Power transformers are usually very expensive and bulky therefore their diagnosis and monitoring systems will be valuable for preventing damages. Partial Discharge (PD) is one of the phenomena that occur in the insulators and gaps between the windings of the power transformer that may be harmful for its health. In this study, a simulation model of an insulator, i.e. epoxy resin, defected with a cylindrical void is taken to generate PD activity and its analysis. Under the analysis PD waveform, PD pulses with higher magnitudes are identified and their characteristics in time and time-frequency transformed domain are studied. Some of the features are amplitude, occurrence, frequency distribution, intensity of the frequencies, etc. PD waveforms are obtained and studied on different supply voltages and different void size. Time domain feature are computed directly from the samples of the PD waveform and Time-Frequency analysis is performed by applying Short Time Fourier Transform.