Algarve, Portugal, Jue -, 008 Review o Sigle-phase Iverters for Grid-coected Applicatios R. ATTANASIO*, M. CACCIATO, F. GENNARO*, G. SCARCELLA Departmet of Electrical, Electroics ad Systems Egieerig Uiversity of Cataia Viale Adrea Doria, 6 - IT955 - Cataia ITALY (*) ST Microelectroics - MPA System Lab Stradale Primosole, 5 - IT9500 Cataia ITALY Abstract: - I this paper, a review of iverters is reported based o coverter characteristics as the umber of eergy processig stages, the presece of a trasformer (high-frequecy or 50Hz trasformer), ad the adopted cofiguratio of the field. I particular, the cosidered solutios have bee evaluated depedig o efficiecy, iput curret ripple ad life-time of the capacitors couplig with the solar paels. Key-Words: - Iverter, curret ripple, power quality, life-time, efficiecy, resoat coverter. Itroductio The growig demad for electricity, the commitmet by the leadig idustrialized coutries to reduce the pollutio ito the atmosphere ad the political istability i the coutries that are the major producers of traditioal fossil fuels, has attracted, over the last decade, the attetio of the public opiio ad scietific worldwide research i the field of reewable eergies []. The preset atioal ad iteratioal eergy policies should carefully cosider ay possible ad practical use of o-pollutig eergy sources. The techologies used to produce eergy from such sources as wid, solar, geothermal or biomass, are ready for the market use i some cases, as for the wid, ad eed further improvemets i other cases, as for PhotoVoltaic(). Electricity productio from small geeratio systems ( to 0 kw), based o paels, is ow more attractive thaks to the presece o the market of plug-ad-play coversio systems while ecoomic icetives are provided by several Europea govermets. The diffusio o a large scale of distributed geeratio systems coected to the grid, ivolves may advatages both for the distributor (or utility) istitutio ad for ed users i terms of optimizatio, flexibility ad stability of the service system. Distributed geeratio systems exploitig solar eergy are based o the use of advaced power electroics, sice the curret coversio processes are associated with switchig power coverters, advaced Pulse Width Modulatio techiques (PWM), ad cotrol systems based o oe or more microcotrollers. Ievitably, the use of power electroics forces to take ito accout such issues as cost, efficiecy of coversio, "Power Quality", ad reliability. Although it is hard to completely solve these issues, it is possible to optimize the geeratio system at the desig stage by adoptig coversio topologies that ca better afford the requiremets imposed by the stadards icludig cost cotaimet. The coversio systems curretly used do ot take ito accout such issues as: curret 'ripple' o the couplig capacitor betwee the field ad the coverter partial solar field shade that ca reduce the performace ot perfect matchig of the modules characteristics whe coected i series or parallel. The most commo power coversio system adopts a sigle- or three-phase voltage source iverter, which is curret regulated through a proper PWM modulatio techique. The iverter is directly coected to the field whose output voltage cotiuously varies betwee 500 ad 700 V, ad is iterfaced to the etwork via a 50 Hz isulatio trasformer. The system is cotrolled through a appropriate algorithm called Maximum Power Poit Tracker (MPPT), able to drive the operatig poit of the solar field at its maximum power uder all coditios of radiatio. I this paper some solutios are preseted that use multiple coverters i order to operate the MPPT for a strig, or eve a pael, to icrease the efficiecy of the system ad the life-time of the couplig capacitors with the solar paels.. Iverter Characteristics ISBN: 978-960-6766-7-8 ISSN: 790-5095
Algarve, Portugal, Jue -, 008 With the aim to compare differet solutios, the most importat techical characteristics of iverters that ca be cosidered are as follow:. Iput/Output Power Accordig to its specific desig, each iverter exploits differet characteristics i terms of iput/output power. Typically the power values are idicated as the rated iput, max iput, ad max AC output.. Topologies I order to avoid circulatio of a curret o the AC side, that causes saturatio ad overheatig of power distributio trasformers, the use of a isolatig trasformer at the output side of the coverter is the simplest ad most adopted solutio to avoid ay icoveiece. However, the Low Frequecy (LF) trasformer i covetioal coverter desig is resposible for losses of aroud % i peak efficiecy ad accouts for the largest part of the iverter s weight ad volume. Desig without LF trasformers is cheaper, more efficiet, ad lighter. O the other had, Trasformer Less (TL) desig eeds additioal electroics ad cotrol compoets to provide protectio