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.

The thesis begins by providing an introduction to transformer theory. An ideal transformer is examined first, followed by full core transformer theory. The partial core transformer is then introduced and compared to the full core design. An introduction to superconductors is then presented where a simplified theory of superconductivity is given. High temperature superconductors are then examined including their physical structure, superconducting properties and the design of the superconducting wire. The early development of high temperature superconducting partial core transformers at the University of Canterbury is then examined. Early partial core development is discussed followed by some material testing at cryogenic temperatures. This work lead into the development of the first high temperature superconducting partial core transformer. This transformer failed during testing and an examination of the failure mechanisms is presented. The results of the failure investigation prompted an alternative winding insulation design which was implemented in a full core superconducting transformer. The modelling used to design a high temperature superconducting partial core transformer is then presented. Based upon the reverse design method, the modelling is used to determine the components of the Steinmetz equivalent transformer circuit. The modelling includes a combination of circuit theory and finite element analysis. An ac loss model for high temperature superconductors is also presented. A new 15 kVA, 230-230V high temperature superconducting partial core transformer was designed, built and tested. The windings are layer wound with first generation Bi2223 high temperature superconductor. The modelling was used to predict the performance of the transformer as well as the ac losses of the high temperature superconductor. A series of electrical tests were performed on the transformer including open circuit, short circuit, resistive load, overload, ac withstand voltage and fault ride through tests. The test results are compared with the model. The transformer was found to be 98.2% efficient at rated power with 2.86% voltage regulation.

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

O transformador de potência é o ativo mais valioso e importante de uma subestação, razão pela qual se faz necessário um acompanhamento especial para estes equipamentos. A operação de qualquer equipamento fora de suas condições nominais é sempre uma situação de risco, porém, tal risco pode ser minimizado através de estudos e análises particulares para cada equipamento.Com os transformadores de potência isso não é diferente, as normas e guias de aplicação de cargas em transformadores prevêem períodos de sobrecarga dentro de algumas condições, porém, há uma grande incerteza nesses dados já que cada transformador envelhece de forma diferente ainda que sejam idênticos de projeto e fabricação. Por essa razão se faz necessária uma análise particular de cada equipamento onde serão identificados os riscos em potencial e a partir daí determinado o grau de confiabilidade do equipamento. Com base em informações históricas dos equipamentos e algoritmos desenvolvidos com base nas normas vigentes, foi desenvolvido o Sistema Integrado de Gestão denominado SGT. Através deste sistema será possível armazenar de forma organizada as informações de todas as unidades fornecendo subsídios para uma tomada de decisão minimizando os riscos. Através do Sistema Integrado será possível estabelecer uma base única de consultas, promovendo assim a otimização destes equipamentos face a regulação do setor elétrico. / The power transformer is the most valuable and important assets of a substation, for that reason is necessary a special attendance for these equipments. The operation of any equipment out of their nominal conditions it is always a risk situation, however, such a risk can be minimized through studies and private analyses for each equipment. Power transformers are not different, the norms and guides of load application foresee overload periods under some conditions, however, and there is a great uncertainty in those data since each transformer ages in a different way although they are identical of project and production. For that reason are necessary peculiar analysis of each equipment where you/they will be identified the potential risks and since then certain the degree of reliability of the equipment. Based in historical informations of the equipments and algorithms developed in agreement with the effective norms, the Integrated System of Management was developed and denominated SGT. Through this system it will be possible to store in an organized way the information of all of the units, supplying subsidies to have a safe decision minimizing the risks. Through the Integrated System it will be possible to establish an only base of consultations, promoting the assets optimization based in the new regulation of the electric section.

A quantidade de falhas recentes em transformadores e suas consequências são alarmantes. As perdas econômicas, os riscos e a falta de confiabilidade decorrentes são inaceitáveis. Face à gravidade da situação um conjunto de recomendações de implementação imediata é estabelecido. O objetivo é assegurar a continuidade da operação confiável dos transformadores em serviço e garantir a especificação segura de transformadores novos. As recomendações envolvem o dimensionamento de enrolamentos levando em conta transitórios rápidos e ressonâncias internas; auditoria e controle de projetos; formas de ondas de ensaio mais representativas de fenômenos transitórios não convencionais; ações associadas à condições de operação; monitoração contínua e aplicação de filtros para a proteção frente a transitórios rápidos. Estas recomendações apoiam-se em análises cuidadosas das condições de projeto, fabricação e ensaios de transformadores complementadas pela análise teórica e de campo de falhas recentes em grandes transformadores de alta tensão. Vários modos de falhas dielétricas de transformadores estão associados a transitórios eletromagnéticos não convencionais. É analisada a modelagem de enrolamentos para transitórios de alta frequência. Os resultados de simulações aplicadas a alguns transformadores de alta tensão são apresentados. A aplicação das recomendações apresentadas deverá resultar na redução de falhas em grandes transformadores de alta tensão. / The number of transformer failures and its consequences are of very high concern. The associated economic losses, risks to life and environment and reduced reliability level are not acceptable any more. From this situation, a set of recommendation with possible immediately implementation is established. The objective is to give a reliable continuous operation condition to the remaining transformers in service and guarantee a safe specification for new transformers. The recommendations are including the dimensioning of transformer windings related to very fast transients and internal resonance; design review; dielectric test waves more representatives of non conventional transients phenomena; operation conditions; continuous monitoring and application of filter as protection device against very fast transients. These recommendations are supported by deep analysis of design conditions, manufacturing and test of power transformers complemented with theoretic and on site analysis of recent failures of high voltage large power transformers. Several dielectric failures modes of transformers are associated with non conventional electromagnetic transients. From this, it is included a modeling of transformer windings for high frequency transients. The results of simulations applied to high voltage transformers are presented. The application of the presented recommendations shall give as result a reduction in number of failures of large power transformers.

