Inspection of Polymer Insulators
A Research Perspective

With the increased use of polymer insulators in the past 10 years, a major research project is being conducted by EPRI that addresses life expectancy, methods to identify high-risk-in-service units, etc. One of the off shoots of EPRI's NCI research effort was to partner with Ofil to develop the DayCor technology.

EPRI has been investigating daytime corona applications: overhead transmission, substations, distribution and airborne, and published two field guides and one training course (“Guide to Corona and Arcing Inspection of Transmission Lines 1001910”, “Guide to Corona and Arcing Inspection of Substation 1001792”, and “Training Course on Corona and Arcing Inspection of Transmission Lines 1006706”). For developing these guides EPRI spent considerable field and laboratory time, a part of which was dedicated to polymer insulators.

There are four main failure modes that affect polymer insulators: brittle fracture, flashunder (also called tracking along or through the rod), destruction of the rod by discharge activity, and mechanical failure of the rod or end fitting. Descriptions of these are given in: “Technical Brief Polymer Insulator Survey” that can be downloaded from www.epri.com.

It has been shown in the field that the DayCor II™ can identify a percentage of units that may fail from some of these failure modes. An example of discharge activity from an in-service unit that was certain to fail is shown in fig 2. Discharge activity is not restricted only to units that are close to failure, incorrectly applied units may also have discharge activity. If discharge activity gets in contact with polymer’s watershed system or with the end-fitting seal, it might result in reduced life expectancy or/and in customer complaints. Examples to incorrectly applied units that have discharge activity are: units that have no corona rings, units that have insufficient corona rings and units that are installed backwards. An example of an in-service polymer insulator with discharge activity because of lack of corona ring is shown in fig 3.

A question frequently asked is "what discharge activity on a polymer insulator is unacceptable?" And the answer: “any discharge activity that is in contact with the rubber weather shed system or the end fitting seal (excluding low levels of discharge activity under wetting conditions).” Another matter which EPRI's Guides attempt to handle is how urgent should such a situation be rectified. Research of this issue is still continuing.
Ongoing EPRI’s research on the use of the technology from airborne platforms, both manual and automated, is currently underway. EPRI has also started a new research project on how to use daytime corona inspection in the determination of whether and when to wash contaminated insulation. These are tall orders but EPRI is up to the tasks!    --up>