Prebreakdown behavior of a composite liquid-solid insulation system under impulse conditions

This paper deals with the prebreakdown and breakdown phenomena of composite liquid-solid insulation systems in strongly non-uniform (point-point geometry) and weakly non-uniform fields (rod-rod geometry) at impulse voltages up to 500 kV. The investigations were carried out in oil-pressboard-oil barrier configurations with electrode gaps up to 22 mm. Streamer inception and propagation were visualized via high-speed shadow-graph photography. Compared with purely liquid insulation systems, the predischarges show identical propagation behavior, i.e. they appear either as primary or secondary streamers depending on the field. However, the propagation ceases at the liquid-solid interface and a luminous micro discharge (MD) without subsequent breakdown occurs. The light intensity of this MD becomes stronger with increasing voltage levels. Further, the MDs lead to surface discharges on the pressboard and, beyond a critical voltage level, Lichtenberg-type figures can be observed on the pressboard surface