The increased need for electric power combined with an aging underground cable infrastructure in a deregulated market environment have forced utilities to refocus their attention on reliability while at the same time reducing maintenance costs as much as possible. This has created a significant need for diagnostic methods and technologies to assess the condition of the underground cable systems. However, while several cable diagnostic technologies are available, they have not all yet been fully accepted in the United States. This is because the different technologies lead to different conclusions for the same cable system, and thus utilities do not completely trust the conclusions. A better understanding of the diagnostic technologies and their correct application is therefore required. The most widely used diagnostic technologies in the United States include dissipation factor (Tan-delta and partial discharge measurements; these tests are therefore, the main focus of this thesis; in particular, when applied to underground extruded cable systems. The purpose of this research is to advance the field of characterization of power cable defects by addressing a number of theoretical and practical diagnostic measurements and their interpretation issues. The discussion is based on data from laboratory experiments and field tests. This thesis consists of two major parts. The first part is devoted to the characterization by Tan-delta measurements in which the major contribution is a new approach for condition assessment using this technology. The second part is devoted to the work on characterization by partial discharge measurements, and the major contribution is a novel approach that is able to analyze, evaluate, and reduce the number of partial discharge diagnostic features.

No description available.

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.

Polymeric insulating materials are being re-evaluated in the context of the re-emergence of HVDC and its advantages in bulk power transfer over long distances. This has been met with new sets of requirement such as; the use of polymeric insulation, compaction of HV equipment (e.g. HV cables), and innovations in converter technology. This equipment requires high power rating and hence will be exposed to high electric stresses. One of the properties of polymeric DC insulation is its ability to retain injected charges at high DC fields leading to local field modification and subsequent breakdown of the insulation through electrical treeing. Electrical treeing is one of the important failure mechanisms of solid polymeric insulations resulting from high voltage stresses and a precursor to failure of electrical equipment. Hence, the performance and reliability of polymeric insulation designs will be affected by electrical treeing. Literature shows that electrical trees initiate easily with switching voltages such as impulses, voltage surges and reversal of power flow direction. Innovations in converter technology employs fast switching devices such as insulated gated bipolar transistors (IGBTs) which generates substantial amount of harmonics and may also impact insulation systems reliability. This research investigates the reliability of epoxy resin (LH/HY 5052) for suitability in HVDC applications due to its excellent properties as jointing compound in medium and high voltage cables systems. The development of test facilities for short term breakdown strength, space charge measurement and electrical treeing experiment have allowed short term breakdown strength on homogeneous layers of thin epoxy-epoxy samples and long term breakdown through electrical treeing under DC, AC and AC superimposed on DC to be investigated so that an understanding of the link between space charge, material strength and life times can be clarified. The results on short term breakdown showed the layered samples have 6% reduction in strength compared to un-layered samples. For long term treeing test, 100% of the samples stressed with negative DC did not fail while 67% of the sample stressed with positive DC failed with average lifetime of 250 minutes. Samples stressed under AC showed forward and reverse directions of tree growth with average lifetime of 143 minutes from 70% failed samples. For AC superimposed on ±DC all samples failed with average lifetimes of 54 and 78 minutes for positive and negative bias tests, respectively. It is concluded that, the differences in lifetime obtained under positive and negative pure DC tests and that of the positive and negative DC bias tests are associated with space charge causing field relief under negative DC and negative bias tests. The huge reduction in lifetimes under AC superimposed on DC as ripples tests highlights the potential threat of power quality issues on the reliability of DC systems. Electrical tree growth from the ground planer electrode (reverse tree) observed under AC test was associated with relatively low voltage under AC test compared with the other tests see Table 8-1 for test voltages employed.

No description available.

