Inception of first mode negative streamers at liquid/solid interfaces

• This paper presents an experimental study on the inception of first mode negative streamers at different mineral-oil/solid interfaces. This study is performed with a point-plane configuration immersed in mineral oil (point cathode). The mineral-oil/solid interface is done by assembling a solid in an inclined position into the point-plane gap. The solid is in contact (or in the proximity) with the point electrode tip. The tested solids are a kraft paper, a paper made from cellulosic micro and nano fibrils and different polymeric films (low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)). These solids have different relative permittivity and structural composition. It is found that the streamer inception voltage is statistically similar for the cases with solids with higher permittivity than mineral oil. The streamer inception voltage for the case without any solid barrier is also statistically similar to the cases with solids with higher permittivity than mineral oil. It is also found that the inception voltage is higher for streamers initiated at permittivity-matched interfaces (cases with LDPE and PTFE). Additionally, the influence of the spatial limitation with the solid surface to the volume where the streamer initiation process takes is performed with PTFE by varying the distance between the point electrode and the solid surface. It is shown that the streamer inception voltage depends on the distance between the point electrode and the solid surface. Furthermore, It is observed that the streamer inception voltage is also influenced when the distance between the point electrode and the surface of the PTFE is several micrometers (twenty times longer than the penetration depth of the avalanche responsible of streamer initiation). Additionally, it is also shown that a recently-proposed streamer inception criterion is unsuitable to predict the streamer inception conditions close to permittivity matched and mismatched liquid/solid interfaces.

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