A peninsular power company's extra high voltage (EHV) transmission grid and the rest of the country behave as a two machine system for the following two types of disturbances loss of a large generator in the southern region of the peninsular power company faults on the 500 kV interconnections between the two systems Whether the two systems will remain stable relative to each other or go unstable depends on the following three factors • severity of the disturbance • loading on the peninsular power company's EHV transmission grid • amount of power imported from the rest of the country For stable oscillations the two systems must remain coupled at them 500 kV interconnections. For separations the two systems should be immediately isolated from one another at their 500 kV interconnections. Since these two systems behave as a two machine system for these two types of disturbances the extended equal area criterion(EEAC) is used to make an extremely quick and accurate prediction of the relative stability between them. For stable oscillations following a disturbance, circuit breakers at the 500 kV interconnections are blocked from tripping. For separations these circuit breakers are tripped. EEAC requires synchronized voltage phasor measurements at two specific locations within the overall electrical power system. The two sites are substations located on opposite sides of the electrical center of the two systems. The voltage angle at each location's electric bus will swing with respect to its equivalent machine. This information is constantly recorded to monitor the relative stability of the overall system. When a disturbance does occur, a prediction is made and the appropriate control actions are issued. / Master of Science

New hybrid Echinacea cultivars, based on crosses of Echinacea purpurea (L.) Moench with several other Echinacea species, have generated interest and excitement in the marketplace due to novel flower colors and forms. However, these cultivars vary significantly in their growth habits and requirements from the species. We examined factors in the production of Echinacea hybrid cultivars to provide guidance to growers. Foliar sprays 600 mg·L⁻¹ benzyladenine (BA) increased numbers of branches between 19% and 83% in Echinacea cultivars while 400 mg·L⁻¹ dikegulac sodium or 500 mg·L⁻¹ ethephon did not improve branching. Of several height control PGRs applied to E. ‘Marmalade,’ only plants treated with two applications of 5000 mg·L⁻¹ daminozide were shorter (24%) compared to untreated controls although flowering was also reduced by 70%. Echinacea ‘Harvest Moon’ plants were shorter in response to all of the PGRs applied, with the best results seen in plants treated with foliar sprays of uniconazole (one application of 30 mg·L⁻¹ or two applications of 15 mg·L⁻¹ ), two applications of 5000 mg·L⁻¹ daminozide, or 4 mg·L⁻¹ paclobutrazol applied once as a drench. Supplying N at 150 mg·L -1 during the growing season provided Echinacea cultivars adequate nutrition and maximized numbers of branches and flowers and shoot dry weight. In overwintering, fertilization treatments that resulted in low substrate electrical conductivity going into dormancy, 5.0 kg·m controlled release fertilizer 15N-3.9P-10K or 150 mg·L⁻¹ N using 15N-2.2P-12.5K applied using constant liquid feed, resulted in the highest survival rates of Echinacea cultivars. As a monitoring tool, SPAD measurements were not successful in predicting tissue N levels in Echinacea hybrids. Twenty-one hybrid cultivars acquired as stage 3 tissue culture plantlets were grown under one of three photoperiods (10-hour, 16-hour, or 24-hour) for 10 weeks before being transplanted to larger containers and grown under natural daylength until flowering. Providing Echinacea hybrid cultivars with a 16-hour photoperiod during liner production resulted in plants which flowered soonest without negative effects on growth. The need for height control PGRs varied by cultivar; however, overall height control PGRs controlled flower stalk height and increased market rating. / Ph. D.

