AC PV MODULE INVERTERS WITH FULL SINE WAVE BURST OPERATION MODE FOR IMPROVED EFFICIENCY OF GRID CONNECTED SYSTEMS AT LOW IRRADIANCE

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1 A V MODULE INVERTERS WITH FULL SINE WAVE BURST OERATION MODE FOR IMROVED EFFIIENY OF RID ONNETED SYSTEMS AT LOW IRRADIANE M. Jansch,.W.. Verhoeve Neherlands Energy Research Foundaion EN,.O. Box 1, NL-1755 Z een, The Neherlands Tel: , Fax: , jansch@ecn.nl ABSTRAT: Inroducing grid conneced V sysems, he difference beween one-phase and hree-phase power delivery is explained, highlighing he issue of buffering sorage in one-phase V inverers. This paper presens and explains burs operaion mode as a way o improve performance of grid-conneced one-phase V inverers wihou need of major addiional invesmens. A low power, inermien inverer operaion a opimum power for a small fracion of ime is preferred over coninuous operaion a low power, hus saving he difference beween operaing and sandby power consumpion for major pars of he operaing ime, and raising par-load efficiency up o opimum values. Keywords: grid-conneced 1 : V-inverer 2 : operaion-mode - 3 : burs - 4 : A-module INTRODUTION hoovolaic producion of elecriciy coninues o gain imporance, as sysem performances increase furher, prices coninue o become lower, and awareness of he high inernal and exernal coss of fossil and nuclear elecriciy generaion grows. While much aenion is paid o dispersed elecriciy producion wihou inerconnecion (e.g., solar home sysems) [1], and righly so, also he grid conneced way of decenralised producion of elecriciy from he sunligh, be i in smaller or bigger unis, increasingly aracs aenion [2]. This is, of course, especially rue for he counries where a dense public elecriciy grid already exiss [3][4]. While echnology for grid conneced V sysems is already well advanced, sandardisaion is being pursued, and here are sill opions, o discover and o es, for furher improvemen of performance and reducion of coss Inverers for rid onneced V Sysems rid conneced V sysems consis, basically, of a V generaor and a V inverer [5],[6] (see figure 1). Figure 1: eneral diagram of grid conneced V sysem The V generaor, he array of phoovolaic cells o conver sunligh ino elecriciy, may be configured in manifold ways, and accordingly he D o A power conversion as well. From single modules wih individual inverers, over srings of modules wih associaed inverer, o large arrays of serial and parallel connecions of modules, wih one or more (e.g. configured in maser-slave mode) cenral inverers; many differen configuraions may be applied. Funcionaliies ha need o be covered include: convering D power ino A power (one or hree phases [7]) wih highes possible efficiency racking he V array's maximum power poin (M) wih highes possible saic and dynamic accuracy (see poser 7B.3 on MT measuremen [8]) sinewave shaping of oupu curren proporional o grid volage wih cos(ϕ) = 1 and lowes possible harmonics produced safey funcions, surveillance and proecion from D side fauls and A side grid irregulariies, including monioring of volage and frequency and consequen sopping of power delivery in case of irregulariies (o avoid undesired islanding) 1.2. Buffering Sorage in One-hase V Inverers V generaors which are operaed a M deliver direc curren, a a volage ha varies wih emperaure o some exen, and a a curren ha is proporional o irradiance. This power delivery is, no regarding slower flucuaions due o irradiance and emperaure changes here, consan in shor-ime (i.e. milliseconds) perspecive (see figure 2). Figure 2: V arrays deliver coninuous power A hree-phase grid connecion wih hree sinusoidal currens of cos(ϕ) = 1, phase-shifed by 120 wih respec o each oher, acceps a coninuous power flow, which is consan in shor-ime (ms) perspecive; i does no oscillae in he rhyhm of he grid frequency. The sum of he hree phase-shifed phase powers yields a value ha is no a funcion of he grid volage sinewave funcion anymore (see figure 3) :

