This paper describes control of two cases steam turbine with synchronous generator which is switched into the island operation. The frequency in an electrical grid is stated on nominal value which is in the Union for the Coordination of the Transmission of Electricity (UCTE) grid 50 Hz. When deviation of frequency is higher then 0.2 Hz, the switching of particular steam units into the island operation is only the chance how to maintain the supply of, at least, some small grids.
The other possibility how to keep power units in operation, to be prepared for the next synchronization to the grid, is to switch them to operation status in which they supply only their self-consumption. This change of the operating state is the most dynamic load change for the control system of the unit. Multiple stages steam turbines are equipped with high pressure hydraulic control and intercept valves which are used for their governing. Simulation of switching of two cases steam turbine with synchronous generator into the island operation is examined in MATLAB Simulink software. Output diagrams of turbine speed, active power, control and intercept valves behaviour are evaluated and plotted.