Silicone coatings have been used at substations for at least 30 years and more recent experience at utilities confirms that they are also an effective approach to solve contamination problems on overhead lines. Moreover, trends suggest that silicone coated toughened glass insulators are now being selected not only to deal with pollution problems on existing lines but also at the design stage of new lines. While this market is expanding, there are still questions related to material selection, performance and longevity. In this context, laboratory research has been undertaken to evaluate electrical performance and there has been field monitoring as well. The following edited contribution by J-M. George, Dr. S. Prat and F. Virlogeux of Sediver in France summarizes this work.
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.
As in the case for the silicone rubber material used in composite insulators, coatings can contain various fillers to increase erosion resistance, e.g. quartz or ATH fillers. For example, among a large diversity of protocols established for accelerated ageing tests, interesting results can be found from the 2000 h multi-stress test that combines UV, rain, salt fog, humidity and voltage on a weekly cycle performed, as per a specification from Terna in Italy (see Fig. 1). Here, there is a clear distinction in performance of different coatings, including those made with different types of ATH filler (Fig. 2).
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:
• Erosion resistance;
• Changes in hydrophobicity under various stress conditions;
• Performance under pollution.
Findings have to be considered with caution given the wide diversity of service situations, e.g. between an environment like the Peruvian desert or along the coast of Sicily in Italy. Similarly, behavior under AC or DC has to be analyzed with respect to the specific implied stresses. Such information on silicone coated toughened glass insulators has been accumulated now for almost 20 years, with a monitoring program that includes yearly evaluation of samples removed from service. These samples demonstrated overall good preservation of hydrophobic properties – including in severe areas where washing cycles had previously been performed almost quarterly. The stress level encountered in such demanding applications translated into some reduction in hydrophobicity around the pin area, which is not surprising given the electric field distribution prevailing in that region. For example, Fig. 3 shows the hydrophobic status of such an insulator after 7 years in a coastal environment with a pollution level equivalent to E7 as described in IEC 60815-1. Fig. 4 shows modeling of electric field around the pin and explains why this section typically suffers more than the rest of the insulator.
It also appears that overall hydrophobicity – even near the live end –is preserved and classified between WC1 and WC3. Moreover, laboratory tests on short strings as well as field observations point out that the complete string never loses overall hydrophobic performance, despite hydrophilic areas in some localized regions of individual units along the string, mostly near the energized side.
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:
• gentle cleaning;
• application of pollution layer.
Insulators are cleaned by washing with warm water and a sponge. Pre-conditioning is then performed by applying a dry inert material in powder form (in this case kaolin) to the clean and dry insulator surface, e.g. using a brush. This layer is applied as uniformly as possible, especially in difficult to reach places. After application, most of the powder is blown off by compressed air until only a thin layer remains on the surface. The adequacy of this layer is controlled both visually and by measurements of Wettability Class (WC) performed according to IEC TS 62073 Method C. This is to ensure that the surface of the insulator is completely hydrophilic (WC=7) after pre-treatment. The insulators are subsequently dipped and twisted in a slurry of tap water, kaolin and salt in order to reach the targeted pollution level (see Fig. 5) and left to dry at room temperature before being tested 2 days later (i.e. a maximum 16+48 h rest time is allowed).
Voltage tests were performed in accordance with the Rapid Flashover Solid Layer Pollution Test Method. The polluted and dried insulators were put into the test hall, as shown in Fig. 6, and wetted according to IEC 60507. After 15 minutes of wetting, voltage was applied. Every 5 minutes voltage was increased in steps of 4 kV (about 5% of estimated flashover voltage). In case of flashover, the voltage was tripped and then immediately applied at a level decreased by one step of 4 kV. The test continued until flashover level starts to increase, indicating cleaning of the insulators. Total test duration was about 100 minutes and each test was repeated on two strings of 5 insulators (with creepage distance of 445 mm per unit). Typical test results are summarized in Fig. 7, where ‘O’ stands for withstand and ‘X’ means flashover.
Results as in Fig. 8 show a substantial increase in withstand voltage compared to a non-coated string, with performance similar or superior to an equivalent composite insulator.
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. A test was performed on a string of 4 units in a clean fog condition, according to the rapid flashover solid layer pollution method, for 100 minutes (as in Fig 9).
The string of 4 units of artificially polluted insulators showed a U50 value of about 72 kV under these test conditions. The 4 units removed from the field after about 20 years of service flashed over at 240 kV and were characterized by a high level of hydrophobicity (see Fig. 10). This finding highlighted the benefit of the RTV silicone coating and the performance achieved even after 20 years exposure in a heavily polluted environment.
DC Salt Fog Pollution Test
Salt fog pollution tests (80g/l) were performed in DC (negative polarity) on silicone coated glass insulators at Sediver’s St. Yorre facility (Fig. 11). To complete the tests series in salt fog conditions, silicone composite insulators with a typical DC housing profile and similar creepage and arcing distance to the glass string were tested as well. Substantial improvements were observed during this test program, with an advantage for silicone coated toughened glass insulators compared to composite insulators. This result is in line with field monitoring on a DC line equipped with silicone coated toughened glass insulators supplied by Sediver.
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 (Fig. 12). So far, only light erosion has been seen (type CE2 in the bottom portion of the string, as per Sediver’s Coating Erosion Class Guide in Fig. 13). 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.
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.