from ijectio. I particular, a groudig detectio circuit is required i the side ad a circuit for detectig the compoet i additio to the AC curret has to be provided. Eve though maufacturers claim that TL iverters ca be made as safe as LF desig, they caot be istalled i such coutries as Italy ad Spai for power up to 0 kw. Below 0 kw, the use of TL coverters is possible by adoptig a protectio that detects a compoet i the output curret. Because high frequecy trasformers are small, lightweight, ad provide electrical isolatio, they are a compromise betwee the covetioal LF ad TL desig. I may istallatios, as differet strigs ca operate uder differet coditios, a separate / coverter for each strig would be used to improve the produced eergy. Multi-strig techology allows multiple (usually two or three) strigs to be coected to a sigle iverter. Such coverters feature a separate MPPT for each strig, esurig maximum eergy yield, reducig coverter costs for systems. Aother feature of last geeratio iverter cosists i master-slave fuctio. A master coverter cotrols the operatio of oe or two slave iverters, such that the slaves are tured off whe the power produced by the field ca be processed by a sigle or two iverters. I this way, iverters always operate at a optimized power level to achieve higher system efficiecy.. Compoets IGBTs are almost exclusively used as power switchig devices at a switchig frequecy aroud 6 to 0 khz. As size, weight ad cost are strictly related with the umber of passive compoets, topologies with less passives are preferred. Moreover, the Coverter lifetime is maily limited by the use of electrolytic capacitors..4 Cotrol strategies PWM switchig strategies ca be used i voltage or curret cotrol scheme depedig if the feedback cotrol sigal is the output voltage or curret. The curret cotrol scheme is maily used, although some iverters employ the voltage cotrol scheme. The curret cotrol scheme is extesively employed because a high power factor ca be obtaied with a simplest cotrol circuit, ad trasiet curret suppressio is possible whe disturbaces such as voltage chages occur i the utility power system. Curret cotrol scheme hardly allows operatio as a isolated power source but there are o problems with grid-coected operatio. Cofiguratio of Systems The problems cocerig the coectio of a distributed geeratio system with the public grid are may ad sometimes of difficult solutio. Amog them, issues as precise cotrol of siusoidal curret ijectio ito the grid ad maximum eergy extractio from a field are very sesitive. As a matter of fact, it is well kow that the characteristics of modules are strogly oliear depedig o the coditios of temperature ad radiatio, as show i Fig.. The effect of such o-liearity is show i the variatio of the available level of output power from the geerator whe a varyig load impedace is coected, for certai coditios of temperature ad solar radiatio. Such a effect may be compesated by usig a / coverter that, exploitig a MPPT cotrol algorithm, realizes the impedace adaptatio ecessary to maximize the eergy produced from the field []. The coectio to the distributio grid is subordiate to the observace of all laws ad regulatios (CEI - 0, IEC 64-8, CEI EN60947-4-, CEI EN 506, EN 6000--, IEEE 547) which establish the criteria for coectio ad discoectio (atiisladig). Some of the mai requiremets that have to be cosidered i case of the coectio of a system ISBN: 978-960-6766-7-8 4 ISSN: 790-5095
Algarve, Portugal, Jue -, 008 with a power peak of,7kw, which are idicated i EN 6000, are show i Table. Fig. - Curret-Voltage(up) ad Power- Voltage(dow) characteristics of a module. Table : Techical specificatios for grid coectio. Specificatios EN 6000 Nomial Power [kw] <=,7 Amplitude of Curret Harmoics [A] (), (5),4 (7)0.77 (9) 0.4 THD <=5% Maximum curret <0. value [A] Voltage rage 0,8, V Frequecy rage [Hz] 49 5 The geeral block scheme of a system icludes photovoltaic paels, which covert solar eergy directly ito electrical form i, a storage system, ad a /AC coverter. /AC coverters used i systems, ca be firstly classified by cosiderig the umber of eergy processig stages used. I particular, it is possible to cosider sigle-, dual-, ad multi-stage iverters. Other features, that allow a further classificatio, cocer the use of a LF or HF trasformer, where the capacitors used to decouple the primary source ad iverter are coected ad, fially, the type of iverter used: voltage or curret source. Moreover, the cofiguratio of photovoltaic field allows a further classificatio because ca cosists i oe or more strigs coected i series ad parallel, or i oe or more paels coected i strigs.. Sigle stage or cetralized iverters I the past, the easiest way to iterface a large umber of modules to the etwork was the use of sigle of three phase cetralized