Some three fourths of electricity generation in Brazil come from renewables. Most of it is supplied by hydro, complemented by biomass-fueled thermal power plants and wind, while solar is still irrelevant. However, in the medium and long terms, a strong growth is expected for wind and solar in the country. Solar and wind resources are variable in time, partially unpredictable and cannot be dispatched to meet the load. These characteristics require system flexibility, which is the capacity of the power grid to adapt to different supply and demand patterns. Given that most thermal plants are not designed for a large frequency of operating cycles, renewables penetration may result in higher maintenance and operating costs, increased fuel consumption and reduced lifetime. Furthermore, some units might be called out-off-merit to maintain voltage and frequency levels. This paper presents preliminary results on the ability of thermal units to provide flexibility to the country´s grid, through an analysis for the Northeast region bounded by transmission constraints among 65 nodes (out of 196 nodes along the country). Results show that the power sector in Brazil might not be well equipped to deal with high penetration rates of variable renewable energy sources, with impacts on the capacity factor, and on the efficiency, of thermal power plants in the country. They also reveal that while wind energy increases the need of ramping capabilities, solar has greater impacts on the number of starts and shutdowns of conventional units.This research was supported by a Marie Curie International Research Staff Exchange Scheme Fellowship within the 7th European Union Framework Programme, under project NETEP- European Brazilian Network on Energy Planning (PIRSES-GA-2013-612263). The authors would also like to express their gratitude to the Coordination for the Improvement of Higher Education Personnel (CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for the essential support given for this work to be carried out
