The Influence of a Dc Bias on Streamers Produced by Step Voltages in Transformer Oil and over Solid-Liquid Interfaces

Using shadowgraphic techniques in conjunction with an Imacon camera, measurements of streamer velocities and minimum voltage required for streamer propagation (MVP) have been made with and without dc superimposed on voltage steps (1.2 ¿s rise time). This has been done in point-plane geometries and semi-uniform fields in a transformer oil alone as well as with pressboard barriers. Velocities and MVP have also been measured for streamers propagating on the surface between cylindrical electrodes perpendicular to insulating sheets having a ground plane on the opposite side. Again the influence of superimposed dc was assessed. The solid materials used were polymethylmethacrylate and pressboard. Measurements with the latter material were made using an optical multichannel analyzer to follow the light emission from the propagating streamer. In most cases both streamer velocities and MVP depend upon the arithmetic sum of the step and dc voltages. With pressboard, however, marked departures from the additivity rule were found for positive streamers. These results will be discussed in terms of space charge due to the dc potential.