Interconnection of PV Modules to the Utility Grid Using an Electronic Inverter Circuit

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1 Inerconnecion of PV Modules o he Uiliy Grid Using an Elecronic Inverer Circui DENIZAR CRUZ MARTINS & ROGERS DEMONTI Deparmen of Elecrical Engineering Power Elecronics Insiue Federal Universiy of Sana Caarina P.O. Box Florianopolis, SC BRAZI Absrac: - In his work i is proposed a simple, however robus, elecronic sysem o exploi he elecrical energy generaed by phoovolaic modules. By means of his sysem he energy is convered, adaped and sen o he elecric uiliy grid. Thus, an amoun of elecrical energy consumed by one given cusomer is produced, having he advanage ha i can be insalled easily in he same place of consumpion, dispensing he use of a ransmission sysem. In order o preserve he qualiy of he produced elecric energy, he sysem samples a reference of he grid volage, obaining, a he oupu, low oal harmonic disorion of he generaed curren. Moreover, oher requiremens are achieved, as he galvanic isolaion beween he modules and he uiliy grid, and he proecion agains energy failure. The power sages, he conrol sraegy, he sysem s operaion principle, he mahemaical analysis and he experimenal resuls acquired wih he prooype consruced in laboraory are presened. Key-Words: - Elecronic Inverer Circui, PV Modules, Elecric Uiliy Grid. Inroducion A phoovolaic module is a device ha, hrough he phooelecric effec, convers luminous energy ino elecric energy. Despie he elecric energy is available in he erminals of he modules in he same insan ha he ligh reaches i, mos of he elecric equipmen of sandard use canno direcly be conneced. This because he generaed power is DC and a low volage (generally beween V and 68 V, depending on he echnology used in he panel consrucion) and he majoriy of he equipmen operaes wih AC, a higher volages ( V o 3 V). As his sysem does no use baeries o sore energy, he generaion depends on exclusively he solar energy availabiliy. Alhough i seems a disadvanage, his opion is economically advanageous. While he module useful life is upper years, a baery operaes for, in he maximum, 5 years and needs periodic mainenance. Currenly, in disribued generaion, i is possible observe a paricular rend in reducing he power of he phoovolaic sysems []. This sraegy allows a sandardizaion and cerificaion, offering possibiliies o esablish warranees o he equipmen. Therefore he designs can be opimized as well as he producion in wide scale and urban inegraion. Based in informaion published in he las years [, 3, 7], i was proposed he sysem here presened. The oupu volage of each panel is abou 5 V in m.p.p (maximum power poin). The power specified o sysem design was W, proper o conver he energy proceeding from wo 5 Wp modules (5 was a W/m insolaion) each one, conneced in series. As he problems relaed o hese ypes of sysems are solved and he modules prices diminish [4, 6], he marke opions become exend. Disribued Power In he las years archiecs around he world began o inegrae a large quaniy of phoovolaic producs ino heir designs, such as modules, inegral roof modules, roofing iles, modules for verical curain wall facades, sloped glazing sysems and skylighs. This characerizes he Build Inegraed Phoovolaic (BIPV). Building applicaion represens one of he mos promising largescale domesic markes for solar echnologies. They serve end-use energy needs a he poin where elecriciy is mos expensive. Boh residenial and commercial buildings offer enormous opporuniy for he building inegraed PV marke. The main benefis of his kind of energy producion are: Consumer-side benefis ower cos elecriciy; Greaer price risk reducion; Greaer reliabiliy and power qualiy; Energy and load managemen; Cogeneraion capabiliy; Grid-side Benefis Reduced elecric line losses; Grid invesmen delay;

