Estimation of Torsional Stiffness of Turbine Generator Lumped Spring Mass Shaft Model Based on Eigenvectors Orthogonality and Newton-Raphson Method

The lumped spring mass model should be used for studying turbine generator torsional vibration and subsynchronous resonance/oscillation (SSR/SSO). It is very important that the torsional stiffness should be calculated accurately. With the given inertia constants and natural modal frequencies, a simple calculation method of the torsional stiffness is proposed. The estimated eigenvectors of torsional vibration modes are obtained from the available data. The modal torsional stiffness constants are estimated by using orthogonality of eigenvectors. Then the torsional stiffnesses could be calculated inversely. Considering possible errors of the results, Newton-Raphson method is used for correction. A case study of No.1 turbine generator unit in Shajiao Plant C is proposed. In this case, due to lack of torsional stiffness data, SSR/SSO cannot be analyzed in detail and the torsional stress relay (TSR) cannot be put into operation. With the proposed method, the problems are solved. The proposed method is simple, efficient, and easy to use. The Newton-Raphson iteration can also be used for correcting spring mass model parameters for TSR after the natural modal frequencies are measured.