This dissertation is the result of research and development of a power conditioning system for Superconductive Magnetic Energy Storage System. The dominant challenge of this research was to develop the power conditioning system that can match slowly varying dc voltage and dc current on the super conductive magnet side with the ac voltages and ac currents on the utility side. At the same time the power conditioning system was required to provide a bi-directional power flow to the superconductive magnet. The focus of this dissertation is a three-level diode clamped dc-ac converter which is a principle part of the power conditioning system. Accordingly, this dissertation deals with the space vector modulation of three-level converters and introduces a computationally very efficient three-level space vector modulation algorithm that is experimentally verified. Furthermore, the proposed space vector modulation algorithm is successfully generalized to allow equally efficient, real time implementation of space vector modulation to dc-ac converters with virtually any number of levels. The most important advantage of the proposed concept is in the fact that the number of instructions required to implement the algorithm is almost independent from the number of levels in a multilevel converter. More on the side of the control of multilevel converters, the particular attention in this dissertation is paid to the problem of charge balance in the split dc-link capacitors of three-level neutral-point-clamped converters. It is a known fact that although the charge balance in the neutral point can be maintained on a line cycle level, a significant third harmonic current flows into the neutral point for certain loading conditions, causing the neutral point voltage ripple. The logical consequence of that ripple is the deteriorated quality of the output voltage waveforms as well as the increased voltage stress on the switching devices. This was the motivation to more carefully explore the loading conditions that cause the unbalance, as well as to study the fundamental limitations of dc-link capacitor charge balancing algorithms. As a part of that work, a new model of the neutral point current in the rotating coordinate frame is developed as a tool in investigation of theoretical limitations and in providing some intuitive insight into the problem. Additionally, the low frequency ripple is quantified and guidelines are offered that can help size the dc-link capacitors. Because the study of the neutral point balance identified the loading conditions, that under some possible system constraints, cause an unavoidable neutral point voltage ripple, a feed forward type of control method is developed next. The proposed feed forward algorithm can effectively prevent the neutral point voltage ripple from creating distortions in the converter output voltage under all loading conditions and without causing additional disturbance in the neutral point voltage. The feed forward method is developed for a sine triangle as well as for the space vector type PWM algorithm. The simulation results that include the full dynamic model of the converter and load system validate the feed forward approach and prove that the feed forward algorithm can effectively compensate the effect of the neutral point voltage ripple. The simulation results are than experimentally verified. / Ph. D.

Terahertz technology is a fast growing field with variety of applications in biology and medicine, medical imaging, material spectroscopy and sensing, monitoring and spectroscopy in pharmaceutical industry, security, and high-data-rate short-range communications. Among different terahertz sources, photomixers are potentially compact, low power consuming, coherent, low-cost, and tunable continuous-wave sources. A terahertz photomixer is a heterodyne scheme, in which two laser beams with their frequency difference falling in the terahertz range mix in a nonlinear medium, such as a photoconductor or a superconductor, and generate a signal, whose frequency is equal to the frequency difference of the two lasers. The frequency of the generated terahertz signal can be tuned by tuning the central frequency of one of the lasers. <br ><br /> In this dissertation, the photomixing in superconductors and photoconductors is studied, and comprehensive analytical models for the interaction of two interfering laser beams with these materials are developed. Integrated photomixer-antenna elements as efficient terahertz sources are introduced and arrays of these elements as high power terahertz sources are designed. Also, an array of photoconductive photomixer-antenna elements with integrated excitation scheme is proposed. <br ><br /> In a photo-excited superconductor, the fundamental equations for the motion of the carriers inside the superconductor material are used in connection with the two-temperature model to find an analytic expression for the generated terahertz photocurrent inside the film. In a photo-excited photoconductor, the continuity equations for the electron and hole densities are solved in their general form along with the appropriate boundary conditions to find photocurrent distribution inside the photoconductor film. It is shown that in a continuous-wave (CW) terahertz photomixing scheme, the resulting photocurrent contains a dc component and a terahertz traveling-wave component. The dependency of the amplitude and the phase of the generated photocurrent on the physical parameters of the photomixer, the parameters of the lasers, the applied dc bias, and the configuration of the device is explored in detail for a photoconductive photomixer made of low-temperature grown (LTG) GaAs and for a high-temperature superconductive photomixer made of YBa₂Cu₃O_7-&delta;. <br ><br /> The developed models for the photoconductive and the superconductive terahertz photomixers are used to design new integrated photomixer-antenna devices. In these devices, the photomixing film simultaneously acts as an efficient radiator at the terahertz frequencies. Integrating the photomixing medium with the antenna not only eliminates any source to antenna coupling problem, but also makes the proposed device attractive for array configurations. <br ><br /> To increase the generated terahertz power, arrays of the photoconductive and the superconductive photomixer-antenna elements are proposed as CW terahertz sources. It is shown that a sub-milliwatt terahertz power is achievable from a typical superconductive photomixer-antenna array structure. The beam steering capability of the proposed devices is also investigated. <br ><br /> A photoconductive photomixer-antenna array with integrated excitation scheme is proposed, in which the laser beams are guided inside the substrate and excite the photomixer elements. In this way the laser power is only being consumed by the photomixer elements, and the photomixer-antenna elements can be integrated with other optical components on a single substrate. The whole structure is robust and less sensitive to vibration and other environmental parameters.

