Thesis (M.S.) University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on August 29, 2007) Includes bibliographical references.

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.

M.Ing. / The study looks into the impact of harmonic distortion on power-plant equipment in general, and then focuses on the impact it has on power transformers operating near the thermal limit. The feasibility of the study is firstly evaluated and then the theory on harmonics and transformer losses is analysed. The study had been narrowed down to power transformers due to the high numbers of failures nationally and internationally attributed to unknown causes. A transformer model is then developed through theoretical considerations. Finally, a case study is done on the capability of a fully loaded transformer under harmonics conditions evaluated through transformer capability calculations and the proposed transformer model. Thereafter the transformer model developed is verified with measured results. The main impact of harmonic current distortion on power transformers is an increase in the rated power losses that results in a temperature rise inside the power transformer. The heat build-up can lead to degradation of insulation, which can shorten the transformer’s life and lead to eventual breakdown. The harmonic current distortion impacts transformer losses – namely, ohmic losses, the winding eddy current losses and other stray losses. All of these harmonic effects on transformer losses are verified theoretically, mathematically and practically. The harmonic impact on the transformer capability is then evaluated through a numerical example of a transformer feeding a harmonic load. The transformer capability is determined via two methods – namely, harmonic capability calculations in the standard “IEEE Recommended Practice for Establishing Transformer Capability when Supplying Nonsinusoidal Load Currents”, [11] and a proposed transformer model derived from theoretical and mathematical analysis. The results show that an increase in the winding eddy current losses can decrease the maximum permissible nonsinusoidal load current substantially. If the load current of the transformer is derated accordingly it translates into a loss of the output power capacity of the power transformer. The standard recommended capability calculations for winding eddy current losses are conservative and not satisfactorily accurate. This results in a large loss of power capacity. The proposed transformer model includes a parameter that estimates the winding eddy current loss in the transformer that results in a smaller loss in power capacity. Furthermore, it was shown that the harmonic current distortion levels could exceed the permissible levels although the harmonic voltage distortion levels are within acceptable levels. The proposed transformer equivalent model is thereafter practically verified with experimental results of papers published by M.A.S. Masoum, E.F. Fuchs and D.J. Roesler, [19], [20] and [29].

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, 2017 / This 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. / CK2018

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. / The sudden failure of oil paper-insulated current transformers has become a problem of considerable concern due to the cost of the resulting destruction, and danger to personnel. The aim of the dissertation is. to determine the most suitable method of detecting lncipient faults in current transformers, test this method on an experimental current transformer, and finally implement this technique in a low cost on-line monitor. A literature survey indicated that hydrogen gas-in-oil on-line monitoring would be the most suitable technique.(Abbreviation abstract) / Andrew Chakane 2019

M. Tech. Electrical Engineering. / Voltage control is an essential part of the electricity transmission and distribution network system for maintaining voltage within statutory limits at the substation bus and at the consumer's point of supply. The most popular voltage control method on the transmission and distribution network system uses the on-load tap changer (OLTC) transformer. This study proposes the use of Fuzzy Logic Control (FLC) systems on the on-load tap changers when two or more mismatched transformers are connected in parallel in order to control the voltage supply to local customers. The actual voltage recordings and on-load tap change operations for each of the paralleled transformers have been analyzed and modelled on MATLAB/Simulink in order to develop an improved control algorithm using Fuzzy Logic Control. With the two-input, one-output FLC system with seven linguistic values giving 49 control rules, the number of tap change operations are minimized and the bus voltage is maintained within the statutory limits.

