Rozprawy doktorskie na temat „HIGH-POWER APPLICATIONS”

The main contribution of this project was to investigate power electronics technology in designing and developing high frequency high power converters for industrial applications. Therefore, the research was conducted at two levels; first at system level which mainly encapsulated the circuit topology and control scheme and second at application level which involves with real-world applications. Pursuing these objectives, varied topologies have been developed and proposed within this research. The main aim was to resolving solid-state switches limited power rating and operating speed while increasing the system flexibility considering the application characteristics. The developed new power converter configurations were applied to pulsed power and high power ultrasound applications for experimental validation.

This thesis introduces a new concept in 3D RF MEMS switches intended for power applications. The novel switch architecture employs electrothermal hydraulic microactuators to provide mechanical actuation and 3D out-of-plane silicon cantilevers that have both spring action and latching mechanisms. This facilitates an OFF-state gap separation distance of -200 μLim between ohmic contacts, without the need for any hold power. Having simple assembly, many of the inherent problems associated with the more traditional suspension bridge and cantilever type beam architectures can be overcome. SPST switches have been developed. With the first revised switch, a novel trench structure was introduced, but the device failed mechanically due to excessive lever stiffness. High RF insertion losses were also found, due to an unwanted oxide layer directly underneath the CPW feed lines. With the second revision in which two types of lever structure were devised, mechanically working devices were achieved. The high losses found previously were significantly reduced due to a revised fabrication process to remove an unwanted oxide layer. Although a superior OFF-state isolation characteristic was achieved, unpredicted ripples were present in the ON-state, causing high insertion loss. It was found that the higher conductivity (than the manufacture's specifications) of the silicon wafer used for the cantilevers caused the ripple by carrying out circuit and electromagnetic modelling. Both the design and fabrication process have been improved through the investigation on failure mechanisms. From the final experiment, the measured ON-state insertion loss and retur loss are less than 0.4 dB and greater than 15 dB up to 12 GHz, respectively, while OFF-state isolation is better than 30 dB up to 12 GHz. The switch works well in both hot and cold-switching modes with 4.6 W of RF power at 10 GHz, without any signs of degradation to the ohmic contacts.

Fiber lasers can be considered a revolutionary technology in the laser field thanks to their unique properties, such as high efficiency, simplicity, compactness and robustness. These features have allowed in the last ten years their outstanding growth both in scientific and industrial applications, eroding the market share of traditional laser sources like solid-state and gas lasers. Fiber lasers power scaling to the kilowatt range is now well established and, thanks to the fiber confinement, excellent output beam quality can be obtained, with a remarkable benefit for applications. Today, high power laser sources are based on ytterbium doped, large mode area fibers because ytterbium can be efficiently pumped in the range 915nm − 975nm (where pump sources are widely available), generating laser action at 1060nm − 1090nm. With this configuration, impressive power scaling has been demonstrated in the last few years. This Ph.D. thesis has been focused on the design and development of high power fiber lasers for a wide range of industrial applications, like cutting, wending, drilling and micro-machining. Both continuous and pulse wave fiber lasers have been demonstrated and particular attention has been devoted to the development of critical technological de-vices like fused fibers combiner, strategic components either for pump light coupling into the laser active fiber (pump combiner) and for power scaling through the beam combining of several fibers lasers (signal combiner). Ytterbium doped fiber lasers have been developed during the Ph.D activity and, in particular, after a theoretical analysis devoted to the modeling of fiber laser cavities and amplifiers, a continuous wave fiber laser and two pulsed laser systems have been demonstrated. The CW fiber laser has been developed with a modular approach: 7 laser modules, capable of emitting hundreds of Watts each, have been coupled together thanks to a fused fiber combiner. A multi-kilowatts output power has been demonstrated. The photo-darkening effect in the active fiber of the laser modules has also been exper-imentally investigated. The pulsed architectures are instead a Q-switched MOPA and a Seed MOPA fiber lasers. The first system is based on a fiber laser oscillator operating in the Q-switching regime, followed by a power amplifier. This laser is capable of delivering 100ns pulses with 10W average power (2kW maximum peak power). The Seed MOPA consists instead of a current modulated laser diode followed by two amplification stages; 2W output average power with adjustable pulse widths from 10 to 100ns has been demonstrated. In the last part of the activity, a preliminary version of a thulium doped fiber laser emitting at about 2000nm (i.e. in the so-called eye-safe region) has been developed. The laser is a Seed MOPA system that has been tested in cw regime but in the near future the pulsed behavior will be investigated.

The use of power electronics in the electrical propulsion systems leads to the optimal and efficient utilization of the traction motors and the energy sources (batteries and/or fuel cells) through the recourse to suitable power converters and their proper control. Power electronics is also used for implementing the multiple conversions of the energy delivered by the sources to feed the various loads, most of them requiring different waveforms of voltage (ac or dc) and/or different levels of voltage. This work focuses on the solutions aimed at improving the efficiency of power converters for vehicular applications, which is of great importance because of the limited amount of energy that can be stored in the electric vehicles. The study takes into consideration both the traction applications and the battery charging applications whether it is done by conductive means or by wireless power transfer (WPT) systems. The improvement in traction drive efficiency results in an increment of the drivetrain efficiency of the vehicle, leading to an extension in the driving range, while the employment of efficient power converters is required to charge batteries with increasingly large capacity. The losses of power devices are even more significant when they operate at high frequencies to compact the size of the filter elements and/or the transformers. The losses of power devices can be minimized by making the commutation soft or by replacing the conventional devices with the new generation devices based on wide bandgap (WBG) semiconductor materials. In this work, the properties of the WBG semiconductor materials are illustrated and the operation of the devices based on these materials are analyzed to grasp better their characteristics and performance. The losses of individual devices (i.e. diode, IGBT, MOSFET) as well as the operation of power converters for various applications are examined in detail. To evaluate the performance of the SiC devices in electric vehicle applications, an AC traction drive for the propulsion of a typical compact C-class electric car has been considered. Two versions of the inverter have been investigated, one built up with conventional Si IGBTs and the other one with SiC MOSFETs, and the losses in the semiconductor devices of the two versions have been found along the standard New European Driving Cycle (NEDC). By comparing the results, it is emerged that the usage of the SiC MOSFETs reduces the losses in the traction inverter of about 5%, yielding an equal increase in the car range. To complete the study, calculation of the efficiency has been extended to the whole traction drive, including the traction motor and the gear. Afterwards, a power factor correction (PFC) circuit, which is commonly used to mitigate the distortion in line current, has been studied. The study is started by considering the basic and the interleaved PFC configurations and by defining their circuit parameters. After selecting the interleaved configuration, the magnitude of voltages and currents in the PFC rectifier has been determined and the values obtained have been verified by a power circuit simulation software. The digital signal processing (DSP) has been also studied as it is used for the control operation of the PFC. At last, a prototype of PFC rectifier with interleaved configuration is designed. The design process and the specification of the components are described in brief. A prototype of synchronous rectifier (SR) is designed for the output stage of a WPT system. With respect to conventional rectifiers, in SRs the diodes are replaced by MOSFETs with their antiparallel diodes. MOSFETs are bidirectional devices that conduct with a low voltage drop. During the dead time, the diodes in antiparallel to the MOSFETs are conducting. At the end of dead-time, signals are applied at the MOSFET gates that make conducting all along the remaining period, thus reducing the conduction losses. The dead-time length is optimized by using fast switching devices based on SiC semiconductor materials. The prototype is designed and tested at the line frequency. The experimental results obtained from the prototype corroborate both the analytical results and the simulation results. As SR exhibits is working with high efficiency at the line frequency, it is expected that at the higher operating frequencies of the WPT systems, the performance of SR will be even better. A DC-DC isolated power converters used to setup the battery charger through wire system are studied. Two topologies of DC-DC converters, i.e. Dual Active Bridge (DAB) and Single Active Bridge (SAB) converters, are considered. For both the topologies operation are described at steady state. For SAB converter, two possible modes of operation are examined: discontinuous current conduction (DCM) and continuous current conduction (CCM). Soft-switching operation of both SAB and DAB converters, obtained by the insertion of auxiliary capacitors, is analyzed. Moreover, the soft-switching operating zone for the two converters are found as a function of the their output voltages and currents. Finally, the comparative analysis of soft-switching operation of SAB versus DAB converter is presented. The thesis work has been carried out at the Laboratory of “Electric Systems for Automation and Automotive” headed by Prof. Giuseppe Buja. The laboratory belongs to the Department of Industrial Engineering of the University of Padova, Italy.
L’utilizzo dell’elettronica di potenza nei sistemi di propulsione elettrica porta all'utilizzo ottimale ed efficiente dei motori di trazione e delle sorgenti di energia (batterie e/o celle a combustibile) attraverso il ricorso a convertitori statici e al loro controllo. L’elettronica di potenza è utilizzata anche per implementare più conversioni dell’energia fornita dalle sorgenti per alimentare i vari carichi, la maggior parte delle quali richiede forme d'onda di tensione diverse (AC o DC) e/o diversi livelli di tensione. Questo elaborato si concentra sulle soluzioni volte a migliorare l'efficienza dei convertitori di potenza per applicazioni veicolari, tema che è di grande interesse per la limitata quantità di energia accumulabile a bordo. Sono prese in considerazione sia le applicazioni di trazione che le applicazioni di ricarica degli accumulatori realizzate con mezzi conduttivi o con i sistemi di trasferimento di potenza senza fili (WPT). Il miglioramento dell’efficienza degli azionamenti di trazione produce un incremento dell'efficienza dell’intero powertrain del veicolo, che si traduce in un incremento dell’autonomia del veicolo, mentre l’impiego di convertitori di potenza efficienti si rende necessario per la ricarica di batterie con capacità sempre maggiori. Le perdite dei dispositivi di potenza sono ancora più significative quando operano ad alte frequenze di lavoro per compattare le dimensioni degli elementi filtranti e/o dei trasformatori. Le perdite nei dispositivi di potenza possono essere minimizzate rendendo la commutazione soft o sostituendo i dispositivi convenzionali con i dispositivi di nuova generazione basati su materiali semiconduttori con ampia banda proibita (WBG). Nell’elaborato, sono illustrate le proprietà dei materiali semiconduttori WBG e si analizza il funzionamento dei dispositivi basati su questi materiali per comprendere le loro caratteristiche e prestazioni. Le perdite di singoli dispositivi (come diodi, IGBT, MOSFET) nonché il funzionamento di convertitori di potenza per varie applicazioni sono esaminati in dettaglio. Per valutare le prestazioni dei dispositivi SiC quando vengano impiegati nei veicoli elettrici, è preso in esame un azionamento di trazione in AC impiegato per la propulsione di una tipica automobile elettrica di classe C. Due versioni di invertitore sono esaminate, una costruita con convenzionali Si IGBT e l'altra con MOSFET SiC, ed è calcolata la potenza persa nei dispositivi a semiconduttore delle due versioni di invertitore mentre l’automobile percorre il ciclo normalizzato di guida europeo (NEDC). Dal confronto dei risultati è emerso che l'utilizzo dei MOSFET SiC riduce le perdite nel convertitore di trazione di circa 5%, ottenendo un uguale incremento dell’autonomia dell’automobile. Per completare lo studio, si è successivamente esteso il calcolo dell’efficienza all’intero azionamento di trazione, comprendente il motore e il riduttore. Si è quindi studiato un raddrizzatore con circuito di correzione del fattore di potenza (PFC), utilizzato per ridurre la distorsione di corrente in linea. Lo studio è iniziato considerando sia la configurazione di base che quella interleaved e individuando i parametri circuitali. Dopo aver scelto la configurazione interleaved, sono determinate le ampiezze delle tensioni e delle correnti presenti nel raddrizzatore PFC e i valori ottenuti sono verificati mediante un software di simulazione di circuiti di potenza. E’ anche studiato un dispositivo per l'elaborazione digitale dei segnali (DSP) nel quale implementare il controllo del raddrizzatore PFC. Infine è progettato un prototipo di raddrizzatore PFC con configurazione interleaved. Il processo di progettazione e le specifiche dei componenti sono brevemente descritti. Un prototipo di rettificatore sincrono (SR) è stato sviluppato per lo stadio di uscita di un sistema WPT. In confronto con i raddrizzatori convenzionali, in un SR i diodi sono sostituiti da MOSFET con diodi in antiparallelo. I MOSFET sono dispositivi bidirezionali caratterizzati da una bassa caduta di tensione e dalla direzionalità nel condurre la corrente. Durante il tempo morto, entrano in conduzione i diodi in antiparallelo ai MOSFET. Al termine del tempo morto, ai MOSFET sono applicati segnali di comando che li portano in conduzione per tutta la restante parte del semiperiodo, riducendo così le perdite di conduzione. La durata del tempo morto è ottimizzata utilizzando dispositivi di commutazione veloci basati su materiali semiconduttori SiC. Il prototipo è stato progettato e sperimentato alla frequenza di rete. I risultati sperimentali ottenuti hanno confermato sia i risultati analitici che le simulazioni. L’elevato valore di efficienza ottenuto sul prototipo operante alla frequenza di rete fanno prevedere che il suo impiego alle alte frequenze operative dei sistemi WPT possa dare risultati ancora migliori. Si sono studiati i convertitori isolati di potenza DC-DC impiegati nei caricabatteria di tipo conduttivo per veicoli elettrici. Si sono prese in considerazione due topologie di convertitori DC-DC, il convertitore con doppio ponte attivo (DAB) e quello con un unico ponte attivo (SAB). Per entrambe le topologie è analizzato il funzionamento in condizioni di regime. Per il convertitore SAB sono esaminate due possibili modalità di funzionamento: conduzione discontinua di corrente (DCM) e conduzione di corrente continua (CCM). Si è analizzato il funzionamento in soft-switching, ottenuto con l’inserzione di condensatori ausiliari, sia del convertitore SAB che di quello DAB. E’ individuata la zona di funzionamento in soft-switching per i due convertitori in funzione delle tensioni e delle correnti di uscita. Infine, è stata eseguita un’analisi comparativa del funzionamento in soft-switching dei due convertitori. Il lavoro di tesi è stato realizzato presso il Laboratorio di "Sistemi Elettrici per l'Automazione e Automotive" diretto dal Prof. Giuseppe Buja. Il laboratorio fa parte del Dipartimento di Ingegneria Industriale dell'Università degli Studi di Padova, Italia.

