Effect of DC bias on surface strength of processed pressboard under lightning and switching impulses

The surface strength of processed transformer pressboard is determined in a practical configuration under additive polarity composite DC plus switching impulse and DC plus lightning impulse voltages. Results obtained with specimens impregnated with field-aged oil are presented. With both types of composite voltage, most breakdowns occurred in the oil away from the interface. There is no significant influence of gap length on the location of the breakdown. With increasing DC bias fewer breakdowns occurred on the surface