For many years, power electronics in the high-power area was performed with extremely slow semiconductor switches. These switches, including the thyristor and the Gate Turn-Off (GTO) thyristor, had the capacity to handle very high voltages and currents but lacked the ability to perform high frequency switching. Low-power converters, such as computer power supplies and low horsepower motor drives, have employed high-frequency switching for years and have benefited from very nice output waveforms, good control dynamic performance, and many other advantages compared to low frequency switching. Recent improvements in high-power semiconductor technology has brought switching performance similar to that of the low-power MOSFETs and IGBTs to the high-power area through the advancement of the IGBT's ratings to create the High Voltage IGBT (HVIGBT) and the development of new GTO-derived devices including the Integrated Gate Commutated Thyristor (IGCT) and the Emitter Turn-Off (ETO) thyristor. These new devices all feature high switching speed and the capability to turn off without the requirement for a turn-off snubber. With these new device technologies the high-power field of power electronics can realize dramatic improvements in the performance of systems for utility applications and motor drives. However, with these high-speed switches come new issues relating to noise, protection, performance of diodes, and thermal management in high-frequency applications. This thesis addresses the application of these new devices, especially the ETO and the IGCT. Examples of each device technology (IGBT, IGCT, and ETO) have been characterized in both their switching performance and conduction loss. The tests performed show how these new devices may be applied to various applications. The switching loss, especially related to turn-off, is the dominant factor in the power dissipation of the high-power switches, so knowledge of these characteristics are very important in the system design. To demonstrate the operation of the ETO, two power converters were constructed. The first was a 100 kW DC/DC converter, which demonstrated the operation of the ETO in a typical building block configuration, the half-bridge. The second system, a 1 MegaVolt-Amp (MVA) three-phase inverter, demonstrated the ETO in an application where the switching frequency and power level were both high. The test results demonstrate the expected characteristics of the high-frequency converters. The development of the ETO's gate driver is described. During the inverter testing, a new failure mode was found involving a parasitic diode within the ETO. This failure mode was analyzed and solutions were proposed. One of the proposed solutions was implemented and there were no more failures of this type. Another possible failure mode regarding a circulating current in an IGCT-based system is also analyzed. Soft-switching techniques can help reduce the switching loss in power semiconductor switches. Several topologies were considered for application in the high-power area, and one was selected for further investigation. A prototype Zero Current Transition (ZCT) circuit was developed using an IGCT as the main switch. The turn-off loss was reduced dramatically through the tested ZCT circuit, and the diode recovery was also alleviated. / Master of Science

DC to DC step-down applications with high input voltage and high voltage conversion ratio operational requirements, such as photovoltaic battery chargers, are subject to high conduction losses, high switching losses and substantial reverse-recovery losses when minority carrier principle diodes are used. The recent introduction of silicon carbide diodes with high breakdown voltages has made possible the elimination of reverse-recovery losses at high voltage levels and as such has sparked interest in their use due to the potential efficiency improvements. This report presents the results of a comprehensive analysis on the use of silicon carbide diodes and their counterparts, ultrafast silicon diodes, in conventional buck converters and isolated current-fed buck converters in high input voltage and high voltage conversion ratio step-down applications. The analysis illustrates both theoretically, with the use of steady-state average models, and experimentally the substantial efficiency benefits of the use of reverse-recovery free silicon carbide diodes in the conventional buck converter and the small but significant improvement in the efficiency of the isolated current-fed buck converter. The improvements of the conventional buck converter paired with silicon carbide diodes are shown to be significant enough to grant the variant the most efficient position for power levels below 1 kW. In addition, the four variants are categorized based on their cost and performance; therefore, providing engineers with a convenient guide to aid their selection of the appropriate converter depending on the operational requirements.

