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Failure of underground cable on San Diego Gas & Electric's electric underground distribution system is an ever increasing problem. While there are a great number of cable diagnostic techniques available, none lend themselves to both an averaged and location specific, on-line implementation. This dissertation demonstrates the development of an on-line suitable technique that utilizes transients and Fast Fourier Transforms to determine a cable section?s impedance magnitude and phase angle as a function of frequency. Simultaneously a theoretical model was developed to simulate various scenarios that an in-service cable might experience. Significant effort was expended developing and optimizing the measurement and data analysis technique. This includes a statistical approach for comparing performance of different cable samples. Both the preliminary and final tests demonstrated the superiority of the frequency domain analysis over comparisons in the time domain. With the effort to date, there appears to be three distinct results: good cable, degraded cable and damaged cable. These differences are statistically significant at the 95% confidence level. Additionally, there appears to be good agreement between the theoretical model and actual test results. Consequently, this measurement methodology continues to hold promise for future practical development.

Les travaux entrepris dans le cadre de cette thèse portent sur l’étude de la dégradation radiolytique de polymères constitutifs de l’enveloppe isolante de câbles électriques contrôle commande. Pour étudier les mécanismes de dégradation de l’isolant du câble, une approche originale à été suivie via l'utilisation de matériaux modèles, i.e un mélange réticulé non chargé à base d’EVA (Ethylène Vinyle Acétate) et d’EPDM (Ethylène Propylène Diène Monomère) et le composite correspondant chargé avec des ATH (trihydroxyde d’aluminium), ont été soumis à une irradiation gamma. Ces travaux s'inscrivent dans la perspective d'un souhait de prolongation de la période d’exploitation des centrales du parc électronucléaire français par l'exploitant EDF. Une analyse multi-échelle de la dégradation des matériaux modèles et de l’isolant a été effectuée pour comprendre l’évolution des propriétés des matériaux polymères induites par différents types de vieillissement (thermooxydatif et/ou radiooxydatif). Dans l’ensemble de ces conditions, Il a été établi que la dégradation du mélange EVA/EPDM et du composite EVA/EPDM/ATH est gouvernée par un mécanisme de coupures de chaînes, le point faible de la matrice polymère étant la partie vinyle acétate; de plus dans le cas de la radiooxydation, les processus purement radiolytiques jouent un rôle clé. À partir de courbes de corrélation, il a été montré que les propriétés physicochimiques des matériaux polymères, telles que l’évaluation par spectroscopie IR de la quantité de produits d’oxydation formés, la détermination de la fraction de gel, et les mesures d’OIT (Temps d’Induction à l’Oxydation), sont des indicateurs sensibles de vieillissement. L’évolution de ces trois critères étant plus précoce et plus progressive que la perte des propriétés mécaniques de l’isolant, on pourrait donc anticiper une perte brutale des propriétés mécaniques, voire de la fonctionnalité du câble, en faisant un suivi de leur évolution. Plus généralement, ces études soulignent l’importance de travailler à des débits de dose très faibles et à température ambiante pour que le vieillissement accéléré soit représentatif, ce qui a été fait dans le cadre de cette thèse. / This study is focused on the radiooxidative degradation of polymeric insulation of Instrument and Controle (I&C) electric cables. In order to investigate the degradation mechanisms of the insulation, an original approach has been implemented thanks to the use of model composites with ATH (Aluminium TriHydrate) filler and blends (without filler) based on a cross-linked mixture of EVA (Ethylene Vinyl Acetate) and EPDM (Ethylene Propylene Diene Monomer) were submitted to gamma-irradiation. This work has been completed within the goal of extending the lifetime operation of French Nuclear Power Plants (NPPs) by EDF. Multi-scale analysis of the model materials and insulation were performed in order to investigate de modifications and the evolution of the polymer properties induced by different kinds of ageing (thermo and/or radiooxidation). After having highlighted that whatever the ageing conditions are, degradation of polymers is governed by scission process, it is shown that purely radiolytic processes play a key role in the overall degradation scenario. These studies based on correlation curves showed that physico-chemical properties of the polymer, such as the evaluation by InfraRed of the amount of the oxidation products formed, gel fraction and Oxidation Induction Time measurements are relevant indicators of ageing. The evolution of these properties during ageing is even, in the case of the insulation, earlier than the loss of mechanical properties, which may allow to anticipate a sudden loss of mechanical properties and therefore of the functionality of the cable insulation. More generally, these studies points out the needs of using the lowest dose rates possible and room temperature, as done in this work.