iverter, depedig if the power were below or above 6 kw. I this cofiguratio, the modules are coected i series to create strigs with output voltage high eough to avoid a additioal voltage boost stage. I order to obtai the desired power level, the strigs are coected i parallel through itercoectio diodes (strig diodes), as show i Fig.. Although this cofiguratio is widely used, especially for high power values of fields, it shows may drawbacks. First of all, the global efficiecy of the geeratio system is effectively reduced []. The mai reaso of reduced performace is due to the cetralized MPPT cotrol that fixes a commo operatig poit for all modules. Because of modules Maximum Power Poit (MPP) variatio with temperature, maufacturig imperfectios, age, ad partial shade, a differet operatig poit should be adopted for each module i order to extract the maximum power from the source. Moreover, some drawbacks are the presece of strig diodes, which itroduce additioal losses, ad the use of a LF trasformer determiig a icrease of cost ad weight of the coverter. Fig. -Cofiguratio phase iverter sigle stage.. Double ad multi stages iverters A more advaced techology is based o the use of photovoltaic fields arraged i strigs rather tha arrays. Each strig is the coected to a double- or a sigle-stage iverter. Accordig to the umber of series coected modules per strig, ad oly if ISBN: 978-960-6766-7-8 5 ISSN: 790-5095
Algarve, Portugal, Jue -, 008 the resultig voltage is sufficietly high (at least 60V), the / coverter ca be omitted. I this last case, a large umber of modules coected i series are required i order to obtai values of ope circuit voltage higher tha 60V. O the other had, with a / boost coverter, the eergy coversio ca also be performed o strigs characterized by low output voltage, resultig from the coectio of a few umber of modules. I ay case, multi-strig iverters, featurig either a double or sigle coversio stage, allow icreasig the efficiecy of the geeratio system. This is maily due to the absece of strig diode losses ad to a more efficiet MPPT performed o each strig. Aother topology adopted by some maufacturers ad still applied to the multi-strig/multi-stage cocept, cosists i the use of a high voltage bus where two or more / outputs are coected accordig to the umber of strigs liked at the iput termials to obtai the desired output power level. The bus voltage is the used to feed a sigle- or three-phase iverter. Actually, this cofiguratio is more complex tha the previous oes but, evertheless, adopted by differet compaies producig photovoltaic iverters. Ideed, the added complexity is compesated by greater flexibility ad better overall performaces. Cosequetly, system efficiecy is higher due to the applicatio of MPPT cotrol o each strig ad higher flexibility comes from the ease of extesios for the photovoltaic field. Eve the so-called "AC modules", obtaied by the combiatio of a photovoltaic module ad a multi-stage iverter for grid coectio are based o the same desig criteria, as show i Fig.. I this case, it is always ecessary to icrease the output voltage of the module with a / boost coverter characterized by high voltage coversio ratio. Such a feature ca be obtaied usig a HF trasformer or by the cascade coectio of two or more amplificatio stages if galvaic isolatio betwee the primary source ad the utility is ot required. 4 Mai Iverter Topologies The choice of coversio topology is, as metioed above, strogly depedet o the field cofiguratio ad its power peak. Geerally, for powers greater tha a few tes of kilowatts, siglestage cofiguratio, as that show i Fig., are preferred. However, most iovative diagrams coversio schemes, as show i Fig. 4, ca also be adopted. The first oe is a multi-phase iverter showig a better efficiecy tha classical half- or full-bridge cofiguratio. The corrected operatios are oly guarateed for a bus voltage value higher tha about 60V. The mai drawback is costituted by the poor maagemet of the eergy that is potetially available from the field. Such a problem is partially solved i the secod cofiguratio of Fig. 4, where the balacig voltage circuit of capacitors C ad C, ad the half-bridge cofiguratio, allows a better use of the available eergy. The MPPT cotrol ca also be applied idepedetly o each strig. Topologies usig a multi-strig cofiguratio for the field ad a multi-stage for the coverter are more coveiet for power levels below 0 kw. S CAP P D DIODE S L9 INDUCTOR CAP P D DIODE S S4 AC AC AC Fig. Sigle coverter (MPPT) for each strig. Lgrid C C Lpv Fig. 4 - No-stadard cofiguratios of sigle stage iverters. ISBN: 978-960-6766-7-8 6 ISSN: 790-5095
