A new generation of real-time digital simulator of the turbine-alternator-grid system (STAR) has been developed by ENEL-R&D-CRA in order to meet the demands of design review, commissioning test, preliminary assessment and final fine-tuning of excitation systems and voltage regulators, as well as of turbine actuators and speed governors. In particular, the electromechanical behavior of the overall turbine-alternator system, with respect to the electrical interconnected grid, may be theoretically studied, by using dynamic simulation models perfected with suitable details. The Control systems performances can also be easily checked and fine-tuned, in presence of small and large perturbations, both in normal and in emergency operating conditions. The simulator STAR, which has been successfully used also for the specialists training, can be effectively considered as a real turbine-alternator system. In fact its inputs may be directly connected to the interfaces of the excitation system and turbine governor; moreover, the simulator provides in output the process variables with the suitable format requested by the different control apparatuses. The paper describes the main design topics of the STAR simulator, as mainly regards its hardware and software architecture and the model framework of the turbine (steam, hydro or gas), alternator (with static and rotating exciter) and grid interconnection. The structure of the excitation control system and turbine speed governor, as well as the parameters configuration, runtime simulation, and data visualization procedures are shortly described. Finally, the STAR performances are shown through significant transient responses.