I found myself walking into an outdoor supply store a few days ago and heard what sounded like electrical buzzing in the power lines above the store’s parking lot. It took me a few moments to realize that what I was listening to was a corona discharge emitting from the power lines. That made me curious about corona, specifically how companies, such as L&S Electric, perform corona testing.

Corona testing plays a large role in determining the health of high voltage electrical systems. When combining corona testing with IR scans, technicians obtain a full picture of the health of electrical equipment. But, what exactly is corona testing?

Wikipedia defines a corona discharge as:

an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized. The discharge will occur when the strength (potential gradient) of the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects.

This discharge may not show up during IR inspections, but is revealed by ultrasonic testing. A corona test looks for disturbances at extremely high frequencies in electrical equipment. Regular testing is particularly useful for industries with continued processes, such as:

> Chemical plants

>Oil refineries

> Food-processing plants

> Paper mills

> Utilities

> Plus, wherever plants run around the clock

This Q & A article at the Plant Maintenance website provides an excellent summary and review of corona testing.

1. What voltage does arcing start at? Potentially as low as 5 volts if there is follow-on current, such as electric arc welding.

2. What voltage does arcing extinguish at? It depends, but most of the time about 50% of the voltage it started at. Remember this when troubleshooting a radio interference problem, if the radio noise is there, turn the line off, if it goes away, turn the line on again, if the noise is not present then it is probably arcing caused by capacitor switching or transients, if the noise is there when the line is turned back on, then it is probably arcing and this can be visible a mile away with a daytime corona camera.

3. What is a good readily available RFI/TVI noise detector? The AM radio in your car or truck tuned where there is no radio station and adjust the volume for comfort, then drive into the radio interference complaint area and listen for the loudest pole. Some noises can be outside the radio reception and will require other detectors.

4. What can cause RFI/TVI? Electrical arcing, micro-arcing, and for a utility items such as loose tie wires at insulators, loose washers or hardware, spark gaps between pieces sometimes rattled by heavy vehicle traffic, in industry or the home causes can range from TV sets, microwaves, transformers, electric motors, vehicles spark plug wiring, over the road diesel trucks with inverters, etc. places where an arc can flash-over momentarily but be limited in current so it doesn’t result in a continuous flash-over.

5. What is electrical corona? Corona is the ionization of the nitrogen in the air, caused by an intense electrical field. Electrical corona can be distinguished from arcing in that corona starts and stops at essentially the same voltage and is invisible during the day and requires darkness to see at night. Arcing starts at a voltage and stops at a voltage about 50% lower and is visible to the naked eye day or night if the gap is large enough (about 5/8" at 3500 volts).

6. What are the indications of electrical corona? A sizzling audible sound, ozone, nitric acid (in the presence of moisture in the air) that accumulates as a white or dirty powder, light (strongest emission in ultraviolet and weaker into visible and near infrared) that can be seen with the naked eye in darkness, ultraviolet cameras, and daylight corona cameras using the solar-blind wavelengths on earth created by the shielding ozone layer surrounding the earth.

7. What damage does corona do? The accumulation of the nitric acid and micro-arcing within it create carbon tracks across insulating materials. Corona can also contribute to the chemical soup destruction of insulating cements on insulators resulting in internal flash-overs. The corona is the only indication. Defects in insulating materials that create an intense electrical field can over time result in corona that creates punctures, carbon tracks and obvious discoloration of NCI insulators.

8. How long does corona require to create visible damage? In a specific substation the corona ring was mistakenly installed backwards on a temporary 500 kV NCI insulator, at the end of two years the NCI insulator was replaced because 1/3 of the insulator was white and the remaining 2/3 was grey.

9. What voltage are corona rings typically installed at? It varies depending upon the configuration of the insulators and the type of insulator, NCI normally start at 160 kV, pin and cap can vary starting at 220 kV or 345 kV depending upon your engineering tolerances and insulators in the strings.

10. What causes flash-over? Flash-over causes are not always easily explained, can be cumulative or stepping stone like, and usually result in an outage and destruction. The first flash-over components are available voltage and the configuration of the energized parts, corona may be present in many areas where the flash-over occurs, flash-over can be excited by stepping stone defects in the insulating path.

11. How to test insulators? Always remember to practice safety procedures for the flash-over voltage distance and use a sturdy enclosure to contain an insulator that may shatter, due to steam build-up from moisture in a cavity, arcing produces intense heat, an AM radio is a good RFI/arcing detection device, a bucket truck AC dielectric test set (130 KV) is a good test set for most pin and cap type insulators. A recent article said the DC voltage required to “search out defects can be 1.9 times the AC voltage. Insulators have a normal operating voltage and a flash-over voltage. Insulators can have internal flash-overs that are/are not present at normal operating voltage. If the RFI is present, de-energize the insulator (line) and if the RFI goes away, suspect the insulator (line). Then there can be insulators that have arcing start when capacitor or other transients happen, stop when the line is de-energized or dropped below 50% of arc ignition voltage. Using a meg-ohm-meter can eliminate defective insulators that will immediately arc-over tripping the test set current overload.

12. Conductor corona is caused by? Corona on a conductor can be due to conductor configuration (design) such as diameter too small for the applied voltage will have corona year-around and extreme losses during wet weather, the opposite occurs during dry weather as the corona produces nitric acid which accumulates and destroys the steel reinforcing cable (ACSR) resulting in the line dropping. Road salts and contaminants can also contribute to starting this deterioration.

13. Why should I care? Insulating materials that are failing can cause flash-overs and outages.

14. Why will my boss (and his boss) care? Any outage costs repair time and lack of customer service, in this age we are all somebody’s customer.

15. How does this cost our company? Lost opportunity revenue, you can’t sell electricity or make products if you don’t have power. An inexpensive part (cotter pin devoured by corona acid) can fail and cost a transformer by the time the sparks quit flying. Customers with continuous process applications, critical care facilities, computing systems typically have uninterruptible power and back-up generating capacity for part of their load for a limited time.

16. How does this affect the reliability of our electrical system? Corona is a symptom, it may be present for years before the component finally fails. Corona can be an indication or the catalyst of the chemical soup that permeates insulator bonding cements preparing them for internal flash-over. How can you detect this, ask me.

17. What tools are available to identify the presence of corona? Corona produces sound, nitric acid (in the presence of moisture), ozone, and ultraviolet light. No RFI/TVI here as this is a chemical reaction.

18. What is flash-over and arcing? Flash-over is an instantaneous event where the voltage exceeds the breakdown potential of the air but does not have the current available to sustain an arc, an arc can have the grid fault current behind it and sustain until the voltage decreases below 50% or until a protective device opens. Flash-over can also occur due to induced voltages in unbonded (loose bolts, washers, etc) power pole or substation hardware, this can create RFI/TVI or radio or tv interference. Arcing can begin at 5 volts on a printed circuit board or as the insulation increases it may require 80 kV AC to create flash-over on a good cap and pin insulator.

19. What are causes of insulator failure? Electrical field intensity producing corona on contaminated areas, water droplets, icicles, corona rings, … This corona activity then contributes nitric acid to form a chemical soup to change the bonding cements and to create carbon tracks, along with ozone and ultraviolet light to change the properties of NCI insulator coverings. Other detrimental effects include water on the surface or sub-surface freezing and expanding when thawing, as a liquid penetrating into a material and then a sudden temperature change causes change of state to a gas and rapid expansion causing fracture or rupture of the material.

If your facility has high voltage electrical systems, using a combination of corona testing and IR thermography may spot trouble long before equipment failure.