Surface tracking on pressboard in natural and synthetic transformer liquids under AC stress

Experimental investigations were carried out to study the tracking process on pressboard in esters under ac voltages. The evolutions of creepage discharges were analyzed by correlating the visual records of surface tracking, the phase resolved PD patterns, the discharge current signals and the high speed shadowgraph images. It is found that the impregnated pressboard is susceptive to discharge erosion occurring on and inside the pressboard, which is characterized by "tree-shaped white and carbonized marks". The gaseous "white mark" channels would attract the subsequent discharges to follow the same routes, and the accumulative energy dissipation in these channels would then result in the carbonization of the channels and the emergence of a "carbonized mark". Once formed, the tree-shaped mark can continue to grow even under a reduced voltage or after a re-impregnation, until it bridges the gap and causes a final flashover. For a mineral-oilimpregnated pressboard, high moisture content (>3.5% by weight) is a necessity to initiate a "white mark"; whereas the "white mark" appears at a lower voltage and develops more easily on a dry ester-impregnated pressboard.