A Method to Perform an Experimental Modal Analysis of a Medium Head Francis Runner in Operation

Background

Hydraulic turbines are increasingly used outside the range of operation for which they were designed due to the growth of electricity produced by wind and solar power sources, which are intermittent. The new operating ranges cause high level of vibrations that reduce significantly the lifespan of the turbine runner. Modal parameters of the runner are influenced by the operating condition, but the extent of this influence remains unknown. Closely spaced modes, which are typical in turbine runners, also remain challenging to identify.

Objective

This paper presents a methodology for conducting an experimental modal analysis of a medium head model Francis turbine runner during operation, with the objective of identifying its natural frequencies and damping ratios while minimizing the impact on the hydraulic surface.

Methods

The runner of a medium head model Francis turbine is instrumented with piezoelectric actuators located on the outer side of the band and semiconductor strain gauges on the blades and the inner side of the band. Classical strain gauges and accelerometers are installed as reference measurements. Piezoelectric actuators are driven by sine sweep signals to generate standing and travelling waves matching the mode shapes of band dominant modes. Natural frequencies and damping ratios are then identified from the frequency response functions.

Results

The energy injected by the piezoelectric actuators is sufficiently high to excite band-dominant natural modes of the runner in operation. The high sensitivity of semiconductor strain gauges allows for low-noise measurements compared to classical strain gauges. The quality of the obtained frequency response functions is high, enabling the identification of natural frequencies and damping ratios and resulting in strong agreement between measured and model-predicted responses. With eight piezoelectric actuators located on the band, the use of traveling wave excitation pattern allows for the individual excitation of the forward and backward components of the first two pairs of band-dominant natural modes.

Conclusion

The proposed method for conducting experimental modal analysis enables the identification of modal parameters of a model Francis turbine runner in operation. By using a traveling wave excitation pattern, the method allows for the individual excitation of the forward and backward components of band-dominant natural modes—provided the number of actuators is sufficiently large—which is particularly beneficial for identifying closely spaced modes when such modes are present.