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Case Study : Loose Parts
During January / February 1999, various diagnostic
technology and techniques developed at the EPRI M&D Center for
Generator Transformers were demonstrated jointly by EPRI M&D,
CENPEEP and Plant teams. A PDM survey was performed on the Generator
Transformers at one of the plants. These transformers were 270 MVA
three phase step-up transformers, oil cooled, and OFAF type.
The survey in Feb 99 indicated the presence
of loose parts which were externally source located, at
load, indicating an internal anomaly near the bottom on the W-phase
low voltage side. (LV side, bottom location 3 & 4). This
location of maximum RMS levels of ultrasonic activity was identified
with the help of Ultrasonic contact test equipmebnt (5550 Fault
Detector) by monitoring ultrasonic RMS levels displayed on the
equipment and listening to the ultrasonic signal, converted to an
audible signal, by laying out a grid. The continuous and cyclic
nature of ultrasonic activity, and the normal dissolved gas in oil
levels indicated an absence of detectable partial discharge.
Vibration spectra (peaks at multiples of odd frequencies: 3X, 5X
etc.) taken at the location indicated the fault to be a possible
loose part. At the bottom most portions, high ultrasonic activity
was also present, and suspected due to flow induced vibrations.
In order to know the condition of the transformer
and calibrate the data an opportunity was availed in April 99,
during the overhaul of the unit. The manufacturer opened the
transformer and found a broken fiberglass mounting nut & bolt
used for mounting the Weidmann Board lying at the bottom of the
transformer tank. The Weidmann board was thus found dismounted from
the mounting bolt at the bottom side on the LV side below W-phase,
the same location where loose part was suspected during the PDM
survey earlier. The board was also loose from the top and was
vibrating. If this condition continued the board would have sheared
off from its mountings over a period of time. After the inspection,
the board and nut / bolt were repaired and restored to original
condition. The details are shown in figure below.
The follow-up survey in May 99, indicated reduced
levels of ultrasonic activity on all sides; reduced levels of
vibration; a major reduction of ultrasonic activity at the defect
location, and the absence of loose part characteristics in the
vibration spectra. At the bottom most part, expected flow induced
ultrasonic activity was observed where the oil flow pipe is located.
The loose board itself was not expected to lead to
any damage or loss of performance of transformer. The shearing off
of the board would have caused secondary damages to the coil, the
extent of which would depend on the location of shear, sharpness of
shear edges, and angle at which the coil is hit. The probability of
damage to coil is difficult to estimate but can not be ruled out.
Assuming minimum secondary damages, and further damage to the board,
(its shearing off) could have caused minimum loss of generation
of five days (about 20 million units at about 85% PLF costing
about Rs 40 million, $910K US Dollars) apart from cost of labor
and material.
In this context, the survey in February 99 could
correctly predict loose parts and follow up survey confirmed the
restoration. This experience has given confidence to both the
station and manufacturer in the benefits achievable using the
techniques, and has proven to be a good confidence
builder.
This case study illustrates the benefits of using multiple
technologies for arriving at equipment condition making decisions
based on periodic surveys, which include the use of vibration
analysis in identifying the nature of fault. | 