Conventionally, for reliability studies in power systems, a constant failure rate is assumed for power generation units. If these units consist of power electronic converters, this assumption might not be valid due to the aging of power components, and it will lead to an unrealistic prediction of reliability. On the other hand, at the system-level, commonly-used reliability calculation tools, such as Monte Carlo Simulation (MCS) and Continuous Markov Process (CMP), are either time-consuming or not possible to be applied to the systems with non-constant failure rates. Therefore, in this paper, a methodology is proposed to calculate system-level reliability for a Power-Electronic-based Power System (PEPS), consisting of several converters with non-constant failure rates. By doing so, not only is the effect of mission profiles integrated into the system-level reliability model, but also the wear-out failures and corrective maintenance are considered. Finally, for a case study PEPS, the system-level indices are calculated using the proposed method. It is shown that assuming constant failure rates for PEPS units is inaccurate and misleading. Moreover, the impacts of various factors, e.g., mission profile, repair rate, topology, and rating of converters, on the system-level reliability are investigated and analyzed.