Dissertations / Theses on the topic 'Neutral Point Clamped (NPC) Converter'

Las topologías de convertidores multinivel han recibido una atención especial durante las dos últimas décadas debido a sus notables ventajas en aplicaciones de alta potencia y media/alta tensión. En estas topologías, y comparadas con el convertidor tradicional de dos niveles, el voltaje que soporta cada dispositivo semiconductor es menor, evitando los problemas asociados con la interconexión serie de dispositivos. La distorsión armónica en la tensión de salida es también menor y la eficiencia mayor. Pero incorporan un número superior de dispositivos semiconductores y la estrategia de modulación resultante es, por tanto, más compleja.<br/>Entre estas topologías, el convertidor cc-ca de tres niveles trifásico con conexión al punto neutro del bus de cc es probablemente el más popular. La aplicación a este convertidor de técnicas de modulación convencionales causa una oscilación de la tensión del punto neutro de baja frecuencia (tres veces la frecuencia fundamental de la tensión de salida). Esta oscilación, a su vez, supone un incremento del estrés de tensión de los dispositivos y provoca la aparición de armónicos de baja frecuencia en la tensión de salida.<br/>Esta tesis presenta una nueva técnica de modulación del pulso de conducción de los dispositivos semiconductores para convertidores de tres niveles trifásicos con conexión a punto neutro, capaz de conseguir un control completo de la tensión del punto neutro con una distorsión armónica reducida en la tensión de salida alrededor de la frecuencia de conmutación. Esta nueva técnica de modulación, basada en la definición de unos vectores espaciales virtuales, garantiza el equilibrado de la tensión del punto neutro con cualquier carga (lineal o no, cualquier factor de potencia) y para todo el rango de tensión de salida, con el único requisito de que la suma de corrientes de fase sea nula.<br/>Las características de la técnica de modulación propuesta y sus beneficios con respecto a otras modulaciones se han verificado a través de simulaciones y experimentos tanto en lazo abierto como en lazo cerrado.<br>Multilevel converter topologies have received special attention during the last two decades due to their significant advantages in high-power medium- and high-voltage applications. In these topologies, and compared to the previous two-level case, the voltage across each semiconductor is reduced, avoiding the problems of the series interconnection of devices. The harmonic distortion of the output voltage is also diminished and the converter efficiency increases. But a larger number of semiconductors is needed and the modulation strategy to control them becomes more complex.<br/>Among these topologies, the three-level three-phase neutral-point-clamped voltage source inverter is probably the most popular. The application of traditional modulation techniques to this converter causes a low frequency (three times the fundamental frequency of the output voltage) oscillation of the neutral-point voltage. This, in turn, increases the voltage stress on the devices and generates low-order harmonics in the output voltage.<br/>This thesis presents a novel pulsewidth modulation for the three-level three-phase neutral-point-clamped converter, able to achieve a complete control of the neutral-point voltage while also having a low output voltage distortion at around the switching frequency. The new modulation, based on a virtual space vector concept, guarantees the balancing of the neutral-point voltage for any load (linear or nonlinear, any load power factor) over the full range of converter output voltage, the only requirement being that the addition of the output three-phase currents equals zero.<br/>The performance of this modulation approach and its benefits over other previously proposed solutions are verified through simulation and experiments in both open- and closed-loop converter configurations.

In this thesis, the neutral point potential drift/fluctuation of the three-level neutral point clamped inverter is analyzed and a novel control algorithm, the two-parameter PWM method is proposed to confine the neutral point potential variation to a very small range. The two-parameter PWM method provides superior neutral point potential control performance even with small DC bus capacitors. The method is based on PWM pulse pattern modification and requires no additional hardware. Detailed analytical models of the neutral point current and potential as a function of the modulation parameters are established and the neutral point potential behavior is thoroughly investigated. Based on the study, the deficiency of the known methods is illustrated and the two-parameter PWM method is developed and its superior performance demonstrated. The performance of the two-parameter PWM method is verified by means of computer simulations utilizing both the per-PWM-cycle average model and the detailed model of the inverter. The results are supported by laboratory experiments involving both an R-L load and an induction motor drive.

In this thesis, field oriented control of permanent magnet synchronous motors using three-level neutral-point-clamped inverter is studied. Permanent magnet synchronous motors are used in high performance drive applications. In this study, the permanent magnet synchronous motor is fed by three-level neutral-point-clamped inverter. For three-level neutral-point-clamped inverter different space vector modulation algorithms, which are reported in literature, are analyzed and compared via computer simulations. The voltage balance on dc-link capacitors is also analyzed and a software control method is implemented in conjunction with the space vector PWM modulation, utilized. Nonlinear effects such as dead-time, semiconductor voltage drop and delays in gate drive circuitries also present in neutral-point-clamped inverter. The effects of these nonlinearities are studied and a compensation method for these nonlinear effects is proposed. The theoretical results are supported with computer simulations and verified with experimental results.

Els convertidors multinivell són topologies de convertidors d’electrònica de potència que poden generar tres o més nivells de voltatge en cadascuna de les fases de sortida. Com a resultat, els voltatges i corrents generats per aquestes topologies presenten una distorsió harmònica baixa. Hi ha diferents configuracions de convertidors multinivell, les quals es basen en connectar dispositius de potència o convertidors en sèrie. El resultat d’aquestes connexions permet obtenir voltatges alts, tant en la part de corrent continu com en la de corrent altern del convertidor. A més, cada dispositiu sols ha de suportar una fracció del voltatge total del bus de corrent continu. Per aquestes raons, els convertidors multinivell són generalment utilitzats en aplicacions d’alta potència. El convertidor de tres nivells amb connexió a punt neutre (neutral-point-clamped) és el més utilitzat. La recerca d’aquesta tesis doctoral està focalitzada en aquesta topologia de convertidor, i el principal objectiu és l’aportació de noves tècniques de modulació. Aquestes tenen en compte diferents aspectes: la velocitat computacional dels algorismes, l’equilibrat de les tensions dels condensadors del bus de contínua, les pèrdues de commutació i les oscil·lacions de baixa freqüència en el punt neutre del convertidor. Totes les estratègies de modulació proposades en aquesta tesis són modulacions d’amplada de polses basades en portadora. En la primera modulació que es presenta, s’injecta un senyal comú (seqüència zero) a totes les moduladores, que es basa en els patrons de la modulació vectorial que utilitza tres vectors dels més propers al de referència (nearest-three-vector modulation). S’estudien i es comparen els resultats d’aquesta modulació amb la seva homòloga, basada en perspectiva vectorial. Una segona proposta és l’anomenada modulació d’amplada de polses de doble senyal (double-signal pulse-width modulation). Aquesta modulació és capaç d’eliminar completament les oscil·lacions de voltatge en el punt neutre del convertidor. No obstant això, es produeix un increment de les pèrdues de commutació en els dispositius de potència i, a més, no hi ha un equilibrat natural de les tensions en els condensadors del bus. Una última estratègia de modulació, anomenada modulació híbrida (hybrid pulse-width modulation), es basa en la combinació de la modulació sinusoïdal (sinusoidal pulse-width modulation) i la de doble senyal. Aquesta presenta una solució de compromís entre reduir les pèrdues de commutació, en detriment d’un augment de l’amplitud de les oscil·lacions de voltatge en el punt neutre. Una segona part d’aquesta tesis es centra en les aplicacions a generació eòlica, ja que els convertidors multinivell estan començant a ser utilitzats en aquest camp. Això es produeix fonamentalment per l’augment continu de les dimensions de les turbines eòliques. En aquesta part de la recerca s’ha considerat la configuració de dos convertidors multinivell connectats a un mateix bus de contínua (back-to-back), tot i que els convertidors han estat estudiats independentment. Inicialment s’ha estudiat el convertidor que va connectat a la xarxa elèctrica i s’ha aplicat l’estratègia de control coneguda com a control orientat a tensió (voltage-oriented control). S’han utilitzat controladors estàndard (proporcional-integral), als quals s’ha afegit un control difús que supervisa i modifica els valors de les constants dels controladors. Aquest supervisor difús millora la dinàmica de la tensió del bus de contínua davant canvis de càrrega quan el convertidor treballa com a rectificador. Per una altra part, s’ha estudiat el control d’una turbina eòlica basada en un generador d’imants permanents. En aquest cas, s’ha aplicat l’estratègia de control coneguda com a control orientat a camp (field-oriented control). S’han avaluat i comparat els avantatges i inconvenients de diferents formes de sintonitzar els controladors.<br>Multilevel converters are power electronic topologies that can generate three or more voltage levels in each output phase. As a result, the voltage and current waveforms generated have lower total harmonic distortion. Multilevel topologies are based on connecting power devices or converters in a series. Consequently, high voltages can be handled on the dc and ac sides of the converter, while each device stands only a fraction of the total dc-link voltage. For these reasons multilevel converters are generally applied to high-power applications. The three-level neutral-point-clamped converter is the most extensively used multilevel topology. This topology is the main focus of research in this dissertation. The main objective is to propose new modulation strategies that are able to meet a compromise solution while considering computational algorithm speed, voltage balance in the dc-link capacitors, switching losses and low frequency voltage oscillations at the neutral point. All the modulation strategies proposed here are based on carrier-based pulsewidth modulation. A new modulation strategy has been implemented using a proper zero-sequence signal injected into the modulation signals. The zero sequence is determined from a space-vector modulation standpoint, particularly the nearest-threevector modulation strategy. The proposed carrier-based technique is compared with its space-vector modulation counterpart. It shows some advantages, such as easier implementation and reduced switching events; however, it still produces oscillations in the neutral-point voltage for some operating conditions. A new modulation strategy able to completely remove such voltage oscillations is also presented. It is called double-signal pulse-width modulation. The main drawback of this strategy is that it increases the switching frequency of the power devices and has no natural capacitor voltage balance. Some balancing strategies are proposed in this dissertation for this specific modulation. Furthermore, a hybrid pulse-width modulation approach is presented which is able to combine sinusoidal pulse-width modulation with doublesignal pulse-width modulation; this represents a compromise solution between switching losses and neutral-point voltage oscillation amplitudes. The second part of this thesis is focused on wind generation applications. Multilevel converters are starting to be used in such a field nowadays, and are expected to be further applied in the near future as the sizes of wind turbines grow. Two back-toback-connected power converters are considered in this application, although they are analyzed independently. First of all, the control of the grid-connected converter is studied. A voltage-oriented control is used with standard proportional-integral controllers. The originality of the method is that a fuzzy supervisor is designed and included in the structure; the fuzzy supervisor is able to modify the proportionalintegral parameters online. It is shown how the control of the total dc-link voltage improves significantly under load changes when the converter is working as a rectifier. On the other hand, a control study is performed on the wind turbine side. The variable speed wind turbine is based on a permanent magnet synchronous generator. A field-oriented control strategy is applied. The controllers are evaluated and compared using different tuning strategies which highlight the advantages and drawbacks of each.