Increased electrical power demands are being experienced on the new generation of aircraft due to an increased reliance on electrical technology of systems such as air conditioning, de-icing systems and electrical flight control actuation. Distribution of power at higher AC and DC voltages is therefore now being seen in modern aircraft to avoid the penalties incurred due to high cable weights. Voltages have increased past the minimum of Paschen's law resulting in a risk that life limiting partial discharge (PD) damage can occur in the insulation systems. This thesis uses a theoretical analysis backed by PD experimental results to investigate the optimal operating voltage of a cabling system. In addition, it proposes a methodology for optimizing the operating voltage level based on an analysis of the power carrying capability of cabling within a fixed and a non-fixed volume system and the derivation of the cable weight as a function of voltage. Furthermore the power carrying capability of a certain round cable system is compared with an insulated flat conductor system as in a printed circuit board (PCB). An initial assessment has been carried out to determine whether more power can be delivered via insulated flat solid conductors as in a PCB, instead of using round cables. The reason why there is a need to investigate this aspect, is because using new PCB technology can offer several advantages over traditional cabling harnesses. The work done has shown that the optimal operating point (e.g. maximum power to weight ratio) for an aircraft power system, does not improve after certain voltage levels. A tradeoff between cable weight and power transfer is required and furthermore the use of DC systems can result in higher power transfers than conventional three phase/400Hz AC systems. The PCB maximum power transfer assessment has also shown that insulated flat conductor systems can offer higher power transfer efficiencies. In addition, experimental AC and DC PD tests on certain unscreened aerospace cables (laid out in different configurations), have shown that the theoretical analysis employed to determine cable safe operating voltages gives conservative results.

Neste trabalho é apresentado um novo método de medição de descargas parciais, através de transformadores de corrente de alta frequência, instalados no condutor de aterramento dos equipamentos do sistema elétrico de potência e acoplados a um osciloscópio. Esse novo método de medição tem como grande ganho o fato dos equipamentos não precisarem ser retirados de operação para realização de medição. Pelo fato das transmissoras serem remuneradas pela disponibilidade das funções transmissão, o diagnóstico dos equipamentos em serviço faz com que os custos sejam reduzidos. Foi realizado um levantamento teórico onde foi definida a curva característica esperada como resposta do transformador de corrente a um sinal impulsivo. Foram levantadas curvas características, em laboratório, através de oscilogramas no domínio do tempo, das assinaturas de descargas parciais em equipamentos com isolação a óleo e a gás SF6, de modo a validar o sinal esperado na fundamentação teórica. Essas assinaturas foram utilizadas em medições em campo, que comprovaram a viabilidade da fundamentação teórica e das assinaturas obtidas em laboratório. Adicionalmente foram levantados fatores que podem influenciar nas medições e mostrados sinais que possuam características diferentes daquelas das descargas parciais, de modo a diferencia-los. / This work presents a new method of measuring partial discharges, through high frequency current transformers, installed in the equipment grounding conductor of the electrical power system and coupled to an oscilloscope. This new measurement method has as great gain, the fact that the equipment doesnt need to be removed from service to perform measurement. Because of the transmission being remunerated by the availability of the transmission functions, diagnosis of equipment in service makes the costs been reduced. A theoretical survey where the characteristic curve expected response from the current transformer to an impulsive signal was set has been done. Characteristic curves were raised in the laboratory through oscillograms in the time domain, of signatures of partial discharges in equipment with SF6 gas and oil isolation, in order to validate expected signal discharges in the theoretical foundation. These signatures were used in field measurements, which proved the feasibility of the theoretical foundation and signatures obtained in the laboratory. Further factors were raised that may influence the measurements and shown signs that have different characteristics from those of partial discharges in order to differentiate them.

The main subject of this diploma’s thesis is the creation of laboratory task on the partial discharge. The thesis is made up of two parts: theoretical and practical. In the theoretical part there are information about partial discharge. One of them is classification partial discharge by type, effects of partial discharge in insulation systems, parameters to assess the quality of electrical insulation of power systems and measurement techniques. Further part is user’s manual for measurement instrument. In practical part is created laboratory task on problematic of the partial discharge measurement. It is demonstrative measure of partial discharge with the aim of understanding problematic of partial discharge, calibration and noise.

No description available.

It is common practice to perform accelerated aging with 60 Hz ac to determine the lifetime characteristics of insulation used in the machine. Comparable breakdown measurements are performed at different voltage levels and temperatures for the polyimide insulated machine winding under normal operating conditions of 60 Hz ac. The result shows that the time to failure can be represented by the inverse power law and the Arrhenius equation with respect to test voltage and temperature respectively. However, the use of power electronic devices causes harmonics, and spikes that electrically degrade the machine winding insulation. Applied high frequency pulse voltages can be used to study the impact of electrical degradation of the machine winding insulation that exists in electrical machines. Evaluation of change in dielectric strength, partial discharge and breakdown voltage is monitored versus aging caused by high frequency pulse voltage at 90% of operating temperature.