The aim of this thesis is to design a controllable DC-link output voltage for isolated slotless tubular linear generators (STLG), which is capable of directly harnessing wave and solar thermal energies. For supplying stable DC-link voltage to load, a suitable voltage regulation circuit is designed based on the integrate system performance assessment. Electrical and mechanical parameters in this refined STLG design are involved to analyze the operational behaviors through magnetic equivalent circuit analysis at different operating modes. From the theoretical modeling and experimental results, both the AC-side and DC-side properties of generator outputs can then be thoroughly investigated. Finally, based on the performance of controllable rectifier model, a three-phase PWM rectifier has been established, and then the regulated DC-link voltage can be implemented using a DSP-based controller combined with required peripheral circuits.

Motor drives represent electric equipment used for speed control of electric motors. Varieties of industrial applications, such as assembly, pumps, fans etc., require motors and they consume huge amount of electric energy. Compared with traditional motor drives, which can only send energy from grid to motor, a regenerative motor drive can achieve bi-directional power flow control between motors and utility grid. Regenerative motor drives are excellent candidates for reducing power loss in motor-related applications. One of the most essential parts of a regenerative motor drive power cell is dc-link capacitors. They create suitable dc-link voltages and smooth the voltage waveforms. Reliability, or lifetime of dc-link capacitors highly affect power cell lifetime, and power loss in dc-link capacitor is also another issue that worth noticing. This thesis focuses on the lifetime modeling and lifetime improvement of dc-link capacitors in a regenerative cascaded H-bridge medium-voltage motor drive. The lifetime modeling bases itself on the mechanisms of dominant lifetime stresses in practical operations. A proposed method is used to reduce a dominant current harmonic component in dc-link capacitors. With the proposed lifetime model and harmonic-reduction method, dc-link capacitor lifetime improvement can be anticipated in this motor drive model. Less power losses in those dc-link capacitor banks can also be achieved. / Thesis / Master of Applied Science (MASc)

No description available.

This thesis proposes novel methods and comprehensive analysis for power loss calculation, DC-link current and voltage ripple estimation, and bus bar design in two-level three-phase voltage source inverters (VSIs). A novel method of MOSFET voltage rise- and fall-time estimations for the switching power loss calculation is developed. The estimation accuracy is significantly improved by the proposed method. In order to provide a reference for thermal management design, inverter power loss analysis is presented. Using the parameters obtained from the semiconductor device datasheets and inverter operating conditions, power loss calculations of three types of devices, namely IGBT, MOSFET, and diode, are discussed. The conduction power loss calculations for these three devices are straightforward; and, the switching power loss of IGBTs and diodes can be obtained from the energy losses given by datasheets. However, many MOSFET datasheets do not provide the switching energy losses directly. Therefore, to acquire MOSFET switching energy losses, switching transient times must be estimated as accurately as possible. The impacts of inverter anti-parallel diode reverse recovery on the DC-link current and voltage ripples are investigated. According to the analysis, the impact of diode reverse recovery on the voltage ripple is negligible, while the RMS value of current ripple is influenced by both diode reverse recovery and inverter switching frequency. A novel method is developed to calculate the ripple current RMS value and the estimation accuracy is significantly improved. Depending on the calculated current and voltage ripples, DC-link capacitor selection is introduced. Generally speaking, failures in the DC-link capacitors take place more frequently than the failures in other parts of the inverter system, and plenty of research has been focusing on minimizing the required DC-link capacitance. As a result, the accurate estimations of DC-link current and voltage ripples are vital in the optimization methods. In addition, with the accurate estimations, the over-design in the DC-link capacitance could be reduced. Finally, the design of a practical bus bar is presented. The DC current distribution is aff ected by the numbers and locations of the DC input tabs, while the AC current distribution is influenced by the numbers and locations of the installation holes for DC-link capacitors and semiconductor devices. Furthermore, parasitic parameters of the bus bar, especially the stray inductance and voltage spikes caused by this inductance during switching turn-o transients, are also discussed from the angle of the design rules and correlation between the parameters and bus bar geometry structure. In the end, a bus bar is designed with balanced current distribution and low stray inductance. / Thesis / Doctor of Philosophy (PhD)