The trend of electrification in transportation applications has led to the fast development of high-power-density power electronics converters. High-switching-frequency and high-temperature operations are the two key factors towards this target. Both requirements, however, are challenging the fundamental limit of silicon (Si) based devices. The emerging wide-bandgap, silicon carbide (SiC) power devices have become the promising solution to meet these requirements. With these advanced devices, the technology barrier has now moved to the compatible integration technology that can make the best of device capabilities in high-power-density converters. Many challenges are present, and some of the most important issues are explored in this dissertation. First of all, the high-temperature performances of the commercial SiC MOSFET are evaluated extensively up to 200 degree C. The static and switching characterizations show that the device has superior electrical performances under elevated temperatures. Meanwhile, the gate oxide stability of the device - a known issue to SiC MOSFETs in general - is also evaluated through both high-temperature gate biasing and gate switching tests. Device degradations are observed from these tests, and a design trade-off between the performance and reliability of the SiC MOSFET is concluded. To understand the interactions between devices and circuit parasitics, an experimental parametric study is performed to investigate the influences of stray inductances on the MOSFETs switching waveforms. A small-signal model is then developed to explain the parasitic ringing in the frequency domain. From this angle, the ringing mechanism can be understood more easily and deeply. With the use of this model, the effects of DC decoupling capacitors in suppressing the ringing can be further explained in a more straightforward way than the traditional time-domain analysis. A rule of thumb regarding the capacitance selection is also derived. A Power Electronics Building Block (PEBB) module is then developed with discrete SiC MOSFETs. Integrating the power stage together with the peripheral functions such as gate drive and protection, the PEBB concept allows the converter to be built quickly and reliably by simply connecting several PEBB modules. The high-speed gate drive and power stage layout designs are presented to enable fast and safe switching of the SiC MOSFET. Based on the PEBB platform, the state-of-the-art Si and SiC power MOSFETs are also compared in the device characteristics, temperature influences, and loss distributions in a high-frequency converter, so that special design considerations can be concluded for the SiC MOSFET. Towards high-temperature, high-frequency and high-power operations, integrated wire-bond phase-leg modules are also developed with SiC MOSFET bare dice. High-temperature packaging materials are carefully selected based on an extensive literature survey. The design considerations of improved substrate layout, laminated bus bars, and embedded decoupling capacitors are all discussed in detail, and are verified through a modeling and simulation approach in the design stage. The 200 degree C, 100 kHz continuous operation is demonstrated on the fabricated module. Through the comparison with a commercial SiC phase-leg module designed in the traditional way, it is also shown that the design considerations proposed in this work allow the SiC devices in the wire-bond structure to be switched twice as fast with only one-third of the parasitic ringing. To further push the performance of SiC power modules, a novel hybrid packaging technology is developed which combines the small parasitics and footprint of a planar module with the easy fabrication of a wire-bond module. The original concept is demonstrated on a high-temperature rectifier module with SiC JFET. A modified structure is then proposed to further improve design flexibility and simplify module fabrication. The SiC MOSFET phase-leg module built in this structure successfully reaches the switching speed limit of the device almost without any parasitic ringing. Finally, a new switching loop snubber circuit is proposed to damp the parasitic ringing through magnetic coupling without affecting either conduction or switching losses of the device. The concept is analyzed theoretically and verified experimentally. The initial integration of such a circuit into the power module is presented, and possible improvements are proposed.
Ph. D.

Three high speed thin film patterning applications have been investigated using a high average power, high peak power laser system. Throughout this work the spatial intensity profile of the laser output was tailored to produce more efficient results. The first application involved the development of rapid laser patterning of an indium tin oxide layer on a glass substrate in order to generate transparent electrodes for a flat panel display. This work showed that the stitch line that occurs in-between adjacent laser pulses was formed by redeposition of material via the plume generated by the second, slightly overlapping pulse which is deposited within the region of overlap, an area which has an increased surface temperature at that time. The second application, laser edge deletion for thin film solar photo-voltaic panels, was an investigation of whether dual wavelength processing was able to avoid introducing micro-cracks into the soda-lime glass substrate. The third application was an examination of high speed removal of an aluminium coating from a stainless steel substrate which demonstrated that the layer could be adequately removed but required a series of highly overlapped pulses.

The present industrial practice of ac to dc rectification uses diode or thyristor bridges which are harmonic polluters and poor power factor sources. The research of this thesis is directed towards exploiting fast, gate-turn-off semiconductor switching devices and PWM techniques in order to develop a new generation of rectifiers with nearly sinusoidal current waveforms operating at unity or even leading power factor. The scope of the investigation is restricted to the Boost rectifier (as opposed to Buck) which has bilateral power transfer capability through bidirectional current flow in the dc link.
The stand-alone, Boost Type PWM Voltage Regulated Rectifier was originally conceived as being Direct Current Controlled. The work of the thesis advances the control methodology by replacing the inner hysteresis current feedback loop by Indirect Current Control, which uses the standard sinusoidal PWM technique. In the process, the cost of two expensive high quality current transducers is avoided. Furthermore, Sinusoidal PWM has more predictable characteristics harmonics for filter design on harmonic elimination purposes.
The thesis addresses the problem of upscaling the voltage and current ratings of the rectifiers. Many semiconductor switching devices have inherent difficulties in voltage and current sharing when connected in series and/or in parallel. These difficulties are avoided by connecting rectifier modules in series and/or in parallel. Different topologies for both series and parallel connections have been analyzed mathematically. Digital simulations and experiments have confirmed the analyses.
The research was carried out by building 2 kW size laboratory models which were subjected to demanding experimental tests. Experimentally justified mathematical models have been developed and have successfully been used in predicting stability boundaries and in the dynamic compensation of feedback control.

