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

Dan Ninedorf, Specialized Camera Sales
Posted 07-25-03

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. 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 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 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 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 of insulator, 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-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 (130KV) 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