Creeping discharges propagating on natural ester oils/pressboard interface under AC and lightning impulse voltages

Abstract

This paper presents the results of experimental characterization of creeping discharges propagating over pressboard immersed in insulating oils, under AC and standard lightning impulse voltages. For comparison purposes, synthetic ester, rape seeds oil (natural ester) and mineral oil were also investigated. Two experimental arrangements were used: (1) a point-to-plane electrodes system where the pressboard is inserted between the electrodes such as the point is perpendicular to pressboard allowing a radial propagation of discharges; and (2) a point-to-bar electrodes system allowing a propagation of discharges in one direction, tangentially to pressboard. The characteristics of discharges propagation, their shape, stopping length (the maximum extension of creeping discharge) as well as the associated current and electrical charge versus the thickness of pressboard and the type of oil are analyzed. It is shown that the magnitude and polarity of voltage, the type of oil as well as the thickness of pressboard influence the characteristics of discharges and especially the stopping length and density of branches. The stopping length of creeping discharges is shorter for pressboard/naturel esters' interfaces than with pressboard/mineral oil interface.