As a result of imperfect manufacturing process or ageing problems, a small gas cavity can be formed between the impregnated paper layers. Partial discharge (PD) activities take place when a localized breakdown occurs inside this embedded void. These PD activities degrade the healthy parts of the surrounding insulating papers and a complete breakdown can take place. Thus, it is important to define the main features of PD activities that occur as a result of a small void embedded between oil impregnated papers in order to distinguish such a defect under the operating conditions in the early stages before the whole insulation failure In this research, a small disk void has been artificially embedded between oil impregnated paper layers. Phase resolved partial discharge pattern (PRPD), PD magnitude and PD repetition rate have been investigated in a voltage level 120% from the inception voltage limit. The degradation that takes place in the surrounding impregnated papers as a result of partial discharge activities has been also detected. The assessment of partial discharge characteristics and the consequent degradation process have been studied under the effect of AC and DC voltage stresses