This paper presents a physical simulator for predicting the off-design and dynamic behaviour of a single shaft heavy-duty gas turbine plant, suitable for gas-steam combined cycles.The mathematical model, which is non linear and based on the lumped parameter approach, is described by a set of firstorder differential and algebraic equations. The plant components are described adding to their steady state characteristics the dynamic equations of mass, momentum and energy balances. The state variables are mass flow rates, static pressures, static temperatures of the fluid, wall temperatures and shaft rotational speed.The analysis has been applied to a 65 MW heavy-duty gas turbine plant with two off-board silo-type combustion chambers. To model the compressor, equipped with variable inlet guide vanes, a subdivision into five partial compressors is adopted, in serial arrangement, separated by dynamic blocks. The turbine is described using a one dimensional row by row mathematical model, that takes into account both the air bleed cooling effect and the mass storage among the stages. The simulation model considers also the air bleed transformations from the compressor down to the turbine.Both combustion chambers have been modelled utilising a sequence of several sub-volumes, to simulate primary and secondary zones in presence of three hybrid burners.A code has been cleated in Simulink environment Some dynamic responses of the simulated plant, equipped with a proportional-integral speed regulator, are presented.

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