This paper assesses the effects of inserting solar and wind power in a hydro-dominated electricity system that has been at risk of outages in the short term. The extent to which renewables could contribute to overcoming potential critical conditions has not received much attention given the intermittency of the sources. In this context, this paper proposes a robust optimization model that, under different scenarios, estimates the implications of building new power plants based on renewables and traditional technologies. It considers costs, average construction times, and blackout events. Hidroituango, a mega hydroelectric project - prone to non-completion due to failures during its construction phase - is considered here because it threatens Colombia's future energy security of supply. Model results indicate that investing in hydro or thermal power units could be risky and twice as expensive as investing in renewables. Findings show that solar and wind technologies, in a ratio of 2.5 to 1, make part of the optimal and secure expansion plan for the Colombian power system. Besides reducing the CO2 emissions of the electricity sector, in the scenarios analyzed, these renewables show fewer blackouts (between 15 and 40% less) than the traditional technologies and more water savings in reservoirs. (C) 2020 Elsevier Ltd. All rights reserved.