DC to DC step-down applications with high input voltage and high voltage conversion ratio operational requirements, such as photovoltaic battery chargers, are subject to high conduction losses, high switching losses and substantial reverse-recovery losses when minority carrier principle diodes are used. The recent introduction of silicon carbide diodes with high breakdown voltages has made possible the elimination of reverse-recovery losses at high voltage levels and as such has sparked interest in their use due to the potential efficiency improvements. This report presents the results of a comprehensive analysis on the use of silicon carbide diodes and their counterparts, ultrafast silicon diodes, in conventional buck converters and isolated current-fed buck converters in high input voltage and high voltage conversion ratio step-down applications. The analysis illustrates both theoretically, with the use of steady-state average models, and experimentally the substantial efficiency benefits of the use of reverse-recovery free silicon carbide diodes in the conventional buck converter and the small but significant improvement in the efficiency of the isolated current-fed buck converter. The improvements of the conventional buck converter paired with silicon carbide diodes are shown to be significant enough to grant the variant the most efficient position for power levels below 1 kW. In addition, the four variants are categorized based on their cost and performance; therefore, providing engineers with a convenient guide to aid their selection of the appropriate converter depending on the operational requirements.

DC to DC step-down applications with high input voltage and high voltage conversion ratio operational requirements, such as photovoltaic battery chargers, are subject to high conduction losses, high switching losses and substantial reverse-recovery losses when minority carrier principle diodes are used. The recent introduction of silicon carbide diodes with high breakdown voltages has made possible the elimination of reverse-recovery losses at high voltage levels and as such has sparked interest in their use due to the potential efficiency improvements. This report presents the results of a comprehensive analysis on the use of silicon carbide diodes and their counterparts, ultrafast silicon diodes, in conventional buck converters and isolated current-fed buck converters in high input voltage and high voltage conversion ratio step-down applications. The analysis illustrates both theoretically, with the use of steady-state average models, and experimentally the substantial efficiency benefits of the use of reverse-recovery free silicon carbide diodes in the conventional buck converter and the small but significant improvement in the efficiency of the isolated current-fed buck converter. The improvements of the conventional buck converter paired with silicon carbide diodes are shown to be significant enough to grant the variant the most efficient position for power levels below 1 kW. In addition, the four variants are categorized based on their cost and performance; therefore, providing engineers with a convenient guide to aid their selection of the appropriate converter depending on the operational requirements. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

In the last few decades India has advanced socioeconomically due to the rapid growth of industries and automobile sector. This in turn increases the use of fossil fuel and diesel. The atmosphere gets polluted due to the harmful substances, which comes from the burning of fuel. These pollutants can be in the form of gaseous, liquid or solid particulate. Air pollution, both indoor and outdoor, is a significant cause of health problems worldwide. In the automobile sector diesel engine exhaust is the major contributor for the air pollution amounting to about 60%. Exhaust generally contains oxides of Carbon, Nitrogen, Sulfur, Soot, Oil mist etc. Due to the heavy consumption of diesel as a fuel there is an urgent need to control diesel exhaust. Successful control of emissions from combustion engines particularly from diesel engines is yet to be achieved. The conventional techniques which are available to control emission now are either difficult to operate or does not satisfy the stringent emission standards. Among the major pollutants of diesel exhaust NOx is one of the pollutants to be taken care of as it is the major contributor for acid rain, photochemical smog etc. Further the NOx cause health problems leading to respiratory diseases, pneumonia, asthma etc. Till date, despite improvement in aftertreatment technologies, diesel engine continues to emit large amounts of NOx. The failure of conventional techniques lead to the development of non-conventional techniques such as high voltage electric discharge based plasma which has already been proved to be economical and highly efficient in Industrial Electrostatic Precipitators. Till now all the research regarding control of NOx has been done at the laboratory level using conventional DC and rotary spark gap based high voltage power supply. However, this is not a feasible option for Automobiles. The objective of this thesis is, therefore, to design and fabricate a solar powered high voltage power supply to be retrofitted into a vehicle. This leads to the development of power supply which will be of smaller size & capable of supplying high voltage to the gas treatment reactor. In the present work emphasis has been made on the design and development of a DC/HVAC power unit based on switch mode power supply approach. The thesis mainly presents the following issues:  Design & fabrication of solar powered high voltage high frequency power supply for the production of Electric Discharge Plasma.  Actual diesel exhausts treatment with the Electric Discharge Plasma.  Exhaust treatment with conventional 3 way CAT.  Exhaust treatment with cascaded plasma & catalyst/adsorbent system  Comparative analysis. The high voltage power supply gives an output of 16kV peak to peak at 12.2 kHz with a 50W power rating. This high frequency high voltage is applied to the gas treatment reactor and studies on actual diesel exhaust cleaning were carried out. It was observed that there was substantial improvement in the NOx removal under this high frequency AC application when compared to conventional 3-way catalytic converter. Studies have also been carried out at different engine loads and a comparative analysis has been made by cascading the NTP with adsorbents & catalysts and results as discussed in detail.

