Wide bandgap (WBG) devices and power electronic converters (PEC) that enable the dynamic control of energy and high-power density designs inevitably contain defects including sharp edges, triple points, and cavities, which result in local electric field enhancements. The intensified local electric stresses cause either immediate dielectric breakdown or partial discharge (PD) that erodes electrical insulators and accelerates device aging. With the goal of addressing these dielectric challenges emerging in power-dense applications, this dissertation focuses on 1) modeling the dielectric characteristics of supercritical fluids (SCFs), which is a new dielectric medium with high dielectric strength and high cooling capability; and 2) establishing the optimal fabrication conditions of electrets, which is a new dielectric solution that neutralizes locally enhanced electric fields. In this dissertation, the dielectric breakdown characteristics of SCFs are modeled as a function of pressure based on the electron scattering cross section data of clusters that vary in size as a function of temperature and pressure around the critical point. The modeled breakdown electric field is compared with the experimental breakdown measurements of supercritical fluids, which show close agreement. In addition, electrets are fabricated based on the triode-corona charging method and their PD mitigation performance is evaluated through a series of PD experiments. Electrets are fabricated under various charging conditions, including charging voltage, duration, polarity, and temperature with the goal of identifying the optimal condition that leads to effective PD mitigation. The PD mitigation performance of electrets fabricated based on these charging conditions is further assessed by investigating the impact of various power electronics voltage characteristics, including dv/dt, polarity, switching frequency, and duty cycle. Electret based electric field neutralization approach is further utilized in increasing the critical flashover voltage associated with the surface flashover voltage. Moreover, due to the high mechanical strength of epoxy composites at cryogenic temperatures, in this dissertation, epoxy-based electrets are fabricated as a solution to PD in high temperature superconducting cables. The experimental demonstrations conducted with electret in this dissertation is dedicated for the establishing the electret based electric field neutralization approach as a dielectric solution for the dielectric challenges in power electronics driven systems.

No description available.

Partial discharge measurement is a common method for monitoring and diagnostics of power transformers, and can detect insulation malfunctions before they lead to failure. Different parameters extracted from the measured PD activity can be correlated to the PD source, and as a result it is possible to identify the PD source by analyzing the PD activity. In this thesis, possible defects that could cause harmful PDs in transformers were investigated. These defects include corona in oil, a void in pressboard, a metal object at floating potential, surface discharge in oil, a free bubble in oil and small free metallic particles in oil. The characteristics of disturbing discharge sources were analyzed, like corona in air, surface discharge in air, and discharge from an unearthed object near to the test setup. The PD activity was recorded both in the time domain and phase domain, and possible characteristics for each PD pattern and waveform were extracted in order to find the best characteristic for the purpose of classification. The results show that in the phase domain parameters such as phase of occurrence, repetition rate and shape of PD Pattern are most suitable for classification while magnitude of discharge can only be useful in specific cases. The results show that the PD waveforms correlated to different defects are similar; however the time domain data include all the information from the phase domain, and also has the power to identify the number of PD sources.  The PD dependency on temperature was investigated on the four test objects including surface discharges in oil, corona in oil, bubble discharges in oil, and metal object at floating potential. The effect of humidity was investigated for corona in oil. The results show that at higher temperature the corona activity in oil and PD activity due to a metal object at floating potential in oil decrease. However, for a bubble in oil and for surface discharge in oil the PD activity increases with the increase of the oil temperature. It was shown that the amount of moisture in oil has a strong impact on number of corona pulses in oil. The last part focused on ageing of oil-impregnated paper due to PD activity. Investigation was made of the behavior of PD activity and its corresponding parameters such as PD repetition rate and magnitude, from inception until complete puncture breakdown. The results show that both the number and magnitude of PD increase over time until they reach to a peak value. After this point over time both curves decrease slowly, and eventually full breakdown occurs. The effect of thermal ageing of oil impregnated paper on time to breakdown and PD parameters was investigated. The results show that thermal aging of oil-impregnated paper increases the number and magnitude of PD. Dielectric spectroscopy was performed on the samples before and after PD ageing and the result was used in order to explain the behavior of PD over time. / <p>QC 20121129</p>

