Corona Inspection

Corona FAQ

The following is meant to be a quick introduction or a quick review regarding the use of corona camera technology to inspect electric power transmission/substation facilities...a dozen questions/answers.

  1. At what voltage does corona become detectable?
    It depends, but most of the time 3500 volts and over at sea level, 70F and fair weather.
  1. What does corona indicate?
    An intense electrical field. Variables that affect when corona will occur include ambient and material temperature, and a few others.
  1. What can cause an intense electrical field?
    Electrical conductivity, conductors and semi-conductors (defective insulating materials), voltage and current all participate in the formation of an electrical energy field.
  1. Why should I care?
    Insulating materials that are failing can cause flash-overs and outages in an electric power grid.
  1. Why will my boss (and his boss) care?
    Any electric power outage costs money in terms of repair time and lack of customer service. In this age we are all consumers of electric energy.
  1. How does this cost our company?
    Lost opportunity revenue, you can't sell electricity or make products if you don't have a reliable power system.
    An inexpensive part can fail and end up destroying a transformer by the time the sparks quit flying.
    Customers with continuous process applications, critical care facilities, computing systems typically require an uninterruptible power source, and they generally have a back-up generating capacity for part of their load for a limited time to protect against power interruptions.
  1. How does this affect the reliability of our electrical system?
    Corona is a symptom of a larger problem. Corona may be present for years before the faulty 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.
  1. What tools are available to identify the presence of corona?
    Corona produces sound, nitric acid (in the presence of moisture), ozone, and ultraviolet light. The sound, nitric acid, and ozone are detectable using microphones and other local detection methods. However, the ultraviolet light can be detected remotely (e.g., from a distance) using a corona camera.
  1. Are there limitations to the tools we need to understand to select the right corona detection tool for our application(s)?
    Yes, please ask the experts.
  1. Is there training available for our staff to better understand how to interpret corona images and the operation and use of the corona tools?
    Yes, please check out the Corona Technology Course and the Corona Sleuth Presentation by clicking here.
  1. What is flash-over and arcing?
    Flash-over is an instantaneous event that occurs when the voltage exceeds the breakdown potential of the air but does not have the current available to sustain an arc. Flash-over can also occur due to induced voltages in un-bonded (loose bolts, washers, etc) power pole or substation hardware. Flash-over can create RFI/TVI or radio/TV interference.

Arcing can begin at 5 volts on a printed circuit board; however as the amount insulation increases, it may require 80kVAC to create flash-over on a good pin and cap insulator.

As photographed by the CoroCAM 4+ corona camera at 30 frames/second NTSC video output, corona appears as a white cloud to indicate the vicinity of the flash-over. The corona energy detected would generally exceed 12,000 photons/second at a distance of less then 10 feet immediately prior to a flash-over event. This could occur on a dry insulator that would typically have 20 to 30 kVAC applied voltage per insulator in a string of insulators. In the video, the arc would produce a visible light image and the corona activity would be visible as an ultraviolet image superimposed on the video image.

  1. What are causes of insulator failure?
    Electrical field intensity produces corona activity in contaminated areas, water droplets, corona rings, ... This corona activity contributes to the formation of nitric acid (e.g., a chemical soup) to change the bonding characteristics of the insulator material. That is, carbon tracks, ozone, and ultraviolet light are produced that change the physical properties of NCI insulator coverings.

Other detrimental effects include water formation on the surface and in small cracks created by weathering [e.g., freezing (contraction) and thawing (expansion)]. As the water penetrates the insulator material, a sudden temperature change (flash-over/corona) causes a rapid change of state (e.g., a liquid to a gas), and the rapid expansion causes the insulator material to fracture or rupture.


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