In this thesis I report experimental studies toward developing versatile, compact, and reliable ultrafast sources in the 1.5 micron wavelength region, and their power scalability. The first part of the thesis reports on the development of a stable all-fiberized wavelength-tunable frequency-shifted feedback (FSF) picosecond laser. Stability of the passive mode-locking mechanism is achieved by combining the effects of nonlinear polarization evolution and a frequency shifting mechanism carried out by an acousto-optic modulator. The novel configuration generates pulses in the range of ~ 34 to 66 ps, depending on the value of the frequency shift applied in the cavity. The cavity allows for continuous wavelength-tuning over 30 nm of the erbium gain bandwidth via a fiberized tunable filter. The stability of the laser cavity allows pulse analysis as a function of different parameters of the laser cavity. Additional extensive numerical analysis, combined with the experimental results, provide novel insights for understanding the dynamics of FSF lasers in the mode-locking regime, which have not been addressed in the literature before. The second part of the thesis reports on the development of a versatile, stable, compact mode-locked fibre laser using nonlinear polarization evolution and a semiconductor saturable absorber mirror(SESAM). The novel cavity can generate pulses with widths between ~ 2.7 and 11 ps over 25 nm of erbium gain bandwidth. This is achieved by integrating in the cavity state-of-the-art optical filters. The performance of this laser is compared to that of the FSF laser in terms of pulse energy, amplitude noise, timing jitter and power scaling. The third part of the thesis reports on the direct amplification of a mode-locked ~ 10 picosecond bandwidth-tunable laser source that I made by means of large-mode area (LMA) erbium-ytterbium co-doped and erbium-doped fibres. While cladding pumping amplification schemes have become a standard option for pulse amplification in the 1.5 μm region, core-pumped amplification in LMA erbium-doped fibres has been less studied. In this thesis, in addition to the amplification of picosecond pulses in an erbiumytterbium co-doped fibre; I present a novel scheme that uses a hybrid co-propagation core-pumped (1480 nm) and counter-propagation cladding-pumped (980 nm) scheme, which compensates for the low cladding absorption at 980 nm of the erbium-doped fibre. Picosecond pulses are amplified up to 1.5 W with peak powers exceeding 11 kW. The last part of this thesis reports on the study of a stable operating regime found in passive mode-locked lasers called noise-like pulses, which can generate broadband spectra directly from the main oscillator. Here, I report the record of a 135-nm bandwidth lineraly polarized noise-like pulse generation in an erbium-doped fibre laser by exploiting the birefringence of the cavity and the Raman gain of a highly nonlinear fibre (HNLF). Noise characterization of the source is carried out and compared to other commercial broadband sources in order to see its applicability in areas such as optical coherence tomography.

The use of high-powered electrical energy systems requires an efficient and capable means to move electrical energy from one location to another while reducing energy losses. This paper describes the design and construction process of a high-powered busbar system that is to be implemented in pulsed-power applications. In order to obtain a robust system capable of handling in excess of 25kJ, both mechanical and electrical analyses were performed to verify a capable design. The following methodology describes how the Lorentz force was balanced with mechanical forces during the design process and then validated after construction was completed using the fundamental Maxwell equations and computer simulations. Main focuses include handling of EMF, high current density concentrations, and overall mechanical stability of the system and how these effects determine the physical design and implementation. In the end, a repeatable methodology is presented in the form of a design process that can be implemented in any system given the design criteria.
Master of Science

Scientists at European Organization for Nuclear Research (CERN) are currently conducting feasibility studies for the Compact linear collider (CLIC); their proposed next experimental setup for gathering information on the fundamental particles of matter. This experiment will involve the simultaneous pulsing of 1300 klystron modulators to produce a 140us, 39GW pulse with a 50 Hz repetition rate. This proposal presents many demands for the connected power system as an effort is made to "hide" this pulse from the local distribution network - instead drawing only the constant average power of approximately 300MW. This challenge is considered in this work. In order to understand the optimal approach both the power system architectures and power electronics interfaces must be considered simultaneously. An approach to the optimisation of the power system architecture is described in this thesis. It is clear from this exercise that the optimum power converter topology for the interface between the electricity distribution network and the klystron modulators is the Modular Multilevel Converter (MMC). This converter is mainly used in modern HVDC transmission circuits as a result of its high efficiency and ability to produce high quality AC waveforms. Pulsing of the klystron modulators does however create further challenges for the inner control loops of an MMC. The placement of the pulse can create imbalances in the DC capacitors of the MMC submodules which may result in tripping of the converter if not corrected. This thesis proposes three arm balancing solutions to be applied together with the decoupled AC and DC side controller designed for the specified application. These proposed solutions to the aforementioned problems are successfully validated using simulation work in PLECS and using data from a laboratory scale prototype of one of the MMC interface power converters.

The continual development of high-power-density power electronic converters is driven particularly by modern transportation applications like electrical vehicles and more electric aircraft where the space and carrier capability is limited. However, there are several challenges related to transportation applications such as fault tolerance for safety concern, high temperature operation in extreme environments and more strict electromagnetic compatibility requirement. These challenges will increase difficulties for more electrical system adoption in the transportation applications. In this dissertation, comprehensive methodologies including more efficient energy storage solution, better power electronics devices capability, better packaging performance and more compact EMI filter design are analyzed and proposed for the goal of high power density converter design in transportation applications.
Ph. D.

The Queensland University of Technology (QUT) allows the presentation of theses for the Degree of Doctor of Philosophy in the format of published or submitted papers, where such papers have been published, accepted or submitted during the period of candidature. This thesis is composed of ten published /submitted papers and book chapters of which nine have been published and one is under review. This project is financially supported by an Australian Research Council (ARC) Discovery Grant with the aim of investigating multilevel topologies for high quality and high power applications, with specific emphasis on renewable energy systems. The rapid evolution of renewable energy within the last several years has resulted in the design of efficient power converters suitable for medium and high-power applications such as wind turbine and photovoltaic (PV) systems. Today, the industrial trend is moving away from heavy and bulky passive components to power converter systems that use more and more semiconductor elements controlled by powerful processor systems. However, it is hard to connect the traditional converters to the high and medium voltage grids, as a single power switch cannot stand at high voltage. For these reasons, a new family of multilevel inverters has appeared as a solution for working with higher voltage levels. Besides this important feature, multilevel converters have the capability to generate stepped waveforms. Consequently, in comparison with conventional two-level inverters, they present lower switching losses, lower voltage stress across loads, lower electromagnetic interference (EMI) and higher quality output waveforms. These properties enable the connection of renewable energy sources directly to the grid without using expensive, bulky, heavy line transformers. Additionally, they minimize the size of the passive filter and increase the durability of electrical devices. However, multilevel converters have only been utilised in very particular applications, mainly due to the structural limitations, high cost and complexity of the multilevel converter system and control. New developments in the fields of power semiconductor switches and processors will favor the multilevel converters for many other fields of application. The main application for the multilevel converter presented in this work is the front-end power converter in renewable energy systems. Diode-clamped and cascade converters are the most common type of multilevel converters widely used in different renewable energy system applications. However, some drawbacks – such as capacitor voltage imbalance, number of components, and complexity of the control system – still exist, and these are investigated in the framework of this thesis. Various simulations using software simulation tools are undertaken and are used to study different cases. The feasibility of the developments is underlined with a series of experimental results. This thesis is divided into two main sections. The first section focuses on solving the capacitor voltage imbalance for a wide range of applications, and on decreasing the complexity of the control strategy on the inverter side. The idea of using sharing switches at the output structure of the DC-DC front-end converters is proposed to balance the series DC link capacitors. A new family of multioutput DC-DC converters is proposed for renewable energy systems connected to the DC link voltage of diode-clamped converters. The main objective of this type of converter is the sharing of the total output voltage into several series voltage levels using sharing switches. This solves the problems associated with capacitor voltage imbalance in diode-clamped multilevel converters. These converters adjust the variable and unregulated DC voltage generated by renewable energy systems (such as PV) to the desirable series multiple voltage levels at the inverter DC side. A multi-output boost (MOB) converter, with one inductor and series output voltage, is presented. This converter is suitable for renewable energy systems based on diode-clamped converters because it boosts the low output voltage and provides the series capacitor at the output side. A simple control strategy using cross voltage control with internal current loop is presented to obtain the desired voltage levels at the output voltage. The proposed topology and control strategy are validated by simulation and hardware results. Using the idea of voltage sharing switches, the circuit structure of different topologies of multi-output DC-DC converters – or multi-output voltage sharing (MOVS) converters – have been proposed. In order to verify the feasibility of this topology and its application, steady state and dynamic analyses have been carried out. Simulation and experiments using the proposed control strategy have verified the mathematical analysis. The second part of this thesis addresses the second problem of multilevel converters: the need to improve their quality with minimum cost and complexity. This is related to utilising asymmetrical multilevel topologies instead of conventional multilevel converters; this can increase the quality of output waveforms with a minimum number of components. It also allows for a reduction in the cost and complexity of systems while maintaining the same output quality, or for an increase in the quality while maintaining the same cost and complexity. Therefore, the asymmetrical configuration for two common types of multilevel converters – diode-clamped and cascade converters – is investigated. Also, as well as addressing the maximisation of the output voltage resolution, some technical issues – such as adjacent switching vectors – should be taken into account in asymmetrical multilevel configurations to keep the total harmonic distortion (THD) and switching losses to a minimum. Thus, the asymmetrical diode-clamped converter is proposed. An appropriate asymmetrical DC link arrangement is presented for four-level diode-clamped converters by keeping adjacent switching vectors. In this way, five-level inverter performance is achieved for the same level of complexity of the four-level inverter. Dealing with the capacitor voltage imbalance problem in asymmetrical diodeclamped converters has inspired the proposal for two different DC-DC topologies with a suitable control strategy. A Triple-Output Boost (TOB) converter and a Boost 3-Output Voltage Sharing (Boost-3OVS) converter connected to the four-level diode-clamped converter are proposed to arrange the proposed asymmetrical DC link for the high modulation indices and unity power factor. Cascade converters have shown their abilities and strengths in medium and high power applications. Using asymmetrical H-bridge inverters, more voltage levels can be generated in output voltage with the same number of components as the symmetrical converters. The concept of cascading multilevel H-bridge cells is used to propose a fifteen-level cascade inverter using a four-level H-bridge symmetrical diode-clamped converter, cascaded with classical two-level Hbridge inverters. A DC voltage ratio of cells is presented to obtain maximum voltage levels on output voltage, with adjacent switching vectors between all possible voltage levels; this can minimize the switching losses. This structure can save five isolated DC sources and twelve switches in comparison to conventional cascade converters with series two-level H bridge inverters. To increase the quality in presented hybrid topology with minimum number of components, a new cascade inverter is verified by cascading an asymmetrical four-level H-bridge diode-clamped inverter. An inverter with nineteen-level performance was achieved. This synthesizes more voltage levels with lower voltage and current THD, rather than using a symmetrical diode-clamped inverter with the same configuration and equivalent number of power components. Two different predictive current control methods for the switching states selection are proposed to minimise either losses or THD of voltage in hybrid converters. High voltage spikes at switching time in experimental results and investigation of a diode-clamped inverter structure raised another problem associated with high-level high voltage multilevel converters. Power switching components with fast switching, combined with hard switched-converters, produce high di/dt during turn off time. Thus, stray inductance of interconnections becomes an important issue and raises overvoltage and EMI issues correlated to the number of components. Planar busbar is a good candidate to reduce interconnection inductance in high power inverters compared with cables. The effect of different transient current loops on busbar physical structure of the high-voltage highlevel diode-clamped converters is highlighted. Design considerations of proper planar busbar are also presented to optimise the overall design of diode-clamped converters.

