POWER January 2016 - 48

EQUIPMENT MONITORING
3. Example time transient of shaft strain following unit trip. Courtesy:
EPRI
nology to the market that directly supports
our long-term goals of reliable operation, "
Pratt said. " Because of research partnerships
like this, we are hopeful to bring products
like this to market when we need them. "
The sensor has also been installed for a
Time (seconds)
4. Spectral plots of shaft strain obtained from generator input shaft at
eight unit load points. Courtesy: EPRI
similar period at Duke Energy's Marshall
Steam Station, and torsional data has been
collected over the load ranges and various
speeds. A Duke spokesperson notes that,
" compared with earlier technologies, the system
is very sensitive and provides more data,
with a higher degree of granularity, which
makes it easier to tell where the torsional
peaks are. The data gives us a very good picture
of each individual torsional node, so we
can identify whether it's a blade stress or predominantly
a rotor-induced torsional stress.
That, in turn, helps us identify what the failure
mechanism would be and how close we
are to that natural frequency. "
Field testing of the prototype system also
provided a valuable basis for assessing performance
of the following key aspects of the
EPRI torsional sensor system:
Frequency (Hz)
5. Example shaft strain spectrogram obtained following a unit trip. Note
the trends in both shaft speed harmonics as well as the natural frequency indications as the
shaft speed declines from bottom to top of plot. Courtesy: EPRI
■ Suitability of the RF excitation to meet the
sensor needs, including the specific transmitter/antenna
components, antennal layout,
and power levels used for both power
transmission and reception.
■ Strength and integrity of the Kevlar-epoxy
infusion used to attach and protect the sensor
system on the shaft.
■ Low noise-floor of the strain spectra, enabling
detection of modes with low elastic
energy at the sensor location.
■ Demonstration that rotor access time durations
required for sensor installation are
compatible with typical time-constrained
outage plans.
■ Functionality and durability of the entire
telemetry system in an enclosed oil-mist
environment.
Frequency (Hz)
that the EPRI equipment was installed toward
the end of the Unit 3 major overhaul
in the spring of 2015. " The equipment was
set up to monitor turbine roll-up of Unit 3
after LP Turbine L-0 bucket upgrades, with
hold points occurring at speed no load and
different loads up to full load. Data was
taken alongside GE's Life Extension Services
program. Both sets of data were used
to identify the torsional frequency and mode
shapes of the shaft system and components.
The results of this data verified that we were
running close to the second harmonic of line
frequency, which in our case is 120 Hz. Several
members of the SRP team-including
Israel Perez (turbine engineer), Jim Colsen
(mechanical engineer), and Joe Frazier (en48
gineering
manager)-along with GE subject
matter experts, met to alleviate the issue. The
solution that emerged from several meetings
resulted in mass reduction along the shaft. "
" In conclusion, " the spokesperson added,
" NGS was able to deviate from the second
line frequency and run Unit 3 without issue,
and this was supported with data that was
taken by both systems. To this day, data can
be taken and monitored with the EPRI equipment
on Unit 3. "
Jim Pratt, senior director of SRP's Base
Load Generation, said it's important for utilities
like SRP to host field tests like this one.
" The results of these field tests have the potential
to benefit the entire industry. This is
one way where EPRI is bringing new techwww.powermag.com
■
Durability of the rotating and stationary telemetry
components exposed to high ambient
temperatures with seismic vibration.
Future Work
Enhancements to the prototype sensor have
been incorporated into an upgraded sensor
design. Once this design is field-tested, the
system will be commercialized. The goal is
to ensure availability of this technology on
a wide basis to meet industry needs. It is anticipated
that highly sensitive and low-noise
shaft strain monitoring will be the basis for
advances in a range of new condition monitoring
applications on a wide array of powergenerating
equipment. ■
-Steve Hesler (shesler@epri.com) is the
manager of EPRI's Steam Turbine-Generators
and Auxiliary Systems Program.
POWER | January 2016
Time (second)
ADC counts (LSBs/Hz)
Strain (LSB)

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POWER January 2016

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