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High Power Factor High-Current Variable-Voltage Rectifiers

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High Power Factor High-Current Variable-Voltage Rectifiers High Power Factor High-Current Variable-Voltage Rectifiers Von der Fakultät für Electrotechnik, Informatik und Mathematik der Universität Paderborn Zur Erlangung des akademischen Grades Doktor der Ingenieurwissenschften (Dr.-Ing.) genehmigte Dissertation von M.-Tech. Jitendra Solanki Erster Gutachter: Prof. Dr.-Ing. Joachim Böcker Zweiter Gutachter: Prof. Dr.-Ing. Marco Liserre Tag der mündliche Prüfung: 11.06.2015 Paderborn 2015 Diss. EIM-E/312 Acknowledgments Firstly, I would like to express my sincere thanks to my advisor Prof. Dr.-Ing. Joachim Böcker. His subject knowledge, guidance and encouragement have played an important role in present work. I want to specially thank Dr.-Ing. Norbert Fröhleke, he has been instrumental in providing me the opportunity to work with LEA. With the in-depth knowledge of power electronics, he guided me through the entire stay in LEA. I would like to thank Prof. Dr. Marco Liserre, as an important member of the committee, he offered time and efforts to review my manuscript. I also want to thank all of the other committee members. It has been a great pleasure to work in the group of LEA. I would like to thank all of my colleagues, especially Dr.-Ing S. Mathapati, Dipl.-Ing. C. Shulte, Dipl.-Ing. K.S. Stille, M.Sc. M. Sun, M.Sc. S. Bolte, M.Sc. K. Dora, Dipl.-Ing. A. Peters and M.Sc. M. Paradkar. I also want to thank the administrative and technical staff members of LEA for their great help. Special thanks are due to Dipl.-Ing. Helmut Foth and Mr. Norbert Sielemann for their helping hand during the phase of laboratory prototype implementation. Thanks belong to the AEG Power Solutions GmbH for funding the project. Especially, I would like to thank Dr.-Ing. P. Wallmeier, Dr.-Ing. A. Averberg, Dr.-Ing. Z. Cao, Dr. L. Bajan and Dipl.-Ing. G. Düppe for their great help and support. I would like to give my sincere thanks to my parents. Last but not the least, I would like to thank my wife Ritu, she was (and continue to be) always there with her love, encouragement, understanding and support. i Abstract High-current variable-voltage (HCVV) rectifiers are used in the metal and chemical industries. Typical power ratings vary from tens of kW to hundreds of MW. Even with the advancement of the power factor correction rectifiers, accepted choices of high-current AC to DC converters remain 6, 12 or 24-pulse thyristor/diode rectifiers, because of high reliability, efficiency and availability of suitable semiconductor devices. The main issues with these rectifiers are poor input power factor, high current harmonic distortion, high-maintenance cost, high weight and large volume. Moreover, to achieve low output voltage ripple, large capacitive filters are required. To tackle these issues, a two-pronged approach is taken in this thesis. First, the power quality issues of thyristor rectifiers are addressed with the help of passive and hybrid filters. Second, completely different, medium-frequency transformer-based topologies are proposed for the HCVV applications. After a due study of the state-of-the-art rectifiers, two core issues pertaining to the power quality of the HCVV rectifiers are identified, i.e. large reactive power rating of passive filters and inability of passive filters to provide varying reactive power compensation. A topology, with a thyristor assisted tap-changing transformer and a diode-bridge rectifier, is proposed that results into reactive power rating reduction of the passive filter. This scheme is optimized to determine an optimum turns-ratio of the transformer and optimum reactive power rating of the passive filter. Experimental results are presented to evaluate the performance of the system. To deal with the issue of fixed compensation of passive filters, a hybrid filter consisting of a parallel combination of a dominant-harmonic passive filter and a distribution static compensator (DSTATCOM) is proposed. The DSTATCOM is used to provide variable reactive power compensation. Along with it, the dominant-harmonic passive filter, with substantial amount of reactive power support, is provided to reduce required rating of the DSTATCOM. An experimental set-up is built to verify the effectiveness of the hybrid filter. A comparison of this hybrid-filter-based rectifier with a state of the art chopper-rectifier is also presented. Diverting away from the bulky thyristor/diode rectifiers, for medium-voltage applications, power supplies with medium-frequency transformer are proposed. Two topologies with different levels of modularity are discussed. Systems utilise a modular multilevel rectifier at the input side for connection to the medium voltage grid. Rectified DC voltage is converted to the low-voltage