Ester liquids including both natural ester and synthetic ester are being considered as potential alternatives to mineral oil, due to their better environmental performance and for some liquids their higher fire point. Although these liquids have been widely used in distribution and traction transformers, it is still a significant step to adopt ester liquids in high-voltage power transformers because the high cost and severe consequence of a factory test failure and the high level of safety and reliability required in service for these units, tend to lead to a cautious approach to any step change in technology. Lightning impulse strength as basic insulation level is of importance for insulation design of power transformers and lightning impulse test is commonly required in the factory routine tests for high-voltage power transformers, so this thesis is aimed to investigate the electrical performances including pre-breakdown and breakdown of natural ester and synthetic ester under impulse voltage. Two types of field geometry were considered in the study, one is sphere-sphere configuration which represents the quasi-uniform fields inside a transformer and another is strongly non-uniform point-plane configuration which represents the situation of a defect or a source of discharge. In quasi-uniform field study, standard breakdown tests were carried out under negative lightning and switching impulse voltages. Influence of various testing methods on the measured lightning breakdown voltage was studied and the 1% lightning withstand voltage was obtained based on Weibull distribution fitting on the cumulative probability plot built up using the approximately 1000 impulse shots. As for strongly non-uniform field study, streamer propagation and breakdown event in ester liquids either with or without pressboard interface were investigated at various gap distances under both positive and negative lightning impulse voltages. A relationship between the results under lightning impulse and previously published results under step voltage was built up to predict the lightning breakdown voltage of ester liquids at very large gaps. The results indicated that impulse strengths of ester liquids for both breakdown and withstand in a quasi-uniform field, are comparable to those of mineral oil. In a strongly non-uniform field, streamers in ester liquids propagate faster and further, than in mineral oil at the same voltage level. Thus breakdown voltages of ester liquids are generally lower than those of mineral oil, which could be as low as 40% at a large gap distance of approximately 1000 mm. Introduction of parallel pressboard interface has no influence on the streamer propagation and thus does not weaken the breakdown voltage, but it tends to reduce the acceleration voltage particularly for mineral oil under positive polarity. Last but not least, a unique phenomenon of secondary reverse streamer (SRS) was observed in ester liquids, which occurs subsequently and well after the extinction of the primary streamer (PS) propagation within a single shot of impulse voltage and has the reverse polarity to the PS. The formation mechanism of SRS is explained mainly due to the reverse electric field induced by the residual space charges left by the PS.

The health of the transmission power transformers in the power system networks is critical to the reliability of electricity supply. Knowing the precise life expectancy of the transmission power transformer is of vital importance as it permits an optimised asset replacement. The traditionally regarded transmission power transformer’s life expectancy of 40 years is considered dated for the transformers in the UK according to the transformer life data in 2010. In this thesis, it is aimed to investigate the life expectancy of the transmission power transformer in the UK from three aspects: statistical analysis on historic transformer life data, thermal modelling of in-service transformers, and through the in-service transformers’ furan measurements.A detailed statistical analysis shows that deriving the transformer’s reliability at a certain age by calculating the hazard rate is inadequate, as the hazard rate at each age has a statistical range in which the confidence band width is related to the amount of the reliability data. The transformer life data in all ages are grouped together to derive a general hazard rate of 0.27%. It is concluded that the transformer life expectancy could not be derived via statistical approaches due to the limited data available at the older transformer ages.As an alternative approach, regarding the life of insulating paper as the ultimate life of a transformer, the thermal model published by the IEC transformer loading guide 60076-7 is reviewed and extended to estimate a transformer’s thermal lifetime. The model is improved in two aspects, such that Arrhenius equation is adopted to consider the paper’s practical ageing mechanism of oxidation and hydrolysis when calculating the paper’s ageing rate; and the model takes consideration of the paper’s moisture accumulation effect.The developed thermal model is used to reversely derive the generally unknown model input – hot-spot factor, by the means of regarding the scrapped transformer’s degree of polymerisation (DP) predicted thermal lives as a benchmark. Assigning the derived hot-spot factor to the field units with regard to the design family, the thermal lives of 106 in-service transformers have been estimated. To enlarge the life sample, the modelling lives are combined with the 79 scrapped transformers’ DP predicted thermal lives. The thermal life expectancy, defined as the median life of the sample set, is derived as 88 years. A series of sensitivity studies are performed to examine the derived life expectancy’s responses on the variations of load, winding-to-oil gradient, top-oil temperature rise, and the setting of winding temperature indicator.As a non-intrusive approach in transformer’s insulating paper assessment, the correlations between the 2-furaldehyde (2FAL) concentration dissolved in transformer oil and paper’s DP derived by different laboratories are reviewed which are found to differ significantly. As a first-time attempt to derive the 2FAL-DP correlation relationship for the field transformers, the paper’s DP is estimated at the age when oil was sampled using the thermal model, and is plotted with the 2FAL measurement. De Pablo’s equation is found to fit the plot of the DP estimates against the 2FAL measurements better than other function formats. The 2FAL concentrations corresponding to the paper’s critical DP levels are given using the developed 2FAL-DP correlation relationship.