High Voltage (HV) cables are designed to transmit electrical power from the source to the placewhere it is consumed. The global trend towards renewable energy sources e.g. wind farms, hy-droelectric and solar panels have led to an increase in the use of HV cables, as these sources aresituated in remote locations far from the areas where they are consumed. Therefore. there is aneed for developing efficient power transmission cables.Mass-impregnated (MI) cable is a type of HV cable which contains an electrical insulation layerof oil or mass impregnated cellulose paper. Due to its excellent dielectric properties, MI cableshas been established as a solution for high voltage electrical transmission system, which has beenused for several decades.The main area of interest in this thesis is the paper insulation in the MI cable. It is importantto ensure that the paper insulation structure is robust enough to avoid any formation of damageand/or defects due to excessive deformations during cable production, handling and installation,which might affect the overall electrical performance of the cable. During handling, the drylapped cable needs to be bent and unbent several times and there is a risk that the insulationwill develop voids due to structural defects, thereby reducing the dielectric capacity of the cable.An example of typical structural defects in MI cables is, so-called, diamond folds, i.e. a collec-tive buckling of several paper strips due to axial compressive load resembling a diamond shapedpattern.The goal of this thesis work is to contribute to the knowledge of the mechanism of diamond foldsin MI cable; the onset of diamond fold and post-buckling responses. The contributions from thecable design or production parameters are also studied. The analyses will be conducted usingnumerical simulation techniques and will be compared to results from characterization tests al-ready conducted in mechanics lab at NKT Technology consulting.The results of this thesis has given an insight about the effect of insulation design parameterssuch as registration, butt-gaps, coefficient of friction and number of layers, on the diamond foldbuckling behavior in an MI cable insulation. It is recommended that these parameters have tobe carefully controlled so as to avoid formation of diamond fold buckling in the insulation.

Polymers are widely used as insulating materials in high voltage power apparatus because of their excellent electrical insulating properties and good thermomechanical behavior. However, under high electrical stress, polymeric materials can get deteriorated which can eventually lead to the failure of the insulation and thereby the power apparatus. Electrical treeing is one such phenomena whereby dendritic paths progressively grow from a region of high electrical stress and branch into conducting channels in a solid dielectric. The propagation of electrical trees is of particular interest for the power industry as it is one of the major causes of failure of high voltage insulation especially in high voltage cables, cast resin transformers as well as rotating machines. To improve the life time of the electrical insulation systems there is a need to improve the electrical treeing resistance of the insulating material for high voltage application. With the development of nanotechnology, polymer nanocomposites containing nano sized particles have drawn much attention as these materials are found to exhibit unique combinations of physical, mechanical and thermal properties that are advantageous as compared to the traditional polymers or their composites. Literature reveals that significant progress has been made with respect to the mechanical, optical, electronic and photonic properties of these functional materials. Some efforts have also been directed towards the study of dielectric/electrical insulation properties of these new types of materials. Considering the above facts, the present research work focuses on utilizing these new opportunities which have been opened up by the advent of nanocomposites to develop tree resistant insulating materials for high voltage power applications. Electrical treeing is a common failure mechanism in most of the polymeric insulation systems and hence electrical treeing studies have been carried out on two types of polymers (viz. polyethylene used in high voltage cable and epoxy used in rotating machines and resin cast transformers) along with three different types of nano-fillers, viz. Al2O3, SiO2 and MgO and with different filler loadings (0.1, 1, 3, 5 wt%). Furthermore, considering the fact that electrical treeing is a discharge phenomenon, the partial discharge characteristics during electrical tree growth in polymer nanocomposites was studied. As morphological changes in the polymer influence the electrical tree growth, the influence of nano-particle induced morphological changes on the electrical treeing has also been studied. Above all, an attempt has also been made to characterize and analyze the interaction dynamics at the interface regions in the polymer nanocomposite and the influence of these interface regions on the tree growth phenomena in polymer nanocomposites. A laboratory based nanocomposite processing method has been successfully designed and adopted to prepare the samples for treeing studies. Treeing experimental results show that there is a significant improvement in tree initiation time as well as tree inception voltage with nano-filler loading in polymer nanocomposites. It is observed that even with the addition of a small amount (0.1 and 1 % by weight) of