Analysis of Failure of Compostie Insulators on the 161kV EHV Lines in Israel

Radu Munteanu, PE
This work was presented at the last UGM

The case

In 1997 IEC had three failures of polymer insulators on a seven-years-old 161kV line and Radu Munteanu of IEC Planning, Development and Technology Department, investigated their causes.

The investigation

The investigation had three stages:
The first stage was inspection of similar neighboring insulators, to find unusual corona activity. Inspections were carried out at nighttime with relative humidity greater than 75%. Since at that time daytime corona visualization technology was not available, the team used long exposure shots (10 minutes each) on a regular 35 mm camera. It was noticed that insulators without corona rings had corona activity between the end fitting and the first shed.
The second stage was a mathematical analysis of the distribution of the electrical field along the insulator, searching for points where the electric field exceeds the value of 25 kV/cm. This value was considered as the threshold electric field for corona activity. The analysis took into account the precise geometry and the dielectric constant of all materials and was carried out with the Coulomb software. The results showed that in the junction of the end-fitting to the polymer housing, a value of 13.8 kV/cm was obtained. But, when adding contamination effect on those insulators that lack corona rings, a value of 36 kV/cm was obtained. Thus it was inferred that the insulators without corona rings were unsuitable for operation at 161kV.
In the third stage, various lab tests were performed on the failed insulators to evaluate their physical structure. All insulators had 1.5 mm thick silicon rubber housing and fiberglass rod
reinforced with Vinylester. In one of the insulators, the silicon-sealing layer at the opening of the end fitting was burnt and partially peeled (see Fig. 23). Sealing test revealed that the sealing of the end fitting allowed moisture to penetrate the rod (Fig. 24). Scanning electron microscopy inspection revealed a smooth fracture surface with no debris, typical to brittle fracture (Fig. 25).
To conclude the analysis Munteanu consulted leading experts, all of which pointed out the clear signs of corona corrosion on the insulator housing and identified the failures of the brittle fracture type “e”.

Summary

A fault developed due to a prolonged corona activity that damaged the silicon housing near the end fitting. Nitric acid (HNO3) was formed on the insulator, in the area exposed to corona discharge, by the corona activity and raindrops, penetrated the rod and started eroding it, causing eventually to the brittle fracture. When the residual strength of the rod was equal to the mechanical tension (static and dynamic) the rod broke causing the line to drop.--up>