Evolution in microprocessor technology poses new challenges for supplying power to these devices. To meet demands for faster and more efficient data processing, modem microprocessors are being designed with lower voltage implementations. More devices will be packed on a single processor chip and the processors will operate at higher frequencies, exceeding 1GHz. New high-performance microprocessors may require from 40 to 80 watts of power for the CPU alone. Load current must be supplied with up to 30A/µs slew rate while keeping the output voltage within tight regulation and response time tolerances. Therefore, special power supplies and Voltage Regulator Modules (VRMs) are needed to provide lower voltage with higher current and fast response. In the part one (chapter 2,3,4) of this dissertation, several low-voltage high-current VRM technologies are proposed for future generation microprocessors and ICs. The developed VRMs with these new technologies have advantages over conventional ones in terms of efficiency, transient response and cost. In most cases, the VRMs draw currents from DC bus for which front-end converters are used as a DC source. As the use of AC/DC frond-end converters continues to increase, more distorted mains current is drawn from the line, resulting in lower power factor and high total harmonic distortion. As a branch of active Power factor correction (PFC) techniques, the single-stage technique receives particular attention because of its low cost implementation. Moreover, with continuously demands for even higher power density, switching mode power supply operating at high-frequency is required because at high switching frequency, the size and weight of circuit components can be remarkably reduced. To boost the switching frequency, the soft-switching technique was introduced to alleviate the switching losses. The part two (chapter 5,6) of the dissertation presents several topologies for this front-end application. The design considerations, simulation results and experimental verification are discussed.

M.S.

The Westinghouse Induction Time Klement Overload Relay, Type CO, a relay operating on the same principle as an alternating current watt-hour meter, is considerably affected in its time of operation when non-sinusoidal waves of potential are applied. If the potential wave applied has a flat top or depressed peak or if the wave has a lagging peak, that is, its maximum instantaneous value occurs more than 90 degrees after the beginning of the cycle, the operation of the relay is accelerated. If the wave of potential applied to the relay reaches its peak more than 90 degrees and less than 180 degrees after the beginning of the cycle, the operation of the relay is retarded from what it would be with a sinusoidal potential applied. The operation of a General Electric Time Current Overload Relay, Type PAC, a plunger type relay with an air bellows time delay, is, in general, retarded in operation by the application of non-sinusoidal potential waves. The operation may be slightly accelerated, however, if the wave distortion is slight, the current setting is low, and the distortion is such that the peak of the wave is flat or lagging. The effect of the wave distortion on other relays requires more study. / Master of Science

Rhizobiales encompass a diverse group of microbes, ranging from free-living, soil-dwelling bacteria to disease-causing, intracellular pathogens. Although the lifestyle of these organisms vary, many genetic systems are well conserved. One system, named the AbcR/VtlR system, is found throughout rhizobiales, and even extends to bacteria in other orders within the Alphaproteobacteria. The AbcR sRNAs are an example of sibling sRNAs, where two copies of the abcR gene are typically present in the genome. The AbcRs are involved in the negative regulation of ABC-type transport systems, which are important components for nutrient acquisition. Although the AbcRs share several features amongst organisms, major differences can be found in their functional and regulatory redundancy, the targets they regulate and how they regulate them. Specifically, one major difference in the AbcRs lies in the nucleotide sequences utilized by the sRNAs to bind mRNA targets. In the present studies, the regulatory mechanisms of the AbcR sRNAs were further characterized in the mammalian pathogen Brucella abortus, and the full regulatory profiles of the AbcRs were defined in the plant pathogen Agrobacterium tumefaciens. As mentioned above, the AbcR sRNAs are important for the proper regulation of nutrient-acquiring transport systems in the Rhizobiales. Since these sRNAs are critical to the lifestyle of a bacterium, proper regulation of this system is key to survival. A LysR-type transcriptional regulator, named VtlR, was found to be the bonefide transcriptional activator of abcR1 in B. abortus. Furthermore, VtlR has been shown to be a key component in host interactions in several rhizobiales. The preset work has shed light on the evolutionary divergence of this regulator in bacteria, and further defined the regulatory capacity of VtlR in Agrobacterium. Overall, the studies described here have made significant advances in our knowledge of the AbcR/VtlR-regulatory systems in the Rhizobiales, and have further defined this system as being a vital part of host-microbe interactions. / PHD