Power transformers are one of the most important components in the electrical energy network. Extending transformer life is very economically valuable due to power outage. Therefore the development of instruments to monitor the transformer condition is of great interest. Detection of partial discharges (PDs) in power transformers is an effective diagnostic because it may reveal and quantify an important aging factor and provide information on the condition of the transformer. However, partial discharge diagnostics are still not effectively used for online monitoring of transformers because of the complexity of PD measurements and difficulties of discriminating of PDs and other noise sources. This thesis presents a further study of detection and location of partial discharges in power transformers based on previous work conducted at the Center for Photonics Technology (CPT) at Virginia Tech. The detection and positioning system consists of multiple extrinsic Fabry-Parot interferometric (EFPI) fiber acoustic sensors which can survive the harsh environment of oil-filled transformers. This thesis work is focused on optimal arrangement of multiple sensors to monitor and locate PD activities in a power transformer. This includes the following aspects. First, the sensor design requirements are discussed in order to successfully detect and accurately position the PD sources. In the following sections, Finite Element Method (FEM) is used to model the EFPI sensor fabricated at CPT. Experiments were conducted to measure the angular dependence of the frequency response of the sensor. It is shown that within the range of ±45º incident angles, the sensitivity varies by 3-5dB. Finally, the thesis demonstrates a PD positioning experiment in a 500 gallon water tank (R à H = 74" à 30" cylinder) using a hyperbolic positioning algorithm and time difference of arrival (TDOA). Finally we demonstrated that 100% of the positioning data is bounded by a 22.7à 4.1à 5.3 mm₃ cube, with a sensing range of 810 mm using the leading edge method with FIR filtering. / Master of Science

A partial discharge (PD) is the dissipation of energy caused by the buildup of localized electric field intensity. In high voltage devices such as transformers, this buildup of charge and its release can be symptomatic of problems associated with aging, such as floating components and insulation breakdown. This is why PD detection is used in power systems to monitor the state of health of high voltage transformers. If such problems are not detected and repaired, the strength and frequency of PDs increases and eventually leads to the catastrophic failure of the transformer, which can cause external equipment damage, fires and loss of revenue due to an unscheduled outage. Reliable online PD detection is a critical need for power companies to improve personnel safety and decrease the potential for loss of service. The PD phenomenon is manifested in a variety of physically observable signals including electric and acoustic pulses and is currently detected using a host of exterior measurement techniques. These techniques include electrical lead tapping and piezoelectric transducer (PZT) based acoustic detection. Many modern systems use a combination of these techniques because electrical detection is an older and proven technology and acoustic detection allows for the source to be located when several sensors are mounted to the exterior of the tank. However, if an acoustic sensor could be placed inside the tank, not only would acoustic detection be easier due to the increased signal amplitude and elimination of multipath interference, but positioning could also be performed with more accuracy in a shorter time. This thesis presents a fiber optic acoustic sensing system design that can be used to detect and locate PD sources within a high voltage transformer. The system is based on an optical acoustic (OA) sensor that is capable of surviving the harsh environment of the transformer interior while not compromising the transformer's functionality, which allows for online detection and positioning. This thesis presents the theoretical functionality and experimental validation of a band-limited OA sensor with a usable range of 100-300 kHz, which is consistent with the frequency content of an acoustic pulse caused by a PD event. It also presents a positioning system using the time difference of arrival (TDOA) of the acoustic pulse with respect to four sensors that is capable of reporting the three-dimensional position of a PD to within ±5cm on any axis. / Master of Science

This thesis focuses on the development of a novel polymer based housing for power thyristor devices typically used in long distance high voltage direct current (HVDC) transmission. Power thyristor devices used in HVDC power conversion stations are typically packaged in a hermetically sealed ceramic housing and have demonstrated an excellent history of reliability and performance. However, to avoid increasing the number of thyristors in future higher powered HVDC schemes thyristors having higher power ratings at 8.5 kV and sizes at 125 mm and 150 mm diameters are sought for implementation to achieve higher transmission ratings of, for example, 4000 A at +/- 800 kV. The main disadvantages of such large ceramic-based packages are higher processing cost and weight whilst robustness is also a concern. To overcome these issues, replacing the current ceramic housing with a polymeric material has been investigated in this project. The advantages it is anticipated such packages will provide include lower cost, less weight, robustness, recyclability, etc. However, some challenges it will also offer are: non-hermeticity i.e. polymers are moisture and gas permeable, potentially more complex manufacturing routes, and different electrical, mechanical and thermal properties compared to ceramic materials. The work presented in this thesis was part of a larger project where these challenges have been addressed by developing and testing a prototype polymeric thyristor housing. The prototype is aimed at demonstrating that polymer packages can deliver performance and reliability comparable to, if not better than, current ceramic packages. In this thesis, it is the package development and reliability related studies that are discussed. Because the housings will experience severe electrical stresses and various thermal excursions during their service life, the electrical and thermo-mechanical behaviour of the polymer housing was studied using finite element analysis to gain an understanding of the effects of various design variables and materials properties on performance and the tradeoffs between performance and manufacturability. From these modelling studies, design guidelines have been established for the future development of polymer housings. On the other hand, to identify the physics-of-failure of the prototype that was manufactured as part of the project, accelerated life tests were performed to study its reliability. The knowledge gained from the polymer prototype development was then applied to the design of a larger 125 mm diameter housing using the Taguchi method of experimental design.

