3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Optimal Transmitter Power Control for indoor RGB-based visible light communication(2018) Kamwangala, ChristianDespite efforts to improve the efficiency and the spacial reuse of the ubiquitous RF frequencies range, the RF spectrum suffers from several issues, namely spectrum congestion and lack of bandwidth for future technologies such as 5G. Visible light communication (VLC) is a wireless technology that uses light as medium of communication. With light emitting diodes (LEDs) lighting becoming common, it is a natural extension of the lighting technology to modulate the LEDs at high speed to enable potentially high bandwidth downlinks. Higher bandwidth becomes a major challenge in the development and implementation of VLC systems as transmission rates increase. One way of increasing bandwidth is through wavelength division multiplexing (WDM) in multicarrier systems that use red, green and blue (RGB) light emitting diodes to produce white light. However, these systems often suffer from low performance because of crosstalk interference, mainly caused by the imperfect nature of optical filters used to discriminate between the colours. The current research project aims to study the effects of optimal transmitter power control on the performance of WDM RGBLEDs VLC systems. The research investigates both centralized and distributed scenarios, using the water-filling algorithm and the Refereed Game Theory to mitigate the interference between the different light colour carriers. Results for the centralized power control show that by using mathematical models that take into consideration the effects of optical filters used for detection of colour signals, one is able to predict the trends in the overall system’s performance. This enables us to adequately, determine the optimal power levels in the red and blue channels that result in the best overall performance of the system. This information is then used to formulate a distributed optimization power control scheme based on the Refereed Game Theory, which shows that cooperation between colour carriers to minimize crosstalk can be enforced by introducing a referee.Item Effect of dielectric thickness on the bandwidth of planar transformers(2017) Vallabhapurapu, Hyma HarishThis research has considered an idealistic non-interleaved planar transformer wherein only the electromagnetic parasitic capacitive and inductive elements arising out of the transformer geometry are taken into account, without considering material limitations. A suitable model for the planar transformer is used to analyse its frequency and power transfer characteristics; this model was validated by three dimensional electromagnetic simulations of various planar transformer structures in FEKO simulation software. The capacitive and inductive parasitics in this model have been found to be functions of the dielectric thickness. The theoretical bandwidth for the planar transformer is defined in this research as a function of dielectric thickness. The effect of dielectric thickness of the transformer windings on the bandwidth of the transformer is analysed, based on the premise that the inherent parasitic capacitive and inductive elements would affect the transfer characteristics of the transformer. Upon conclusion of this analysis, it is found that the dielectric thickness of a planar transformer can be optimised such as to present an optimised bandwidth. A closed form analytic expression for the optimum dielectric thickness value is derived and presented in this research. In a design example of a 4:1 50W transformer presented in this research, it has been shown that the bandwidth can be improved by 384%, along with a power density improvement of 45%, upon choosing of an optimum dielectric thickness of 0.156mm to replace a standard 0.4mm thick dielectric. It should be noted that the results derived in this research are purely theoretical, justified by many idealisations and assumptions that are argued throughout the research. It is thus expected that practical results should at best approach the theoretical results, due to the known non-ideal nature of reality.Item Investigation into the correlation between paper insulation thermal ageing estimation using the arrhenius equation and other methods for generator transformers(2015) Metebe, Michael TebogoMany generator transformers were installed many years ago during the initial commissioning of Eskom’s power stations. Many of these transformers have started showing signs of significant ageing of the paper insulation and hence require regular monitoring. There are two methods that are currently being employed to assess the degree of ageing of the paper insulation in a generator transformer, which are paper sampling and furan level measurement. This dissertation investigates an alternative method of predicting the degree of ageing of the paper insulation instead of what is used currently. This method uses the Arrhenius equation that relates time and temperature to determine the degree of degradation of organic materials. The reliability of the Arrhenius estimation method is assessed by comparing the predicted DP (Degree of polymerisation) values with the measured DP values of the same transformer paper insulation. The results obtained showed that there is reasonable correlation between the DP values estimated from the Arrhenius equation and the DP values estimated from the measured furan levels. The accuracy of the prediction method is reduced when the oil temperature greatly differs from the paper insulation temperature. The application of the Arrhenius equation to estimate the ageing of paper insulation is a great milestone in the quest to predict the remaining life of a transformer. It is the only method available to do this prediction and using online temperature measurement on transformers makes the method more reliable.Item Electrical transient interaction between transformers and the power system: case study of an onshore wind farm(2016) Banda, Cedric AmittaiThrough the Renewable Energy Independent Power Producer Procurement Program (REIPPPP) the South African government has awarded opportunities for growth of renewable energy through bidding rounds. Round 1 saw a total capacity of 397 MW being awarded to independent power producers (IPP). Subsequently Rounds 2, 3 and 4 each had a total capacity of 333 MW auctioned. The advent of renewables on the market has brought upon its own associated problems with regards to power quality issues and failure of HV equipment. This thesis will address transformer failures that occurred at an onshore wind farm. The nature of the transformer failures suggest transient overvoltages are mainly to blame. A comparison between transformer failures in South African and Brazil suggest a common failure mechanism. The failure starts with an inter-turn insulation failure which propagates to an inter-layer insulation. In worst cases the failure mode results in a puncture through the LV-HV barrier and punctures through the LV winding. An extensive literature review was performed to find appropriate methods to predict and explain the failure mode in wind turbine LV-MV step-up transformers. Of the different models which were reviewed the most notable was the Multi-conductor Transmission Line (MTL) model which was chosen as the preferred model due to its ability to predict the inter-turn/inter-layer voltage stresses. Verification of the developed MTL model by the author was then compared to published results of an MTL model of a disc winding transformer. The results of the comparison revealed a relatively good agreement between the developed model and the published model. The application of the MTL model to represent the voltage stresses in transformer windings was then extended to two specially constructed wind turbine step-up transformer prototypes. The prototypes differed in the winding arrangement of the MV coil. The other used two separate MV coils separated by an oil gap whereas the other had a single MV coil. To validate the model accuracy, a comparison of measured results versus those obtained analytically was done for the two prototypes. The analytical and measured results also had a relatively good agreement for the two prototypes considered. Measurement of switching surges was done on-site at the wind farm to understand the nature of the transients. Using analysis tools such as FFT and frequency domain severity factor it was possible to understand the impact the nature of these transients would pose on the transformer insulation. Different mitigation techniques which can be used to alleviate the transient overvoltages to within safe levels were investigated. The most notable protection device considered was the RL choke device which offered a significant reduction of the pre-strikes and is virtually transparent under power frequency operation.