published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

The relationship between impulse breakdown stress and stressed oil volume has been extensively investigated for uniform field electrodes. In large high voltage transformers it is essential to extend this relationship to paper insulated non-uniform field situations which are more frequently encountered in practice. This paper develops the stated relationship by experimental tests, using insulated cable to plane configurations, and statistical methods. These tests show that the impulse breakdown strength of an insulated cable to plane or nonuniform field system has a two parameter Weibull distribution, when the oil is considered to be the weak-link of the system. A non-linear regression analysis is then used to find that the Weibull parameters for insulated non-uniform field electrodes are approximately the same as the parameters for the uninsulated uniform field electrodes provided their stressed oil volumes are equivalent. Statistical calculations are used to derive the fundamental relationship between the Weibull parameters for any stressed oil volume and the Weibull parameters for the unit oil volume. With this relationship it is possible for design engineers to approximately determine the probability of electrical breakdown of any insulated cable to plane configuration in large power transformers.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering in the High Voltage Research Group School of Electrical and Information Engineering Johannesburg, June 2017 South Africa / Servitude availability in space-constrained built-up areas within the Johannesburg or Central Load Network (CLN) poses every-day challenges for power system engineers. Strengthening the backbone 88/275 kV transmission system within the CLN becomes even more difficult when multi-circuit transmission lines are required for increased power transfer capabilities. When uprating is considered to increase the power transfer capability, the withstand levels of existing external insulation demands an optimisation to find a new stress versus strength balance that allows reliable operation of substations at higher voltages. The research includes primarily an investigative simulation study to evaluate the current Eskom available design clearances in terms of their withstand capability when subjected to over-voltage transients. Two voltage range classes were evaluated and the results are discussed. For voltage range 1, it was found that the over-voltage stress was low enough to allow for a higher nominal operating voltage while maintaining the existing clearances. For voltage range 2, existing clearances are also found to be conservative and smaller safety margins will most likely be acceptable. From a transient analysis evaluation, voltage uprating is considered as a very attractive option to increase the power transfer capability of existing substations. Current Eskom clearances for 88 kV and 275 kV are expected to perform well during transients generated in uprated systems. Electrode grading to improve the field gradients in the substation will require attention to increase gap factors. Additional surge arresters are considered to be a cost effective solution to control over-voltages throughout the whole uprated substation. The physical modification of substations to replace strung conductors with tubular conductors, ensuring sufficient outage time to refurbish and rebuild with new equipment will be the most challenging part of uprating existing substations. / MT 2017

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Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Physics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Bibliography: leaves 225-239. / Ph.D.

Advancements in power electronics to higher power levels and faster switching times allow new machine and systems designs, but also create higher stresses on electric machinery insulation. High performance, pulse-width modulated (PWM) inverters are now available for medium voltage drive systems, and are being considered by the U.S. Navy as they move to the "all-electric" ship. If this process is to be successful, a necessary component will be to understand the impact of partial discharge (PD) generation on electric drive systems. Out of the many PD influencing parameters, voltage level, voltage rise-time, switching frequency, and temperature were chosen to be investigated with regards to their influence on PD generation in a comprehensive research project in the Motor Systems Research Facility (MSRF) at Oregon State University (OSU). The tests were performed on representative propulsion coils employing two different 4160 V insulation systems and were evaluated by both an optical and electrical PD detection method. A highly flexible test configuration was developed, capable of adjusting each of the four test parameters independently over a wide range of appropriate values. The developed test program enabled the analysis of the influence of the parameters on the generation of PD, as well as an evaluation of the test coils and PD instrumentation used. It is concluded that, as expected, voltage level is the most significant parameter affecting PD production. However, there is a surprising interdependence of rise-time and pulse-width that requires further investigation. Multiple-cycling tests are seen as appropriate to determine the effect of temperature. Based on the subjective nature of the findings from the test program an improved PD instrument is proposed, which would increase the capabilities and objectivity of the PD detection process. / Graduation date: 2004