The demand for electric vehicles has expanded rapidly for both industrial and transportation applications. In parallel, new battery technologies have been introduced which are capable of deep-discharge and powering electric vehicles for long periods of time. Due to the increasing complexity of charging algorithms, battery chargers are exposed to demanding operating requirements. In battery charger applications, power converters should not only regulate the battery voltage and power over a wide range, all the way from complete discharge up to the charged floating voltage but also respond to the input voltage variation period. It is also important to work at high efficiency and with low switching noise and charging current ripple. This work studies different problems regarding DC-DC power converters with wide voltage regulation as battery chargers and investigates the application of novel high-order resonant power converters (fourth and fifth-order) and modulation strategies at various power levels. As a solution for high power applications, this work first introduces a modified full bridge LLC resonant power converter driven by both variable frequency and phase shift modulation. The proposed modulation strategy along with the modified resonant circuit exhibits excellent performance for a 3kW resonant power converter, without taking advantage of burst mode strategy. The second part of this work introduces a novel fifth-order L3C2 resonant converter for medium power level applications, that can regulate the battery voltage from near zero output voltage, zero output current to maximum output power. A 950W design example demonstrates a wide output voltage regulation with maximum efficiency of 96%. Finally, a fourth order L3C resonant converter is proposed for electric vehicles with roof-top solar photovoltaic panels, which can not only regulate the battery voltage in a wide range but also track the input voltage variation for extracting the maximum available power from the PV panel. All results from this work have been confirmed experimentally, which highlight the exceptional regulation capability of the proposed resonant power converters and modulation techniques.
Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate

In this dissertation, a novel semiconductor mode-locked oscillator which is an extension of eXtreme Chirped Pulse Amplification (XCPA) is investigated. An eXtreme Chirped Pulse Oscillator (XCPO) implemented with a Theta cavity also based on a semiconductor gain is presented for generating more than 30ns frequency-swept pulses with more than 100pJ of pulse energy and 3.6ps compressed pulses directly from the oscillator. The XCPO shows the two distinct characteristics which are the scalability of the output energy and the mode-locked spectrum with respect to repetition rate. The laser cavity design allows for low repetition rate operation <100MHz. The cavity significantly reduces nonlinear carrier dynamics, integrated self phase modulation (SPM), and fast gain recovery in a Semiconductor optical Amplifier (SOA). Secondly, a functional device, called a Grating Coupled Surface Emitting Laser (GCSEL) is investigated. For the first time, passive and hybrid mode-locking of a GCSEL is achieved by using saturable absorption in the passive section of GCSEL. To verify the present limitation of the GCSEL for passive and hybrid mode-locking, a dispersion matched cavity is explored. In addition, a Grating Coupled surface emitting Semiconductor Optical Amplifier (GCSOA) is also investigated to achieve high energy pulse. An energy extraction experiment for GCSOA using stretched pulses generated from the colliding pulse semiconductor mode-locked laser via a chirped fiber bragg grating, which exploits the XCPA advantages is also demonstrated. Finally, passive optical cavity amplification using an enhancement cavity is presented. In order to achieve the interferometric stability, the Hänsch-Couillaud Method is employed to stabilize the passive optical cavity. The astigmatism-free optical cavity employing an acousto-optic modulator (AOM) is designed and demonstrated. In the passive optical cavity, a 7.2 of amplification factor is achieved with a 50 KHz dumping rate.
Ph.D.
Optics and Photonics
Optics and Photonics
Optics PhD

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.
Includes bibliographical references (p. 217-218).
This thesis presents the development of a microinverter for single-phase photovoltaic applications that is suitable for conversion from low-voltage (25-40V) DC to high voltage AC (e.g. 240VAC,RMS). The circuit topology is based on a full-bridge series resonant inverter, a high-frequency transformer, and a novel half-wave cyclo-converter. The operational characteristics are analyzed, and a multidimensional control technique is utilized to achieve high efficiency, encompassing frequency control and inverter and cyclo-converter phase shift control. An experimental prototype is demonstrated in DC/DC conversion mode for a wide range of output voltages. The proposed control strategy is shown to allow for accurate power delivery with minimal steps taken towards correction. The prototype achieves a CEC averaged efficiency of approximately 95.1%. Guidelines for optimization are presented along with experimental results which validate the method.
by Aleksey Trubitsyn.
S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 202-207).
Converting electricity to mechanical motion is a foundation of modern civilization. A controllable "knob" is often necessary in these electromechanical energy conversion systems to achieve adjustable motion or a process control. An energy-efficient approach to realize this "knob" is through variable-speed drives (VSD), which are power-electronic based converters with associated control operating as an interface between the electrical machine and the electrical source. These drives are not only critical in a wide range of applications including industrial processes, electric propulsion systems, and power generation plants but also becoming increasingly relevant for optimizing energy consumption. For example, a motor without a VSD running at fixed speed can potentially waste 30% to 80% of energy in mechanical throttles located upstream from a compressor or downstream of a pump. In addition to being a controllable knob for energy conversion, these VSDs are configurable to support the electrical source, e.g., the electric grid, through appropriate reactive power support and controllable power factor - a vital feature required for the future electric grid comprising more complex electrical networks. However, merely 13% of global loads in mega-watt class high-power applications are driven by VSDs. At these higher power levels, the VSD design is significantly challenging due to the limited available power-electronic device ratings and allowable switching frequency leading to design trade-offs among size, efficiency, performance, reliability, and cost. This dissertation proposes a switched-doubly-fed machine (switched-DFM) drive that uses a parallel architecture for electromechanical energy conversion to reduce the required power processing capability of the power-electronic converter by two-thirds while operating seamlessly over a wide speed range. Additionally, the proposed architecture provides exciting opportunities for supporting the electric grid with reactive power not only through the VSD but also using the electrical machine. The approach confronts the challenges of high power electromechanical energy conversion from the perspective of electromagnetics, power electronics, circuit designs, embedded computing, and control to push the trade-off boundary for the VSD to be physically small, efficient, reliable, flexible, inexpensive, and electric-grid friendly. The thesis contributions include a design procedure for the proposed switched-DFM drive based on a required drive-torque-speed capability, a control architecture that can achieve seamless performance across the entire speed range from the perspectives of the electrical grid and the mechanical load, multiple transfer-switch circuit topologies enabling uninterrupted on-the-fly reconfiguration of the DFM, steady-state and dynamic performance comparison between different switched-DFM drive topologies, and an exploration of DFM electromagnetic design considerations that suit the proposed architecture. A lab-scale experimental setup that emulates an entire power system from generation to consumption is designed and built to demonstrate seamless, wide-speed range, and four-quadrant operation of the proposed switched-DFM drive. The proposed methodologies open up opportunities to create efficient, cost-effective, and sustainable solutions for high power electromechanical energy conversion systems.
by Arijit Banerjee.
Ph. D.

The majority of the worldwide installed power inverters today are voltage source inverters followed by current source inverters where the concluding decision lies with the performance of the applications besides the usual economic reasons. Recent active development in the current source inverter areas has seen the emerging of various generalized multilevel current source inverter topologies analogous to the existing multilevel voltage source inverter families. To date, the multilevel current source inverter families have been classified principally by the physical appearance of their basic structures and also by the number of current sources employed. The existing multilevel current source inverter topologies are unpopular for present applications due to reasons such as big sizes, high control complexity and low reliability; which circumstances are often associated to massive component counts and multiple requirements of current sources. Therefore, this research has been focused on the single-phase single-source generalized multilevel current source inverter for this apparent advantage; where this thesis proposed a novel generalized multilevel current-source inverter topology with the lowest component utilization while employing just a single current source. In addition, the proposed topology can conveniently achieved dc current balance with a simple low frequency switching strategy for the five- and nine-level current outputs. From comparison analysis, the proposed topology has significantly less number of components employed compared to the nearest topology, which implies low implementation cost. The experimental results verify the characteristics and performances of the proposed topology acquired by computer simulations.

