Today’s electrical maintenance and reliability teams are faced with the stresses and challenges of the modern workplace. Electrical work teams specifically are faced with the loss of skilled electrical trades workers, aging electrical equipment and infrastructure, emerging technologies, an increased load on our current electrical grid, and the continued threat of hazards and injuries from electrical incidents.
According to research by the National Fire Protection Agency (NFPA), from 2012 to 2016, a total of 739 workers died from exposure to electricity. Eighty percent of the fatal injuries were from direct exposure to electricity while performing construction, maintenance, or cleaning activities. The majority of the locations where the fatalities happened were industrial facilities. Although the numbers of fatal injuries are still high, the numbers have trended downwards over the last 10 years (see Figure 1).
Electrical maintenance
Airborne ultrasound has become one of the must-have technologies for the inspection of energized electrical equipment. Airborne ultrasound is also a perfect complementary technology to traditional infrared thermography scans of electrical assets. At any voltage, thermal anomalies and sources of ultrasound such as tracking and arcing can occur. Corona can also occur at 1000 V and greater. Any of these conditions threaten the reliability of the equipment being inspected.
Typical electrical components that can be inspected with ultrasound include switchgear, load interrupter switches, breakers, transformers, motor control centers, and terminal transition cabinets. Airborne ultrasound can be used for electrical inspections and for some failure modes, can allow for more problems to be found sooner. Also, because airborne ultrasound can be used to listen first before opening electrical assets for further inspection, safety is increased and the risk for an arc flash incident is greatly reduced.
As a further complement to traditional infrared inspections, and to aid in the proper diagnosis of the condition heard, the sound file of the anomaly detected by the airborne ultrasound instrument can be recorded and then analyzed in both FFT and Time Wave Form in spectrum analysis software in order to properly diagnose the electrical fault. This form of analysis is referred to as ultrasound imaging.
Airborne ultrasound technology
Hand-held airborne and structure-borne ultrasound instruments sense and receive high frequency sound waves that are produced from various sources, including 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 with the naked ear. The instrument receives the high frequency sound and through a process called heterodyning, translates the high frequency sound into an audible sound heard through the headset by the inspector. The sound is then measured as a decibel (dB) on the display panel of the instrument.
Ultrasound is considered by many to be the most versatile of any PdM technologies. Typical applications for ultrasound include for 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, just to name a few. Ultrasound instruments can be used to inspect energized electrical components that are 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 because, as mentioned above, an ultrasound inspection of electrical equipment can be performed without opening the cabinet or enclosure.
Corona, tracking, and arcing
One electrical anomaly that ultrasound will detect is corona. Even though corona produces little to no heat, it does, however, produce ultrasonic emissions. If the inspector’s ultrasound instrument has onboard sound recording capability, the ultrasound emission from corona can be recorded and further analyzed for a correct diagnosis (see Figure 2). A note of importance with corona is the fact that it is only present in voltages at or above 1000 V, where air becomes a conductor and hence the ionization of air surrounding a connection can occur. If inspection is being done on voltages below 1000 V, and an ultrasound is heard, the inspector can rule out corona as a possible diagnosis.
When the recorded ultrasound of corona is looked at in a spectrum analysis software, very prominent 60 Hz harmonics can be noted. If the sound recording is done outside of North America, one would see very dominant 50 Hz harmonics. Additionally, in between the 60 Hz harmonics, you would see what is referred to as frequency content (see Figure 3). Frequency content is basically harmonic activity between the more dominant harmonics. As the condition worsens, there will be a loss of the dominant 60 Hz harmonics, and uniformity in the amplitude of the recorded ultrasound will decrease.
Tracking occurs when there is a low current pathway to ground across an insulator (see Figure 4). Many people 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, mid, and high voltages and characterized as a steady buzzing sound with periodic “crackling” and “popping” sounds. Further damage is done when tracking is not corrected, and will rather quickly lead to arcing.
The move from corona to tracking leads to a destructive path across the insulation, and creates pin-holes and spider web like tracking that causes surface deterioration. When visually inspected, one can see a very obvious tracking path on the surrounding surfaces. Also, a conductive cloud of ionized air surrounds the connections. Flash-over can now occur once a tracking path is complete from phase to phase or phase to ground.
Finally, arcing happens when there is a discharge to ground across an insulator. Arcing causes 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 rather erratic bursts of discharges and popping sounds. These are identifiable when looking at a recorded ultrasound of arcing in the Time Wave Form (see Figure 5).
Conclusion
Ultrasound instruments are versatile and easy to use, and can greatly enhance inspections on almost any electrical equipment. With the onset of new methodologies in physical asset management like IIoT and artificial intelligence, ultrasound sensors can be utilized to allow for 24/7 monitoring of critical electrical equipment and enhanced diagnostics through machine learning techniques.
In the end, it’s all about safety. Ultrasound inspections can be done prior to opening the energized gear to scan with infrared or for visual inspections. If an ultrasonic emission is heard, then the proper precautions can be taken before opening the energized cabinet. Also, for those that rely on the services of an outside contractor to perform infrared scans and other electrical maintenance, an ultrasound scan can be done in between the service provider’s work to listen for any changes that may have occurred and to detect anomalies.
When ultrasound is paired with traditional infrared scans of electrical equipment, an inspector is given a greater chance of detecting failure modes that could potentially be missed when relying on just one single technology. For best results, analyzing recorded ultrasounds in either the FFT or Time Wave Form view is the recommended method of diagnosing electrical anomalies heard with ultrasound.