In this thesis the concept of inhomogeneous dielectrics is demonstrated for various optical coating applications. Compositionally-varying silicon oxynitride (SiON) dielectric layers, with the refractive index varying as a function of position, are grown by computer-controlled plasma-enhanced chemical vapor deposition (PECVD) using silane, nitrogen, and nitrous oxide reactant gases. Compositionally graded and superlattice-like SiON layers are grown and their compositional profiles are confirmed by Auger electron spectroscopy sputter profiling. Inhomogeneous antireflection coatings and rugate filters, with sinusoidally varying refractive index profiles, are designed and fabricated and their measured spectral responses are found to be in excellent agreement with simulated results. Alternating-current thin film electroluminescent (ACTFEL) devices with multiple layer dielectrics also are designed, fabricated, and the insulating layers are shown to increase the optical outcoupling efficiency of an ACTFEL devices by approximately 14 % compared to that of a conventional ACTFEL structure. / Graduation date: 1993

Hydrogen, one of the cleanest energies, is very attractive in the near future. However, it could be hazardous to store, transport and use hydrogen gas because leakage can cause explosion if sparks appear. Therefore, it is essential to develop sensors to detect the hydrogen leakage in order to prevent potential accidents. In this research, Metal-Insulator-Semiconductor (MIS) Schottky-diode hydrogen sensors with different gate insulators (Ta2O5, La2O3, LaTiON, and HfTiO) or substrates (Si, SiC, and InGaN/GaN MQW) were prepared in order to study their hydrogen sensing performances. Firstly, two sensors based on Si and SiC with Ta2O5 as gate insulator were prepared and compared. Owing to high permittivity (~25), good thermal stability and low electrical defects, Ta2O5 was chosen as the insulator. The differences in sensitivity and response time between the two sensors were ascribed to the difference in the surface morphology of Ta2O5 between the SiC sensor (mean surface roughness was 0.39 nm) and its Si counterpart (mean surface roughness was 0.22 nm). Secondly, due to the high permittivity (~25) and good thermal stability of La2O3, the high permittivity (~20), low interface-state density, and low leakage current of LaTiON, Si sensors with these two dielectrics as gate insulator were developed. The sensitivity of the La2O3 sensor could exceed 7.0 at 150 oC, and the sensor exhibited good hydrogen sensing performance at up to 250 oC. On the other hand, the maximum sensitivity of the LaTiON sensor could reach 2.5 at 100 oC. For the LaTiON sensor, the Poole-Frenkel model controlled the carrier transport at high temperatures (150 ~ 200 oC) while the thermionic emission was the dominant conduction mechanism at lower temperatures (from room temperature to 150 oC). For the La2O3 sensor, the hydrogen reaction kinetics was confirmed, and an activation energy of 10.9 kcal/mol was obtained for this sensor. Thirdly, the La2O3 gate insulator used in the previous work was applied to make MIS sensor on SiC substrate for higher-temperature applications. Its maximum sensitivity and response time at high temperature (260 oC) are 4.6 and 20 s, respectively. The electrical conduction mechanisms were explained in terms of Fowler-Nordheim tunneling (below 120 oC) and the Poole-Frenkel effect (above 120 oC). Finally, in order to see whether the unique structure of InGaN/GaN multiple quantum wells (MQWs) can be utilized for the MIS Schottky-diode hydrogen sensor, three sensors were made on InGaN/GaN MQWs substrate, one without gate insulator, one Finally, in order to see whether the unique structure of InGaN/GaN multiple quantum wells (MQWs) can be utilized for the MIS Schottky-diode hydrogen sensor, three sensors were made on InGaN/GaN MQWs substrate, one without gate insulator, one In summary, the quality of the gate insulator plays an important part in the performance of the hydrogen sensors. SiC and InGaN/GaN MQW substrates are suitable for high-temperature (from ~200 to ~500 oC) applications while the low-cost sensors based on Si substrate can function well below about 200 oC. Hydrogen sensors with these high-k materials (Ta2O5, La2O3, LaTiON, and HfTiO) as gate insulator can produce good electrical characteristics, high sensitivity, and fast response. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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

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