Turbine-generator design, analysis, and testing for prevention of double-frequency torsional resonance

Prior to the mid-1970's, there existed little or no industry experience which would allow a meaningful assessment of the relationship between power system disturbances and their effects on turbine blade stresses. At the time, the most sophisticated rotor torsional analytical tools involved a lumped mass representation which typically was only applicable for predicting stresses in shafts and couplings for subsynchronous (less than 60 Hz) torsional vibrations. When incidents of lower pressure (LP) turbine blade distress due to double-frequency (120 Hz) torsional resonance were successfully diagnosed and corrected, the need for enhanced analytical and experimental methods was apparent. This paper describes various analytical and experimental developments in the area of coupled rotor/blade torsional response. It also describes ongoing efforts and developments in the analysis, detection and prevention of double-frequency torsional resonance.