This dissertation analyses the reliability of power transformers and its impact of failure on system performance. Eskom Distribution, Eastern Region is used as a practical case study, which has an installed transformer base of 6066MVA comprising of 428 transformers ranging from 1 MVA to 8OMVA with voltage levels of 6.6kV to I 32kV. The literature review illustrates the theory and principles of transformers, evolution and changes in design criteria, the function of cellulose and insulating oil, failure modes. operations and maintenance practices and factors affecting the distribution systems performance. This study included a conditional assessment and an oil analysis review of transformers at Eskom. A method to trend multiple oil samples was developed and illustrated. The research further investigates the reliability of series and parallel systems using actual component reliability values. A study was conducted to establish the degree of network firmness. Transformer failure data was analysed and were shown to be characteristic of a bathtub curve. Defects from on site inspections were analysed and identified oil leaks as a maintenance focus area. The Distribution Supply Loss Index was determined to be the major impact Key Performance Index due to transformer failures. Transformer failures using statistical methods, showed HV/LV winding to be the main component to fail. The cost of a transformer failure to Eskom and the customer was determined. International Benchmarking was investigated to establish the criteria for network reliability indices and to compare the network infrastructure and performance of international utilities and Eskom. The later part of the study involved the analysis of a risk ranking methodology to establish a risk ranking matrix. The transformers were ranked according to the matrix, identifying the high risk focus areas. Projects were raised within Eskom to replace the identified high risk transformers. This study has concluded that the reliability of transformers is impacted by the changes in transformer design, increased maintenance defects and inadequate transformer protection at substations. The reduced oil volume per kVA, increased hot spot and ambient temperature, and compact tank designs have resulted in the cellulose being overheated and fault gases being produced during normal operating conditions. The increase in load demand from the existing transformer fleet and a reduction in capital expenditure to maintain and build additional substations have also contributed to accelerated aging, since the transformers are forced to operate at 100 percent loading. There is an increase in transformer maintenance defects due to in sufficient operational staff, high staff turnover, reduced skills transfer. and insufficient network contingencies to allow for planned outages to clear the defects identified. The failure analysis showed that the main component to fail is HV/LV windings. The winding failures were traced to there being no or inadequate transformer protection at -20% of substations. / Thesis (M.Sc.)-University of KwaZulu-Natal, 2007.

The successes or failures one experiences in life depend greatly on the decisions that one makes. This is not only true in one’s personal life but is also the case in the business environment. In this modern world, simple and complex decisions are the key elements for a business to be successful in the competitive global environment. Effective decision making is an intricate process. In Eskom it is important to integrate the technological and business aspects to support the decision making process. Research methodology provides one with the necessary tools to support this decision making. The main focus of the study is the development of an Optimisation Model for the Management of Transformers in the Eskom East Grid Transmission Sysytem. In the Eskom Transmission system, there are a large number of power transformers which are the most expensive and strategically important component of the Transmission system. There were three main objectives. The first objective included the forecasting of transformer failures using the forecasting techniques of moving average, weighted moving average, exponential smoothing and regression analysis. The second objective focused on the investigation of the re – location of the tap changer maintenance team to a new area using the mathematical and statistical methods of simulation and decision tree analysis. The third objective included the investigation of an inventory control and management model where the optimum number of spares pertaining to transformer units that should be made available was determined using the mathematical and statistical model called the economic order quantity. These objectives were then used to investigate the development of the optimisation model pertaining to transformers. The results of the study concluded that the operating research techniques which included the forecasting methods and the economic order quantity models were suitable for research in Eskom. However it is important to note that the Eskom system and the environment in which transformers operate in is dynamic and has some factors that cannot be controlled. These factors must be taken into consideration when the various models are used in the investigation of the optimisation model. The introduction of these external factors is beyond the scope of this study and is not included. It was concluded that the simulation and decision tree analysis could be used as an integral part of the optimisation model successfully. The limitations that were highlighted included the integrity of the secondary data (sample size, the source of the secondary data, data quality and data governance), the limitations associated with forecasting, the limitations of the operations research, mathematical and statistical models and the fact that the Eskom network is dynamic. . The recommendations included the application of the forecasting techniques and the inventory control model to a larger population size which was that of the transformers in the entire Transmission system. It was indicated that Eskom resources, time and money must be used to support the business’s strategy to train and develop employees to an acceptable competency level. Future studies should include the factors that affect forecasting and the implementation of the inventory control model to increase the accuracy of the results. The introduction of these external factors is beyond the scope of this study and was not included in the models. / Thesis (MBA)-University of KwaZulu-Natal, Westville, 2009.

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Thesis (Ph. D.)--Victoria University (Melbourne, Vic.), 2007. / Includes bibliographical references.