Corona inception voltage (CIV) and breakdown strength of the composite oil formed with mineral and ester oil increase when the percentage of ester oil in the composite oil is increased. Corona inception voltage is higher under negative DC voltage compared with AC and positive DC voltages. Also, the variation in the breakdown voltage of composite oil under AC and positive DC is nearly the same and negative DC voltage has higher breakdown strength. The viscosity of the composite oil increases with increase in the percentage of ester oil in the composite oil. FTIR analysis provides functional groups present in the composite oil. UHF signal formed during corona activity in composite oil is much similar to what is observed in mineral oil/ester oil. The dominant frequency content in UHF signal formed is at 0.9 GHz. Spectrum analyzer results indicate that intermittent discharges occur due to corona in composite oil under AC voltages and burst type discharges occur under DC voltages and the factor is more predominant under negative DC voltages. © 2017 IEEE.