DOCTORAT DE L'UNIVERSITÉ DE TOULOUSE

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1 En vue de l'obtenton du DOCTORAT DE L'UNIVERSITÉ DE TOULOUSE Délvré par : Insttut Natonal Polytechnque de Toulouse (INP Toulouse Dscplne ou spécalté : Géne Électrque Présentée et soutenue par : M. NICOLA SERBIA le mercred 9 janver 4 Ttre : CONVERTISSEURS MODULAIRES MULTINIVEAUX POUR LE TRANSPORT D'ENERGIE ELECTRIQUE EN COURANT CONTINU HAUTE TENSION. Ecole doctorale : Géne Electrque, Electronque, Télécommuncatons (GEET Unté de recherche : Laboratore Plasma et converson d'energe (LAPLACE Drecteur(s de Thèse : M. PHILIPPE LADOUX M. POMPEO MARINO Rapporteurs : M. ANDREA DEL PIZZO, UNIV. DEGLI STUDI DI NAPOLI FEDERICO II M. MOHAMED MACHMOUM, UNIVERSITE DE NANTES Membre(s du jury : M. MOHAMED MACHMOUM, UNIVERSITE DE NANTES, Présdent M. PHILIPPE EGROT, EDF R&D MORET SUR LOING, Membre M. PHILIPPE LADOUX, INP TOULOUSE, Membre M. POMPEO MARINO, SECONDA UNIVERSITA DI NAPOLI, Membre M. VINCENZO IMPROTA, SOCIETE ANSALDOBREDA NAPLES, Membre

2 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Acknowledgements The Phd thess s not just an ntensve work perod of three years. Ths s an experence of lfe where I had the opportunty to ncrease my personal and professonal experence. In these three years through the nterfacng wth persons that asssted me and wth whch I had the honor to share ths perod. Wth them (drectly and un-drectly, I had the opportunty to grow up my professonal knowledge and to mprove my personalty. Frstly I would lke to thanks the members of the jury: Mr. Mohamed MACHMOUM, Professor at the Polytech Nantes and Mr. Andrea DEL PIZZO, Professor at the Unverstà degl Stud d Napol Federco II, for havng read n detals ths dssertaton and for havng wrote a very detaled and exhaustve report about the manuscrpt as examners of my thess by contrbutng to the mprovement of the work. Mr. Phlppe EGROT, engneer at EDF n the HVDC frame for havng accepted to revew my thess and havng been part of the jury. Mr. Vncenzo IMPROTA, Engneer at Ansaldobreda SpA for havng accepted to revew my thess and havng been part of the jury. Then I would lke to thanks Mr. Paolo BORDIGNON, executve vce presdent of Rongxn Power Electronc Co., Ltd for hs economc (by means the company and professonal support to the development of the work Then I lke to say thank you to my two supervsors. Peoples that I'll never stop thankng. I can consder these two persons the only guys who attended to my professonal development and they prncpally contrbuted to my professonal and recently personal maturaton. I know that s very dffcult to meet persons lke them. These two persons are alphabetcally sorted below. Thank you to Mr. Phlppe LADOUX, Professor at the Insttut Natonal Polytechnque of Toulouse and drector of my thess. Frstly for havng accepted me as Ph.D. student and to have beleved that for me was possble facng ths much hard and trcky subject. Thank for gvng me hs support and hs methods, and for best havng supervsed my studes. He taught me how the complex world of engneerng can become very easy and pleasurable. For me was an honor workng wth hm and I hope that a new workng adventure s gong to start. Besdes the professonal aspect I found a person wth a great humanty and frendshp. It was able to make easy all the dffcultes that I had at my arrvng n France. Thank you, Phlppe hs famly, because they spent a lot of tme wth me gvng me the honor to meet me at ther home. Wth ther help I never had any dffcultes. Thank you for ther precous frendshp.

3 Ncola Serba Thank you to Mr. Pompeo MARINO, Professor at the Seconda Unverstà degl Stud d Napol, supervsor of my fnal project of the Laurea degree n 7, supervsor of my fnal project of the Laurea Magstrale degree n and fnally my PhD thess drector. Thank you for trustng n me. Thank you havng gave me the opportunty. Thank for your very precous suggestons and your ntutons. He taught me that everythng s possble by workng hard and fne. Under the human aspect I thnk that there are many thngs to learn by hm. He made my Ph.D. perod a superlatve experence. He shared wth me hs passon for the salng whch for me was a very formatve and fantastc experence. So, of course I m sure that a very great frendshp has been consoldated. It s always an honor spendng tme wth hm, thank you Prof.! Thanks to the LAPLACE, I have met a lot of people, and I have to say thanks to someone of them: All the members of the group Convertsseur Statque, expecally the responsble Mr. Frédérc RICHARDEAU. Thank you Les Super Flles! Carne BASTIE, Lea BOULANGER, Cécle DAGUILLANES, Catherne MOLL-MAZELLA and Valére SCHWARZ. Besdes ther mpeccable professonalsm they made the tme at the laboratory more pleasant and cheerful (partcularly the Valere sneezes. Mr Jean-Marc BLAQUIERE for hs great techncal experence and for havng helped me n the realzaton of the prototype. Mr Jeaques BENAIOUN for hs great techncal nformatcs experence. He was very effcent to solve my nformatcs problem. All the PhD students, Post-Doc and others that shared all the good tmes, especally: Andre DE ANDRADE (dedé, he helped me especally at the begnnng. Thank for your frendshp. He never left me alone and Julo BRANDELERO, Clement NADAL and Damen BIDART, les colocatares plus emportants». Franços PIGACHE, a very good frend always helpful and frendly. Then Johannes SCHELLER and Samer YAMMINE, I found very good frends; they are prncpally responsble of my ntensve week-ends. Alberto ROSSI, Etenne FOURNIER (thank you for your help for the language, Jule EGALON, Sebasten SANCHEZ (mon professeur de toulousan, Maud TAUZIA (ma prof de Franças Amanda VELAZQUEZ SALAZAR ( la nna, Mustapha DEBBOU, Bernardo COUGO, Eduard Hernando SOLANO SAENZ, Julan Andres SUAREZ, Benedkt BYRNE sorry f I ve forgotten someone.

4 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Then I say thank you to Gulano RAIMONDO, thank to hs help especally at the begnnng. It was prncpal responsble of my ntegraton. Thank you for your frendshp too. I have also to say thanks to the people of DITEN from the Unversty of Genova, especally to Ganluca PARODI, Lus VACCARO and Prof. Maro MARCHESONI. I have also to say thanks to the people of the Dpartmento d Ingegnera Industrale e dell Informazone of the Seconda Unverstà degl Stud d Napol, especally to: Ncola GRELLA and Angela BRUNITTO for ther effcency n all the bulky admnstratve procedures. My Italans colleagues: Mchele FIORETTO for hs frendshp and for hs tme spent together Lug and Gudo RUBINO, for ther techncal experence and ther very precous help. Especally lately. Felce Andreozz, Marco BALATO and Lug FEOLA. Professors Roberto LANGELLA and Alfredo TESTA for havng shared several tme of hs research wth me, and all the others PhD students I ve met n Aversa. At the end I d lke to say thank you to my FAMILY, they supported always my choces. They are responsble of my ambton. Thank you for havng always beleved n me and for me s an honor beng your son. I hope and I know that ths my new goal helps your happness. Then at the end there s ROSSELLA, despte the dstance she s able to make me a happy man agan. Moreover she makes me want be a better man.

5 Ncola Serba SUMMARY Ths work was performed n the frame of collaboraton between the Laboratory on Plasma and Energy Converson (LAPLACE, Unversty of Toulouse, and the Second Unversty of Naples (SUN. Ths work was supported by Rongxn Power Electronc Company (Chna and concerns the use of multlevel converters n Hgh Voltage Drect Current (HVDC transmsson. For more than one hundred years, the generaton, the transmsson, dstrbuton and uses of electrcal energy were prncpally based on AC systems. HVDC systems were consdered some 5 years ago for techncal and economc reasons. Nowadays, t s well known that HVDC s more convenent than AC for overhead transmsson lnes from 8 - km long. Ths break-even dstance decreases up to 5 km for underground or submarne cables. Over the twenty-frst century, HVDC transmssons wll be a key pont n green electrc energy development. Due to the lmtaton n current capablty of semconductors and electrcal cables, hgh power applcatons requre hgh voltage converters. Thanks to the development of hgh voltage semconductor devces, t s now possble to acheve hgh power converters for AC/DC converson n the GW power range. For several years, multlevel voltage source converters allow workng at hgh voltage level and draw a quas-snusodal voltage waveform. Classcal multlevel topologes such as NPC and Flyng Capactor VSIs were ntroduced twenty years ago and are nowadays wdely used n Medum Power applcatons such as tracton drves. In the scope of Hgh Voltage AC/DC converters, the Modular Multlevel Converter (MMC, proposed ten years ago by Professor R. Marquardt from the Unversty of Munch (Germany, appeared partcularly nterestng for HVDC transmssons. On the base of the MMC prncple, ths thess consders dfferent topologes of elementary cells whch make the Hgh Voltage AC/DC converter more flexble and easy sutable respect to dfferent voltage and current levels. The document s organzed as follow. Frstly, HVDC power systems are ntroduced. Conventonal confguratons of Current Source Converters (CSCs and Voltage Source Converters (VSCs are shown. The most attractve topologes for VSC-HVDC systems are analyzed. The operatng prncple of the MMC s presented and the szng of reactve devces s developed by consderng an open loop and a closed loop control. Dfferent topologes of elementary cells offer varous propertes n current or voltage reversblty on the DC sde. To compare the dfferent topologes, an analytcal approach on the power losses evaluaton s acheved whch made the calculaton very fast and drect. A HVDC lnk to connect an off-shore wnd farm platform s consdered as a case study. The nomnal power level s MW wth a DC voltage of 6 kv. The MMC s rated consderng press-packed IGBT and IGCT devces. Smulatons valdate the calculatons and v

6 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS also allow analyzng fault condtons. The study s carred out by consderng a classcal PWM control wth an nterleavng of the cells. In order to valdate calculaton and the smulaton results, a kw three-phase prototype was bult. It ncludes 8 commutaton cells and ts control system s based on a DSP-FGPA platform. v

7 Ncola Serba RESUME Les travaux présentés dans ce mémore ont été réalsés dans le cadre d une collaboraton entre le LAboratore PLAsma et Converson d Énerge (LAPLACE, Unversté de Toulouse, et la Seconde Unversté de Naples (SUN. Ce traval a reçu le souten de la socété Rongxn Power Electroncs (Chne et trate de l utlsaton des convertsseurs mult-nveaux pour le transport d énerge électrque en courant contnu Haute Tenson (HVDC. Depus plus d un sècle, la génératon, la transmsson, la dstrbuton et l utlsaton de l énerge électrque sont prncpalement basées sur des systèmes alternatfs. Les systèmes HVDC ont été envsagés pour des rasons technques et économques dès les années 6. Aujourd hu l est unanmement reconnu que ces systèmes de transport d électrcté sont plus approprés pour les lgnes aérennes au-delà de 8 km de long. Cette dstance lmte de rentablté dmnue à 5 km pour les lasons enterrées ou sous-marnes. Les lasons HVDC consttuent un élément clé du développement de l énerge électrque verte pour le XXIème sècle. En rason des lmtatons en courant des sem-conducteurs et des câbles électrques, les applcatons à forte pussance nécesstent l utlsaton de convertsseurs haute tenson (jusqu à 5 kv. Grâce au développement de composants sem-conducteurs haute tenson et aux archtectures multcellulares, l est désormas possble de réalser des convertsseurs AC/DC d une pussance allant jusqu au GW. Les convertsseurs mult-nveaux permettent de travaller en haute tenson tout en délvrant une tenson quas-snusoïdale. Les topologes mult-nveaux classques de type NPC ou «Flyng Capactor» ont été ntrodutes dans les années 99 et sont aujourd hu couramment utlsées dans les applcatons de moyenne pussance comme les systèmes de tracton. Dans le domane des convertsseurs AC/DC haute tenson, la topologe MMC (Modular Multlevel Converter, proposée par le professeur R. Marquardt (Unversté de Munch, Allemagne l y a dx ans, semble partculèrement ntéressante pour les lasons HVDC. Sur le prncpe d une archtecture de type MMC, le traval de cette thèse propose dfférentes topologes de blocs élémentares permettant de rendre le convertsseur AC/DC haute tenson plus flexble du pont de vue des réversbltés en courant et en tenson. Ce document est organsé de la manère suvante. Les systèmes HVDC actuellement utlsés sont tout d abord présentés. Les confguratons conventonnelles des convertsseurs de type onduleur de tenson (VSCs ou de type onduleur de courant (CSCs sont ntrodutes et les topologes pour les systèmes VSC sont ensute plus partculèrement analysées. Le prncpe de fonctonnement de la topologe MMC est ensute présenté et le dmensonnement des éléments réactfs est développé en consdérant une commande en boucle ouverte pus une commande en boucle fermée. Pluseurs topologes de cellules élémentares sont proposées afn d offrr dfférentes possbltés de réversblté du courant ou de la tenson du côté contnu. Afn de comparer ces structures, une approche analytque de v

8 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS l estmaton des pertes est développée. Elle permet de réalser un calcul rapde et drect du rendement du système. Une étude de cas est réalsée en consdérant la connexon HVDC d une plateforme éolenne off-shore. La pussance nomnale du système étudé est de MW avec une tenson de bus contnu égale à 6 kv. Les dfférentes topologes MMC sont évaluées en utlsant des IGBT ou des IGCT en boter pressé. Les smulatons réalsées valdent l approche analytque fate précédemment et permettent également d analyser les modes de défallance. L étude est menée dans le cas d une commande MLI classque avec entrelacement des porteuses. Enfn, un prototype trphasé de kw est ms en place afn de valder les résultats obtenus par smulaton. Le système expérmental comporte 8 cellules de commutatons et utlse une plate-forme DSP-FPGA pour l mplantaton des algorthmes de commande. v

9 Ncola Serba RIASSUNTO Durante tre ann del corso d Dottorato d Rcerca n Conversone dell Energa, l attvtà s è svluppata nel quadro d una collaborazone tra la Seconda Unverstà degl Stud d Napol, l laboratoro LAPLACE (Laboratore Plasma et Converson d'energe dell Unverstà d Toulouse. Il lavoro d tes è stato noltre supportato dalla Rongxn Power Electronc (Cna e concerne l mpego del converttore multlvello per le trasmsson n corrente contnua ad alta tensone comunemente conoscute n letteratura come Hgh Voltage Drect Currents (HVDC. Nell ultmo secolo, la generazone, la trasmssone, la dstrbuzone ed l consumo d energa è stato prncpalmente basato su sstem n corrente alternata (AC. I sstem d tpo HVDC s sono res attrattv negl ultm 5 ann per una sere d ragon d natura tecnca ed economca. Ogg, è ben noto che le connesson HVDC sono pù convenent rspetto a quelle AC per dstanze superor a lnee comprese tra 8 km. Questa dstanza d sogla s rduce quando s parla d trasmsson sottomarne. Nel ventunesmo secolo, le trasmsson HVDC saranno un punto chave anche per lo svluppo e l ntegrazone con l preesstente sstema elettrco delle energe rnnovabl. A causa della lmtazone n corrente de dspostv semconduttor e de cav d trasmssone, l mpego d alte potenze s traduce nell mpego d converttor ad alte tenson. Graze alo svluppo d dspostv semconduttor, è ogg possble ottenere converson AC/DC per alte potenze dell ordne de GW. Per dvers ann, converttor Multlvello d tpo sorgente d tensone, n letteratura not come voltage source converters (VSC, consentono d lavorare ad alt lvell d tensone e d mporre una forma d onda d tensone al lato AC pressoché snusodale. Le classche topologe come NPC e Flyng Capactors t tpo VSI sono state ntrodotte crca vent ann addetro ed ogg sono generalmente utlzzate n applcazon d meda potenza come gl azonament delle macchne elettrche. Per la conversone AC/DC ad alta tensone, l converttore modulare multlvello (MMC, proposto crca dec ann fa dal professore R. Marquardt della Unverstà d Monaco (Germana, è sembrato partcolarmente attrattvo ed nteressante per le trasmsson HVDC. Partendo dalla struttura HVDC, s sono consderate all nterno del lavoro d dfferent topologe d celle elementar che rendono l converttore pù flessble e pù faclmente adattable rspetto a dfferent lvell d tensone e corrente. Il lavoro d tes s è svolto secondo l seguente ordne: n prms, sstem HVDC sono stat ntrodott. Le confgurazon convenzonal basate su converttor a sorgente d corrente (CSC e quelle basate su converttor a sorgente d tensone (VSC sono state descrtte. In entramb cas l prncpo d funzonamento sul quale s basa l trasfermento d potenza è stato descrtto. Parallelamente è stato effettuato uno studo sullo stato dell arte de semconduttor mpegat nella elettronca d potenza e sono state tratte v

10 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS valutazon su meglo adattabl alle connesson HVDC. S è evnto dedotto che l orentamento delle trasmsson HVDC è basato sulla conversone VSC. Per tale motvo ha analzzato le topologe multlvello pù attrattve. I prncp d funzonamento dell MMC sono stat studat e l dmensonamento de component reattv è stato proposto consderando due dfferent approcc a seconda del controllo potzzato per l sstema. Nel corso del suo studo s è noltre evnto che dfferent topologe d celle elementar offrono vare propretà reversbltà d corrente o d tensone sul lato DC. Al fne d comparare le dfferent topologe, s è proposto un nuovo approcco analtco per lo studo delle perdte ha reso l calcolo veloce e dretto. In tale ambto una nuova struttura multlvello è stata ntrodotta. Tale topologa è stata pensata per sstem AC/DC basata su raddrzzator a ponte d dod. Tal sstem nfatt sono compost da trasformator d rete d tpo ZgZag confgurat n tal modo da compensare le component contnue della corrente ntrodotte dal raddrzzatore a ponte lato AC. La topologa proposta nel lavoro d tes è pensata per rmpazzare vecch raddrzzator obsolet e poco versatl con una struttura multlvello capace d avere un mpatto armonco rdotto ed un funzonamento a quattro quadrant n termn d potenza. In una fase successva gl stud sono stat valdat attraverso una campagna d smulazon. Il caso consderato è quello d un sstema HVDC-VSC multtermnal chamato ad nterfaccare un parco eolco off-shore sto n Cna. La potenza del sstema è d MW con una tensone DC d 6 kv. Il converttore MMC è stato dmensonato consderando de dspostv IGBT d tpo press-pack e de dspostv IGCT. Le smulazon hanno valdato le smulazon anche n condzon d fault. Lo studo del controllo per l sstema è stato effettuato n prma battuta consderando la classca modulazone PWM. Tale modulazone è stata mplementata sfasando le portant tra le celle che compongono la struttura. Al fne d valdare lo studo e rsultat d smulazone, un prototpo trfase da kw è stato realzzato. Tale prototpo è formato da 8 celle d commutazone d tpo semplc. Il sstema d controllo è stato mplementato graze una pattaforma basata su logca DSP-FPGA. x

11 Ncola Serba RESUME DE LA THESE EN LANGUE FRANÇAISE Chaptre I : Les systèmes HVDC Ce chaptre présente les systèmes pour le transport d énerge électrque en courant contnu à haute tenson (HVDC et soulgne leur rôle clé dans le développement des énerges renouvelables. Ces 4 dernères années, les systèmes HVDC ont été développés pour le transport de l électrcté compte tenu des consdératons technques et économques suvantes : Par rapport aux systèmes en courant alternatf, la transmsson en courant contnu, malgré le coût addtonnel des sous-statons de converson, est économquement ntéressante pour des dstances supéreures à 8 km dans le cas des lgnes aérennes et 5 km pour les lgnes enterrées ou sous-marnes (Fgure I-. Les systèmes en courant contnu permettent les nterconnexons entre des réseaux hétérogènes qu peuvent être asynchrones entre eux, et/ou à fréquences dfférentes. L améloraton constante de la technologe des dspostfs sem-conducteurs a perms d attendre des nveaux de pussance de l ordre du GW. Nous llustrons la descrpton des prncpes de connexon HVDC en fasant référence aux prncpales nstallatons actuelles. Deux prncpaux types de connexon HVDC sont utlsés. Celles basées sur des convertsseurs AC/DC de type onduleur de courant (CSC et celles basées sur des convertsseurs AC/DC de type onduleur de tenson (VSC. Avant d entrer dans les détals de fonctonnement de ces lasons HVDC, nous décrvons les prncpaux dspostfs sem-conducteurs dsponbles sur le marché et employés pour les applcatons «haute tenson». Nous donnons en partculer une descrpton détallée des technologes en boter pressé (press-pack, qu peuvent être consdérées comme les melleures canddates pour la mse en œuvre de sem-conducteurs en haute tenson et fort courant. Nous donnons ensute une descrpton des convertsseurs CSC à base de thyrstors et présentons les prncpes de réglage de la pussance. Du fat que les thyrstors ne présentent pas de problèmes de mse en sére drecte, les convertsseurs peuvent attendre des tensons de l ordre de 5 kv. Ben que smple et robuste, la topologe de type CSC ne permet pas un contrôle ndépendant des pussances actve et réactve et absorbe également des courants non snusoïdaux qu nécesstent des dspostfs de fltrage occupant à 3% de la superfce totale d une sous-staton (Fgure I-4. Les convertsseurs de type VSC commandés en modulaton de largeur d mpulson (MLI sont basés sur des sem-conducteurs à amorçage et blocage commandées (IGBT ou IGCT. Les topologes HVDC-VSC permettent d effectuer le transport d énerge en courant contnu en offrant, vs-à-vs des réseaux AC, des réglages ndépendants des pussances actve et réactve. La mse en sére drecte d IGBT étant très délcate, la tenson reste aujourd hu lmtée à 3 kv pour une topologe classque à tros nveaux de tenson par bras. x

12 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Nous décrvons ensute des topologes multnveaux qu sont adaptées à la haute tenson. Par rapport aux structures classques, elles peuvent garantr une forme d onde quas snusoïdale en rédusant les harmonques et en permettant une réducton des éléments de fltrage. Parm ces topologes multnveaux, nous présentons le prncpe de base du convertsseur modulare multnveaux (MMC qu sera développé dans la sute de la thèse. Cette structure consste en la mse en sére de blocs élémentares dentques (Fgure I-6. Elle est aujourd hu préférée aux structures tradtonnelles car elle garantt une modularté en termes de producton ndustrelle et n a théorquement pas de lmte supéreure pour la valeur de la tenson DC pusqu l est toujours possble d ajouter des blocs élémentares en sére. Chaptre II : Le convertsseur modulare multnveaux (MMC Nous étudons dans ce chaptre le convertsseur AC/DC modulare multnveaux. Le crcut trphasé est formé de la connexon de deux bras par phase (Fgure II-. Chaque bras mpose la moté de la tenson DC ans que la tenson AC. Chacun des bras condut également un ters du courant DC et la moté du courant AC. La combnason des deux bras nous permet d obtenr les courants et tensons AC et DC nécessares au transfert de pussance par la lason DC. Après une premère analyse du fonctonnement, nous proposons un modèle moyen de la structure (macro modèle afn de smplfer l étude de dmensonnement. Ce modèle ne prend pas en consdératon les effets des harmonques dus aux dspostfs de commutaton mas garantt une plus grande rapdté dans les smulatons, les calculs étant smplfés. L étude est en outre valable quelle que sot la topologe des blocs élémentares et consdère une commande MLI classque avec entrelacement des porteuses. Nous effectuons une analyse prélmnare des courants et tensons du convertsseur. Du pont de vue des harmonques de courant, outre les composantes DC et AC, chacun des bras condut une composante au double de la fréquence fondamentale (Fgure II-9. Cette composante découle de l équlbrage énergétque entre les deux bras qu composent chaque phase. La mnmsaton de cette composante jouant un rôle fondamental dans le dmensonnement des éléments de fltrage, nous étudons par la sute deux possbltés lées au plotage de la structure. Des smulatons sur un système de MW composé de 64 convertsseurs élémentares par bras valdent l étude. Dans le premer cas, nous adoptons un contrôle de la structure qu ne permet pas de supprmer l harmonque de second ordre du courant de bras. La lmtaton de son ampltude est alors effectuée exclusvement par les composants passfs. Ans, en augmentant la capacté du condensateur de chaque bloc élémentare et l nductance sére de chaque bras, l ampltude de cet harmonque peut être dmnuée. Pour ne pas lmter la plage de réglage du convertsseur à cause des valeurs élevées de l nductance de bras, nous proposons alors d utlser deux nductances couplées par phase. Elles sont couplées de manère à présenter une valeur élevée vs-à-vs de l harmonque de courant d ordre deux tands qu une valeur fable est présentée vsà-vs de la composante fondamentale de courant. Cette approche requert ben entendu une structure plus coûteuse mas un crcut de contrôle plus smple. Le second cas consdère une commande plus complexe capable de contrôler chaque courant de bras de façon à obtenr la référence désrée à la fréquence fondamentale tout en x