2 Improved grid reliabiliy; Auxiliary services, such as volage suppor and sabiliy. 3 Elecronic Circui Presenaion and Operaion Principle The elecronic circui is composed by wo disinc processing energy sages as shown in Fig.. 3. Flyback Converer The firs sage is a flyback converer responsible o increase modules volage. Moreover, his converer makes possible he galvanic isolaion beween he modules and he uiliy grid, propiiaing more securiy o he sysem in case of amospheric discharges and people conac wih he modules. Even i prevens leakage currens and generaion of elecromagneic inerference. The phoovolaic modules direcly feed he inpu of flyback. Is inpu volage is approximaely 3 V and will be increased o 37 V a he oupu. The converer operaes in coninuous conducion mode. This choice allows he lowes RMS S swich curren reducing is conducion losses. Anoher advanage in he coninuous conducion mode is ha he oupu volage depends only on duy cycle D, imposed for he conrol sysem. A regeneraive snubber [8] is used for S swich proecion. + Vpfv C sage one :a S Vg() N Tr N D C + S sage wo D S D - - Cf phoovolaic modules S3 D3 S4 D4 uiliy grid Fig. : Represenaion of he uiliy conneced wo sages power circui. 3. Full Bridge Volage Inverer The second sage is he full bridge volage inverer ha performs wo basic funcions: he coninuous volage inversion and he oupu curren sinusoidal modulaion. The inpu characerisic of his inverer is volage source and he oupu is curren source. Thus i can be conneced o flyback, which he oupu is in volage, and o he commercial uiliy grid, ha has volage source characerisic. The full bridge volage inverer can produce a curren sinusoidal waveform wih low harmonic disorion a he oupu when modulaed in a convenien way Boh sages operae a 5 khz frequencies. Then he soring energy componens (capaciors and inducors) and he ransformer have small dimensions, yielding reduced weigh and size o he sysem. The oupu inverer, which he modulaion is made in wo levels, is conneced direcly o he elecric grid. 4 Mahemaical Analysis 4. Flyback Converer Fig. shows he graphic represenaion of he main waveforms of he flyback converer in coninuous conducion mode. drive signal imax imin imax imin Vg() i () i () c a Ts Ts Fig. : Idealized waveforms of he flyback converer in coninuous conducion mode. (a) Drive signal; (b) Primary winding curren a ransformer; (c) Secondary curren a ransformer. (a) (b) (c)

3 Definiion of he variables used in he mahemaical analysis: Flyback oupu volage; Vpfv a D fb c S a S Phoovolaic module volage; Turns raio ransformer; Duy cycle of flyback converer; S swich conducion ime; S swich opening ime; Vg() Drive signal volage applied o S ; i max Maximum curren in primary winding of he ransformer Tr ; i min Minimum curren in primary winding of he ransformer Tr; i () Curren in primary winding of he ransformer; i () Curren in secondary winding of he ransformer; Turns number of he primary winding; N N V V Ts fb fs fb and Turns number of he secondary winding; Volage a primary; Volage a secondary; Swiching period of flyback converer, Ts fb = ; fs Swiching frequency of flyback converer. From Fig. and Fig. we have Vpfv i = c () i = a () Defining he ransformer urn raio as N V a = N V = (3) I I a = (4) and subsiuing () and () in (3) and (4) resul: Vpfv cs a = (5) as and Defining he duy cycle as cs D fb = (6) Ts Ts = c + a (7) fb S S And subsiuing (6) and (7) in (5) yeld a Vpfv D fb fb = (8) Simplifying resuls in Vpfv Ts ( D fb ) Tsfb D fb = a D The equaion (9) makes he associaion beween he inpu volage Vpfv and he oupu of he flyback converer in coninuous conducion mode, as a funcion of he duy cycle D and he ransformer urns raio a. 4. Full Bridge Volage Inverer The DC volage in he oupu capacior C is processed resuling, in he oupu, an AC curren. This curren is modulaed in he inverer using a grid volage reference obaining a high power facor and low harmonic disorion. Operaing ogeher wih he inducor, he inverer assumes a curren oupu characerisic, allowing is connecion o he grid, which has a source volage characerisic. Through he correc choose of he inducor i is possible o define he maximum ripple of he injeced curren. A small filer capacior (Cf) is added in order o filer he high frequency componen. The role of he diodes D o D 4 (Fig. ) is o conduc he curren hrough inducor during he dead ime beween he swiching of S -S 4 and S -S 3. This is because i canno be simulaneous conducion of he branch S -S 3, he same occurring for S -S 4. Therefore he swiches are urned on and off in pairs (S -S 4 ) and (S -S 3 ). This procedure permis o produce wo volages levels modulaion. Fig. 3 shows he main waveforms of he volage inverer. The ime inerval depiced is ha where he grid volage is near o he maximum posiive value. Definiion of he variables used in he mahemaical analysis: Vr Grid volage; c Swiching conducing ime of S e S 4 ; a Opening ime of S e S 4 ; fb (9)

