5] M.. A. Nunes, H. H. Zün an U. H. Bezea, Tansient Stability Magin of aiable vesus Fixe Spee win Systems in Electical Gis, Poceeings of 2003 IEEE Bologna Powe Tech Confeence, June 23th 26 th, Bologna Italy. [6]. Akhmatov, H. Knusen an A. H. Nielsen Avance Simulation of Winmills in the Electic Powe Supply, Electical Powe & Enegy System, vol. 22, pp , [7] P. Kunu, Powe System Stability an Contol. USA: McGawHill, 994. [8] M.. A. Nunes, J.P.A. ieia an U. H. Bezea, New Contol to Impove the oltage Stability of the Win Geneatos in the Failue Events, In IASTED Intenational Confeence on Powe an Enegy Systems, Benalmáena, Spain, June, [9] A. Feijoo, J. Cias an C. Caillo, A thi oe moel fo the oublyfe inuction machine, Electic Powe Systems Reseach, ol 56, pp 227, [0] P. Søensen et al., Simulation of Inteaction between Win Fam an Powe System, Risø National Laboatoy, Roskile, RisøR28(EN), Denmak, Decembe 200. [] S. Heie, Gi Integation of Win Enegy Convesion Systems, Chiceste, UK: John Wiley & Sons Lt., 998. [2] N. Jenkins, R. Allan, P. Cossley, D. Kischen, an G. Stbac, Embee Geneation Lonon, U.K.: Inst. Elect. Eng., [3] M.. A. Nunes, J.P.A. ieia an U. H. Bezea et al, New Mathematical Moels to Repesent aiable Spee Win Geneation Systems in Tansient Stability Stuies, 5th Powe Systems Computation Confeence (PSCC05), Liège, Belgium, [4] Achilles, S., Polle, M., Synchonous Machine Moels fo Stability Assessment of Win Fams In Poc. of Fouth Intenational Wokshop on LageScale Integation of Win Powe an Tansmission Netwoks fo Offshoe Win Fams, Billun, Denmak, [5] Kause, P.C.; Wasynczuk, O.; Suhoff, S.D.; 994. Analisys of Electic Machiney. Piscataway, NJ: IEEE PRESS.