Electrical Equipment Reliability with Ultrasound
Airborne & structure-borne ultrasound is the most versatile PdM technology available. Typical applications include compressed air leak detection, vacuum leak detection, steam traps, valves, and various condition monitoring applications. One application in particular that has seen the most increase in usage in over the last few years is for electrical inspection of energized electrical components.
Ultrasound is becoming a must have technology for electrical inspections because of how complementary it is to traditional infrared inspections of electrical gear, as well as safety. Ultrasound inspections can typically be done prior to opening energized electrical cabinets. Electrical anomalies detected by ultrasound include corona, tracking, and arcing, and partial discharge.
This paper will discuss what ultrasound technology is and how it is used to inspect electrical equipment such as transformers, switchgear, and substations. Screenshots from a spectrum analysis software of recorded ultrasounds will be inserted to show how proper diagnosis of electrical anomalies is done through ultrasound imaging. Ultrasound Imaging is a term that is used when ultrasounds are recorded onboard an ultrasound instrument and played back in a spectrum analysis software, or viewed in real time onboard the instrument. In either case, they can be viewed in either the time series or the FFT.
Electrical Equipment & the 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. An ultrasound inspection of electrical equipment can be performed without opening the cabinet or enclosure.
Corona refers to the ionization of air surrounding an electrical connection. Corona, by nature does not produce significant heat to be detected by an infrared camera. If left undetected, and if corona should continue to occur, it can lead to more severe problems in metal-clad switchgear. By-products of corona include ozone, electromagnetic emissions, nitric acid, and ultraviolet light. More prominent signs of corona include a smell of ozone in the area of electrical gear, unusual weathering patterns on bus, an accumulation of white powder and dust on insulators, carbon tracks on insulators, and discoloration & deterioration of insulated conductors.
Even though corona produces little to no heat, it does however produce ultrasonic emissions. If the inspector’s ultrasound instrument has on-board sound recording capability, the ultrasound emission from corona can be recorded and further analyzed for a correct diagnosis. A note of importance on corona is the fact that it is only present in voltage above 1000 volts. At 1000 volts and greater, air becomes a conductor and hence the ionization of air surrounding a connection can occur. If inspection is being done on voltages below 1000 volts, 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 60Hz harmonics can be noted. If the sound recording is done outside of North America, one would see very dominant 50Hz harmonics. Additionally, in between the 60Hz harmonics, you would see what is referred to as frequency content. Frequency content is basically harmonic activity between the more dominant harmonics. As the condition worsens, there will be a loss of the dominant 60Hz 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. Many will 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 which 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 a rather erratic bursts of discharges and popping sounds. These are identifiable when looking at a recorded ultrasound of arcing in the Time Wave Form.
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. 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, 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 is given a greater chance of detecting anomalies 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.