Thesis (MScEng)--Stellenbosch University, 2003.<br>ENGLISH ABSTRACT: The versatility of power electronic converters has made them a dominant force in the current electrical and electronic engineering industry. So too industry presents a wider range of applications, forever demanding operation at higher power levels. To meet this need a variety of multilevel converters have evolved. The challenge often lies in the selection of the appropriate topology for a specific application. This thesis presents a practical comparison between the Series-Stacked and Neutral Point Clamped multilevel converter topologies as candidates for Medium Voltage Direct Current and Traction applications. Their configurations, characteristics, switching techniques and practical performances are compared, in order to aid the topology selection process.<br>AFRIKAANSE OPSOMMING: As gevolg van die veelsydigheid van drywings elektroniese omsetters word dit geimplementeer in n toenemende hoeveelheid toepassings met toenemende drywings vlakke in die elektriese en elektroniese industrie. Om aan hierdie behoefte te voorsien het n reeks veelvlak omsetters ontstaan. Die uitdaging hiermee is die keuse van die toepaslike topologie vir n spesifieke doelwit. Hierdie proefskrif vergelyk die Serie Gekoppelde en die Geklemde Neutrale Punt omsetters ten opsigte van konfigirasie, karakteristieke, skakel tegnieke en praktiese werksverrigting om die toepaslikheid te bepaal vir Medium Spanning Gelykstroom en Traksie toepassings.

This thesis describes multilevel converters (MLC) designed for use with a repurposed electronic vehicle battery (battery 2nd life). MLC is of particular interest due to the low harmonic distortion content and reduced voltage stress in the switching devices. A detailed study of the MLC topologies and modulation techniques is presented. Space vector modulation is analysed and implemented to evaluate the converter. A comprehensive assessment of the MLC is presented using wide bandgap (WBG) devices highlighting the devices’ thermal and high switching frequency features.

Cette thèse vise à élaborer de nouvelles stratégies de commande basées sur la commande prédictive pour le système de génération d’énergie éolienne. La topologie des systèmes de production éolienne basées sur le Générateur Asynchrone à Double Alimentation (GADA) qui convient à des plateformes de génération dans la gamme de puissance de 1.5 à 6 MW est abordée. Du point de vue technologique, le convertisseur à trois niveaux et clampé par le neutre (3L-NPC) est considéré comme une bonne solution pour une puissance élevée en raison de ses avantages: capacité à réduire la distorsion harmonique de la tension de sortie et du courant, et augmentation de la capacité du convertisseur grâce à une tension réduite appliquée à chaque semi-conducteur de puissance. Une description détaillée de la commande prédictive à ensemble de commande fini (FCS-MPC) avec un horizon de prédiction de deux pas est présentée pour deux boucles de régulation: celle liée au convertisseur connecté au réseau et celle du convertisseur connecté au GADA. Le principe de la commande repose sur l’utilisation d’un modèle de prédiction permettant de prédire le comportement du système pour chaque état de commutation du convertisseur. La minimisation d’une fonction de coût appropriée prédéfinie permet d’obtenir la commutation optimale à appliquer au convertisseur. La thèse étudie premièrement les problèmes liées à la compensation du temps de calcul de la commande et au choix et aux pondérations de la fonction de coût. Ensuite, le problème de stabilité de la commande FCS-MPC est abordé en considérant une fonction de Lyapunov dans la minimisation de la fonction de coût. Finalement, une étude sur la compensation des effets des temps morts du convertisseur est présentée<br>This thesis aims to elaborate new control strategies based on Model Predictive control for wind energy generation system. We addressed the topology of doubly fed induction generator (DFIG) based wind generation systems which is suitable for generation platform power in the range in 1.5-6 MW. Furthermore, from the technological point of view, the three-level neutral-point clamped (3L-NPC) inverter configuration is considered a good solution for high power due to its advantages: capability to reduce the harmonic distortion of the output voltage and current, and increase the capacity of the converter thanks to a decreased voltage applied to each power semiconductor.In this thesis, we presented a detailed description of finite control set model predictive control (FCS-MPC) with two step horizon for two control schemes: grid and DFIG connected 3L-NPC inverter. The principle of the proposed control scheme is to use system model to predict the behaviour of the system for every switching states of the inverter. Then, the optimal switching state that minimizes an appropriate predefined cost function is selected and applied directly to the inverter.The study of issues such as delay compensation, computational burden and selection of weighting factor are also addressed in this thesis. In addition, the stability problem of FCS-MPC is solved by considering the control Lyapunov function in the design procedure. The latter study is focused on the compensation of dead-time effect of power converter

As wide-bandgap devices continue to experience deeper penetration in commercial applications, there are still a number of factors which make the adoption of such technologies difficult. One of the most notable issues with the application of wide-bandgap technologies is meeting existing noise requirements and regulations. Due to the faster dv/dt and di/dt of SiC devices, more noise is generated in comparison to Si IGBTs. Therefore, in order to fully experience the benefits offered by this new technology, the noise must either be filtered or mitigated by other means. A survey of various DC/DC topologies was conducted in order to find a candidate for a battery interface in a UPS system. A three-level NPC topology was explored for its potential benefit in terms of noise, efficiency, and additional features. This converter topology was modeled, simulated, and a hardware prototype constructed for evaluation within a UPS system, although its uses are not limited to such applications. A UPS system is a good example of an application with strict noise requirements which must be fulfilled according to IEC standards. Based on a newly devised mode of operation, this converter was verified to produce no common-mode voltage under ideal conditions, and was able to provide a 6 dB reduction in common-mode voltage emissions in the UPS prototype. This was done while achieving a peak efficiency in excess of 99% with the ability to provide bidirectional power flow between the UPS and battery backup. The converter was verified to operate at the rated UPS conditions of 20 kW while converting between a total DC bus voltage of 800 V and a nominal battery voltage of 540 V.<br>Master of Science<br>As material advancements allow for the creation of devices with superior electrical characteristics compared to their predecessors, there are still a number of factors which cause these devices to see limited usage in commercial applications. These devices, typically referred to as wide-bandgap devices, include silicon carbide (SiC) transistors. These SiC devices allow for much faster switching speeds, greater efficiencies, and lower system volume compared to their silicon counterparts. However, due to the faster switching of these devices, there is more electromagnetic noise generated. In many applications, this noise must be filtered or otherwise mitigated in order to meet international standards for commercial use. Consequently, new converter topologies and configurations are necessary to provide the most benefit of the new wide-bandgap devices while still meeting the strict noise requirements. A survey of topologies was conducted and the modeling, design, and testing of one topology was performed for use in an uninterruptible power supply (UPS). This converter was able to provide a noticeable reduction in noise compared to standard topologies while still achieving very high efficiency at rated conditions. This converter was also verified to provide power bidirectionally—both when the UPS is charging the battery backup, and when the battery is supplying power to the load.

Made available in DSpace on 2016-12-12T20:27:38Z (GMT). No. of bitstreams: 1 Tiago Lemes.pdf: 2229669 bytes, checksum: 9b9cd44356c6d0002fccf67fe418f52b (MD5) Previous issue date: 2014-09-26<br>Coordenação de Aperfeiçoamento de Pessoal de Nível Superior<br>The main subject of this work is the study of a 5 levels T-Type NPC inverter topology, which is applied in photovoltaic energy processing for power generation. The grid power injection is done controlling converter current, which is injected into grid. This work presents equations, component-designs and their validation, which are necessary for the Inverter s power structure implementation. Also inverter modeling and design of implemented controllers are described. Through this study, it was possible to build a 3 kW prototype, which besides the current control, has a system to balance the differential voltage of bus capacitors. Through the prototype, experimental results were acquired.<br>O objeto de estudo deste trabalho é a topologia inversora NPC T-Type 5 níveis, aplicada no processamento da energia fotovoltaica, sendo o principal objetivo a geração de energia elétrica por meio do controle da corrente aplicada à rede. Este trabalho apresenta o equacionamento, projeto dos componentes e sua validação, que fazem parte da estrutura de potência do inversor, bem como a sua modelagem e projeto dos controladores implementados. Por intermédio deste estudo foi possível construir um protótipo com potência nominal de 3 kW, que além do controle da corrente, apresenta uma malha de equilíbrio da tensão diferencial do barramento. Mediante construção desse protótipo, foram extraídos os resultados experimentais.