A quantidade de falhas recentes em transformadores e suas consequências são alarmantes. As perdas econômicas, os riscos e a falta de confiabilidade decorrentes são inaceitáveis. Face à gravidade da situação um conjunto de recomendações de implementação imediata é estabelecido. O objetivo é assegurar a continuidade da operação confiável dos transformadores em serviço e garantir a especificação segura de transformadores novos. As recomendações envolvem o dimensionamento de enrolamentos levando em conta transitórios rápidos e ressonâncias internas; auditoria e controle de projetos; formas de ondas de ensaio mais representativas de fenômenos transitórios não convencionais; ações associadas à condições de operação; monitoração contínua e aplicação de filtros para a proteção frente a transitórios rápidos. Estas recomendações apoiam-se em análises cuidadosas das condições de projeto, fabricação e ensaios de transformadores complementadas pela análise teórica e de campo de falhas recentes em grandes transformadores de alta tensão. Vários modos de falhas dielétricas de transformadores estão associados a transitórios eletromagnéticos não convencionais. É analisada a modelagem de enrolamentos para transitórios de alta frequência. Os resultados de simulações aplicadas a alguns transformadores de alta tensão são apresentados. A aplicação das recomendações apresentadas deverá resultar na redução de falhas em grandes transformadores de alta tensão. / The number of transformer failures and its consequences are of very high concern. The associated economic losses, risks to life and environment and reduced reliability level are not acceptable any more. From this situation, a set of recommendation with possible immediately implementation is established. The objective is to give a reliable continuous operation condition to the remaining transformers in service and guarantee a safe specification for new transformers. The recommendations are including the dimensioning of transformer windings related to very fast transients and internal resonance; design review; dielectric test waves more representatives of non conventional transients phenomena; operation conditions; continuous monitoring and application of filter as protection device against very fast transients. These recommendations are supported by deep analysis of design conditions, manufacturing and test of power transformers complemented with theoretic and on site analysis of recent failures of high voltage large power transformers. Several dielectric failures modes of transformers are associated with non conventional electromagnetic transients. From this, it is included a modeling of transformer windings for high frequency transients. The results of simulations applied to high voltage transformers are presented. The application of the presented recommendations shall give as result a reduction in number of failures of large power transformers.

O transformador de potência é o ativo mais valioso e importante de uma subestação, razão pela qual se faz necessário um acompanhamento especial para estes equipamentos. A operação de qualquer equipamento fora de suas condições nominais é sempre uma situação de risco, porém, tal risco pode ser minimizado através de estudos e análises particulares para cada equipamento.Com os transformadores de potência isso não é diferente, as normas e guias de aplicação de cargas em transformadores prevêem períodos de sobrecarga dentro de algumas condições, porém, há uma grande incerteza nesses dados já que cada transformador envelhece de forma diferente ainda que sejam idênticos de projeto e fabricação. Por essa razão se faz necessária uma análise particular de cada equipamento onde serão identificados os riscos em potencial e a partir daí determinado o grau de confiabilidade do equipamento. Com base em informações históricas dos equipamentos e algoritmos desenvolvidos com base nas normas vigentes, foi desenvolvido o Sistema Integrado de Gestão denominado SGT. Através deste sistema será possível armazenar de forma organizada as informações de todas as unidades fornecendo subsídios para uma tomada de decisão minimizando os riscos. Através do Sistema Integrado será possível estabelecer uma base única de consultas, promovendo assim a otimização destes equipamentos face a regulação do setor elétrico. / The power transformer is the most valuable and important assets of a substation, for that reason is necessary a special attendance for these equipments. The operation of any equipment out of their nominal conditions it is always a risk situation, however, such a risk can be minimized through studies and private analyses for each equipment. Power transformers are not different, the norms and guides of load application foresee overload periods under some conditions, however, and there is a great uncertainty in those data since each transformer ages in a different way although they are identical of project and production. For that reason are necessary peculiar analysis of each equipment where you/they will be identified the potential risks and since then certain the degree of reliability of the equipment. Based in historical informations of the equipments and algorithms developed in agreement with the effective norms, the Integrated System of Management was developed and denominated SGT. Through this system it will be possible to store in an organized way the information of all of the units, supplying subsidies to have a safe decision minimizing the risks. Through the Integrated System it will be possible to establish an only base of consultations, promoting the assets optimization based in the new regulation of the electric section.

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.