The contents of this work investigate the capability of integrating the PT in applications by invoking the ratio of the throughput power to volume represented by the term: power density. The fundamentals of the PT are introduced in chapter two. In chapter three, the fundamental limitations of the PT's capability of transferring power to the load are studied. There are three major limitations: temperature rise due to losses during operation, electromechanical limits of material, and interactions with output rectifier. The analysis and estimation are then verified by experiments and calculations implemented on three different PT samples fabricated from three different manufacturers. The subject of chapter four is the behavior of the PT's power amplifier. This chapter concentrates on two main amplifier topologies, optimized based on the simplicity of structure and minimization of components (passive and active): class D and class E amplifiers. The operational characteristics of these amplifiers with the PT are then comparison. Methods to track the optimum frequency and discontinuous working mode of the PT are proposed as the approaches to improve the energy transfer of the PT. In chapter five, prototypes of four devices using a PT are developed and introduced as illustrations of the integration of PTs into practical applications: an igniter for high intensity discharge (HID) lamps, high DC voltage power supplies, and electronic ballasts for LEDs, and stand-alone ionizers for food sterilizers. Some concluding statements and ideas for future works are located in the last chapter - chapter six.

This dissertation presents theoretical and experimental results from a research program that was aimed at finding practical ways of transferring energy to various loads, mainly from an inductive energy store fluxed by a primary store such as a capacitor bank. The main obj ecti ves of the work were to investigate and develop high power opening and closing switches, together with the transfer circuits needed to generate the fast (less than lOOns duration) high energy, pulses required in many applications. The study was to include a feasibility study of the use of the Plasma Erosion Opening Switch (PEOS) in such a system. To produce the large fast pulses required, an opening switch is required that: * Carry a current of the order of several kA during the inductor storage time. It should also be able to interrupt this current and to withstand the high voltage it will experience as the current is subsequently transferred to a load. * Conduct for as long as possible (up to one quarter period of the current waveform), to maximise the inductively stored energy which can be transferred to xhe load when the switch opens. * Open to an impedance that is large compared to the load impedance . This ensures that most of the inductively stored energy is transferred to the load. Open sufficiently rapidly to produce the required sharp pulse of voltage. In pulsed-power applications, energy is usually supplied from slow and relatively inexpensive power sources such as a capacitor bank, or an explosive flux-compression generator, which deliver large quantities of energy in the lO-lOO time range. Although no single switch is currently available which has such a long conduction time, together with a nanosecond opening time, the PEOS is a potential candidate. To overcome its short conduction time, while still obtaining an opening time of less than lOOns, the PEOS is used together with an additional slower stage or stages of switching. The key to this method is that each successive switching stage produces a considerably increased voltage. Various different types of switch were investigated and these are described in the thesis. Particular consideration is paid to the performance of the PEOS, as the final conditioning stage. Exploding foils are also investigated, together with a novel Automatic Exploding Foil Change-Over Switch, since an exploding foil opening switch is needed to condition the output of the capacitor bank before the PEOS. The initial resistance of the PEOS is very low, and the change-over switch is required to ensure that the current transfer takes place when the voltage across the fuse approaches its peak value. An important part of the investigation was to develop a mathematical model of the PEOS, as a part of the power condi tioning circuit, in order to simulate the system for different load conditions. The thesis explains the design, operation and performance characteristics of the various pulsed-power components, such as capacitor banks, closing and opening switches, pulse transformer, the vacuum system required for a PEOS, and high voltage and current measurement techniques.

This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Thesis: M. Eng., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2019
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 72-74).
Miniaturization of power electronics can improve the performance of many applications, such as renewable energy systems, data centers, and aerospace systems. Operation in the high frequency (HF) regime (3{ 30 MHz) has potential for miniaturizing power electronics, but designing small, efficient inductors at HF can be challenging. At these frequencies, losses due to skin and proximity effects are difficult to reduce, and gaps needed to keep B fields low in the core add fringing field loss. This thesis aims to improve the design of HF inductors. A low-loss inductor structure for HF applications and associated design guidelines that optimize for loss have been developed. The structure achieves low loss through quasi-distributed gaps and a new field shaping technique that achieves low winding loss through double-sided conduction. An example ~15 [mu]H inductor designed using the proposed guidelines achieved an experimental quality factor of 720 at 3MHz and 2A (peak) of ac current.
In some cases, litz wire may further improve the performance of the proposed structure. With litz wire, the example inductor achieved an improved quality factor of 980. The proposed structure also has great design and application flexibility. Core sets for this structure can be scaled by a factor-of-four in volume and still cover a large, continuous range of inductor requirements, e.g. power handling and inductances. A wide range of requirements can therefore be achieved with a small set of core pieces. The proposed inductor structure and design techniques thus have greater potential for commercial adoption to facilitate the design of low-loss HF inductors. The design techniques used in the proposed structure can also be extended to high-power radio-frequency (RF) applications, such as RF power amplifiers for industrial plasma generation. A modified version of the proposed structure, along with modified design guidelines, can achieve low loss in this operating space.
Simulations show that an example ~600 nH inductor achieves a quality factor of 1900 at 13:56MHz and 78A (peak). Therefore, the developed design techniques and inductor structures are suitable for small, highly-efficient inductors at HF, and can thereby help realize high-frequency miniaturization of power electronics.
by Rachel S. Yang.
M. Eng.
M.Eng. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science

Demands for delivering high instantaneous power in a compressed form (pulse shape) have widely increased during recent decades. The flexible shapes with variable pulse specifications offered by pulsed power have made it a practical and effective supply method for an extensive range of applications. In particular, the release of basic subatomic particles (i.e. electron, proton and neutron) in an atom (ionization process) and the synthesizing of molecules to form ions or other molecules are among those reactions that necessitate large amount of instantaneous power. In addition to the decomposition process, there have recently been requests for pulsed power in other areas such as in the combination of molecules (i.e. fusion, material joining), gessoes radiations (i.e. electron beams, laser, and radar), explosions (i.e. concrete recycling), wastewater, exhausted gas, and material surface treatments. These pulses are widely employed in the silent discharge process in all types of materials (including gas, fluid and solid); in some cases, to form the plasma and consequently accelerate the associated process. Due to this fast growing demand for pulsed power in industrial and environmental applications, the exigency of having more efficient and flexible pulse modulators is now receiving greater consideration. Sensitive applications, such as plasma fusion and laser guns also require more precisely produced repetitive pulses with a higher quality. Many research studies are being conducted in different areas that need a flexible pulse modulator to vary pulse features to investigate the influence of these variations on the application. In addition, there is the need to prevent the waste of a considerable amount of energy caused by the arc phenomena that frequently occur after the plasma process. The control over power flow during the supply process is a critical skill that enables the pulse supply to halt the supply process at any stage. Different pulse modulators which utilise different accumulation techniques including Marx Generators (MG), Magnetic Pulse Compressors (MPC), Pulse Forming Networks (PFN) and Multistage Blumlein Lines (MBL) are currently employed to supply a wide range of applications. Gas/Magnetic switching technologies (such as spark gap and hydrogen thyratron) have conventionally been used as switching devices in pulse modulator structures because of their high voltage ratings and considerably low rising times. However, they also suffer from serious drawbacks such as, their low efficiency, reliability and repetition rate, and also their short life span. Being bulky, heavy and expensive are the other disadvantages associated with these devices. Recently developed solid-state switching technology is an appropriate substitution for these switching devices due to the benefits they bring to the pulse supplies. Besides being compact, efficient, reasonable and reliable, and having a long life span, their high frequency switching skill allows repetitive operation of pulsed power supply. The main concerns in using solid-state transistors are the voltage rating and the rising time of available switches that, in some cases, cannot satisfy the application’s requirements. However, there are several power electronics configurations and techniques that make solid-state utilisation feasible for high voltage pulse generation. Therefore, the design and development of novel methods and topologies with higher efficiency and flexibility for pulsed power generators have been considered as the main scope of this research work. This aim is pursued through several innovative proposals that can be classified under the following two principal objectives. • To innovate and develop novel solid-state based topologies for pulsed power generation • To improve available technologies that have the potential to accommodate solid-state technology by revising, reconfiguring and adjusting their structure and control algorithms. The quest to distinguish novel topologies for a proper pulsed power production was begun with a deep and through review of conventional pulse generators and useful power electronics topologies. As a result of this study, it appears that efficiency and flexibility are the most significant demands of plasma applications that have not been met by state-of-the-art methods. Many solid-state based configurations were considered and simulated in order to evaluate their potential to be utilised in the pulsed power area. Parts of this literature review are documented in Chapter 1 of this thesis. Current source topologies demonstrate valuable advantages in supplying the loads with capacitive characteristics such as plasma applications. To investigate the influence of switching transients associated with solid-state devices on rise time of pulses, simulation based studies have been undertaken. A variable current source is considered to pump different current levels to a capacitive load, and it was evident that dissimilar dv/dts are produced at the output. Thereby, transient effects on pulse rising time are denied regarding the evidence acquired from this examination. A detailed report of this study is given in Chapter 6 of this thesis. This study inspired the design of a solid-state based topology that take advantage of both current and voltage sources. A series of switch-resistor-capacitor units at the output splits the produced voltage to lower levels, so it can be shared by the switches. A smart but complicated switching strategy is also designed to discharge the residual energy after each supply cycle. To prevent reverse power flow and to reduce the complexity of the control algorithm in this system, the resistors in common paths of units are substituted with diode rectifiers (switch-diode-capacitor). This modification not only gives the feasibility of stopping the load supply process to the supplier at any stage (and consequently saving energy), but also enables the converter to operate in a two-stroke mode with asymmetrical capacitors. The components’ determination and exchanging energy calculations are accomplished with respect to application specifications and demands. Both topologies were simply modelled and simulation studies have been carried out with the simplified models. Experimental assessments were also executed on implemented hardware and the approaches verified the initial analysis. Reports on details of both converters are thoroughly discussed in Chapters 2 and 3 of the thesis. Conventional MGs have been recently modified to use solid-state transistors (i.e. Insulated gate bipolar transistors) instead of magnetic/gas switching devices. Resistive insulators previously used in their structures are substituted by diode rectifiers to adjust MGs for a proper voltage sharing. However, despite utilizing solid-state technology in MGs configurations, further design and control amendments can still be made to achieve an improved performance with fewer components. Considering a number of charging techniques, resonant phenomenon is adopted in a proposal to charge the capacitors. In addition to charging the capacitors at twice the input voltage, triggering switches at the moment at which the conducted current through switches is zero significantly reduces the switching losses. Another configuration is also introduced in this research for Marx topology based on commutation circuits that use a current source to charge the capacitors. According to this design, diode-capacitor units, each including two Marx stages, are connected in cascade through solid-state devices and aggregate the voltages across the capacitors to produce a high voltage pulse. The polarity of voltage across one capacitor in each unit is reversed in an intermediate mode by connecting the commutation circuit to the capacitor. The insulation of input side from load side is provided in this topology by disconnecting the load from the current source during the supply process. Furthermore, the number of required fast switching devices in both designs is reduced to half of the number used in a conventional MG; they are replaced with slower switches (such as Thyristors) that need simpler driving modules. In addition, the contributing switches in discharging paths are decreased to half; this decrease leads to a reduction in conduction losses. Associated models are simulated, and hardware tests are performed to verify the validity of proposed topologies. Chapters 4, 5 and 7 of the thesis present all relevant analysis and approaches according to these topologies.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.
Includes bibliographical references (p. 86-87).
A multistage bandgap circuit with very high power supply rejection ratio was designed and simulated. The key features of this bandgap include multiple power modes, low power consumption and a novel resistor trimming strategy. This design was completed in deep submicron CMOS technology, and is especially suited for portable applications. The bandgap designed achieves over 90 dB of power supply rejection and less than 17 microvolts of noise without any external filtering. With an external filtering capacitor, this performance is significantly enhanced. In addition, the design includes an efficient voltage-to-current converter and a fast-charge circuit for charging the external capacitor.
by Siddharth Sundar.
M.Eng.

