In this thesis, partial discharge (PD) phenomenon in oil-impregnated paper (OIP) is investigated under accelerated electrical stress. The thesis is mainly focused on the characteristic of PD activity and the influence it has on the insulation properties of OIP. PD source was created by introducing an air filled cavity embedded between layers of OIP. PD activity is investigated from the initiation up to final puncture breakdown of the OIP. The time-evolution of number, maximum magnitude and average magnitude of PD is investigated for cavities with different diameter and height. It was found that time to breakdown is shorter if the cavity diameter is larger and cavities with higher depth produce larger PDs. Comparison between PD activity in three cases, i.e. unaged OIP, thermally aged OIP and OIP samples with higher moisture content is performed. In general, it is found that for all cases the number and the maximum magnitude of PD follows a similar trend versus ageing time. During the very beginning of the experiment large discharges occur and they disappear after a short ageing time. Number and maximum magnitude of PD increase with time until reaching a peak value. Finally both parameters decrease with time and puncture breakdown occurs in the sample. Even though PD activity in thermally aged OIP is higher compared to the unaged OIP samples, the time to breakdown for new and thermally aged OIP samples is similar while it is shorter for OIP samples with higher moisture content. In this thesis, partial discharge (PD) phenomenon in oil-impregnated paper (OIP) is investigated under accelerated electrical stress. The thesis is mainly focused on the characteristic of PD activity and the influence it has on the insulation properties of OIP. PD source was created by introducing an air filled cavity embedded between layers of OIP. PD activity is investigated from the initiation up to final puncture breakdown of the OIP. The time-evolution of number, maximum magnitude and average magnitude of PD is investigated for cavities with different diameter and height. It was found that time to breakdown is shorter if the cavity diameter is larger and cavities with higher depth produce larger PDs. Comparison between PD activity in three cases, i.e. unaged OIP, thermally aged OIP and OIP samples with higher moisture content is performed. In general, it is found that for all cases the number and the maximum magnitude of PD follows a similar trend versus ageing time. During the very beginning of the experiment large discharges occur and they disappear after a short ageing time. Number and maximum magnitude of PD increase with time until reaching a peak value. Finally both parameters decrease with time and puncture breakdown occurs in the sample. Even though PD activity in thermally aged OIP is higher compared to the unaged OIP samples, the time to breakdown for new and thermally aged OIP samples is similar while it is shorter for OIP samples with higher moisture content. Breakdown strength of OIP samples is measured before and after ageing with PDs. It is found that the breakdown strength of OIP samples decreases by around 40% after the sample is exposed to accelerated electrical ageing. Furthermore a thermal model was developed to investigate the possible transition of breakdown mechanism from erosion to thermal breakdown in OIP dielectrics. It was found that PD activity can lower the thermal breakdown voltage of OIP up to four times. / <p>QC 20150206</p>

L'électroluminescence (EL) de isolants polymères est étudiée car elle peut permettre d'approcher les phénomènes de vieillissement électrique en fournissant la signature optique d'espèces excitées sous champ électrique. Le vieillissement et la rupture diélectrique dans les isolants polymères est d'un intérêt fondamental pour les chercheurs, concepteurs et fabricants de dispositif du génie électrique. À cet égard, les décharges partielles (DPs) sont un des principaux processus conduisant au vieillissement et à la défaillance des isolants. Cependant, avec le développement des matériaux et procédés, les DPs sont évitées dans certaines situations, par exemple, les câbles haute tension, les condensateurs, etc. Par conséquent, le besoin reste prégnant pour la compréhension des mécanismes de dégradation électrique sous forte contrainte électrique, qui peut être initiée par des porteurs énergétiques. Dans ce travail, l'EL, la cathodoluminescence (CL) excitée sous faisceau d'électrons, ainsi que d'autres techniques de luminescence ont été appliquées à la caractérisation de polyoléfines et d'autres polymères isolants. Afin de comprendre la formation d'excitons dans des films minces de Polypropylène (PP) et Polyéthylène (PE), la dépendance en champ de l'EL et du courant sous contrainte continue, et de l'EL et de sa résolution selon la phase sous contrainte AC, sont étudiées. Les spectres d'EL du PP et du PE ont le même pic principal à environ 570 nm, ce qui implique des structures et des défauts chimiques similaires pour les deux matériaux, et le même processus de dégradation. Le pic principal peut être complété par une émission à environ 750 nm dominante à faible champ. L'impact de la nature des électrodes a été étudiée sur du PEN pour comprendre l'origine de l'émission dans le rouge. A travers la dépendance en champ de l'EL et sa résolution selon la phase avec des métallisations or et ITO, on montre que l'émission dans le rouge est liée à la nature des électrodes et correspond à l'excitation de plasmons de surface ou d'états d'interface. Une étude plus approfondie est effectuée sur la cathodoluminescence