Consistent international efforts have been made over the past few decades to move the world towards an environmentally sustainable society. Wind energy conversion systems (WECSs) are one of the largest contributors within this movement. Furthermore amongst existing wind turbine power generation technologies, the doubly fed induction generator (DFIG) has been distinctively popular for its lower capital costs especially in higher power applications. In order to study the integration of this type of generator into the grid, a laboratory based DFIG test rig was developed where its complete design process is presented in this dissertation. Mathematical modelling of related system components were thoroughly investigated so as to facilitate controller design based on the internal model control (IMC) methodology. In addition, a complete soft grid synchronisation procedure for the DFIG was investigated. It was found that the application of active damping within the IMC control law resulted in reduced stator current transients during synchronisation. Control voltage excitation for the DFIG rotor circuit was achieved by the implementation of two voltage source converters (VSC’s) connected in a back-to-back configuration via a common DC-link. The rotorside converter (RSC) was responsible for regulating the machine speed whereas the grid-side converter (GSC) was responsible for regulating the DC-link voltage. In addition, these converters provided decoupled and bidirectional power flow control which enabled the DFIG to operate at sub synchronous and super synchronous speeds. A three-level VSC was chosen for the GSC control, where a resource conservative modulation algorithm that eliminates DC-link neutral voltage unbalance was implemented. The DFIG system design was simulated, and the results were verified through experimental tests performed on a 1.5kW wound rotor induction machine (WRIM). A detailed description of the laboratory setup of the DFIG is presented, and various practical limitations are discussed. It was found that the performance of the developed DFIG test rig correlated well with results of the simulations. Stable operation was achieved for various system test conditions, which indicated the system’s robustness to serve as a practical platform for future DFIG related research.

For decades, the Silicon-based semiconductors have been the solution for power electronics applications. However, these semiconductors have approached their limits of operation in blocking voltage, working temperature and switching frequency. Due to material superiority, the relatively-new wide-bandgap semiconductors such as Silicon-Carbide (SiC) MOSFETs enable higher voltages, switching frequencies and operating temperatures when compared to Silicon technology, resulting in improved converter specifications. The current study tries to investigate the impact of emerging medium-voltage SiC MOSFETs on industrial motor drive application, where over a quarter of the total electricity in the world is being consumed. Firstly, non-commercial SiC MOSFETs at 3.3 kV and 400 A rating are characterized to enable converter design and simulation based on them. In order to feature the best performance out of the devices under test, an intelligent high-performance gate driver is designed embedding required functionalities and protections. Secondly, total of three converters are targeted for industrial motor drive application at medium-voltage and high-power range. For this purpose the cascaded H-bridge, the modular multilevel converter and the 5-L active neutral point clamped converters are designed at 4.16-, 6.9- and 13.8 kV voltage ratings and 3- and 5 MVA power ratings. Selection of different voltage and power levels is done to elucidate variation of different parameters within the converters versus operating point. Later, comparisons are done between the surveyed topologies designed at different operating points based on Si IGBTs and SiC MOSFETs. The comparison includes different aspects such as efficiency, power density, semiconductor utilization, energy stored in converter structure, fault containment, low-speed operation capability and parts count (for a measure of reliability). Having the comparisons done based on simulation data, an H-bridge cell is implemented using 3.3 kV 400 A SiC MOSFETs to evaluate validity of the conducted simulations. Finally, a novel method is proposed for series-connecting individual SiC MOSFETs to reach higher voltage devices. Considering the fact that currently the SiC MOSFETs are not commercially available at voltages higher above 1.7 kV, this will enable implementation of converters using medium-voltage SiC MOSFETs that are achieved by stacking commercially-available 1.7 kV MOSFETs. The proposed method is specifically developed for SiC MOSFETs with high dv/dt rates, while majority of the existing solutions could only work merely with slow Si-based semiconductors.<br>Ph. D.

The main focus of this thesis is on the power electronic converter system challenges associated with the grid integration of variable-renewable-energy (VRE) sources like wave, marine current, tidal, wind, solar etc. Wave energy conversion with grid integration is used as the key reference, considering its high energy potential to support the future clean energy requirements and due the availability of a test facility at Uppsala University. The emphasis is on the DC-link power conditioning and grid coupling of direct driven wave energy converters (DDWECs). The DDWEC reflects the random nature of its input energy to its output voltage wave shape. Thereby, it demands for intelligent power conversion techniques to facilitate the grid connection. One option is to improve and adapt an already existing, simple and reliable multilevel power converter technology, using smart control strategies. The proposed WECs to grid interconnection system consists of uncontrolled three-phase rectifiers, three-level boost converter(TLBC) or three-level buck-boost converter (TLBBC) and a three-level neutral point clamped (TLNPC) inverter. A new method for pulse delay control for the active balancing of DC-link capacitor voltages by using TLBC/TLBBC is presented. Duty-ratio and pulse delay control methods are combined for obtaining better voltage regulation at the DC-link and for achieving higher controllability range. The classic voltage balancing problem of the NPC inverter input, is solved efficiently using the above technique. A synchronous current compensator is used for the NPC inverter based grid coupling. Various results from both simulation and hardware testing show that the required power conditioning and power flow control can be obtained from the proposed multilevel multistage converter system. The entire control strategies are implemented in Xilinx Virtex 5 FPGA, inside National Instruments’ CompactRIO system using LabVIEW. A contour based dead-time harmonic analysis method for TLNPC and the possibilities of having various interconnection strategies of WEC-rectifier units to complement the power converter efforts for stabilizing the DC-link, are also presented. An advanced future AC2AC direct power converter system based on Modular multilevel converter (MMC) structure developed at Siemens AG is presented briefly to demonstrate the future trends in this area.

This PhD dissertation has presented a number of scenarios in which Energy Storage Systems (ESSs) can be usefully employed for increasing energy system performances. Particularly, after introducing the State-of-the-Art of ESS technologies (Chapter 1), reference has been made to some stationary and automotive applications. Stationary applications have regarded Renewable Energy Sources (RESs) exploitation issues and EV integration within micro-grids (Chapter 2). It has been shown that ESSs are particularly useful in compensating for RES forecasting errors, whereas they are much less effective as energy buffers. In addition, Vehicle to Grid (V2G) has also been revealed as an alternative and viable solution for increasing RES penetration level and micro-grid autonomy, even in presence of small EV fleets. The promising results obtained in the energy management of power systems by means of the use of V2G and G2V paradigm have suggested the integration of Electric Vehicles (EVs) into the power system. This requires that EV energy storage systems should satisfy both electric propulsion and power system requirements. With this aim, the design and management of a novel Hybrid Energy Storage System (HESS) for EVs has been considered (Chapter 3). Particularly, the proposed configuration allows the reduction of the peak current delivered by EV batteries, thus preserving their rated performances and increasing their lifetime. This goal has been achieved by means of a suitable management of the energy flows provided by the HESS, leading to a good exploitation of the proposed topology. The effectiveness of the proposed solutions has been verified through several extensive simulation studies, which have been carried out in the Matlab environment. In conclusion, it can be stated that all cases have revealed the need of carefully sizing and managing ESSs in order to achieve optimal results. In this context, it is worth noting that the employment of large ESS easily leads to enhanced performances but also to significant increased costs. This drawback cannot be sustained, especially in automotive applications, in which EV competitiveness is strictly related to a decrease of ESS size, weight and costs. On the other hand, small ESSs do not generally guarantee the same performances but they can be quite similar if optimal management and control strategies are employed. These last thus will cover a fundamental role in making ESS more widespread, enabling an optimal trade-off among increased performances, costs, management and control issues.