Thesis (S.M. in Naval Architecture and Marine Engineering)--Massachusetts Institute of Technology, Dept. of Ocean Engineering; and, (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002.
Includes bibliographical references (leaves 96-102).
by Ronald J. Rutan.
S.M.in Naval Architecture and Marine Engineering
S.M.

With the development of solar energy, power conversion systems responsible for energy delivering from photovoltaic (PV) modules to ac or dc grid attract wide attentions and have significantly increased installations worldwide. Modular power conversion system has the highest efficiency of maximum power point tacking (MPPT), which can transfer more solar power to electricity. However, this system suffers the drawbacks of low power conversion efficiency and high cost due to a large number of power electronics converters. High-power density can provide potentials to reduce cost through the reduction of components and potting materials. Nowadays, the power electronics converters with the conventional silicon (Si) based power semiconductor devices are developed maturely and have limited improvements regarding in power conversion efficiency and power density. With the availability of wide bandgap devices, the power electronics converters have extended opportunities to achieve higher efficiency and higher power density due to the desirable features of wide bandgap devices, such as low on-state resistance, small junction capacitance and high switching speed. This dissertation focuses on the application of wide bandgap devices to the dc-dc power conversion for the modular PV applications in an effort to improve the power conversion efficiency and power density. Firstly, the structure of gallium-nitride (GaN) device is studied theoretically and characteristics of GaN device are evaluated under testing with both hard-switching and soft-switching conditions. The device performance during steady-state and transitions are explored under different power level conditions and compared with Si based devices. Secondly, an isolated high-efficiency GaN-based dc-dc converter with capability of wide range regulation is proposed for modular PV applications. The circuit configuration of secondary side is a proposed active-boost-rectifier, which merges a Boost circuit and a voltage-doubler rectifier. With implementation of the proposed double-pulse duty cycle modulation method, the active-boost-rectifier can not only serve for synchronous rectification but also achieve the voltage boost function. The proposed converter can achieve zero-voltage-switching (ZVS) of primary side switches and zero-current-switching (ZCS) of secondary side switches regardless of the input voltages or output power levels. Therefore, the proposed converter not only keeps the benefits of highly-efficient series resonant converter (SRC) but also achieves a higher voltage gain than SRC and a wide range regulation ability without adding additional switches while operating under the fixed-frequency condition. GaN devices are utilized in both primary and secondary sides. A 300-W hardware prototype is built to achieve a peak efficiency of 98.9% and a California Energy Commission (CEC) weighted efficiency of 98.7% under nominal input voltage condition. Finally, the proposed converter is designed and optimized at 1-MHz switching frequency to pursue the feature of high-power density. Considering the ac effects under high frequency, the magnetic components and PCB structure are optimized with finite element method (FEM) simulations. Compared with 140-kHz design, the volume of 1-MHz design can reduce more than 70%, while the CEC efficiency only drops 0.8% at nominal input voltage condition. There are also key findings on circuit design techniques to reduce parasitic effects. The parasitic inductances induced from PCB layout of primary side circuit can cause the unbalanced resonant current between positive and negative half cycles if the power loops of two half cycles have asymmetrical parasitic inductances. Moreover, these parasitic inductances reflecting to secondary side should be considered into the design of resonant inductance. The parasitic capacitances of secondary side could affect ZVS transitions and increase the required magnetizing current. Because of large parasitic capacitances, the dead-time period occupies a large percentage of entire switching period in MHz operations, which should be taken into consideration when designing the resonant frequency of resonant network.
Doctor of Philosophy
Solar energy is one of the most promising renewable energies to replace the conventional fossils. Power electronics converters are necessary to transfer power from solar panels to dc or ac grid. Since the output of solar panel is low voltage with a wide range and the grid side is high voltage, this power converter should meet the basic requirements of high step up and wide range regulation. Additionally, high power conversion efficiency is an important design purpose in order to save energy. The existing solutions have limitations of narrow regulating range, low efficiency or complicated circuit structure. Recently, the third-generation power semiconductors attract more and more attentions who can help to reduce the power loss. They are named as wide band gap devices. This dissertation proposed a wide band gap devices based power converter with ability of wide regulating range, high power conversion efficiency and simple circuit structure. Moreover, this proposed converter is further designed for high power density, which reduces more than 70% of volume. In this way, small power converter can merge into the junction box of solar panel, which can reduce cost and be convenient for installations.

Silicon carbide is a promising wide bandgap semiconductor for high-power, high-temperature and high frequency devices, owing to its high breakdown electric field strength, high thermal conductivity and ability to grow high quality SiO2 layers by thermal oxidation. Although the SiC power MOSFET (metal-oxide-semiconductor field effect transistor) is preferred as a power switch, it has suffered from low channel mobility with only single digit field effect mobility achieved using standard oxidation process (1200◦C thermal oxidation). As such, this thesis is focussed on the development of 4H-SiC MOSFETs (both lateral and vertical MOSFETs) to improve the channel mobility and breakdown characteristics of these devices. In this work, high temperature nitridation using N2O has been investigated on MOS capacitors and MOSFETs, both with gate oxides grown directly in N2O environment or in a O2 ambient followed by a N2O post-oxidation annealing process. Results have demonstrated that at high temperature (>1200◦C) there is a significant improvement in the interface trap density to as low as (1.5x10^11cm-2eV-1) and field effect channel mobility (19cm2/V.s) of 4H-SiC MOSFET compare with a lower temperature (between 800 and 1200◦C) oxidation (1x10^12cm-2eV-1 and 4cm2/V.s). Nitridation temperatures of 1300◦C was found to be the most effective method for increasing the field effect channel mobility and reducing threshold voltage. The number of working devices per sample also increased after N2O nitridation at 1300◦C as observed for both lateral and vertical MOSFETs. Other post oxidation techniques have also been investigated such as phosphorous passivation using solid SiP2O7 planar diffusion source (PDS). The peak value of the field effect mobility for 4H-SiC MOSFET after phosphorus passivation is approximately 80cm2/V.s, which is four times more than the valued obtained using high temperature N2O annealing. Different JTE structures have been designed and simulated including single-zone JTE, space modulated JTE (SMJTE) and the novel two-step mesa JTE structures. It was found that for the same doping concentration the SM two-zone JTE and SMJTE have higher breakdown voltage than the single zone JTE. With SMJTE, the device could achieve more than 90% of the ideal parallel plane voltage from simulations and 86% from the breakdown test of the fabricated devices.

