Streamer formation and subsequent propagation is an important precursor to determine the breakdown characteristics of many high voltage electrode configurations under different circumstances. Therefore, streamers can be used as an effective probe for providing an insight into the condition of discharge propagation along insulating materials. Experiments were performed in a parallel plane electrode arrangement under a uniform electric field. Different specimens were examined. Tests were carried out with applied fields higher than the minimum required for streamer stable propagation, in this way the dependence of the propagation velocity on electric field was investigated with emphasis given on the effect of the surface insulating properties. Regarding the insulator bushing simulations, the influence of internal metallic conductor within the specimen insulators can result in increased propagation fields and decreased propagation velocities in comparison to the corresponding 'pure' insulators.
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