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What you cant hear can hurt you! All sound is not created equal,
and we must make distinctions.
Joe Gierlach ~ Vice President, Technical Training and
Support
TEGG Corporation ~ Pittsburgh, PA
ABSTRACT: Ultrasonic testing has been around for many years
throughout different disciplines and industries. With its unique
ability to detect high frequency emissions well above the human
range of hearing, it has allowed many to benchmark and identify
problematic areas in many venues. Although no one system typically
works without the other in most commercial and industrial
facilities, there is a common component that is universally
present, and loss of it would result in EVERY system in a facility
to cease. Electricity! Without it, nothing works, generally
speaking. Despite beliefs that there are no moving parts in
electrical systems, in fact, depending on where you are located,
there are movements that occur 50/60 times every second! This
creates stresses on components, and can cause deterioration of
conductors and insulators on low, medium, and high voltage items.
Lets take a look at a few examples of where this technology has
identified deficiencies in systems and prevented the loss of
electrical supply. We will also examine the necessary training to
identify characteristic footprints of these deficiencies through
spectral analysis.
INTRODUCTION: Reliability! This is the name of the game in
industry today if companies want to remain competitive on a global
scale. Aside from production and manufacturing, there are also the
needs for data processing, banking, internet service providers,
radio and television broadcasting, educational facilities, call
centers, health care facilities and places of public assembly. The
one commonality within all of these arenas is the absolute need for
constant electrical supply. Without it, nothing will work, and the
consequences range from lost revenue, lost customer loyalty,
increased maintenance budgets due to cost of failed components, and
credibility damage within your discipline. Because many components
make up the electrical distribution system, they are often
overlooked in contingency planning where maintenance is concerned.
Many planners operate under the pretense that if it isnt broke,
doesnt fix it, and this allows a perceived option to exclude
attention to most of the electrical system. Many types of
facilities listed above typically have redundancy built into the
electrical system to ensure a reliable supply of power in the event
of an interruption for short or long periods of time. This is a
prudent approach given the mission of any given facility; however,
it can provide a false sense of security in respect to an
uninterrupted supply of electricity. Consideration is seldom given
to component failures in either the normal or standby power
systems, where a lack of preventive measures to ensure equipment
health could very well result in an interruption or failure of some
sort. Even those facilities that do have the foresight to be
proactive with electrical system maintenance tend to address it
only half-heartedly. This is to say, in many cases, there may be an
annual shutdown where the in-house maintenance staff or contracted
personnel goes through all the gear to clean, vacuum, tighten and
torque all the connections; however, this approach could also
create some unintended consequences. Deficiencies can be created
simply by the task of torquing connections beyond the recommended
specification, causing a degradation of that connection point once
the system is restored.
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Some facilities that cannot shut down employ the use of infrared
thermography as a tool and technology in an effort to identify hot
spots within the system components. To find problems before they
materialize, there are many organizations throughout the world
providing these services, but the infrared cameras are just one
tool that can effectively be used on todays electrical systems. The
complexities in todays systems mandate that personnel performing
this service are trained and knowledgeable of the construction and
operation of the electrical equipment, and trained on the safety
hazards involved. There are many idiosyncrasies within the systems
that only a trained eye will detect. Many deficiencies are
overlooked by untrained personnel using even state-of-the-art
infrared imagers. This paper looks at some past examples where
ultrasound has proved effective in identifying issues when other
technologies have failed. We also discuss the differences on 50 Hz
and 60 Hz systems, what is problematic, and what is normal. This is
taking place in many corners of the globe, as you will see, and
illustrates an unconventional approach to maintenance programs.
Case History #1 It all starts with transformers in any facility,
whether it is commercial, industrial or residential occupancies.
