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Geomagnetically induced currents (GIC) could saturate a transformer core. Two significant effects are the abnormal stray flux in transformers and extremely large harmonic contents in excitation currents, which can lead to serious equipment damage and power system misoperation. Such incidents have occurred during the March 1989 K-9 solar magnetic disturbance. This dissertation starts with a systematically study of transformer magnetization under GIC. It reviews both dc and ac saturation patterns of five transformer core designs. Magnetic fields along various traverses for dc excitation are presented. Impedance matrix entries of a single phase transformer are compared for normal and dc operations. New observations have been formed based on the simulation results. The study helps to reveal the fundamental transformer magnetization mechanism under GIC in order to assess potential stray flux heating possibilities of geologically vulnerable transformer units. Based on the finite element analysis, an improved method of modeling transformer excitation under dc bias using equivalent magnetic circuit is developed. There are two unique points in this approach: first, information of 3D finite element magnetic flux distribution analysis is used to construct and verify the circuit model; second, the effect of the transformer tank: is included The model is capable of simulating transformer excitation currents under different levels of dc bias with good accuracy. As a consequence, the complete variations of excitation current harmonics with respect to an extended range of dc bias are revealed. The sensitivity of transformer winding impedances and core loss on the excitation characteristics are examined. The saturated transformer under no-load and various loading conditions is simulated. A laboratory test is performed on a small scale transformer and compared with the model results. Excitation harmonics generated from dc biased three phase transformer banks with different types of equivalent loads are also simulated The effect of both unbalanced dc excitations and unbalanced loads are investigated The results of this study contribute in understanding transformers as harmonic sources and the impact on power systems during a geomagnetic disturbance. / Ph. D.

High frequency induced electrical energy has been used by medical science for several years to produce heat internally in the human body. The uses of such a process are many and varied. They range all the way from producing artificial fever in the entire body to heating one certain afflicted spot such as a rheumatic joint. / Master of Science

published_or_final_version / abstract / Electrical and Electronic Engineering / Master / Master of Philosophy

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographies. / by Martin Frederick Schlecht. / Sc.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Bibliography: leaves 91-93. / by Byung Hoon Kim. / M.S.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by David Alan Stephenson. / B.S.

published_or_final_version / Electrical Engineering / Master / Master of Philosophy

Electrical systems have become ubiquitous, and with them come the need to accurately monitor electric current. The aerospace industry is no exception. Modern aircraft may contain more than one hundred current sensors, each one critical to a properly functioning vehicle. While these sensors function acceptably, several areas have been identified for improvement: size, weight, and cost. Each sensor is bulky, taking up valuable space. They are also costly to manufacture. The existing design is based on the Hall effect, and has remained fundamentally unchanged for decades. With the recent progress in manufacturing techniques and materials, it would be beneficial to reexamine these sensors and determine if improvements can be made using the accomplishments of recent years. Of particular interest are microelectromechanical systems, also known as MEMS. Using a sensor based on MEMS technologies in which design, function, and fabrication are closely intertwined would automatically meet two of the three goals: reducing size and weight. MEMS additionally have the potential to allow existing systems to be miniaturized. Also of interest are advanced materials, some of which can behave as transducers, linking different physical phenomenon. The goal of this dissertation is to use advances in manufacturing techniques and materials, specifically those discussed above, to design a better current sensor. As part of this goal, several potential solutions were studied and optimized. Finally, proof-of-concept prototypes were fabricated and tested to validate the feasibility of the designs and offer insight into continued sensor development.

Ohmic heating is an electrical resistance heating resulting from the passage of electrical current through an electrically conductive material offering some resistance. Most food products possess sufficient electrical conductivity to be able to take advantage of ohmic heating. This research was undertaken to evaluate the quality changes associated with fine ham emulsions prepared under ohmic heating conditions as compared to those produced by conventional heating. / In the first part of the study, kinetics of quality changes (color, texture, water holding capacity, cooking loss and water activity) associated with fine ham emulsion subjected to conventional heating conditions were evaluated. The second part of the study was to standardize ohmic heating conditions for achieving target time-temperature combinations. It was desired to achieve cooking conditions (temperature come-up and hold times) both similar to and faster than conventional heating so that meaningful quality comparisons could be made between the two. / The final part of the study was to compare the quality changes of fine ham emulsion subjected to ohmic heating under different rates, and to compare them with conventional heating. (Abstract shortened by UMI.)