La stabilité est un facteur très important dans tous les modes de fonctionnement pour un Système à Puissance Distribué (SPD). En SPD, les charges sont connectées au bus DC à travers d’un filtre entré LC. La plupart des charges de SPD d'avions présents une caractéristique de charge à puissance constante dans un domaine de fonctionnement dans laquelle ils sont étroitement contrôlés. Ainsi, elles peuvent être modélisées comme une résistance négative. Changement de la charge dans un sous-système peut conduire un système stable dans l'instabilité.Une solution pratique pour diminuer le risque d'instabilité est présentée dans cette thèse qui consiste à modifier le contrôle des convertisseurs ou système onduleur-moteur connecté au bus DC. Cette solution permet de stabiliser le système, même avec un condensateur plus petit. Dans la première partie de la thèse, une méthode linéaire est présentée qui permet étudier la stabilité locale d'un système onduleur-moteur connecté au réseau par un filtre LC et un redresseur. Une technique de compensation d’oscillation est utilisée pour améliorer la marge de stabilité du système et la taille de la capacité dc-link sans modifier la structure des boucles de courant ou de couple. Cette technique consiste à superposer une puissance stabilisant sur la puissance absorbée par le drive. Bien que les modèles linéaires puissent être employées avec succès pour décrire le comportement d'un système physique au niveau local, ils échouent souvent de fournir une caractérisation satisfaisante de large-signal. Dans la deuxième partie, deux méthodes pour la stabilisation large-signal du système électrique sont présentées. Dans la dernière partie, une nouvelle méthode, basée sur les spécifications dynamiques est proposée pour étudier la stabilité d'un système électrique en cascade / Stability is the first and very important factor in all modes of operation for a Distributed Power System (DPS). In DPS, loads are connected to the DC-bus through an input LC filter. Most of the loads in DPS of aircraft present a constant power load characteristic within a domain of operation in which they are tightly controlled. So they can be modeled as negative resistance. Change of the load in one subsystem may lead a stable system into instability. A practical solution to decrease the risk of instability presented in this thesis which consists in modifying the control of the converters or inverter-motor drive system connected to the DC-bus. This solution permits to stabilize the system even with a smaller size of capacitor. In the first part of the thesis, a linear method is presented which allows investigating local stability of an inverter-motor-drive system connected to the grid through an LC filter and a rectifier. An oscillation compensation technique is used to improve the stability margin of the system and the size of the dc-link capacitance without modifying structure of the torque or current loops. This technique consists in superposing a stabilizing power on the absorbed power by the drive. Although linear models can be successfully employed to locally describe the behavior of a physical system, they often fail to provide a satisfactory large signal characterization. In the second part, two methods for the large signal stabilization of the electrical system are presented. In the last part, a new method, based on dynamic specifications, is proposed to study the stability of a cascaded electric system

Ingeniero Civil Eléctrico / En los últimos años, la integración de fuentes de generación distribuida dentro los sistemas eléctricos ha emergido como un tema crítico impulsado principalmente por motivaciones ambientales e incentivos económicos. En este contexto, la utilización de convertidores fuente de voltaje (VSC, Voltage Source Converters) como interfaz generador-red ha cobrado cada vez más importancia, pues permiten operar y controlar los sistemas de generación, desempeñando un rol esencial en redes desbalanceadas. En el contexto de una red desbalanceada, aparecerán oscilaciones de potencia de doble frecuencia en la red, siendo reflejadas como ripple en el DC-link de un VSC. En la red, las oscilaciones de potencia activa pueden causar excesivo ruido y estrés mecánico en generadores sincrónicos, mientras que la potencia reactiva oscilante puede causar pérdidas excesivas sobre el sistema eléctrico. Análogamente las oscilaciones de potencia en el DC-link pueden afectar negativamente al desempeño de sistemas energizados por paneles solares, baterías y celdas de combustibles, reduciendo su eficiencia, vida útil y la confiabilidad en el largo plazo. Además, a causa de estas oscilaciones podría haber un eventual disparo de las protecciones por sobre tensión en el caso del DC-link, o por sobre corrientes en el convertidor lado red. En virtud de lo anterior, se han desarrollado distintas estrategias para controlar los VSC usando principalmente corrientes de secuencia positiva y negativa. Mediante estas corrientes los objetivos de control que se buscan conseguir son: compensación de oscilaciones de potencia activa en la red, potencia reactiva constante, corrientes balanceadas, DC-link sin ripple, entre otros. Dado que los grados de libertad que otorgan estas corrientes no son suficientes para alcanzar todos los objetivos de control que se quisieran, se podrían presentar significativas oscilaciones de potencia activa y/o reactiva de doble frecuencia de red o corrientes desbalanceadas de elevada magnitud. De manera alternativa se han propuesto nuevas estrategias de control usando corriente de secuencia cero y de este modo aumentar los grados de libertad, sin embargo la principal desventaja de estas estrategias es que pueden llegar a necesitar elevadas corrientes de secuencia cero, lo cual no siempre es posible de obtener según los límites nominales del sistema En este trabajo se propone una extensión de la estrategia de control que utiliza secuencia cero, permitiendo trabajar con corrientes de secuencia positiva, negativa y cero dentro de un esquema colaborativo. La estrategia propuesta tiene la capacidad de compensar las oscilaciones de potencia en el DC-link manteniendo las corrientes de fase y del neutro dentro del límite nominal, y mitigar las oscilaciones de potencia activa y reactiva de la red. Para lograr esto, la estrategia es diseñada utilizando MATLAB y simulada en el software PLECS con el fin de estudiar su comportamiento, y luego validada experimentalmente usando la plataforma de control dSPACE DS1103.

