The current state of electricity generation systems in Ukraine is reviewed and analyzed. The dynamics of volumes and structure of electricity production and the specific fuel consumption for its supply are presented. Technologies and measures to improve the efficiency of thermal power plants (TPP) are presented with regard for current conditions of rapid development of renewable energy. The possible transition of country's power industry from fossil fuels to renewable sources (solar and wind energy) is considered in details. Such a transition is accompanied by the integration of a significant part of the variable power generation from renewable energy sources with unguaranteed generation of electricity (solar and wind power plants) into the electric grid. The proposed scenario of transition to 100% renewable electricity generation is divided to four stages with the development and modernization of TPP based on fossil fuels is analyzed as a necessity for the implementation of this scenario. It is substantiated that the modern electric system should combine all types of generation and any types of consumers responsible for situational demand for their services, change in real time the parameters and topology of the electric grid according to the current operating conditions. The flexibility and reliability of the Integrated Power System of Ukraine during the transformational transition can be provided by highly maneuverable combined-cycle power plants and fast-start power plants using fossil fuels.