Ultrasound Inspections of Electrical Distribution Equipment
Ultrasound Electrical and Mechanical Inspection Overview
How does it work?
Noise produced by electrical emissions, deterioration of rotating mechanical equipment and most leakage problems contains a broad range of frequencies including the ultrasonic. This ultrasonic noise is shorter in wave length and more directional in nature and is therefore significantly easier to distinguish from other background noise and then be precisely identified and located.
We utilize Ultrasound or ultrasonic detection to facilitate electrical inspections of both low voltage (Tracking and Arcing at 480-1kv) and high voltage (Corona Discharge at 1kv and up) equipment without opening the equipment enclosure. We also utilize Ultrasound as primary mode leak detection for compressed gas, steam, vacuum and dense fluids.
Detectable ultrasonic noise generated during some undesirable events points the way to timely and effective repairs:
- In high voltage applications, the passage of undesirable electric current through the air creates ionization of the air (corona discharge) and ultrasonic noise
- In lower range voltage applications, arcing from hidden and defective electrical component connections produces both audible and ultrasonic noise
- Vibration resulting from deteriorating or poorly maintained rotating equipment bearings produces noise in the ultrasonic region that is proportional to the degree of deterioration
- Vibration that results in movement of adjacent attached metallic components creates noise that has harmonic components in the ultrasonic region
- Compressed gas transition from higher to lower pressure as from a pipe or pressure vessel leak creates turbulence and ultrasonic noise
- Water (or other dense fluid) leakage from a pipe creates turbulence at the point of leakage and more ultrasonic noise
Armco Infrared utilizes Ultrasound Detection to promote inspector and plant electrical safety by:
1. Pre-emptive scanning of older or harsh environment switchgear operating at 1kv and below where the probability of equipment contamination and arcing (and possible arc-flash) is higher. This service may be comprehensive and part of the job scope as agreed with the client or it may be on an item by item basis decided at the time of inspection in conversation with the Qualified Assistant and facility guide.
2. Scanning of higher voltage or main switchgear in all environments for arcing, tracking and corona discharge without the need to remove live equipment covers. Where covers can be removed, both Ultrasound and Infrared scanning will likely be employed.
The higher the voltage or potential difference between the equipment and ground, the greater the possibility of corona discharge or other form of breakdown that can result in flashover or an arc-flash incident when the panel cover is removed. For this reason, facility operators are increasingly reluctant as voltages increase beyond 1kv to open enclosures for routine inspections including infrared thermography. Problems that are developing (deterioration of insulation, etc) can go undetected up to the point of exceedingly expensive or catastrophic failure. In increasing cases, operators are installing inspection windows to allow both infrared and line-of-sight ultrasound inspections.
High Voltage Corona Discharge Detection with Ultrasound
Equipment energized with high voltage (1kv and higher) alternating (non- uniform) electric fields have the potential for ionization of the surrounding air and corona discharge. In high-voltage switchgear, corona is the indication of a potential problem and these indications will be present continuously until component failure.
Corona activity may occur from sharp edges on energized hardware, broken conductor strands or defective insulators. When corona occurs it creates sound, uv radiation, ozone, nitric acid and electromagnetic emissions. Nitric acid so produced may remove plating resulting in corrosion and poor connection of steel parts as well as progressive damage to insulating materials. Insulator deterioration may continue to complete breakdown with the resulting flashover and serious economic losses due to downtime as well as premature repairs and replacement of switchgear components. Plant safety is also compromised. Losses of this type may be prevented by regular checking of switchgear for the presence of corona.
Fortunately, with an unopened enclosure, Ultrasound can be used to safely detect the presence of corona discharge either at the enclosure door gaps or through direct contact. The “Qualified Person” or assistant with the inspector may, with proper attire, then open the enclosure with greater confidence, have a noted condition monitored or report the condition for immediate action.
Click the blue spot on the illustration below left to hear what Corona Discharge sounds like through the detector headset. The image below right shows a spectrum analyzer display of a corona discharge – You should notice a similarity of the sound and the repetitive spikes of the time domain response
UE Spectralyzer spectral display showing a Corona Discharge example note spikes at multiples of 60 hz and relatively constant harmonic level
Corona discharge (spikes) occur on the peaks of the waveform when the potential difference between conductor and earth or equipment ground is the highest and therefore occur at a relatively consistent 60 pulses per second (on the negative swing of the cycle). Especially for critical equipment, this condition would be monitored for a change in the character of the sound along with a change in the spectral (or time) response that would indicate a progression of the underlying condition to Tracking or Arcing.
The pc based UE Spectralyzer software shows the typical time domain “graph” of a corona discharge in the image below:
Also note the constant level of “white-noise” present as a background to the spikes-
With proper training and suitable equipment, corona discharge can be detected and correctly identified as compared to other types of electrical faults including “tracking” and “arcing” which are the subjects of the next sections.
Tracking Detection in Electrical Switchgear with Ultrasound
Whereas corona discharge is generally a function of the stresses resulting from high voltages present, Tracking (also known as “mini-arcing”) may be a function of voltage stress but also may occur at lower voltages due to deterioration or weakness in the insulating material. Weakness may derive from improper installation or damage due to mechanical influence as well the influence of nitric acid as mentioned in the section on corona discharge.
Tracking may be identified as distinct from corona discharge both aurally as well as well as from spectrum response. Note that harmonic spikes at 60 hz progressively decrease in amplitude and the higher harmonics are essentially missing. The time domain response shows Tracking as erratic bursts of energy that occur fairly quickly and at varying intensity or level. The aural example sounds as you would expect.
UE Spectralyzer spectral display showing a Tracking example – note fewer spikes at multiples of 60 hz and decreasing harmonic level
The illustration below left shows the typical erratic behavior of the Tracking event both in time as well as level. It is clear that this event is not limited to high voltage applications as it will occur at portions of the wave or cycle where the line voltage is approaching relatively low levels (120vac is low compared to a line to earth of 1kv, etc).
Note that tracking may occur at lower voltage points of the cycle and random positions in time
A Spectralyzer Time Domain display showing a Tracking example – note random spikes of longer duration and possibly higher energy also note background noise similar to corona discharge
Tracking also may be considered a progression toward eventual failure from that of corona discharge alone. Tracking events, due to the increased energy involved at lower voltages may well include higher currents which will translate into heating and accelerated deterioration. This effect of this type of event may be more readily observed and confirmed with a thermal imager.
Arcing Detection in Electrical Switchgear with Ultrasound
Detection of arcing as distinct from Tracking and Corona discharge can be made initially by the aural patterns as it is typically random bursts of high intensity and relatively long duration at random positions in time. Listen to the example provided below left. The spectral response as seen below is similar to Tracking. For this reason, the Time domain response is utilized to make the distinction.
Spectralyzer Spectral Display showing an Arcing example – note random spikes with no harmonic repetition – fault type is not clear
The Illustration below left shows a representation of arcing on the 60hz waveform. Note the similarity to Tracking in the random position of the event(s) – both in time and at voltage level with respect to earth or ground. Also note the increased duration of the event – far more energy involved with resulting current flow and heating.
A Spectralyzer Time Domain display showing an Arcing example random spikes of far longer duration and higher energy – this is a clear indicator of the event compared to the spectral display
Arcing also may be considered a progression toward highly probable failure from that of a Tracking event. Arcing, due to the significantly increased energy involved at lower voltages will include progressively higher currents which will translate into higher heating and progressively higher rates deterioration. This effect of this type of event will be even more readily observed and confirmed with a thermal imager.