Transformers reduce utility supplies to usable voltages for
distribution throughout a facility. A failure or malfunction with
the transformer will affect everything downstream. In a properly
operating transformer, one would expect a number of basic
observations using several testing technologies. Listed below are
several of these benchmarks:
1) Even and uniform thermal patterns when viewed with an
infrared camera on the vessel, insulators, mechanical lugs, and
windings (if visible, dry type unit)
2) A characteristic hum audibly with signature spectrums in both
frequency and time domain 3) Balanced supply voltages and currents
on the primary side 4) Balanced distribution voltages and currents
on the secondary 5) No evidence of leaks with respect to insulating
oil 6) No discoloration of conductors and insulators
When we think of infrared, a properly functioning unit should
exhibit a consistent, uniform pattern in the camera as mentioned
above. A couple of examples of this are seen below in Figure 1 and
Figure 2:
Figure 1
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Figure 2
In the infrared world, this is a typical pattern one would hope
to see if there were no critical issues internally which would
result in localized temperature increases that dissipate very
quickly due to the indirect nature of the measurement. With respect
to ultrasonic analysis, the spectrums that are atypical for the
same unit are illustrated below in Figure 3 and Figure 4:
Figure 3
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Figure 4
As you can see, the frequency spectrum (Figure 3) indicates the
60 Hz peaks with little to no noise content in between each peak,
which is a key indicator for electrical anomalies. Additionally,
the time domain (Figure 4) displays a typical broadband of white
noise in the middle, with the amplitude of each excursion having
little to no variation throughout the spectrum. These examples show
clearly what a normal unit should look like in the ultrasonic
world. Finally, the amplitude of either frequency or time domain,
or even on the instrument itself when recording, should NEVER be
used as a gauge for severity. This only represents how close one is
to the source of the emission. Reciprocally, there can be tell-tale
signs in these analysis tools that DO indicate when an issue is
present. In one example, a service transformer supplying power to a
major university did have a problem that was only identified by the
use of ultrasonic testing. The key on this was the time series as
you see below:
Figure 5
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Figure 6
The frequency spectrum shows a classic response for a 50 Hz
(Australia) transformer that is in operation. The time series, on
the other hand, indicates something different. The classic
broadband of white noise is displayed in the yellow box. However,
there are pronounced excursions of varying amplitude at different
points throughout the recording, which indicate electrical
discharges associated with an arc. Two are vastly deeper than the
rest, providing the exculpatory evidence necessary for an accurate
analysis (see red arrows). The spectrum above was recorded on a
liquid-filled transformer with an 11 KV primary supply voltage and
a 400 volt secondary voltage. The primary feeders were encapsulated
in a tar resin, making airborne measurements ineffective. Contact
measurements were taken near the junction box, and the
determination was made that the emission was originating internally
on one of the stud connections to the bushing. At this point, an
oil sample was necessary to determine dissolved gas in oil content
where levels of certain combustible gases would support the
analysis. A digital image of these types of transformers is
below:
Figure 7 Figure 8
EXCURSIONS
BROADBAND WHITE NOISE
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Of course there are other items visible in this digital
illustration which would need attention, such as the leaking
insulating oil. It is never good for a cooling medium to run low
and affect the capacity of such an important piece of
equipment.
Case History #2 Even on much smaller devices, there can be
problems internally which do not show up solely using infrared
thermography. The indirect nature of measurements makes the
effectiveness of cameras limited at best, particularly when we are
dealing with smaller, lightly-loaded devices, or much larger
devices where the thermal resistance is increased by mass. During a
visit to Manchester, England in 2005, we had the opportunity to
test our theories on several items in which panel covers may or may
not have been removed due to access limitations. In both instances,
the ultrasonic technology proved invaluable in identification of
issues that would have otherwise been overlooked. The prime example
surrounds a 32 amp, single pole circuit breaker feeding a critical
circuit in a high occupancy hotel. During the application of the
service, it was determined that the device was running well below
its rated capacity at 17 amps, and the infrared image displayed
nothing that would have been cause for alarm. (See the infrared and
digital images below for perspective and location.)
Figure 7
The infrared image indicates what one would expect from a device
that has approximately 45% of its rated current flow, and no
localized anomalies that would raise a red flag prompting further
investigation. Using contact ultrasonic measurements, however, told
a different story. Not only was the sound suspect, but the
definitive proof was again in the frequency and time domain
analysis. Using comparisons to known samples of tracking, we
observed the characteristic frequency spectrum which had
fundamental 50 Hz fault, along with the first several harmonics of
50 Hz; then further out on the spectrum, no discernable faults that
were multiples of the fundamental frequency. Additionally, it was
not quiet in between the faults, as the frequency noise noted is
indicative of tracking type discharges inside this circuit breaker.