2 V V EFF Accep i: one phase inverers wih buffering sorage (common sae of he ar [6]) Avoid i: hree-phase inverers wihou buffering sorage (see poser 7B.6 for furher deails [7]) EFF Figure 3: Three phases accep coninuous power () = Σ i=1..3 [ i()] Exploi i: longer buffering duraion possible a lower power, inermien energy delivery o grid connecion, improved efficiency a low irradiance (his lecure L5.3) This aricle examines he hird opion a lile furher. () = Σ i=1..3 [ 2 / 3 V EFF I EFF sin 2 (ω+ϕ i)] = EFF Due o his coninuous power flow (= EFF) no elecrolye capacior is needed in case of a hree phase grid connecion, and he sizing of componens can be considerably reduced (see poser 7B.6 for furher deails on hree phase inverer opologies [7]). A one-phase grid connecion wih one sinusoidal curren of cos(ϕ) = 1 acceps power proporional o sin 2 (ω), which oscillaes every 10 ms beween zero and wo imes he effecive power ( EFF, see figure 4) : () = 2 V EFF I EFF sin 2 (ω) = 2 EFF sin 2 (ω) V V EFF EFF Figure 4: One phase acceps oscillaing power 2. BURST OERATION MODE The erm "burs operaion mode" denoes inermien power delivery from he inverer oupu o he uiliy grid usually performed a power levels considerably below raed power Moivaion Burs operaion mode inends o avoid energy losses which occur due o long ime periods of full operaion of he inverer a low irradiance and hus low power levels. I is expeced ha he high power conversion efficiencies a medium and high power levels can his way be mainained down o very low power levels rinciple Basic idea is, in case of low irradiance, insead of operaing he inverer all he ime a a small fracion of is raed power, o operae i a opimum power level for a small fracion of he ime (see figure 6). onsequenly, for one-phase inverers, a buffering sorage (mosly an elecrolye capacior) is necessary, and he peak power sress MAX on semiconducors and magneic componens reaches 200% of EFF. When momenary A power is high, he capacior suppors he V array feeding he inverer; when i is low, he V array charges he capacior (see figure 5). 2 Figure 5: Momenary A power high and low 0 The oscillaing curren flowing hrough he capacior causes a volage ripple on he capacior volage V. To summarise wih respec o buffering sorage in V inverers, following principal opions are available: Figure 6: oninuous and burs mode inverer operaion Burss of power delivery alernae wih pauses where he inverer sleeps in a sandby mode wih very low self consumpion, in a way ha he effecive power maches he acual V power generaion, hus keeping V array operaion volage V a he M (see figure 7, compare figures 8 and 9). A low irradiance and power he oscillaing (100Hz) curren hrough is smaller, resuling in a smaller volage ripple on V as well. The low slew rae of V, when he capacior is charged from V array curren a low irradiance, allows o omi one or more full sine waves of inverer oupu curren, wihou V exceeding he specified volage window around he M. Is he upper limi

3 of he volage window around he M approached, he inverer can be acivaed for one or more burss of power delivery, pulling he capacior volage V down again o he lower allowed limi. Such an operaion mode resuls in inermien power delivery o he grid, where burss and pauses of one or more full sine waves lengh follow each oher. Medium irradiance and power leads o longer burss and shorer pauses, while a low irradiance burss of jus one full sine wave can be separaed by long pauses (see figures 8 and 9). The burss of oupu curren can be processed a opimum power BURST = OT where he inverer's efficiency η has is maximum value η OT. During he pauses wihou power delivery o he grid, major pars of he inverer are deacivaed for energy saving. sleep while V V < V M + V burs while V > V M - V Figure 7: The capacior volage V, including ripple, needs o remain close enough o he V array's M V > M < M Figure 8: Rare burss and long pauses a low irradiance V > M < M Figure 9: Frequen burss and shor pauses a medium irradiance Dependen on he design consrains and choices, he opimum power may be found beween 20% and 100% of raed power. Lenghs of burss and pauses follow as a funcion of irradiance level. The upper par of figure 10 illusraes in principle he improvemens in effecive power conversion efficiency, resuling from burs operaion mode. According o he fracion of ime passed in sandby, and o he exen ha self consumpion is acually avoided during burs pauses, energy savings can be achieved, resuling in improved par load efficiency. η EFF 90% 80% BURST opimum efficiency opimum power EFF EFF Figure 10: Saving self consumpion in burs pauses improves effecive efficiency, burss keep opimum power while irradiance is low The lower par of figure 10 shows he relaion beween effecive power and burs power. I is possible o work wih variable or fixed burs power level, he laer being shown in figure 10. Falling effecive power level is mached hrough reducing burs lengh and increasing pause duraion Implemenaion To achieve energy savings, he inverer circuis need o be designed in such a way ha inernal power supply of all pars ha are non-operaing in burs pauses can be deacivaed efficienly for shor periods of ime, i.e. for muliples of 20 milliseconds. Self consumpion of execuive pars is saved during burs pauses, only supervisory pars keep operaing (see figure 11). Execuive ars Supervisory ars Figure 11: Inverer design mus suppor efficien shorime de-acivaion of execuive pars. To implemen burs mode conrol efficienly, some redesign of inernal power may be necessary, including ways o deacivae relevan pars of he circuiry for shor imes o save heir power consumpion. No furher addiional hardware is necessary. The buffering capacior ha is presen anyway does no need o be enlarged. A low irradiance levels, is capaciy is sufficien o bridge inverer oupu pauses of lenghs of muliple full sinewaves wihou problem. Such a burs operaion mode can be easily inegraed ino he micro-conroller's program for MT-conrol, sinewave-generaion and day-nigh sarup-shudown. Wih burs pauses allowed o become very long, highly efficien M-racking and power conversion can be con-

