Understanding impulse surface flashover phenomenon on thermally aged oil impregnated pressboard material

Surface flashover phenomena under lightening impulse voltage of positive and negative polarity on thermally aged Oil Impregnated Pressboard (OIP) material used in transformer is studied. OIP material with surface deposited Cu at different concentrations are also used for comparison. Total energy deposited on the OIP material and the magnitude of discharge current increased with ageing duration. The decay characteristics concerning optical emission from the surface flashover matched with the temporal decay of discharge current. The study also revealed some of the pre-breakdown phenomena before the main flashover breakdown. Optical Emission Spectroscopy (OES) revealed signature of diffused Cu and S in aged OIP material. Higher emission intensity for Cu was observed for higher aged OIP material, indicating larger possibility of degradation. OIP material collected from a failed transformer also showed optical emission lines matching with Cu emission, indicating possibility of employing the OES technique in contaminant detection. The temperature of the plasma during the surface flashover was also estimated.