What you can t 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. Let s 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 isn t broke, doesn t 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.
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 today s electrical systems. The complexities in today s 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
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
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
EXCURSIONS BROADBAND WHITE NOISE 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
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.
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
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
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: 25 Hz Fault Fundamental 50 Hz Elements 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.
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.
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.
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.
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 system s 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.