Abstract: The main steam flow rate is one of the key parameters in performance calculation of steam turbine unit. The use of existing steam turbine high pressure governing valve models to calculate the main steam flow rate poses problems such as exhausting modeling, model complexity, and difficulty in obtaining parameters of the calculation process. The valve with variable effective flow area is modeled by elliptic equations and tested with open literature experimental data. Method a calculation model for the off-design performance of a 660 MW ultra supercritical steam turbine unit with steam distribution combined the valve model. Then this unit model is used to explore the efficiency of HP cylinder and the unit's economic under different loads with different operation strategies. Analysis shows that the characteristics which the ellipse equations reflected are consistent with the literature experimental data and this method is simple in computation, high accuracy in work area. The efficiency of the HP cylinder of the unit decreases as the opening degree of the steam turbine high pressure governing valve decreases. The unit heat rate is the lowest when adopting the sliding pressure operation and the valve in 30%-45% opening should be recommended for operation when taking into account the request of frequency adjustment of electric network. The proposed operation strategies can improve the units' economic performance and enhanced the ability of in depth peak load cycling at the request of frequency adjustment of electric network. It provides reference for the operation strategy of similar units.

Key words: Plane curved beam, Reciprocating bending, Setting round process, Straightening process, Uniform curvature, economic operation, elliptic equations, steam distribution, valve characteristics