It is intended to present a procedure for obtaining optimum economic dispatch with consideration of control over the generating units during change of the load in thermal power systems. Therefore, a mathematical model, which takes the transmission losses into account, was built. The deviation in each of the rotational speed, system frequency, electrical and mechanical power in addition to the deviation in transmission losses, generated output powers of each unit and in the corresponding generation costs can be computed during the change of the load from a time interval to the next one. The steady-state deviations of the above mentioned items can be computed, also, in each time interval. The results, which were obtained from the application of this model on a power system, have been presented and compared with other results.