Creepage discharge on pressboards in synthetic and natural ester transformer liquids under ac stress

This study reports experimental studies of creepage discharges on the pressboard (PB) surface in esters under ac divergent electric field. With the help of commercial partial discharges detector, wideband current measurement and high‐speed image recorder, the differences of discharge patterns between open gap and on PB surface were studied in esters as well as in mineral oil. The results show that the presence of PB surface tends to promote discharges, especially negative discharges, and enables more discharges to occur at smaller phase angles. It is believed that the promotion effect of PB surface on discharges is caused by surface charges and bubble residences; because of more intense discharges and higher viscosity of esters, this effect is more significant in esters than in mineral oil.