DC using suitable isolated DC-DC converters. These configurations provide unique advantages like good efficiency, high input power factor, good input current quality and controlled output current (with low output voltage ripple) over the full operating range. Moreover, these topologies are modular and result into size reduction as compared to the conventional rectifiers. Circuit configurations, design, control, simulation results and performance (power factor, current THD, efficiency and volume) of the proposed system are discussed in detail. A comparison of these topologies with the state of the art chopper-rectifier is also carried out. ii Zusammenfassung Hochstrom-Gleichrichter mit variabler Ausgangsspannung (HCVV-Rectifiers) werden in der Metall- und der Chemischen-Industrie verwendet. Die typische Nennleistung beträgt zwischen mehreren zehn Kilowatt bis einigen hundert Megawatt. Selbst mit der Weiterentwicklung durch Leistungsfaktorkorrektur-Gleichrichter bleiben 6-, 12- oder 24-pulsige Thyristor-/Dioden- Gleichrichter der Stand der Technik bei AC/DC Wandlern. Die Hauptgründe sind ihre hohe Zuverlässigkeit, der hohe Wirkungsgrad und die Verfügbarkeit geeigneter Halbleiter- Bauelemente. Die größten Probleme sind ihr geringer Eingangs-Leistungsfaktor, die hohe Verzerrung des Stroms, ein hohes Gewicht und ihre Baugröße. Um diesen Herausforderungen zu begegnen, werden in dieser Arbeit zwei Ansätze verfolgt. Als erstes werden Probleme des Thyristor-Gleichrichters bezüglich der Leistungsqualität durch passive und hybride Filter angegangen. Im zweiten Teil der Arbeit, werden unterschiedliche Transformator-basierte Topologien, die mit mittlerer Frequenz arbeiten, für HCVV-Lasten vorgestellt. Nach eingehender Analyse von Gleichrichtern auf dem Stand der Technik, können zwei Kernprobleme identifiziert werden, welche die Leistungsqualität dieser Gleichrichter beeinträchtigen. Dies sind die Reduktion der Filtergröße und die Tatsache, dass passive Filter nicht in der Lage sind für eine variable Blindleistungskompensation zu sorgen. Ein Transformator mit mehreren Thyristor gesteuerten Anzapfungen- und einem nachgeschalteten Dioden-Gleichrichter wird vorgeschlagen. Diese Konfiguration bietet eine reduzierte Blindleistung durch den passiven Filter. Sie ist darauf ausgelegt, ein optimales Windungsverhältnis des Transformators und eine optimale Blindleistungsreduktion des passiven Filters zu erreichen. Versuchsergebnisse werden präsentiert, um die Leistungsfähigkeit des Systems zu bewerten. Um dem Problem der festen Kompensation des passiven Filters zu begegnen, wird ein hybrider Filter vorgeschlagen, welcher aus einer parallelen Zusammenstellung eines passiven Filters zur Unterdrückung der dominanten Harmonischen und einem statistischen Blindleistungskompensator (DSTATCOM) besteht. Der statistische Blindleistungskompensator sorgt für eine variable Blindleistungskompensation. Dank dem passiven Filter zur Unterdrückung der Harmonischen kann ein hoher Anteil der Blindleistung kompensiert werden, was die Anforderungen an den statistischen Blindleistungskompensator verringert. Ein Versuchsaufbau wird genutzt, um die Wirksamkeit des Hybridfilters zu bestätigen. Zudem wird ein Vergleich zwischen einem Gleichrichter mit Hybridfilter und einem Gleichrichter nach dem aktuellen Stand der Technik durchgeführt. Abweichend von voluminösen Thyristor-/Dioden-Gleichrichtern werden Netzteile mit Mittelfrequenz-Transformator vorgestellt. Zwei Topologien mit verschiedenen Modulationsgraden werden behandelt. Diese Systeme verwenden einen modularen mehrstufigen Gleichrichter, einen Umrichter, einen Mittelfrequenz-Transformator und einen Dioden- Gleichrichter um eine Mittel-Wechselspannung in eine Nieder-Gleichspannung über einen Mittelfrequenz-Wechselspannungs-Zwischenkreis zu wandeln. Die Konfigurationen bieten spezifische Vorteile wie z.B. eine hohe Effizienz, einen großen Leistungsfaktor am Eingang, eine gute Eingangsspannungsqualität und einen regelbaren Ausgangsstrom über den gesamten Betriebsbereich. Zusätzlich sind die Konfigurationen modular und führen zu einer Verkleinerung der Baugröße im Vergleich zu konventionellen Gleichrichtern. Die Konfigurationen der Schaltung, Auslegung, Reglung, Ergebnisse der Simulation und die Leistungsfähigkeit iii (Leistungsfaktor, Verzerrung des Stroms, Wirkungsgrad und Baugröße) des vorgestellten Systems werden im Detail beschrieben. iv Table of Contents Nomenclature ................................................................................................................................. ix 1. Introduction ............................................................................................................................. 1 2. State of The Art: High-Current Variable-Voltage Rectifiers .................................................. 5 2.1 High-Current Variable-Voltage
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