Das Ziel der Arbeit war die Untersuchung eines neuartigen Phasenbausteins mit der Topologie des Dreipunkt – Neutral Point Clamped – Stromrichters mit Spannungszwischenkreis (3L-NPC-VSC) für Windkraftanwendungen. Wichtige Anforderungen an den Phasenbaustein und daraus resultierende Herausforderungen, sowie Lösungen für ausgewählte Teilprobleme werden präsentiert. Um die Vorteile des 3L-NPC-VSC für Hersteller von Windkraftanlagen zugänglich zu machen, ist es sinnvoll, einen neuartigen Phasenbaustein zu entwickeln. Der Phasenbaustein soll einfach in Systeme zu integrieren sein, in denen gegenwärtig Zweipunktstromrichter (2L-VSC) zum Einsatz kommen. Da sich Modulation, Zwischenkreisbalancierung und Kurzschlussschutz vom 2L-VSC unterscheiden, soll der Phasenbaustein diese Herausforderungen eigenständig bewältigen. Die Arbeit beschreibt die Konzeption eines solchen Phasenbausteins und behandelt insbesondere die Modulation, die Zwischenkreisbalancierung und den Kurzschlussschutz des 3L-NPC-VSC. Ein Vergleich verschiedener Modulationsverfahren wurde durchgeführt und die am besten geeigneten Verfahren für die Implementation in den Phasenbaustein ausgewählt. Eine Anforderung war, dass dieser Signale einer übergeordneten Regelung verarbeiten kann, welche für einen 2L-VSC berechnet wurden. Ein Überblick der Zwischenkreisbalancierungsverfahren zeigte, dass nahezu alle den Nachteil einer zusätzlich benötigten Strommessung haben. Die Untersuchung einer neuen an der Professur Leistungselektronik der TU Dresden entwickelten Methode ohne den Bedarf der Strommessung zeigte, dass diese anwendbar ist. Der Algorithmus wurde simuliert, implementiert und experimentell getestet und zeigte gute Resultate. Die Aufgabe eines komplett unabhängigen Kurzschlussschutzes war die schwierigste. Alle möglichen Fehler innerhalb eines Moduls wurden analysiert und kategorisiert. Einige Fehlertypen können innerhalb einer Phase behandelt werden. Entsprechende Algorithmen wurden entwickelt und getestet. Allerdings gibt es Fehlertypen, die nicht durch die Steuerung einer einzelnen Phase behandelt werden können. Eine schnelle Kommunikation zwischen den drei Phasen des Konverters wäre notwendig. Alternativ könnte eine übergeordnete Steuerung diese Fehler behandeln. Zum Schluss wurde ein Demonstrator des Phasenbausteins aufgebaut und experimentell untersucht. Einige Messergebnisse werden gezeigt, um die Funktion zu verifizieren.:1 Einleitung 1.1 Motivation 1.2 Zielstellung 1.3 Inhalt der Arbeit 2 Stromrichter für Windkraftanlagen 2.1 Stand der Technik 2.1.1 Zweipunktstromrichter mit Spannungszwischenkreis 2.1.2 Dreipunkt-Neutral-Point-Clamped-Stromrichter mit Spannungszwischenkreis 2.1.3 Kommerziell verfügbare Stromrichter für WKA 2.2 Vollumrichterlösung mit erhöhter Ausgangsspannung 2.2.1 Motivation und Anforderungen 2.2.2 Vereinfachter Vergleich von Zwei- und Dreipunktstromrichtern 2.3 Herausforderungen bei der Realisierung des 3L-NPC-VSC 3 Struktur und Funktion eines neuartigen 3L-NPC-Phasenbausteins 3.1 Struktur und Schnittstellen 3.1.1 Stand der Technik für 3L-NPC Phasenbausteine 3.1.2 Neuartiger 3L-NPC-VSC-Phasenbaustein 3.2 Realisierung 3.2.1 Anforderungen 3.2.2 Technische Realisierung 3.3 Experimentelle Verifikation 3.3.1 Versuchsstand 3.3.2 Messergebnisse 4 Modulation und Zwischenkreisbalancierung eines 3L-NPC-VSC 4.1 Modulationsarten im Überblick 4.1.1 Trägerbasierte Modulation für den 3L-NPC-VSC 4.2 Ausgewählte Modulation für den neuartigen Phasenbaustein 4.2.1 Zweipunktraumzeigermodulation in einem Trägerband 2L-SVM 4.2.2 Dreipunktraumzeigermodulation 3L-SVM 4.3 Stand der Technik bei Zwischenkreisbalancierungsverfahren 4.4 Die direkte Totzeitregelung zur Zwischenkreisbalancierung 4.4.1 Theoretische Grundlagen 4.4.2 Simulative Verifikation der direkten Totzeitregelung 4.4.3 Experimentelle Verifikation der DDTC 5 Kurzschlussschutz eines 3L-NPC-VSC-Phasenbausteins 5.1 Versuchsstand I5.2 Kurzschlussfehler einer 3L-NPC-VSC-Phase 5.2.1 Kategorisierung der Kurzschlüsse 5.2.2 Untersuchte Bauteilfehler innerhalb einer 3L-NPC-VSC-Phase 5.3 Kurzschlussbehandlungsmethoden 5.3.1 Stand der Technik 5.3.2 Schutzmaßnahmen für 3L-NPC-VSC 5.4 Analyse von Kurzschlüssen und Ableitung von Behandlungsmaßnahmen 5.4.1 Fehler eines äußeren IGBTs 5.4.2 Fehler eines inneren IGBTs 5.4.3 Fehler einer Clampdiode 5.5 Maßnahmen zur sicheren Behandlung von Kurzschlüssen in 3L-NPC-VSC 6 Zusammenfassung