Submitted by Bruno Albuquerque de Castro null (bruno.castro@feb.unesp.br) on 2016-06-01T17:52:34Z No. of bitstreams: 1 Bruno_Albuquerque_de_Castro_MestradoEE.pdf: 3788528 bytes, checksum: 28b69717116921fa999eda08254ac160 (MD5) / Approved for entry into archive by Ana Paula Grisoto (grisotoana@reitoria.unesp.br) on 2016-06-02T19:10:47Z (GMT) No. of bitstreams: 1 castro_ba_me_bauru.pdf: 3788528 bytes, checksum: 28b69717116921fa999eda08254ac160 (MD5) / Made available in DSpace on 2016-06-02T19:10:47Z (GMT). No. of bitstreams: 1 castro_ba_me_bauru.pdf: 3788528 bytes, checksum: 28b69717116921fa999eda08254ac160 (MD5) Previous issue date: 2016-05-25 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O monitoramento e a localização precoce de descargas parciais em aparelhos de alta tensão, como nos transformadores de potência, são de fundamental importância para a prevenção de problemas funcionais associados à degradação contínua dos materiais que compõe o isolamento elétrico destes tipos de aparelho. Alguns fatores críticos na operação dos transformadores, como o funcionamento em sobrecarga, superaquecimento, transitórios e sinais elétricos de grande conteúdo harmônico vinculados a sistemas chaveados, em longo prazo, fazem com que o sistema de isolação de um transformador apresente degradação de suas propriedades físicas e químicas intrínsecas aos diversos tipos de materiais utilizados para esta finalidade e, deste modo, surjam descargas parciais. Este trabalho teve como objetivo aplicar sensores piezelétricos de baixo custo para a identificação e localização de descargas parciais em transformadores de potência por meio de sistemas inteligentes do tipo Redes Neurais e sistema de inferência neuro fuzzy adaptativos. Ambos os sistemas foram treinados com algumas métricas de processamento de sinais e os resultados de erro médio de localização chegaram na casa dos milímetros. Variou-se o número de sensores acoplados e foi realizado um estudo sobre os resultados de localização obtidos. / Partial discharge damages in power transformers require high cost monitoring procedures based on corrective maintenance or even interruptions of the power system. The development of online non-invasive monitoring systems to detect partial discharges in power transformers has great relevance since it can reduce significant maintenance costs. Some critical factors in the operation of transformers such as overload, nonlinear loads, transient voltage surges by atmospheric origin and switching, can make the insulation system of transformers to lose their physical and chemical properties. Therefore, these operating conditions can cause early deterioration of the insulation, causing internal partial discharges that may develop into major defects and thus shorten the useful life of electrical equipment. This research aimed to apply a low cost piezoelectric sensors for partial discharge identification and location in power transformers through intelligent systems such as neural networks and adaptive fuzzy inference system. Both systems were trained with some signal processing metrics and the results for location error was in the region of millimeters. It was varied the number of coupled sensors and a study was conducted on the obtained location results.

No description available.

