A method of online calculation steam turbine exhaust enthalpy is presented in this paper. We consider the steam turbine, feedwater heating system, and the auxiliary heating system and the condenser as a closed thermodynamic system. Using online measuring parameters, we can accurately get the main steam flow rate, the reheated steam flow rate and the exhaust steam flow rate by the Flugel formula and the improved Flugel formula, and then work out the turbine exhaust enthalpy according to energy equilibrium equation of this closed system. To this closed thermodynamic system, the energy carried by the feedwater extraction steam, and gate pole steam leak and shaft sealing steam leak circulates inside the system, therefore there is no need to consider this part of energy and it will not cause any errors. As a result, not only is the calculation simplified, but also the accumulative errors small due to a few measurement parameters needed. The calculation result of different operating conditions of a certain N200-12.7/535/535 type steam turbine has proved that the method has a higher precision and it is an effective method for online calculation condensing steam turbine exhaust enthalpy.