Among the multilevel (ML)-voltage source converters (VSCs) for medium voltage (MV) and high power (HP) applications, the most used power topology is the three level (3L)-neutral point clamped (NPC)-VSC, due to its features such as common direct current (DC)-bus capability with medium point, absence of switches in series-connection, low part count, and straightforward control. The use of MV-insulated gate bipolar transistor (IGBT) modules as power switches offers further advantages like inexpensive gate drivers and survival capability after short-circuit. However, the IGBT modules have a reduced life cycle due to thermal stress generated by load cycles. Despite the advantages of the 3L-NPC-VSC, its main drawback is the uneven power loss distribution among its power devices. To address this issue and to improve other characteristics, more advanced ML converters have been developed. The 3L-active neutral point clamped (ANPC)-VSC allows an improved power loss distribution thanks to its additional IGBTs, which increase the number of feasible zero-states, but needs a loss balancing scheme to choose the proper redundant zero-state and a more complex commutation sequence between states. The 3L-neutral point piloted (NPP)-VSC improves the power loss distribution thanks to the use of series-connected switches between the output terminal and the positive and negative DC-link terminals. Other advanced power topologies with higher amount of levels include the 5L-ANPC-VSC, which has a flying capacitor per phase to generate the additional levels; and the 5L-stacked multicell converter (SMC), which needs two flying capacitors per phase. The goal of this work is to is to evaluate the performance of the aforementioned NPC-type ML converters with common DC-link, included the ones with flying capacitors, in terms of the power loss distribution and the junction temperature of the most stressed devices, which define, along with the nominal output voltage, the maximum power the converter can deliver. A second objective of this work is to describe the commutations of a MV 3L-ANPC-VSC phase leg prototype with IGBT modules, including all the intermediate switching states to generate the desired commutations.:Figures and Tables V Glossary XIII 1. Introduction 1 2. State of the art of medium voltage source converters and power semiconductors 5 2.1. Overview of medium voltage source converters 5 2.1.1. Multilevel Voltage Source Converter topologies 6 2.1.2. Application oriented basic characteristic of IGCTs and IGBTs 10 2.1.3. Market overview of ML-VSCs 11 2.2. IGBT modules for MV applications 12 2.2.1. Structure and Function 12 2.2.2. Electrical characteristics of the IGBT modules 15 2.2.3. Power losses and junction temperatures estimation 17 2.2.4. Packaging 19 2.2.5. Reliability and Life cycle of IGBT modules 21 2.2.6. Market Overview 23 2.3. Summary of Chapter 2 23 3. Structure, function and characteristics of NPC-based VSCs 25 3.1. The 3L-NPC-VSC 25 3.1.1. Power Topology 25 3.1.2. Switching states, current paths and blocking voltage distribution 26 3.1.3. Modulation of three-level inverters 28 3.1.4. Power loss distribution 32 3.1.5. “Short” and “long” commutation paths 33 3.2. The 3L-NPP-VSC 34 3.2.1. Power Topology 34 3.2.2. Switching states, current paths and blocking voltage distribution 35 3.2.3. Power Loss distribution 36 3.3. The 3L-ANPC-VSC 37 3.3.1. Power Topology 37 3.3.2. Switching states, current paths and blocking voltage distribution 38 3.3.3. Commutations and power loss distribution 39 3.3.4. Loss balancing schemes 57 3.4. The 5L-ANPC-VSC 60 3.4.1. Power Topology 60 3.4.2. Switching states, current paths and blocking voltage distribution 61 3.4.3. Commutation sequences 62 3.4.4. Power Loss distribution 70 3.4.5. Modulation and balancing strategies of capacitor voltages 70 3.5. The 5L-SMC 74 3.5.1. Power Topology 74 3.5.2. Switching states, current paths and blocking voltage distribution 75 3.5.3. Commutations and power loss distribution 78 3.5.4. Modulation and balancing strategies of capacitor voltages 80 3.6. Summary of Chapter 3 81 4. Comparative evaluation and performance of NPC-based converters 83 4.1. Motivation and goal of the comparisons 83 4.2. Basis of the comparison 83 4.2.1. Simulation scheme 85 4.2.2. Losses and thermal models for (4.5 kV, 1.2 kA) IGBT modules 86 4.2.3. Operating points, modulation, controllers and general parameters 88 4.2.4. Life cycle estimation 94 4.3. Simulation results of the 3.3 kV 3L-VSCs 97 4.3.1. Loss distribution and temperature at equal phase current 97 4.3.2. Maximum phase current 109 4.3.3. Life cycle 111 4.4. Simulation results of the 6.6 kV 5L and 3L-VSCs 115 4.4.1. Loss distribution and temperature at equal phase current 115 4.4.2. Maximum phase current 120 4.4.3. Life cycle 128 4.5. Summary of Chapter 4 132 5. Experimental investigation of the 3L-ANPC-VSC with IGBT modules 135 5.1. Goal of the work 135 5.2. Description of the 3L-ANPC-VSC test bench 136 5.2.1. Medium voltage stage 136 5.2.2. Gate drivers and digital signal handling 138 5.2.3. Measurement equipment 139 5.3. Double-pulse test and commutation sequences 140 5.3.1. Description of the double-pulse test for the 3L-ANPC-VSC 140 5.3.2. Commutation sequences for the double-pulse test 142 5.4. Commutation measurements 142 5.4.1. Switching and transition times 144 5.4.2. Type I commutations 145 5.4.3. Type I-U commutations 150 5.4.4. Type II commutations 150 5.4.5. Type III commutations 157 5.4.6. Comparison of the commutation times 157 5.4.7. Stray inductances of the “short” and “long” commutations 163 5.5. Summary of Chapter 5 167 6. Conclusions 169 Appendices 173 A. Thermal model of IGBT modules 175 A.1. General “Y” model 175 A.2. “Foster” thermal circuit 177 A.3. “Cauer” thermal circuit 178 A.4. From “Foster” to “Cauer” 179 A.5. Temperature comparison using “Foster” and “Cauer” networks 181 B. The “Rainflow” cycle counting algorithm 183 C. Description of the wind generator example 187 C.1. Simulation models 188 C.1.1. Wind turbine 188 C.1.2. Synchronous generator, grid and choke filter 189 C.1.3. Converters 189 C.2. Controllers 190 C.2.1. MPPT scheme 190 C.2.2. Pitch angle controller 191 C.2.3. Generator side VSC 192 C.2.4. Grid side VSC 193 D. 3D-surfaces of the maximum load currents in NPC-based converters 195 Bibliography 201 Bibliography 201<br>Unter den Multilevel-Spannungsumrichtern für Mittelspannungs- und Hochleistungsanwendungen ist die am häufigsten verwendete Leistungstopologie der NPC-VSC, wegen seinen Merkmalen wie die Gleichstrom-Bus fähigkeit mit mittlerem Punkt, das Fehlen von Schaltern in Reihenschaltung, eine geringe Anzahl von Bauteilen und eine einfache Steuerung. Die Verwendung von Bipolartransistor Modulen mit isolierter Gate-Elektrode als Leistungsschalter bietet weitere Vorteile wie kostengünstige Gatetreiber und Überlebensfähigkeit nach einem Kurzschluss. Die IGBT-Module haben jedoch aufgrund der durch Lastzyklen erzeugten thermischen Belastung eine verkürzte Lebensdauer. Trotz der Vorteile des 3L-NPC-VSC ist der Hauptnachteil die ungleichmäßige Verteilung der Leistungsverluste zwischen den Leistungsgeräten. Um dieses Problem zu beheben und andere Eigenschaften zu verbessern, wurden fortgeschrittenere ML-Konverter entwickelt. Das 3L-ANPC-VSC ermöglicht dank seiner zusätzlichen IGBTs eine verbesserte Verlustleistungsverteilung, wodurch die Anzahl der möglichen Null-Zustände erhöht wird, es ist jedoch ein Verlustausgleichsschema erforderlich, um den richtigen redundanten Null-Zustand, und benötigt auszuwählende komplexere Kommutierungssequenz zwischen Zuständen. Das 3L-NPP-VSC verbessert die Verlustleistungsverteilung durch die Verwendung von in Reihe geschalteten Schaltern zwischen der Ausgangsklemme und den positiven und negativen Zwischenkreisklemmen. Andere fortgeschrittene Leistungstopologien mit einer höheren Anzahl von Stufen umfassen den 5L-ANPC-VSC, der pro Phase einen fliegenden Kondensator zur Erzeugung der zusätzlichen Stufen aufweist; und den 5L-SMC, der pro Phase zwei fliegende Kondensatoren benötigt. Das Ziel dieser Arbeit ist es, die Leistung der oben genannten NPC-VSC, einschließlich der mit fliegenden Kondensatoren, hinsichtlich der Verlustleistungsverteilung und der Sperrschichttemperatur der am stärksten beanspruchten Geräte zu bewerten. Diese definieren zusammen mit der Nennausgangsspannung die maximale Leistung, die der Umrichter liefern kann. Ein zweites Ziel dieser Arbeit ist die Beschreibung der Kommutierungen eines MV 3L-ANPC-VSC- Prototyps mit IGBT-Modulen einschließlich aller Zwischenschaltzustände, um die gewünschten Kommutierungen zu erzeugen.:Figures and Tables V Glossary XIII 1. Introduction 1 2. State of the art of medium voltage source converters and power semiconductors 5 2.1. Overview of medium voltage source converters 5 2.1.1. Multilevel Voltage Source Converter topologies 6 2.1.2. Application oriented basic characteristic of IGCTs and IGBTs 10 2.1.3. Market overview of ML-VSCs 11 2.2. IGBT modules for MV applications 12 2.2.1. Structure and Function 12 2.2.2. Electrical characteristics of the IGBT modules 15 2.2.3. Power losses and junction temperatures estimation 17 2.2.4. Packaging 19 2.2.5. Reliability and Life cycle of IGBT modules 21 2.2.6. Market Overview 23 2.3. Summary of Chapter 2 23 3. Structure, function and characteristics of NPC-based VSCs 25 3.1. The 3L-NPC-VSC 25 3.1.1. Power Topology 25 3.1.2. Switching states, current paths and blocking voltage distribution 26 3.1.3. Modulation of three-level inverters 28 3.1.4. Power loss distribution 32 3.1.5. “Short” and “long” commutation paths 33 3.2. The 3L-NPP-VSC 34 3.2.1. Power Topology 34 3.2.2. Switching states, current paths and blocking voltage distribution 35 3.2.3. Power Loss distribution 36 3.3. The 3L-ANPC-VSC 37 3.3.1. Power Topology 37 3.3.2. Switching states, current paths and blocking voltage distribution 38 3.3.3. Commutations and power loss distribution 39 3.3.4. Loss balancing schemes 57 3.4. The 5L-ANPC-VSC 60 3.4.1. Power Topology 60 3.4.2. Switching states, current paths and blocking voltage distribution 61 3.4.3. Commutation sequences 62 3.4.4. Power Loss distribution 70 3.4.5. Modulation and balancing strategies of capacitor voltages 70 3.5. The 5L-SMC 74 3.5.1. Power Topology 74 3.5.2. Switching states, current paths and blocking voltage distribution 75 3.5.3. Commutations and power loss distribution 78 3.5.4. Modulation and balancing strategies of capacitor voltages 80 3.6. Summary of Chapter 3 81 4. Comparative evaluation and performance of NPC-based converters 83 4.1. Motivation and goal of the comparisons 83 4.2. Basis of the comparison 83 4.2.1. Simulation scheme 85 4.2.2. Losses and thermal models for (4.5 kV, 1.2 kA) IGBT modules 86 4.2.3. Operating points, modulation, controllers and general parameters 88 4.2.4. Life cycle estimation 94 4.3. Simulation results of the 3.3 kV 3L-VSCs 97 4.3.1. Loss distribution and temperature at equal phase current 97 4.3.2. Maximum phase current 109 4.3.3. Life cycle 111 4.4. Simulation results of the 6.6 kV 5L and 3L-VSCs 115 4.4.1. Loss distribution and temperature at equal phase current 115 4.4.2. Maximum phase current 120 4.4.3. Life cycle 128 4.5. Summary of Chapter 4 132 5. Experimental investigation of the 3L-ANPC-VSC with IGBT modules 135 5.1. Goal of the work 135 5.2. Description of the 3L-ANPC-VSC test bench 136 5.2.1. Medium voltage stage 136 5.2.2. Gate drivers and digital signal handling 138 5.2.3. Measurement equipment 139 5.3. Double-pulse test and commutation sequences 140 5.3.1. Description of the double-pulse test for the 3L-ANPC-VSC 140 5.3.2. Commutation sequences for the double-pulse test 142 5.4. Commutation measurements 142 5.4.1. Switching and transition times 144 5.4.2. Type I commutations 145 5.4.3. Type I-U commutations 150 5.4.4. Type II commutations 150 5.4.5. Type III commutations 157 5.4.6. Comparison of the commutation times 157 5.4.7. Stray inductances of the “short” and “long” commutations 163 5.5. Summary of Chapter 5 167 6. Conclusions 169 Appendices 173 A. Thermal model of IGBT modules 175 A.1. General “Y” model 175 A.2. “Foster” thermal circuit 177 A.3. “Cauer” thermal circuit 178 A.4. From “Foster” to “Cauer” 179 A.5. Temperature comparison using “Foster” and “Cauer” networks 181 B. The “Rainflow” cycle counting algorithm 183 C. Description of the wind generator example 187 C.1. Simulation models 188 C.1.1. Wind turbine 188 C.1.2. Synchronous generator, grid and choke filter 189 C.1.3. Converters 189 C.2. Controllers 190 C.2.1. MPPT scheme 190 C.2.2. Pitch angle controller 191 C.2.3. Generator side VSC 192 C.2.4. Grid side VSC 193 D. 3D-surfaces of the maximum load currents in NPC-based converters 195 Bibliography 201 Bibliography 201

Dissertação de Mestrado Integrado em Engenharia Electrotécnica e de Computadores apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra.<br>Nos últimos anos, o mundo tem assistido a um grande desenvolvimento tecnológico marcado sobretudo pela proliferação de sistemas informáticos e de accionamentos eléctricos. No entanto, a utilização massiva deste tipo de cargas, ditas não-lineares, acarreta grandes problemas no que toca à qualidade da energia eléctrica, criando uma elevada poluição harmónica. Na tentativa de minimizar estes efeitos nocivos surgiram os filtros activos de potência, tradicionalmente compostos por um conversor de 2 níveis. Porém, devido aos recentes desenvolvimentos nas topologias multinível, estas começaram a integrar os sistemas de filtragem, conferindo-lhes um melhor desempenho. Contudo, uma vez que é usado um maior número de semicondutores, a probabilidade de ocorrência de uma falha de circuito aberto ou de curto-circuito num deles aumenta consideravelmente. Deste modo, surge a necessidade de desenvolver estratégias que permitam identificar estas avarias e actuar imediatamente de forma a que o sistema possa permanecer em funcionamento. No seguimento desta problemática, na presente dissertação irão ser propostos métodos de diagnóstico e de tolerância a falhas para filtros activos de potência paralelos de 3 e 4 fios, baseados num conversor NPC de 3 níveis. A validade das técnicas apresentadas irá ser comprovada através de simulações computacionais e de ensaios experimentais.<br>In the past few years, the world has witnessed a great technological development, where the computer systems and the electric drives play an important role. However, the massive widespread of this so called non-linear loads brings many problems regarding the electric power quality, creating a high harmonic pollution. In order to mitigate this harmful effect, the active power filters have appeared, traditionally composed by a 2 level converter. Nevertheless, due to recent development of multilevel topologies, they are becoming to integrate these filtering systems, improving their performance. However, since it is needed a greater number of switching devices, the probability of an open circuit or a short-circuit failure in one of them is significantly higher. Thus, it is important to develop strategies to identify these faults and act immediately in order to keep the system operational. Then, in this dissertation will be proposed fault diagnosis and fault tolerance methods for 3 and 4 wire shunt active power filters, based on a 3 level NPC converter. The effectiveness of these techniques will be demonstrated by simulation and experimental results.

