This paper proposes a dynamic optimization mathematical approach to obtain the optimum mix of distributed generation plants with renewable energy resources (RER) necessary to provide reliability to critical electrical systems. This model takes as inputs the energy potential ofwater, wind and solar resources available and injectable in each area, investment and operation and maintenance (OM) cost ofthese plants and related infrastructure; as well as parameters of existing electrical grid and conventional generation plants. The proposed model is performed for the 8760 hours of a typical year within which it is considered out-of-service states for existing transmission lines and power plants. A robust approach is contemplated though a cost sensibility analysis. The results of the application performed in a real case in Puno, Peru evidence an improvement of approximately 81% and 68% in reliability indexes SAIDI and SAIFI, respectively.