Module integrated converters (MIC), also called micro inverter, in single phase have witnessed recent market success due to unique features (1) improved energy harvest, (2) improved system efficiency, (3) lower installation costs, (4) plug-N-play operation, (5) and enhanced flexibility and modularity. The MIC sector has grown from a niche market to mainstream, especially in the United States. Due to the fact that two-stage architecture is commonly used for single phase MIC application. A DC-DC stage with maximum power point tracking to boost the output voltage of the Photovoltaic (PV) panel is employed in the first stage, DC-AC stage is used for use to connect the grid or the residential application. As well known, the cost of MIC is key issue compared to convention PV system, such as the architecture: string inverter or central inverter. A high efficiency and density DC-DC converter is proposed and dedicated for MIC application. Assuming further expansion of the MIC market, this dissertation presents the micro-inverter concept incorporated in large size PV installations such as MW-class solar farms where a three phase AC connection is employed. A high efficiency three phase MIC with two-stage ZVS operation for grid tied photovoltaic system is proposed which will reduce cost per watt, improve reliability, and increase scalability of MW-class solar farms through the development of new solar farm system architectures. This dissertation presents modeling and triple-loop control for a high efficiency three-phase four-wire inverter for use in grid-connected two-stage micro inverter applications. An average signal model based on a synchronous rotation frame for a three-phase four-wire inverter has been developed. The inner current loop consists of a variable frequency bidirectional current mode (VFBCM) controller which regulates output filter inductor current thereby achieving ZVS, improved system response, and reduced grid current THD. Active damping of the LCL output filter using filter inductor current feedback is discussed along with small signal modeling of the proposed control method. Since the DC-link capacitor plays a critical role in two-stage micro inverter applications, a DC-link controller is implemented outside of the two current control loops to keep the bus voltage constant. In the end, simulation and experimental results from a 400 watt prototype are presented to verify the validity of the theoretical analysis.

The past few decades have seen a rapid sales increase and technological development of electric vehicles (EVs). As the key part of the electrical powertrain systems, the traction machine drive systems in modern EVs are composed of voltage source inverters (VSI) and electric machines. In this thesis, multiphase VSIs are studied and designed to achieve volume reductions when compared with existing 3-phase benchmark VSIs. Different existing switching strategies for arbitrary phase number multiphase VSIs are investigated resulting in an understanding of best practice and a newly proposed switching strategy. Thus, the first contribution of this thesis is switching strategies that support subsequent investigations and experimental validation. DC-link capacitor and heat sink are two bulkiest components in VSIs and hence it is more efficient to decrease their volumes to achieve the compactness improvement. The investigation methodology and procedure for arbitrary phase number VSI DC-link capacitor requirements, i.e. capacitance and RMS current ratings, are firstly developed. Increased phase number decreases the DC-link capacitor requirements and hence the VSI volume significantly. Throughout this analysis, the connected multiphase machine is considered appropriately, though no electric machine design is described in the thesis. While other authors have studied DC-link current ripple, this thesis qualifies and quantifies the system benefits. This is the second contribution. Multiphase VSIs thermal models are built and their respective thermal performances studied and evaluated against a reference 3-phase benchmark VSI. The power loss deviation among different semiconductor dies is lower or even eliminated in the multiphase VSIs. Furthermore, the multiphase integrated design VSIs have a significant heat sink volume reduction when compared to the 3-phase benchmark VSI. This study and concluding benefits are the third contribution. Finally, comparative test validations are made on an experimental set-up designed to illustrate the benefits of a 9-phase against a reference 3-phase system. Here, the test hardware and implementation are carefully designed to representatively illustrate performance benefits. / Thesis / Doctor of Philosophy (PhD)