Terahertz technology is a fast growing field with variety of applications in biology and medicine, medical imaging, material spectroscopy and sensing, monitoring and spectroscopy in pharmaceutical industry, security, and high-data-rate short-range communications. Among different terahertz sources, photomixers are potentially compact, low power consuming, coherent, low-cost, and tunable continuous-wave sources. A terahertz photomixer is a heterodyne scheme, in which two laser beams with their frequency difference falling in the terahertz range mix in a nonlinear medium, such as a photoconductor or a superconductor, and generate a signal, whose frequency is equal to the frequency difference of the two lasers. The frequency of the generated terahertz signal can be tuned by tuning the central frequency of one of the lasers. <br ><br /> In this dissertation, the photomixing in superconductors and photoconductors is studied, and comprehensive analytical models for the interaction of two interfering laser beams with these materials are developed. Integrated photomixer-antenna elements as efficient terahertz sources are introduced and arrays of these elements as high power terahertz sources are designed. Also, an array of photoconductive photomixer-antenna elements with integrated excitation scheme is proposed. <br ><br /> In a photo-excited superconductor, the fundamental equations for the motion of the carriers inside the superconductor material are used in connection with the two-temperature model to find an analytic expression for the generated terahertz photocurrent inside the film. In a photo-excited photoconductor, the continuity equations for the electron and hole densities are solved in their general form along with the appropriate boundary conditions to find photocurrent distribution inside the photoconductor film. It is shown that in a continuous-wave (CW) terahertz photomixing scheme, the resulting photocurrent contains a dc component and a terahertz traveling-wave component. The dependency of the amplitude and the phase of the generated photocurrent on the physical parameters of the photomixer, the parameters of the lasers, the applied dc bias, and the configuration of the device is explored in detail for a photoconductive photomixer made of low-temperature grown (LTG) GaAs and for a high-temperature superconductive photomixer made of YBa₂Cu₃O_7-&delta;. <br ><br /> The developed models for the photoconductive and the superconductive terahertz photomixers are used to design new integrated photomixer-antenna devices. In these devices, the photomixing film simultaneously acts as an efficient radiator at the terahertz frequencies. Integrating the photomixing medium with the antenna not only eliminates any source to antenna coupling problem, but also makes the proposed device attractive for array configurations. <br ><br /> To increase the generated terahertz power, arrays of the photoconductive and the superconductive photomixer-antenna elements are proposed as CW terahertz sources. It is shown that a sub-milliwatt terahertz power is achievable from a typical superconductive photomixer-antenna array structure. The beam steering capability of the proposed devices is also investigated. <br ><br /> A photoconductive photomixer-antenna array with integrated excitation scheme is proposed, in which the laser beams are guided inside the substrate and excite the photomixer elements. In this way the laser power is only being consumed by the photomixer elements, and the photomixer-antenna elements can be integrated with other optical components on a single substrate. The whole structure is robust and less sensitive to vibration and other environmental parameters.

No description available.

Abstract Recently, the expansion in power system capacities leads to the development of large-scale steam turbine generator units. As a result, a fault on the power system may induce large fault current and give rise to serious torque vibrations on turbine shafts and blades, which ought to be improved in order for the reliable operation of a turbine-generator system. In the thesis, countermeasures are proposed from electrical viewing-point and from mechanical viewing-point respectively. Based on electrical viewing-point, the apparatus in the generator stator side and in the rotor side respectively is applied to suppress the induced disturbing source. The high temperature superconductive fault current limiter bank introduces a large normal-state resistance to restrict the dc component of stator fault current. The choke coil acts as a low pass filter to restrict the system-frequency component of field fault current. Both of them lead to the reduction in electromagnetic torque of system-frequency and effectively improve the vibrating behavior of blades. Based on mechanical viewing-point, it is found from the electromechanical analysis that the Generator/LP-Turbine shaft stiffness and the Generator rotor inertia constant determine the responses of all turbine blades. Once the stiffness on this shaft section is reduced by replacing the rigid shaft coupling with a flexible one or the inertia constant is augmented by a system-frequency mechanical filter, the blades become intrinsically less responsive to electrical disturbances. As a result, the blades will bear less stress impact and can be designed with smaller safety factor. On the other hand, LP-turbine long blades operated in corrosive environment and underwent the statistical stress impact due to randomly distributed negative sequence current is studied also. In such situation, the blades may be subjected to corrosion fatigue and the long term effects of power system unbalance may become the cause of fatigue damage on blades though the negative sequence current is still within the limitation of generator thermo-rating. As a result, turbine blades are possibly unprotected by traditional system unbalance protection scheme. Therefore, it will depend on the operating environments and the blade materials whether such long-term stress can be neglected or not. If there is the potential of blade damage, one has to reconsider the I2 protection settings and rearrange the load distribution to limit the system unbalance.

In this thesis, the superconductive and superresistive properties of thin percolating films of lead nanoclusters are presented. The samples were created by depositing clusters from an inert gas aggregation cluster source onto substrates held at either room temperature or 10K. Observations of the characteristic behaviours of the samples were made through R(T ) and V (I) measurements. Several interesting features were observed - smooth and discrete steps in the R(I) curves, hysteresis between increasing and decreasing bias currents, and non-zero resistances at superconducting temperatures. Explanations are proposed in terms of theoretical models of several phenomena - phase slips, phase slip centres and hotspots - which have seen little prior application to percolating systems in literature.