Nowadays, where a profound transformation and climate change adaptation are crossing the energy industry, optimal expansion planning is critical for the more needed renewable and clean energy integration in medium and large systems. Moreover, we need to do this reliably and economically suitable for the final customers and the companies serving them.This work presents the results of applying the FESOP (Fabulous Energy System Optimizer) tool to analyze the Aysén electric system's optimal expansion. We developed FESOP to extend the long-term operational planning and include capacity expansion, variable energy resource representation, and modern flexibility systems capabilities, such as manageable storage systems and spinning reserve constraints.The resulting optimal expansion plans fulfill the expected demand for the next 30 years of the study, considering four growing scenarios. In the process, more renewable energy is integrated into the system. Moreover, given the spinning reserve requirement, the system always has enough reserve to cover full wind and solar generation.