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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.