The ability to withstand a short circuit is the most essential feature of a power transformer. The most important reason to design short-circuits proof transformers is to ensure the reliability of the power grid (avoiding black outs etc.) and safety (fire and explosion in case of failure). During short circuit, the most effected winding is the LV winding due to the flow high currents even during the normal working condition. So during a short circuit large forces are generated which act on the winding and these forces can reach hundreds of tons in fraction of a second, so the transformer must be properly designed in order to withstand these forces or the transformer can fail in different ways. One of the possible failure modes called “Spiraling” is discussed and analyzed in this thesis. Spiraling Occurs when the LV winding twists tangentially in the opposite direction at the ends due to radial short circuit forces. From literature study the transient forces acting on the winding during a 3-phase short circuit was determined and these transient forces were used to perform simulations on the model. The axial and radial forces applied on the model were such that it has a uniform magnitude per each turn. Various analysis was performed on the model which includes the Static, Modal and Transient Structural analysis in Ansys Workbench and each analysis involved parametric analysis where the deformations and the torsional mode shapes were determined

Orientador: André Luiz Andreoli / Resumo: Os transformadores são equipamentos de suma importância no sistema elétrico de potência. Qualquer falha destes equipamentos, pode acarretar prejuízos e consequências graves tanto para as concessionárias de energia, quanto aos seus consumidores. A ocorrência de descargas parciais em transformadores de potência está diretamente relacionada às condições de seu sistema de isolamento. Elas podem se desenvolver ao longo do tempo e culminar na perda total do sistema de isolamento. Desta forma, o monitoramento das descargas parciais torna-se essencial para o planejamento de uma manutenção e a garantia do funcionamento adequado destes equipamentos. Neste contexto, o objetivo desta pesquisa é apresentar um método de localização e identificação de descargas parciais em transformador de potência operando com carregamento nominal, através de sensores piezelétricos de baixo custo e ferramentas como a transformada wavelet e os sistemas de inferência neuro-difuso adaptativos. / Mestre

With deregulation, diagnosis and maintenance of power equipment, especially power transformers, become increasingly important to keep power systems in reliable operation. This dissertation systematically studied two kinds of transformer failure and abnormality cases, and then developed a new Internet based Virtual Hospital (VH) for power equipment to help power equipment diagnosis and maintenance. A practical case of generator-step-up (GSU) transformer failures in a pumped storage plant was extensively studied. Abnormal electrical phenomena associated with GSU transformers, including switching transients and very fast transients (VFT), and lightning, were analyzed. Simulation showed that circuit breaker restriking could be a major cause of transformer successive failures, and current surge arrester configuration did not provide enough lightning protection to GSU transformers. Mitigation of abnormal electrical phenomena effects on GSU transformers was proposed and discussed. The study can be a complete reference of troubleshooting of other similar transformer failures. Geomagnetically induced current (GIC) is another possible cause of transformer abnormality. A simplified method based on the equivalent magnetizing curve for transformers with different core design was developed and validated to estimate harmonic currents and MVar drawn by power transformers with a given GIC. An effective indicator was proposed using partial harmonic distortion, PHD, to show when the transformer begins saturating with the input GIC. The developed method has been applied to a real time GIC monitoring system last year for a large power network with thousands of transformers. A new Internet based Virtual Hospital (VH) for Power Equipment was conceptually developed to share experience of power equipment diagnosis and maintenance, and update the existing diagnostic techniques and maintenance strategies, and a comprehensive information model was developed for data organization, access, and archiving related to equipment diagnosis and maintenance. An Internet based interactive fault diagnostic tool has been launched for power transformers based on dissolved gas analysis (DGA). The above results and findings can help improving power equipment diagnosis and utility maintenance strategies. / Ph. D.