13 Ncola Serba supprmant la composante de rang deux. Dans ces condtons, le dmensonnement des composants passfs est rédut pusque seul l harmonque de courant à fréquence fondamentale est consdéré. La complcaton du contrôle n est pas aujourd hu un problème grâce au large chox de dspostfs numérques de commande dsponbles sur le marché. Ans, avons-nous prvlégé ce cas dans la sute du traval de thèse. x Chaptre III : Nouvelles topologes de convertsseurs modulares multnveaux. Dans ce chaptre, nous proposons et étudons et dfférentes topologes pour le convertsseur modulare multnveaux afn d obtenr dfférentes proprétés en termes de réversblté de tenson ou de courant. La premère topologe consdérée pour réalser un bloc élémentare est une smple cellule de commutaton. C est celle qu est utlsée dans la verson de base du MMC (Fgure III-5. Cette topologe est bdrectonnelle en courant et unpolare en tenson. Pour cette rason, en cas de court-crcut sur le côté DC, le système multnveaux n est pas en mesure de lmter le courant ce qu rsque de détrure les sem-conducteurs. Seules les cellules bpolares sont en mesure de lmter le courant en cas de court-crcut sur le côté DC. Dans ce but, nous ntrodusons le pont asymétrque et le pont complet. La premère structure (Fgure III-8 est bpolare en tenson mas undrectonnelle en courant. Cette topologe rend le MMC peu adaptée au réglage de la pussance réactve mas dans le cas où le facteur de pussance est untare, cette topologe étant undrectonnelle en courant, le système effectue l nverson de la pussance en nversant la tenson DC, ce qu est typque des CSC à thyrstors. Pour cette rason, une telle structure peut être utlsée pour le remplacement mmédat des convertsseurs à thyrstors. Par la sute, nous consdérons également le pont complet (Fgure III-. Assurément, cette structure est la plus flexble car elle est smultanément bdrectonnelle en courant et en tenson, mas par rapport aux deux précédentes elle exge le double de composants sem-conducteurs. Dans ce chaptre, nous présentons une approche analytque pour le calcul des pertes dans les sem-conducteurs. Elle permet par la sute une évaluaton drecte et rapde du rendement du convertsseur AC/DC. Jusqu à présent, dans la lttérature, une telle approche n avat pas été proposée pour le MMC car la forme d onde du courant dans les sem-conducteurs rend le calcul des pertes très complexe. A la sute de la valdaton des formules analytques par des smulatons avec les modules de calcul de pertes du logcel PSIM, nous effectuons une comparason du rendement du système en consdérant l utlsaton des tros topologes mse en avant c-dessus. La comparason est effectuée pour une pussance de MW et une tenson DC de 6 kv. En termes de rendement, la structure à smples cellules est la mons dsspatve. Les deux autres à base de cellules bpolares présentent des pertes plus élevées car elles requèrent au fnal plus de composants sem-conducteurs. Ben que ces topologes permettent au système de meux gérer les condtons de court-crcut DC, une basse même mnme au nveau du rendement (de l ordre,5% est dffclement acceptable compte tenu des nveaux de pussance ms en jeu. Nous présentons ensute une nouvelle structure modulare multnveaux (Fgure III-8 de convertsseur AC/DC. Contrarement à la verson tradtonnelle, cette topologe adopte pour

14 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS chaque phase une seule branche de blocs élémentares et une seule boucle de contrôle du courant. D autre part, celle-c est connectée avec le réseau alternatf trphasé à travers un transformateur zg-zag. Le dmensonnement des composants réactfs et des sem-conducteurs est dentque à la verson de base. A ttre d exemple, nous proposons cette nouvelle structure pour remplacer les ancens redresseurs à base de dodes ou thyrstors (Fgure III-3. Nous développons ce remplacement en conservant le même transformateur de lgne et ans les mêmes nveaux de courant et tenson. Nous effectuons les smulatons en consdérant un système de MVA. Chaptre IV : Commande MLI pour les convertsseurs modulares multnveaux Nous développons dans ce chaptre le contrôle pour les structures MMC en consdérant une modulaton (commande MLI classque avec entrelacement des porteuses. A chaque fos, les smulatons valdent l étude en consdérant un système de MW avec une tenson de 6 kv sur le côté contnu et côté alternatf un fonctonnement à facteur de pussance untare en mode onduleur ou redresseur. La commande pour convertsseurs modulares multnveaux proposée dans ce chaptre comporte tros boucles de contrôle (Fgure IV-3 : Le contrôle du courant assure que chaque courant de branche at les bonnes valeurs des composantes AC et DC nécessares pour obtenr la pussance requse. Après avor établ les équatons électrques du MMC trphasé, nous exprmons les grandeurs électrques dans un repère tournant dq synchronsé sur le réseau alternatf. Une fos les équatons étables dans ce nouveau repère, nous effectuons la synthèse des régulateurs PI de manère à ce que le système sot stable, capable de suvre la consgne de courant à la fréquence fondamentale et de supprmer la composante harmonque de courant de rang. En amont du contrôle de courant l faut assurer l équlbre des énerges stockées dans les condensateurs. Compte tenu de la pussance mse en jeu côté contnu, cette parte du contrôle adapte la pussance actve afn de mantenr constantes les tensons sur les condensateurs des blocs. A cet effet un correcteur PI, dont nous donnons la synthèse, assure pour les branches postve et négatve le contrôle de la valeur moyenne des tensons condensateurs. Dans une branche du convertsseur, les tensons sur chaque bloc peuvent être déséqulbrées à cause des dspersons sur les valeurs des composants passfs et des pertes dfférentes dans les sem-conducteurs. Pour cela, dans le but de réguler chaque tenson condensateur à la valeur désrée, nous prévoyons un contrôle local basé sur un correcteur proportonnel qu agt sur le sgnal modulant au nveau de chaque bloc élémentare. Des smulatons, basées sur un convertsseur ayant des branches avec des pertes par blocs dfférentes, valdent l effcacté de ce réglage. Chaptre V : Prototype de convertsseur modulare multnveaux de kw. Afn de valder les résultats de calcul et de smulaton, nous avons réalsé un prototype à pussance rédute. La structure nclut 8 cellules de commutaton, elle est prévue pour fonctonner avec une tenson DC de 6 V pour une pussance nomnale de kw (Fgure V- x

15 Ncola Serba. Cette maquette a été conçue et réalsée au LAPLACE. Le contrôle est mplanté sur une plateforme DSP-FPGA. Nous testons une premère confguraton conformément à la Fgure V-5. Nous consdérons une branche unque par phase avec une charge RL trphasé de 4 kw en sére, le tout est almenté par une source de tenson contnue de 6 V. Après avor étudé les boucles de contrôle et réalsé des smulatons prélmnares, nous effectuons les tests en boucle fermée. Cette confguraton a été ntalement chose car nous savons que la structure MMC classque, à deux branches par phase, peut dffclement lmter le courant en condton de défaut. Ans, la présence de la charge RL en sére dans chaque branche lmte «naturellement» le courant et permet sans danger la mse au pont des chanes de mesure des sgnaux et la valdaton de la synthèse des régulateurs. La bonne correspondance entre les résultats expérmentaux et les smulatons nous permet alors d aborder le fonctonnement dans une confguraton MMC classque mas dans un premer temps avec un contrôle en boucle ouverte (Fgure V- sur une charge RL trphasée de 5 kw. Des smulatons en boucle fermée avec un contrôle en boucle fermée dans un repère dq valdent ensute la synthèse des correcteurs pour le système de kw (Fgure V-6. Les smulatons sont effectuées sur une charge RL trphasée (Fgure V-. Il nous reste à effectuer les tests en boucle fermée sur la maquette. Conclusons et Perspectves Aujourd hu, les connexons HVDC consttuent un élément de réponse aux besons énergétques mondaux crossants. La technologe multnveaux, assocé au développement de sem-conducteurs haute tenson contrôlés au blocage, va permettre aux convertsseurs de type onduleur de tenson (VSC de devenr la topologe la plus employée dans les systèmes HVDC. Toutefos, grâce aux avantages découlant de la faclté de mse en sére des thyrstors, les structures CSC restent encore meux adaptées aux tensons élevées. A court terme, l écart entre les deux topologes pourrat se rédure de manère sgnfcatve grâce aux performances offertes par les thyrstors blocables de type IGCT. Ces composants en boter pressé présentent par rapport aux modules classques pluseurs avantages : En cas de court-crcut dans une cellule, l n y a pas de rsque d exploson du boter et la structure monolthque de l IGCT (sngle wafer est plus adaptée pour l encapsulaton en boter pressé qu un ensemble de pettes puces IGBT. Ce traval de thèse a porté sur des topologes convertsseurs modulares multnveaux. Pour les études prélmnares, nous avons proposé un «macro modèle», ndépendant de la topologe des blocs élémentares, qu a perms une analyse drecte du fonctonnement et des smulatons plus rapdes. Le dmensonnement du convertsseur a été effectué pour deux stratéges de contrôle. La premère consdère seulement un contrôle de la composante fondamentale du courant de sorte mas entraîne la crculaton d un harmonque de rang deux dans les branches du crcut. La mse en œuvre d nductances couplées dans les branches du convertsseur pourrat xv

16 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS consttuer une bonne soluton pour lmter ce courant mas dans la gamme de pussance vsée (GW, une telle technologe augmenterat les coûts de manère consdérable. En revanche, la seconde approche, consste à contrôler le courant dans chacune des branches mas requert un système de contrôle plus performant basé sur une commande en dq. A cette condton, la composante harmonque de courant de rang deux est supprmée, ce qu permet de mnmser le volume et donc le coût des éléments réactfs. L emplo des dfférentes topologes de bloc élémentare rend le MMC plus flexble en termes de réversblté en tenson et en courant. En termes de pertes, à nveaux de pussance et de tenson contnue dentques, la smple cellule est la plus ntéressante. Cependant, les autres topologes qu fournssent une tenson de sorte bpolare (pont asymétrque et pont complet rendent la structure apte à lmter le courant en cas de court-crcut côté contnu. La commande MLI classque avec entrelacement des porteuses permet une réducton de la fréquence de commutaton ce qu mnmse les pertes dans les sem-conducteurs. Toutefos cette technque de modulaton présente une lmte nféreure en fréquence de commutaton de l ordre de Hz. Quand le nombre de nveaux est très élevé, la modulaton de la tenson en «marches d escaler» peut être très ntéressante. Une étude de cette technque de modulaton (marge d escaler sera développée prochanement. En effet, l nous reste à analyser l mpact de cette stratége de modulaton sur le dmensonnement des éléments réactfs et les pertes dans les sem-conducteurs pour la comparer à la commande MLI classque avec entrelacement des porteuses. Dfférents aspects pourraent rendre le pont asymétrque ntéressant dans les applcatons HVDC. En effet, par rapport à la structure classque, pour une même ampltude relatve d ondulaton de tenson, la capacté du condensateur de chaque bloc peut être rédute. De plus, comme le système effectue l nverson du flux de pussance par le changement de polarté de la tenson DC, cette topologe peut être employée pour remplacer les structures CSC dans des sous-statons HVDC avec l avantage de travaller à facteur de pussance untare. La nouvelle structure à une seule branche par phase (sngle loop proposée dans le chaptre III permet un contrôle plus smple. Elle ne requert pas d nductances en sére dans les branches pusqu elle utlse drectement l nductance de fute du transformateur dont le secondare dot être couplé en zgzag pour annuler la composante contnue du flux dans les colonnes. De plus l solement du transformateur est dmensonné unquement pour la tenson du réseau alternatf. Cec n est pas le cas de la confguraton classque du MMC où, en plus de la composante alternatve de tenson, le transformateur dot supporter une tenson d solement contnue égale à la moté de la tenson sur le len DC (composante homopolare. Au-delà de ces consdératons, et de manère plus générale, l utlsaton de cette nouvelle structure pourrat être ntéressante pour remplacer d ancens redresseurs à dodes ou à thyrstors, en apportant les avantages découlant de la structure VSC. Un prototype de kw a été développé au laboratore LAPLACE. Afn d nterfacer le crcut de pussance avec le système de contrôle, un ensemble de cartes «Interface Hardware» a été réalsée. Cet ensemble de cartes adapte les nveaux des sgnaux provenant des capteurs du prototype aux nveaux des tensons d entrée du dspostf de commande. Il permet auss le xv

17 Ncola Serba fltrage du brut pour les sgnaux analogques. En ce qu concerne les sgnaux numérques de commande en provenance du dspostf de contrôle, ceux-c sont transms aux cellules de commutaton va des fbres optques. Avant de démarrer les essas en pussance, une procédure prélmnare de test a été effectuée. Tous les capteurs ont été calbrés et toutes les connexons de la chane de mesure ont été vérfées. Enfn, l nterconnexon des masses de tout le système a été effectuée pett à pett afn d évter tout problème de compatblté électromagnétque (CEM. Les résultats expérmentaux avec une commande MLI classque avec entrelacement des porteuses ont été obtenus pour la structure à boucle smple et la structure classque. Le bon fonctonnement des boucles de contrôle a perms de valder le modèle du système et la synthèse des régulateurs. Prochanement, ce prototype permettra d une part de tester la structure à une branche par phase avec le transformateur à secondare couplé en zgzag et d autre part le fonctonnement en boucle fermée avec la commande en dq pus la modulaton en «marche d escaler». xv

18 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS RIASSUNTO DETTAGLIATO DELLA TESI IN LINGUA ITALIANA Captolo I: A proposto d HVDC Questo captolo tratta sstem HVDC (le transport d énerge électrque en courant contnu Haute Tenson andandone ad evdenzare l ruolo chave che hanno nel campo della trasmssone dell energa elettrca attraverso la consultazone d crca 4 rferment bblografc. L adozone d sstem HVDC negl ultm 4 ann ha avuto un ruolo fondamentale per sstem d trasmssone per una sere d consderazon tecnche ed economche. Rspetto a sstem d trasmssone AC, trasmettere n corrente contnua, nonostante l costo addzonale dovuto alle sottostazon d conversone, nza a dventare convenente per dstanze maggor d 8 km per lnee aeree e 5 km per lnee sottomarne (Fgure I-. Il contnuo mgloramento delle tecnologe de dspostv semconduttor essendo al cuore delle tecnologe HVDC Sstem n corrente contnua consentono nterconnesson tra ret eterogenee che possono essere asncrone tra loro e/o a frequenza dversa. Una descrzone su prncp d connessone HVDC è stata llustrata facendo rfermento alle prncpal nstallazon attualmente esstent cascuna delle qual n grado d gestre potenze dell ordne de GWs. Due tp d connessone HVDC sono utlzzate. Quelle basate su converttor AC/DC d corrente (CSC e quelle basate su converttor AC/DC d tensone (VSC. I lavor present nella letteratura corrente fno ad ogg s sono sempre focalzzat sulla topologa de converttor, n questo lavoro una delucdazone esaustva sul concetto d trasfermento d potenza d tpo HVDC sa per strutture CSC che VSC è stata data. Prma d scendere nel dettaglo prncpal dspostv semconduttor dsponbl sul mercato ed mpegat per le alte tenson sono stat descrtt attraverso la consultazone d crca vent rferment bblografc. Lo studo ha messo n luce le vare evoluzon de dspostv dandone un ordne d grandezza sulle tenson e corrent nomnal sostenbl per cascuno d ess. Inoltre una descrzone dettaglata della tecnca presspack è stata data gacché tale tecnologa può essere consderata la meglo canddata per l utlzzo e l mpego d dspostv semconduttor nelle alte potenze. Una descrzone de converttor CSC basat su trstor è stata fornta al fne d rendere meglo comprensble l approcco adottato per lo scambo d potenza per queste strutture. Graze al fatto che trstor non presentano problem d connessone dretta n sere, sstem CSC-HVDC rescono a raggungere tenson dell ordne de 5 kv. Ovvamente la topolga non consente l controllo ndpendente della potenza attva e reattva ed noltre esbsce un xv

19 Ncola Serba contenuto armonco n corrente tale da rchedere dspostv d fltraggo che occupano l - 3 % della superfce totale d una sottostazone (Fgure I-4. Una descrzone de converttor VSC modulat PWM (MLI basat su dspostv d commutazone controllabl sa n apertura che n chusura (IGBT è stata fornta. Dopodché la trasmssone d energa HVDC basata su converttor VSC è stata llustrata. Le topologe VSC rescono ad effettuare l trasfermento d potenza attva e reattva n manera ndpendente. D altro canto per topologe classche a due lvell s resce al massmo ad operare a 3 kv a causa de problem dovut alla messa n sere d dspostv IGBT. Le topologe VSC sono state prese n consderazone nel lavoro. In partcolare una descrzone delle strutture multlvello è stata data graze alla loro capactà d lavorare ad alte tenson. Tal topologe rspetto a quelle tradzonal rescono a garantre una forma d onda quas snusodale rducendo l contenuto armonco e permettendo una rduzone degl element d fltraggo. All nterno delle topologe multlvello la struttura modulare multlvello (MMC è stata presentata nel captolo e studata nel lavoro d tes. Tale struttura consste nella messa n sere d converttor elementar (Fgure I-6 normalmente dentc (per questo modulare. Tale struttura è stata preferta a quelle tradzonal gacché garantsce una modulartà n termn d produzone ndustrale e non ha lmtazon superor sul valore della tensone DC poché è possble sempre aggungere converttor elementar n sere. xv Captolo II: Strutture modular multlvello La struttura modulare multlvello è stata studata n questo captolo. La confgurazone trfase per questa struttura è formata dalla connessone d due ram per fase (Fgure II-. Ogn ramo mpone metà della tensone DC e la tensone al lato AC. Cascun ramo noltre conduce un terzo della corrente DC e la metà della corrente AC. La combnazone tra due ram fa s che s ottengano le corrent e tenson AC e DC necessare al trasfermento d potenza rchesto. Per l anals un modello medo della struttura è stato estratto (macro modello al fne d semplfcare le consderazon prelmnar. Tale modello non consdera gl effett delle armonche dovut a dspostv d commutazone ma garantsce una maggore veloctà nelle smulazon poché semplfca calcol. Lo studo noltre è stato ottenuto ndpententemente dalla scelta della topologa per l converttore elementare e consderando una phase shfted snusodal PWM (commande MLI classque avec entrelacement des porteuses. La potenzaltà dello studo, oltre alla semplfcazone della comprensone, sta nel fatto che tale struttura è stata resa altamente flessble e versatle n termn d tenson e corrent gestte. In una anals prelmnare corrent e tenson del sstema sono state analzzate. Dal punto d vsta armonco d corrente, cascun ramo, oltre alle component DC ed AC conduce una componente AC al doppo della frequenza fondamentale che rmane all nterno della struttura (corrente crcolante n Fgure II-9. Questa componente derva dal blancamento energetco tra due bracc che compongono ogn fase. La soppressone d tale componente goca un ruolo fondamentale nel dmensonamento de component reattv che è stata effettuata consderando due cas. Il prmo caso consdera un sstema controllato n manera tale per cu non s è n grado d sopprmere l armonca d II ordne della corrente d ramo. Per tale motvo la compensazone è

20 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS effettuata n manera hardware. Al crescere del condensatore posto n parallelo a cascun converttore elementare e dell nduttore d ramo tale armonca s rduce. Per problem d controllabltà del sstema dovut ad elevat valor dell nduttore d ramo una nuova confgurazone d tpo trpolare per l nduttore è stata proposta nel lavoro. Tale nduttore è confgurato n manera tale da mporre una elevata nduttanza d ramo al fne d sopprmere la II armnca nella corrente ed una rdotta nduttanza d uscta garantendo la controllabltà del sstema. Tale approcco ovvamente rchede un hardware pù costoso ma un controllo pù semplce. Il secondo caso consdera un controllo leggermente pù complesso capace d controllare cascuna corrente d ramo n manera tale da ottenere l rfermento desderato alla armonca fondamentale d corrente e d sopprmere anche la corrente crcolante nel ramo alla seconda armonca della fondamentale. In tal condzon l dmensonamento de component passv s rduce gacché solo l armonca d corrente a frequenza fondamentale è consderata. La complcatezza del controllo non è un problema al gorno d ogg graze alla vasta scelta d dspostv d controllo dsponbl sul mercato. Per questo motvo è stato preferto n questo lavoro d tes. Per tutt cas, smulazon su un sstema da MW composto da 64 converttor elementar per ramo hanno valdato lo studo. Captolo III: Nuove topologe multlvello per sottostazon HVDC In questa parte del lavoro nuove confgurazon per la struttura modulare multlvello sono state studate e proposte. Cò è avvenuto andando a cambare d volta n volta la topologa del converttore elementare. Dsporre d dfferent topologe ha reso la struttura MMC pù versatle e flessble ne confront de lvell d tensone e corrente. La prma topologa consderata come converttore elementare è la cella semplce che rappresenta la versone base dell MMC. Questa topologa è bdrezonale n corrente ma unpolare n tensone. Per tale motvo n condzon d fault DC l sstema multlvello non è n grado d lmtare la correne d corto crcuto rschando d danneggare dspostv semconduttor. Solo celle bpolar sono n grado d meglo lmtare la corrente n condzon d fault DC. A tale scopo l ponte asmmetrco ad H ed l ponte ntero ad H sono stat consderat. La prma struttura (Fgure III-8 è bpolare n tensone ma undrezonale n corrente. Per tale motvo l mpego d questa topologa rende la struttura MMC poco adatta a scamb d potenza reattva. In condzon d fattore d potenza untaro, essendo tale topologa undrezonale n corrente, l sstema effettua l nversone della potenza tramte l nversone della tensone che è tpco de sstem CSC. Per questo motvo tale struttura può essere anche utlzzata per l rmpazzo mmedato d converttor basat su trstor. Infne tra le celle bpolar anche l ponte ad H è stato consderato (Fgure III-. Ovvamente tale struttura è la pù flessble delle prme due essendo anche bdrezonale n corrente ma esge l doppo de component. In questa parte un approcco analtco per l calcolo delle perdte ne dspostv è stato dato. Tale approcco ha reso la valutazone della effcenza del sstema dretta veloce. Tale approcco era stato evtato n letteratura gacché la componente contnua della forma d xx

21 Ncola Serba corrente ne dspostv, dovuta alla struttura MMC, rendeva l calcolo delle perdte molto complesso. Nel presente lavoro nvece la formalzzazone analtca delle perdte è stata formalzzata e valdata. A valle della valdazone delle forme analtche tramte l software PSIM un confronto sul rendmento del sstema è stato effettuato consderando l uso delle tre topologe evdenzate sopra. Il confronto è stato effettuato a partà d potenza ( MW e d tensone DC (6 kv. In termn d rendmento la sngola cella è la meno dsspatva. Le altre due celle b-polar hanno un ncremento delle perdte gacché rchedono un ncremento de component, tal perdte non sono accettabl per lvell d potenza gestta. Ovvamente tal topologe permettono al sstema d gestre meglo le condzon d faults. Successvamente nel captolo una nuova struttura modulare multlvello è presentata (Fgure III-8 chamata Raddrzzatore a sngola semonda. Per ogn fase questa topologa adotta un solo ramo rspetto alla versone tradzonale. D altro canto s nterfacca con la rete attraverso un trasformatore zg-zag. Il dmensonamento de component reattv e de dspostv semconduttor è lo stesso della versone base. Al fne d valdare lo studo del macromodello questa nuova struttura è stata proposta per rmpazzare vecch raddrzzator basat su dod o trstor (Fgure III-3. Il rmpazzo è stato svluppato conservando lo stesso trasformatore d lnea e dunque gl stess lvell d corrente e tensone. Smulazon sono state effettuate consderando un sstema da MVA. Captolo IV: Un nuovo controllo PWM per le strutture modular multlvello Un nuovo controllo per le strutture MMC è stato svluppato n questo captolo consderando una modulazone (commande MLI classque avec entrelacement des porteuses. Volta per volta smulazon hanno valdato lo studo consderando un sstema da MW con una tensone DC d 6 kv. Le smulazon sono state noltre fornte per condzon d funzonamento a fattore d potenza untaro n modaltà nverter e raddrzzatore. Il controllo de sstem MMC n letteratura hanno sempre cercato d sopprmere la seconda armonca d ramo della corrente n manera parallela al controllo tradzonale. C sono numeros lavor che adottano tale sstema rendendo l controllo alquanto complesso sa alla comprensone che all mplementazone [49]-[5]. Il controllo tpco per sstem multlvello è costtuto da tre ccl d controllo fondamental (Fgure IV-3. Il controllo d corrente, asscura che cascuna corrente d ramo abba gust valor per le component AC e DC necessare ad ottenere la potenza rchesta. L approcco per tale controllo è stato effettuato tramte una sovrapposzone degl effett. Dopo aver mpostato le equazon caratterzzant l sstema è stata effettuata una trasformazone delle grandezze nel sstema d rfermento ad ass rotant DQ. Una volta defnte le equazon, la sntes de regolator PI è stata effettuata n manera tale rendere l sstema n grado d nsegure la corrente desderata e sopprmere la seconda armonca d corrente (corrente d rcrcolo ne margn d stabltà. xx

22 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Il sstema d controllo proposto nel lavoro d tes è lneare ed, attraverso la taratura de regolator, agsce n manera tale da sopprmere anche la seconda armonca a frequenza fondamentale d corrente. L nnovazone sta nel fatto che l tutto è effettuato attraverso un sngolo cclo senza l aggunta d loops addzonal adottat n [5]-[5]. A monte del controllo d corrente è posto l blanco d energa d ramo. Tale parte d controllo regola la potenza attva necessara a mantenere tutte le tenson su condensator d ramo costant al valore desderato. La sntes de regolator PI è stata fornta. Cascuna cella d ramo può essere sblancata a causa delle dfferent tolleranze de component passv, conduzon dsegual e/o perdte dfferent ne dspostv semconduttor ed nfne dfferent rsoluzon de sensor. Per questo motvo al fne d blancare ogn cella al valore d tensone desderato, un controllo locale è stato prevsto defnto come l blanco della tensone d cella. Un correttore proporzonale per ogn converttore elementare è stato adottato e sntetzzato. Tale controllo n manera ndpendente da due precedent agsce drettamente sull ndce d modulazone. Smulazon n condzon d celle sblancate hanno valdato lo studo. Captolo V: Il prototpo MMC da kw Al fne d valdare rsultat analtc e smulatv un prototpo a potenza rdotta è stato realzzato. Tale struttura nclude 8 celle d commutazone d tpo semplce (smple cell, una tensone DC d 6 V ed una potenza nomnale d kw (Fgure V-. Il prototpo è stato progettato e realzzato presso l LAPLACE. Inoltre al fne d effettuare test spermental l controllo è stato mplementata tramte pattaforma DSP-FPGA. I lvell d potenza e d tensone scelt per l prototpo sono abbastanza alt per una rspetto a quell adottat n letteratura per la spermentazone da laboratoro d sstem MMC. Una prma confgurazone n modaltà sngle loop è stata testata (Fgure V-5. In partcolare solo un ramo per fase s è consderato con n sere un carco RL da 4 kw. Il tutto n parallelo alla sorgente DC. Dopo la sntes del controllo e smulazon prelmnar, sono stat effettuat tests a cclo chuso. Questa è una confgurazone ntermeda che ha un duplce scopo. È noto che la classca struttura MMC è poco capace d lmtare la corrente d ramo n condzon d faults, per questo motvo non è stata preferta come prma prova. La presenza del carco RL n sere al ramo nfatt lmta la corrente nel ramo garantendo lo stesso un set-up delle catene d segnale e la valdazone della sntes de regolator n condzon d scurezza. È defnta confgurazone ntermeda gacché a causa delle propretà unpolar della cella semplce al carco vene mposta anche una componente DC all nterno del ramo. In ogn caso la buona corrspondenza tra smulazon e prove spermental hanno reso l passaggo alla confgurazone con trasformatore zg-zag mmedato. In un secondo step la struttura MMC a cclo aperto è stata consderata (Fgure V-. Smulazon prelmnar a cclo chuso con un controllo nel sstema d rfermento rotante DQ hanno valdato la sntes de controllor per l sstema da kw (Fgure V-6. Le smulazon sono state effettuate mponendo la rete al lato AC. Il sstema è stato dunque testato n xx