In the past decade, there has been an increased demand to develop compact power supplies with high efficiency and high power density in order to revolutionise traditional approaches for high power radio frequency (RF) applications, such as long pulse modulators used for high energy physics acceleration experiments. Resonant technology has been considered to meet these challenges, due to its soft-switching characteristic at high operating frequency. The aim of this research project is to provide further knowledge in series resonant parallel loaded (SRPL) resonant converters for long pulse modulators, and to design advanced closed-loop control strategies for voltage pulse regulation. The proposed converter comprises three single phase SRPL resonant output stages, which guarantees a strong ability of overcoming the influence of tank unbalances and enables independent modulation procedures. A DQ modelling approach was utilized to analyse the converter. Based on it, a PI + repetitive control strategy was designed for voltage pulse regulation to obtain good dynamics and steady state performance. A combined frequency and phase shift modulation method was selected to control the converter so that soft-switching of semiconductor devices can be always achieved, even in the presence of large tank unbalances. Good correlation between simulation and experimental results has been demonstrated, which validates the converter circuit design, modelling approach, control strategy and modulation method

The main objective of this thesis is to explore the use of volume holographic elements recorded in photo-thermo-refractive (PTR) glass for power scaling of narrow linewidth diffraction-limited fiber lasers to harness high average power and high brightness beams. Single fiber lasers enable kW level output powers limited by optical damage, thermal effects and non-linear effects. Output powers can be further scaled using large mode area fibers, however, at the cost of beam quality and instabilities due to the presence of higher order modes. The mechanisms limiting the performance of narrow-linewidth large mode area fiber lasers are investigated and solutions using intra-cavity volume Bragg gratings (VBG) proposed. Self-pulsations-free, completely continuous-wave operation of a VBG-stabilized unidirectional fiber ring laser is demonstrated with quasi single-frequency (< 7.5 MHz) output. A method for transverse mode selection in multimode fiber lasers to reduce higher order mode content and stabilize the output beam profile is developed using angular selectivity of reflecting VBGs. By placing the VBG output coupler in a convergent beam, stabilization of the far-field beam profile of a 20 ?m core large mode area fiber laser is demonstrated. Beam combining techniques are essential to power scale beyond the limitations of single laser sources. Several beam combining techniques relevant to fiber lasers were compared in this study and found to be lacking in one or more of the following aspects: the coherence of the individual sources is compromised, the far-field beam quality is highly degraded with significant power in side lobes, spectrally broad and unstable, and uncertainty over scaling to larger arrays and higher power. Keeping in mind the key requirements of coherence, good far-field beam quality, narrow and stable spectra, and scalability in both array size and power, a new passive coherent beam combining technique using multiplexed volume Bragg gratings (M-VBGs) is proposed. In order to understand the mechanism of radiation exchange between multiple beams via these complex holographic optical elements, the spectral and beam splitting properties a 2nd order reflecting M-VBG recorded in PTR glass is experimentally investigated using a tunable single frequency seed laser. Two single-mode Yb-doped fiber lasers are then coherently combined using reflecting M-VBGs in both linear and unidirectional-ring resonators with >90% combining efficiency and diffraction-limited beam quality. It is demonstrated that the combining bandwidth can be controlled in the range of 100s of pm to a few pm by angular detuning of the M-VBG. Very narrow-linewidth (< 210 MHz) operation in a linear cavity and possibility of single-frequency operation in a unidirectional ring cavity of the coherently combined system is demonstrated using this technique. It is theoretically derived and experimentally demonstrated that high combining efficiency can be achieved even by multiplexing low-efficiency VBGs, with the required diffraction efficiency of individual VBGs decreasing as array size increases. Scaling of passive coherent beam combining to four fiber lasers is demonstrated using a 4th order transmitting M-VBG. Power scaling of this technique to 10 W level combined powers with 88% combining efficiency is demonstrated by passively combining two large mode area fiber lasers using a 2nd order reflecting M-VBG in a unidirectional ring resonator. High energy compact single-frequency sources are highly desired for several applications – one of which is as a seed for high power fiber amplifiers. Towards achieving the goal of a monolithic solid-state laser, a new gain medium having both photosensitive and luminescence properties is investigated – rare-earth doped PTR glass. First lasing is demonstrated in this new gain element in a VBG-stabilized external cavity.
Ph.D.
Doctorate
Optics and Photonics
Optics and Photonics
Optics

This work describes the design, development and characterisation of high efficiency photovoltaics (laser power converters) for the conversion of monochromatic light from a laser source into electrical energy. The technology provides a means of transmitting power wirelessly through free-space, for applications in the remote powering of electrical devices and systems. It also provides a means of efficiently transmitting power though fibre-optic cables, allowing electrical power to be delivered free from electromagnetic interference. The design of the laser power converter is considered for efficient conversion of monochromatic light at a target wavelength of 1550nm. This wavelength was chosen based on its ability to transmit through the atmosphere and silica-based fibre-optics with minimal losses. It also allows for the maximum exposure limit of 1kWm^-2 to be transmitted in free-space, which is eye- and skin-safe. Various semiconductor materials were explored for this design in terms of their maturity, band-gap tunability and lattice matching to common substrates. The laser power converter was then developed based on the material system InGaAsP/InP with a band-gap tuned to match the incident target wavelength. These cells were then characterised using a tunable laser source and the best cell achieved a conversion efficiency (at 20 degrees) of 38.9% at an irradiance of 0.73kWm^-2 at the target wavelength. However, earlier field tests conducted by Dr. Jayanta Mukherjee demonstrated an efficiency of 45% at 1kWm^-2, which is much higher than conventional single-junction solar cells and currently holds the record for monochromatic PVs operating at 1550nm. The various carrier recombination mechanisms that limit the efficiency are then investigated by measuring the cell performance down to temperatures of 100K. In this measurement the efficiency at 39Wm^-2 is shown to increase from 28.6% to 72% over the temperature range 300-100K and approaches the theoretical detailed-balance limit. An advanced temperature-dependent diode and resistance model is then formulated to predict the dominant carrier loss mechanisms at room temperature. It was found that (to a first approximation) defect-related carrier recombination dominates over the temperature range with a lifetime of 5us at room temperature. The model also determined a carrier mobility at room temperature of 12.4 cm^2V^-1s^-1 in the emitter layer, which results in a high sheet resistance and limits carrier transport to the contacts. Finally, the effects of non-uniform illumination (due to the Gaussian laser beam profile) on the device performance is investigated. A detailed carrier transport model is devised to understand the implication of non-uniform illumination on the diffusion and recombination of carriers generated in the top emitter layer. A light-beam-induced-current scan and a carrier-time-of-flight scan across the cell surface is then conducted to determine local changes in the device performance and obtain the carrier transport properties. From this the emitter diffusion coefficient and SRH lifetime (to a first approximation) were found to be 3.96cm^2s^-1 and 5us, which is in good agreement with the temperature-dependent illumination study. This work then proposes a new top contact design, which overcomes the impact of non-uniform illumination and sheet resistance.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (p. 109).
This thesis presents a new topology for a high efficiency dc/dc resonant power converter that utilizes a resistance compression network to provide simultaneous zero voltage switching and near zero current switching across a wide range of input voltage, output voltage and power level. The resistance compression network maintains desired current waveforms over a wide range of voltage operating conditions. The use of on/off control in conjunction with narrowband frequency control enables high efficiency to be maintained across a wide range of power levels. The converter implementation provides galvanic isolation and enables large (greater than 1:10) voltage conversion ratios, making the system suitable for large step-up conversion in applications such as distributed photovoltaic converters. Three 200 W prototypes were designed, built and tested. The first prototype was made as a proof of concept and operated at a switching frequency of 100 kHz. It had an efficiency of 93.5% (at 25 V input and 400 V output). The second prototype was operated at a switching frequency of 500 kHz and had an efficiency of 93% (at 25 V input and 400 V output). The high frequency losses caused by the ringing in voltage and current due to the resonating parasitics of the transformer were removed with the help of a matching network in the third prototype. This final prototype operated at a switching frequency of 500 kHz and showed that over 95% efficiency is maintained across an input voltage range of 25 V - 40 V (at 400 V output) and over 93.7 % efficiency across a wide output voltage range of 250 V - 400 V (at 25 V input). These experimental results demonstrated the effectiveness of the proposed design.
by Wardah Inam.
S.M.

This thesis contains two projects - the development of a Raman fibre amplifier system for gas sensing and the development of a fibre-in beam-out isolator for use with fibre lasers. The first project developed a fibre Raman amplifier which amplified a 10mW narrow linewidth DFB laser at 1651nm to an output power of over 2W. To achieve this high output power from a narrow linewidth Raman amplifier mechanisms to suppress stimulated Brillouin scattering had to be employed. This amplifier system was packaged into a portable 19” rack enclosure and used to demonstrate remote, single-end, tuneable diode laser spectroscopy of methane. The packaged system detected methane concentrations of 100ppm.m at over 100m during challenging field trials. Extrapolation of lab based measurements to longer distances predicts an ultimate sensitivity of the system of 100ppm.m at greater than 200m. The second project developed a Faraday isolator to be used at the output of industrial fibre lasers. The isolator had an isolation of greater than 30dB, an insertion loss of less than 0.5dB and a return loss of greater than 50dB. The isolator used a dual Faraday rotator design with a half wave plate between the rotators to compensate for thermal stress induced birefringence. Using this approach the isolation was shown to be constant to greater than 60W. The isolator was also compensated for thermal lensing by balancing the positive thermal lens produced in the Faraday rotator with a negative thermal lens in a DKDP crystal. This reduced the thermal lens of the isolator from ~9 Rayleigh length per kW intrinsic to below 2 Rayleigh lengths per kW.

High-power, high-voltage and high voltage-conversion ratio DC-DC converters are an enabling technology for offshore DC grids of the future. These converters are required to interface between offshore wind farms and an offshore DC grid and a key design issue is the size and weight of the converter, which significantly impacts the cost of the associated off-shore platform. In addition to this application, some rural communities, particularly in Canada, Australia and South Africa,which are located far away from the electrical power generators, can take the advantages of this technology by tapping into existing HVDC transmission line using a high voltage-conversion ratio DC-DC converter. The work described in this thesis is an investigation as to how such DC-DC converters may be realised for these applications. First a review of existing DC-DC converters was carried out to assess their suitability for the target applications. A classification of DC-DC converters into Direct and Indirect converters was proposed in this work based on the manner in which the energy is transferred from the input to the output terminal of the converter. Direct DC-DC converters, particularly Switched Capacitor(SC) converters are more promising for high-voltage, high-power and high voltage-conversion ratio applications, since the converter can interface between the low-voltage and the high-voltage terminals using low-voltage and low-power power electronic modules. Existing SC topologies were examined to identify the most promising candidate circuits for the target applications. Four SC synthesis techniques were proposed in order to derive new SC circuits from existing topologies. A new 2-Leg Ladder, modular 2-Leg Ladder and bi-pole 2-Leg Ladder were devised, which had significant benefits in terms of size and weight when compared with existing circuits. A scaled power 1 kW converter was built in the laboratory in order to validate the analysis and compare the performance of the new 2-Leg ladder circuit against a conventional Ladder circuit, where it was shown that the new circuit had higher efficiency, smaller size and lower output voltage ripple than the Ladder converter.

