This paper presents models of Boiler and Steam Turbine of combined cycle plants that can be used with any type of Gas Turbine models for grid dynamics studies. These models were obtained through a review of characteristics of large steam turbine and heat recovery boilers that define the behavior of combined cycle plants. Also is presented a Gas Turbine model of a combined cycle plant that was validated by tests developed and applied to evaluate reliability of combined cycle plants that are being installed in Argentinean electric system. Reliability test goals are the behavior characterization of combined cycle plants during large systems disturbances.