Measuring Torsional Natural Frequencies of Turbine Generators by On-Line Monitoring

Abstract

Large turbine generators have torsional modes of vibration that can be excited from the electrical grid by torques applied through the generator. The most significant of these torques has a frequency at twice the grid frequency and is due to the negative sequence current in the generator caused by operation at unbalanced load or during grid transients. When the twisting modes of the low pressure turbine rotors combine with the vibratory modes of the last few stages of blade rows, and the frequency of the combined torsional mode is close to the frequency of the exciting torques, significant vibratory response of the shaft and blades can occur. The accumulated fatigue damage caused by such vibration over time can result in failure of the blades. Since this low damped torsional vibration can not be seen on any of the plant instrumentation, it can result in the loss of low pressure blades with little or no warning. To ensure that the turbine generator is not susceptible to damage from the torsional vibratory response of these modes, it is necessary to confirm that the torsional frequencies are sufficiently removed from the frequency of the exciting torques when the turbine generator is operating. For a large turbine generator, the torsional modes of concern are often between the 15th to 25th mode of vibration. Analysis techniques may not be able to determine the frequency of these modes within the accuracy required to ensure that they are not excited. The only reliable way to determine the natural frequencies of such modes with sufficient accuracy is to measure them directly while the turbine generator is operating. On-line monitoring is often the preferred approach for such measurements since it does not impact the operation of the plant and it determines the torsional natural frequencies at the plant operating conditions. Torsional natural frequencies tend to change as a function of turbine generator speed while the turbine generator is off-line and as a function of power while the turbine generator is on-line. On-line monitoring uses sensitive instrumentation and time averaging techniques to determine the torsional natural frequencies of a turbine generator from random vibration of the shaft while the turbine generator is operating. Identifying the torsional mode that is associated with each measured frequency requires the combination of a good analytic model of the turbine generator and an understanding of how the torsional frequencies react to specific changes in operating parameters. The analytic and measurement techniques that have been developed through experience and implemented during numerous on-line measurements are described in this paper. These techniques have also been used to determine blade stress response levels to torsional excitation in order to evaluate the susceptibility of a specific turbine generator to damage from torsional vibration. In this regard, there is some evidence that the torsional response of the turbine generator can be amplified by the steam flow through the blade path. Finally, these techniques can be used to evaluate any specific transient that occurs during operation of the plant with respect to its impact on fatigue usage of the turbine blades and shaft. If necessary, modifications can be designed to shift the torsional natural frequencies away from the problem torques once the complete response of the turbine generator to torsional excitation is understood.