Calculation and Measurement of Torsional Vibrations in Large Steam Turbosets – New Technique

Abstract

ABB has been carrying out torsional vibration test on steam turbine sets for many years, with sizes ranging from 350MW coal fired steam turbines to 1000MW nuclear steam turbines. Rated speeds have been 1,800 rpm and 3600 rpm. Torsional shaft vibrations are excited in large steam turbine generators sets by electrical disturbance torques occurring in the active part of the generators. Due to the low torsional damping of the shaft trains, resonance can cause high torsional vibration amplitudes. Some thermal power plant operators require experimental verification of the natural torsional shaft frequencies. ABB has developed a new measurement technique which allows torsional vibration to be measured in a selected measurement plane along the shaft. With today's highly sensitive sensors and modern measurement and data analysis equipment, the excitation due to normal random disturbances in the electrical grid provides sufficiently good results. All natural torsional frequencies in the range of interest are obtained with an accuracy of ± 0.2 Hz and identified using the calculated natural frequencies. The new measurement technique, places no restrictions on normal power plant operation.