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Corona

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Corona emissions are an unwanted side effect on High-Voltage (HV) electrical installations. It occurs on equipment operating at voltages of 11 kV and greater.

Corona emissions occur when the air in the vicinity of a conductor, or on any piece of electrical equipment, is ionized, due to the high electrical potential. When the electrical field strength around the object exceeds 3kV/mm, excess energy is radiated in the form of light and electromagnetic waves.

Corona emission causes

  • Energy losses in the electrical network. Energy losses as high as 11.5 kW/km/phase have been measured on a 400 kV power line during moist atmospheric conditions.
  • Has a damaging effect, causing early ageing of insulators and other HV equipment that result in the reduction of the components life span.
  • A chemical process causing Nitric Acid to form which adversely affects most surfaces.
  • Radio interference.

Characteristics of Corona

  • Corona tends to accumulate at sharp points/protrusions.
  • Dry band arcing: When the insulator’s surface has it’s resistance reduced and the insulator becomes contaminated with pollutes it starts to conduct electricity, dry bands are formed, interrupting the flow of current due to leakage which leads to arcing. A surge of leakage current occurs every time the dry bands on an insulator spark over.
  • Sparking, Flashover or Arcing: This occurs when there is insufficient insulation between two electrodes. If the resistance is reduced by pollutes, the arcs bridging the dry bands are able to burn continuously and can extend along the insulator; thereby spanning more and more of its surface. This can eventually lead to spanning the complete insulator and a flashover may occur.
  • Corona emissions are not visible to the spectral range of the naked human eye, as it occurs in the UV band.

Reducing excess Corona emission to acceptable levels by

  • Regularly scheduling preventative maintenance of power lines.
  • Regularly washing insulators that are prone to contamination, caused by external conductive sources such as salt, sand and industrial pollution.
  • Proper design and good construction practices.

Corona Detection

  • Because Corona is not visible to the naked human eye, a camera is required to efficiently visualize the Corona source operating in the UV band.
  • UV radiation can be detected and located quickly by scanning all the equipment with an UV camera without having to inspect all hardware in detail.
  • Scanning electrical equipment with an UV camera from the ground is possible. However, experience has shown that:
    • Backlight has a negative impact on the effectiveness of the camera equipment.
    • The closer the camera is to the source of the Corona emissions, the more effective it becomes.

Advantages of aerial Corona scan

  • Maintenance costs and time are saved due to locating problem areas quickly and identifying the cause of the UV radiation more easily and effectively.
  • Identifying polluted (weakened) insulators.
  • Small corona discharges can be detected at a very early stage, which are the primary causes of a possible breakdown.
  • The helicopter provides an elevated platform that is closer to the electrical system compared to ground based scans. Therefore conductor damage, which may not otherwise be detected by the naked eye, would be located with an UV camera.

Typical UV / Corona sources on High-Voltage systems

  • Broken or damaged conductor strands.
  • Damaged insulator discs.
  • Spark gaps on loose clamps.
  • Pollution on conductors or insulators.
  • Missing or damaged corona rings.
  • Incorrect arc horns.
  • Flashover paths.
  • Sharp hardware points or protrusions.
  • Corroded conductors.
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