Estabilidade da geração de energia em usinas hidrelétricas com turbinas Francis reversíveis: estudo de caso UHE Tucuruí

Abstract

Reversible hydroelectric power plant projects have been consolidated on the international energy scene since the 1970s and are increasingly common as a solution in ancillary services for integration with intermittent energy sources (FEI) during peak hours. In Brazil, the large hydroelectric generation base allowed this need to be postponed for a long period. HPP Tucuruí, the last hydroelectric plant in the Tocantins River cascade, is a landmark in national engineering. Equipped with two powerhouses with an installed capacity of 8,535 MW, it has been largely responsible for regulating the FEI in Brazil, especially wind farms in the northeast region. Its Francis turbines at Powerhouse 1 have a nominal power of 350 MW each, maximum flows of 600 m3/s, Francis rotors with a diameter of 8.15m and 12 blades and a specific speed of 86.9 rpm.m3/4 are machines of considerable capacity and has been widely used by the ONS for power regulation. However, the model used for load dispatch shows clear signs of exhaustion, when comparing the growth in the accumulation of electromechanical damage in the generating units. One possibility of mitigating the severity of damage and increasing generation stability is the partial conversion of generating units to TR (Reversible Turbine) operating in hydraulic short circuit, with the aim of stabilizing dispatched power and consequently reducing the frequency of starts and stops. . This work presents the development of a methodology to predict the hydromechanical behavior of Francis turbines converted into reversible turbines in hydroelectric plants. A one-dimensional mathematical model based on the application of the Euler equation for the turbine and pump modes is preliminarily used. The triangle of speeds at the rotor inlet and outlet is formed, for a preliminary analysis of the flow in the Francis rotor. Characteristic curves of flow, torque and power of reversible turbines are produced, based on statistical correlations of existing machines, obtained from the literature. In the end, the hydro energy model is validated and a case study is presented at HPP Tucuruí. The results indicate the technical feasibility of operating stability with a reduction in starts and stops of unconverted generating units. In the dimensionless curves generated, it was not possible to obtain the optimal performance condition in pump mode, with the adoption of the same existing Francis rotor. Therefore, modifications to the mechanical and electrical design of the generating units must be carried out to achieve more stable and safe operation. The use of reversible turbines in this work is not intended to increase energy storage, but rather for the purpose of stability of machine operation and reduction of electromechanical damage, which for this purpose is technically viable.