This thesis proposes one comparison methodology for single and three-phase high power high frequency transformers in power conversion systems. The objective is to compare the volume of the transformers. And single and three-phase Dual Active Bridge Converter (DAB1 and DAB3) topologies with single and three-phase isolating transformers are selected for the transformer comparison. Design optimization of power transformer has been studied and simplified models have been built for the single and three-phase transformer design optimization in this work, including assumptions for core shapes, materials, winding structures and thermal model. Two design methods have been proposed according to different design constraints, named T – B Method and J – B Method separately. T – B Method is based on feature of the core, which has the major limits of maximum flux density and temperature rise. The flux density should not reach the saturation value of the core, and temperature rise should meet specifications in different applications to assure the performance of the core (permeability, saturation flux density, and core loss) and the insulation of the wire. And J – B Method starts from the comparison of area product in conventional design method. The relationship between area product of transformer cores and the flux and current of the transformer in design is analyzed. There is specified relationship between area product of single and three-phase transformers if flux and current densities are specified for both. Thus J – B Method is proposed with the design constraints of specified current and flux density. Both design methods include both single and three-phase transformer design. One example case for single and three-phase transformer comparison is selected as high power high frequency DAB conversion system. Operation principles are studied for both DAB1 and DAB3 based on previous work. And transformer design based on the T – B and J – B Methods are carried out and transformer volumes are compared. And results show that three-phase transformer has little benefit in volume or thermal than single-phase transformer, when they are utilized in single-phase DAB and three-phase DAB converters separately. Scaled-down single and three-phase DAB systems have been built and volume and thermal tests have been carried out. / Master of Science

In this work, we first analyze the behavior of the acoustic wave from the theoretical point of view using a simplified 1-dimensional model. The model was developed based on the conservation of mass, the conservation of momentum, and the state equation; in addition, the fluid medium obeys Stokes assumption and it is homogeneous, adiabatic and isentropic. Experiment and simulation results show consistency to theoretical calculation. The second part of this thesis focuses on the PD signal analysis from an on-site PD measurement of the in-house design fiber optic sensors (by Virginia Tech, Center for Photonics Technology). Several commercial piezoelectric transducers (PZTs) were also used to compare the measurement results. The signal analysis employs the application of wavelet-based denoising technique to remove the noises, which mainly came from vibration, EMI, and light sources, embedded in the PD signal. The denoising technique includes the discrete wavelet transform (DWT) decomposition, thresh-holding of wavelet coefficients, and signal recovery by inverse discrete wavelet transform. Several approaches were compared to determine the optimal mother wavelet. The threshold limits are selected to remove the maximum Gaussian noises for each level of wavelet coefficients. The results indicate that this method could extract the PD spike from the noisy measurement effectively. The frequency of the PD pulse is also analyzed; it is shown that the frequencies lie in the range of 70 kHz to 250 kHz. In addition, with the assumed acoustic wave propagation delay between PD source and sensors, it was found that all PD activities occur in the first and third quadrant in reference to the applied sinusoidal transformer voltage. / Master of Science