Recently in more or all electric vehicles, higher voltage batteries are used which employ large number of cells in series. Series connection among cells may lead to single point of failures, safety and charge equalization issues that demand complex control and costly and/or lossy battery management methods. Most present day high voltage batteries use dissipative-charge balancing methods, which result in poor efficiency, additional thermal management burden and lower overall vehicle range. Furthermore, the output voltages of such batteries remain unregulated and may widely change with load and environmental conditions, complicating the overall power pass design of the electrical power system. As a step forward to address these issues, this thesis studies a fault-tolerant regulated active high voltage electric accumulator with integrated power electronics for safe charge and discharge of the high voltage energy storage system. / Thesis / Master of Applied Science (MASc)

The research programme at the University of Canterbury includes the development and applications of partial core inductors and transformers for high voltage testing of generator insulation. Unlike a conventional full core transformer, a partial core transformer has no limbs and yokes. A partial core transformer is a compromise between a full core and coreless transformer. It is superior to its full core counterpart as far as cost, weight and ease of transportation are concerned. Partial core transformers have a low magnetising reactance and hence draw a high magnetising current. This characteristic makes them a perfect fit in applications where the load is capacitive in nature, such as a.c. power frequency high voltage testing of generator insulation and cable testing etc. The work carried out for this thesis focuses on automatically controlling the amount of reactive power on the supply side of a partial core transformer. The considered design includes a third winding around the existing two windings. A power electronic controller is connected to the third winding, which modifies the VAr absorption characteristics of the magnetically coupled supply winding. Two options are considered to achieve continuous reactive power control in the partial core transformer as explained below. First, a thyristor controlled reactor (TCR) is proposed as the VAr controller. It is modelled using PSCAD/EMTDC software. Simulations reveal the design criteria, overall performance and the limitations of the suggested proposal. The TCR connected tertiary winding takes the capacitive burden of the supply. The model demonstrates the ability of the automatically controlled TCR to provide a continuous variation of reactive power without significant under or over compensation. This feature limits the supply current to its real component only, so the supply provides only the losses of the system. Second, a voltage source converter is considered as the VAr controller. This is modelled in PSCAD/EMTDC and a hardware prototype is designed and built. Based on the analysis, the control algorithm (including a digital PI controller) is implemented using an 8 bit micro-controller, PIC18LF4680. The prototype is tested in the laboratory for both active and inductive load conditions as seen from the supply side. Performance of the hardware prototype is discussed in detail. The PSCAD/EMTDC model and the hardware prototype successfully demonstrate the feasibility of a STATCOM controlled partial core transformer. The proposed system is capable of compensating a wide range of capacitive loads as compared with its TCR counterpart. It is proved that the system is very robust and remains dynamically stable for a large system disturbance such as change in load from full capacitive to inductive and vice versa. This confirms that the system is capable of providing continuous VAr control.

