Using Ultrasound To Identify Electrical Faults
EP Editorial Staff | January 17, 2013
Seeing is believing. So is hearing. And with electrical systems, two predictive/diagnostic technologies can be better than one.
By Joseph D. Gierlach, Jr., TEGG Service – An ABM Company
When it comes to complex electrical systems, all components have value. But some, such as transformers, circuit breakers, insulators, switchgear, etc., can take entire systems down—with disastrous results. These “high-value” assets are the ones that demand the most scrutiny.
There are myriad things that can and do go wrong with electrical systems, and “fixing” them isn’t possible without first diagnosing them. Interestingly, infrared and ultrasound technologies are often used in tandem to identify potential faults. Since infrared technology doesn’t “see” through enclosed panels unless they’re equipped with infrared windows, ultrasound technology can play a key role in safety and convenience. In fact, this is where ultrasound really proves its value as a predictive and diagnostic tool.
Listening first and looking later
The uses of infrared technology in managing equipment assets are typically well known. Some maintenance professionals, however, may not be as familiar with the role of ultrasound technology in diagnosing complex system malfunctions. The two technologies work well together because one “sees” and the other “hears” what humans are incapable of sensing.
One of the keys to successful deployment of ultrasound is the ability to identify the sound signatures of electrical emissions, such as arcing, tracking and corona. This becomes easier with experience and practice. (There is also substantial information on sound signatures available via the Internet. One helpful, free resource is a sound-recording library from UE Systems at www.uesystems.com).
Ultrasound is particularly useful in helping avoid potentially lethal events such as arc flash. By first listening with an ultrasound instrument, a user can detect anomalies—i.e., the tell-tale “bursts” of arcing or the steady, continuous “frying-egg” sound of corona—that clearly say to him or her, “Don’t open this unit (unless, that is, you’re ready to be exposed to temperatures as hot as the sun itself and an almost-inevitably fatal explosion).” It’s early-warning signs like these that bring us to the example of how ultrasound was instrumental in diagnosing a potentially disastrous failure in a critical-care unit of an Ohio hospital.
The hospital’s problem…
Two transformers, manufactured in 2004, were inspected in October 2011. Virtually identical, the units were used to power a critical-care unit at the hospital.
Transformers are crucial to the entire performance of an electrical system and, if properly maintained, should last about 30 years. Regular maintenance is paramount, as transformers are subject to scarcity: The lead-time to obtain one is approximately 10 months.
The hospital had hired its local TEGG Service provider to inspect the transformers annually. The previous year, there had been no maintenance problems noted. Unfortunately, that was not the case with the facility’s October 2011 energized services inspection.
The inspection’s findings…
An ultrasound examination of one of the units revealed a suspect sonic signature in the B phase coil. For comparison, an ultrasonic sound recording was taken of the sister unit, which was under similar operating conditions.
— The B phase coil of the suspect unit had a distinctly different and substantially louder sonic signature when compared with that of the “good” unit. This was the first indication of a potential malfunction within the unit. But, ultrasound by itself can’t tell exactly what a problem is, especially when relying on decibel level alone.
Fig. 1. There’s clearly a qualitative difference in the FFT display comparing the “B” phase coils of the hospital’s two transformers.
— An FFT (fast Fourier transform) analysis of the ultrasound recording was conducted that showed a visually qualitative difference between the wave patterns of the two units’ B phase coils (Fig. 1.) Further, a time domain analysis between the two coils showed a distinct difference in the peaks and valleys of the sonic signature.
Fig. 2. Time domain for the suspect unit’s coil
Fig. 3. Time domain for the comparative “good” sister unit’s coil
— The “good” B coil showed a flat line, without peaks and valleys, while the “bad” coil showed dramatic rises and falls, confirming visually what the diagnostician heard with the Ultraprobe ultrasonic testing instrument (Fig. 2 and Fig. 3.) This situation underscores the importance of making sure ultrasonic recordings are long enough (30 seconds is generally sufficient) to detect such anomalies, which may not be as readily detectable in shorter recordings.
— The FFT revealed fault frequencies that were not 60 Hertz harmonics—indicating a problem that was something other than electrical. But there was significant frequency content throughout the spectrum (which did indicate something electrical).
Based on this information, one thing was certain: The B phase coil in the suspect system was clearly exhibiting a problem. But what was it? The next step was infrared imaging.
Infrared imaging comparing the good and bad coils showed higher heat saturation on the bad one (Fig. 4). With the dynamic range of the infrared camera exceeded, it was determined that the suspect winding was hotter than 150 C. This evidence was compelling enough to recommend shutting down the equipment and performing a thorough visual inspection to determine the cause of the increased ultrasonic emission and thermal output of the B phase coil. The necessary (normal) electrical tests—insulation resistance, winding resistance, turns-ratio testing and power-factor dissipation—subsequently revealed no abnormalities and showed all values to be within industry-standard limits. Good news for the customer, right? Only if you ignore what the ultrasound and infrared analysis revealed.
We had heard sounds. We had seen the heat signatures. It was time to use something a little old-fashioned but, in this case, more reliable: our eyes.
Getting the whole picture
A visual inspection of the B phase coil showed that, contrary to the normal electrical tests, all was not well. With the A phase coil, all pieces were flush, as one would expect: no distortion among the joints and miters, and all sections were even and flat. The B phase coil was a different story altogether. A visual inspection showed that the steel pieces were not flush, indicating a distortion of the top horizontal section bowing out on the B phase winding, low-voltage side (Fig. 5). The defect was present exactly where the ultrasonic emission was greatest, and there was visual evidence of excessive heating on the core laminations, backing up what the infrared camera saw. This is proof of the fallibility of electrical tests alone—and proof of the value of ultrasound and infrared in detecting a potentially catastrophic problem. Such a problem could have been dismissed altogether if the electrical tests alone had been trusted.
Additional examination showed more lamination separation further in the core toward the high-voltage side, as well as thermal effects on the varnish of the core laminations. The miter joint that connected the center phase to the top horizontal laminations was no longer properly connected due to “bowing” of the joint, thereby disturbing the magnetic flux path, which was the likely cause of the increased ultrasonic emission and heating. More problematic, though, was the fact that the “good” sister unit, upon visual inspection, was beginning to show the same types of as problems as the bad unit—which was confirmed by a third party brought in to evaluate our findings. The conclusion? The hospital’s transformers had a manufacturer defect.
Why ultrasound mattered
If the hospital had relied on the electrical tests alone—which showed no anomalies—the defects in the transformers would have continued unnoticed, probably worsened, and led to a disastrous outcome: a power outage in a critical-care unit. While ultrasound alone did not “diagnose” the problem, telltale sounds that the emissions provided did prompt further investigation. In turn, that additional investigation uncovered nasty issues with the potential to adversely affect a life-saving facility. As this case study illustrates, ultrasound should be an integral part of the toolkits of those responsible for the diagnosis, maintenance and repair of critical electrical assets. MT
Joseph D. Gierlach, Jr., is Vice President, Technical Training and Support, for TEGG Service – An ABM Company, based in Pittsburgh, PA. To contact him directly, email: Joseph.Gierlach@abm.com.
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