Synchronous Generators (SGs) play a vital role in energy production as well as for the Industry. The insulation system of them, of which epoxy resin and mica are the most used insulation materials, plays the most significant role in proper operation and in extending its lifetime. Epoxy resin and mica have characteristics, which make them very good materials for a reliable SG insulation. Partial Discharges (PDs) are one of the most serious problems, because they can cause problems on the SG insulation. PDs are both a symptom of insulation degradation and a means to identify possible insulation faults. So, it is very important to detect PDs with offline or/and online PD Tests. A comparison of different MATLAB/Simulink PD models are presented in this paper. Epoxy resin, mica and a combination of these two insulation materials are used for simulations in order to investigate factors, such as the applied voltage, the geometry of the void inside the insulation, and how these affect the condition of the materials, and how these are related to PDs and flashover voltages, which may appear also in electrical machines’ insulation. The aforementioned factors can be used in order to check which of the materials is affected the most and which one is the most proper for SGs’ insulation system.