In this paper, we present a comprehensive, multi-timescale approach to evaluate energy transition policies aiming at fully renewable generation in power systems of non-interconnected areas, typically islands or remote regions. The approach links three dynamic models: (i) a capacity expansion model, ETEM-SG, proposes an investment and generation plan for typical days, up to 2030; (ii) a simplified dispatch model is used to validate the generation plan for a full year of weather data and demand variations; and (iii) a static and dynamic analysis of the power system is used to assess the stability of the new power system for rapid events such as a sudden reduction of renewable generation. The proposed three-step approach generates a cost-effective long-term investment planning that ensures the balance between supply and demand at an hourly time step and the stability of the power system at a timescale of milliseconds. The presentation is based on a case study fully described in a report made with ADEME, the French Agency for Ecological Transition, for the French island of La Réunion. It shows how a reliable 100% renewable power supply is achievable by 2030, in this area.