9.3.5 Case study: Torsional test measurement plane selection

It may be helpful to the reader to review Sec. 13.4, dealing with torsional vibration testing, before starting this case study.

For the turbine-generator defined in Fig. 7.5 and Table 7.2, suppose that it is a contractual obligation to perform a torsional vibration test following start-up of the unit. The purpose is to measure as many of the subsynchronous torsional natural frequencies and modal damping values as possible that can be stimulated by torques emanating from the generator rotor. Also suppose that it is cost-effective to use only toothed wheels and magnetic pickups to measure the speed oscillations following planned disturbances to the machine to determine the required torsional parameters. This may be because toothed wheels and magnetic pickups are hardware already planned to be supplied for use in control system equipment.

The questions to be answered are:

• Is measuring at one or both locations adequate to meet the test objectives? If only one measurement is to be taken, which turbine-generator end would be best?

• What other torsional vibration calculations should be performed to provide added assurance of meeting test objectives?

Solution to Case Study 9.3.5. The first step is to calculate the subsyn-chronous torsional natural frequencies and mode shapes. The mode shapes provide easy-to-obtain information that defines on a relative basis how responsive the modes are at all nodal positions, including in this case the proposed measurement locations.

The subsynchronous natural frequencies shown in Table 9.3 indicate...