Some basic principles about RTV coatings
Coating insulators with RTV silicone rubber coatings to improve stain resistance is certainly not a new maintenance technology. But while RTV silicone rubber coatings have been used for decades, not all electrical engineers fully understand how they work and what determines their efficacy.
At the same time, one might well ask, why use RTV coatings for glass and porcelain insulators when composite insulators are already available? The reasons are as follows:
First, the vast majority of existing electrical installations worldwide are glass or porcelain insulators, even in heavily polluted areas, most of which have been in operation for many years. If properly designed for the operating environment, natural rain or wind self-cleans well and requires little maintenance other than regular inspections. However, in many cases, power plants or lines are built in areas where pollution continues to increase over time, such as nearby industrial areas and highways. This type of construction can significantly alter the level of fouling that negatively affects the line or plant. We have to do something about it, or we will increase the risk of flashover. In addition, the increasing cost of cleaning ceramic insulators in heavily polluted areas is another factor driving the application of antifouling coatings. As freshwater resources become more expensive, the labor cost of using trucks or helicopters to flush each year is also rising, and the economics of RTV coatings are increasingly highlighted relative to cleaning. Second, the RTV coating has both the electrical performance advantages of silicone rubber and the mechanical performance advantages of ceramics. Also, not all utilities are confident in the long-term operational performance of composite insulators, such as in areas where persistently strong UV rays or bird pecks are a serious problem, or in some special applications, such as in the case of tensile strings. It is believed that composite insulators may be damaged by line workers performing routine maintenance work on the conductors. The last point is that insulators will eventually get covered in conductive contamination, which can trigger problems when they get wet. Coating a thin layer of RTV material on the surface of electric porcelain and glass insulators makes the originally hydrophilic surface hydrophobic, thereby greatly improving its stain resistance. In addition, since the low molecular weight (LMW) chains contained in the silicone rubber continue to migrate to the surface, the ceramic insulator remains hydrophobic even if the surface is covered with a contamination layer.
Do I need to wash after using RTV paint?
Usually not required, although cleaning depends largely on the type of surface contamination and the rate of accumulation. In the exceptional case of a large accumulation of one-time contamination, washing is recommended, as water repellency may be temporarily reduced or even lost. But high pressure water washing should always be avoided, as high pressure water washing may damage the coating. Obviously, if the frequency of cleaning is still close to that of unpainted insulators after applying RTV paint, it is not very logical to use paint.
How long are RTV coatings valid for?
This is of course the key question when we compare the cost of RTV coatings to alternative measures such as water wash or silicone grease. But there is no definite answer as it depends a lot on the quality of the RTV coating and the level of operation at the time of application. Fortunately, these variables are within the control of utility operators.
It is certainly reasonable to expect an effective operating life of at least 10-12 years, possibly even longer, assuming that the RTV silicone rubber material from a qualified supplier is properly formulated and applied to the insulator by a trained professional in the specified environment.. For example, at a 230kV substation in a heavily polluted area of Hamilton, Ontario, Canada, RTV coatings applied in the late 1980s/early 1990s are still in use today.
What are the requirements to apply paint effectively?
Ideally, the application of RTV coatings is best done under "factory conditions" with tight control over their thickness. However, this is obviously not possible for substations, so applying the correct method is key. This includes cleaning the insulator surface prior to coating for optimum bond strength. Pre-treatment of the surface is a crucial step, so ideally it is closely monitored. Since traditional adhesion testing methods used by the coatings industry are generally not suitable for silicone rubber, other methods, such as cross-hatch testing or water spray adhesion testing, must be used. At the same time, it is also necessary to ensure that the coating of the optimum thickness covers the surface evenly to avoid uneven voltage distribution on the surface of the insulator in a humid environment. Coatings that are too thick or too thin are not optimal. This is why some paint suppliers recommend that users have their own experienced personnel to apply the paint.
What are the signs of paint failure?
Visual inspection alone can provide obvious clues to the condition of the coating, especially if there is clear evidence of extensive coating peeling. Furthermore, since the main advantage of the coating is to convert the hydrophilic ceramic surface to hydrophobic, an effective means of detecting end of life is to measure whether the hydrophobicity is permanently lost. It can be easily monitored by simply measuring hydrophobicity during routine power plant outages. End of life can also be monitored by direct measurement of leakage current and indirectly by thermal imaging, preferably when the surface is lightly wetted. Because the purpose of the coating is to limit leakage currents, indirect thermal imaging can show the presence or absence of surface currents. Monitoring the end-of-life of RTV coatings on overhead line insulators is more challenging, and random sampling of samples from different areas of the line will provide useful information on the hydrophobicity residual.