23 Ncola Serba modaltà raddrzzatore e nverter a fattore d potenza untaro. Successvamente prove spermental a cclo aperto sono state effettuate mponendo al sstema un carco RL d crca 5 kw (Fgure V- al fne d valdare la gusta modulazone e l corretto dmensonamento de component reattv (condensator ed nduttor. Resta da effettuare ovvamente l passaggo de test a cclo chuso per la struttura MMC. xx Concluson e Prospettve Al gorno d ogg le connesson HVDC sono una buona rsposta alla fabbsogno energetco mondale che è sempre pù crescente. Le topologe multlvello stanno rendendo Voltage source converters (VSC tra pù mpegat ne sstem HVDC. Lo svluppo de dspostv semconduttor controllat n fase d spegnmento ed mpegat per alte tensone hanno reso queste strutture molto nteressant. D altro canto graze a vantagg dervant dalla facltà della messa n sere d trstor, le strutture CSC gestscono meglo le alte tenson. Nel prossmo futuro, l dvaro tra le due topologe verrà decsamente rdotto graze alle prestazon offerte da dspostv IGCT sa nella fase d accensone che d spegnmento. L nscatolamento a pressone (press pack porta noltre una sere d vantagg rspetto a modul classc specalmente n condzon d emergenza dove c è l rscho d esplosone. La struttura a sngolo tassello (sngle wafer rende l IGCT pù adatto per l nscatolamento a pressone rspetto all IGBT. Per queste ragon l IGCT sembra essere l dspostvo pù attrattvo n applcazon VSC-HVDC. Il lavoro d tes è stato focalzzato su converttor modular multlvello. Per stud prelmnar l macromodello ha consentto valutazon drette e smulazon pù veloc. Inoltre ha reso l modello ndpendente dalla partcolare topologa. Il dmensonamento del sstema è stato effettuato attraverso due approcc d controllo. Il prmo consdera solo un controllo sulla corrente d uscta AC che determna una consderevole seconda armonca nel ramo. L nduttore trpolare accoppato potrebbe essere una buona soluzone al fne d lmtare questa corrente ma nel campo delle applcazon d alta potenza, la partcolartà dell hardware accresce cost n manera consderevole. Il secondo approcco nvece consste nel controllo della corrente d cascun ramo, noltre esso rchede un sstema d controllo pù effcente basato sul sstema d rfermento rotante DQ. Sotto questa condzone la seconda componente armonca della corrente è cancellata andando a rdurre cost degl element reattv. L mpego d dfferent topologe come converttore elementare rende l MMC pù flessble n termn d reversbltà d tensone e corrente. In termn d perdte a partà d potenza e tensone DC, la cella semplce è pù convenente. Le altre topologe però che fornscono una tensone bpolare (HB asmmetrco e ponte ad H rendono la struttura capace d lmtare la corrente d corto crcuto n caso d fault DC. La commande MLI classque avec entrelacement des porteuses porta ad una rduzone della frequenza d swtchng e dunque rduce le perdte ne dspostv. Certamente questa tecnca d modulazone presenta un lmte nferore sulla frequenza d swtchng. Quando l numero de lvell è molto elevato la modulazone sta case (marge d escaler può essere molto nteressante per strutture multlvello. Uno studo della modulazone (marge d escaler sarà

24 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS presto svluppato. Infatt rmane da fare una nvestgazone sugl effett della modulazone ne confront del dmensonamento d element reattv e ne confront delle perdte ne dspostv rspetto alla (commande MLI classque avec entrelacement des porteuses. Dfferent aspett potrebbero rendere l ponte asmmetrco ad H nteressante nelle applcazon HVDC. Se questa topologa è scelta, l condensatore d cella potrebbe essere rdotto a partà d ampezza d oscllazone della tensone. Sccome l sstema effettua l nversone del flusso d potenza tramte l cambamento della polartà della tensone DC, questa topologa può essere mpegata per rmpazzare strutture CSC per sottostazon HVDC con l vantaggo d lavorare a fattore d potenza untaro. La struttura nuova a controllo unco (sngle loop proposta nel captolo III permette un controllo pù semplce. La topologe non rchede l doppo nduttore poché utlzza l nduttore parassta posto n sere al trasformatore accoppato zg-zag. l accoppamento del trasformatore rchede pù rame d un classco avvolgmento. L solamento del trasformatore deve essere effettuato solo per la tensone AC. Questo non è l caso della classca confgurazone per l MMC dove l trasformatore deve sostenere un solamento DC par alla metà della tensone sullo DC lnk (sequenza omopolare. Oltre a queste consderazon, l uso d questa nuova struttura potrebbe essere nteressante per rmpazzare vecch raddrzzator garantendo vantagg dervant dalle strutture VSC. Un prototpo da kw è stato svluppato nel laboratoro LAPLACE. Al fne d nterfaccare l crcuto d potenza con l sstema d ptototpazone rapda, una pattaforma d schede pazzate sulla struttura chamata Hardware d Interfacca è stata realzzata. Questo hardware adatta lvell de segnal provenent da sensor del prototpo verso l lvello delle tenson d ngresso del dspostvo d pototpazone rapda. Inoltre tale hardware fornsce l fltraggo del rumore per segnal analogc. Anche per segnal dgtal provenent dal dspostvo d prototpazone, una conversone elettro-ottca è stata fornta dall hardware d nterfacca al fne d controllare le celle. Prma d avvare le prove n potenza, una procedura prelmnare è state eseguta. Tutt sensor sono stat calbrat e la gusta connessone della catena d segnale è stata verfcata. Infne l ottmzzazone della confgurazone delle masse d tutto l sstema è stata effettuata passo dopo passo al fne evtare tutt probleme dovut alla compatbltà elettromagnetca (EMI Rsultat spermental n (commande MLI classque avec entrelacement des porteuses sono stat ottenut per la struttura a sngolo cclo e quella classca. Il buon funzonamento de ccl d controllo ha valdato l modello del sstema e la sntes de regolator. Prossmamente, questo prototpo permetterà d testare la struttura a sngolo cclo con l trasformatore zg-zag, l funzonamento a cclo chuso nel sstema d rfermento dq e la modulazone a (marge d escaler. xx

25 Ncola Serba Content Chapter I. HVDC SYSTEMS... I. About HVDC... I. HVDC Connecton Systems... 3 I.. The concept of a HVDC connecton... 3 I.. HVDC Confguratons... 7 I..3 Semconductor devces for HVDC systems... 8 I..4 CSC-Phase controlled converters... 6 I..5 CSC-HVDC SYSTEMS... I..6 VSC-PWM based AC/DC converters... I..7 VSC-HVDC systems... 6 I.3 VSC-HVDC multlevel topologes... 7 I.3. Neutral Pont Clamped (NPC... 9 I.3. Flyng capactor... 9 I.3.3 Cascaded Multlevel Inverters... 3 I.4 Conclusons... 3 Chapter II. MMC systems II. The Macro Model II.. Macro model valdaton II.. Study of the MMC basc structure II. Output current mposton... 4 II.. Cell capactor... 4 II.. Branch nductor II..3 Smulatons II.3 Branch current mposton II.3. Szng II.3. Smulatons II.4 Conclusons Chapter III. New multlevel topologes xxv

26 Content MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS III. Elementary converters for the MMC Structure... 6 III.. Sngle Cell... 6 III.. Asymmetrcal H-brdge III..3 H-brdge III. Effcency for multlevel structure III.. The analytcal approach III.. System ratng III..3 Sngle cell& Full H-Brdge III..4 Asymmetrcal H-Brdge III.3 Conclusons III.4 New Modular Multlevel Half Wave topology wth zgzag transformer III.4. 3 MMC Half Wave topology to upgrade obsolete dode/thyrstor rectfers. 8 III.4. Conclusons Chapter IV. PWM Control for Modular Multlevel Converter IV. Introducton IV. Prncple of the Phase shfted PWM for MMCs IV.. Current control loop... 9 IV.. dq reference frame IV..3 Branch energy balancng... IV..4 Cell voltage balancng... 5 IV.3 Conclusons... 8 Chapter V. The kw modular multlevel prototype... 9 V. The prototype confguraton... V.. Reactve elements desgn... V.. Hardware In the Loop confguraton... V. Sngle Loop Confguraton... 4 V.. The control... 6 V.. Smulatons... 8 V..3 Expermental results... V.3 MMC confguraton... V.3. Smulatons n Closed Loop operaton... V.3. Open Loop-Tests... 4 V.4 Conclusons... 3 xxv

27 Ncola Serba A. The Elementary cell V.4. Measurement Cards A. The Frame A.. Acquston Card... 4 A.. Optcal emtter... 4 A. HIL Box A.3 PIN tables xxv

28 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Chapter I. HVDC SYSTEMS Ths chapter presents the HVDC systems by pontng out the key role that they play n the feld of electrcal energy transmsson. After a chronologcal descrpton of the penetraton of the HVDC system n the transmsson grd scenaro, the most employed structures are depcted and ther advantages/drawbacks are descrbed. A comparson s acheved between the Current Source Converter and Voltage Source Converter based HVDC. Nowadays, regardng economc and techncal consderatons VSC-HVDC systems are most popular. Then, ths work focuses on the topology based on Modular Multlevel Converters (MMCs whch s more and more often chosen for VSC-HVDC power statons I. About HVDC The world energy consumpton s expected to ncrease by more than 54% every ten years []. Moreover, populaton growth and the development of new economes requre energy sharng that has to keep n step to guarantee electrcal grd voltage stablty. On the other hand, the Kyoto protocol to the Unted Natons framework conventon on clmate change defned the ways and the constrants of regulatng energy producton. Those n attendance at ths meetng consdered renewable energy sources as a good way to acheve the goal. Snce the begnnng of the st century, many countres have chosen to deregulate the electrcty sector. Ths has created a more flexble mx of energy sources by encouragng hgher effcences, partcularly wth the ntroducton of prvate nvestments n the energy market. In the scenaro of electrcal energy transmsson growth, HVDC systems seem to best meet the purposes gven. As affrmed n [], thanks to ther nherent power flow control capablty and asynchronous feature, HVDC systems assocated wth flexble AC transmsson systems (FACTS are spreadng all over the world. In the last 4 years, HVDC has played a key role n transmsson systems wth a seres of economc and techncal consderatons: As shown n Fgure I-, compared to AC transmsson systems, HVDC transmsson systems become more convenent for a dstance dependng on the lne technology (around 8 km for overhead lne and 5 km for underground or

29 Chapter I HVDC Systems submarne cables. Despte the fact that HVDC converter statons are expensve, the transmsson lne requres a reduced number of conductors whch approxmately leads to a reducton of one thrd of the cost. Cost Break Even Dstance DC converter staton DC AC staton AC 8- km Overhead Lne 5 km Submarne cables Dstance Fgure I- - Estmaton of the coasts for AC and DC transmsson The ever-ncreasng mprovements n power electroncs devces, more partcularly n the feld of turn-off controlled semconductors, are at the heart of HVDC technologes []. HVDC systems allow nterconnectons between mscellaneous grds whch can be asynchronous or wth dfferent operatng frequences. They facltate ntegraton of renewable sources lke wnd farms or photovoltac plants. Untl 5, accordng to [3], the total power nstalled n HVDC systems was around 55 GW, amountng to.4% of the worldwde nstalled generaton capacty. The curve shown n Fgure I- shows the trend of the man nstallatons acheved n the world snce 97. In the next years, 48 GW of HVDC nstalled statons are expected by Chna alone. A detaled overvew on the exstng project can be further found n [4].

30 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS GW Years Fgure I-: Power provded by HVDC transmssons I. HVDC Connecton Systems I.. The concept of a HVDC connecton The evoluton of the sold state devces essentally made possble the concept of the AC/DC converson. The mercury arc valves were replaced by sold-state devces named thyrstors snce 97s. The frst thyrstor employment was the Eel Rver n Canada based on Lne-Commutated Converter (LCC, whch was bult by General Electrc and went nto servce n 97 [5]. Snce that tme onwards, the thyrstor LCCs or Current Source Converters (CSCs have been contnuously dffusng and developng for HVDC applcatons, lke the transmsson systems for whch the basc confguraton s shown n Fgure I-3. The typcal CSC based HVDC connecton assumes n steady state that the power flow s regulated by changng the sgn of the averaged value of the voltages (V out and V out mposed on the DC lne. The system s adopted for hgh power levels, beyond GW, for nstallatons lke the Itapu system n Brazl (6.3 GW [6], or the longest power transmsson that lnks Xangjaba to Shangha [7]. Reactve Power v = v out I DC v out = v Reactve Power Real Power Fgure I-3 - CSC-HVDC system base lay out 3

31 Chapter I HVDC Systems In TABLE I- examples of exstng CSC-HVDC connectons are lsted. The table gves an dea of the power, level voltages and the transmsson dstances nsured by these systems. Project Name France England Shn-Shnano Sakuma Norway Netherland Xangjaba Shangha Completaton Year Power Ratng DC voltage Covered Dstance 986 GW ±7 kv 7 km Maker Alstom Grd Semconductor devces Thyrstors 993,3 GW ±5 kv Mtsubsh Thyrstors 8,7 GW ±45 kv 58 km ABB Thyrstors 7. GW ±8 kv 9 km ABB Thyrstors TABLE I-: Example of Exstng CSC-HVDC connectons The huge DC voltage that these converters can reach s allowed thanks to the drect seres connecton of the thyrstors. More detaled descrptons on the devces are gven n the next secton. Just to gve an dea of the huge physcal structure, an example n seres connecton of 336 thyrstors such as the Shn-Shnano substaton s gven n Fgure I-4. Another example s also gven n Fgure I-5. It corresponds to the substaton located n France (Les mandarns of the France England HVDC nterconnecton. Fgure I-4: thyrstor tower for the frequency converter on Shn-Shnano sde Fgure I-5: Thyrstor tower of the France-England connecton on the french sde However, the development of hgh rated fully controllable swtches, whch are descrbed n the next secton, such as nsulated gate bpolar transstors (IGBTs and gate-commutated 4

32 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS thyrstors (IGCTs let the Voltage Source Converters (VSCs be an attractve alternatve to the CSCs for HVDC applcatons. The level of power afforded for these systems goes hand n hand wth the evoluton of the VSC topologes and the voltage whch the semconductor devce s able to sustan. A basc confguraton of these connectons s shown n Fgure I-6. In steady state the power flow s regulated by changng the sgn of the averaged value of the currents mposed on the DC lne. The frst VSC-HVDC nstallaton, whch consoldated the success of these systems, was the HVDC Hellsjön Grängesberg (Sweden from ABB, called HVDC Lght [8]. It s a PWMcontrolled system bult at the begnnng of 997 []. The power ratng s about 3 MW wth a voltage of kv. Many other nstallatons are lsted n [9]. AC Reactve Power = v out + - V DC + - out = v AC Real Power Fgure I-6 - VSC-HVDC system base layout Frst VSCs for HVDC applcatons were two-level nverters whle three-level nverters (Neutral Pont Clamped topology were ntroduced later. To sustan the huge voltage the nstallatons are composed by seres connected IGBTs. ABB s the only maker whch provdes ths confguraton (Fgure I-7 and Fgure I-8 []. The confguraton allows each VSC to sustan maxmum 3 kv wth a maxmum power ratng at one GW. Fgure I-8: IGBTs Seres connected lay-out Fgure I-7: Typcal seres valve tower of ABB 5

33 Chapter I HVDC Systems The last generaton of multlevel VSC for HVDC applcatons s the Modular Multlevel Converter (MMC or MC, ntroduced by Marquardt and Lesncar n, []. The frst applcaton of MMC for HVDC was the Trans Bay Cable n San Francsco, Calforna, powered by Semens n November, []. Another example of VSC HVDC connecton based on MMC topology s the France-Span nterconnecton INELFE [3] (Fgure I-9. The connecton s acheved wth a DC voltage of ±3 kv and nvolves two converters of GW each one. In Fgure I- s depcted a typcal tower provded for a phase of the MMC. An overvew of a typcal MMC based substaton for HVDC connectons s shown n Fgure I-9. Fgure I-9: Lay out of a HVDC sub-staton based on MMC topology proposed by SIEMENS Fgure I-: typcal MMC tower of a phase based on IGBT and proposed by SIEMENS In [] s affrmed that the CSC-HVDC systems, also called classc HVDC, can be consdered as mature technologes today [4]-[8]. Advantages nclude ther natural ablty to lmt the currents n fault condtons on the DC lnk. In the past, the nablty of VSC structures to lmt DC currents under fault condtons lmted ther adopton. Of course the CSC-HVDC connectons allow the system workng at hgher DC voltages such as ±5 kv. Ths because the seres connecton of thyrstors s well mastered. Today, for these reasons, only Ultra HVDCs are based on thyrstor converters [6]. A more detaled descrpton of the problems whch the seres connectons of IGBTs lead s gven n [8]. There are many reasons whch justfy the success of VSC structures. We can menton new developments n crcut breakers (CB, n control systems whch are able to regulate the DC voltages not also n ordnary condtons [9]. Moreover systems whch allow 6

34 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS the power reversblty through the changng of the current drecton requre a less expensve cable technology than the others [] (CSC-HVDC systems allow the change of the power flow through the changng of the DC voltage polarty. I.. HVDC Confguratons Dfferent confguratons of HVDC systems can be determned accordng to the partcular applcaton and the project consdered. The man confguraton lay outs are shown n ths secton. Then the methods to regulate the power flow are descrbed for CSC and VSC systems. Back-to-back systems are composed of two converter statons. The converson takes place n the man locaton, and these systems are not sutable for long-dstance transmsson. The block dagram depcted n Fgure I- shows AC/DC converson. Ths facltates the connecton between asynchronous grds. Ths knd of connecton s also known as a unpolar system. Unpolar systems can be employed also for submarne connectons by usng the ground to return current. On the other hand many problems can be led from ths knd of employment [9]. = = Fgure I- BB HVDC system One of the most used confguratons s as shown n Fgure I-. These systems are manly employed to transmt power n overhead lnes. Also called bpolar systems, these are composed of two unpolar structures. Usually the double structure can be consdered to be a redundancy. Of course f one of the two converters turns off, half part of the total power can be guaranteed on the lne [9]. Ths structures use the ground as potental reference. 7

35 Chapter I HVDC Systems = = AC = AC = Fgure I-: Bpolar system By connectng more than two sets of converters, t s possble to arrange mult-termnal connectons Fgure I-3. For the partcular depcted confguraton converters, f CSC based connectons are consdered, and 3 operate as rectfers whle converter can operate as nverter. By mechancally swtchng the connectons of a gven converter, other combnatons can be acheved [9]. For VSC based connectons the swtch s not necessary due to the sgn of the DC voltage s kept. AC AC 3 = = = = = = AC Fgure I-3: Mult-termnal connecton I..3 Semconductor devces for HVDC systems Despte the huge cost of devces employed for the medum and hgh power applcatons, ths knd of applcaton covers only a much reduced part of the semconductor total market [] as shown n Fgure I-4. The dstrbuton and the trend of the semconductors n power electronc feld are reported n Fgure I-5. The descrpton ndcates the manufacturer and places the semconductor devces accordng to voltage and current ratng. 8

36 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS MW ( B$ kw Total (B$ W mw Fgure I-4: Total semconductor market ( Fgure I-5: Dstrbuton and trend for semconductor devces n power electronc feld [] An nvestgaton on the most used semconductor devces was provded for the HVDC connecton's feld. For each devce the operatng range was gven n terms of managed power, moreover advantages and drawbacks whch decded the replacement of one respect to one another were hghlghted. In [3] s affrmed that the devce manufacturers have developed dfferent technologes for addressng the demand for an ncreasng relablty and lfetme. In ths context, the devce packagng assumes a crtcal role. Several manufacturers prefer power modules wth bonded nterconnectons even though these bondng wres and solder layers are susceptble to thermomechancal stress and ultmately falure when subjected to power cyclng. In the hgh power electronc, partcularly n HVDC feld, a consoldated packagng structure s the press contact assembly technology [4] called Press-pack (PP. Ths technology acheves the conducton on the power sde of the semconductor juncton through pressure contact surfaces. Ths leads to elmnate bondng wres and solder layers; t offers an mproved power cyclng lfetme [3]. Accordng to the type of devce, dfferent technologes were developed by the makers such as sngle wafer (Fgure I-6 PP or mult-de devce PP (Fgure I-7. Fgure I-7: Example of Press-Pack mult-de devce 9

37 Chapter I HVDC Systems Fgure I-6:Cross secton of a press-pack sngle wafer (monolthc Typcal frame assembles are provded for the sngle wafer press-pack and for mult de devce n Fgure I-8 and Fgure I-9. Fgure I-8: Press-pack sngle wafer tower by ABB Fgure I-9: Assembled Press-Pack mult-de devce I..3. Dode For HVDC connectons the fast dodes for the free-wheelng and the clampng ones are used accordng to the topology. The operatng voltage range for the sngle devce s about - kv. Moreover these devces can reach currents of -7 ka. The devce s almost composed by a monolthc juncton even for Press-Pack structures. Problems due to the reverse recovery are well treated n lterature especally for the free wllng dodes. Unexpected problems are caused by ths phenomenon such as overvoltage and HF oscllatons whch lead to EMI problems. The most frequent problems are the Snappy Recovery (Fgure I--a and Reverse Recovery Dynamc Avalanche (Fgure I--b. The study of these phenomena were been consoldated n [6] whch showed that under adverse combnatons of hgh commutatng d/dt, large crcut stray nductance, low forward current and low juncton temperature, t s lkely that all fast power dodes produce excessve voltage spkes due to snappy recovery. One of the last hgh voltage dode technology s proposed by [6] and exhbts soft recovery performance under all operatng condtons. Ths dode structure s capable of provdng the necessary charge for soft recovery behavor by employng the new Feld Charge Extracton (FCE technology. More detaled aspects are treated n []-[7].

38 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Fgure I-: Reverse recovery voltage and current n the FCE dode Fgure I-: Reverse recovery voltages and current n the dodes (ABB Ths FCE dode provded a new performance for hgh voltage fast recovery dodes and t can be consdered as the most employed n applcatons based on fully-controlled semconductor devces such as VSCs. I..3. Thyrstor These devces that can sustan voltages n the range of kv are matched for HVDC applcatons. On the market t can be found devces whch can conduct current levels up to 5kA. Nevertheless, the thyrstor s not a fully controllable swtch. For HVDC applcatons, ths devce s usually provded n a Press-Pack sngle wafer structure (monolthc (Fgure I-. Evolutons of power ratng and wafer dmenson versus tme are reported n Fgure I-3. Fgure I-: Typcal commercal thyrstors (Infneon Fgure I-3: The most powerful semconductor type Thyrstors can reach very hgh voltage levels, they are very fast durng turn-on and they don t show overvoltage problems n seres connecton [8]-[].

39 Chapter I HVDC Systems I..3.3 IGBT The Insulate Gate Bpolar Transstor was ntroduced n 98 combnng a MOS gate wth a bpolar transstor for hgh voltage sustanng and smple gate drvng. Actually on the market there are devces whch can sustan a voltage up to 6.5 kv and swtch a current up to 75 A. Ths devce thanks to the MOS gate can be controlled wth a small power level. Moreover the MOS structure, dstrbuted over the entre chp, allows full area conducton of the bpolar transstor.. For hgh hgh voltage and hgh current applcatons modules are based on multchp substrates (Fgure I-4. The b-drectonalty n current s guaranteed by the reverse dode whch s ncluded n the structure. An example of mult-chp packagng s gven n Fgure I-5. In many cases the fault of ths component due to over-current makes the devce always opened, whch leads to an exploson [9]. For seres connecton of these devces, an external mechancal swtch or a semconductor devce are always added to by-pass the broken devce [3]. Fgure I-4: Mult chp confguraton proposed by ABB Fgure I-5: External package proposed by ABB for a mult chp confguraton For HVDC applcatons also the Press-Pack could be adopted. Nevertheless, as dscussed n [3] the sngle wafer constructon cannot be drectly transferred to the manufacturng of IGBTs. Indeed, t s stll not feasble to produce a large IGBT wafer for hgh power applcatons due to the fne pattern of the IGBT cell structure. To overcome ths technologcal lmtaton, the press-pack housng for IGBT was developed as a mult-de devce. Nowadays two Press Pack technologes can be found on the market [], [3]. Drect Pressure Proposed by Toshba and Westcode, the system conssts of several chp stacks each composed of an IGBT de, supportng molybdenum dsks and a chp frame to algn these dsks. The external lds are also acheved wth mult-blocks (stamps transferrng the external force to the ndvdual IGBT stacks [].