4 i i drive signal S ; S 4 Ts Ts Vr i = c () drive signal S ; S 3 i Ir min c dead ime a + Vr i = c () Considering null he losses i can be said ha he power in he inpu and he oupu are he same, or eiher Pi = Pr () i i S Ir min i S (collecor) i D v collecor-emier) S Ir min Ir min Fig. 3: Main waveforms of he volage inverer. Pi Inverer inpu power; ; Pr Ii med Ir med Ir min Vr D inv Ts Inverer oupu power; Average curren in he inverer inpu during one swiching period; Average curren sen o grid during one swiching period; The maximum value of he curren in inducor in one swiching period Ts, (ha is he same curren sen o he grid); The minimum value of he curren in inducor in one swiching period Ts.; Grid volage; Duy cycle of he full bridge volage inverer; Swiching period of he full bridge volage inverer. 4.3 Saic gain In order o deermine he saic gain equaion of his sage i mus observe he behaviour of he curren sen o he grid during a whole swiching ime. Therefore he mahemaical consideraions o follow are abou a period Ts of ime. Following he curren i showed in Fig. 3 here is a posiive variaion and anoher negaive one called i e i respecively. As Fig. 3 Pi =.Ii med (3) Pr = Vr.Ir med (4) Ts Ii med = Ii( ) d (5) Ts Ts Ir med = Ir( ) d (6) Ts being he currens and Ii Ir Ir Ir Ir = (7) c a max min max min ( ) + Irmin + Irmax Irmax Irmin Irmax Irmin Ir ( ) = + Irmin + Irmax (8) c a In ha manner, solving he inegrals (5) and (6) resul Ii med = ( Dinv ) ( Irmax + Irmin) (9) Ir med = ( Irmax + Irmin) () Applying he equaions (9) and () in (3), (4) e () resuls in: V V r i = Dinv () The equaion () represens he saic gain of he full bridge volage inverer wih an inducor in he oupu, for he operaion mode showed in Fig Conrol Sraegy Observing he circui in Fig., i is possible o see ha he volage is seady as well as, for one swiching period, he volage Vr. Hence, his wo volages canno be modified by he duy cycle. The duy cycle changing hen increase or decrease he curren sen o he grid. Thus, calculaing he derivaive of he inducor curren, and wih (), resuls:

5 di d [( D ) V ] [ V ] inv i r = [A/s] () If he duy cycle will be such ha produces a value of (D inv -) equal o Vr, he rise raio will be null, di ha is, =. If so, he curren sen o he grid is d consan. The value of D inv for a null rise raio is D V + V i r inv = (3) Accordingly, he curren modulaion chose o conrol his sage was he comparison of is insananeous average values wih a grid sample reference. Then an adequae PWM signal is generaed o he swiches. The modulaion of duy cycle D inv occurs in sinusoidal mode. The swiching frequency is consan and is defined in 5 khz. The employed conrol sraegy is shown in Fig. 3. flyback R R Full bridge volage inverer drive circui comparaor + - riangular waveform inpu B A.B grid curren sample duy cycle muliplicaor inpu A subracor Fig. 3: Employed conrol sraegy. - + R3 R4 volage sample In cases of volage failure he sysem sops o feed he grid because here is no reference o generae he oupu curren. This is a desirable feaure since elecric uiliy grid conneced sysems of energy producion mus hal he generaion in order o avoid he "islanding effec" [5], where pars of he uiliy grid are energized even wih he oal disconnecion of he elecrical sysem by he uiliy. The second conrol loop ha composes he inpu B hrough he resisors R and R was incorporaed o compensae evenual variaions of he uiliy grid volage. The firs sage operaes in open loop, however sudies are being made for he implemenaion of a m.p.p.. sysem (maximum power poin racker - sysem ha searches he maximum power poin) wih he aim o improve he energy colleced from he modules. V ref 6 Design Procedure Design procedure for he flyback converer. Daa: P = W = 37 V fs fb = 5 khz Vpfv = 3 V D fb =,8 Using (9) we have 37,8 = a a = 3,83 3,8 Seing he value of N = 5 urns we ge wih (3) N = 75 urns. These values are used for he consrucion of he ransformer Tr. The chosen core is of he ype EE4/. For a duy cycle of,8 we have, from (6), cs = 3 ms and from (7), as = 8 ms. The capacior C has only he funcion o eliminaes evenual inducance in he wires beween he modules and he converer, herefore is value is very small, resuling in a value of µf 36 V. The capacior C is calculaed wih he equaions o follow: The volage ripple is kep low o avoid large variaions a he oupu. VC = V C Ii c = VC max The curren in he oupu of he firs sage is Pi Ii = Ii = =. 7 A C = = µf Design procedure for he full bridge inverer. Daa: fs inv = 5 khz Vr = V I is assumed ha he maximum value of he volage Vr is V, or, Vr p = 3 V. By considering a modulaion index of 9% in inverer, we have c =, 9 Ts, or, c =,9 4µ s c = 36µ s. Applying in he equaion (), resuls in: 37 3 i = 36 6

