In the present work, thermal ageing of oil impregnated pressboard material in mineral oil and nanofluid was carried out and fundamental characteristics viz surface discharge inception voltage under high frequency AC voltages and with harmonic voltages with different THDs are studied. It is observed that SDIV increases with increase in supply voltage frequency. Also, on thermal ageing, reduction in SDIV is observed, irrespective of frequency of the AC voltage. Also, increase in harmonic frequency with increase in THDs showed reduction in SDIV. In addition, charge accumulation studies are carried out. It is observed that three layers of OIPs store more charge and dissipates slowly. Also, negative charges get accumulated more than positive charges. PRPD analysis shows that the surface discharge activity on OIPs occurs near to the peak of the AC voltage and with harmonic AC voltages when dv/dt is high. © 2017 IEEE.