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We study streamer initiation in liquid cyclohexane, in a point-plane geometry under impulse voltage. A new differential charge measurement system, allowing a high sensitivity to be obtained under impulse voltage (below 0.1 pC), is presented. The investigations presented concern the influence of applied voltage, impulse rise time and hydrostatic pressure. Under DC voltage, the negative streamer initiation is due to an avalanche in the liquid phase, and is not affected by pressure. In contrast, positive streamer initiation shows some variation when the pressure is increased.

This paper reports the results of experimental investigations about prebreakdown phenomena in liquid nitrogen, in point-plane electrode geometry (gap distance: 2 mm) under impulse voltage (up to 30 kV). As in hydrocarbon liquids, slow "bush like" streamers and fast filamentary streamers are observed in positive polarity. The spectral analysis of the emitted light during the propagation of filamentary streamers shows the second positive band emission system of nitrogen (C/sup 3/II/sub u//spl rarr/B/sup 3/II/sub g/) in the region 330 nm to 400 nm. The rotational and vibrational temperature T/sub r/ and T/sub v/ can be evaluated from a comparison of experimental and simulated spectra of vibrational bands for the transition /spl Delta/v=-2 of the second positive system. From these measurements, we can conclude that the temperature of gaseous filaments is about 540 K, i.e. a quite cold plasma.

This paper presents experimental investigations concerning the influence of space charge on the ac breakdown of moving mineral oil, between metallic electrodes. Space charges are injected in oil with sharp metallic points, and carried into the breakdown test cell through a perforated ground electrode. Breakdown voltages are measured under ac voltage in rod-sphere or sphere-sphere electrode at 5 and 10 cm gap distances. The results show that, whatever the flow rate of oil and the amount of charge injected, no significant influence on ac breakdown voltage is observed. As a consequence, it appears that the deleterious effect of static electrification of oil in devices like power transformers cannot be simply ascribed to a degradation of the intrinsic oil performance.