Corona is the process by which current, perhaps sustained, develops from an electrode with a high potential in a neutral fluid, usually air, by ionizing that fluid so as to create plasma around the electrode. The ions generated eventually pass charge to nearby areas of lower potential, or recombine to form neutral gas molecules. Plasma is a fourth state of matter (after solid, liquid, and gas). It is like a gas or liquid, but molecules are separated into atoms, and the outer electrons are stripped off and are freed into the plasma. In comparison, the sun is essentially a big ball of very hot plasma.
It is also the flow of electrical energy from a high voltage conductor to the surrounding air. This effect only becomes significant for potentials higher than 1000V (1kV).It is characterized by a faint glow, a crackling noise and conversion of atmospheric oxygen to ozone. The audible buzz sound becomes louder is there is increased moisture & pollutants in the air.
Causes of Corona
Corona is causes by the following reasons:
The natural electric field caused by the flow of electrons in the conductor. Interaction with surrounding air.
Poor or no insulation is not a major cause but increases corona.
The use of D.C (Direct Current) for transmission.(Reason why most transmission is done in form of AC)
Effects of Corona
- Line Loss - Loss of energy because some energy is used up to cause vibration of the air particles.
- Long term exposure to these radiations may not be good to health (yet to be proven).
- Audible Noise
- Electromagnetic Interference to telecommunication systems
- Ozone Gas production
- Damage to insulation of conductor.
Minimizing Corona Effects
Installing corona rings at the end of transmission lines.A corona ring, also called anti-corona ring, is a toroid of (typically) conductive material located in the vicinity of a terminal of a high voltage device. It is electrically insulated. Stacks of more spaced rings are often used. The role of the corona ring is to distribute the electric field gradient and lower its maximum values below the corona threshold, preventing the corona discharge.
Question & Answer about Corona
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 adjusts 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 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 somebodys 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 there 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. 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 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/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 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.