40 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Fgure I-6: 3D lay out of a Drect pressure IGBT (Westcode Fgure I-7: Westcode nternal dsposton of a IGBT In [3] t was proved that the press-pack package shows excellent performances n terms of relablty and thermo mechancal-behavors. On the other hand, accordng to the structure lay-out shown n Fgure I-8, the drect transmsson of the pressure to the sngle chp requres a calbraton of the strength wth hgh resoluton due to the fragle structure of the sngle chp. Moreover an unbalanced dstrbuton of the pressure among the chps drectly decreases the relablty. Fgure I-8: Lay out of the IGBT n the frame Indrect pressure Ths technology was ntroduced by ABB that s the only maker, the structure s also called Press Pack Indrect (PPI. As reported n [3] - [33] a module conssts of a number of parallel connected subassembles, called sub-modules nsde a rgd frame. As reported n Fgure I-9, when the module s mechancally clamped, each of the press-pns s subject to a force F=c x, where c s the sprng constant and x s the travel dstance. The surplus force, exceedng the sum of all forces on the chps, s sustaned by the rgd frame. In ths way, the dfference n the force on the chps no longer depends on the pressure homogenety n the stack, but only on nternal tolerances of sprng constant and travel dstance, whch can be accurately controlled.thus, even long stacks, wth ther nherent problem of havng 3

41 Chapter I HVDC Systems nhomogeneous pressure dstrbuton across the PPI, become easy to assemble. In Fgure I-3 a typcal press-pack made by ABB and a tower of seres connected IGBSs are reported. Fgure I-9: Three IGBT chps wth an ndvdual press-pn each Fgure I-3: Pctures of a module stack, an open ndvdual module, and a sub-module nsde the module It s well known that the Press-Pack IGBT modules proposed by ABB have a good resstance to the thermal cyclng and allow a stable short crcut n fault condtons [34]. The ndrect pressure of the constructon presents better performances n case of hgh number of seres connected devces. The homogeneous pressure dstrbuton guarantees mprovements n terms of thermal behavors and gves to the structure a good robustness toward the vbratons. The modular structure leads to a good effcency n terms of ndustral producton. I..3.4 IGCT The Integrated Gate-Commutated Thyrstor s exclusvely used for very hgh power applcatons such as medum voltage drves or wnd turbne converters n the mult-megawatt range. These devces can turn-off up to 6 ka under 4.5 kv. In the future, ths devce could be the best canddate for HVDC systems based on VSCs. In the world-wde, there are only three producton stes whch are located n Japan (Mtsubsh, n Swtzerland (ABB and n Czec Republc (ABB. The IGCT presents a Monolthc structure (Fgure I-3 always n press pack packagng as shown n Fgure I-3. 4

42 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Fgure I-3: 4" IGCT (5.5 ka/4.5 kv (ABB Fgure I-3: 6" IGBT wafer As reported n [35] n the conductng state, an IGCT s a regeneratve thyrstor swtch lke a SCR or a GTO as llustrated n Fgure I-33. In the blockng state, the gate cathode juncton s reverse-based and s effectvely out of operaton so the resultant devce s that of Fgure I-34. Fgure I-33: Lay out n conducton mode for the IGCT Fgure I-34: Lay out of the IGCT n blockng mode Fgure I-33 and Fgure I-34 also represent the conductng and blockng states of GTOs wth one major dfference, namely that the IGCT can transt from a state to other one nstantaneously [35]. A typcal turn off phase for an IGCT devce s shown n Fgure I-35.. The IGCT technology allows elmnatng the GTO zone [8] so the devce becomes a transstor pror havng to sustan any blockng voltage. Because turn-off occurs after the devce has become a transstor, no external dv/dt lmtaton s requred and the IGCT may operate wthout snubber as does a MOSFET or an IGBT [35]. 5

43 Chapter I HVDC Systems Fgure I-35: Typcal currents and voltages for a IGCT n turn off mode More detaled aspect n terms of devces presents on the market and ther dmenson, operatve voltage and current are hghlghted n [35]. Of course at the moment of ts ntroducton the IGCT requred a more complcated producton costs. Actually the IGCT can be consdered very smple because of the development of the makers. Ths devce s affrmed on the market also for ts avalablty because there are not many thngs that fal ts [35]. Moreover about ts juncton aspects the IGCT s not senstve to dv/dt and d/dt problems [35]. I..4 CSC-Phase controlled converters CSCs are the most affrmed structures n the feld of HVDC systems [9]. The basc converter s depcted n Fgure I-36, ths s a classcal 6-pulse topology. out v OUT v l n Fgure I-36: Rectfer brdge based on phase-controlled thyrstors 6

44 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Typcal voltage waveforms are presented n Fgure I-37. The DC waveform depends on the lne-to-lne AC voltages. The average value on the DC voltage ( can be fxed by controllng the turn-on angle of the semconductor devces respect to the lne-to-lne voltage. Vuv Vvw Vwu -Vuv -Vvw -Vwu Vo Tme (s Fgure I-37: Lne to lne voltage waveforms for the 6-pulse converter The relatonshp between the turn-on angle and the ampltude of the DC voltage mposed by ths system s descrbed n Fgure I-38. v V.sn( t ( 3 V 6.cos OUT V 3 6 V <V OUT > Rectfer ( Inverter ( 3 6 V Fgure I-38: DC voltage regulaton accordng to the phase angle The turn-on angle ψ determnes also the phase between the current and the lne-neutral voltage. A typcal current waveform of a 6-pulse thyrstor converter s depcted n Fgure I-39. At hgh current level, the nductance on the ac-sde l cannot be gnored. In fact for a gven angle ψ, the current commutaton takes a sgnfcant commutaton nterval whch nfluences also the maxmum negatve lmt on the DC voltage [36]. 7

45 Chapter I HVDC Systems 5 v +I [p.u.] Tme [s] Fgure I-39: Typcal nput current waveform for a 6-pulse rectfer compared wth the AC phase voltage -I The AC current waveform can be decomposed n a Fourer seres (3. The fundamental component shows a phase shft respect to the phase voltage. Ths means that the regulaton of the DC voltage determnes the actve (4 and reactve power (5 transmsson. 3 I. sn(.t.sn[( 6h (.t ] h 6h P V. I.cos (4 Q V. I.sn (5 (3 The typcal spectral content s shown n Fgure I-4. Nevertheless, t s well known that the harmonc spectrum can be mproved by nterleavng thyrstor commutatons wth multwndng transformers to acheve -pulse or 4-pulse rectfers [36]-[37]. 3.I [A] frequency Hz Fgure I-4: Harmonc content for a 6 pulse thyrstor rectfer 8

46 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS The presence of the AC lne nductor lmts the voltage capablty of the converter dependng on the lne current as ( v OUT V 6.cos.. I (6 The output characterstc of the converter s gven n Fgure I-4. The curves are marked for dfferent values of ψ. For values greater than 9, the system works n nverter mode (4 th quadrant. In ths case there s an extncton tme nterval, t nv, durng whch the voltage across the thyrstor s negatve and beyond whch t becomes postve. Tme nterval t nv should be greater than the thyrstor mnmum turn-off tme t q. Otherwse, the thyrstor wll prematurely turn-on, leadng to a loss on the current control whch can be destructve. 3 V 6 <v OUT > ψ= I 3 V 6 ψ= 8 t nv Fgure I-4: DC voltage capablty vs AC current The current drawn by the power staton has to be fltered by LC shunt crcuts tuned on the typcal frequences of the low rank harmoncs [38]. These flters take-up -3 % of the surface employed for the substaton whch s not neglgble. As example, Fgure I-4 shows the feld of LC flters assocated to the AC/DC converter staton of the France-England connecton ( GW. 9

47 Chapter I HVDC Systems Fgure I-4: LC shunt flters at «Les Madarns» converter staton n France - England Interconnecton I..5 CSC-HVDC SYSTEMS In ths secton s shown the operatng prncple for the converters whch characterze the CSC-HVDC lnk. The basc connecton s composed by a voltage rectfer whch provdes the power necessary to the transmsson lne through the DC current mposton. The regulaton s acheved through the phase angle n-pulse v v OUT P v ψ out Latched on v ref vout PI - + ref Fgure I-43: Basc HVDC connecton based on CSC systems By consderng the reference current the converter whch s n rectfer mode works on the red characterstc n Fgure I-44. The operatng power dagram s shown n Fgure I-45 and t s mportant to note that the control of the actve power affects the nput reactve power.

48 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS V Carachterstc of Q - v red Characterstc of t nv lmt Inverter Rectfer DC current ψ ref Fgure I-44: I-V dagram for the two converters 3 6 V.I 3 6 V. I Fgure I-45: PQ dagram P The converter whch gets the power and works n nverter mode s depcted n Fgure I-46. The converter must regulate the voltage phase angle to operate at the mnmum turn-off tme t nv and then lmt the reactve power receved. As shown n Fgure I-44, the operatng pont s gven by the ntersecton of characterstcs for the frst converter (n red and the recever converter (n blue. out = ref n-pulse v OUT + - v v ψ ref vout v Fgure I-46: thyrstor converter n nverter operaton To change the drecton of the power flow t s necessary that the two converters swtch the roles. Ths means an nverson of the DC voltage polartes. I..6 VSC-PWM based AC/DC converters In ths secton, after a bref revew on the VSC SPWM converters the prncples of the HVDC transmsson are explaned. The so-called Voltage Source Converter based on the Snusodal Pulse Wdth Modulaton allows the AC/DC converson (and vce-versa by regulatng the actve and the reactve power ndependently. The basc structure for the

49 Chapter I HVDC Systems converson s shown n Fgure I-47. If the AC voltage s mposed the power flow s regulated va duty cycle α whch determnes the sgn of the current. v v = α d v d Fgure I-47: basc structure for the AC/DC converson based on VSC systems The basc control dagram for a 3-phase VSC s depcted n Fgure I-48. A generator synchronzed on the grd voltage v s determnes the AC current reference whch s composed by actve and reactve components. The duty cycle α determnes the voltage v r whch draw the desred AC current va nductor L [39]. d v s L v r Vd n g g 6 PLL ref reactve act ref PI α PWM Fgure I-48: Dagram of the control for a VSC PWM converter Accordng to the duty cycle varaton the pulse wdth modulaton s determned as shown n Fgure I-49. The phase voltage waveform mposed by the three phase converter on the AC sde reaches values between -/3V d and +/3V d [37]. The fundamental component can reach maxmum ampltude equal to V d /.

50 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS [p.u.].5 Pulses Tme [s] Tme [s] Fgure I-49: swtch pulses and duty cycle; phase voltage mposed by the converter and ts fundamental component waveform Inductor L determnes the current waveform accordng to the sngle phase equvalent crcut n Fgure I-5. v r Vd (+/*V ( d I X L Δ V AC grd V s V r VSC voltage Fgure I-5: Sngle phase Equvalent crcut VSC-HVDC connecton Ampltude and phase of the current depends on the actve and reactve powers provded to the grd. (Fgure I-5. 3

51 Chapter I 5 HVDC Systems v [p.u.] -5 - φ Tme [s] Fgure I-5: AC current and grd voltage As t s shown n Fgure I-5, the lne current waveform shows a spectrum whch has nterharmoncs centered on the multple of the swtchng frequency [4] h s the rato between swtchng frequency and fundamental frequency..5 Current [p.u.] (h-.f.5 (h+.f (h-.f (h+.f hf 4 6 Frequency (Hz f 3hf Fgure I-5: AC current harmonc spectrum for a PWM VSC structure Due to the undrectonal DC voltage, the drecton of the power flow regulated on the AC sde leads on the DC sde to a change on the sgn of the current averaged value. An example of postve averaged value of the DC current s shown n Fgure I-53. Each converter mposes the desred DC voltage through the averaged value of the DC current. 4

52 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS 6 4 [p.u.] Tme [s] Fgure I-53: d current Regardng the sngle phase equvalent crcut at the fundamental frequency (Fgure I-5, the correspondng vector dagram converter s gven n Fgure I-54. The DC voltage determnes the output voltage lmt of the converter as marked on black rng whle the AC current the lmt (red rng s fxed by the semconductor devces. Then the area correspondng to the ntersecton of the two rngs gves the operatve range of the DC/AC converter. Accordng to the references made n Fgure I-5 the sgn of the power flow s also determned. P Converter Voltage Lmt V r I act V s jx L I Q I react I RMS Current Lmt Fgure I-54: Fresnel dagram for a AC/DC VSC structure 5

53 Chapter I HVDC Systems The synchronous reference frame s tuned on the voltage grd v. By varyng the ampltude and the phase γ of V r the vector jx L I s placed to acheve the desred actve (7 and reactve (8 power. P V V sn X S r 3 (7 L Q V cos V r S 3 VS (8 X L In the next secton the prncple of control to acheve the power sharng s provded for the HVDC lnk based on VSC structures. I..7 VSC-HVDC systems The basc lay out of a VSC-HVDC lnk s hghlghted n Fgure I-55. The approach supposes that the power s transferred from the source (on the left to the source (on the rght. v s v s v s L v r = v P d + d - V d = v = v r L v s α α PLL ref reactve act ref PI PI ref reactve act ref PI V d ref Fgure I-55: smple control strategy for a VSC based HVDC connecton The two termnals share the same voltage on the DC lnk. The recever converter (rght draw the currents n grd whch fxes the actve power level. To balance the DC voltage, the converter absorbs the requred actve power on grd. The reactve power controls on each 6

54 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS sde are completely ndependent. The stablty of the DC voltage s ensured by the frst converter. The DC voltage control generates the reference for the actve part of the current necessary to keep the requred DC voltage. Accordng to the clams made up to ths pont, VSC-HVDC systems can be chosen rather than CSC-HVDC ones because of a seres of factors, such as: Falures of the commutatons due to AC network dsturbances that could be avoded. Independent managng of the actve and reactve power. The use of modulatons such as PWM, whch guarantees frequency very low harmonc dstorton on the currents. The AC flter sze can be greatly reduced. That s why n the followng secton, multlevel VSC topologes, able to operate n hgh voltage applcatons, are consdered. I.3 VSC-HVDC multlevel topologes Due to the lmted current capablty of the cables and semconductor devces, HVDC systems requre converters able to operate on around a hundred volts. In Fgure I-56, the man topologes of voltage source nverters are remnded. DC voltage + V C V out + V C AC Voltage a + V C + V C V out + V C b V out + V C c Fgure I-56: Basc schema of VSC topologes for a smple one a, -VSI b, and a multlevel topology c The smplest VSC topology s the two-level, three-phase brdge [4]. If ths soluton s adopted, many seres-connected IGBTs are used to compose one devce as shown n Fgure I-57. As treated prevously there s just one manufacturer avalable on the market for ths soluton. 7

55 Chapter I HVDC Systems V DC A B C Fgure I-57: VSI -level topology for hgh voltage employment The connecton of seres devces leads to output voltage waveforms, whch show hgh dv/dt, whch s a man constrant for transmsson lne transformers. Moreover, for hghpower converters, the swtchng frequency s very low due to power losses and the lmtatons of the semconductor devce. To keep the harmonc mpact under the lmt mposed by the standards, an AC flter s necessary. The use of multlevel converters enables work at a hgh-voltage level, wth a hgh-waveform qualty. The man feature of these converters s that they draw a quas-snusodal voltage waveform from several levels obtaned from flyng capactors (lke flyng cap converters [4] connected to each commutaton cell. In multlevel structures, due to the nterleaved modulaton technque, t s possble to acheve a seres of advantages [4] - [43], such as: Quas-snusodal AC voltage waveform Low harmonc mpact Reduced costs for the flterng elements Possble drect connecton to the MV grd Reducton of semconductor losses due to a very low sngle-swtchng frequency per devce An overvew s gven n the next secton on the multlevel topologes canddate to be employed for hgh-power transmsson. 8

56 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS I.3. Neutral Pont Clamped (NPC One of the topologes whch lterature started to consder s the NPC [43]. In Fgure I-58, a three-level verson s shown, but t s possble to add the components and place them correctly to ncrease the number of levels. T D D C T D D C T D D C C V DC L T D T D T D T 3 D 3 D C T 3 D 3 D C T 3 D 3 D C T 4 D 4 T 4 D 4 T 4 D 4 C V DC leg leg leg3 Fgure I-58: Three-level three-phase NPC topology The component whch characterzes ths topology s the dode necessary to clamp the swtchng voltage to the half level of the DC bus, whch s splt nto three levels by two seres of connected bulk capactors. In ths topology, the mddle pont s also called the neutral pont, outlned n Fgure I-58 as the ground. By ncreasng the number of levels, the voltage whch the dodes have to sustan rses. If the voltage ratng of each dode s kept, more devces are necessary for the whole voltage. For ths reason, f the number of voltage levels that the system can mpose s N, (N- dodes are necessary. For hgh-dc voltages, the system becomes less convenent due to the huge number of dodes. I.3. Flyng capactor Another multlevel topology whch s sutable for hgh-power applcatons s the Flyng Capactor structure wth N mbrcated cells (Fgure I-59. The output nductor value s calculated to lmt the output current rpple at the equvalent swtchng frequency. [44] The FC topology ncludes N- flyng capactors, and the operatng voltage of each cell s V d /N [44]. One drawback of ths topology s the stored energy n the flyng capactors close to the DC bus (voltage and energy ncrease wth ndex. However, t s possble to connect capactors n the seres to sustan hgh voltage, but t s not certan that the voltage wll be equally shared between them. 9

57 Chapter I HVDC Systems Cell u C N- C I d u L f V d C N- C v v w L f L f C N- C w Fgure I-59: Three-phase flyng capactor converter The two topologes analyzed present a better reducton n the harmoncs. Despte the mprovements whch they are able to reach, these knds of multlevel converters present a seres of lmtatons/drawbacks. For ths reason, they dd not succeed n these HV-applcaton demands [47]. Not sutable for the ndustral seres producton (thanks to the modular constructon n order to enable scalng to dfferent power and voltage levels, usng the same hardware [48] Unwanted EMI dsturbances generated by a very hgh slope (d/dt of the arm currents The DC bulk capactor stores a huge quantty of energy whch leads to damages under faulty condtons The stored energy of the concentrated DC capactor at the DC-Bus results n extremely hgh surge currents and subsequent damage f short crcuts at the DC-Bus cannot be excluded Harmoncs on the AC current must always be suppressed 3

58 Ncola Serba I.3.3 MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Cascaded Multlevel Inverters These structures are characterzed by a seres connecton of elementary converters that are normally dentcal, as shown n Fgure I-6. Each cell corresponds to a voltage level. Accordng to the partcular modulaton technque, t s possble to acheve the desred voltage waveform accordng to the mposed reference (Fgure I-6. E u = v o Elementary Converters v out E un = v on Fgure I-6: Cascaded multlevel stage Fgure I-6: Example of multlevel waveform Wth respect to the tradtonal topologes, cascaded structures ensure the modularty of the system by ensurng seres ndustral producton. Due to the modularty, they do not present upper-dc voltage lmts. In fact, t s possble to add more seres cells to sustan the desred voltage. A topology whch has been affrmed n the last decade s the Modular Multlevel Converter. Ths structure s more and more often chosen for VSC-HVDC power statons [45]. The converter s a composton of seres-connected elementary cells (Fgure I-6. Ths converter offers the possblty to regulate the actve and reactve power ndependently. Each phase s composed by two groups of elementary cells (..N and N+ N, called branches. Each branch conducts the half-phase current. As affrmed n [47], At frst glance, when beng compared to conventonal VSC or multlevel VSC, the new topology offers several features whch may seem strange or defntely wrong. Thanks to a seres of advantages lsted above, the next chapter pays attenton to the man topology and szng aspects for ths structure: 3

59 Chapter I HVDC Systems Each arm conducts half current and n contnuous conducton mode Arm nductances contrbute to lmt faulty condtons The bulk capactor s not necessary because there are two termnal cells Each capactor cell voltage can be controlled very slowly wth respect to the current regulator The DC lnk voltage can be controlled by the converter EC EC EC EC N EC N EC N w u v EC N+ EC N+ EC N+ EC N EC N EC N V d Fgure I-6 : Modular Multlevel Converter base schema I.4 Conclusons The development of semconductor turn-off devces and the success of the multlevel topologes n recent years have made VSC-HVDC structures the most employed n HVDC systems. CSC structures can manage hgh voltages because are composed by thyrstor rectfers. Ths devce does not suffer the seres connecton. On the other hand, VSC structures can control the actve and reactve power ndependently. Moreover the SPWM based structures make the flterng stage very small respect to the CSC. VSC structures allow also slanded operaton. Compared to the tradtonal multlevel structures, Cascaded multlevel converters, due to ther modularty, are matched to seres ndustral producton. Moreover they do not present upper-dc voltage lmts. In fact, t s possble to add more seres cells to sustan the desred voltage. 3

60 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Chapter II. MMC systems In the frst part of ths chapter a macro model s provded for the MMC structure. The socalled averaged model facltates consderatons and nvestgatons wthout takng nto account the effects of the harmoncs at the swtchng frequency, makng the study fast and drect. Szng parameters are provded for the reactve elements (branch nductors and cell capactors for two dfferent approaches. A frst approach supposes a current control, whch acts drectly on the AC output current (one current control per phase. Under ths condton, a new confguraton s also proposed for the branch nductor n order to mprove the system performances. The second approach proposes two current loops per phase, each of whch acts on the current of each nductor. For both approaches, smulatons are performed to valdate the study. The consdered structure n the work s depcted n Fgure II-. Each phase of the system s composed by two branches. Each branch s a connected seres of N elementary cells (EC and the branch nductor L. Each phase contans N elementary cells. At the top are the negatve branches (n, and at the bottom are the postve ones (p. I d VCu C EC VCw C EC VCv C EC v nu v nv v nw VCu C EC N VCNw C EC N C EC N VCNv V d nu nv L L nw L u v w L L L v u v v v w V d VC(N+u C EC N+ pu v pu VC(N+w C EC N+ pv v pv VC(N+v C EC N+ pw v pw VC(Nu C EC N VC(Nw C EC N VC(Nv C EC N Fgure II-: Modular Multlevel Converter n three-phase confguraton 33

61 Chapter II MMC Systems II. The Macro Model To make prelmnary consderatons and gan an easer understandng of the system, a model at low frequency was extracted. A Snusodal Pulse Wdth Modulaton (SPWM s assumed. Ths approach does not consder the swtchng contrbuton on the spectral content for the voltages and currents of the system. Moreover, ths approach makes the study of the MMC structure ndependent from the partcularly topology of the seres connected elementary converters. v C (t (t C Elementary Converters of the o ( t ( t branch o (t v C (t C (t o ( t f ( t av (t o v C ( t ( t v o (t v C ( t f ( t v av o (t Fgure II-: Low-frequency model b of an elementary swtchng cell a Each commutaton cell can be seen as a -port devce (Fgure II- a. The nput sde s characterzed by the voltage and current for the cell capactor. The output sde carres out the voltage cell and the branch nductor. The relatons between currents and voltages of the cell are at ( and (, respectvely, where f(t s the modulaton functon, dependng on the modulaton sgnal and the topology of the elementary converter. Thus, the averaged model of the cell s depcted n Fgure II- b. av ( t ( t f ( t (9 C o av v ( t v ( t f ( t ( o C a b By startng from the dagram gven n Fgure II-, t s possble to extract the equvalent averaged crcut (Fgure II-3 vald for the MMC system; the equatons whch characterze the structure don t consder the partcularly topology of the elementary converter. Moreover each capactor resumes the total capactance for the N converters whch compose the branch. The equvalent value s C/N. 34

62 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS I d av V cun av V cvn av V cwn C/N C/N C/N av av v v nu nv av v nw Vd/ f nu av nu f nv av nv f nw av nw N L av u L av v L av w L v u L v v L v w Vd/ av pu av pv av pw n av V cup av V cvp av V cwp C/N av C/N C/N v av pu v pv av v pw f pu f pv f pw Fgure II-3: Averaged model for the MMC To make the study not-dependent from the partcular topology, defntons on the nomenclature about the functons has to be gven n Table II-. Table II-: Elementary converters functon defnton f(t k(t α(t Functon whch multpled for the averaged cell gan gves the averaged voltage waveform of the cell output AC part of f(t Averaged value of the swtchng functon. (for a unpolar elementary converter f(t=α(t; for a bpolar elementary converter α(t=(±f(t/ All of the consderatons were made just for one phase. Each branch mposes a voltage ( whch s the equvalent sum of the output voltage of each seres connected cell. Moreover each cell mposes a voltage dependng on the modulaton functon and the voltage capactor. By supposng the voltages on the capactors for all the cells of the branch equal between them and the same for all the modulaton functons, each branch mposes the voltage n ( Accordng to the dagram shown n Fgure II-3, the phase voltage s acheved n by neglectng the voltage drop on the branch nductor L (3. Branch currents determne the current n the phases (4. 35

63 Chapter II MMC Systems N av av vnu( t V ju ( t j av av N ; V ju ( t f j ( t Vcj ( av av vpu( t V ju ( t j av av vnu VCn fn( t av av vpu VCp f p( t ( av av vpu( t vnu( t vu ( t (3 ( t ( t ( t (4 u nu pu The nstantaneous and averaged voltage waveforms are compared n the next secton. Ths facltates better understandng of the dfference between the two models. II.. Macro model valdaton The typcal voltages and currents of an MMC system are consdered to demonstrate the relablty of the macro model. The smulaton results are reported n p.u., because the partcular case study s shown after the szng consderatons acheved n the next sectons. The swtchng frequency for the sngle cell s 45 Hz for the nstantaneous model. The voltage and currents of the nstantaneous model are compared to the averaged ones. Moreover, consderatons for the spectral content are acheved to show the frequency lmts of the model wth respect to the nstantaneous one. In Fgure II-4, a typcal voltage waveform of the capactor of the elementary converter s shown. The nstantaneous model seems to match well wth the averaged one. Ths s consoldated also by the spectral content comparson n Fgure II-5. Capactor Voltage [p.u.] Instantaneous Model Averaged model tme [s].98 Fgure II-4: Instantaneous and averaged model comparson of a voltage on a cell capactor 36

64 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Averaged [p.u.] Instantaneous [p.u.] Frequency (Hz Fgure II-5: Spectral content comparson of a capactor voltage for a cell A more evdent matchng between the two models s shown by consderng the output voltage of the elementary cell ( n Fgure II-6. In ths case, the matchng between the two models s more evdent n ther low-frequency spectral content compared n Fgure II-7. The averaged model s not able to take nto account the swtchng frequency.. Instantaneous Averaged Voltage [p.u.] tme [s] Fgure II-6: output voltage mposed by an elementary cell; comparson between nstantaneous model and averaged model 37

