The paper considers the factors stimulating the development of new, more efficient power unit control systems. A diagram of a flexible modeling system is presented with a breakdown into functional blocks corresponding to the systems of a power unit with a VVER-1000 type reactor. The paper presents a mathematical model of a steam generator, part of a flexible modeling system. The mathematical model describes the thermohydraulic state of steam generators and steam pipelines connecting them to the turbine. An algorithm for calculating the parameters of steam generators and steam pipelines at a new time step is presented. Verification of the presented model of the steam generator in the dynamic mode of the NPP power unit with a VVER-1000 reactor with the isolation valve of the steam generator at the stage of mastering the power of 75% was verified and showed good agreement between the calculated and experimental data.