Electroporation is the process where externally applied electric fields cause significantly increased permeability of the cell membrane. The increased permeability allows the transport of external compounds into the cell. This is important for applications in electrochemotheropy, electrofusion and drug delivery. Electroporation also has applications in the disinfection of liquids. Given a high enough electric field across the cell membrane, the electroporation process can become irreversible, leading to cell destruction. With the cell membrane under an intense electric field, the cell membrane structure fails causing the cell to die. Conventional liquid beverage disinfection systems rely on slow heating methods requiring large power requirements; this can reduce the taste and quality of some liquids. Pulse generators provide the necessary electric fields to produce the required voltage potential across the cell membrane. The usefulness of electroporation depends on several parameters such as amplitude, frequency and rise/fall times of the electric field. The wave shape also has a bearing on performance, and is limited by the pulse generator topology. A multilevel bipolar waveform is desired with operating frequencies above about 1 kHz. The cascaded H-bridge or full-bridge topology is the most useful as it capable of producing multilevel bipolar waveforms at high frequency. This thesis presents the design and implementation of a multilevel high-voltage pulse generator, capable of creating very high-voltage AC pulses. MOSFET switching devices in conjunction with good layout practices were used to provide required fast switching speeds. The full-bridge topology is used to create a multilevel output profile through cascading of multiple stages. As a full-bridge topology inherently creates a RCL resonant network, there are many challenges associated with mitigating high-frequency noise sources. Two separate stages are built, a low voltage stage capable of outputting up to 200 Vp and a high voltage stage capable of switching up to 1 kVp. A control board was also built for pulse signal generation and user configuration of the output waveforms. The designed pulse generator can produce short pulses of up to 1.4 kVp at frequencies of up to 350 kHz using primarily resistive loads (that simulate a conductive liquid load). Little high frequency switching noise was observable on the output waveform. A single stage pulse generator was also tested with actual liquid loads using an electrode chamber, demonstrating electroporation. The liquid load testing was performed on water and milk derived from milk powder. Results showed that the liquid loads were consistent with primarily resistive loads.

Design, development, and testing of a comprehensive and automated classification system for single and multiple PD source identification based on the relationship between the variation of PRPD patterns and the sources of PD is proposed. The proposed system consists of feature extraction methods and classifier algorithms that are implemented for recognition of partial discharge patterns. For single PD source identification, twelve high performance, applicable feature extraction techniques on PRPD patterns are employed to extract features. In order to present a comprehensive classification system, 10 well-known algorithms for the classification of PD sources have then been used. To evaluate the performance of the classification system, three laboratory test setups are designed and built to simulate various types of PD activities. The first test setup is designed to model common sources of PD in air, oil, and SF6. Using this setup, the application of automated classification system on different sources of PD in different HV insulation media is investigated. The second and third test setups are designed to test the classification system on identification of different sources of PD in oil-immersed insulation and power transformer cellulose insulation under both electrical and thermal stresses, respectively. In many practical situations, the interest lies in the identification of multiple, simultaneously activated PD sources in insulation. Multi-source PDs sometimes results in partially overlapped patterns, which makes them hard to be identified by single source identification techniques. To further enhance the proposed classification system, a novel algorithm to identify Multi-source PDs is developed and appended to the system. To evaluate the performance of this algorithm, a number of multi-source PD models have been designed. The overall results show that the classification system is well able to identify the single and multi-source of partial discharges. More importantly, this identification system is able to assign a ``degree of membership" to each PRPD pattern, besides assigning a class label to it. This enables probabilistic interpretation of a new PRPD pattern that is being classified and results in safer decision making based on the risk associated with different sources of PD. The results of this research is beneficial for the design of a solid basis for an automated, continuous 24/7 monitoring of equipment, which facilitates PD source identification in early stages and safe operation of HV apparatus. / October 2016

Thesis (M.Sc.), Faculty of Engineering (Electrical Engineering), University of the Witwatersrand, 1994