65 Chapter II MMC Systems Instantaneous [p.u.] Averaged [p.u.] ² 5 5 Frequency (Hz Fgure II-7: Spectral content comparson for the output cell voltage The macro model s very useful n makng all prelmnary consderatons about the reactve element szng and testng the control. The model mplementaton s very drect, and the smulatons are very fast because the swtchng frequency s not consdered for the tme step choce. By now, f t s not specfed, all the smulatons are performed by consderng the averaged model. Before the szng approach proposton, the elementary cell topology s defned n the next secton. The sngle cell characterzes the MMC base structure [49] by consderng a SPWM. A more detaled study on the choce of the cell topology s acheved n chapter III. In ths chapter the study n gven for the averaged model f the adaptaton of the nstantaneous one s not specfed. II.. Study of the MMC basc structure The basc verson of the MMC structure s composed by sngle cells (Fgure II-8. The modulaton sgnal gven n (5 consders ω to be the fundamental frequency and cos(φ the power factor. Negatve and postve branches have duty cycle (6 and (7, complementary between them. I d V d SM SM N nu Vu VNu L v nu VCju C Devce Devce nu Vju k( t M sn( t ; M (5 k( t fn n (6 f p p fn( t (7 v u Fgure II-8: Smple u cell adopted for the MMC structure u L V d 38 pu SM N+ V(N+u SM N VNu v pu

66 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS From the assumptons gven for the elementary cells n ( and ( and based on consderaton for the modulaton rato assumed for the sngle cell, the branch voltages and currents for the u-phase are carred out n (8 and (9, the postve part s symmetrcal to the negatve one. The presence of a second harmonc component s confrmed n the works [5]- [4] and s hghlghted as follows. v v un up un pn ( t Vd ( t Vd I ( t I sn( t I I ( t I sn( t I ( M sn( t ( M sn( t f f V f V f cos( t cos( t sn( t sn( t I I d 3 (8 (9 Each branch current s composed of three terms: The DC component, whch follows the flow shown n Fgure II-9 a The AC component at the fundamental frequency; accordng to the flow depcted n Fgure II-9 b, each branch conducts half of the phase output current The second harmonc component, whch s kept n the branches (Fgure II-9 c and represents the energy balance between the negatve and the postve branch for each phase [49]-[4]. Moreover, ths component doesn t flow on the DC sde because t s a negatve sequence. V d I d 3 Inu L u Vnu I d 3 Inv Vnv L v I d 3 Inw L Vnw u Inu L u Vnu v Inv L Vnv w Inw L Vnw Vnu Inu nd u L u Vnv Inv nd v L v Vnw Inw nd w L w L vu L w v v L v w L v w v u L v v L v w L vu L vv L vw V d Ipu Vpu Ipv Vpv Ipw Vpw n u Ipu Vpu v Ipv Vpv w Ipw Vpw n Ipu Vpu Ipv Vpv Ipw Vpw n a b c Fgure II-9: Current flow n the MMC structure The study of the system was conducted by makng two dfferent assumptons accordng to the consdered control approach to the AC current. The two cases are descrbed below: 39

67 Chapter II MMC Systems The output AC current mposton permts control of the system by consderng the AC output current u. In ths case, t s not possble to regulate the branch currents composed also by the nd harmonc component, whch has to be consdered n the study (Fgure II-. In ths hypothess, a method for the passve components szng s gven and, upon consderaton for a case study, smulatons were acheved. The branch current mposton operates drectly on the branch currents nu and pu and supposes the AC voltage mposed on the AC output sde. In ths case, t s possble to acheve the AC output current desred and to suppress the second harmonc components (Fgure II-. Szng parameters are gven n the study, and one more tme, the smulaton results valdated the study. Id Id Vnu Vnu V d Inu V d Inu * nu PI Vu L v Cu Vu L V d u L Ipu * u PI V d u L Ipu * pu PI Vpu Vpu Fgure II-: MMC elementary crcut by mposng the AC output current Fgure II-: MMC elementary by mposng each branch current II. Output current mposton In ths secton, the crcut presented n Fgure II- s studed. The szng parameters were acheved by startng from consderatons made n (8 and (9. Evaluatons gave the dependence of voltages and currents on the branch nductors and cell capactors. By suppressng the current source (Fgure II-, the ampltudes of the nd components of the branch voltages and currents are reported n ( [49]. harmonc V f I f ( t sn( t I f cos( t ( 4 L 4

68 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS The capactor current c (t s cn ( t un( t ( M sn( t cn ( t I I M I MI sn( t sn( t sn( t sn( t 4 4 I f MI f cos( t cos( t sn( t By usng the Werner formulas: cn I I M I MI MI ( t sn( t sn( t cos( cos( t I f MI f MI f cos( t sn(3 t sn( t 4 4 The current capactor has also a thrd harmonc component thus, the capactor current s acheved n ( accordng to (. cn( t cn( t cn( t st cn( t nd cn ( t ( st 4 I M ( I I M cos( I M sn( t sn( cos( t cn cn ( t ( t nd 3rd cn ( t I MI cos( 8 f ( MI 4I f cos(t 8 MI f sn(3 t 4 ( The fundamental current component s suffcent to generate the second harmonc component. Moreover, accordng to (3, to keep a constant DC component of the capactor voltage, the DC component of the capactor current has to be equal to zero, so t s possble to acheve a relatonshp between the I and I n (4. It s possble to verfy that the ampltude of the thrd harmonc for the capactor voltage can be neglected wth respect to the other ones. Thus the capactor voltage (5 and the total branch voltage (6-(7 can be obtaned. 4

69 Chapter II MMC Systems 4 dt t C N V t V cn d cn ( ( (3 cos( 4 MI I (4 sn( 4 ( 8 ( cos( ( cos 4 sn( sn( cos( 8 ( ( t I MI C t V t M I M I C t M I C t V N V t V f nd cn f st cn d cn (5 sn( ( t M t V N V cn un (6 ( cos( 8 sn( cos( sn( ( cos ( 4 6 ( cos( 8 ( cos 4 64 (4 sn( sn( cos( 6 sn( cos( ( sn( 4 ( cos 3 8 ( 3 t C N M I t M I MI M M I I C N t V t C M I M I N C MI I MN MV t V C MN I M t V M I M I C NM V t V f f nd un f f d d st un f d un (7 By fxng the apparent power of the system, V d and therefore N, I, and M and varyng the power factor, the varaton of the ampltude of V un (t nd s not very senstve to ts second term, so t s possble to rewrte V un (t nd as (8. The ampltude of the nd harmonc component of the current s therefore extracted n (9. ( cos M I MI M( M I I C N (t V wth I L (t V f f nd un f nd un 4 6 (8 N LC NM MN N M I I f ( cos 3 (9 Ths study permts evaluaton of the nd harmonc ampltude of the branch currents and voltages from the knowledge of the power rate of the system, V d and therefore N, I, and M. II.. Cell capactor The value of the capactor s extracted accordng to the rpple ampltude of the voltage at low frequency. By consderng equaton (5, the components of the voltage do not depend

70 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS only on the capactor value but also on the I f, whose extracton s n (9. The evaluaton of the capactor becomes a non-lnear problem. For ths reason, the evaluaton of the cell capactor was acheved by mplementng the formulas gven prevously n an teratve procedure shown n the flow chart n Fgure II-. C=; ΔC defnton C=C+ΔC V C =f(c,l ΔV des Vc MAX -Vc MIN? No Yes Fgure II-: Iteraton method for the capactor evaluaton II.. Branch nductor When the second harmonc component n the current s consdered, t s necessary to defne a lmt range wthn whch the branch nductor has to be defned. The nferor lmt s gven by the rpple ampltude of the branch current due to the commutatons of the devces. By assumng a snusodal pulse wdth modulaton and consderng that all of the cells are nterleaved between them, the maxmum ampltude of the branch current rpple s ΔI MAX. In (3 s reported the nferor lmt of the nductor. L Vd (3 8 N f c I MAX The second harmonc component of the branch current contrbutes to ncrease the total rms value. If ths component s consdered, an overszng of semconductor devces and the copper of the nductor must be takng nto account. Accordng to (9, to lmt the ampltude of the 43

71 Chapter II MMC Systems second harmonc component of the branch current, t s necessary to ncrease the value of the nductor. If the nductor value s sgnfcant, ts voltage drop at the fundamental frequency could become very great. Usually, the voltage drop on the nductor, ΔV L, has to be kept under a small percentage of the AC voltage to ensure the controllablty of the system at network frequency (3. V L I LMAX (3 II... Consderatons The only way to lmt the branch second harmonc both for the voltage and the current s to sze the reactve elements as bg as possble. For the current controller, t s not possble to operate on ths component, whch flows only n the branch (look the grey loop n Fgure II-9. The dynamc response of the equvalent nternal current loop has to be studed. To extract the equvalent capactor of each branch the relatonshp between the output voltage of the v un/p (3 and the branch current u(n/p (33 of the averaged branch shown n Fgure II-3, f the averaged duty cycle of the cell s consdered equal to ½. The branch capactance s extracted n (38 and for the phase s (39. C dvcu ( t un( t (3 vcu vun N dt (33 4C dvun 4C ( t un( t (34 C eq N dt N (35 The equvalent crcut of the loop depcted n Fgure II- s acheved n Fgure II-3 for the small sgnal approxmaton. I h V h C/4N=C eq L=L eq R T =R eq I jce q H( j (36 V LC jr C eq r (37 LeqCeq R Ceq (38 L eq eq eq Fgure II-3: Equvalent RLC crcut of a sngle phase 44

72 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS The RLC crcut ncludes the capactor, whch represents the equvalent capactance n the phase; then there are the negatve and the postve nductors and the resstance, whch resumes the dsspatve part of the semconductors and the copper losses. The system s studed lke a typcal second-order crcut of whch the transfer functon s (36. Accordng to the partcular applcaton, t s possble to acheve some consderaton for the bode dagram of the magntude (Fgure II-4, partcularly about the frequency resonance (37 and the dampng factor (38. If the consdered system has to sustan hgh voltages, the number of cell capactors N can be consdered huge. If the typcal value of the cell capactor s around mf [5], the C eq s relatvely small. Thus, n most cases the frequency resonance f r s greater than the fundamental frequency. Moreover, the branch nductor s around mh f the second harmonc has to be attenuated [5]. Ths leads to a dumpng factor (38 smaller than. For these reasons, the fundamental frequency of the system s placed n the postve slope of the magntude curve (Fgure II-4. Ths means a further ncrease for the ampltudes of greater harmoncs. db [p.u.] f f fres f Frequency [p.u.] Fgure II-4: Bode dagram of a RLC crcut for a typcal hgh voltage system Wth these consderatons, for ths control approach the nductor szng has to respect the condton on the resonant pulsaton gven n (38. 45

73 Chapter II MMC Systems r (39 L eqceq Ths condton requres an ncrement of the mathematcal product on the denomnator. The chance to ncrement the value of the capactor s greatly reduced because of the physcal sze of ths element whch s ncluded on each elementary converter. It s not possble to further ncrement the branch nductor because of the voltage drop condton (3 at the fundamental frequency. For these reasons, n the next secton s gven a new confguraton of the nductor whch can better manage the two contrastng condtons gven untl now. II... Coupled nductors The am of ths confguraton s to acheve an nductor whch offers a small seres mpedance to respect the condton (3 and a hgh mpedance at.f r to meet the condton (39. Consderatons start from the classcal confguraton of the 4-port model of a transformer depcted n Fgure II-5. The equatons whch characterze the real transformer are shown n (4. Ī Ī L T R T a=.. L T R T V V L M Fgure II-5: -port model of a transformer V V j jl ( LT LM RT I jlm I M I j( LT LM RT I (4 If the transformer s connected n the confguraton depcted n Fgure II-6, t can be consdered a trpolar component of whch equatons are reported n (4 and (4. 46

74 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS L T R T a=.. Ī Ī L T R T L M V V Fgure II-6: transformer model n trpolar confguraton V V V I I I R RT L LT 4L M V ( R jl I (4 (4 The reactve element provdes results that can be consdered a smple connected nductor seres element for whch the schema s shown n Fgure II-7. Ths crcut has a seres nductor whch has four tmes the magnetzaton nductance L M and output mpedance L T, whch typcally s very low as descrbed n (43. R T L T L M L T R RT L LT 4L M V ( R jl I (43 R T Fgure II-7: Equvalent trpolar element of the coupled transformer The coupled transformer requres more complcated buldng costs; on the other hand t presents an equvalent branch nductance whch s four tmes that of the classcal verson. Ths can be acheved f the magnetzaton nductance s szed wth the same number of turns of the smple branch nductor. 47

75 Chapter II MMC Systems II..3 Smulatons After the choce of the system power rate, n ths secton, smulatons performed valdated the szng parameters gven above. Moreover, the mprovements through the coupled reactor are shown. A HVDC lnk to connect an off-shore wnd farm platform s consdered as a case study. The nomnal power level s MW, wth a DC voltage of 6 kv. The MMC s rated consderng press-packed IGBT. The study s carred out by consderng a classcal PWM control wth an nterleavng of the cells. All of the smulatons are performed by consderng the macro model. The reference structure s depcted n Fgure II-3, and t consders just sx averaged cells (one per branch, each of whch resumes the N nterleaved nstantaneous cells. Each capactor corresponds to the whole capactance of each branch and has to sustan the DC voltage V d. II..3. MMC system wth classcal branch nductor The man parameters of the system are gven n Table II-. I DC EC V u V DC EC N V Nu v nu V DC nu u R S pu EC N+ L u L R S V (N+u Table II-: system power rate System Power Rate Nomnal power MW Phase to phase Grd voltage V ll 83 kv V d 6 kv Number of sub-modules N=64 Voltage capactor.5 kv Inductor resstance R S 6 mω EC N V Nu Fgure II-8: Case study system v pu In ths case, an nductor equal to 5mH was chosen, and a cell capactor of 7mF s pcked to acheve a voltage rpple around the %. Accordng to condton (39, a resonance frequency of 34 Hz was chosen. In Fgure II-9, the capactor voltages are reported only for a sngle phase. The chosen capactor keeps the rpple ampltude under the mposed value. 48

76 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS 6 Cell Voltage [V] V Cun /N V Cup /N Tme [s] Fgure II-9: Cell voltage normalzed on the number of cells The currents flowng (Fgure II- n the branch nductors have a lmted second harmonc component around %, as shown n Fgure II-. Moreover, other harmoncs are not amplfed, so condton (39 s met. Current [A] 5 un vn wn tme [s] Current [A] 5 up vp wp tme [s] Fgure II-: branch currents 49

77 Chapter II MMC Systems Current % Frequency [Hz] Fgure II-: Spectral content of the branch current n percentage wth respect to the fundamental component The huge nductor value makes the system unable to meet condton (3. Fgure II- reports a huge voltage drop on the nductor, whch could cause the system to lose the current controllablty. Under these assumptons, the system s not able to qute match all of the condtons mposed n the prevous secton. 8 V% [V] frequency [Hz] Fgure II-: Percentage spectral content of the voltage drop on the branch nductor In the next paragraph, the nterphase transformer s employed for the MMC structure. Ths reactor s called to replace the tradtonal branch nductor to overcome the contrastng condtons gven before. II..3. Coupled transformer for MMC structure The nterphase transformer s chosen wth a magnetzaton nductor L M of 5 mh. The nductor L T canddate to defne the seres voltage drop V L at the fundamental frequency s chosen to be tmes smaller than the prevous one. A cell capactor of 6mF s proposed always to acheve a voltage rpple around the % mark. By consderng condton (39, the resonance frequency of the crcut s 36 Hz. 5

78 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS EC V u V d EC N V Nu v nu nu R T V d L M pu EC N+ L T u u L T R T V (N+u Table II-3: Reactve elements szng Cell Capactor 6 mf L I MAX.5 H 5 mh L M EC N V Nu v pu Fgure II-3: MMC phase wth coupled nductors Fgure II-4 reports the capactor voltages. The chosen capactor keeps the rpple ampltude under the mposed values. Voltage [V] V Cun /N V Cup /N frequency [Hz] Fgure II-4: Cell voltage normalzed on the number of cells Also, n ths case, the branch nductors (Fgure II-5 are able to lmt the second harmonc component as shown n Fgure II-. Moreover, other harmoncs are not amplfed so that condton (39 s met. 5

79 Chapter II MMC Systems Current [A] tme [s] un vn wn Current [A] tme [s] Fgure II-5: Branch currents up vp wp Current % Frequency [Hz] Fgure II-6: Spectral content of the branch current n percentage wth respect to the fundamental component The employment of coupled nductors (nterphase transformer better manages the tradeoff between the frequency response condton affrmed n (39 and a low-voltage drop on the seres nductor L T depcted by (3. Fgure II-7 reports the voltage drop wth respect to the phase voltage ampltude provded for the classcal branch nductor employment and the coupled nductors employment. The ampltude of the voltage drop s reduced about ten tmes that of the classcal verson. 5

80 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS 8 Tradtonal Inductor Coupled Trpolar Inductor V L % Frequency [Hz] Fgure II-7: Comparson of the harmonc content of the voltage drop on the tradtonal branch nductor of the prevous case and the seres nductor L T For MMC structures, the use of the coupled transformer better manages the hard trade-off between the controllablty of the system and the dynamc response. If ths approach s adopted, the nterphase transformer employment s suggested. In ths applcaton, only one current control loop s necessary wth respect for other tradtonal approaches. On the other hand, more complex hardware s necessary. II.3 Branch current mposton For ths approach, there are two controls per phase that operate drectly on the branch currents nu and pu, and the AC voltage s mposed on the AC output sde. In ths case, t s possble to acheve the AC output current desred and to suppress the second harmonc components (Fgure II-. A detaled descrpton of the control s proposed n Chapter IV. Thus, the voltages v (n/pu (44-(45 and the currents (n/pu (46-(47 are reported wthout the second harmonc component. As evaluated n ( the current has a DC component whch s related to the fundamental one to keep constant the voltage capactor n the cell. A clearer understandng on the components of the branch currents can be acheved by lookng Fgure II-9. u nu Vd ( t ( m( t ; u d ( t ( m( t pu V (44 upu ( t unu ( t Vd uu ( t m( t; (45 53

81 Chapter II nu pu MI ( t 4 MI ( t 4 I cos( sn( t I cos( sn( t MMC Systems (46 ( t ( t ( t I sn( t u nu pu (47 II.3. Szng The value of capactor C s chosen to lmt the voltage rpple at the fundamental frequency V C provded by (5 and wth a neglected second harmonc component. The maxmum rpple ampltude s acheved n a pure reactve operatng mode, when sn(φ=. So the capactor evaluaton n ths case s a lnear problem and can be extracted from (48.. I C ; V V C C Vd N (48 II.3.. Consderatons on the arm nductor For systems wth hgh power and hgh voltage, the number of sub modules employed becomes very large. For ths reason, the equvalent swtchng frequency s very hgh f nterleaved modulatons are mplemented. Ths allows a very low swtchng frequency f sw for each devce, whch leads to a sgnfcant reducton n the swtchng losses. At ths condton, the branch nductor value, whch s calculated from (3, s very low for a huge number of cells. The second harmonc suppresson n the branch s not consdered, because t s automatcally managed by the current control loop. Problems due to the dynamc response no longer exst because the control approach s dfferent. On the other hand, the branch nductor plays a key role n the lmtaton of short crcut currents under faulty condtons. For ths reason, a low branch nductor means huge short crcut currents. If a branch current control approach s consdered, the choce of ths reactve element s acheved n order to lmt the short crcut current. Ths aspect s taken nto account n V.II. For a lmted number of sub modules, usually the branch nductor value gven by (3 succeeds n managng a lmted short crcut current, too. II.3. Smulatons Accordng to the consderatons provded prevously, an nductor equal to mh was chosen, the rpple ampltude of the current s kept around 7% (Fgure II-8. Only n ths case nstantaneous model s consdered to acheve the current rpple. A cell capactor of 6mF s consdered to acheve a voltage rpple around % of the DC value (V d /N. Smulatons are 54

82 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS performed by consderng the averaged model shown n Fgure II-3. The capactor voltages reported n Fgure II-9 just for a sngle phase show that the ampltude of the voltage rpple s kept under the mposed value. Fgure II-8: Current rpple on the phase current from the nstantaneous model 6 V Cnu /N V Cpu /N 55 Voltage [V] Tme [s] Fgure II-9: Voltage capactor normalzed on the number of cells 55

83 Chapter II MMC Systems The currents flowng n the branch nductors (Fgure II-3 can be consdered snusodal. The second harmonc component s reduced and can be consdered neglgble wth respect to the fundamental. Current [A] 5 un vn wn tme [s] Current [A] 5 up vp wp tme [s] Fgure II-3: Branch currents The second harmonc component s greatly reduced f compared to the prevous case (output current mposton as shown n Fgure II-3. The current control loop n the branch drectly lmts the component. 5 4 Sngle Output Current Loop Branch current loops Current [A] frequency [Hz] Fgure II-3: Comparson of the branch current -harmonc content 56

84 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS II.4 Conclusons The macro-model makes the prelmnary evaluatons drect and allows for very fast smulatons. The control of the AC output current can lead to a huge second harmonc of the fundamental component n the branch current and an amplfcaton of the greater harmoncs. The smple approach of the control has to be compensated by hardware whch becomes more complcated and expensve. On the other hand, the second approach facltates the employment of more smple hardware. Of course, t requres a slghtly more complcated control that s not a problem today thanks to the large choce of control devces avalable on the market. For these reasons, the second control approach was chosen heren the thess. 57

85 Chapter II MMC Systems 58

86 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Chapter III. New multlevel topologes A topologcal study s consoldated and extended n ths chapter on three dfferent cell topologes. The study of each cell s acheved, and ther employment for the MMC structure s treated. For each cell, topology advantages and lmts are dscussed n terms of current and voltage. By fxng the nomnal power of the system and the DC lnk voltage, the cells are studed and compared n terms of current and voltage on the AC output. It s shown that also mprovements on the ratng of the reactve elements can be acheved f other topologes are chosen as the elementary cell wth respect for the basc cell employed. An analytc approach for the power losses evaluaton s gven. The losses n the semconductor devces are evaluated for each topology. In the second part of ths chapter a new multlevel structure s presented. For each phase ths topology adopts just one branch and nterfaces tself wth the grd through the zg-zag transformer. The new structure s compared wth the MMC one n terms of szng. Moreover the so-called multlevel Half wave structure s proposed n order to upgrade the old three phase rectfers based on dodes/thyrstors. The upgratng s acheved by keepng the same grd transformer and therefore the voltage and current levels. Smulatons are performed n order to valdate the study and evaluate the advantages. 59

87 Chapter III III. New multlevel topologes Elementary converters for the MMC Structure The choce of the elementary converter depends on the current and voltage whch the MMC system has to conduct and to mpose. A seres of prelmnary consderatons on the power flow and the control approach are necessary to determne the voltage and current waveforms. As t was ntroduced n the prevous chapter, the branch current control approach s chosen. For VSC-HVDC systems, as dscussed n the chapter I, the grd voltage s mposed so the power flow s regulated through the current regulaton as shown n the base crcut reported n Fgure III-. The basc control for an MMC structure s depcted n Fgure III- accordng to the choce made n prevous secton. To smplfy the representaton, only one phase s depcted; the consdered phase voltage and current are shown n (49-(5. The averaged model s consdered for the analyss, and the controlled sources (v nu and v pu resume the N seres connected converters. I d u u X L AC grd Transmsson V d I nu v nu ref nu PI v u N v u L u N V d u L Fgure III-: AC sde of the DC-AC transmsson v pu u I sn( t (49 v u V sn( t (5 I pu ref pu PI Fgure III-: Current regulaton for the power flow provded for a MMC structure Consderatons regardng voltages and currents are gven by splttng the crcut n the AC and DC part (prncple of superposton. The study s performed only for one phase. Accordng to the AC crcut of the MMC structure (Fgure III-3, the branch currents (5 and voltages (53 are provded. Ths s vald f the branch nductors have the same value and a neglgble voltage drop. For the DC part of the system (Fgure III-4, each branch conducts the thrd part of the DC current (5 because of the three phases. The controlled voltage source n the branch must balance the DC lnk voltage accordng to (54. 6

88 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS AC v nu I d DC V nu N v u AC nu L u V d N DC I nu L u u L L AC pu AC v pu V d DC I pu DC V pu Fgure III-3: AC crcut of the MMC structure Fgure III-4: DC crcut of the MMC structure AC nu AC pu v v AC nu AC pu u u v v u u (5 (53 I I V V DC nu DC pu DC nu DC pu I d 3 I d 3 Vd Vd (5 (54 To balance the AC and the DC sdes, each branch must conduct the currents n (55 and mpose the voltages n (56. pu nu u I d 3 u I d 3 (55 v v nu pu Vd vu Vd vu (56 The actve and the reactve power on the AC sde are reported n (57. P AC 3VI cos( Q 3VI sn( (57 On the DC sde the actve power s acheved n (58. 6

89 Chapter III New multlevel topologes P V I (58 DC d d From a power balance between the DC and AC sde the (59 s carred out. V d I d 3VI cos( (59 In the next three sectons, dfferent topologes for the elementary converters are presented as canddates to be employed for MMC structures. Lmts and advantages are hghlghted for each topology. The approach supposes that each branch voltage s the equvalent sum of the N seres converters as depcted n the prevous secton. The averaged model was consdered for the study to make the analyss fast and drect. Moreover, a snusodal pulse wdth modulaton s supposed. III.. Sngle Cell Ths s the base cell topology used for MMC structures. Ths cell presents two transstors wth ant-parallel dodes (Fgure III-5. Ths topology allows the mposton of a monopolar voltage and the conductng of a b-drectonal current. The study s provded for the negatve part of the system; consderatons on the postve part are drectly deducted for symmetry. V C C nu Monopolar voltage V ju B-drectonal current Fgure III-5: Sngle cell topology Ths structure can mpose only a postve voltage as shown n Fgure III-6. If the consdered branch voltage s (56, the condton (6 must be met. The voltage on the cell capactor V C must make the system able to reach all the voltage levels requred as depcted by (6. 6

90 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS v un N.V C V V d t Vd V (6 V N V. d C (6 Fgure III-6: Averaged voltage waveform f the sngle cell s consdered So the modulaton sgnal s reported n (6 f the value of M s consdered maxmum. V k ( t sn( t, V d M V (6 V d The power balance n (59 s consdered between the AC and the DC sde to carry out the current waveform. The relatonshp between the AC and the DC currents s performed f the AC voltage ampltude n (6 s consdered. The current waveform s shown n Fgure III-7 by employng the sngle cell for an MMC structure. un I d I d 3 I I d 3 Fgure III-7: Averaged current waveform f the sngle cell s consdered t 3 I d M I cos( (63 4 Ths balance s also necessary to ensure a constant dc voltage across the capactor. The rght power balance also ndcates a constant voltage capactor n the cell and therefore the rght energy balance n the branch. 63

