The paper presents development of mathematical model of steam power plant for studies of reactive power - voltage (Q-V) control. The model of realistic six generator power plant is developed in Matlab-Simulink environment. All relevant components of the system studied are modeled in sufficient detail including synchronous generators, turbines and governors, exciters and AVRs, transformers, and the network. Several case studies are used to investigate potential problems that could occur if manual control was performed by the plant operator to restore the voltage at a point of common coupling or to produce the required reactive power set by the system operator. Results of simulations are compared with recorded responses at the power plant and confirmed the accuracy of developed mathematical model.