This was not all that was necessary to make the determination,
though, as one must also use time domain to ascertain if the
signature of this type of problem is also present. Aside from the
characteristic broadband of white noise mentioned earlier, a
tracking recording would have excursions above and below the middle
band as the discharges occur and generate the ultrasonic
emission.
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These would also have a pattern that is non-uniform in nature
and amplitudes that vary throughout the recording. An example of
this is seen below.
Figure 8
The recorded sample, indicated in red, is very similar to the
known sample. As mentioned above, there are several harmonics of
the fundamental frequency and then few as you move toward the
higher end. Noise content is also evident in the recorded sample as
well. This type of frequency spectrum is common in electrical with
many different noises one might hear, and could be mistaken without
the use of the time domain; although this is a starting point to
make the determination of is it electrical, mechanical, vibration,
problematic, etc For this reason, you must use both tools and
attempt to identify the footprints that would be present on
tracking in this case. In the illustration below, this is the
case:
Figure 9
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Clearly, the band of white noise is seen with excursions
occurring throughout the recording and amplitudes of no discernibly
similar intensities. Also noteworthy, the gauge for severity of
electrical issues comes not from the decibel level of the
recording, but in the frequency content of faults and the frequency
of the discharges in the time domain. This means the more compact
the excursions appear; the more frequently the discharges are
occurring. Once again, ultrasonic emissions identified a potential
problem that could have been overlooked.
Case History #3 Another example of how ultrasound is used would
be to disprove the presence of an apparent problem. As there are
many sounds that are naturally occurring in electrical equipment,
one must be able to discern what is to be passed over and what
should be addressed. In this next case, we have a transformer from
the United Kingdom in which audible and ultrasonic sounds were
intermixed, and the question was do we have a problem? In the
recorded wave files, one can hear normal 50 cycle hum, which is
expected with transformers. Additionally, there were other elements
in the recording that raised suspicion that a deeper problem may be
present. Once the proper analysis was conducted, there could be no
mistaking what was present.
Figure 10
Figure 11
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As you can see in the FFT display, each 50 Hz harmonic is
present across the entire spectrum with little to no frequency
content in between each peak. This is characteristic of a normally
functioning transformer; however, in this case, the recorded sound
contained something different in the audible qualities, which
raised suspicion that it was not normal. The time domain also
supported the notion that all was OK. This required a closer look
at the FFT, and through this observation, there was in fact an
additional element in the frequency display that was consistent
with the difference in the sound quality. There was also a 25 Hz
harmonic which is not common with electrical related deficiencies.
In fact, this is key in making the correct determination that the
additional elements in the recording were mechanical in nature, and
ultimately were identified as core delamination. See below for the
illustration:
The revelation of this 25 Hz harmonic allowed for a precise
diagnosis on this particular unit. Although there were no
indications with the infrared camera that the vibrations within the
unit core were causing any adverse heating effects, this was
identified early enough to develop an action plan to coordinate an
outage with the customer and appropriately make adjustments and
repairs to the iron core. Familiarity with the equipment you are
performing maintenance on and knowing the signatures or footprints
of what is expected and what is out of place is crucial to making
proper judgments in real life situations. It can make the
difference as to whether or not a unit will continue to function
properly, fail due to a missed deficiency, keep a budget within
plan, or just as importantly, either increase or damage YOUR
credibility with the customer or maintenance staff.
25 Hz Fault Fundamental 50 Hz Elements
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Case History #4 The last example in this paper is from several
years back, but it is too good not to share once again for the
benefit of those who may have missed the story. A mall complex in
Milwaukee, WI had a distribution set-up of a north and south feeder
loop for redundancy to ensure a constant supply of power.
Additionally, there were two twin bank towers 12 stories each on
either end of the mall complex. The feeder loops entered each tower
into junction boxes which provided an interface with the puffer
switches used for selecting which loop would supply power at any
given time. In the north tower, the main electric room was in the
basement, and when we entered to perform our maintenance, the sight
was something to behold:
Figure 12
In the photo above, the junction box cover is to the left of the
exposed feeder loop conductors, and for a supply voltage of 4160
volts, it is not a safe situation by any stretch of the
imagination; not to mention that there is seepage from the
conduits, which are run over 800 feet underneath a parking lot
between the tower and the main mall complex. The bucket which is in
the image was placed there with a plastic valve and Teflon tubing
to drain the water from the box to a floor drain in the corner.