nano-particles to epoxy results in the improvement of electrical treeing resistance as compared to the unfilled epoxy. In fact, different tree growth patterns were observed for the unfilled epoxy and epoxy nanocomposites. Surprisingly, even though there is not much improvement in tree inception time, a saturation tendency in tree growth with time was observed at higher filler loadings. To understand the influence of nano-particles on electrical treeing, the interaction dynamics in the epoxy nanocomposites were studied and it was shown that the nature of the bonding at the interface play an important role on the electrical tree growth in epoxy nanocomposites. The results of electrical treeing experiments in polyethylene nanocomposites obtained in this study also reveal some interesting findings. An improved performance of polyethylene against electrical treeing with the inclusion of nano-fillers is observed. It is observed that there is a significant improvement in the tree inception voltage even with low nano-filler loadings in polyethylene. Other interesting results such as change in tree growth pattern from branch to bush as well as slower tree growth with increase in filler loading were also observed. Another peculiar observation is that tree inception voltage increased with increase in filler loading upto a certain filler loadings (3 % by weight) and then decreased in its value at high filler loading. The morphology of polyethylene nanocomposites was studied and a good correlation between morphological changes and treeing results was observed. Effect of cross-linking on electrical treeing has also been studied and a better performance of cross-linking of nano-filled polyethylene samples as compared to the polyethylene samples without cross-linking was observed. The partial discharge (PD) activity during electrical tree growth was monitored and different PD characteristics for unfilled and nano-filled polyethylene samples were observed. Interestingly, a decrease in PD magnitude as well as the number of PD pulses with electrical tree growth in polyethylene nanocomposites was observed. It is known that PD activity depends on the tree channel conductivity, charge trapping and gas pressure inside the tree channel. The ingress of nano-particles into the tree channel influences the above known phenomena and affects the PD activity during electrical tree growth. The observed decrease in PD magnitude with increase in filler loading leads to the slow propagation of electrical trees in polyethylene nanocomposites. In summary, it can be concluded that polymer nanocomposites performed better against electrical treeing as compared to the unfilled and the conventional micron sized filled polymer composites. Even with low filler loading an improved electrical treeing resistance was observed in polymer nanocomposites. An optimum filler loading and a suitable filler to inhibit electrical treeing in the polymers studied are proposed. This work also establishes the fact that the characteristics of the interface region and the induced morphological changes have a strong influence on the electrical treeing behaviors of nanocomposites. These encouraging results showed that epoxy and polyethylene nanocomposites can be used as tree resistant insulating materials for high voltage applications. These results also contribute to widen the scope of applications of polymer nanocomposites in electrical power sector as well as development of multifunctional insulation systems.

Since its first introduction in 1998, extruded direct current (DC) cable technology has been growing rapidly leading to many cable system installations with operation voltages up to 320 kV. Cable manufacturers invest heavily on technology development in this field and today extruded DC cable systems for operation voltages as high as 525 kV are commercially available. The electrical field distribution in electrical insulation under DC voltage is mainly determined by the conduction physics, therefore a good understanding of the DC conduction is necessary. In case of Cross-linked Polyethylene (XLPE) insulation, the presence of the peroxide decomposition products (PDP) is believed to influence its electrical properties. The PDP are volatile and therefore they may diffuse out of the samples during sample preparation and testing. Besides, the morphology of the XLPE is known to evolve over time even at moderate temperatures. Since the material may change during preparation, storage and even measurement, the procedure during all stages of the study should be chosen carefully. In this work, the physics of the dielectric response and conduction in XLPE is briefly discussed. The existing measurement techniques relevant to characterization of DC conduction in XLPE insulation materials are reviewed. The procedure for high field DC conductivity measurement is evaluated and recommendations for obtaining reproducible results are listed. Two types of samples are studied, i.e. thick press molded samples and thick plaque samples obtained from the insulation of in-factory extruded cables. For press molded samples, the influence of the press film used during press molding and the effect of heat-treatment on the electrical properties of XLPE and LDPE are studied. High field DC conductivity of XLPE plaque samples is measured with a dynamic electrode temperature to simulate the standard thermal cycles. Investigations show that using PET film during press molding leads to higher apparent DC conductivity and dielectric losses when compared to using aluminum foil. The influence of heat-treatment