One of the causes for the failure of power transformers is due to formation of partial discharges. Partial discharge taking place in transformer oil under AC voltages were studied by researchers worldwide in detail. But for the PD formation under transient voltages, the results are scanty. In the present work, three different defects were considered viz. corona discharge, discharge initiated due to floating particle and discharge formation due to particle movement and the discharges initiated by these defects under standard lightning impulse were studied adopting acoustic emission technique. The FFT analysis of AE signal obtained due to different defects has close resemblance and hence the ternary plots were obtained using the FFT output of the AE signal and classified. MRSD technique was adopted to remove the noise signal in the AE signal formed due to the discharges. The AE signal obtained due to discharges under AC voltages were compared with the signals obtained under standard lightning impulse voltage. © 2011 IEEE.