91 Chapter III New multlevel topologes III.. Asymmetrcal H-brdge Ths topology guarantees the bpolar voltage, but t can only conduct a undrectonal current. The consdered crcut s shown n Fgure III-8. T D D 3 V ju nu T D V C B-polar voltage Monodrectonal current D 4 Fgure III-8: Asymmetrcal H-brdge topology For ths topology, the branch current has to be kept as expressed n (64. Ths condton leads to a balance between the AC and DC components of the currents carred out n (65. I d 3 un I I d 3 t Id un ( t I sn( t 3 (64 3 I I d (65 Fgure III-9: Averaged current waveform f Asymmetrcal H-converter s consdered The choce of a current wth a hgh DC component ncreases the rms value. Ths means an overszng of the semconductor devces. To optmze the semconductor choce, the balance (65 s chosen so that the DC component s equal to the AC one. Under ths condton, by substtutng (65 n the power balance (6, the relatonshp between the AC and DC voltages s acheved n (66. Ths means that the use of the asymmetrcal HB-topology s not suggested for reactve power compensatons because of the hgh DC lnk voltage levels requred. Then, the followng study was performed only wth the unt power factor. Vd Vˆ (66 cos 64

92 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS The voltages mposed by each branch are expressed by (67 where N s the number of elementary converters per branch. The total number of elementary converters has to be chosen accordng to (68 where f s the modulaton sgnal. ˆ Vd vun( t N V ju Vd sn( t ( ˆ Vd vup t N V ju Vd sn( t (67 Vd f ( t n sn( t NVC Vd, M Vd fˆ( t p sn( t N V C NV (68 C To ensure a lnear SPWM modulaton the peak value of f(t has to be less than. The rato between the number of elementary converters and the DC voltage s carred out n (69. v un 3. V d V N.V C V d V d -N.V C Fgure III-: Averaged voltage waveform f asymmetrcal H-brdge topology t 3 N V C V d, (69 In ths case, the voltage level NV C, whch enables the system to reach the maxmum ampltude of the branch voltage, s.5 tmes that of the DC voltage. Ths means a 5% ncrease for the number of elementary converters per branch wth respect to the employment of the smple cell. On the other hand, consderatons on the szng capactor make ths topology more attractve. III... The capactor of the Asymmetrcal H topology and the AC current The voltage mposed by the Asymmetrcal H-topology s double that of the smple one. By fxng the values of the DC voltage and the system power rate, the actve power s provded for the sngle elementary converter (7 and for the asymmetrcal H brdge topology (7. In the comparson between the two powers n (7, t s shown that, f the asymmetrcal H brdge 65

93 Chapter III New multlevel topologes topology s employed, half of the current s necessary to acheve the same power, also decreased by M. P P AC AC 3 M 3 Vd I SC V I AH d SC (7 (7 The ampltude of the voltage rpple on the capactors depends on the AC current (48. Thus, to acheve the same ampltude of voltage rpple, at party of capactor voltage of the elementary converter, less than half of the capactance s necessary n terms of sngle cell use (7. ISC C I AH M SC SC CAH M (7 SC III..3 H-brdge The so-called four-quadrant converter can manage the b-drectonal proprety n the current of the sngle cell, and t can mpose a b-polar voltage as the asymmetrcal H-brdge topology. On the other hand, the H-brdge topology requres double the components of the others, four transstors and four dodes, as depcted n Fgure III-. T 4 D 4 T D V C T 3 D 3 V ju un T D B-polar voltage B-drectonal current Fgure III-: H-brdge topology In (73 the voltage condton to respect s reported. The equvalent sum of the output voltages waveform s hghlghted n Fgure III-. 66

94 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS V d V (73 v un 3. V d V N.V C V d V d -N.V C Fgure III-: Averaged output voltage of the H- brdge converter t I d I d 3 un I I d 3 Fgure III-3: Averaged current waveform n the H- brdge converter t In (74, the current condton s waveform s hghlghted n Fgure III-. I I. (74 3 cos d Under ordnary operatve condtons, the H-brdge structure s not necessary for the MMC systems. On the other hand, f ths cell s confgured lke a sngle cell, the b-polarty can be used n the case of DC fault, whch s addressed n the chapter V. III. Effcency for multlevel structure Regardng the power level, % of losses s not neglgble ( GW correspond to MW of losses. For ths reason many semconductor producers consder effcency as an alternatve fuel. Thus, the power losses evaluaton s not a secondary problem. The montorng of the losses becomes more attractve for hgh-voltage applcatons when the number of devces s very hgh. In ths chapter, the modular multlevel structure s studed n terms of effcency. An nvestgaton on the power losses n semconductor devces s carred out by takng nto account the prevously descrbed cells. An analytcal approach was gven for each case. After fxng the nomnal power and the DC voltage for the converter, the effcency of the system s evaluated for each knd of employed cell. The analytcal results are valdated by smulatons performed by PSIM software. 67

95 Chapter III New multlevel topologes III.. The analytcal approach A three-phase balanced operaton s assumed and then only one elementary converter s consdered. The current n the devces should be carefully determned and the defnton of the conducton ntervals s fundamental to evaluate conducton and swtchng losses [5]-[55]. Conducton losses (76 are evaluated by consderng a pece-wse lnear approxmaton of the forward characterstc (75 r s on state resstance and V the voltage threshold [56]. Averaged and RMS values are calculated accordng to expressons (77 and (78 where α s the duty cycle of the control sgnal. Assumng θ=ω t, θ -θ s the conducton angle of the devces. V r I V (75 T / D T / D C T / D rms avg ( I V I cond P T / D rt / D T / D T / D T / D (76 (77 avg IDT / ( t ( t d (78 rms IT ( t ( t d For the calculaton of the swtchng losses, we consdered, for the energy curves gven by the manufacturer datasheets, a second order approxmaton [56]. Coeffcents a dev, b dev and c dev n (79 come from ths approxmaton. P sw T / D fsw ( a ( t b ( t c dev dev dev V V ref d (79 Conducton and swtchng losses extracted n ths secton suppose a commutaton frequency much hgher than the fundamental frequency [56]. III.. System ratng In Table III- the nomnal power and the devces are defned. Moreover from the semconductor datasheet the lnear coeffcents of the conducton curve and the coeffcents of second order of the energy curve are shown. The number of cells, AC output current and voltages are extracted accordng to the cell topology chosen for the system. 68

96 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS DC voltage V d Cell voltage Swtchng frequency f sw = Hz Table III-: Case study POWER CONVERTER S = MVA IGBT V CC =.8 kv TRANSISTOR TOSHIBA S6X6B r T =.3mΩ; V T =.5 V 6 kv.5 kv DIODE EUROPEC D 33 SH r D =.3mΩ; V D =.V a on +a off =-5n; b on +b off =.m; c on +c off =-.4m a rec =-µ; b rec =5.m c rec =.57 III..3 Sngle cell& Full H-Brdge The performed study s vald for both the sngle and for the full H-brdge cell. Ths last one n ordnary condtons s confgured lke a sngle cell. The bpolar voltage s provded only for faulty condtons. The losses of both the swtchng and conducton depend on the duty cycle and on the current n the devce. The RMS and the average values of the current n the devces are carred out n ths secton. III..3. Current calculaton As t was shown n Fgure III-4, for the study the negatve branch s chosen. All capactor voltages are consdered wth a constant value V C =V d /N moreover for the current we consder a waveform wthout rpple at the swtchng frequency. For the H-brdge cell (Fgure III-5, n normal operaton, the devce 3 s always on whle the devce 4 s always off. In terms of power losses, devces and are analyzed accordng to the analytcal method adopted for the smple cell. In normal condton the devce 4 s always opened whle the devce 3 conducts always wthout commutatons. Always opened T D T 4 D 4 T D V C un V C un T D T 3 D 3 T D Fgure III-4: Sngle cell confguraton Always closed Fgure III-5: H-brdge confguraton The duty cycle s gven by (8. 69

97 Chapter III New multlevel topologes k( t n ( t ; p( t n( t (8 In Fgure III-6, the modulaton sgnal f(t and the current n a cell of the negatve branch nu (46 are reported. Due to the current waveforms, the conducton nterval of the devces depend on the sgn of cos(φ. The DC component of the current determnes dfferent losses between the two swtches of the cell M cos( arcsn( φ nu (t M sn(ωt+φ I u M cos( 4 Fgure III-6: Modulaton sgnal and cell current ω t ω t M cos( arcsn( M cos( arcsn( Dev. T D T D TABLE III- Conducton ntervals of the devces Conducton Interval [θ ; θ ] [π+asn(mcos(φ/ ; Current Mod. ndex π-asn(mcos(φ/] un (t α n (t [-asn(mcos(φ/; π+asn(mcos(φ/] un (t α n (t [-asn(mcos(φ/; π+asn(mcos(φ/] un (t α p (t [π+asn(mcos(φ/ ; π-asn(mcos(φ/] un (t α p (t TABLE III- gves for each devce the conducton nterval and the assocated current. The current averaged values were evaluated by solvng ntegrals (77 and (78 accordng to TABLE III-. The results are gven n expressons (8, (8 and (83. I avg T ( 4 M cos ( avg I M cos ( ID (8 6 4 I avg T I avg D M cos ( ( 4 M cos I 4 6 M cos( M cos( a sn M cos ( ( 4 M cos I 4 6 M cos( M cos( a sn (8 (83 7

98 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS 7 In a smlar way, ntegral (78 allows calculatng current RMS value for each devce. The results are gven from expresson (84 to expresson (87 ( ( ( cos 3 4 cos cos( cos cos( sn cos M M M M M a M I I rms T (84 ( ( ( cos 3 4 cos cos( cos cos( sn cos M M M M M a M I I rms D (85 ( ( ( cos 9 4 cos cos( 34 cos cos( sn cos M M M M M a M I I rms T (86 ( ( ( cos 9 4 cos cos cos( 34 cos( sn ( cos M M M M M a M I I rms D (87 ( cos sn cos sn ( ( M a M a ref C dev dev dev d V V c t b t a g ( ref C dev dev dev V V M a c M M M a M I b M M M M a M I a cos( sn cos( 4 cos 4 cos( sn cos( 4 ( cos 4 cos cos( 6 cos( sn cos 4 6 (88 ( cos sn cos sn ( ( M a M a ref C dev dev dev d V V c t b t a g (89

99 Chapter III New multlevel topologes a dev I 6 6M cos( M a sn cos( M cos M cos ( 4 ( 4 M cos I M M VC b cos dev M cos( a sn cos( 4 M cos( 4 4 Vref M c dev a sn cos( adev aon aoff a sw fsw sw fsw g bdev bon boff P D g b cdev con c off c PT adev aon a a off sw f sw sw fsw T g bdev bon b P off D g b cdev con c off c P adev aon aoff a sw fsw sw fsw g bdev bon boff P D g b cdev con c off c PT adev aon a a off sw f sw sw fsw T g bdev bon b P off D g b cdev con c off c P dev dev dev dev dev dev dev dev dev a b c a b c dev dev dev a b c rec rec rec rec rec rec a b c rec rec rec rec rec rec (9 (9 (9 (93 The unsymmetrcal current waveform makes the calculaton more complcated compared to a classcal Voltage Source Inverter. Knowng the RMS and averaged values of the currents the conducton losses can be easly evaluated consderng expresson (76. Swtchng losses are calculated by consderng ntegral (79 and conducton ntervals depcted n TABLE III-. It s possble to defne functons g and g as reported n (88 and (89. Nevertheless, swtchng losses change accordng to the sgn of cos(φ as t s shown n expressons (9 to (93. III..3. Case study The power losses evaluaton was performed n dfferent operatng modes: nverter and rectfer at unt power factor and reactve power compensaton. By keepng the same semconductor devces, analytcal calculatons were valdated by PSIM software. Accordng to the analyss of the cell topology, the parameters n TABLE III-3 are extracted to carry out the power requred by the system (Table III-. I u [rms] 693 A V ll [rms] 83 kv α MAX.95 N 64 TABLE III-3 system parameters f the sngle cell or the H-brdge cell are used 7

100 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS III..3.3 Results for Sngle Cell As t s shown n Fgure III-7 and Fgure III-8, n the unt power factor operaton the MMC presents a strong dsperson of the power losses between the devces as expected from the prevous secton. In the reactve power compensaton, the currents n the branches of the MMC become symmetrcal and the losses calculaton s then equvalent to a classcal VSI, moreover the nductor case s the same of the capactor case (Fgure III-9. Power [W] Conductng Total Swtchng Power [W] Conductng Total Swtchng T T D D T T D D Fgure III-7: Cell power losses for MMC n Inverter operatng mode Fgure III-8: Cell power losses for MMC n Rectfer operatng mode Power [W] Conductng Total Swtchng T T D D Fgure III-9:Cell power losses for MMC n reactve operatng mode 73

101 Chapter III New multlevel topologes III..3.4 Results for the full H-brdge Analytcal results are reported n Fgure III- to Fgure III-. For the swtchng devces and the power losses evaluaton was performed as n the elementary cell. Just conducton losses for S 3 are added. Up Arm Cell, I rms =69.96,fsw= Hz, Inverter mode Up Arm Cell, I rms =693.4,fsw= Hz, Rectfer mode 4 8 Conductng Total Swtchng Conductng Total Swtchng Power [W] 6 4 Power [W] T T T3 D D D3 T T T3 D D D3 Fgure III-: Cell power losses for MMC n Inverter operatng mode Fgure III-: Cell power losses for MMC n Rectfer operatng mode Up Arm Cell, I rms =693.4,fsw= Hz, Inductve mode 8 7 Power [W] Conductng Total Swtchng T T T3 D D D3 Fgure III-: Cell power losses for MMC n reactve operatng mode The addton of semconductor devces makes the full H-brdge more expensve n terms of losses. For ths reason an nvestgaton on the total losses of ths cell respect to the smple cell was carred out. In Fgure III-3 and Fgure III-4 the ncreases are shown. 74

102 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Id=65,fsw= Hz for DS33SH. Tradtonal Cell H-Cell Power % Fgure III-3: Increase of the losses for a 3-Phase system by usng full H brdge cells respect to a classcal system INV REC IND CAP Fgure III-4: Total losses comparson for a 3-phase system by consderng the classcal cell and the full H brdge cell III..4 Asymmetrcal H-Brdge Wth respect to the prevous topologes, the asymmetrcal H-Brdge soluton shows dfferent current waveforms n semconductor devces and dfferent modulaton ratos. Due to the symmetry, just the negatve branch s consdered; the reference crcut s reported n Fgure III-5. The current has only one drecton as n (94. For ths reason, dodes D and D n nomnal operatng condton are not used. For each par of devces, n (95 and (96 are reported the modulaton rato necessary for the calculaton of RMS and average currents. V C T D D 4 Fgure III-5: Asymmetrcal H-Brdge employed for the negatve branch D 3 T D un (t Iˆ ( t ( sn( un (94 n( t M sn( t D 3,D 4 (95 ' ' n( t n( t M sn( t (96 T,T III..4. Current calculaton Accordng to (77, the average value of the devces s extracted n (97, whle the RMS values are reported n (98 and (99. Accordng to the current drecton, shown n Fgure III-5, only T, T, D 3 and D 4 are conductng. 75

103 Chapter III New multlevel topologes I avg D/ T Iˆ ( ( un t t d 4 (97 I I rms D rms T ˆ I 3 M un( t n ( t d 4 ˆ ' I 3 M un( t n( t d 4 (98 (99 Accordng to (78 the swtchng losses can be expressed by (. P T / D sw f sw a Iˆ 8 3 Iˆ V b c V C CC ( To calculate the total losses, an equvalent case study wth same power ratng and same DC voltage level was consdered (see Table III-. Accordng to the asymmetrcal H-brdge propertes, the AC voltage value s ncreased wth respect to a topology usng smple cells (66. III..4. Results As we sad before, the power losses evaluaton was performed only for a unt power factor. By keepng the same semconductor devces and ratngs (Table III-, analytcal calculatons were valdated by PSIM software by makng ratng adaptatons n TABLE III-4 due to ths knd of topology. I u [rms] 95 A V ll [rms] kv α MAX.95 V C.5 kv N 3 TABLE III-4 System parameters Losses for ths elementary converter are balanced between the components as shown n Fgure III-6. Of course the total losses are the double (Fgure III-7 respect to the sngle cells due to the number of elementary converters necessary for the employment of ths topology. 76

104 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Power [W] Conductng Total Swtchng Total Losses %.5.5 T T D3 D4 Fgure III-6: Sngle elementary converter losses Asymmetrc HB Smple Cell Fgure III-7: Total losses percentage comparson wth the sngle cell III.3 Conclusons The MMC structure by Professor R. Marquardt s based on sngle-cell topologes. Ths cell cannot mpose negatve voltages; thus, n the case of DC faults, only the branch nductor can lmt the current. The asymmetrcal H-brdge topology needs the same number of devces per elementary converter. It can mpose a b-polar voltage that better lmts the over-currents n faulty condtons. Moreover, at the party of power szng and capactor voltage, ths topology allows a reduced value of the capactance. On the other hand, the number of elementary converter s around the double rather than the sngle cell use; moreover, ths topology s not sutable for reactve power compensatons. Of course ths topology can be employed for CSC based applcatons where the power reversblty s acheved wth the DC voltage reversblty. If the four-quadrant operaton s requred wth a good lmtaton of over-currents, then the use of the H-brdge cell s suggested. Moreover, ths cell causes the MMC structure not to depend on the voltage and current levels; ths aspect s well consoldated n the next secton. Of course, the employment of double the components ncreases the costs. The power reversblty wth ths topology can be acheved wth both the DC voltage and the DC current. The analytcal power losses study makes the evaluaton fast and drect. It was not easy to carry out the formulas due to the DC component n the devce s current. If the DC voltage value s mantaned, the Asymmetrcal H-brdge s not so convenent n terms of power losses, despte the lower RMS AC current. Ths s because, n the Asymmetrcal H-brdge, each devce conducts durng the whole perod. Nevertheless, f ths cell s employed, a reduced capactor value can be acheved. Fnally, the 5% ncreasng losses wth respect to the snglecell employment s not acceptable for the HVDC employments whch usually requre almost % of the total losses (ncludng losses n the reactve elements. For many other applcatons wth a relatvely reduced power the full H-brdge topology s recommended, because t can be employed lke a sngle cell n ordnary operatng condtons, and t can lmt the current n faulty operatng condtons. 77

105 Chapter III New multlevel topologes III.4 New Modular Multlevel Half Wave topology wth zgzag transformer As shown prevously, each branch of the MMC structure has a DC and AC component n the voltage and current. The combnaton of two branches per phase makes possble the rght power transferrng of the two components accordng to the assumptons made n the prevous chapter and the crcut dagrams gven n Fgure III-3 and Fgure III-4. The proposed structure requres just the upper part of the MMC structure as shown n Fgure III-8. Ths confguraton absorbs on the AC sde currents wth a DC zero sequence. To avod the saturaton of the magnetc core a secondary wndng wth a zg-zag confguraton s used [57]-[58]. Thanks to ths couplng, secondary voltages are balanced whle the DC zero sequence current s canceled on the prmary sde. On each phase, several DC/AC cells (converters are connected n seres. Output currents u, v and w are mposed by three ndependent control loops whch provde a Pulse Wdth Modulaton to the DC/AC converters (. The phase voltage s chosen accordng to ( Id I I ' u Iˆcos( t d ( ; ' v Iˆcos( t ; d ' w Iˆcos( t v' ( ˆ u t V sn( t ( I d V Cu = Elementary Converters V Cv = V Cw = v bu v bv v bw V CuN = V CvN = V CwN = u v u N N 3 N u V d v v v v v v v u w v w v w w v v N n v w Fgure III-8 Upgraded AC/DC converter From Fgure III-8 the averaged model presented n Fgure III-9 s developed and vald at the fundamental frequency. By neglectng the voltage drop across the leakage nductor of the transformer L S, voltagev bu can be expressed by (3. 78

106 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS f ( t m ( t u u I d V Cu C/N Vbu V Cv C/N Vbv V Cw C/N V bw v u L S u f v ( t mv ( t f ( t m ( t w w V d v v L S v v u v w L S w v v n Secondary of the Transformer v w Fgure III-9 - Smplfed crcut on the AC sde wth averaged model of the Half Wave multlevel converter V Bu ( t V Vˆ sn( t (3 d Each elementary converter has to conduct the thrd part of the DC current and the AC current necessary to acheve the power requred f a untary transformaton rato s provded for the zg-zag transformer. By consderng the same capactor voltages V C and the same DC current, f V d s the DC voltage for the tradtonal structure a fast comparson n terms of szng can be acheved for the same cell topology (Sngle Cell. Moreover for the zg-zag transformer a untary transformaton rato N /N s chosen. The voltage mposed by each branch s evaluated n (8 to keep a unpolar voltage; the modulaton sgnal s so extracted n (9. ( sn( V ( sn( V Bu ( t V d t (4 f ( t N V t (5 C d The szng comparson n Table III-5 hghlghts that for the modular multlevel Half Wave the double of the cells per phase are necessary respect to the classcal MMC. The sources V bx n 79

107 Chapter III New multlevel topologes fact have to sustan all the DC voltage. Moreover the half of the current s mposed on the AC sde because the AC voltage s the double respect to the classcal MMC structure. Number of cells AC rms voltage AC rms current MMC Mult HW wth zg-zag Rato Mult HW/MMC Vd Vd. N. N. V V V d V M. P I 3. V C C V d V M. P I ½ 3. V Table III-5: Comparson on the szng of the classcal MMC structure and the Multlevel Actve Front End In many cases the new structure can be used to replace systems where there s a pre-exstent zg-zag confguraton such as the applcaton chosen as case study n the next secton. III.4. 3 MMC Half Wave topology to upgrade obsolete dode/thyrstor rectfers In ths secton a partcular assocaton of AC/DC multlevel converters s proposed to update classcal 3-pulse dode or thyrstor rectfers. The approach s made by keepng the preexstng transformer and voltage values both on the AC and DC sde. The use of 3-pulse dode or thyrstor rectfers can be consdered one of the frst structures to acheve a AC/DC converson. In lterature, t s well known that the smplcty of ths topology leads to a seres of drawbacks [59]. Due to current waveforms show on the AC sde, these rectfers present a poor power factor. Due to severe constrants on Power Qualty, these knds of topologes are assocated to harmonc flters whch ncrease the complexty of the converson system [6]. Moreover half wave rectfers show a DC current component on the three phase currents that nfluences the transformer ratng. At ths effect, a transformer wth a zgzag couplng can be used [59]. Nowadays, n the frame of Medum and Hgh Power applcatons, new requrements on power qualty lead to draw a quas-snusodal current waveform wth a four quadrant operaton. For ths reason, classcal dode/thyrstor rectfers are obsolete and should be replaced. Wth the vew to save money, t s necessary to update the tradtonal topology by keepng the same transformer and the same output voltage level. Nevertheless, actve front end solutons based on multlevel voltage source nverters [6] are not sutable to control the DC voltage n the same condtons as a thyrstor rectfer (the average output voltage s always lower than the nput voltage. 8

108 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS III.4.. Three phase half wave rectfer The 3 pulse brdge rectfer presented n Fgure III-3 absorbs on the AC sde currents wth a DC zero sequence as shown n Fgure III-3-a [6].To avod the saturaton of the magnetc core a secondary wndng wth a zg-zag confguraton s used [57]-[58]. Thanks to ths couplng, secondary voltages are balanced whle the DC zero sequence current s canceled on the prmary sde as t s shown n Fgure III-3-b. u v u u v N N 3 N v v v v u w v w w v v V d v w N n Fgure III-3: Three phasehalf brdge rectfer wth zgzag transformer. a Current [A] I_AV ' u ' v ' w Tme [s] b Current [A] I_AV= u v w Tme [s] Fgure III-3 AC Current waveforms at secondary sde (a and prmary sde (b of the transformer 8

109 Chapter III New multlevel topologes III.4.. The case study In a half wave three phase rectfer, voltages on AC and DC sde are respectvely defned by expressons (6 and (7 by consderng the averaged crcut n Fgure III-9. Thus, the upgraded system has to be able to manage the power wth the same voltage level. So the branch voltage (8 and the modulaton sgnal (9-( are evaluated. ' ( t Vˆ sn( t (6 v u 3 3 V d Vˆ (7 ˆ 3 3 VBu ( t Vd V sn( t wth V d Vˆ (8 3 3 fˆ( t N V ˆ C V sn( t (9 Vˆ 3 3 Vˆ 3 3 m( t N V C N VC ( To keep constant the capactor voltage on each elementary converter, the power level has to be the same on DC and AC sde (. Ths statement leads to a drect relaton between I d and Î (. Thus, expresson ( shows that the current waveform has postve and negatve values requrng a converter topology bdrectonal n current. PDC Vd Id P ˆˆ DC PAC wth VI ( PAC 3 cos( Id I ˆcos( ( 3 As dscussed n the prevous secton, the multlevel AC/DC converter has to be based on four quadrants elementary converters as depcted n Fgure III-. Left column of TABLE III-6 shows the man parameters consdered for the multlevel converter ratng. Consderng 4.5 kv IGBTs or IGCTs devces and accordng to (9.The swtchng frequency s fxed to 35 Hz and the value of the capactor s calculated to acheve a voltage rpple under %. 8

110 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS DC lnk POWER CONVERTER S N = MVA DC Lnk Voltage (V d kv Transformer secondary voltage V ll 3 kv Zgzag Transformer DC capactor voltage V C.6 kv Magnetzaton Inductance L M H Number of Cells per phase N Total Leakage Inductance.7 mh Cell Capactor mf Wndng Resstance.7Ω Swtchng Frequency 35 Hz Turn rato N /N = N /N TABLE III-6 Converter Ratng To valdate ths study a model based on the tme-doman smulaton tool PSIM was developed. Smulaton results are presented below. In Fgure III-3-a, the multlevel voltage waveforms mposed by the converters are shown. As t was hghlghted n expresson (8, the DC offset makes these waveforms unsymmetrcal. Fgure III-3-b show capactor voltages and valdate the choce of the capactor value whch guarantees a voltage rpple of %. x 4 a Voltage [V] Voltage [V] Voltage [V] Tme [s].5 x b Tme [s] c Tme [s] Fgure III-3: Smulaton Results Voltage waveforms. V Cu V Cv V Cw V d E u E v E w 83

