The Price of Incorrect Insulator Selection in Areas of High Pollution – Learning from Brazil

When TAESA acquired the 500kV Xingó – Angelim II Transmission Line in 2010, they set about investigating the ongoing issues the line had experienced since it began operation in 2002. What was uncovered is a misspecification lesson for all utilities.

Background

Transmission line failure due to natural pollution is common, accounting for a significant percentage of power outages globally. The problem is often attributed to rising pollution levels caused by evolving environmental conditions, which deem the original specification parameters ineffective. Occasionally, the root cause lies in a fundamental misspecification of parameters in the development stages of a transmission line, which has a knock-on effect on performance that declines further over time due to increasing line weakness[1].

The Effect of Cumulative Pollution:
Salt and other conductive contaminants accumulate on the exterior of the insulator. As rain, fog, or dew dampen the insulator surface, the layer of pollution becomes conductive due to the presence of ionic solids. The leakage current flows over the polluted layer, forming high-resistance dry bands which damage the insulator and, in some cases, result in a flashover [2].

The Application of RTV Coating

In 2007, the Xingó – Angelim II line underwent extensive maintenance, with more than 13,000 insulators replaced with new glass bells. An RTV coating was applied to 60% of the new insulators to offer protection against flashovers. In spite of this, just three years later, another failure occurred in the same area. Upon inspection, engineers noticed that the RTV coating was flaking on some of the new insulators, and further testing revealed a loss of hydrophobicity.

Switching from Glass to Polymeric Insulators

By this time, the Xingó-Angelim II had been acquired by TAESA, who had successfully used composite polymeric insulators in neighbouring transmission lines in the past. In 2015, the decision was taken to replace the insulators in the São Pedro region, and 618 glass strings were removed.  Silicone suspension and strain insulators specifically designed to withstand high-intensity pollution were installed across the high-altitude towers, enhancing the leakage distance by 80% compared to the glass bells. 

The polymeric insulators increased the specific leakage distance of the transmission line from 15.12 mm/kV to 27.3 mm/kV, meaning the line was now resilient to very heavy pollution levels. Other benefits of the polymeric insulators included enhanced hydrophobicity and a 75% reduction in weight.

It was initially thought that the new insulators had solved the issue. However, the Xingó-Angelim II outages resurfaced after a year, this time occurring in areas outside of the São Pedro mountains. Maintenance crews used corona cameras to inspect sample areas on the line, finding that the level of corona activity had increased dramatically across the glass strings.
 
A review of weather conditions over the previous year showed a decreased level of rainfall, which resulted in increased dirt accumulation, leading the engineers to determine that pollution was responsible for the failures.

The O&M department determined that the outages had not occurred in these areas earlier because the towers in the São Pedro region were the weakest point. Once the polymeric insulators were installed in São Pedro, other weak spots emerged in lines in the surrounding area. The engineering team determined that the ideal solution would be to replace the remaining glass bells with polymeric insulators at some stage in the future.

Maintenance Considerations of Composite Insulators

Although the polymeric insulators have resolved the pollution and failure issues in the São Pedro towers, there are other factors to be considered when switching from glass to composite polymeric insulators.

Additional maintenance is required to inspect the polymeric insulators for signs of contamination buildup periodically. This can be done easily using the corona camera. More comprehensive inspection is also required, as polymeric insulators have been shown to be vulnerable to brittle fractures in areas of high humidity[3].  TAESA are addressing this by visually inspecting the insulators and taking samples for PET scan and X-ray on a scheduled basis.

A less maintenance-heavy solution, such as porcelain long rod insulators bolstered by semi-conductive glaze could potentially offer equal performance in similar environmental conditions.

Conclusion

A failure to recognise the level of pollution in the São Pedro region during specification caused significant issues with the Xingó – Angelim II TL. This resulted in substantial manpower and material costs as a solution was sought, and despite solving the issue along the affected stretch of line, the weakness has now spread to the glass insulators in the surrounding area. This study highlights the critical importance of considering environmental pollution along the entirety of the line during specification.

*The information provided in this content is for informational purposes only and should not be considered professional advice. We make no warranties or guarantees, express or implied, and are not responsible for any losses or damages resulting from your use of this information.

[1] Berrêdo et. al., “Case Study for Using & Monitoring Composite Insulators on Overhead Lines under Natural Pollution”, TAESA, Rio de Janeiro, Brazil. https://ofilsystems.com/wp-content/uploads/2020/01/Case-study-for-monitoring-NCI-under-pollution.pdf

[2] Gonos et. al., “Dielectric behaviour of polluted porcelain insulators”, IEEE Proceedings – General Transmission and Distribution. https://www.researchgate.net/publication/3354708_Dielectric_behaviour_of_polluted_porcelain_insulators

[3] INMR, “Stress Corrosion Cracking & Brittle Fracture of 400 kV Insulator in Humid Environment”. https://www.inmr.com/stress-corrosion-cracking-brittle-fracture-of-400-kv-insulator-in-humid-environment/