Ester liquids including both natural ester and synthetic ester are being considered as potential alternatives to mineral oil, due to their better environmental performance and for some liquids their higher fire point. Although these liquids have been widely used in distribution and traction transformers, it is still a significant step to adopt ester liquids in high-voltage power transformers because the high cost and severe consequence of a factory test failure and the high level of safety and reliability required in service for these units, tend to lead to a cautious approach to any step change in technology. Lightning impulse strength as basic insulation level is of importance for insulation design of power transformers and lightning impulse test is commonly required in the factory routine tests for high-voltage power transformers, so this thesis is aimed to investigate the electrical performances including pre-breakdown and breakdown of natural ester and synthetic ester under impulse voltage.Two types of field geometry were considered in the study, one is sphere-sphere configuration which represents the quasi-uniform fields inside a transformer and another is strongly non-uniform point-plane configuration which represents the situation of a defect or a source of discharge. In quasi-uniform field study, standard breakdown tests were carried out under negative lightning and switching impulse voltages. Influence of various testing methods on the measured lightning breakdown voltage was studied and the 1% lightning withstand voltage was obtained based on Weibull distribution fitting on the cumulative probability plot built up using the approximately 1000 impulse shots. As for strongly non-uniform field study, streamer propagation and breakdown event in ester liquids either with or without pressboard interface were investigated at various gap distances under both positive and negative lightning impulse voltages. A relationship between the results under lightning impulse and previously published results under step voltage was built up to predict the lightning breakdown voltage of ester liquids at very large gaps.The results indicated that impulse strengths of ester liquids for both breakdown and withstand in a quasi-uniform field, are comparable to those of mineral oil. In a strongly non-uniform field, streamers in ester liquids propagate faster and further, than in mineral oil at the same voltage level. Thus breakdown voltages of ester liquids are generally lower than those of mineral oil, which could be as low as 40% at a large gap distance of approximately 1000 mm. Introduction of parallel pressboard interface has no influence on the streamer propagation and thus does not weaken the breakdown voltage, but it tends to reduce the acceleration voltage particularly for mineral oil under positive polarity.Last but not least, a unique phenomenon of secondary reverse streamer (SRS) was observed in ester liquids, which occurs subsequently and well after the extinction of the primary streamer (PS) propagation within a single shot of impulse voltage and has the reverse polarity to the PS. The formation mechanism of SRS is explained mainly due to the reverse electric field induced by the residual space charges left by the PS.