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Looking at the infrared image below, you can see there is
nothing alarming about the thermal patterns that are visible. On
both of the cables, there is even, uniform and load related energy
which does not exceed 60 degrees Celsius, well within the
insulation rating of the cable.
Figure 13
Any knowledgeable thermographer could agree that this is exactly
the type of pattern one would expect on a cable of this type with
moderate load demand present. The insulation has an absolute rating
of 90 degrees Celsius, and is in no jeopardy of damage with this
operating condition. While conducting the routine service of the
equipment in the room, the safety ultrasonic scan was conducted
with the airborne module, instrument set at 40 kHz, and maximum
sensitivity. During the survey of the puffer switch, which is
physically located to the left of the junction box, an unusual
sound was detected with the probe. The quality of the sound
suggested a mix of arcing and tracking, but we could not be sure
unless recorded and analyzed with the Spectralyzer. As you can see
in the FFT display, the characteristics of tracking appear with the
first couple of 60 Hz harmonics, elevated noise content between the
peaks, and similar to the known sample of tracking shown in
white.
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The time domain was a bit dicey, as the signal was overloaded at
the time of recording, and there was a chopping of the wave form at
each peak, making an assessment of severity by comparison of the
differences in amplitude a bit difficult.
The signature band of white noise is again present throughout
the recording. The excursions are present, but not necessarily
exhibiting a uniform level or spacing, which was preserved in the
original recording. Based on this and coupled with the analysis of
the frequency content, it was determined that tracking was
occurring on the cable insulation between the outer jacket and the
semi-conductive layer underneath.
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This situation was sure to deteriorate further and ultimately
cause a failure of the insulation system, resulting in the cable
failure. It was also likely that the neighboring cable would have
been exposed to the effects of such a failure, and in all
probability, would have resulted in a failure of that cable. This
is the key to a preventive mindset and the approach to identify and
address problem areas before a failure stage.
SUMMARY: When it comes to equipment operation and health, no one
can afford to be complacent. To minimize operation and maintenance
costs, as well as remain competitive in whatever arena one may
participate, a well-administered program is certain to pay
dividends and provide peace of mind. With all the available tools
and test equipment at the disposal of personnel today, it would be
foolish to rely on any one technology and put all the eggs in one
basket. Infrared thermography, for example, has been around for
many years and has gained markedly increased notoriety over the
last decade. This is a fantastic tool, but we must keep perspective
and realize that it is just that, one tool. Notwithstanding the
limitations of a single technology methodology, a preventive
maintenance program would be incomplete at best without using all
of the tools available. It would be like taking a vehicle in for a
full-service oil change and having only 2 quarts of oil replaced
instead of 5 quarts, or inflating 2 tires with the recommended 35
PSI and the other 2 with only 10 PSI. Ultrasound provides an
excellent augment to infrared that cannot be overstated. There are
too many limitations with infrared regarding line of sight, access,
and many other variables present in electrical systems and
components. In order to have the most comprehensive assessment of a
systems health, there is no other option than to, at a minimum,
keep these two instruments attached at the hip when selecting a
course of action. Finally, with the increased attention to
electrical arc flash, why would anyone risk performing maintenance
on any piece of equipment without first listening to see if
something could be lurking behind closed doors that, when opened,
could explode with a fury not seen by most people. About the
author: Mr. Gierlach has worked in the maintenance electrical arena
as a journeyman electrician for over 20 years. He has been involved
with many industries, including the U.S. Air Force, naval destroyer
and cruiser construction and commissioning; phosphate, chemical,
fertilizer and co-generation plants; steel and titanium mills; and
most recently with TEGG Corporation in Pittsburgh, PA. Mr. Gierlach
is responsible for the Technical Training and Support Department,
and has been intimately involved with the development,
implementation, and application of preventive maintenance programs
of electrical distribution systems. He has presented at numerous
technical conferences, authored technical articles, and
participates in the development of standards in the US. and abroad.
He is an ITC Level III certified thermographer, a Level II
certified ultrasonic inspector, and possesses several other
certifications in state-of-the-art technologies utilized in
electrical preventive maintenance services.