111 Chapter III New multlevel topologes Fgure III-33-a show the secondary current waveforms wth a DC component. As expected, the prmary current waveforms presented n Fgure III-33-b are quas-snusodal wth a very low harmonc dstorton. Current [A] Current [A] 8 ' 6 u 4 ' v ' w Tme [s] Tme [s] u v w Fgure III-33 Smulaton Results Current Waveforms Improvements on the AC sde n terms of current are shown n Fgure III-34. The sngle loop topology allows acheve neglgble fundamental greater harmoncs. Ths leads to a bg reducton of the flterng elements. 4 3 Spettro della corrente d fase SngleLoop 3p hb Current [A] Frequency [Hz] Fgure III-34: comparson spectral content of the phase current 84

112 Ncola Serba III.4. MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Conclusons In terms of szng the MMC Half Wave structure s the same respect to the MMC, of course the new topology does not requre the branch nductor because t utlzes the leakage nductance of the zg-zag transformer. In a classcal MMC structure the transformer has to sustan a DC nsulaton equal to the half of the DC voltage. Ths s not the case for the new topology, a classcal nsulaton s suffcent. The multlevel converter proposed to upgrade old rectfers seems attractve. It draws on the AC sde quas-snusodal current waveforms. Furthermore, thanks to the cascaded H Brdge, a four quadrant operaton can be acheved on the DC sde. Nevertheless, the number of requred devces could be very expensve. On the other hand, changng the transformer turns rato could be a cheaper soluton requrng a lower number of sem-conductor devces for the same output voltage. 85

113 Chapter III New multlevel topologes 86

114 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Chapter IV. PWM Control for Modular Multlevel Converter A control strategy for modular multlevel structures s proposed n ths chapter. The scenaro of the modulatons technques s descrbed consderng Phase Shft Modulaton PWM. In a classcal VSI, the control mposes the desred output currents and keeps a constant voltage on the DC lnk. For multcellular structures the rght balance among the voltages of each elementary converter s necessary, too. For PS-PWM employment, the control for multlevel structure s carred out through three man control loops whch are descrbed hereafter. The regulators are defned and synthetzed for each control loop. A system of MW composed by 64 elementary converters per branch s consdered as a case study for whch szng parameters were provded n the second chapter. Smulatons were performed to valdate the control strategy by ensurng the rght set-up of the regulators. The chosen smulaton envronment s MATLAB-PSIM. 87

115 Chapter IV PWM Control for Modular Multlevel Converter IV. Introducton In the last decade the attenton of research and development for modulaton technques have played a key role for multlevel structures [39], [7]-[7]. The goal was to extend tradtonal technques to multlevel cases. The control of multlevel structures requres more complex strateges to drve more semconductor devces. On the other hand, benefts can be derved from managng multple degrees of freedom (the avalablty and redundancy for nstance. The development and the detaled descrpton of the man modulaton technques n the frame of the multlevel structures are well defned n [7]. The dagram n Fgure IV- depcts the scenaro. In ths work the study of modulaton technques s focused on Phase Shfted PWM, derved from the extenson to multlevel structures of the tradtonal PWM technque. Fgure IV-: Scenaro of the modulaton technques for multlevel structures [7] The strategy adopted n ths work for mult-cell topologes assocates each carrer to a partcular elementary converter or power cell to be modulated ndependently usng snusodal PWM respectvely. The modulaton provdes also an even power dstrbuton among the cells. For a converter wth N cells, a carrer phase shft of 36 /N s ntroduced across the cells to generate the quas-snusodal output waveform [73]. 88

116 Ncola Serba IV. MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Prncple of the Phase shfted PWM for MMCs One more tme the averaged model of the structure shown n Fgure IV- s proposed n order to make the prelmnary study fast and drect. The subscrpt av n the formulas s neglected to better vsualze the equatons. The nstantaneous model s used only f specfed. I DC av V cun av av V C/N cvn V cwn C/N C/N av av v v nu nv av v nw V DC / f nu av nu f nv av nv f nw av nw N L av u L av v L av w L v u L v v L v w V DC / av pu av pv av pw n av V cup av av V cvp V cwp C/N av C/N C/N v av pu v pv av v pw f pu f pv f pw Fgure IV-: Averaged model for a MMC structure The control for SPWM modulated multlevel structures has to ensure that: The system s able to mpose the desred current to acheve the requred power In each phase the capactors are on the desred voltage level The voltages among the capactors of each elementary converter are balanced between them. The unbalancng can be caused by dfferent tolerances of passve components, unequal conducton and swtchng losses n the semconductor devces or sgnal mbalance and resoluton ssues nherent n the control crcut ncludng voltage/current sensors [63]. For these reasons the control approach needs three controllers whch are arranged accordng to the dagram n Fgure IV-3. 89

117 Chapter IV PWM Control for Modular Multlevel Converter V C V * V DC ( n / p... CN ( n / p Branch Energy Balancng I I * ( n / p n / p Actve Branch Current Control f ( n/ p fα adapt ( n/ p * V DC /N Cell Balancng ( n/ p N f N ( n/ p N Fgure IV-3: SPWM Control approach for a multcellular structure The energy balancng and the current control follow the classcal cascaded dsposton of the VSC based structures. The energy balancng provdes the requred actve power, through a current reference, n order to keep the capactors charged. In deal condtons where all the cells of the system are equal and balanced, wth the same losses and wth sensors wth the same characterstcs, the cell balancng control could be neglected. Ths control only regulates the voltage of each capactor around the rght level, whch s just reached by the energy-balancng controller. Thus ths loop adjusts the voltage on the capactor by drectly nterferng on the modulaton sgnal. The control approach does not depend from the partcular topology of the elementary converter. The scalng between f(t and the duty cycle α(t s mmedate. Each controller of Fgure IV-3 was presented and developed for the MMC structure. Moreover smulatons were performed to valdate the study on the MW system presented n the prevous chapters and recalled n Table II-. The system was szed accordng to the consderatons carred out n secton II.3. For a better understandng the averaged model was consdered. 9

118 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS I DC EC V u V DC V DC EC N nu u R S pu EC N+ V Nu R S L u L V (N+u v nu Table IV-: system power rate System Power Rate Nomnal power MW Phase to phase Grd voltage V ll 83 kv V d 6 kv Number of sub-modules N=64 Branch nductor L mh Cell Capactor C 6 mf Voltage capactor.5 kv Cell swtchng frequency Hz Inductor resstance R S 6 mω EC N V Nu v pu Fgure IV-4: Case study system IV.. Current control loop The study supposes that the averaged model of the MMC (shown n Fgure IV- s connected on the AC sde wth a balanced (6 and symmetrcal three-phase grd (4. v u ( t V sn( t v v ( t V snt v ( t V sn (3 v t 3 3 ( t I sn( t v ( t I snt w ( t I snt (4 3 3 u As dscussed n the second chapter the supermposton approach facltates the study and allows for easer understandng. In Fgure IV-5 the control strategy for the AC part of the system s depcted. The system supposes that there s not DC current for the symmetry condton. The structure can be seen as two ndependent STATCOMs (negatve and postve. 9

119 Chapter IV PWM Control for Modular Multlevel Converter I DC = AC v nu AC v nv AC v nw AC nu L * u PI AC nv L * v PI AC nw L u v u v v v w L L L v w AC pu * u PI AC pv * v PI AC pw n AC v pu AC v pv AC v pw Fgure IV-5: Control approach for the AC part for a MMC structure Each voltage of the branch generator can be regulated to mpose the requred current through the branch nductor. The AC part of the current for each branch s requred to be the half of the phase current. Accordng to the symmetry condtons one of the branch currents per STATCOM depends on the other ones as wrtten n (5. AC nw AC AC AC AC AC ( ( nv nu pw (5 pv pu Ths means that for each STATCOM just two branch voltage generators can be controlled accordng to the smplfed schema shown n Fgure IV-6 for the negatve part and n Fgure IV-7 for the postve part. AC AC L ( u/ v n ( u/ v p L AC v ( u/ v n v (u/v (t AC v ( u / v p v (u/v (t n Fgure IV-6: Smplfed crcut for the negatve part of the structure n Fgure IV-7: Smplfed crcut for the postve part of the structure 9

120 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS For each part the equatons (6 and (7 are acheved by consderng the branch nductors wthout copper losses. d L dt d L dt AC nu AC nv ( t ( v ( t ( v AC nu AC nv ( t v u ( t ( t v ( t v (6 d L dt d L dt AC pu AC pv ( t v ( t v AC pu AC pv ( t v u ( t ( t v ( t v (7 In order to control the AC part of the MMC structure t s necessary to respect the four equatons ndependently. The control strategy for the DC part of the structure s shown n the layout n Fgure IV-8. L L I DC * I DC PI DC v nu DC v nv DC v nw V DC DC nu L DC nv L DC nw L n L L L V DC DC pu DC pv DC pw DC v pu DC v pv DC v pw Fgure IV-8: Current control loop for the DC part of the MMC structure The u and v current controls are supposed to work well. The control must provde just the balance n (8. The current loop n Fgure IV-8 s acheved n (. Snce the two controlled generators are drven by the same loop the ( s developed nto (. I DC DC DC DC t ( t ( t ( t ( t ( (8 u( n/ p ( v( n/ p w( n/ p u( n/ p v( n/ p w( n/ p t 93

121 Chapter IV PWM Control for Modular Multlevel Converter I 3 v DC DC wn v DC DC DC u( n / p ( t v( n / p ( t w( n / p ( t DC d L d DC DC ( t vwp ( t VDC LL I DC I DC ; vwn ( t vwp ( t dt 3 dt DC L d ( t vwp ( t VDC LL I DC 3 dt DC wn (9 ( ( Fnally for the current control, the branch generators have to be drven to comply wth the fve equatons summarzed n (6, (7 and (. IV.. dq reference frame The advantages comng from the control mplemented n the dq reference frame wth respect to the tme doman are well known and descrbed n the lterature [64], [65]. In ths secton the control approach represented n the dq reference frame s shown and then smulatons are performed to valdate ths study. Durng the steady state of the system the currents n the branches are thereby extracted n ( for the negatve part and n (3 for the postve part of the system. nu nv nw ( t ( t ( t AC nu AC nu AC nu I ( t 3 I ( t 3 I ( t 3 DC DC DC ( pu pv pw ( t ( t ( t AC pu AC pu AC pu I ( t 3 I ( t 3 I ( t 3 DC DC DC (3 Accordng to the park transformaton the dq components are evaluated accordng to (4 and (5 [66]. dn ( t ( t ( t 3 sn( t sn( t sn( t 3 3 cos( cos( cos( t t t 3 3 / / / qn nv n nw t dp ( t sn( t sn( t sn( t 3 3 ( t 3 cos( cos( cos( t t t ( t 3 3 / / / qp pv p pw t nu pu ( t ( t ( ( t ( t ( (4 (5 94

122 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS 95 The dq components of the current for each part (negatve and postve of the MMC are so extracted n (6. Of course, the dq components depend on the AC part of the branch currents because the second part of the sum s equal to zero. To perform the current control loop the dq transform of the dervate has to be carred out accordng to (6, (7 and (. For ths reason the dervate n the tme doman for (4 and (5 are acheved n (6. The components are evaluated by consderng the (8 and (9 DC p n w p n v p n u p n AC p n w AC p n v AC p n v AC p n w AC p n v AC p n u q p n AC p n w AC p n v AC p n u AC p n w AC p n v AC p n u d p n I dt d dt d dt d dt d dt d t t t t dt d t dt d t dt d dt d t t t t dt d t dt d t dt d dt d cos( 3 cos( cos( 3 3 sn( 3 sn( sn( 3 3 cos( 3 cos( cos( 3 3 sn( 3 sn( sn( 3 / ( / ( / ( / ( / ( / ( / ( / ( / ( / ( / ( / ( / ( / ( / ( / ( / ( / ( (6 In order to acheve a better vsualzaton, (6 was rearranged n the form of (7 for the negatve part of the system and (8 for the postve one. dn q AC n nq nq d AC n nd dt d t dt d dt d t dt d ( ( (7 pd q AC p pq pq d AC p pd dt d t dt d dt d t dt d ( ( (8 The obtaned equatons show the dependency between the d and q components, as descrbed n [63]-[65]. So the results n (6 and (7 and ( n the dq reference frame become (9 and (3. ( ( 3 ( ( ( ( ( ( ( ( ( t v V t I dt d L L t v t v L t dt d L t v t v L t dt d L DC DC L q nq nd nq d nd nq nd (9 ( ( 3 ( ( ( ( ( ( ( t v V t I dt d L L t v t v L t dt d L t v t v L t dt d L DC DC L q pq pd pq d pd pq pd (3

123 Chapter IV PWM Control for Modular Multlevel Converter Evaluatons showed that the d and q components of the current are coupled between them [64]. In the next secton d and q components are decoupled to smplfy the regulator synthess. IV... The regulator synthess In order to test only the current control loop, the system n Fgure IV-9, the averaged branch, s consdered by substtutng the capactor wth a voltage source. V C ( n / p V N DC ( n/ p t ( t f ( ( n/ p t ( V ( t f( C ( n / p t Fgure IV-9: Averaged model of the MMC branch to test the control loop The PLL adopted s the Feed Forward q-pll whch generates the drect component synchronous reference frame. The chosen q-pll was consoldated n [68] for ts fast and robust latchng. As descrbed before the dq current controls are performed for the postve and negatve part of the structure. Accordng to (9 and (3 the decouplng layouts are acheved n Fgure IV- for the negatve part and n Fgure IV- for the postve part. 96

124 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS nu nv nw v u v PLL u, v, w v t v d,q, w I n * nd * nq Current controller v Negatve part d - + PI nd nq + - ω L ω L PI - v q - - v nd v d + v q - + vnq sl ω L ω L sl Plant Negatve part nd nq Fgure IV-: Layout of the control for the negatve part n dq reference frame pu pv pw v u v PLL u, v, w v t v d,q, w I p * pd * pq Current controller v postve part d + + PI pd pq + - ω L ω L PI - v q - + v pd v d + v q + + vpq sl ω L ω L sl Plant postve part pd pq Fgure IV-: Current controller plant n dq reference frame for the postve part of the structure After the decouplng the d and q components can be ndependently treated as depcted n Fgure IV- and Fgure IV-3 [69]. 97

125 Chapter IV PWM Control for Modular Multlevel Converter * nd nd C (s V DC v d sl * pd pd C (s V DC v d sl * nq nq C (s V DC v q sl * pq pq C (s V DC v q sl Fgure IV-: Current loops for the negatve part after the de-couplng Fgure IV-3: Current loops for the postve part after the de-couplng In the dq reference frame, the control of the fundamental component means controllng a constant varable n the tme doman. Moreover, a second harmonc component due to the crculatng currents, treated n the second chapter, has to be suppressed (Fgure II-9. In dq ths component becomes the fundamental one. The PI regulator (3 s synthetzed accordng to the open loop transfer functon shown n (3. The gan k s evaluated n order to acheve a crossng frequency ten tmes the fundamental component whle the tme constant T s defned to acheve a phase margn of 6 n order to guarantee the stablty. st C ( s k st VC st H( s k. sl st VC H( s k. C CL H( jc a tan 3 CT 3 wth ( f C T C (3 (3 (33 The layout of the component control s defned n Fgure IV-4 by mplementng the thrd equaton of (9 or (3. 98

126 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS I * * DC 3I ( n / p Controller DC part + - PI - V DC + v( n / p + +V DC 6 s L ( 3L L Plant DC part 3I( n/ p Fgure IV-4: Current controller plant for the component of the structure The layout of the control loop s depcted n Fgure IV-5. I * * DC 3I ( n / p V DC C ( s 6 s L ( 3L L 3I( n/ p Fgure IV-5: Control loop for the component The regulator (34 makes the system able to follow the power requred, for ths reason; the dynamc propretes of the open loop transfer functon (35 are slower than the current loop. Therefore, the cuttng frequency s around Hz whle the tme constant s chosen to guarantee the stablty (36. st C ( s k st 6 st H ( s k VDC. s(3l L st H( s H( j C C L 6 k VDC. C (3LL L a tan 3 CT 3 wth C 5 T C (34 (35 (36 99

127 Chapter IV PWM Control for Modular Multlevel Converter IV... Smulatons The current control loops are valdated va smulatons by consderng the averaged system wth the man parameters lsted n Table II-. The smulatons are performed accordng to the power excurson n Fgure IV-6, from nverter to rectfer operatng mode always at unt power factor. The dq currents measured n the system seem to match qute well wth the references. The stablty and the crossng frequency mposed by the regulators guarantee the stablty and good shape of the waveforms nd* [A] nd [A] pd* [A] pd [A] M 5M M -5M -M P [W] Tme (s Fgure IV-6: d-currents wth the references and actve power In Fgure IV-8 and Fgure IV-8 the good synthess of the regulators s valdated even for the q component whch s mposed to zero and for the component whch s the thrd part of the DC current.

128 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS nq [A] - -4 pq [A] 4 nd* [A] nd [A] pd* [A] pd [A] Iq* [A] nq [A] pq [A].5 3 Tme (s Fgure IV-7: Components for q-currents - - In Ip Tme (s Fgure IV-8: q-current references and measurements Fnally the u, v, w current components n the branches are reported for the two parts of the system. As verfed before, the component regulator was also very well syntheszed.

129 Chapter IV PWM Control for Modular Multlevel Converter nu [A] nv [A] nw [A] I DC Id/3 [A] 5-5 pu [A] pv [A] pw [A] IId/3 DC [A] Tme (s Fgure IV-9: u, v, w current components n the branches and contnuous current IV..3 Branch energy balancng Ths part of the control regulates the actve power necessary to keep the capactor voltages on a requred level. The averaged system n Fgure IV- s consdered. For each part of the system (postve and negatve the DC mean capactor voltage of the three branches V Cn/p s gven by (37. V C n 3 ( V V V Cnu Cnv Cnw ; V ( V V V 3 C p Cpu Cpv Cpw (37 In the dq reference frame, consderng the PLL latched on the phase voltage, the actve and reactve powers, managed by each part of the structure, are acheved n (38. The acheved balance affrms that the actve power depends only on the drect component of the AC current and of the AC voltage. The AC voltage s mposed by the grd. The control of the actve power s acheved by the regulaton of the drect component of the current. P Q AC ( n / p ( n / p 3 3( vd ( n / p d vq( n / p q ( PAC ( n / p v v Q d ( n / p q q ( n / p d ( n / p 3v 3v d ( n / p d d ( n / p q ; (38 Assumng that the negatve and postve parts of the structure are balanced between them, the actve power s shared n the structure accordng to the (39. The equvalent capactance

130 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS of each branch s consdered as dscussed n the second chapter (C/N by takng nto account the AC n (38 and the balance (39 becomes the (4. P P DC DC C d PACn PACp VC ( n/ p ( t VC ( n/ p ( t ; P N dt d 6vd( n/ p d C VC ( n/ p ( t dt ACn PAC PACp (39 (4 A constant value of the capactor voltage requres a current wth a null DC component as dscussed n the second chapter. Hence, we have a drect relatonshp between the DC and AC currents of the system recalled by (4 n the dq reference frame. AC and DC current components n the system are dependent. M d cos( (4 4 IV..3. Desgn of the controller The synthess was developed by consderng the layout n Fgure IV- vald for the negatve and the postve structure accordng to the (4. The gan of the loop was fxed by consderng v d equal to the peak value of the voltage (6. * ( V C ( n / p ( V C ( n / p C V (s * ( n / p d 6 V P DC N sc Fgure IV-: Branch energy control loop By consderng the voltage regulator (3, the open loop transfer functon of the system s reported n (43. The control system guarantees the rght energy balancng, for ths reason the PI regulator s chosen to acheve a low crossng frequency and by ensurng the stablty (44. 3

131 Chapter IV PWM Control for Modular Multlevel Converter C st v v ( s kv (4 stv Hv N st v ( s kv 6 V. (43 sc stv N H( s kv 6 V. C CvC Hv( jc a tan 3 CvT 3 v wth C T Cv v (44 IV..3. Smulatons Smulatons were carred out n order to verfy the good synthess of the energy balancng regulators. The man parameters of the system are reported n Table II-. Smulatons are performed by consderng the power excurson n Fgure IV-, keepng a null reactve power. The drect components of the current references are generated by the voltage control as depcted by the macro-layout for of the control n Fgure IV-3. By requrng a DC current of 65 A (necessary to acheve MW, the voltage controllers generate the rght reference by ensurng the rght level of the d-component for each branch current (4 for the two parts of the structure (Fgure IV-. M 5M M -5M -M -5M Pa nd [A] nd* [A] pd [A] pd* [A] Tme (s Fgure IV-: Output of the energy balancng controller and drect component of the current for the negatve and postve part; power flow 4

132 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS The voltage value on the averaged cell capactors s kept constant as shown n Fgure IV-. Vcnu [V] Vcnv [V] Vcnw [V] 7K 65K 6K 55K 7K 65K 6K 55K Vcpu [V] Vcpv [V] Vcpw [V] Tme (s Fgure IV-: Averaged capactor voltages IV..4 Cell voltage balancng To balance each cell on the desred voltage value a proportonal corrector s chosen as was descrbed n [63]. For each branch the controllers are acheved accordng to Fgure IV-3. The control adds an offset dα to the duty cycle mposed by the prevous controller and t s placed accordng to the control plant n Fgure IV-3. Because the balance locally nterferes on the sngle cell t s necessary to take n account the drecton of the current. In ths way, accordng to the references mposed n Fgure IV-, f the current s postve and the voltage of the cell capactor s low, the regulator ncreases the tme n whch the capactor s connected to the branch untl when s reached the requred value and vce-versa. 5

133 Chapter IV PWM Control for Modular Multlevel Converter I DC Comp Sgn V DC /N V C K C X dα V c V CN K C X dα N V cn Fgure IV-3: Capactor voltage balancng dagram n a branch The constant of the regulator k C (45 s evaluated to acheve at most 5% of the maxmum value of the duty cycle where VC s the ampltude of the capactor voltage determned by the value of the capactance of the cell (6 mf to acheve % of voltage rpple. k C V C max 5% (45 IV..4. Smulatons To perform the smulatons, the nstantaneous model n Fgure IV-4 was consdered. More detals are reported n Table II-. A resstance s mposed n parallel (.5 kω for a cell capactor n order to unbalance ts voltage V Cu to 375 V. On the other hand the energy balancng loop forces the voltage on another cell of the same branch, n ths case V Cu, to ncrease up to 65 V n order to reestablsh the balance. The smulaton vew n Fgure IV-4 starts wth the cell balance enablng. Accordng the sgn of the current, Fgure IV-4 shows the nterventon of the frst regulator dα u n order to decrease the voltage ampltude by consderng a postve DC current. The opposte nterventon, for a less voltage ampltude, s carred out for V Cu by dα u. The same results, startng from the same capactor voltage value, for a negatve DC current are shown n Fgure IV-5. 6

134 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS 3 Voltage [V] Tme [s].5 V Cu V Cu d u d u Tme [s] Fgure IV-4: Capactor voltages and outputs of the cell balancng regulator dα for a postve DC current 3 Voltage [V] 5 V Cu V Cu Tme [s].5 d u -.5 d u Tme [s] Fgure IV-5: Capactor voltages and outputs of the cell balancng regulator dα for a negatve DC current 7

135 Chapter IV PWM Control for Modular Multlevel Converter IV.3 Conclusons For multlevel structures a very low swtchng frequency can be obtaned for each elementary converter due to the phase shfted carrers of the PWM modulaton. For a hgh number of levels however there s an nferor lmt of the swtchng frequency per swtchng devce. In fact the averaged value of the capactor current of the elementary converter has to be kept at zero. Ths s guaranteed for a swtchng frequency not less than Hz. Ths means that at current party, the devce losses can t be further reduced. For starcase based modulatons, partcularly when the number of levels s very hgh, the equvalent swtchng frequency can be further reduced up to 9 Hz [75]. Ths mproves performances of the devces n terms of losses. For both the modulaton technques a centralzed control s necessary. Ths means that all the sgnals of the system, currents and capactor voltages, must be connected to a central controller. For a hgh number of levels a very complex hardware s requred to wre each capactor voltage to the central controller and, vce-versa, to wre the drvng sgnal from the controller to the swtchng devces. For these reasons the trend could be to provde a central controller whch manages just the control of the currents and each cell provdes tself wth voltage, maybe accordng the surroundng ones. 8

136 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Chapter V. The kw modular multlevel prototype In order to valdate the szng of the components and the control approach a three phase prototype of kw was made. The structure s composed of 8 elementary cells. Each cell s szed to sustan a V capactor voltage. IGBTs were chosen as swtchng devces. The converter s confgured to test the sngle loop structure proposed n the thrd chapter and then, to valdate the control loop n the dq reference frame and descrbed n the prevous chapter. In a frst step, the converter was confgured n a sngle loop structure usng for each branch a RL load n seres to the elementary cells. Ths s an ntermedate confguraton, whch serves a double purpose. The classcal MMC s well known for ts much-reduced capablty of lmtng the branch current n faulty condtons [76]-[77] so t was not preferred for a frst test. The RL seres connected load nstead lmts the current n the branches by guaranteeng the setup of the sgnal chans and the valdaton of the synthess of the regulators n safety condtons. Moreover, ths confguraton renforced the study of the sngle loop topology for whch expermental results are presented. In a second step, the classcal MMC s tested n open loop condtons and expermental results are reported. 9

137 Chapter V The kw modular multlevel prototype V. The prototype confguraton The prototype was developed at the LAPLACE laboratory. It s a kva three-phase modular multlevel converter composed of 8 swtchng cells. On ths bass each branch has 4 voltage levels (, V DCcell, V DCcell, 3V DCcell. Otherwse for a sngle loop confguratons, two branches are drectly n seres and 7 voltage levels can be acheved (, V DCcell,, 6V DCcell. A dagram of the converter s shown n Fgure V-. The layout also shows the nstalled voltage and current sensors. The ratng data s summarzed n TABLE V-. I DC V Elementary Cell IL IL 3 IL 5 V 7 Elementary Cell 7 V 3 Elementary Cell 3 V Elementary Cell V 8 Elementary Cell 8 V 4 Elementary Cell 4 V 3 Elementary Cell 3 V 9 Elementary Cell 9 V 5 Elementary Cell 5 V DC V AB V BC V 4 Elementary Cell 4 V Elementary Cell V 6 Elementary Cell 6 V 5 Elementary Cell 5 V Elementary Cell V 7 Elementary Cell 7 V 6 Elementary Cell 6 V Elementary Cell V 8 Elementary Cell 8 IL IL 4 IL 6 V CA Fgure V-: Layout of the prototype kw system The Power supply Power Rate kva Model TDK-Lambda Genesys V DC 6V Power 5 kw V DCcell V DC max. out voltage 6 V L 5mH DC max. out. current 8.5 A C mf Cell f sw khz IGBT IRGP35B6PDPBF 6A 6V TO 47 Case TABLE V-: kw system parametres

138 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS The power supply can reach up to 5 kw that s the half of the power ratng of the system. Ths value s suffcent to acheve the prelmnary tests whch concern the set-up of the sensor chans and the valdaton of the control loops. V.. Reactve elements desgn The desgn was acheved accordng to the consderatons carred out n Chapter II. The value of the nductor L s chosen n order to lmt the current rpple n the branch at swtchng frequency. Partcularly, for ths prototype a maxmum current rpple of % was allowed. v u s the phase voltage mposed by the converter on each phase. Equaton (46 consders a maxmum modulaton ndex M of.9. V VDC.9 9V (46 The rms current on the AC sde at fundamental frequency can be expressed as (47 where P s the power of the converter. P I 5A 3V 7. (47 As prevously evaluated, the nductor must respect the balance n (48, for practcal reasons an nductance L=5 mh was chosen wth a ratng of A VDC 6 L 3. 4mH 4N f I 83.4 (48 c max A capactor of mf was chosen as t lmts the voltage rpple at fundamental frequency under %. Each cell s desgned to requre an optcal sgnal for the drvng of the semconductor devces. The desgn of the frame, the PCB of the sngle cell, the arrangement of the sensors and the power supply for the sgnal management are descrbed n Appendx A. The fnal frame for the power sde of the converter s shown n Fgure V-.

