Corona leads to insulation failure. That much has long been known. However key aspects of the problem are still being researched. These include magnitude and duration of corona that will initiate degradation, most suitable detection methods and development of accelerated tests to predict service performance in its presence.
Corona is the halo or circle of light seen around the sun during a full eclipse. Electrical engineers borrowed the term to describe the glow around a conductor subjected to sufficiently high voltage. The glow is caused by ionization of gas and subsequent release of light when electrons that have gained energy from the electric field revert to their original stable state. Since the discharge does not bridge the entire space between the electrodes, corona is sometimes also referred to as partial discharge. The reason the glowing is localized only around the source is that the insulation provides a barrier to further ionization while the electric field, which decays rapidly with increasing distance from the source, is unable to sustain the ionization.
When it comes to composite insulators, corona activity can originate from hardware, voids within the material or from defects in the interfaces. Most of the light produced by such corona has a wavelength shorter than 400 nm and therefore falls in the UV range. By contrast, most solar radiation is in the visible range (400-700 nm), the shorter wavelengths being filtered by the earth’s ozone layer. In fact, some peaks in the UV region of the corona spectrum match or exceed those in the solar spectrum. Polymeric materials are more susceptible to degradation from UV produced by corona than from solar radiation, especially if the corona is close to the material.
Corona ruptures stable oxygen molecules (O2) to create radicals, which combine with these molecules to form ozone (O3). The ozone then attacks unsaturated (i.e. double and triple bond) sites in elastomeric materials resulting in cracking, as commonly observed on rubber tires, gaskets and seals. Even minute amounts of ozone (i.e. in the ppm range) are sufficient to initiate cracking, however the time required depends on material formulation. While most modern elastomers are stabilized, some may eventually succumb to ozone attack should its concentration become sufficiently high.
Corona also produces acids, both organic (oxalic) and inorganic (nitric) in the presence of moisture (e.g. from ambient humidity, dew, fog or water on the surface). Depending on concentration (pH), this can also locally degrade polymers. Corona can even ‘drill’ holes in a material, suggesting that degradation is not solely due to chemical attack by ozone. For example, researchers have calculated the temperature at the tip of the discharge and shown it to be high enough to cause ‘evaporation’ of even inorganic materials such as mica. Of course, this requires repeated bombardment in always the same location. There has also been suggestion of mechanical attack, much like sandblasting, due to the impact of repeated discharges on a material. In fact, it’s amazing that any single physical phenomenon can be the trigger of so many different modes of degradation – all at the same time.
For all practical purposes, one cannot see or hear corona (at least not without specialized equipment). Since degradation is initiated at the molecular level, inorganic dielectrics with strong chemical bonds – such as porcelain or glass – have a higher resistance to it than do polymers. But this fact should definitely not lead to the conclusion that corona will always limit the service life of composite insulators on HV transmission lines. That is because corona can be mitigated or even completely eliminated through proper insulator design and manufacturing. But it’s also important to realize that, if there is sustained corona on a housing, the life of such insulators will be limited.
To illustrate, a past research project on this topic involved ground inspection using a corona camera and binoculars of composite insulators on 115 kV, 230 kV and 500 kV lines. Several of the 230 kV towers were equipped with composite insulators with and without corona rings on adjacent phases. The 115 kV insulators did not have any corona rings, whereas the 500 kV insulators had rings on the line as well as ground end hardware.
Given the dry conditions of the service territory, the utility involved in this project decided not to replace the 230 kV insulators not equipped with a corona ring. This allowed a comparative study of the effects of corona on composite insulators since all other factors (i.e. design, material, manufacturing details, location and system voltage) were identical. The corona observed on these insulators – many of which had been in service for over 25 years – was sporadic and originated from the hardware.
The 230 kV insulators being evaluated comprised three generations of composite insulator technology. Some were removed for further examination and it was found that the shed closest to the line end without a corona ring exhibited minor to serious changes in the form of hardening, cracking and discoloration. It was clear that these insulators were approaching their end of useful life and indeed probably would have already failed in locations having more precipitation. By contrast, none of the insulators equipped with corona rings showed any evidence of degradation.
This research demonstrated that when it comes to composite insulators, design and the need for a corona ring are dependent on voltage level and location. Moreover, for insulators in special locations (e.g. heavy contamination, high altitude, heavy wetting) it is prudent to have a line end corona ring at voltages below 230 kV – a system voltage below which corona rings were not seen as required in the past. In such cases, depending on location, high quality composite insulators manufactured using a variety of materials and processes can be expected to last many years.
Professor Ravi S. Gorur