4 inued down o exremely low irradiance and effecive power levels EFF. To achieve his, insananeous D curren and volage are measured beween V array and buffering capacior by he microconroller for conrol. In fac, MT and nigh-day ransiions can hus be execued in an inegraed manner hrough he same conrol approach: sriving o keep V wihin he specified volage window around he M hrough sleeping as long as volage is oo low, and bursing whenever volage reaches upper limi, and increasing power level from opimum value o maximum value in coninuous mode when high irradiance demands his (see figure 7, 8 and 9). Firs seps owards burs operaion mode have already been aken by V inverer indusry: The A module inverer OK4-100 [9] operaes in burs operaion mode for effecive power below 3% of raed power. Burs operaion mode is no coninued up o medium power levels here, and burs power level is kep quie low. This choice helps o avoid flicker problems (see below), while i may be possible o furher improve par load efficiency. Opimum choices of parameers for burs operaion mode remain o be revealed by sudies which are no ye compleed ossible Drawbacks Full sine wave burs conrol makes sure ha no addiional harmonics or D currens are inroduced ino he uiliy grid due o non-sinusoidal waveform. However, he inermien delivery of sinewave burss may give rise o flicker problems, which depend on grid impedance and on presence of possibly harmed elecriciy-consumers on he same grid segmen. In fac, already a a few hundred was of burs power could flicker problems arise under very unfavourable condiions, while in oher cases he criical power level is much higher. ossible remedies include a separae grid connecion (e.g. a V power plan) and reducing he burs power level (i.e. a design parameer). I can also be considered ha burs conribuions of large numbers of small A V module inverers, operaing independenly in parallel, combine o a coninuous sinewave curren delivery wih saisically flucuaing ampliude. Thus can he disurbance by flicker phenomena be expeced o reduce according o saisical laws wih rising number of parallel devices. Though in pracice hardly feasible, smar co-ordinaion of many parallel devices, inerlacing heir burs conribuions, could yield o virually coninuous power delivery in oal Expeced Achievemens Furher degrees of freedom for design rade-offs are inroduced hrough considering burs operaion mode. V The difference in power self-consumpion beween operaing and sandby mode of he inverer can be saved for major pars of he operaing ime. Down o very low effecive power levels EFF, he effecive efficiency η a parial load can ideally be raised up o he inverer's opimum efficiency value η OT, which occurs around medium power levels. Regarding he considerable fracion of ime per year passed a low irradiance values, a subsanial par of he self-consumpion can be saved, which may ranslae ino a reducion of he enire inverer losses of abou %. Of course, compared wih an already very efficien inverer a regular operaion mode, he resuling increase of performance raio will hardly be much larger han approx. 1%. However, his improvemen of energy yield can be achieved wihou major addiional invesmens. 3. SUMMARY AND ONLUSIONS Inroducing grid conneced V sysems, he difference beween one-phase and hree-phase power delivery has been explained, highlighing he issue of buffering sorage in one-phase V inverers. Burs operaion mode is presened and explained, elaboraing on moivaion, principle, implemenaion, possible drawbacks, and expeced achievemens in his paper. Burs operaion mode can improve performance of gridconneced one-phase V inverers wihou need of major addiional invesmens. 4. REFERENES [1] H. abler, H.J.M. Beurskens: "Rural Elecrificaion wih hoovolaics", EuroSun'96, Freiburg, ermany, 1996 [2] A. Derrick, R. Barlow, L. Francheschi, A. Sorokin, J. Bonda: "Solar hoovolaics o he Year 2010: A lan for Europe", 13 h E VSE Nice, France, 1995 [3] K. Kiefer, T. Koerkel, A. Reinders, E. Roessler, E. Wiemken: "2250 V-Roofs in ermany - Operaing Resuls from Inensive Monioring and Analysis hrough Numerical Modelling", 13 h E VSE Nice, France, 1995 [4] J. Boumans,.W.A. Balus, R.v. Zolingen, T.v.d. Weiden: "Overview and erformances of rid onneced V Sysems in The Neherlands", 13 h E VSE Nice, France, 1995 [5] M. Jansch: "Nuzungsgrad phoovolaischer Anlagen", VDI-Verlag, Reihe 6, Nr. 336, 1996 [6] H. Wilk: "Inverers for hoovolaic Sysems" in "hoovolaic Sysems", ome-book, Fraunhofer- ISE, Freiburg, ermany, 1995

5 [7] M. Jansch,.W.. Verhoeve: "Inverers wih Three hase Oupu and wihou Elecrolye apacior for Improved Lifeime, Efficiency and oss of rid onneced Sysems", 7B.6, 14h E VSE Barcelona, Spain, 1997 [8] M. Jansch, M. Real, H. Haeberlin,. Whiaker, K. Kurokawa,. Blaesser,. Kremer,.W.. Verhoeve: "Measuremen of Maximum ower oin Tracking Accuracy, Error, and Efficiency", 7B.3, 14h E VSE Barcelona, Spain, 1997 [9] Discussion wih Mr. H. Oldenkamp from NKF Kabel B.V., Elecronics roup - hoovolaics, Delf, The Neherlands, 1997

P. Bruschi: Project guidelines PSM Project guidelines.

P. Bruschi: Project guidelines PSM Project guidelines. Projec guidelines. 1. Rules for he execuion of he projecs Projecs are opional. Their aim is o improve he sudens knowledge of he basic full-cusom design flow. The final score of he exam is no affeced by