Summary:This doctoral dissertation has been performed theoretical and experimental research of condensing steam turbine with controlled extraction in different operation mode.Theoretical examinations have been made on the basis of the presented mathematical model. Mathematical model of a steam turbine is divided into high and low pressure part. High-pressure turbine part includes the area of flow of steam through regulating wheel and a second stage, that is up the regulated extraction, while the low pressure part implies the area after the regulated extraction to the exhaust steam into the condenser. Based on the results of research, there has been constructed the diagram of changing operation mode depending on the input quantity of steam, the regulated extraction and electrical power.In the experimental part of the doctoral dissertation there have been performed some measurements in objective operative conditions under various regimes of load turbogenerator plant which is installed in CHP power plant of Refinery Rijeka Mlaka. There have been defined forty-two operation modes based on available data on energy consumption in process plants. The obtained results of experimental measurements are relevant indicators of the tested plants. The analysis of the obtained results has been found certain theoretical deviations from the experimental studies. With regard to the obtained deviations the mathematical model has been corrected according to correction factors for the specific consumption of steam and produced electric power towards the results obtained by experimental measurements. These deviations exist because the mathematical model does not consider specificities of individual design steam turbine that affect the flow losses. These factors give the contribution to calculation in the steam turbine cogeneration system with varying operating conditions.