Estimation of Structural Mechanical Damping in Multi-mass of Turbine Generator for SSTI Study

This paper provides a practical method for estimating structural mechanical damping in sub-synchronous torsional interaction (SSTI) analysis with limited information on the generators. The risk of SSTI is generally assessed by comparing the damping of the turbine generator with that of the electrical system on a problematic mode in the frequency domain followed by time domain simulation studies to confirm the occurrence of the torsional interaction. Unfortunately, mechanical damping is often neglected in the SSTI studies simply because the structural mechanical damping data for the turbine generator is unavailable. Only modal damping ratio is provided in practice, and study results with incorrect mechanical damping should be misleading. This paper thus presents a method to accurately estimate the structural mechanical damping of the multi-mass model based on Rayleigh and Caughey damping models. Comprehensive studies in both frequency and time domain using PSCAD/EMTDC demonstrate the accuracy and efficacy of the proposed estimation method. Theoretical aspects and applicability of two damping models for the SSTI analysis are also discussed.