In the past two decades the core aircraft technology is going through a drastic change. The traditional technologies that is almost half a century old, is going through a complete revamp. In the new “More Electric Aircraft” technology many mechanical, pneumatic and hydraulic systems are being replaced by electrical and power electronic systems. Airbus-A380, Boeing B-787 are the pioneers in the family of these new breed of aircrafts. As the aircraft technology is moving towards “More Electric”, more and more electric motors and motor controllers are being used in new aircrafts. Number of electric motor drive systems has increased by about ten times in more electric aircrafts compared to traditional aircrafts. Weight of any electric component that goes into aircraft needs to be low to reduce the overall weight of aircraft so as to improve the fuel efficiency of the aircraft. Hence there is an increased need to reduce weight of motors and motor controllers in commercial aircraft. High speed ironless axial flux permanent magnet brushless dc motors are becoming popular in the new more-electric aircrafts because of their ability to meet the demand of light weight, high power density, high efficiency and high reliability. However, these motors come with very low inductance, which poses a big challenge to the motor controllers in controlling the ripple current in motor windings. Multilevel inverters can solve this problem. Three-level inverters are proposed in this thesis for driving axial flux BLDC motors in aircraft. Majority of the motors in new more electric aircrafts are in the power range of 2kW to 20kW, while a few motor applications being in the range of 100kW to 150kW. Motor controllers in these applications run from 270Vdc or 540Vdc bus which is the standard in new more electric aircraft architecture. Multilevel Inverter is popular in the industry for high power and high voltage applications, where high-voltage power switching devices like IGBT, GTO are popularly used. However multilevel inverters have not been tried in the low power range which is appropriate for aircraft applications. A detail analysis of practical feasibility of constructing three-level inverter in lower power and voltage level is presented in this thesis. Analysis is presented that verify the advantages of driving low voltage and low power (300Vdc to 600Vdc and less than 100kW) motors with multilevel inverters. Practical considerations for design of MOSFET based three-level inverter are investigated and topological modifications are suggested. The effect of clamping diodes in the diode clamped multilevel inverters play an important role in determining its efficiency. SiC diodes are proposed to be used as clamping diodes. Further, it is realised that power loss introduced by reverse recovery of MOSFET body diode prohibits use of MOSFET in hard switched inverter legs. Hence, a technique of avoiding the reverse recovery losses of MOSFET body diode in three-level NPC inverter is conceived. The use of proposed multilevel inverter topology enables operation at high switching frequency without sacrificing efficiency. High switching frequency of operation reduces the output filter requirement, which in turn helps reducing size of the inverter. In this research work elaborate trade-off analysis is done to quantify the suitability of multilevel inverters in the low power applications. For successful operation of three-level NPC inverter in aircraft electrical system, it is important for the DC bus structure in aircraft electric primary distribution system to be compatible to drive NPC inverters. Hence a detail study of AC to DC power conversion system as applied to commercial aircraft electrical system is done. Multi-pulse rectifiers using autotransformers are used in aircrafts. Investigation is done to improve these rectifiers for future aircrafts, such that they can support new technologies of future generation motor controllers. A new 24-pulse isolated transformer rectifier topology is proposed. From two 15º displaced 6-phase systems feeding two 12-pulse rectifiers that are series connected, a 24-pulse rectifier topology is obtained. Though, windings of each 12-pulse rectifiers are isolated from primary, the 6-phase generation is done without any isolation of the transformer windings. The new 24-pulse transformer topology has lower VA rating compared to standard 12-pulse rectifiers. Though the new 24-pulse transformer-rectifier solution is robust and simple, it adds to the weight of the overall system, as compared to the present architecture as the proposed topology uses isolated transformer. Non-isolated autotransformer cannot provide split voltage at the dc-link that creates a stable mid-point voltage as required by the three-level NPC inverter. Hence, a new front-end AC-DC power conversion system with switched capacitor is conceived that can support motor controllers driven by three-level inverters. Laboratory experimental results are presented to validate the new proposed topology. In this proposed topology, the inverter dc-link voltage is double the input dc-link voltage. An intense research work is performed to understand the operation of Trapezoidal Back EMF BLDC motor driven by three-Level NPC inverter. Operation of BLDC motor from three-Level inverter is primarily advantageous for low inductance motors, like ironless axial flux motors. For low inductance BLDC motor, very high switching frequency is required to limit the magnitude of ripple current in motor winding. Three-level inverters help limiting the magnitude of motor ripple current without increasing the switching frequency to very high value. Further, it is analysed that dc link mid-point current in three-level NPC inverter for driving trapezoidal BLDC motor has a zero average current with fundamental frequency same as switching frequency. Because of this, trapezoidal BLDC motors can easily be operated from three-level NPC inverter without any special attention given to mid-point voltage unbalance. One non-ideal condition arrives in practical implementation of the inverter that leads to non-zero average mid point current. Unequal gate drive dead time delays from one leg to other leg of inverter introduce dc-link mid-point voltage unbalance. For the motoring mode operation of trapezoidal BLDC motor drive, simple gate drive logic is researched that eliminates need of the gate drive dead-time, and hence solves the mid-point voltage unbalance issue. Simple closed loop control scheme for mid-point voltage balancing also is also proposed. This control scheme may be used in applications where very precise control of speed and torque ripple is warranted. All the investigations reported in this thesis are simulated extensively on MATHCAD and MATLAB platform using SIMULINK toolbox. A laboratory experimental set-up of three-Level inverter driving axial flux BLDC motor is built. The three-level inverter, operating from 300Vdc bus is built using 500V MOSFETs and 600V SiC diodes. All the control schemes are implemented digitally on digital signal processor TMS320F2812 DSP platform and GAL22V10B platforms. Experimental results are collected to validate the theoretical propositions made in the present research work. At the end, in chapter 5, some future works are proposed. A new external voltage balance circuit is proposed where the inverter dc-link voltage is same as the input dc-link voltage. This topology is based on the resonant converter principle and uses a lighter resonant inductor than prior arts available in literature. Detail simulation and experimentation of this topology may be carried out to validate the industrial benefits of this circuit. It is also thought that current source inverters may work as an alternative to voltage source inverters for driving BLDC motors. Current source inverters eliminate use of bulky DC-link capacitors. Long term reliability of current source inverters is higher than voltage source inverters due to the absence of possibility of shoot-through. Further, in voltage source inverters, the voltage at the motor terminal is limited by the source voltage (dc-link voltage). This issue is eliminated in current source inverters. An interface circuit is conceived to reduce the size of dc-link inductors in current source inverters, pending detail analysis and experimental verification. The interface circuit bases its fundamentals on the principles of operation of multilevel inverters for BLDC motors that is presented in this thesis.