In this thesis, I report experimental studies towards power scaling of ultrashort fibre-based sources designed for applications including high average power femtosecond pulse generation and nonlinear frequency conversion. While the power produced by rare-earth doped fibre lasers operating in continuous-wave has dramatically increased within a few years to exceed the kilowatt level, pulsed fibre sources have been limited to tens of watts due to the onset of nonlinearities in fibre amplifiers. Therefore the aim is to manage fibre nonlinearities to achieve specific output properties at high average power. An innovative aspect of this work lies in the remarkable combination of telecom-grade semiconductor laser sources and high-power Yb-doped fibre amplifier technologies to produce short pulses at very high average power. Fibre nonlinear effects are often detrimental to the performance of fibre systems but can also provide an attractive tool to generate new useful wavelengths. The final part of this thesis describes efficient white light generation produced by a microstructure fibre pumped by the previously described green fibre source. Furthermore, I investigated a novel fibre source configuration for guide star application. The source I developed produced 1W at 589 nm through frequency doubling of 1178 nm radiation produced by pulsed Raman amplification in an Yb-doped fibre amplifier.

This thesis details my work on the development of fabrication techniques for high power doped fibre lasers, using novel fibre geometries, and their demonstration. The main methods for increasing output power were using helical cores, multiple cores and large cores. A method for fabricating helical core fibres was proposed and implemented. Core and cladding pumped devices were successfully constructed, and were then tested in collaboration. A cladding pumped helical core fibre laser improved the beam from a 30μm core from an M2 of 3.3 to < 1.4, with a maximum output power of 64W and slope efficiency of 84%. A ribbon fibre with multiple laser emitting cores was fabricated. The difficulties in the fabri- cation of such a fibre required extensive research, and arise from the large aspect ratio of the preform and fibre, and the low size reduction during fibre drawing. Through many steps a ten core ribbon fibre was fabricated, which was used, in collaboration, to achieve an output laser power of 250W, with a slope efficiency of 65%. Using a spectral beam combination technique, 5 cores were locked together. The output from a number of cores within a circular fibre with no combination is a method for attaining stable high powers with moderate beam quality. A three core fibre was fabricated, and showed an M2 of 5 and slope efficiency of 75%. This fibre was then tapered down from 150μm in order to improve the beam quality further, and for 125μm and 100μm tapers, the M2 values measured were improved to 4.3 and 3.5, respectively. An Yb-doped rod was fabricated, with a large core of 140μm diameter, and machined flats to break the cladding symmetry. The power attained was only 13.4W with a slope efficiency of 20%, due to surface imperfections. A fibre created with altered fabrication techniques showed no surface defects and showed over 90% transmission for 800nm light. Experiments performed by a colleague showed a slope efficiency of 69% and an M2 of 11, indicating that the fabrication method could create an effective cladding pumped rod laser.

The aim of my project is to develop pulsed Ytterbium (Yb) doped fibre master oscillator power amplifier (MOPA) systems seeded by semiconductor lasers. I was principally focused on two specific projects aligned to sponsored programs of research within the ORC pulsed fibre laser group: the first project, TSB funded project LAMPS, aimed to develop an important class of next generation laser system capable of average output powers of more than 100 W when operating in both the nanosecond and picosecond regimes. The goal was to develop a fully fiberized, polarisation maintaining, single transverse mode system. The full project included the development of the necessary diode & micro-optic systems, fibre beam delivery technology and with application focused evaluations in collaboration with our industrial partners. The main project partners were BAE Systems, Selex, Ceram, Intense Photonics, ORC, Herriot Watt University, Power Photonics, OptoCap and Rofin Sinar. I contributed to the development of the single transverse mode Ytterbium (Yb)-doped fibre system and achieved the full target specifications of 100 W of output power with single mode and single polarisation operation in both the nanosecond and picosecond regimes. In addition, second harmonic generation pumped by the fundamental beam at 1.06 μm was also achieved. In order to transfer from picosecond pulses to nanosecond pulses it is only necessary to switch the seed laser, the power amplifier system remaining unchanged making for a highly flexible system. Both fundamental and second harmonic beam were successfully used to do material processing and various high power frequency conversion experiments (visible, broadband supercontinuum and mid-IR). The second project, called HEGAC (also funded by the TSB), was a collaboration with the University of Cambridge and SPI Lasers Ltd. The aim of the HEGAC project was to develop a high power nanosecond fibre laser with an active pulse shaping capability suitable for cutting metals. This project targeted mJ pulses with more than 100 W average power at the final output – with a 200 W stretch objective. We first achieved more than 310 W using a free space seeding and pumping configuration in our laboratories proving power scaling of our proposed approach. I subsequently rebuilt and improved this system and developed a fully- fiberized version (including all pump launches). The laser was capable of generating >100 W of output power and pulse energies up to 2.5 mJ. This project also involved spatial mode as well as temporal pulse shaping. Using a pair of axicon lenses the normal Gaussian beam profile was converted to a ring shaped profile as required and the system tested up to average powers of 100 W. In addition to the normal temporal pulse shapes required using our pulse shaping system (square, triangle and step), I also achieved high average power pulses with smooth shaped pulses (Parabolic and Gaussian) using an adaptive pulse shaping technique. The laser was transported and successfully used in materials processing experiments at Cambridge, proving the robustness of the design and implementation. I also did some novel experiments on high efficiency Raman conversion exploiting the square shaped pulses possible using this laser

There is a trend in the power industry for high temperature components (such as steam pipe work) to be operated in an increasingly arduous fashion. This would involve the use of elevated steam temperatures/pressures and a greater frequency of start up/shut down cycles. Such generation strategies are being adopted due to the need for thermally efficient power supply that can match fluctuating market demands. If these generation strategies are to be implemented safely it is critical that careful analysis of the system components is conducted in order to ensure that premature failure does not occur. The advanced material models and techniques that are used in academia to simulate these components are often out of reach of the engineers working in industry. The present work describes the development of an analysis “toolbox” that takes several advanced material models (which can accommodate complex loading conditions) and applies them in numerical (finite element analysis, FEA) and approximate life estimation methods. The toolbox comprises several modules, each of which relates to a specific aspect of component analysis. In this thesis, the fundamental procedures behind these modules are developed in novel ways in addition to the development of the toolbox as a whole. The toolbox modules may be roughly divided into the definition of a component’s material, geometry and loading condition, followed by some form of analysis procedure and a report of the key results. A material’s behaviour is commonly determined from mechanical tests. For in service components, scoop sampling is an exciting new method to extract small amounts of material which may then be tested using several novel small specimen techniques. An investigation has been conduced in the present work that verifies the safety of this method and allows the localised stress behaviour around an excavation to be estimated. Material constants in material behaviour models are usually determined by fitting the outputs of the model to experimental data in an optimisation procedure. A great deal of work has been completed on this topic using the complex Chaboche unified visco-plasticity model. This has led to the formation of the combined parallel optimisation strategy and the development of data cleaning for the determination of material constants in any model. Due to the high temperature conditions power plant components operate in, creep is a major concern. Several damage material models have been compared which can represent failure due to creep. Generally, these models can be divided into power law and hyperbolic sine functions. Through a comparative investigation using multiple component geometries, it has been found that the hyperbolic sine function creep law gives lower predictions of failure time than the power law models at realistic stress levels. Hyperbolic sine function failure lives were also more representative of reality. It is therefore critical when performing component analysis to consider the form of a material model as well as the loading range its material constants are applicable to. The Chaboche unified visco-plasticity model has also been discussed. Using this model, both hardening due to the accumulation of plastic strain and viscous effects (such as creep stress relaxation) may be described. Models like this will play an important role in the analysis of high temperature components as they experience fluctuations in both load and temperature. Although it appears simple, the geometry of a high temperature pipe bend in a power plant is actually complex due to the manufacturing process employed (a straight pipe section is heated through induction coils and bent using a fixed radius arm). The pipe’s wall thickness not only varies circumferentially around the pipe’s cross section but also around the bend itself. Through the analysis of industrial data (collected by ultrasonic measurement of components during outage inspections) several novel geometry factors have been developed that quantify this dimension variation. A new method to analyse such pipe bends has also been created that interpolates the stress states between two dimensional (2D) models that represent the cross section of a pipe bend at several key locations. Once a geometry, loading condition and material has been defined, an analysis procedure may be employed in order to assess the condition of the component. As creep is a key concern under high temperature conditions, most of the analysis procedures discussed in the present work are focused on the prediction of peak rupture stresses (δR) which may be used to estimate failure lives due to creep. Several approximate (errors are typically less than 5%) parametric relationships have been developed that allow peak rupture stresses to be determined based on, for example, pipe bend geometry factors. In addition, to aid in bespoke FEA analyses, a collection of routines with a graphical user interface (GUI) have been created that can write input files for a commercial FEA code (ABAQUS), run the job and post process the results. This can save a great amount of user effort when attempting to analyse components. Finally, an original neural network (that uses a partially connected, multiple input node architecture) has been proposed that predicts δR in pipe bends operating under steady-state creep conditions. Both internal pressure and system loads have been incorporated as inputs for this neural network. This has required the definition of several new load factors that describe the system loads acting on a component. Recommendations for future developments based on this research have also been given. Future developments may look to include fatigue effects in parametric equations, as well as considering the effect of varying loading conditions (possibly through a damage fraction approach). The Chaboche model (or similar unified model) may be modified to include temperature dependency and damage effects (allowing for a wider application to component analysis). The effect of geometry variation may be included in the neural network, again extending its applicability, and stresses due to temperature distributions in the piping components may be incorporated (at present, these have not been considered, however system loads may be thermally driven). The work presented in this thesis addresses a complete analysis procedure, from collecting material information from a component through scoop sampling, to determining material constants for this material by an optimisation procedure and analysing the component using either numerical or approximate methods. Although pipe bends have been considered for the significant part of this work due to the relatively small amount of research reported in literature, similar methodologies may be applied to other power plant components of interest, such as welds, steam headers or branch pipes.