Analytical-approximation mathematical model of a 500 MW-class CCGT unit for simulation and prediction applications

The principal objective of this paper was to formulate an original methodology for the development of mathematical models of Combined Cycle Gas Turbine (CCGT) units, using as a case study a 500 MW-class unit operating in cogeneration technology, resulting in the simultaneous production of electricity and district heat, with the additional capability of exporting process steam. The modelling scope addressed key challenges, including: performance maps, expansion lines of exhaust gas and steam, and heat transfer in the heat recovery steam generator and district water heaters. The model was developed using an analytical-approximation approach and included technological limitations imposed by the implemented automatic control systems. A comprehensive validation of the model was conducted, and selected performance characteristics of the cycle were determined. The model demonstrated a highly accurate representation of the CCGT unit's performance, with mean absolute and relative errors for the electrical power output of the gas turbine and steam turbine units equal to 1.18 MW (0.54 %) and 0.63 MW (0.55 %), respectively. The proposed methodology enables off-design simulation of the unit's operation, supports thermal diagnostics of the individual thermal-flow systems, and allows for multi-criteria evaluation of the cycle's performance as a function of varying external conditions.