Prebreakdown events and breakdown involving solid-liquid interfaces

Recently, we have correlated impulse strength of oils with measurements of prebreakdown streamer velocities.
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This paper extends such work to composite solid-liquid systems. The methods for measuring streamer velocities by shadowgraphy have been described previously.
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The creepage fixture utilizes two cylindrical electrodes (1.27 cm diameter) resting on the solid dielectric whose other surface is a ground plane (cf. Figure 1). In order to facilitate streamer initiation, a 3 µm radius point was inset into the high voltage electrode to protrude 0.5 mm into the gap, 0.25 mm above the insulating surface. Because shadowgraphy requires transparency, most work has been done using a polymethyl methacrylate (PMMA) slab with an indium-tin oxide transparent ground electrode. In one case, streamer velocities were measured on paper by laying 10 µm thick capacitor tissue on the PMMA surface, adhesion being provided by the surface tension of the oil.