is different depending on the press film. High field DC conductivity measurements and chemical composition measurement of samples obtained from the cable insulation are in good agreement with the results obtained from the full scale measurements. Finally a non-monotonic dependence of apparent DC conductivity to temperature of some samples pressed with PET film is discovered which to the author’s best of knowledge has not been previously reported in the literature. / Sedan det första införandet i 1998 har extruderad likspänning (DC) kabeltekniken vuxit snabbt och har lett till många existerande kabelsysteminstallationer med driftspänningar upp till 320 kV. Kabeltillverkare investerar kraftigt i teknikutveckling inom detta område och idag finns extruderade DC kabelsystemen tillgängliga för driftspänningar så höga som 525 kV. Elektrisk fältfördelning i isolationsmaterial under hög DC spänning, beror framförallt på materialets elektriska ledningsfysik, därför är en bra förståelse av DC ledningsförmåga nödvändig. Isolationsmaterial av tvärbunden polyeten (PEX) innehåller tvärbindningsrestgaser som tros påverka materialets elektriska egenskaper. Restgaserna är flyktiga och kan diffundera bort från proven under preparering och mätning, även under måttliga temperaturer. PEX materialets morfologi ändras även med tiden. Med tanke på att materialet kan ändras under provpreparering, lagring och även vid mätning, så måste samtliga steg ovan väljas mycket försiktigt. I detta arbete diskuteras grundläggande fysik för dielektrisk polarisering och ledningsförmåga i PEX-isolation tillsammans med granskning av existerande mätteknik relevant för karakterisering av ledningsförmåga i PEX. Procedurer för mätning av DC ledningsförmåga under höga elektriska fält är undersökta och rekommendationer för reproducerbar mätningar är framtagna. Två typer av prover är studerade, tjocka pressade plattor och tjocka plattor som ursvarvats från kommersiell tillverkade högspänningskablar. För pressade plattor, studerades effekten utav press-filmens påverkan på de elektriska egenskaperna hos PEX och LDPE. Påverkan av värmebehandling på DC ledningsförmåga av PEX plattor studerades också. Slutligen studerades DC ledningsförmåga av PEX och LDPE plattor under höga DC fält och med dynamisk temperatur på elektroderna med syftet att efterlikna standardvärmecyklingar. Undersökningarna visade att användningen av PET filmer under pressning av plattor ledde till högre DC ledningsförmåga och högre dielektriska förluster i proven i jämförelse med användning av aluminiumfolie. Påverkan utav värmebehandling är olika beroende på typ av film som används pressningen. Det finns en stark korrelation mellan resultaten från DC konduktivitet och kemisk komposition mätningar från plattor skaffat från kabelisolation och resultaten från fullskaliga kabelmätningar. Slutligen, upptäcktes ett icke monotont beroende av DC konduktivitet hos PEX och LDPE plattor på temperatur som tidigare inte rapporterats i litteraturen. / <p>QC 20160125</p>

Partial discharge (PD) measurements have been widely accepted as an efficient online insulation condition assessment method in high voltage equipment. Two sets of experimental PD measuring setups were established with the aim to study the variations in the partial discharge characteristics over the insulation degradation in terms of the physical phenomena taking place in PD sources, up to the point of failure. Probabilistic lifetime modeling techniques based on classification, regression and multivariate time series analysis were performed for a system of PD response variables, i.e. average charge, pulse repetition rate, average charge current, and largest repetitive discharge magnitude over the data acquisition period. Experimental lifelong PD data obtained from samples subjected to accelerated degradation was used to study the dynamic trends and relationships among those aforementioned response variables. Distinguishable data clusters detected by the T-Stochastics Neighborhood Embedding (tSNE) algorithm allows for the examination of the state-of-the-art modeling techniques over PD data. The response behavior of trained models allows for distinguishing the different stages of the insulation degradation. An alternative approach utilizing a multivariate time series analysis was performed in parallel with Classification and Regression models for the purpose of forecasting PD activity (PD response variables corresponding to insulation degradation). True observed data and forecasted data mean values lie within the 95th percentile confidence interval responses for a definite horizon period, which demonstrates the soundness and accuracy of models. A life-predicting model based on the cointegrated relations between the multiple response variables, trained model responses correlated with experimentally evaluated time-to-breakdown values and well-known physical discharge mechanisms, can be used to set an emergent alarming trigger and as a step towards establishing long-term continuous monitoring of partial discharge activity. Furthermore, this dissertation also proposes an effective PD monitoring system based on wavelet and deflation compression techniques required for an optimal data acquisition as well as an algorithm for high-scale, big data reduction to minimize PD data size and account only for the useful PD information. This historically recorded useful information can thus be used for, not only postault diagnostics, but also for the purpose of improving the performance of modelling algorithms as well as for an accurate threshold detection.