corona testing

8/3/2019 Corona Testing

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Corona & Testing - Who, What, When, Where & Why

The following is meant to be a quick introduction or a quick review...a corona, arcing,

flash-over Q and A.

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. Rememberthis 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 bevisible 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 andlisten 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 sometimesrattled 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-overmomentarily but be limited in current flow 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 electricalfield. 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?


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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 thenaked 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 anintense 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 atemporary 500kV 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 ofinsulator, NCI normally start at 160kV, pin and cap can vary starting at 220kV or

345kV 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-overcomponents 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 distanceand 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 (130KV) isa 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.


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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. Are there presentations of the tools we need to understand to select the right

corona or arcing detection tool?

Yes, please contact me for a Corona CD with presentations, articles, and learn

how we locate corona and prevent electrical outages.

19. Is there training available for our staff in understanding corona

interpretation and the operation and use of the corona tools?

Yes, please see the Corona Technology Course and also the Corona Sleuth

Presentation and ask me. http://www.corona-technology-course.com/

20. 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 arccan 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 substationhardware, this can create RFI/TVI or radio/tv interference. Arcing can begin at 5

volts on a printed circuit board or as the insulation increases it may require

80kVAC to create flash-over on a good cap and pin insulator.

21. 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 insulatorcoverings. 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 rapidexpansion causing fracture or rupture of the material.
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