In the past two decades the core aircraft technology is going through a drastic change. The traditional technologies that is almost half a century old, is going through a complete revamp. In the new “More Electric Aircraft” technology many mechanical, pneumatic and hydraulic systems are being replaced by electrical and power electronic systems. Airbus-A380, Boeing B-787 are the pioneers in the family of these new breed of aircrafts. As the aircraft technology is moving towards “More Electric”, more and more electric motors and motor controllers are being used in new aircrafts. Number of electric motor drive systems has increased by about ten times in more electric aircrafts compared to traditional aircrafts. Weight of any electric component that goes into aircraft needs to be low to reduce the overall weight of aircraft so as to improve the fuel efficiency of the aircraft. Hence there is an increased need to reduce weight of motors and motor controllers in commercial aircraft. High speed ironless axial flux permanent magnet brushless dc motors are becoming popular in the new more-electric aircrafts because of their ability to meet the demand of light weight, high power density, high efficiency and high reliability. However, these motors come with very low inductance, which poses a big challenge to the motor controllers in controlling the ripple current in motor windings. Multilevel inverters can solve this problem. Three-level inverters are proposed in this thesis for driving axial flux BLDC motors in aircraft. Majority of the motors in new more electric aircrafts are in the power range of 2kW to 20kW, while a few motor applications being in the range of 100kW to 150kW. Motor controllers in these applications run from 270Vdc or 540Vdc bus which is the standard in new more electric aircraft architecture. Multilevel Inverter is popular in the industry for high power and high voltage applications, where high-voltage power switching devices like IGBT, GTO are popularly used. However multilevel inverters have not been tried in the low power range which is appropriate for aircraft applications. A detail analysis of practical feasibility of constructing three-level inverter in lower power and voltage level is presented in this thesis. Analysis is presented that verify the advantages of driving low voltage and low power (300Vdc to 600Vdc and less than 100kW) motors with multilevel inverters. Practical considerations for design of MOSFET based three-level inverter are investigated and topological modifications are suggested. The effect of clamping diodes in the diode clamped multilevel inverters play an important role in determining its efficiency. SiC diodes are proposed to be used as clamping diodes. Further, it is realised that power loss introduced by reverse recovery of MOSFET body diode prohibits use of MOSFET in hard switched inverter legs. Hence, a technique of avoiding the reverse recovery losses of MOSFET body diode in three-level NPC inverter is conceived. The use of proposed multilevel inverter topology enables operation at high switching frequency without sacrificing efficiency. High switching frequency of operation reduces the output filter requirement, which in turn helps reducing size of the inverter. In this research work elaborate trade-off analysis is done to quantify the suitability of multilevel inverters in the low power applications. For successful operation of three-level NPC inverter in aircraft electrical system, it is important for the DC bus structure in aircraft electric primary distribution system to be compatible to drive NPC inverters. Hence a detail study of AC to DC power conversion system as applied to commercial aircraft electrical system is done. Multi-pulse rectifiers using autotransformers are used in aircrafts. Investigation is done to improve these rectifiers for future aircrafts, such that they can support new technologies of future generation motor controllers. A new 24-pulse isolated transformer rectifier topology is proposed. From two 15º displaced 6-phase systems feeding two 12-pulse rectifiers that are series connected, a 24-pulse rectifier topology is obtained. Though, windings of each 12-pulse rectifiers are isolated from primary, the 6-phase generation is done without any isolation of the transformer windings. The new 24-pulse transformer topology has lower VA rating compared to standard 12-pulse rectifiers. Though the new 24-pulse transformer-rectifier solution is robust and simple, it adds to the weight of the overall system, as compared to the present architecture as the proposed topology uses isolated transformer. Non-isolated autotransformer cannot provide split voltage at the dc-link that creates a stable mid-point voltage as required by the three-level NPC inverter. Hence, a new front-end AC-DC power conversion system with switched capacitor is conceived that can support motor controllers driven by three-level inverters. Laboratory experimental results are presented to validate the new proposed topology. In this proposed topology, the inverter dc-link voltage is double the input dc-link voltage. An intense research work is performed to understand the operation of Trapezoidal Back EMF BLDC motor driven by three-Level NPC inverter. Operation of BLDC motor from three-Level inverter is primarily advantageous for low inductance motors, like ironless axial flux motors. For low inductance BLDC motor, very high switching frequency is required to limit the magnitude of ripple current in motor winding. Three-level inverters help limiting the magnitude of motor ripple current without increasing the switching frequency to very high value. Further, it is analysed that dc link mid-point current in three-level NPC inverter for driving trapezoidal BLDC motor has a zero average current with fundamental frequency same as switching frequency. Because of this, trapezoidal BLDC motors can easily be operated from three-level NPC inverter without any special attention given to mid-point voltage unbalance. One non-ideal condition arrives in practical implementation of the inverter that leads to non-zero average mid point current. Unequal gate drive dead time delays from one leg to other leg of inverter introduce dc-link mid-point voltage unbalance. For the motoring mode operation of trapezoidal BLDC motor drive, simple gate drive logic is researched that eliminates need of the gate drive dead-time, and hence solves the mid-point voltage unbalance issue. Simple closed loop control scheme for mid-point voltage balancing also is also proposed. This control scheme may be used in applications where very precise control of speed and torque ripple is warranted. All the investigations reported in this thesis are simulated extensively on MATHCAD and MATLAB platform using SIMULINK toolbox. A laboratory experimental set-up of three-Level inverter driving axial flux BLDC motor is built. The three-level inverter, operating from 300Vdc bus is built using 500V MOSFETs and 600V SiC diodes. All the control schemes are implemented digitally on digital signal processor TMS320F2812 DSP platform and GAL22V10B platforms. Experimental results are collected to validate the theoretical propositions made in the present research work. At the end, in chapter 5, some future works are proposed. A new external voltage balance circuit is proposed where the inverter dc-link voltage is same as the input dc-link voltage. This topology is based on the resonant converter principle and uses a lighter resonant inductor than prior arts available in literature. Detail simulation and experimentation of this topology may be carried out to validate the industrial benefits of this circuit. It is also thought that current source inverters may work as an alternative to voltage source inverters for driving BLDC motors. Current source inverters eliminate use of bulky DC-link capacitors. Long term reliability of current source inverters is higher than voltage source inverters due to the absence of possibility of shoot-through. Further, in voltage source inverters, the voltage at the motor terminal is limited by the source voltage (dc-link voltage). This issue is eliminated in current source inverters. An interface circuit is conceived to reduce the size of dc-link inductors in current source inverters, pending detail analysis and experimental verification. The interface circuit bases its fundamentals on the principles of operation of multilevel inverters for BLDC motors that is presented in this thesis.

碩士<br>國立雲林科技大學<br>電機工程系碩士班<br>89<br>The widespread use of solid-state converters have resulted in many power quality issues such as harmonic distortion in the line current, low power factor, low efficiency and voltage flicker, etc. In this thesis, a multilevel pulse width modulation (PWM) converter which is suitable for high voltage / high power applications is proposed. With the advantages of low voltage harmonic distortion, low EMI, low voltage rating of the switching devices, power reversible, easy implementation and reduction in size and weight, the proposed neutral point diode clamped power converter was applied to the following applications: power factor correction, active power filter, voltage regulator and on line uninterruptible power supply. The simulation (based on Matlab/Simulink) and experimental results are presented. The author employed the TMS320C240 EVM board with some expanded peripheral circuit to realize the modular control circuit. The results revealed that the adopted multilevel technology has the characteristics of high power factor, and high efficiency, and can be used in the medium voltage and high power applications.

碩士<br>國立雲林科技大學<br>電機工程系碩士班<br>90<br>Power electronic products have been widely used in the industry. But these products produce the serious power pollution. The power pollution problems results in low power quality and low efficiency. The power quality include the cases of under-voltage, over-voltage, outage, surge, dip, voltage sag, voltage swell, harmonics, electromagnetic inference (EMI), flicker, voltage frequency variation and unbalance. In the recent market products, we can not find a perfect and low cost product to solve above mentioned problems for power pollution. Large current harmonics, low power factor and low efficiency are the main drawbacks of diode or phase-controlled rectifier. In this thesis, a multilevel pulse width modulation (PWM) converter used in the medium voltage and high power applications is proposed to replace the conventional power converter. With the advantages of low harmonic distortion, low EMI, low voltage rating of the switching devices, power reversible and reduction in size and weight, the neutral-point switch-clamped converter is proposed. The proposed neutral-point-clamped converter can be operated as an active power filter, power factor corrector, on line uninterruptible power supply and voltage regulator. To verify the proposed operation scheme, performance characteristics are given by the computer simulation and experimental results.