Effects of media, soil moisture, fertility rate, and plant growth regulators on plant growth were investigated for 13 taxa of succulents. Media: Liners were grown in five common greenhouse substrates: 80% peat, 60% bark + 30% peat moss, 80% pine bark/20% Permatil (v/v), 100% composted pine bark, or whole tree substrate until market ready. Overall, higher percentage bark mixes yielded smaller plants, with lower shoot dry weights, shoot heights and widths. Soil Moisture: Liners were potted into a 60% bark/30% peat soilless potting mix. In group 1 , irrigation to container capacity occurred when volumetric soil moisture content fell below 30%, 20%, or 10%,. Group two (seven species) irrigation thresholds were shifted to 35%, 25%, and 15%. Effects of irrigation rate were significant in three of the 13 species studied, and those effects were species-specific. Fertility Rate: Liners were potted into60% bark/30% peat substrate. Fertility reatments in group were 0, 50, 100, or 200 mg.L-1 nitrogen. Group 2 plants received treatments of 50, 150, 250, or 350 mg.L-1 nitrogen. Four of the 11 species studied were affected by nitrogen rate, with rates up to 200 mg.L-1 generally producing the largest plants. PGRs: Seven species were potted into a 60% bark/30% peat substrate. Group one plants were treated with a foliar application of benzyladenine (Configure) at rates of 0, 400, 800, or 1600 mg.L-1. Group 2 plants were treated either BA at 0, 250, 500, or 1000 mg.L-1, dikegulac sodium (Augeo) at 400, 800, or 1600 mg.L-1, or a tank mix of 500 mg.L-1 Configure and 800 mg.L-1 Augeo. BA caused an increase in branches leaders or offsets in two species. / Master of Science

An input filter is often required between a switching regulator and its power source due to the need of preventing the regulator switching current from being reflected back into the source. The presence of the input filter often results in various performance difficulties such as loop instability, degradation of transient response, audiosusceptibility and output impedance characteristics. These problems are caused mainly by the interaction between the peaking of the output impedance of the input filter and the regulator control loop. Conventional single-stage and two-stage input filters can be designed to minimize the peaking effect, however this often results in a penalty of weight or loss increase in the input filter. A novel input filter compensation scheme for a buck regulator that eliminates the interaction between the input filter output impedance and the regulator control loop is presented. The scheme is implemented using a feedforward loop that senses the input filter state variables and uses this information to modulate the duty cycle signal. The feedforward design process presented is seen to be straightforward and the feedforward easy to implement. Extensive experimental data supported by analytical results show that significant performance improvement is achieved with the use of feedforward in the following performance categories: loop stability, audiosusceptibility, output impedance and transient response. The use of feedforward results in isolating the switching regulator from its power source thus eliminating all interaction between the regulator and equipment upstream. In addition the use of feedforward removes some of the input filter design constraints and makes the input filter design process simpler thus making it possible to optimize the input filter. The concept of feedforward compensation can also be extended to other types of switching regulators. / Ph. D.

No / The epidermal differentiation program is regulated at several levels including signaling pathways, lineage-specific transcription factors, and epigenetic regulators that establish well-coordinated process of terminal differentiation resulting in formation of the epidermal barrier. The epigenetic regulatory machinery operates at several levels including modulation of covalent DNA/histone modifications, as well as through higher-order chromatin remodeling to establish long-range topological interactions between the genes and their enhancer elements. Epigenetic regulators exhibit both activating and repressive effects on chromatin in keratinocytes (KCs): whereas some of them promote terminal differentiation, the others stimulate proliferation of progenitor cells, as well as inhibit premature activation of terminal differentiation-associated genes. Transcription factor-regulated and epigenetic mechanisms are highly connected, and the p63 transcription factor has an important role in the higher-order chromatin remodeling of the KC-specific gene loci via direct control of the genome organizer Satb1 and ATP-dependent chromatin remodeler Brg1. However, additional efforts are required to fully understand the complexity of interactions between distinct transcription factors and epigenetic regulators in the control of KC differentiation. Further understanding of these interactions and their alterations in different pathological skin conditions will help to progress toward the development of novel approaches for the treatment of skin disorders by targeting epigenetic regulators and modulating chromatin organization in KCs. / National Alopecia Areata Foundation; (R13AR067088-01) from the National Institute of Arthritis and Musculoskeletal and Skin Diseases; and the National Center for Advancing Translational Sciences

Thesis (M.Sc. (Horticulture)) -- University of Limpopo, 2017 / Refer to document / National Research Foundation of South Africa, and Agricultural Research Council-Universities Collaboration Centreꞌ for scholarship and research