There is a growing interest in a time-domain simulation of active power balancing and frequency control to examine the frequency stability in long-term time scale (up to several hours or a day). In order to perform the accurate simulation, proper models of thermal power plants that play principal roles in the balancing and frequency control are essential.  In this paper, new models of conventional steam power plants and gas turbine combined cycle power plants suitable for balancing and frequency control analysis are presented. In the developed models, thermal power plant dynamics and plant control systems are simplified without affecting the accuracy. This simplification decreases the number of parameters and reduces the computational burden. The developed models were validated through the comparison between the model response and the measured response of representative thermal power plants.