Le développement d’avions plus électriques se traduit par une plus grande utilisation d'actionneurs électriques qui remplacent des systèmes hydrauliques et pneumatiques existants ou répondent à de nouveaux besoins. La distribution de l'énergie à bord est basée sur un réseau continu 540V connecté à des convertisseurs électroniques de puissance. Les composants électroniques de puissance à grands gaps (SiC, GaN) améliorent les performances des convertisseurs mais les fronts de tension très raides sont imposés aux bobinages des actionneurs électriques. Chaque front excite un régime transitoire comportant une surtension importante et donc des champs électriques intenses dans l’isolation inter-spires des bobines. Lorsque cette contrainte électrique répétitive dépasse le seuil d'apparition des décharges partielles (PDIV), la durée de vie des couches organiques qui isolent les spires est fortement réduite. L'étude détaillée des décharges partielles (DP) dans l'espace qui sépare les fils émaillés des bobinages permet de situer les zones dans lesquelles les DP se produisent. Elle montre que le thermocollage, avec des surépaisseurs adaptées de thermocolle, permet d'élever le PDIV au-delà des pointes répétitives de tension pour les actionneurs qui fonctionnent dans les parties pressurisées des avions. Des solutions ordonnées de bobinages ont été expérimentées sur des bobines cylindriques imprégnées. Cette analyse montre que l'imprégnation augmente légèrement le PDIV de l'isolation inter-spires dans des proportions qui ne correspondent pas totalement aux caractéristiques intrinsèques du vernis utilisé. Par conséquent, les bobines imprégnées doivent être conçues sur la base des performances de l'isolation primaire du fil émaillé. L’arrangement des spires dans une bobine ordonnée permet de répartir les contraintes et donc de concevoir des bobines qui résistent aux pointes de tension répétitives imposées par l'onduleur aux basses pressions correspondant aux zones non pressurisées d'un avion en vol. Un modèle, basé sur un schéma équivalent HF prenant en considération les deux premières résonances des bobines élémentaires, permet d'analyser la répartition des contraintes entre les bobines connectées en série d'une phase. / The More Electric Aircraft development is reflected by a bigger use of the electrical actuators, which replace the hydraulic or pneumatic system existing or they can also answer to new needs. The energy distribution on board is based on a high voltage continuous bus of 540V connected to electrical power converter. The electric power wide band gap components (SiC, GaN) improve the converters performances but very steep of voltage edge is imposed on the windings of the electric actuators. Each edge excites a transitory regime including an important surge which corresponds to intense electric fields in the inter-turns insulation of coils. When this repetitive electrical constraint exceeds the partial discharges inception voltage (PDIV), the life time of the organic layer between the turns is strongly reduced. The detailed study of the partial discharges (PD) in the area which separate the enamel wire of the windings allows to locate the area where the PD appears. The use of thermo-bonding, with a bonding thickness adapted, allows to raise PDIV beyond the repetitive edge of voltage for the actuators which work in the pressurized parts of the planes. The orderly windings solutions were tested on impregnated cylindrical coils. This analysis shows that the impregnation increases slightly the PDIV from the inter-turns insulation in proportions that do not totally correspond to the intrinsic characteristics of the used varnish. Therefore, the impregnated coils must be conceived on the basis of the primary insulation performance of the enamelled wire. The turn’s arrangement in an orderly coil allows to distribute the constraints and so to conceive the windings which can resist the compulsory repetitive of voltage spikes by the inverter in low pressures corresponding to not pressurized zones in a plane during the flight. A model, based on HF equivalent schema taking in consideration the first two resonances of the elementary coils, allows to analyse the constraints distribution between the coils connected in series of a machine phase.

The thesis discusses the design and experimental verification of a new method for detecting partial discharges in oil-cooled high-power transformers, emphasizing the origin of the measured signal from the outer and inner regions of the transformer tank. The detection and spatial localization of partial discharges utilizes UHF (Ultra High Frequency) electromagnetic signal measurement. The UHF partial discharge detection method, also referred as UHF PD, is one of the most advanced techniques for the accurate spatial localization of the signal source position. The focus of the thesis lies in the development of technical and software solutions for the separation of partial discharge signals originating from the internal space of the transformer and the spurious signals from the connection lines. The proposed technical and program solutions constitute a signal discriminatory method. The precise and repeatable diagnostics of the transformer state are guaranteed by observing the measurement methodology and special setting of the diagnostic system. The functionality of the signal discriminatory method was verified during real measurement of the oil-cooled power transformers at the Dukovany nuclear power plant in operation. The output of the designed methodology is to set up a calibration procedure and follow-up steps to ensure objective, repeatable, and comparable measurements for the purposes of regular predictive maintenance in transformers.