Isolated inverters feature the freedom in voltage step-up/down, electrical safety, and modularity. Among them, pseudo-dc-link inverters have the advantage of high efficiency due to their single-stage structure. Traditionally, pseudo-dc-link inverters are based on pulse-width-modulated converters, which suffer from hard switching, the need for auxiliary components, and/or high current stresses. Meanwhile, the series resonant converter has been prevalent in past decades due to its simplicity and high efficiency. Therefore, it is intriguing to design a single-stage inverter based on a series resonant converter. However, there are limited papers regarding such an inverter topology. To figure out the reason, basic modulation methods proposed or implied in the literature are summarized and evaluated through circuit simulation software. It turns out each basic modulation method has at least one critical drawback in modulation range, hard switching, and/or high current stresses. Given the deficiencies in the basic modulation methods, a hybrid modulation method is proposed here. The proposed method combines variable-frequency modulation in the high-output region and short pulse-density modulation in the low-output region. In this way, all the aforementioned critical drawbacks can be greatly alleviated. The hybrid modulation method is compared to the basic modulation methods based on three design metrics: the rms value of the resonant current, the magnetic flux of the transformer, and the turn-off current. By these design metrics that directly related to power losses, the benefit of the proposed method in terms of efficiency can be explained. Moreover, a power loss model is also established to provide more insights into the inverter's efficiency performance. It helps demonstrate how the selection of resonant tank and other factors affects the power loss distribution. Also, an inverter design procedure is introduced and a prototype is built to verify the proposed modulation method. The results show that the switching losses, especially the turn-on loss, can be well suppressed, and the losses in other passive components are well restrained. This implies the proposed method is suitable for high-frequency applications. Other than efficiency, output waveform quality is also important for an inverter. However, the changing plant model makes the controller design difficult. Therefore, a third-order model established by other researchers has been adopted to identify the pole locations. In addition, a gain-varying method is proposed for the compensator to reduce the gain variance caused by different operating conditions. The experimental results show that without the gain-varying method, the inverter may have issues in slow tracking and/or instability. Finally, in some scenarios, the inverter based on a series resonant converter can be regarded as a module. A multi-modular inverter can be formed by connecting the modules in an input-parallel-output-series configuration. In this case, a technique termed sequential waveform synthesis can be applied. The proposed technique can extend the region of variable-frequency modulation and shorten the region of short pulse-density modulation. This is beneficial to efficiency based on an analysis. With more than a certain amount of modules connected, the short pulse-density modulation can even be waived, which means the multi-modular inverter can be free from turn-on loss. In summary, this dissertation focuses on developing modulation methods for inverters based on the series resonant converter. Soft-switching feature and high efficiency are the two top priorities. The analytic and experimental results are provided based on standalone applications. / Doctor of Philosophy / Inverters are an important part of a modern electric power system, as they convert dc electric power into ac electric power. In some applications, inverters with electrical insulation (isolated inverters) are preferred due to the need for engineering freedom, safety, and other reasons. However, each conventional isolated inverter has some of the following drawbacks: hard-switching in semiconductor devices, high circulating current, poor transformer utilization, and high complexity. These drawbacks limit the efficiency and compactness of an inverter system, making the system less attractive to practical applications. An inverter based on a series resonant converter seems to be a solution because the series resonant converter is known for being simple and highly-efficient. However, there has yet to be a proper modulation method for it. Therefore, the main contribution of this dissertation is to propose a hybrid modulation method. With the proposed method, the inverter can operate with high efficiency. Furthermore, the hard-switching can be well suppressed, which means a high-frequency, compact design is possible. Besides the theory of the proposed method, this dissertation also includes a power loss model, a hardware design procedure, and analytic comparisons with other methods. In addition, a digital approach to control the inverter is proposed. Without it, the output voltage waveform may be highly distorted. Finally, another sequential control strategy is proposed in this dissertation for an integrated system. The integrated system is composed of multiple inverters based on a series resonant converter. With the sequential control strategy, the overall output waveform quality of the integrated system can be improved.

AC/DC adapters mostly employ two-stage topology: Power Factor Correction (PFC) pre-regulation stage followed by an isolated DC/DC converter stage. Low power AC/DC adapters require a small size to be competitive. Among their components, the bulk DC-link capacitor is one of the largest because it should keep the output voltage with low ripple. Also, the size of this capacitor is penalized due to the universal line voltage application. Synchronization through employing leading edge modulation for the first PFC stage and trailing edge modulation for the second DC/DC converter stage can significantly reduce the ripple current and ripple voltage of the DC-link capacitor. Thus, a smaller DC-link capacitance can be used, lowering the cost and size of the AC/DC adapter. Benefits of the synchronous switching scheme were already demonstrated experimentally. However, no mathematical analysis was presented. In this thesis, detailed mathematical analyses in per-unit quantity are given to facilitate the calculation of the DC-link capacitor ripple current reduction with Leading/Trailing Edge Modulation strategies. One of the limitations of leading/trailing edge modulation is that the switching frequencies of the two stages need to be equal to achieve the best reduction of the DC-link capacitor ripple current. The DC-link capacitor ripple current will become larger if the switching frequency of the DC/DC converter is larger than that of the PFC pre-regulator, which blocks us to employ higher frequency for isolated DC/DC converter to reduce its transformer size. This thesis proposed a new Leading/Trailing Edge Modulation strategy to further reduce the DC-link bulk capacitor ripple current when switching frequency of DC/DC converter stage is twice the switching frequency of PFC stage. This proposed pulse width modulation scheme was verified by simulation. Experimental results obtained through digital control based on FPGA are also presented in this thesis.