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Streamer characteristics, breakdown strengths and gassing behaviour of insulating liquids under electric stresses are taken into account for a reliable design and safe operation of the transformer. Ester liquids which are biodegradable and have high fire point have been widely used in distribution transformers and some power transformers in recent years. It is also interesting to introduce ester liquids into High Voltage Direct Current (HVDC) converter transformers due to the fast development of HVDC transmission lines. Therefore, this thesis aims to investigate the pre-breakdown, breakdown characteristics and gassing behaviour of a synthetic ester liquid under DC and various impulse voltages where a mineral oil is tested as the benchmark. A comprehensive study of streamer characteristics and breakdown strength of the mineral oil and the synthetic ester liquid under both positive and negative DC voltages was carried out in the point-plane electric fields. Characteristics of streamer length, propagation velocity and shape were analysed based on shadowgraph images obtained at a gap distance of 10 mm, using a multi-channel ultra-high speed camera. Streamer inception voltages with the tip radii of 5 µm, 10 µm, 20 µm and 50 µm and breakdown voltages at various gaps of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm were also investigated. The results indicate that there is no obvious streamer propagation (less than about 10% of the gap distance) under negative polarity even when the applied voltage approaches breakdown voltage. At the same applied voltage level, the streamer in the synthetic ester liquid propagates faster and further than that in the mineral oil. As a result, the breakdown voltages of the synthetic ester liquid are lower than those of the mineral oil at all the gap distances investigated under both polarities. Experimental and modelling studies of pre-breakdown and breakdown phenomena in the mineral oil and the synthetic ester liquid under impulse waveforms with different tail-time were carried out in the point-plane electric fields. A compact solid-state switch based impulse generator was used to provide different impulse waveforms from short tail-time to 'step-like' tail-time: 0.8/8 µs, 0.8/14 µs, 0.8/30 µs and 0.8/3200 µs. A point-plane electrode configuration with a small gap distance of 10 mm and a tip radius of 10 µm was used. The results indicate that the shorter tail-time impulse waveform results in a shorter stopping length and higher breakdown voltage; however it does not affect the instantaneous breakdown voltage and time to breakdown. A mathematical model is therefore described to predict the breakdown voltage under different impulse waveforms. In addition, with the similar stopping length, higher energy injected from the short tail-time impulse caused the streamers to have more branches than those under the long tail-time impulse. The characteristics of fault gas generation in the mineral oil and the synthetic ester liquid under various levels of electrical faults were studied. A test platform with functions of automatic spark fault control and data acquisition was developed. The effects of spark numbers (from 20 to 500), gap distance (5 mm and 10 mm) and voltage levels (Vb-99.9% and 1.5Vb-99.9%) on fault gas generation in liquids were studied. The key gases in the mineral oil are H2 and C2H2, while the key gases in the synthetic ester liquid are H2, C2H2 and CO. The amount of fault gas generation increases linearly with the number of sparks. However, the number of sparks does not have an obvious effect on fault gas pattern and gas generation per unit fault energy in µL/J. Spark at a larger gap distance or under a higher applied breakdown voltage generates more fault gases due to higher injected fault energy.

Electrical insulating liquids, organic esters, conductivity, dielectric, permittivity

Le SF6 (Hexafluorure de soufre) a été utilisé depuis longtemps comme un gaz d'isolation très adapté pour les appareillages électriques. Cependant, son impact sur le réchauffement climatique, estimé 24000 fois plus actif que le CO2 sur 100 ans, pose le problème de son remplacement à moyen terme. L'objectif de cette thèse est d'étudier deux solutions de remplacement du SF6 : les liquides biodégradables et les mélanges de gaz CF3I/N2. Les deux phénomènes physiques conduisant à un claquage : la génération d'une décharge et sa propagation, ont été étudiés séparément. L'influence des différents paramètres tels que la pression, distance, distribution de champ, teneur en particules ou humidité relative…a été également mise en évidence. La fonction d'isolation est en fin validée dans un maquette disjoncteur moyenne tension qui représente un cas d'application réelle. / SF6 gaz (sulfur hexafluoride) has been widely used as a very suitable insulation material for HV apparatus. However, it is also known as a remarkable green house gas, which Global Warming Potential is 24000 times greater than CO2. Consequently, its utilisation in the electrical industry should be reduced. The aim of this work is to study two alternatives to SF6: biodegradable liquids and CF3I/N2 gas mixtures. The two physical phenomena leading to a breakdown: the generation of a discharge and its propagation have been studied separately. The influence of various parameters such as pressure, distance, field distribution ,particle content and relative humidity ... was also highlighted. The insulation function is then validated in a MV circuit breaker, which represent a real application case.

This labor deals with features of alternative electrical insulating liquids and also petroleum-base liquids. It examines their usage in the electrotechnical industry. The practical part of the thesis deals with measurement of absorption charakteristics of biodegradable electrical insulating liquid MIDEL 7131 and the petroleum - based transformer oil TRAFO N. The results of the measurement are examined by the help of substitutional math function.

This master thesis deals with clasification of electrical insulating liquids and analysis their reological properties. Theoretical part describes viscosity and possibilities of its measurement, electrical and non-electricl properties of electrical insulating liquids and themselves electrical insulating liquids. Practical part describes measurement of viscosity and density of chosen vegetable oils, their components and transformer oils in wide temperature range. At the conclusion of the thesis, analysis of measurement results is done.

This master's thesis deals with the electrical insulation fluids used in electronics industry. The analysis of the current state of research and innovations in modern, biodegradable fluids in terms of their use as diagnostics. It is described by the physical nature of polarization and conductivity mechanisms, which are applied in liquids. It was compiled automated workstation for monitoring components of the complex permittivity over a wide temperature range. In the experimental part we analyzed the behavior of liquids selected alternative based on natural esters when exposed to an alternating electric field at various temperatures. Evaluated results are compared with values for conventional mineral oil.