Inspections

Damage Caused by Corona Discharges

Why Do Inspections?

Corona discharges is indicative of:
  • Bad hardware design
  • Incorrect hardware installation
  • Damage to hardware
  • Contamination of hardware
  • Corroded hardware

Damage Caused by Corona Discharges

Corona discharges has a number of damage modes:

1. Audible Noise & Radio Interference

Corona discharges causes a signature buzzing noise, this is not actually damaging, but may be an irritation to humans and animals in the vicinity. If the harmonics give positive interference then it will cause vibration in the conductor.
The radio interference is unlikely to directly cause damage, but it may cause electronic devices to fail to operate correctly or perform erratically, which may cause damage to the device.

2. UV Light

The UV light produced by corona discharges will cause accelerated aging of polymer hardware, which will in turn cause the hardware to fail sooner than expected.
The UV light can also affect various animals which can see into the UVc range.
This has caused issues in Reindeer migration.

3. Ionization

THE PROCESS OF IONIZATION STARTS WITH CORONA DISCHARGE:
Corona is a type of Partial Discharge where electrons are emitted into the air. The mechanism of electron emission is in the physics domain.
The whole process starts with cosmic rays ionizing single molecules in the air around HV hardware by knocking an electron our of its orbit.
At this stage the collision causes the molecule to become excited. The molecule wants to return to its neutral state, it does this by emitting the excess energy as light photons. The frequency of the photons depends on the atoms and bond types of the molecule which has been excited.
The free electron liberated in the process is repelled by the electric field around the hardware during its negative phase, and attracted during the positive phase.
If the repulsion force pushes the electron to a high enough speed, then it will cause more than one electron to be kicked free during a collision with another air molecule.
When two or more electrons are liberated by a collision then an avalanche has formed.
The avalanche will continue until the electric field has decreased in strength to the point where it does not accelerate the electron to a fast-enough speed to liberate an electron during a collision.
In the case of N2 (the main component of the air around us) the light frequencies emitted is mainly in the UV range with just 5% above 400nm, which is considered the lowest end of visible blue.
The light spectrum of N2 fluorescence, after it has been vertically stretched 1000x times larger.
The ionized molecules can also merge with others to share their remaining electrons.
i.e.
O2 + e- = 2O-
O- + O2 = O3 (O-Zone)
O-zone is very reactive and will oxidize metals, causing rust.

N2 & O2 + e- = 2N+ + 2O-
N+ + 5O- = NO2 + O3
NO2 + H2O + O2 = HNO3
Nitric acid appeats as white dust on the hardware.

Nitric acid will remove the plating on metal parts and attacks coatings on all parts.
A variety of other molecules may be formed by the ionization activity, i.e
1. Oxalic Acid
2. Carbonic Acid

DETECTING CORONA DISCHARGES

The Ozone layer only allows some frequencies to pass in various volumes.
Below 280nm the light from the N2 fluorescence is uniquely visible. This is called the solar blind range.
To be able to visualize the UVC and visible images in a useful way 2 types of cameras have to be used along with suitable optics.
The detected UV image is overlaid onto a visible image to show where the discharge is occurring. The UV Blob is false coloured to be observable

EPRI Assessment Overview

Once a discharge has been found the immediate question from users are: “Is this discharge an important fault?”.

This is not quite the right question, all faults are important to some extent. The question should rather be: “What is the maintenance priority of this fault?” The difference being, a fault may be important but may not require immediate attention as there are more severe faults or the expected time to failure will results in a failure long after the next maintenance date for the site.
The question then becomes how to assign maintenance priority? Many users immediately look at the “count” attained by the camera, but this is not correct as the counts are not easily comparable (due to the many factors influencing it). The count is at best useful as rate of damage indicator at the time of inspection, if it is back calculated to a standard condition. The count changes based on the ambient weather, this also means that the count cannot be used as part of a trending exercise – unless inspections can be done under exactly the same conditions – very unlikely.
The most reliable way of assigning a maintenance priority is by using the Maintenance Priority Rating system developed by the US Electrical Power Research Institute (EPRI). The system assigns a rating based on the discharge’s ability to damage the hardware it is on or near, the presence of physical damage and the expected timeframe until failure of the hardware.

The Maintenance Priority Rating levels and their associated questions are re-interpreted as a decision tree below.
The US EPRI Field Guide: Daytime Discharge Inspection of Transmission and Distribution Overhead Lines and Substations contains an image library of a variety of components with examples of what discharges on these components look like, along with a description of the optimal conditions under which to do an inspection, a description of the activity which would result in a discharge detected on the hardware location, what will happen if no action is taken, the time frame until failure and what action should be taken to correct the fault.

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