Ceramic insulators are often damaged by vandalism or accidental contact. Large substation bushing are particularly vulnerable, especially the sheds. Damage can range form small chipping and loss of surface glaze to complete destruction.

Glaze loss reduces the strength of the porcelain and also makes it difficult to clean. Loss of design leakage distance for the insulator may affect its ability to withstand flashover voltages. Bushings with damaged sheds should be either replaced or repaired.

It is often not feasible to replace the damaged insulator. The product may no longer be manufactured and other bushings with similar dimensions may be difficult to locate. Even if the product is still available, it often takes months for delivery and custom orders are very costly.

The policy has been to repair damaged insulators by coating sheds that have been slightly damaged with an insulating coating such as glyptol lacquer. However, this coating has not performed well, due to disintegration when exposed to weather and ultraviolet.

Sheds that have more serious damage have been repaired with a filled epoxy resin, which can easily be casted in place and shaped. This resin, however, does not survive low temperatures due to its coefficient of thermal expansion being very different from that of porcelain. The repair rarely lasts more than two years in service.

The first step is to assess the extent of damage with site personnel and make a repair or replace decision, taking into account the availability of replacement bushings and the need to limit the length of an outage.

The repair method is to coat any sheds that have minor damage with a high voltage insulator coating. Sheds with major damage are nondestructively examined for extent of damage. The sheds are then rebuilt to reproduce the original leakage distance using one of two different ceramic materials. These ceramic materials have similar coefficients of thermal expansion to porcelain, hence , they are immune to thermal cycling degradation once the repair is sealed with the high voltage insulating coating.

The repair will extend the life of a damaged bushing and avoid the cost of replacement.

 

(From internet)