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This paper deals with study of the breakdown and prebreakdown behavior of two mineral based and a silicone oil carried out using uniform and non-uniform electrode geometrics under power frequency voltage. The results show that the probability of occurrence of breakdown in mineral based oil under negative polarity is much higher than in silicone oil. The differences in the pre-breakdown streamers in both oils using shadowgraphic optical technique are investigated. Though the shape of the streamers observed are found to be similar to those reported here and in literature for solid olefinic insulations, yet the two phenomena differ considerably in time scale and underlying physical details. The influence of the investigated parameters related to the chemical nature of two oils and polymeric solids are discussed.

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This experimental investigation explores the effect of high voltage lightning surges on the insulating oil used in the oil filled extra high voltage cables that serve electric power networks worldwide. Using non-uniform electrode configuration, the breakdown voltage and corresponding discharge propagation velocities in oil samples that are doped with trace amounts of I-Methylnaphthalene are investigated under both polarities of applied stresses in gap spacing of up to 10 mm. Two distinct ranges of additive concentrations are observed which effect these characteristics. Generally, the positive discharge growth decreases with increase in gap spacing while the negative ones show opposite behaviour. Anomalous behaviour of discharge growth is observed at an optimum concentration of additive, as it attains supersonic velocity which increases with the increase in gap spacing. A correlation between the positive impulse breakdown voltage and the discharge propagation velociry is also established.

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