139 Chapter V The kw modular multlevel prototype Fgure V-: Power Hardware frame V.. Hardware In the Loop confguraton In ths secton a descrpton s gven on the most mportant parts of the system, whch allows the control mplementaton and tests. As shown n Fgure V-3, the confguraton, besdes the multlevel prototype, s composed of a HIL box, whch allows mplementng the control system through a PC. The HIL box accepts analogcal sgnals and sends dgtal sgnals to the prototype through the nterfacng hardware desgned for the purpose. The nterfacng hardware allows managng the sgnals n two drectons. In one drecton t processes and adapts the analog sgnals comng from the prototypes sensors for the HIL box nput. In the other drecton the nterfacng hardware converts the dgtal drvers comng from the HIL box n optcal sgnals to control the cells. A more detaled descrpton of these components s gven n Appendx A.

140 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS HIL System Interfacng Hardware kw Converter Prototype Fgure V-3: HIL confguraton Lay out The fnal assembly of the system wth a passve RL load s shown n Fgure V-4. Fgure V-4: Fnal assembly for the system 3

141 Chapter V The kw modular multlevel prototype In the next secton the modular structure s confgured n sngle loop modalty. A passve load s connected to the system. After a bref explcaton of the load choce, the control approach s shown and the synthess of the regulators s carred out. Expermental results are used to valdate the study. V. Sngle Loop Confguraton The structure s organzed as shown n Fgure V-5. Due to the unpolar propretes of the sngle cell ths system mposes also a DC component on the load. For ths reason ths arrangement can be consdered a prelmnary confguraton before connectng the zg-zag transformer ntroduced n Secton III-. The confguraton was useful to set-up the sensors and to confrm the good correspondence between the smulatons and the expermental results wth respect to the regulator synthess. For the tests, a 4 kw three-phase load was used. I DC V Elementary Cell L Elementary L 3 Elementary L 5 V 7 Cell 7 V3 Cell 3 V Elementary Cell V 8 Elementary Cell 8 V4 Elementary Cell 4 V 3 Elementary Cell 3 v bu V 9 Elementary Cell 9 v bv V5 Elementary Cell 5 vbw V 4 Elementary Cell 4 V Elementary Cell V6 Elementary Cell 6 V DC V 5 Elementary Cell 5 V Elementary Cell V7 Elementary Cell 7 V 6 Elementary Cell 6 V Elementary Cell V8 Elementary Cell 8 L L L L L L R L R L R L Fgure V-5: Prototype n Sngle Loop Confguraton In order to evaluate the value of the resstance R L to acheve the fxed power, the branch currents (49 and voltages (5 are defned by neglectng the voltage drop on the nductor. 4

142 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS L3 L5 I DC L I sn( t 3 I DC I sn( t 3 3 I DC I sn( t 3 3 (49 v v bv bw vbu VDC V sn( t VDC V sn( t 3 VDC V sn( t 3 (5 The power of each branch s balanced accordng to (5 (n each cell AC and DC power must be balanced accordng to III., so the balance for the total actve power s gven by (5. By consderng a SPWM modulaton and the relatonshps acheved n Chapter II for the sngle cell, M s consdered the ampltude of the modulaton ndex. P V cell DC DC cell VDC PAC VI ; V M (5 I DC I DC I DC 3Pcell 3 RL 3 RLI 3VI VDC I DC 3 RL 3 RLI 9 9 (5 By takng n account (5, n (53 the value of I s reported. I I DC (53 3M By substtutng (53 n (5, (54 s acheved. VDC RLI DC R 3 3M ; eq RL (54 3 3M So the control matches the mpedance through the varaton of M n order to acheve the requred power. In ths case R L s defned by the resstor bench avalable n the laboratory whch allows up to 4 kw operatng power. Accordng to the relatonshps acheved before, TABLE V- reports the man operatng parameters. The nductor L L was chosen to test the good current rpple around the 5%. 5

143 Chapter V The kw modular multlevel prototype Sngle loop system parameters Operatng power 4 kw V DC 6 V R L 4 Ω R eq 9 Ω I DC 6.7 A M.6 L L 5 mh Number of cells N per branch 6 TABLE V-: system parameters for the sngle loop confguraton In the next secton the control approach s descrbed and the synthess of the controller s carred out. V.. The control A smple u, v, w frame s consdered and a superposton approach s used to smplfy the study. By consderng the averaged model, the DC current loops are shown n Fgure V-6 whle the AC loops are reported n Fgure V-7. I DC I DC = L L DC vbu I DC 3 L L DC vbv I DC 3 L L DC v bw I DC 3 V DC AC I L L L AC v bu AC I L 3 L L AC vbv AC I L 5 L L AC v bw R L R L R L R L R L R L Fgure V-6: Layout of the DC part of the system Fgure V-7: Layout of the AC part of the system By consderng the sngle cell topology the voltage mposed by each branch s reported n (55 accordng to the averaged system n Fgure V-8 for a generc phase (u, v or w. On the DC approach, the branch mposes the voltage to balance the DC sde accordng to (56. The AC voltage value s determned by the modulaton ndex M gven n (57. The voltage on the equvalent capactor s provded by (58. 6

144 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS L Vc V DC C/N α v b L L R L NVC vb ( M sn( t (55 DC v V NV V (56 v V b AC b DC C DC M NV sn( t (57 N C C V Cj N j (58 Fgure V-8: Averaged model of the branch cells Accordng to the control strategy for the multlevel structures depcted n Fgure IV-3 (prevous chapter PI regulators are synthetzed. The current control loop s hghlghted n Fgure V-9, where n ths case the gan of the system s V DC. The PI regulator s synthetzed n order to acheve a crossng frequency of khz whle the tme constant s defned to acheve a phase margn of 6 n order to guarantee the stablty. * L C (s V DC sl L Fgure V-9: Current control loop The branch energy balancng generates the actve reference current necessary to keep a total voltage on the capactors of V DC where C/N s the equvalent capactance of each branch. The PI regulator s chosen to acheve a low crossng frequency by ensurng the stablty. 7

145 Chapter V The kw modular multlevel prototype * ( V DC ( V C - C V (s I * ( u, v, w 6 P DC N sc By consderng the nstantaneous model, the same strategy already descrbed n the prevous chapter was adopted to keep a constant voltage of V DC /N on each cell capactor. Ths because the cell voltage balancng s a parallel loop whch depends nether from the current nor from the branch energy balancng loops but t drectly nterferes on the modulaton ndex. One more tme each cell capactor has V voltage. V.. Smulatons For the sngle loop system prevously descrbed, TABLE V- reports the man parameters and ts layout s hghlghted n Fgure V-5. The levels mposed by the cells (Fgure V- n the system are lmted to 5 levels because the modulaton ndex M s equal to.6. The maxmum number of levels 7 s reached for a value of M almost equal to Vbu [V] Vbv [V] Vbw [V] Tme (s Fgure V-: Voltage mposed by each branch 8

146 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS The currents n the branches and the DC current are shown n Fgure V-. The current rpple, kept below the desred value, confrms the rght evaluaton of the branch nductor. L [A] L3 [A] L5 [A] dc [A] Tme (s Fgure V-: Branch currents and DC current The voltage on the cell capactors kept at the desred value valdates the sngle cell balancng as shown n Fgure V V [V] V [V] V3 [V] V4 [V] V5 [V] V6 [V] V7 [V] V8 [V] V9 [V] V [V] V [V] V [V] V3 [V] V4 [V] V5 [V] V6 [V] V7 [V] V8 [V] Tme (s Fgure V-: Voltages on the cell capactors 9

147 Chapter V The kw modular multlevel prototype The performed smulatons valdate the study for ths confguraton of the system. Expermental results are reported n the next secton. V..3 Expermental results The tests confrm the good operaton of the sensor chans and the good synthess of the regulators. The acqustons are dsplayed by consderng a tme scale of 5ms/dv. v bu v bv 5ms I DC u Fgure V-3: Branch voltages and currents on DC sde and phase u. v bu v bv 5ms v u Fgure V-4: Branch voltages and currents

148 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS 8 Voltage [V] V V V 3 V 4 V 5 V tme [s] Fgure V-5: Cell capactor voltages The expermental results seem to match qute well wth the smulatons. The voltages mposed by the branches confrm one more tme the correct phase shft between the carrers. Moreover all the levels are not reached because the ampltude of the modulaton ndex s around 6%. The achevng of the power requred valdates the good choce of gans for both the regulators for the energy balancng and current loops. The stablty of the system s guaranteed by the phase margn whch valdates the values of the tme constants. Fnally the parallel loop for the cell voltage balancng nterferes wthout nfluencng the stablty. V.3 MMC confguraton The MMC confguraton of the prototype s consdered. For the smulatons n closed loop, the multlevel structure s connected to a three-phase voltage source (Fgure V-6. For the expermental results, an open loop control s consdered wth a RL load (Fgure V-. V.3. Smulatons n Closed Loop operaton Because of the network connecton the smulatons were performed for the full kw power system of whch parameters are reported n V.. An excurson s carred out at unty power factor by leadng the system durng operaton from nverter to rectfer mode.

149 Chapter V The kw modular multlevel prototype I DC V Elementary Cell IL IL 3 IL 5 V 7 Elementary Cell 7 V 3 Elementary Cell 3 V Elementary Cell v nu V 8 Elementary Cell 8 v nv V 4 Elementary Cell 4 v nw V 3 Elementary Cell 3 V 9 Elementary Cell 9 V 5 Elementary Cell 5 V DC V 4 Elementary Cell 4 V Elementary Cell V 6 Elementary Cell 6 V 5 Elementary Cell 5 v pu V Elementary Cell v pv V 7 Elementary Cell 7 v pw V 6 Elementary Cell 6 V Elementary Cell V 8 Elementary Cell 8 IL IL 4 IL 6 v u v v v w Fgure V-6: MMC system confgured for the smulatons The u, v, w currents are shown n Fgure V-7. Durng the excurson the stablty of the system s mantaned. Also the stablty of the DC current confrms the good synthess of the current regulators. Moreover the rght actve power s requred by the regulator synthetzed for branch energy balancng loop.

150 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS L [A] L3 [A] L5 [A] Idc - - L [A] L4 [A] L6 [A] Idc [A] Tme (s Fgure V-7: Branch currents and DC current The branch voltages n Fgure V-8 show how all the four swtchng levels are reached. Ths valdates the good phase delay between the SPWM modulaton carrers. 6 Vnu [V] Vnv [V] Vnw [V] 4 6 Vpu [V] Vpv [V] Vpw [V] Tme (s Fgure V-8: Voltages mposed by the branches 3

151 Chapter V The kw modular multlevel prototype Fnally the averaged voltages on the cell capactors are kept on V. Ths valdates the balancng of the sngle cell voltage. voltage [V] Tme [s] V V V 3 V 4 V 5 V 6 5 voltage [V] Tme [s] V 7 V 8 V 9 V V V voltage [V] Tme [s] V 3 V 4 V 5 V 6 V 7 V 8 Fgure V-9: Voltages on the cell capactors V.3. Open Loop-Tests The tests were carred out for the maxmum capablty of the power supply (5 kw. The MMC confguraton s connected to the RL load accordng to the layout shown n Fgure V-. The resstve load s composed of two 4 kw test benches n parallel to acheve the power 4

152 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS necessary for the tests. The fnal resstance star confgured has a 4 Ω value n order to acheve 4.8 kw. IDC V Elementary Cell IL IL3 IL5 V7 Elementary Cell 7 V3 Elementary Cell 3 V Elementary Cell v nu V8 Elementary Cell 8 v nv V4 Elementary Cell 4 v nw V3 Elementary Cell 3 V9 Elementary Cell 9 V5 Elementary Cell 5 V DC V4 Elementary Cell 4 V Elementary Cell V6 Elementary Cell 6 V5 Elementary Cell 5 v pu V Elementary Cell v pv V7 Elementary Cell 7 v pw V6 Elementary Cell 6 V Elementary Cell V8 Elementary Cell 8 IL IL4 IL6 L L L L L L v u v v v w R L R L R L Fgure V-: MMC system confgured for the expermental tests The currents n the negatve branches are depcted n Fgure V-. In open loop each branch current presents a DC and a second harmonc component of the fundamental.. 5

153 Chapter V The kw modular multlevel prototype 5ms L L3 L5 Fgure V-:Negatve branch current As shown n Fgure V-, the currents n the negatve and postve branches are n phase opposton n terms of fundamental component. Also the sum between them s reported accordng to the references shown n Fgure V-. The sum has a DC and a second harmonc component whch has to be suppressed by the closed loop control. L L L+ L 5ms Fgure V-: Negatve, postve branch current and the sum 6

154 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS The value of the DC current around 8A confrms an operatng power of the system around 4.8 kw (Fgure V-3. The output AC currents (Fgure V-3 have a snusodal waveform. Ths means that both the components DC and second harmonc are kept n the branch for each phase. The current rpple s further reduced because of the phase shft modulaton technque. In fact the group of carrers for the cells of the postve part s phase shfted respect to the group of the negatve part. w v u 5ms DC Fgure V-3: Output AC currents and DC current The 4 voltage levels acheved for the branch voltages valdate the correct phase shft among the carrers of each cell (Fgure V-4. 7

155 Chapter V The kw modular multlevel prototype v nu 5ms v nv v nw L Fgure V-4: Negatve branch voltages and current n the negatve branch Fgure V-5 shows the postve and negatve branch voltages for the u-phase. As expected the voltages have the same DC component. The AC components complement each other at the fundamental frequency. v nu 5ms v pu L Fgure V-5: Negatve and postve branch voltages, branch current 8

156 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS The zoom of Fgure V-5 shown n Fgure V-6 valdates the rght phase shft between the cells of the negatve branch and the cells of the postve branch. Furthermore, the current rpple ampltude confrms the rght szng of the branch nductors whch guarantees a current rpple under % (the nductor s rated for a current of A., ms A Fgure V-6: Zoom on voltage and current waveforms Fnally some capactor voltages are shown n Fgure V-7. The voltages are balanced between them, they have the same DC component value of V. v 4 v 8 5ms v 6 L Fgure V-7: Capactor voltages and branch current 9

157 Chapter V The kw modular multlevel prototype V.4 Conclusons Expermental valdatons for the sngle loop structure wll allow an mmedate transton to the confguraton wth the zg-zag transformer. The closed loop tests ensured the good correspondence between the sensors and the nput analog sgnals to the controller (HIL Box. Moreover, each output modulaton sgnal comng from the controller drves the rght devce. The wrng of the swtchng sgnals by optcal fber consderably reduced the EMI problems. The stablty of the tested closed loop system valdates the relablty of the smulaton results by confrmng the rght synthess of the regulators. Ths aspect wll facltate the closed loop tests for the MMC structure. Of course ths case s always more dangerous just because the branch nductor lmts the current n emergency condtons (dvergence of the control, faults etc. These tests wll be performed n the comng month. 3

158 Ncola Serba NEW TOPOLOGIES FOR HVDC POWER STATIONS Conclusons & Future Prospects Nowadays HVDC connectons are an approprate answer to the more and more ncreasng world energetc demand. Multlevel topologes are gong to make VSC converters the most employed n HVDC systems. The development of hgh voltage controlled turn-off devces made these structures very attractve. On the other hand because of advantages comng from the easly seres connectons of thyrstors, CSC structures can better manage hgh voltages. In the near future, the gap between VSC and CSC structures wll be much reduced thanks to the performances offer by IGCT devces n terms of on-state current ratng and blockng voltage. The press packagng leads a seres of advantages respect to the classcal modules especally n fault condton where there s a rsk of exploson. The sngle wafer feature makes the IGCT more sutable for the press pack packagng respect to the IGBT. For these reasons the IGCT seems to be the most attractve devce n VSC-HVDC applcatons. Ths thess focused on the VSC based Modular Multlevel Structure. For prelmnary studes the macro-model allowed drect evaluatons and very fast smulatons especally as ths model s not dependent on the elementary converter topology. The ratng of the system was carred out through two control approaches. The frst consders just a control on the AC output current whch leads to a huge second harmonc current n the branch. We showed that Coupled nductors could be a good soluton to lmt ths current but n the feld of hgh power applcatons, the partcularty of ths hardware ncreases absolutely the cost. Thus, the second approach conssts n a control of each current n the branch, despte t requres a more effcent control system based on a dq reference frame. Under ths condton the second harmonc component of the current s cancelled whch cut down the ratng of the passve components. The employment of dfferent topologes as elementary converter made the MMC more flexble n terms of voltage and current reversblty. In terms of losses at party of power and DC voltage, the smple cell s more convenent. Unlke topologes whch provde bpolar voltage (Asymmetrcal HB and full H-brdge make the structure able to lmt the short-crcut current n case of fault on the DC lnk. The Phase Shfted PWM led to a reducton of the swtchng frequency and then the semconductor losses. Of course ths modulaton technque presents an nferor lmt on the swtchng frequency. When the number of levels s very huge the starcase modulaton could be very attractve for multlevel structures. A study of the starcase modulaton for the MMC structure s gong to be soon developed. In fact an nvestgaton on the nfluence of the modulaton versus the ratng of the reactve elements and versus power losses of the devces compared to the PS PWM left to be done. Few aspects could make the Asymmetrcal HB attractve n terms of HVDC applcatons. If ths topology s chosen, the cell capactor can be reduced at party of voltage rpple ampltude. 3

159 Conclusons & Future Prospects Snce the system acheves the nverson of the power flow by changng the polarty of the DC voltage, ths topology can be employed to replace CSC based HVDC power statons wth the advantage of a unt power factor operaton. The new sngle loop structure proposed n chapter III allows an easer control system. The topology does not requre the double branch nductor because t uses the leakage nductor of the zg-zag couplng transformer. Although ths couplng requres more copper than a classcal wndng, the nsulaton of the transformer has to be rated only for the AC voltage. Ths s not the case of a classcal MMC arrangement where the transformer has to sustan a DC nsulaton half of the DC voltage (DC zero sequence component. Beyond these consderatons, the use of ths new structure could become very attractve to upgrade old rectfers by guaranteeng advantages comng from VSC structures. A kw prototype was developed n the LAPLACE laboratory. In order to nterface the power crcut wth the Hardware In the Loop system a boards console placed on the frame called Interfacng Hardware was acheved. The nterfacng hardware adapts the voltage levels of the sgnals comng from the sensors of the prototype to the nput voltage level at of the HIL box. Moreover t provdes also the nose flterng for the analogcal sgnals. Even for the output dgtal sgnals comng from the HIL box an electrc-optcal converson s provded by the Interfacng Hardware to control the cells. Before startng the power tests, a prelmnary procedure was carred out. All the sensors were calbrated and the good wrng of the sgnal chan was verfed. Fnally the optmzaton of the groundng confguraton of all the system was mproved step by step n order to avod EMI problems. Expermental valdatons n SPWM were acheved for the sngle loop topology and the classcal structure. The good operaton of the control loops valdated the system modelng approach and the regulator synthess. In the near future, ths prototype wll allow testng the sngle loop structure wth the zg-zag transformer, the closed loop operaton n a dq frame and the starcase modulaton. 3

160 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS APPENDIX A - The Prototype: Desgn & Development A. The Elementary cell In ths secton the elementary swtchng cell of the MMC converter prototype s descrbed. A smple scheme s reported n Fgure A -. +Vn Optc Fber RX Dead Tme Crcut Dead Tme Crcut IGBTs DRIVER Vpm C VOLTAGE SENSOR -Vn Fgure A - : Scheme of the elementary swtchng cell Each of the 8 swtchng cell s composed by the followng man components: IGBT IRGP35B6PDPBF 6A 6V TO 47 Case IGBTs Drver CONCEPT SC8T Voltage sensors Capactor mf (xmf (45V optc fber recever for swtchng sgnal Power supply TRACO TMS 55 The elementary swtchng cell s equpped by a sngle optc fber recever. On the cell, a logc crcut generates the complementary swtchng sgnals for the BOT and the TOP IGBT. Moreover, a crcut for managng the dead tme s present. Partcularly ths s desgned to gve a dead tme of µs. 33

161 APPENDIX A The prototype: Desgn & Development Each cell s equpped by a LEM voltage sensor that measures the capactor voltage. The LEM sensor gves the measure n current. It s desgned to gve 5mA for a measured voltage of V. All the electroncs on the PCB s suppled by a TRACO connected to the 3V 5Hz network. Fgure A - shows the dmensons of the cell PCB wth the components dsposton. Moreover Fgure A - 3 and Fgure A - 4 report the TOP and BOTTOM layers of the PCB. The fnal realzaton for the cell s reported n Fgure A - 5. Fgure A - : Dsposton of components on the cell 34

162 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Fgure A - 3: PCB Top layer Fgure A - 4: PCB Bottom layer The cells are posed n group of 3 on a sngle heat snk. Fnally the heat snk are assembled n group of 3 formng a matrx of cells 3x3 as shown n the pcture of Fgure A -. Fgure A - 5: Photo of a cell Fgure A - 6: Cells on the heatsnks V.4. Measurement Cards Each board s equpped wth a voltage sensor and two current sensors, to acheve all the requred measurements four boards were nstalled. Adaptaton and flterng stages are acheved by the acquston cards whch are detaled below. The reference crcut for the measurements acheved on the prototype s reported n Fgure A

163 APPENDIX A The prototype: Desgn & Development Fgure A - 7: Measurement card lay-out and ts defntve realzaton A. The Frame From Fgure A - 8 to Fgure A - few vews of layout of the frame prototype are reported. The detaled descrpton of the power stage wth the elementary cells features was gven n R-. Capactor voltages and branch currents are provded by sensors (LEM and measurement cards adapt output sensor voltages to the hardware nputs. 36

164 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS Fgure A - 8: Top vew of the frame 37

165 APPENDIX A The prototype: Desgn & Development Fgure A - 9: Vew of the desgn for the frame 38

166 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS kw Prototype Measurement Cards Interfacng HW Fgure A - : CAD desgn of the system The nterfacng hardware platform shown n Fgure A - s composed by sortng card whch provdes to adapt the analogcal sgnals comng from the acquston cards, sorted n groups of four, to the DB37 connector. Moreover the dgtal sgnals comng from the DB37 connector are dvded n groups of eght sgnals whch are converted n optcal sgnals by the optcal emtter. In the next paragraphs each board of the nterfacng platform s descrbed. 39

167 APPENDIX A The prototype: Desgn & Development 8 Voe [...5] Voe [...5] Voe [...5] Optcal Emtter Optcal Emtter Optcal Emtter Aquston Card 4 Aquston Card 3 Aquston Card Aquston Card 4 JB VOUT 5..8 JA VOUT..4 Sortng Card DB37M 4 DB37M 3 DB37M DB37M From OPAL-RT To OPAL-RT From Measurements Fgure A - : Interfacng Hardware platform 4

168 Ncola Serba MODULAR MULTILEVEL CONVERTERS FOR HVDC POWER STATIONS A.. Acquston Card Each board allows adaptng the sgnals comng from the measurement cards to the rght level [±5V] for the analog nput sde of the OP534 for the OPAL-RT frame. Each card s able to process eght sgnals by elmnatng the nose wht tunable hgh frequency actve flters. To process 9 analog measurements four acquston cards are employed. The PCB crcut and ts fnal layout are depcted n Fgure A -. Fgure A - Acquston card layout. 4

169 APPENDIX A The prototype: Desgn & Development A.. Optcal emtter The dgtal sgnals comng from the OPAL-RT provdes the ON/OFF state of the semconductor devces. The sgnals are arranged n three groups, one per phase. The output card OP5354 of OPAL-RT puts out [, +5 V] sgnals. Each optcal emtter s desgned to convert n optcal sgnal up to eght electrcal nputs. In the confguraton adopted for the prototype just sx ways are cabled (Fgure A - 3 whch corresponds to the number of cells per phase of the MMC prototype. Fgure A - 3: Optcal emtter layout For the nterfacng hardware two tables are provded. One table s provded for the analogcal sgnal and another one for the dgtal sgnals. Each table reports the correspondences, by consderng the levels of connecton per sgnal. The levels are: OPAL-RT number channel 4