d'isolants polymères. Des couches minces de PP, PE, ainsi que de Polyethylene Naphthalate (PEN) et de Polyether Ether Ketone (PEEK) ont été irradiés par faisceau d'électrons jusqu'à 5 keV. Nous avons pu reconstruire les spectres de CL et d'EL du PE et du PP à partir de quatre composants élémentaires: fluorescence, chimiluminescence, luminescence induite par recombinaison, et composante principale du spectre d'EL à 570nm décrite plus haut et considérée comme signature du vieillissement. Pour la première fois, la nature de l'EL et de la CL de polyoléfines est décomposée en quatre composantes de base avec des contributions relatives différentes. L'identification de ces composantes spectrales est utile pour interpréter la luminescence de polyoléfines et autres isolants polymères, et établir les liens entre distribution de charge d'espace et vieillissement diélectrique. A travers ces recherches sur l'EL et la CL dans plusieurs isolants polymères, i.e. polyoléfines ou polyesters, la formation d'excitons et les processus de relaxation d'énergie sous contrainte électrique et électrons énergétiques sont mis en évidence. Surtout, l'analyse en composantes spectrales et la reconstruction des spectres donne accès aux mécanismes d'excitation de la luminescence et à une corrélation avec le vieillissement électrique. A l'avenir, les mesures de luminescence peuvent devenir une méthode standard pour sonder et analyser les isolants polymères. / Electroluminescence (EL) of insulating polymers is a subject of great interest because it is associated with electrical ageing and could provide the signature of excited species under electric field. Electrical ageing and breakdown in insulating polymers is of fundamental interest to the researchers, the design engineers, the manufacturers and the customers of electrical apparatus. In this respect, Partial Discharge (PD) is a harmful process leading to ageing and failure of insulating polymers. However, with the development of the materials and apparatus, PDs can be weakened or avoided in some situations, e.g. extra high voltage cables, capacitors, etc. Therefore, there is urgent demand for understanding electrical degradation mechanisms under high electric field, which can be triggered by energetic charge carriers. In this work, Electroluminescence, EL, and cathodoluminescence, CL, excited under electron beam, along with other luminescence-family techniques are carried out for probing polyolefins and other insulating polymers. In order to uncover the excitons formation in Polypropylene (PP) and Polyethylene (PE) thin films, the field dependence of EL and current under DC stress and field dependence of EL and phase-resolved EL under AC stress, are investigated. The EL spectra of both PP and PE have the same main peak at approximately 570 nm, pointing towards similar chemical structures and defects in both polyolefins, and same route to degradation. This main peak can be complemented by an emission at approximately 750 nm dominating at low field. Electrode effect on the EL of Polyethylene Naphthalte (PEN) was investigated to understand the origin of the red emission at 750 nm. Through field dependence of EL and phase-resolved EL of Au or ITO electrodes, we proved the red component is due to the nature of electrode, more precisely Surface Plasmons and/or interface states. Further thorough study was carried out on cathodoluminescence of insulating polymers. Thin films of PP, PE, along with Polyethylene Naphthalate (PEN) and Polyether Ether Ketone (PEEK) were irradiated under electron beam up to 5 keV to be excited. We could reconstruct EL and CL spectra of both PE and PP using four elementary components: i.e. Fluorescence, Chemiluminescence, Recombination-induced Luminescence, and main component of the EL spectrum at 570 nm reported above and constituting an ageing marker. For the first time the nature of both EL and CL in polyolefins is uncovered, containing four basic components with different relative contributions. Identification of these spectral components is helpful to interpret the nature of light emission from polyolefins and other insulating polymers and to bridge the gap between space charge distribution and electrical ageing or breakdown. Through researches on EL and CL in several insulating polymers, i.e. polyolefins and a polyester, excitons formation and relaxation processes under electric stress and kinetic electrons are evidenced. More importantly, the spectral components analyses and reconstruction uncovers the nature of luminescence and its correlation to electrical ageing. In the future, luminescence measurement can be developed to be a standard method to probe and analyze insulating polymers.

Submitted work deals with monitoring impact of thermal and electric ageing on the dielectric properties of insulating material NKN 0887. In the course of the experiment five sets of dielectric insulating materials were aged. The voltage range has been set from 1,5 kV till 2,2 kV and temperatures were 23 °C and 200 °C. The capacity and loss factor has been directly measured by RLC meter Agilent with attached electrode system. The electrode system operates on the principle of a plate capacitor. The values of relative permittivity and loss number has been calculated from values of capacity and loss factor. For easier confrontation has been frequency dependences of these values plotted in a chart. The Cole-Cole diagrams has been created and factors of Havriliak-Negami function has been calculated. Data has been compared with each other and the effect of ageing factors has been deduced.