Die vorliegende Arbeit befasst sich mit einem detaillierten Vergleich von Mehrpunkt-Schaltungstopologien mit zentralem Gleichspannungszwischenkreis für den Einsatz in Mittelspannungsanwendungen. Im Rahmen dieser Untersuchungen wird die 3-Level Neutral Point Clamped Spannungswechselrichter Schaltungstopologie (3L-NPC VSC) sowohl mit Multilevel Flying Capacitor (FLC) als auch mit Multilevel Stacked Multicell (SMC) Schaltungstopologien verglichen, wobei unter Verwendung von aktuell verfügbaren IGBT-Modulen Stromrichterausgangsspannungen von 2.3 kV, 4.16 kV und 6.6 kV betrachtet werden. Neben der grundlegenden Funktionsweise wird die Auslegung der aktiven Leistungshalbleiter und der passiven Energiespeicher (Zwischenkreiskondensatoren, Flying Capacitors) für die untersuchten Stromrichtertopologien dargestellt. Unter Berücksichtigung verschiedener Modulationsverfahren und Schaltfrequenzen werden Kennwerte für den Oberschwingungsgehalt in der Ausgangsspannung und dem Ausgangsstrom vergleichend evaluiert. Die installierte Schalterleistungen, die Halbleiterausnutzungsfaktoren, die Stromrichterverlustleistungen sowie die Verlustleistungsverteilungen werden für die betrachteten Stromrichtertopologien detailliert gegenübergestellt und bewertet.:Inhaltsverzeichnis Liste der Variablen i Liste der Abkürzungen v 1 Einleitung 1 2 Überblick von Mittelspannungsstromrichtertopologien und Leistungshalbleitern 3 2.1 Mittelspannungsumrichtertopologien 3 2.2 Leistungshalbleiter 8 3 Aufbau und Funktion von Mittelspannungsstromrichtertopologien 10 3.1 Neutral Point Clamped Stromrichter (NPC) 10 3.1.1 3-Level Neutral Point Clamped Stromrichter (3L-NPC) 10 3.1.2 Mehrstufige NPC-Umrichter 21 3.2 Flying Capacitor Stromrichter (FLC) 23 3.2.1 3-Level Flying Capacitor Stromrichter (3L-FLC) 23 3.2.2 4-Level Flying Capacitor-Stromrichter (4L-FLC) 33 3.2.3 Mehrstufige Flying Capacitor-Stromrichter (NL-FLC) 39 3.3 Stacked Multicell Stromrichter (SMC) 43 3.3.1 5L-Stacked Multicell Stromrichter (5L-SMC) 43 3.3.2 N-Level Stacked Multicell Umrichter (NL-SMC) 51 4 Modellierung und Auslegung der Stromrichter 59 4.1 Verlustmodell 59 4.1.1 Sperrschichttemperaturen 64 4.2 Auslegung der Leistungshalbleiter 65 4.2.1 Stromauslegung 67 4.2.2 Worst-Case Arbeitspunkte 69 4.3 Auslegung der Zwischenkreiskondensatoren 75 4.3.1 Spannungszwischenkreis 76 4.3.2 Lastseitige Strombelastung und resultierende Spannungswelligkeit im Spannungszwischenkreis 77 4.3.3 Abhängigkeit der Strombelastung und der Spannungswelligkeit im Spannungszwischenkreis vom Frequenzverhältnis mf 95 4.3.4 Netzseitige Zwischenkreiseinspeisung 97 4.3.4.1 Zwischenkreiseinspeisung mit idealisiertem Transformatormodell 98 4.3.4.2 Zwischenkreiseinspeisung mit erweitertem Transformatormodell 101 4.3.5 Simulation des Gesamtsystems 104 4.4 Auslegung der Flying Capacitors 107 4.4.1 Strombelastung der Flying Capacitors 109 4.4.2 Spannungswelligkeit über den Flying Capacitors 113 4.4.3 Abhängigkeit der Spannungswelligkeit der Flying Capacitors vom Frequenzverhältnis mf 124 4.4.4 Auswirkung der Spannungswelligkeit der Flying Capacitors auf die Ausgangsspannungen 126 5 Vergleich der Stromrichtertopologien 129 5.1 Daten für den Stromrichtervergleich 129 5.2 Basis des Vergleiches 132 5.3 Vergleich für einen 2,3 kV Mittelspannungsstromrichter 134 5.3.1 Vergleich bei verschiedenen Schaltfrequenzen 134 5.3.2 Vergleich bei maximaler Trägerfrequenz 142 5.4 Vergleich für einen 4,16 kV Mittelspannungsstromrichter 146 5.4.1 Vergleich bei verschiedenen Schaltfrequenzen 146 5.4.2 Vergleich bei maximaler Trägerfrequenz 153 5.5 Vergleich für einen 6,6 kV Mittelspannungsstromrichter 156 5.5.1 Vergleich bei verschiedenen Schaltfrequenzen 156 5.5.2 Vergleich bei maximaler Trägerfrequenz 162 5.6 Vergleich von 2,3 kV, 4,16 kV und 6,6 kV Mittelspannungsstromrichtern 165 5.6.1 Vergleich bei identischer installierter Schalterleistung SS 165 5.6.2 Vergleich bei einer identischen Ausgangsleistung 167 6 Zusammenfassung und Bewertung 171 Anhang 175 A. Halbleiterverlustmodell 175 Referenzen 177<br>The thesis deals with a detailed comparison of voltage source converter topologies with a central dc-link energy storage device for medium voltage applications. The Three-Level Neutral Point Clamped Voltage Source Converter (3L-NPC VSC) is compared with multilevel Flying Capacitor (FLC) and Stacked Multicell (SMC) Voltage Source Converters (VSC) for output voltages of 2.3 kV, 4.16 kV and 6.6 kV by using state-of-the-art 6.5 kV, 3.3 kV, 4.5 kV and 1.7kV IGBTs. The fundamental functionality of the investigated converter topologies as well as the design of the power semiconductors and of the energy storage devices (Flying Capacitors and Dc-Link capacitors) is described. The installed switch power, converter losses, the semiconductor loss distribution, modulation strategies and the harmonic spectra are compared in detail.:Inhaltsverzeichnis Liste der Variablen i Liste der Abkürzungen v 1 Einleitung 1 2 Überblick von Mittelspannungsstromrichtertopologien und Leistungshalbleitern 3 2.1 Mittelspannungsumrichtertopologien 3 2.2 Leistungshalbleiter 8 3 Aufbau und Funktion von Mittelspannungsstromrichtertopologien 10 3.1 Neutral Point Clamped Stromrichter (NPC) 10 3.1.1 3-Level Neutral Point Clamped Stromrichter (3L-NPC) 10 3.1.2 Mehrstufige NPC-Umrichter 21 3.2 Flying Capacitor Stromrichter (FLC) 23 3.2.1 3-Level Flying Capacitor Stromrichter (3L-FLC) 23 3.2.2 4-Level Flying Capacitor-Stromrichter (4L-FLC) 33 3.2.3 Mehrstufige Flying Capacitor-Stromrichter (NL-FLC) 39 3.3 Stacked Multicell Stromrichter (SMC) 43 3.3.1 5L-Stacked Multicell Stromrichter (5L-SMC) 43 3.3.2 N-Level Stacked Multicell Umrichter (NL-SMC) 51 4 Modellierung und Auslegung der Stromrichter 59 4.1 Verlustmodell 59 4.1.1 Sperrschichttemperaturen 64 4.2 Auslegung der Leistungshalbleiter 65 4.2.1 Stromauslegung 67 4.2.2 Worst-Case Arbeitspunkte 69 4.3 Auslegung der Zwischenkreiskondensatoren 75 4.3.1 Spannungszwischenkreis 76 4.3.2 Lastseitige Strombelastung und resultierende Spannungswelligkeit im Spannungszwischenkreis 77 4.3.3 Abhängigkeit der Strombelastung und der Spannungswelligkeit im Spannungszwischenkreis vom Frequenzverhältnis mf 95 4.3.4 Netzseitige Zwischenkreiseinspeisung 97 4.3.4.1 Zwischenkreiseinspeisung mit idealisiertem Transformatormodell 98 4.3.4.2 Zwischenkreiseinspeisung mit erweitertem Transformatormodell 101 4.3.5 Simulation des Gesamtsystems 104 4.4 Auslegung der Flying Capacitors 107 4.4.1 Strombelastung der Flying Capacitors 109 4.4.2 Spannungswelligkeit über den Flying Capacitors 113 4.4.3 Abhängigkeit der Spannungswelligkeit der Flying Capacitors vom Frequenzverhältnis mf 124 4.4.4 Auswirkung der Spannungswelligkeit der Flying Capacitors auf die Ausgangsspannungen 126 5 Vergleich der Stromrichtertopologien 129 5.1 Daten für den Stromrichtervergleich 129 5.2 Basis des Vergleiches 132 5.3 Vergleich für einen 2,3 kV Mittelspannungsstromrichter 134 5.3.1 Vergleich bei verschiedenen Schaltfrequenzen 134 5.3.2 Vergleich bei maximaler Trägerfrequenz 142 5.4 Vergleich für einen 4,16 kV Mittelspannungsstromrichter 146 5.4.1 Vergleich bei verschiedenen Schaltfrequenzen 146 5.4.2 Vergleich bei maximaler Trägerfrequenz 153 5.5 Vergleich für einen 6,6 kV Mittelspannungsstromrichter 156 5.5.1 Vergleich bei verschiedenen Schaltfrequenzen 156 5.5.2 Vergleich bei maximaler Trägerfrequenz 162 5.6 Vergleich von 2,3 kV, 4,16 kV und 6,6 kV Mittelspannungsstromrichtern 165 5.6.1 Vergleich bei identischer installierter Schalterleistung SS 165 5.6.2 Vergleich bei einer identischen Ausgangsleistung 167 6 Zusammenfassung und Bewertung 171 Anhang 175 A. Halbleiterverlustmodell 175 Referenzen 177

碩士<br>國立中正大學<br>電機工程研究所<br>101<br>This thesis presents three-phase bi-directional neutral-point clamped converter, include grid-connection mode and retification mode. The converter is divided into two parts of which,one is power stagewith neutral-point clamped configuration,theother is control stage with micro-controller RX-62T to realize thefeedforward current control law. In grid-connection mode,power is transformed into AC, and injected into current to the utility grid. In rectification mode, the grid current is transformed into DC to supply rectified load. Simulated and experimental results measured from a three-phase bi-direction neutral-point clamped converterhave been presented to confirm its feasibility. Keyword: neutral-point clamped converter, high power system

碩士<br>國立中正大學<br>電機工程研究所<br>102<br>This thesis presents design and implementation of a three-phase bi-directional neutral-point clamped converter, including two operation modes: grid-connection mode and rectification mode. The converter is divided into two parts, of which one is a power stage with neutral-point clamped configuration, the other is a control stage with a single-chip micro-controller Renesas RX-62T to realize the division-summation (D-Σ) digital control law. In grid-connection mode, power is transformed into AC and injected into the utility grid. In rectification mode, the utility power is transformed to DC and supply rectified loads. Simulated and experimental results measured from a three-phase bi-directional neutral-point clamped converter have been presented to con-firm the analysis, discussion and feasibility.

碩士<br>國立臺灣科技大學<br>電機工程系<br>107<br>This thesis proposes a control strategy for a three-level diode clamped ac/dc converter to achieve balanced line current under unbalanced ac source. Consequently, through the control scheme proposed, in this paper, it can not only balance input current but also reduce dc output voltage ripple when three-phase neutral-point clamped rectifier works on input voltage unbalance condition. The proposed strategy is based on the ripple characteristics in the dc filter capacitors deployed in the dc output. The ac ripples contained in the dc voltages of the filter capacitors are highly affected by the balanced condition of the ac source. Under ideally ac source, only sixth harmonics will exixt, while second and third harmonics will emerge under unbalanced ac source. The latter two harmonics will change the ripple waveforms dramatically and then makes power quality deterioration. These multi-harmonic ripple waveforms can be used to compensate the unbalanced line current caused by unbalanced ac source, and no need to use coordinate transformation, positive and negative phase sequence disconnecting method, direct power control that calculations and analysis can control the current imbalance. The converter dc output voltage is regulated by the coltroller with unity power factor. The ripple waveforms are trsansfered to pulse type, form which the pulse wides are used to estimate the unbalanced condition of the ac sourcecontrol system and then control scheme is planned to control device to generate the compensated comment of line cueernt, at last, through current control loop control three-phase input current, using pulse width modulation generate switch signal, so it can improve and modulate input ac current unbalance factor. The control scheme of this thesis using Matlab/Simulink simulation system to evaluate the performance of the proposed scheme. Moreover, the a 750VDC/2kW, PF 0.99 three-phase NPC rectifier prototype based on digital signal processor TMS320F28069 also is established to demonstrate the vadility of the proposed system. Both simulation and experimental results show the correctness and practicality of proposed control scheme.