4th IASME/WSEAS Iteratioal Coferece o ENERGY, ENVIRONMENT, ECOSYSTEMS ad SUSTAINABLE DEVELOPMENT (EEESD'08) Algarve, Portugal, Jue -, 008 Two of the most iterestig solutios, already o the market, are show i Fig. 5. Both solutios have three / boost coverters, without isolatio i the first case, ad isolated through a HF trasformer i the secod, while the /AC coverter ca be half- or full-bridge type. The decouplig capacitors, which are the weak compoets of the system, are i both cases, coected o low voltage side i parallel with the strigs. A topology that allows to reduce the size of decouplig capacitors, through a reductio of the curret ripple, is show i Fig. 6. The reduced curret ripple is achieved thaks to the iterleaved Vboost Vboost Lgrid C C modulatio of the power devices i the / stage [4][5]. A particular class of iverters is that featurig HF-lik [6]. I this case, the / coverter is suitably cotrolled i order to properly shape the curret accordig to a siusoidal evelope. Such a curret waveform, adequately filtered, is ijected ito the grid through a iverter switchig at 50Hz, i.e. at low frequecies, greatly reducig the commutatio losses of the power devices. Aother class of coverters, whose performace evaluatio has bee recetly started by maufactures, is represeted by Zero Voltage Trasitio (ZVT) phase shift coverters. Amog those, the FB phase-shift coverter is already used i telecom ad server applicatios where high power desity ad high efficiecy are madatory. Active bridges, firstly itroduced i [7] also exploit the phase shift cocept i order to achieve Zero Voltage Switchig (ZVS) for the power devices. A iovative coverter has bee proposed i [8], achievig sigificat reductio of productio costs ad high efficiecy. The coverter scheme is show i Fig. 8 ad is composed of a iput bridge coected to a cotrolled voltage doubler (M5, M6) through a HF trasformer, ad a FB iverter used to geerate a cotrolled 0Vrms, 50Hz siusoidal voltage. The voltage doubler, coected to the secodary side of the trasformer, is adopted to decrease the trasformer turs ratio, simplifyig the widig structure ad miimizig the required widow area. Vboost Fig. 5 Multi-strig/multi-stage cofiguratios. D5 D6 L6 5 Coclusio I the paper, some of the most iterestig ad relevat coverters for applicatios are cosidered ad classified. Some issues, like costs reductio ad efficiecy maximizatio, has bee addressed ad may solutios have bee proposed. Other problems, like the coverter life-time especially for some critical compoets as electrolytic capacitors, are still beig studied ad ot completely solved. Ii L5 0 0 L L C CFc C SC M M D TX D Vbus Gz Gz Z C5 Gz Gz Z4 Z L L4 Z C6 RL C M M4 M D M D4 D D D9 D0 C5 Lgrid Fig. 6 Coverter for ripple reducig of the iput curret. Fig. 7 HF-lik topology. ISBN: 978-960-6766-7-8 7 ISSN: 790-5095
Algarve, Portugal, Jue -, 008 + M M4 M M Llk TX C C M6 M5 Fig. 8 - Zero voltage switchig phase-shift coverter. Today, topologies with classic sigle-stage iverter appear replaced by multi-strig cofiguratios with dual-stage iverter. It is foreseeable that future techological developmets will eable the itegratio of the coversio system withi the photovoltaic module. I such a way, for small power field, the coverter will ot exists aymore as a separate compoet. Refereces: [] T.J. Hammos, Greehouse gas emissios from power geeratio i Europe, 9 th Iteratioal Uiversities Power Egieerig Coferece, UPEC 004, Bristol Uited Kigdom, Sept. 004. [] E. Koutroulis, K. Kalaitzakis, N.C. Voulgaris, Developmet of a microcotroller-based, photovoltaic maximum power poit trackig cotrol system, IEEE Trasactios o Power Electroics, Vol.6,., Ja. 00, pp. 46 54. Z Z4 Z Z LF CF LC V grid [] S. Kjaer, J. Pederse, F. BlaaBjerg : A Review of Sigle Phase Grid coected Iverters for Photovoltaic Modules IEEE trasactio o Idustry Applicatios, vol. 4. 5 october 005, pp. 9-06. [4] R. Attaasio, M. Cacciato, A. Cosoli, G. Scarcella, F. Gearo, A. Testa A Novel Coverter System for Fuel Cell Distributed Eergy Geeratio, IEEE 6th Aual Power Electroics Specialist Coferece, PESC 05, -6 Jue 005, Recife, Brazil, pp. 6-67. [5] M. Cacciato, A. Cosoli, G. Scarcella, A. Testa A Multi-Phase / Coverter for Automotive Dual-voltage Power Systems IEEE Idustry Applicatios Magazie, November/December 004, pp. -9. [6] P.T. Krei, R.S Balog, High-frequecy lik iverter for fuel cells based o multiple-carrier PWM, IEEE Trasactios o Power Electroics volume 9, issue 4, sept. 004 pp:79 88. [7] M.N. Kheraluwala, R.W Gascoige, D.M Diva, E.D. Bauma, Performace characterizatio of a high-power dual active bridge -to- coverter IEEE Trasactios o Idustry Applicatios; volume 8,Issue 6, Nov.-Dec. 99 pp:94 0. [8] M. Cacciato, A. Cosoli, N. Aiello, R. Attaasio, F. Gearo, G. Macia " A Digitally Cotrolled Double Stage Soft-switchig Coverter for Grid-coected Photovoltaic Applicatios ", IEEE Applied Power Electroics Coferece, APEC 008, 4-8 February 008, Austi, TX (USA). ISBN: 978-960-6766-7-8 8 ISSN: 790-5095