6 ,4 = i 3[ V s] [ A] imiing he maximum excursion of he curren i a,% resuls in: [ W ] i =,% =, 9A [ W ] Wih his 3,4 = =, 4,9 [ H ] ha suggesed in a inducor projec of 5 mh. The design of he capacior Cf followed he procedures already known in lier aure for calculaion of high frequency filers. The calculaed value was Cf = 33 nf. 7 Simulaions Resuls Simulaions were made in order o verify he behaviour of he sysem and validaion of he mahemaical sudy. Some resuls are presened below ms V(D7:)/ m 3.8ms 3.ms 3.6ms I(D7) Fig. 7: Swich S 3 drains volage and curren. 6.56ms 6.6ms 6.64ms 6.68ms 6.7ms V(7:,R38:)/37+.5 I(7) Fig. 8: Inducor volage and curren. High frequency. -5m ms 3.8ms 3.85ms ms -I(V) V(D5:)/ Fig. 4: Inverer inpu volage and curren. 6.4ms.ms 5.ms 3.ms 3.8ms V(V:+,V:-)/37 -I(V) Fig. 9: Elecric uiliy grid volage and injeced curren. 8 Experimenal Resuls A laboraory prooype was consruced wih he objecive o confirm he operaion of he sysem. We can observe ha he experimenal resuls of he flyback and he volage inverer maches, respecively, o hose obained by simulaions..6ms I(Rs4).8ms.ms.ms.4ms.588ms V(M3:d)/+5 Fig. 5: Swich S drains volage and curren. volage curren.ms.4ms.6ms.8ms.ms I(Rs4) V(M3:d)/ Fig. 6: Swich S drains volage and curren. Swiching deail. Fig. : Inverer inpu volage and curren. High frequency deail. Scales: V/div; ma/div; µs/div.

7 volage volage curren curren Fig. : Swich S drain volage and curren. Scales: 5 V/div; A/div; µs/div. Fig. 4: Inducor volage and curren. Scales: 5 V/div; ma/div; µs/div. volage curren volage curren Fig. : Swich S drain volage and curren. Swiching deail. Scales: 5 V/div; A/div; µs/div. volage curren Fig. 3: Swich S 3 drain volage and curren. Scales: V/div; ma/div; µs/div. Fig. 5: Elecric uiliy grid volage and injeced curren. Scales: V/div; ma/div; ms/div. 9 Conclusion In his work a simple, however robus sysem was presened. The sysem does no need baeries since i operaes conneced o elecric uiliy grid. The energy supply occurs in periods where he sunligh is presen, being he sysem in wai sae when i does no have ligh. An immediae applicaion for his ype of sysem can be made in places ha need refrigeraion due o he hea produced for he sun, for example, in air -condiioning sysem, where i has coincidence beween he demand of energy for refrigeraion and he generaion of elecric energy by he phoovolaic sysem. The adoped conrol sraegy allowed he producion of a curren wih lile harmonic disorion, simplifying and reducing he size and he number of componens, as much of he conrol, as of he oupu filer. The high frequency operaion allowed he reducion of he magneic componens and he capaciors [7]. This sysem presens some imporan posiive feaures such as he naural isolaion beween he modules and he uiliy grid, robusness operaion,

8 simpliciy in he power sages and in he conrol sraegy, possibiliy of inerconnecion wih oher unis (connecion in parallel) and wide useful life, since no mobile par exiss. This sysem operaes wih commercially available modules being necessary no adapaion of hese o be conneced. References: [] G. J. van der Merwe, and. van der Merwe, 5 W Inverer An Opimal Design for Use in Solar Home Sysems, Proceedings of he IEEE Inernaional Symposium on Indusrial Elecronics, ISIE, Vol, 998, pp [] H. Marsman, A. J. K. Kil, B. J. Hoeksra, K. J. R. Hommerson, and H. Oldenkamp, Design and Operaional Experience wih Small and Medium Sized Inverers in he Neherlands. [3] M. Andersen, and B. Alvsen, W ow Cos Module Inegraed Uiliy Inerface for Modular Phoovolaic Energy Sysems, IECON95 Vol G-, 995, pp [4] R. Rüher, Phoovolaic Solar Energy Elecic Power Generaion, Workshop Solar Energy Elerosul, 998. [5] R. A. Jones, T. R. Sims, and A. F. Imece, Invesigaion of Poenial Islanding of a Self- Commuaed Saic Power Converer In Phoovolaic Sysems, IEEE Transacion on Energy Conversion, Vol.5, No.4, 99, pp [6] E. A. Alsema, P. Frankl, and K. Kao, Energy Pay- Back of Phoovolaic Energy Sysems: Presen Saus and Prospecs, nd World Conference and Exhibiion on Phoovolaic Solar Energy Conversion, 998, pp [7] R. Demoni, and D. C. Marins, Inerconnecion of a Phoovolaic Panels Array o a Single-Phase Uiliy ine from a Saic Conversion Sysem, PESC Power Elecronic Specialiss Conference,, pp. 7-. [8] B. M. Bird, K. G. King, D. A. G. Pedder, An Inroducion o Power Elecronics, John Wiley & Sons. Second Ediion. Grea Briain. 983.