Electrical equipment reliability with ultrasound and infrared technology

BY ADRIAN MESSER, UE Systems, Inc.

Ultrasound and infrared technologies are a perfect match when conducting inspections of electrical equipment. At any voltage, thermal anomalies and sources of ultrasound such as tracking and arcing can occur. Corona can also occur at 1,000 volts and greater. Any of these conditions threatens the reliability of the equipment being inspected.

Typical electrical components that can be inspected with ultrasound and infrared include switchgear, load interrupter switches, breakers, transformers, motor control centers, and terminal transition cabinets. This article will provide information on how using both infrared and ultrasound for electrical inspections can identify more problems sooner. Further, it will show how safety is increased when using ultrasound to scan enclosed electrical gear prior to opening for additional inspection.

As a complement to infrared inspections and to aid in the proper diagnosis of the condition, recorded ultrasonic examples will be shown in fast Fourier transform and time wave form from spectrum analysis software to show how to properly diagnose electrical anomalies. This form of analysis is referred to as ultrasound imaging.

WHAT IS ULTRASOUND?

Hand-held airborne and structure-borne ultrasound instruments sense and receive high-frequency sound waves produced from various sources that include turbulence such as a compressed air leak, friction (as in an under-lubricated bearing), and ionization in electrical discharges. These high-frequency sounds are above the range of normal human hearing and therefore cannot be heard. The instrument receives the high-frequency sound, and through a process called heterodyning, translates the high-frequency sound into an audible one heard through the inspector’s headset. The sound is then presented as a decibel (dB) reading on the display panel of the instrument.

Ultrasound is probably the most versatile of any predictive maintenance technology. Typical applications for ultrasound include compressed air and gas leak detection, bearings, motors, gearboxes, valves, steam traps, hydraulic applications, and condition-based lubrication of bearings and rotating equipment.

When it comes to electrical inspection, ultrasound instrumentation can be used on almost any energized electrical equipment including metal-clad switchgear, transformers, substations, relays, and motor control centers. Ultrasound instruments inspect energized electrical components on low-, medium-, and high-voltage systems.

Traditional inspection of energized electrical equipment has been performed by noncontact infrared cameras. However, in recent years, ultrasound instruments have been added to these inspections for various reasons. One of the main reasons has been safety. An ultrasound inspection of electrical equipment can be performed without opening the cabinet or enclosure.

ULTRASOUND AND IR

One electrical anomaly that ultrasound will detect is corona (Figure 1 and Figure 2). Even though corona produces little to no heat, it does produce ultrasonic emissions. If the inspector’s ultrasound instrument has on-board sound recording capability, the ultrasonic emission from corona can be recorded and further analyzed for a correct diagnosis. A note of importance on corona: It is only present in voltage above 1,000 volts. At 1,000 volts and greater, the dielectric strength of air can be exceeded, and ionization of air surrounding a connection can occur. If inspection is done on voltages below 1,000 volts and ultrasound is heard, the inspector can rule out corona as a possible diagnosis.

When the recorded ultrasonic signature of corona is looked at in spectrum analysis software (Figure 3), very prominent 60 Hz harmonics are noticeable. If the sound recording is done outside of North America, one would see very dominant 50 Hz harmonics. Between the 60Hz harmonics, you would see frequency content. Frequency content is harmonic activity between the more dominant harmonics. As the condition worsens, a loss of the dominant 60Hz harmonics occurs, and uniformity in the amplitude of the recorded ultrasound decreases.

Tracking (Figure 4) occurs when there is a low-current pathway to ground across an insulator. Many refer to tracking as baby-arcing. This event is common where there is severe breakdown of the insulating material and loose connections. Tracking can occur in low, medium, and high voltages and is characterized as a steady buzzing sound with periodic crackling and popping sounds. Further damage is done when tracking is not corrected, and it will quickly lead to arcing.

The transition from corona to tracking can lead to a destructive path across the insulation that creates pin-holes or spider web-like patterns that cause surface deterioration. When visually inspected, one can see an obvious tracking path on the surrounding surfaces. A conductive cloud of ionized air surrounds the connections. Once a tracking path is complete from phase-to-phase or phase-to-ground, flashover can occur.

Finally, arcing happens when there is a discharge to ground across an insulator. Arcing can cause severe damage to equipment, plant/facility operations, and people. Melting of connectors, damage or loss of insulation, and fires usually result from electrical arcs. Arcing can easily be heard and detected with ultrasound. The sound characteristic for arcing is a rather erratic burst of discharges and popping sounds. These are identifiable when looking at a recorded ultrasound of arcing in the time wave form (Figure 5).

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EXAMPLES

Arcing (Figure 6) was detected on the b-phase line side of this 2,000 amp main breaker (Figure 7). The arcing was worse when the load increased. The arcing had severely deteriorated the internal contacts, which eventually would become so deteriorated that the voltage drop could affect the downstream load. At this particular facility, the replacement cost for this item was approximately $20,000.

The next example is from a 2,000 KVA 11KV-415v cast-resin transformer (Figure 8a and Figure 8b). An inspection on this equipment was requested after audible noise in the area increased. The inspection was done during the winter months; for this facility, this transformer typically operates at reduced loading, as it supplies chillers and associated other plant equipment that normally does not work as hard during the winter months. During the inspection, it was noted that the load was around 420 Amps per phase (Figure 8a, Figure 8b, Figure 9, Figure 10).

The next example, from a contactor, is on a piece of equipment called an orbit motor (Figure 11a). A routine airborne ultrasound inspection was done, and distinct sounds of tracking were heard. A follow-up inspection with infrared was performed, and the diagnosis was severe tracking (Figure 11b and Figure 12).

CONCLUSION

Ultrasound instruments are versatile and easy to use and can greatly enhance inspections on almost any electrical equipment. In the end, it’s all about safety. Ultrasound inspections can be done prior to opening the energized gear to scan with infrared technology. If an ultrasonic emission is heard, then the proper precautions can be taken before opening the energized cabinet. For those who rely on the services of an outside contractor to perform infrared scans, an ultrasound scan can be done in between the annual infrared scan to see if any emissions are heard.

When ultrasound and infrared are used together, an inspector has a greater chance of detecting anomalies that could potentially be missed when relying on a single technology. For best results, analyzing recorded ultrasounds in either the fast Fourier transform or time wave form view is the recommended method of diagnosing electrical anomalies heard with ultrasound.

Adrian Messer, CMRP is the Manager of U.S. Operations for UE Systems, Inc. For more than a decade, Adrian has helped facilities around the country transform their reliability programs by successfully implementing ultrasound technology for condition monitoring and energy conservation applications. As a subject matter expert on ultrasound technology and implementation best practices, Adrian has been a featured speaker at numerous industry events. He is a graduate of Clemson University and maintains close ties to the university, assisting current students in a mentorship program through the College of Business & Behavioral Science. Adrian is a Certified Maintenance & Reliability Professional (CMRP) through the Society of Maintenance & Reliability Professionals (SMRP), a charter member of the Carolinas Chapter of SMRP, and the current interim Chairman of the chapter.

For more information, visit www.uesystems.com/ev or contact us at info@uesystems.com.

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