The application of RTV silicone coatings to porcelain and glass insulators in order to increase performance under pollution is not a new maintenance technology. Yet, while available for decades, not all power engineers fully understand its basics and what determines efficacy. At the same time, given the availability of composite insulators, one might ask why such coatings are still offered. Why do power companies not just specify silicone insulators for service areas where there are serious pollution concerns? The following will help to answer this and other questions:
Why is there a need for RTV coatings?
The simple answer is that the vast majority of existing power installations across the globe – even those built in zones of high contamination – still feature porcelain and glass insulators, most of them put in service years ago. Depending on how well designed these have been for their operating environment and on the efficacy of natural cleaning by rain or wind, they should require little maintenance beyond periodic inspection. However, there are situations where substations or lines are built in areas where pollution exposure has increased, due for example to construction of nearby industrial areas or highways. This type can dramatically change the level of pollution affecting a line or substation and something must be done or there will be elevated risk of flashovers.
Another consideration promoting continued interest in coatings is the cost of cleaning ceramic insulators installed in highly polluted service areas. With fresh water an increasingly costly resource and with labor costs of annual washing from truck or helicopter also going up, the relative economic benefit of RTV coatings versus cleaning is growing. Yet another factor behind more and more utilities opting for RTV coatings onto porcelain and glass insulators is because these offers the electrical benefits of silicone with the mechanical benefits of ceramics. Finally, not all power utilities are confident in the long-term performance of composite insulators, e.g. in areas with persistently high UV or where bird-pecking is a problem or in applications, such as tension, where there is a perception that they may be damaged by workers carrying out routine maintenance on conductors.
What is the principle behind RTV coatings?
A thin layer of RTV material adhering to porcelain and glass insulators vastly increases pollution withstand since the silicone imparts hydrophobicity to what would otherwise be a hydrophilic surface. Moreover, since the silicone contains low molecular weight (LMW) chains that continually migrate to the surface, the ceramic insulator will remain hydrophobic, even when covered by a layer of pollution.
How long does an RTV coating remain effective?
This is a key question when comparing the relative costs of coating by RTV versus alternatives such as water washing or using silicone grease. There is no consistent answer since much will depend on the quality of the coating material and also how well it has been applied. Fortunately, these variables are both under the control of utility maintenance people. Assuming the RTV silicone material has been well formulated by a competent supplier and is applied by trained personnel under controlled conditions, it is certainly reasonable to expect at least 10 to 12 years of effective service life, possibly longer. For example, coatings applied in the late 1980s/early 1990s on bushings at a highly polluted 230 kV substation near Hamilton, Ontario operated for much longer than originally expected.
What is required for effective coating application?
In the best of cases, RTV coating would be applied under ‘factory conditions’, allowing careful monitoring of thickness. However, this is clearly not possible for substations and therefore the key in this case is good coating practice. This includes cleaning the ceramic insulator surface beforehand for best adhesion. Surface preparation is a critical step and therefore should ideally be monitored closely. Since conventional adhesion tests employed by the coatings industry at large do not work for silicones, other methods such as the cross hatch test or water blast adhesion test must be used. It is also necessary to ensure optimal spray thickness with even coverage in order to avoid uneven voltage distribution developing along the insulator surface under wetting conditions. Too thin or too thick a layer are both sub-optimal. That is why some suppliers of coatings recommend using their experienced staff to carry out installation.
Do RTV coatings ever need to be cleaned?
Generally, no, although much will depend on the type and rate of pollution that accumulates on them. If there is an exceptional event that deposits a great deal of pollution at one time, cleaning may be advisable since hydrophobicity could temporarily be reduced or even lost. High pressure washing, however, should always be avoided since this can damage the coating. Clearly, if RTV coated porcelain needed to be cleaned anywhere near as frequently as uncoated insulators, there would be little logic in using coatings.
What are the signs of end of life?
Visual inspection can provide obvious clues regarding the condition of a coating, especially is there is evidence of large portions flaking off. Also, since the major benefit of the coating lies in transforming a previously hydrophilic ceramic surface to one that has hydrophobicity, one useful measure of end of life would be permanent loss of hydrophobicity. This can easily be monitored by simple hydrophobicity measurements conducted during routine station maintenance shutdowns. End of life can also be monitored via direct leakage current measurements and indirectly via thermal imaging, best taken during periods of light surface wetting. Since the goal of the coating is to limit leakage current, indirect thermal imaging gives an indication of the presence of surface currents. Monitoring end of life of RTV coatings on overhead line insulators is more challenging. Removing sample insulators at random from different sections of line will provide useful information on residual hydrophobicity.