A singular flashover path observed on the surface of synthetic-resin-bonded paper cylinders immersed in transformer oil under switching impulse voltage conditions

In order to clarify basic surface flashover phenomena in transformer oil under switching impulse voltage, the flashover characteristics on four kinds of large insulating cylinders are studied for positive lightning impulse of 1.2/50 μs, positive switching impulse of 500/3600 μs, and 50 Hz alternating voltage. The insulating cylinders are made from material containing about 40 wt.% phenol resin and 60 wt.% kraft paper. A singular surface flashover path is observed under switching impulse voltage which includes a partial jump of the discharge channel around the HV electrode. The peak value of the flashover voltage for the switching impulse is nearly equal to that of a 50 Hz voltage; this was not expected before the experiment, because breakdown voltage of switching impulse for oil-immersed insulation is generally intermediate between the lightning implulse and AC voltage. The singular flashover path of the switching impulse is explained by an electric field analysis taking account of the remaining charge of 100 pC/mm³ which has the same polarity as the applied voltage and is spread out along about 10 to 40% of the distance between the HV electrode to the grounded electrode