Surface treeing on pressboard barriers in synthetic and natural ester liquids under AC stress

This paper reports experimental investigations on pressboard surface treeing processes and the accompanying creepage discharge patterns. With the help of fast digital video recorder, wideband discharge current sampling resistor and commercial PD detector, it is found that the liquid impregnated pressboard as composite insulation is susceptible to erosion giving rise to white marks due to intense discharges occurring on/near to the pressboard surface. The white mark is visible after discharges evaporate the liquid and moisture and takes the form of trapped gaseous bubbles within the surface layers of pressboard. Once the white mark is formed, it can continue to grow even under a reduced voltage level until it bridges the gap and finally causes flashover. The white mark appears at a lower voltage and propagates more readily on the ester impregnated pressboard, due to more intense discharges, more gas generation and the higher viscosity of esters than mineral oil.