Handbook of Turbomachinery Second Edition Revised - Ventech!

System Disruptions Leading to Rotor Torsional Modes. Rotor torsional
vibrations can be excited by electrical faults in the distribution system. These
torsional vibrations can lead to considerable damage to the rotor and
blading and have led to a number of blade failures in the power industry
[55]. The turbine generator system may respond to subsynchronous,
synchronous, or supersynchronous (typically two times the running speed,
e.g., 120 Hz for 60-Hz turbines) frequencies.
Design of pre-1975 machines typically evaluated the potential for
synchronous vibrations, but were not designed for frequency resonances
equal to two times the running speed. These resonances occur as a result of
load imbalance on the three phases which can induce secondary currents in
the generator stator. These stator currents, termed ‘‘negative sequence
currents,’’ cause an alternating torque to act on the rotor system at twice the
transmission grid frequency of 60 Hz. Sources of such load imbalance can
include: cutoff of load, line accidents, subsynchronous resonance, or a short
in the generator terminals [73, 74].
Torsional modes near this frequency tend to be highly coupled to
disk–blade modes and, as a result, considerable high cycle fatigue damage
can be imparted to LP blades, particularly in the last stages, from the
phenomenon illustrated schematically in Fig. 42. Failures caused by this
mechanism have been reported in fossil units and in nuclear units.
A number of researchers have reported on the analysis of disk–blade
interactions, specifically related to avoiding unwanted resonances (see, for
example, Refs. [55–63]). The aims of such analyses are to evaluate blade and
disk dynamics and to avoid resonances with the electrical grid frequency.
Two examples illustrate the typical methodology.
Higuchi and Tsuda performed a torsional vibration analysis of the LP
turbine in a 1,160-MW nuclear unit; of particular concern were frequencies
near 120 Hz. They found as many as 45 torsional natural modes in the
frequency range below 180 Hz. The natural modes included fundamental
modes, higher modes of the shaft, and blade–shaft interactions. Torsional
modes above 70 Hz were mostly caused by coupled effects between the
blade, shaft, and disk. Field tests measured the blade induced vibrations
caused by short circuits induced at the generator terminals and concluded
that, in this case, the blades of the L-0 row were tuned well away from
120 Hz.
Reid provided a detailed case study that evaluated coupled blade–
rotor torsional vibration in anticipation of a major low-pressure blade
retrofit. That analysis analyzed the natural frequencies and response stresses
for a system that included a combined HP, IP and two LP turbines along
with a generator and exciter. Bladed disks were modeled as branch elements.
The model developed was confirmed using impact or ‘‘bump’’ testing on the
Copyright © 2003 Marcel Dekker, Inc.