Discharge characteristics of oil-pressboard insulation under impulse voltage waveforms with different parameters

Due to the arrangement of the substations and the inner resonance of windings, transformers in field will encounter lightning overvoltage with waveforms quite different from that of the standard lightning impulse. Moreover, very fast transient overvoltage (VFTO) caused by the switching operations of disconnectors in gas insulated stwichgears (GISs) brings increasing threat to transformers. Therefore, we established an oil-pressboard insulation model to simulate the typical insulation configuration of transformer windings, and used it to experimentally studied the influences of impulse voltage's front time and oscillation frequency on the typical insulation's breakdown characteristics, as well as the breakdown characteristics under VFTO and the influences on them from the pressboard's placement. It is obtained that the breakdown voltage increases either with the decrease of front time or the increase of oscillation frequency, and it can be up to 30% higher than that under standard lightning impulses. The breakdown voltage under VFTO is 10% higher than that that under standard lightning impulses. For the model with a single pressboard, its breakdown voltage decreases with increasing the distance from the sphere electrode to the pressboard.