Material Selection
The reason silicone coatings can make a decisive difference in withstand capability under extreme pollution is because of their hydrophobic properties. However, longevity, performance and ageing depend greatly on selecting an optimum formulation, application method and other quality criteria. Indeed, silicone coatings can have a wide spectrum of different chemistries and, as is the case for composite insulators, there is a need to evaluate the impact of environmental and electrical stresses on specific coatings in order to select the ideal formulation. While tests such as the inclined plane are still controversial for silicone rubber and coatings, there are several other evaluation methods to distinguish among alternative silicone coatings.
Ageing & Longevity
Ageing is a central issue whenever dealing with polymeric materials. Composite insulators have been around long enough to give an answer and weaknesses identified have come mainly from erosion of rubber housings or in the seals that prevent the core becoming exposed to moisture ingress. In this regard, coating over toughened glass is fundamentally different. While looking for the best possible material with respect to erosion resistance, the fact is that, even when a coating is damaged, the integrity of the glass insulator is not at risk. Nor is there compromise in the inherent properties of the glass, which continues to function as a normal uncoated insulator. R&D work has therefore concentrated on three aspects:
1.Erosion resistance;
2.Changes in hydrophobicity under various stress conditions;
3.Performance under pollution.
Pollution Performance
Most service experience has confirmed that coatings eliminate the need for frequent washing and also risks of flashover. Since the main function of the coating is its ability to prevent contamination flashover, special focus has been placed on artificial pollution tests.
Clean Fog Pollution Tests with Solid Deposit Layers
Clean fog pollution tests with solid deposit layers were performed in partnership with STRI’s test laboratory in Sweden and results were later confirmed at the CEB High Voltage Laboratory in Bazet, France. One of the difficulties has been in the preparation and deposition of contamination on a hydrophobic surface prior to testing, with or without recovery, in order to repetitively obtain a uniform pollution layer. The approach by STRI is now internationally accepted and used as a basis for Round Robin Test within CIGRE WG C4 303. The test procedure is comprised of the following:
1.gentle cleaning;
2.pre-conditioning;
3.application of pollution layer.
Clean Fog Pollution Test on Samples Removed After ca. 20 Years service.
A clean fog pollution test was performed in the CEB High Voltage Laboratory to evaluate performance of silicone-coated insulators removed after about 20 years in service in an environment characterized by a mix of desert, marine and industrial pollution. ESDD and NSSD were measured on one insulator sample in order to apply a similar level of pollution to new glass insulators (i.e. ESDD of 0.1 mg/cm² and NSDD of 0.1 mg/cm²) for the purpose of comparing performance.
Field Monitoring
As discussed, R&D work is being devoted both to laboratory tests and field monitoring. In this regard, various aspects are being monitored, including overall insulator condition, adherence, thickness, comparative hydrophobicity, recovery time, pollution levels and contaminant conductivities, etc. Besides the benefit of being able to calibrate laboratory test procedures to findings from the field, so-called gradient in stress on the coating along the string has also been measured. While some units can be partially WC5, the string remains fully hydrophobic overall. This has been confirmed in all service environments being investigated. To help monitor the evolution along a string, a geometric approach has been established that sees the string divided into 3 sections: bottom 25%; top 25%; and middle 50% of the string length.Similarly, in terms of hydrophobicity, only some areas in the same portion have been affected. This demonstrates a strong buffer and high resilience of silicone coating when applied to glass insulators. No flashovers have so far been encountered on coated insulators and with no need for washing.
Conclusions
Silicone material is being used on overhead transmission lines mostly either as rubber housing for a composite insulator or as a coating over a traditional glass or porcelain insulator. In the case of polymeric insulators, any damage, erosion or reduction to hydrophobicity can lead to premature ageing and eventual risk of failure. For a silicone coated toughened glass insulator, there is no such critical condition since below the coating is a toughened glass dielectric body that is resistant to environmental conditions.
Pollution performance of silicone coated toughened glass insulators in AC and DC have been demonstrated by Rapid Flashover Solid Layer Pollution Tests either on artificially polluted units or from naturally polluted samples removed from the field. Field monitoring of the performance of coated insulators has also been initiated. So far, what appears clear from strings removed after more than a decade of observation is partial reduction of hydrophobicity but units removed after 20 years of service have maintained excellent hydrophobic properties. Overall, the insulator string always remains fully hydrophobic. This is the key factor impacting service performance along with eliminating the need for washing.
From: http://www.inmr.com/silicone-coatings-toughened-glass-insulators/