Os transformadores de potência elétrica produzem um elevado nível de ruído que em alguns casos gera desconforto à comunidade vizinha. O presente trabalho desenvolve um modelo estrutural de elementos nitos que, com carregamento adequado, é capaz de reproduzir o deslocamento, medido durante a operação, de um transformador de potência elétrica. Este modelo estrutural é utilizado para calcular a redução do deslocamento das superfícies deste transformador decorrentes do emprego de absorvedores dinâmicos de vibração. Este modelo estrutural foi desenvolvido para estimar o número de absorvedores dinâmicos, sua massa e sua localização que permitiria reduzir o ruído acústico emitido por este transformador de potência até alcançar níveis de intensidade de som em comformidade com a legislação brasileira. / Electrical power transformers produce a high level of noise, which, in some cases, generate discomfort to the surrounding community. The present work develops a structural model by the Finite Element Method that, with adequate loading conditions, is capable of reproducing the displacement, measured during operation, of the transformer tank. The structural model has been used to compute the displacement reduction of the transformer surfaces by using dynamic vibration absorbers. The structural model was developed to estimate the number of dynamic absorbers, the mass and localization that could reduce the acoustic noise to levels of intensity of sound in accordance to the Brazilian law.

O crescente aumento na demanda de energia elétrica nacional, associada às alterações regulamentares do setor, em que o tempo que um equipamento permanece indisponível para o sistema, aguardando manutenção significa perda de receita para as companhias de energia, motivou a busca por diagnósticos precisos e utilização de técnicas não invasivas que possam ser aplicadas em transformadores em serviço. Assim, o foco desta pesquisa foi o desenvolvimento de uma arquitetura de sistema inteligente baseado em Redes Neurais Artificiais, que a partir de características extraídas de sinais de emissão acústicas provenientes de sensores distribuídos espacialmente no tanque de transformadores de potência, possa identificar internamente o local de ocorrência das descargas parciais e fornecer as distâncias estimadas entre os sensores e o ponto dessa descarga, e com essas distâncias, utilizando técnicas numéricas de triangulação, o sistema fornece também a coordenada espacial da falha auxiliando no diagnostico de defeito do transformador e no processo de tomada de decisões. / The increasing in the demand for national electrical energy coupled with alterations in the regulation of the sector, where the time which a piece of the electrical system equipment stays out of service means loss of income for the electrical companies, has motivated the search for correct diagnostics and usage of non-invasive technics that can be used in tranformers which are in operation. Therefore, the aim of this research was to develop the design of an intelligent system based on Artificial Neural Nets, which through the characteristics extracted from the acoustic emission signals coming from the sensors spatially distributed in the power transformer tank can identify internally the place of the occourrence of the partial discharges and provide the estimate distances between the sensors and the discharges point, so with these distances using triangle technical analysis the system will also provide the spatial coordinate of the flaw for diagnosing the problem with the transformer and help the process of decision making.

Esta pesquisa apresenta uma metodologia, baseada em duas técnicas não invasivas para identificação e diagnóstico de pontos quentes em transformadores de potência durante plena operação ou em fase de projeto. A primeira técnica é baseada na medição de radiação infravermelha, emitida pelo equipamento em funcionamento e registrada por meio de análise termográfica. A segunda técnica é possível a partir do conhecimento prévio das características construtivas do transformador, fazendo uso do Método dos Elementos Finitos (MEF). A segunda técnica pode ser validada a partir das medições realizadas utilizando a primeira técnica. A formação de gases no interior dos transformadores de potência, devido à elevação da temperatura do óleo isolante em função dos pontos quentes, é discutida em detalhes com base nas normas técnicas estabelecidas pelo IEEE e IEC. As técnicas e procedimentos abordados ao longo dessa pesquisa foram obtidos a partir de um transformador com potência nominal de 120 MVA e relação de tensão 13,8/230 kV, projetado para integrar uma fazenda eólica ao Sistema Interligado Nacional (SIN). / This research presents a methodology based on two noninvasive techniques for identification and diagnostic of hot spots in power transformers during operation or project development. The first is based on measurements of infrared radiation from the equipment during operation and recording by thermography. The second technique is possible from the previous knowing of the constructive characteristics of the power transformer, by using the Finite Element Method (FEM). The second technique can be validated from measurements obtained using the first technique. The gas formation into the power transformers, because of the high temperatures in the insulating oil due to the hot spots, is discussed in details based on normative recommendations well established by the IEEE and IEC. All techniques and procedures to be approached in this research were obtained using a 120-MVA power transformer with voltage relationship of 13.8/230 kV that was projected to interconnect a wind farm to the Interconnected Brazilian System (Sistema Interligado Nacional - SIN).