In recent times, ester oil is used as insulation in transformers. In order to improve the performance characteristics, silicondioxide nano particles were dispersed in the oil. An attempt has been made to understand the corona activity under high frequency AC and harmonic AC voltages with different level of distortion. No characteristic variation in corona inception voltage is observed with increase in supply voltage frequency. The UHF signal formed during corona discharge activity has dominant frequency at near 1 GHz An attempt has been made to understand the surface charge accumulation characteristics of paper insulation impregnated in nano particle dispersed ester oil. It is observed that in paper insulation in nano ester oil, the initial charge accumulation is low. Phase resolved PD analysis was carried out to understand corona activity in ester oil and nano particle dispersed ester oil, under AC and harmonic AC voltages. Fourier Transform Infrared spectroscopy (FTIR) analysis was carried out in order to quantify effect of nano particles on surface characteristics of oil impregnated pressboard (OIP) sample. © 2017 Institute of Electrical Engineers of Japan. All rights reserved.