When an electrical network which has operated for years without problems suddenly starts to experience a growing number of insulator flashovers this is generally the result of service conditions that have deteriorated due to increased pollution. Should such problems occur on overhead lines, there are a number of countermeasures, including washing, increasing the specific creepage distance of the insulators (assuming that this does not diminish line to ground clearances) or, if the affected insulators are ceramic, switching to composite designs.
If the flashovers occur at substations, the problem may seem more manageable since all the affected insulators are located relatively close to one another. However, unlike for overhead lines, it is generally not feasible to replace porcelain insulators, especially those which form an integral part of large equipment. Washing substation insulators, while costly, typically presents fewer obstacles than is the case on overhead lines. But it is difficult to establish when is the critical time to wash. Applying silicone grease to substation insulators is more practical than for line insulators, but the application process requires the insulators to be de-energized. Moreover, the grease has to be renewed regularly, probably every second year. As is the case with washing, it is not always easy to predict the optimal time to re-grease.
In South Africa, flashover problems due to deteriorating pollution conditions at certain substations created just this type of situation requiring remedial action. About 30 years ago, the country’s principal utility, Eskom, as well as several local utilities began to consider an alternative to washing and greasing – namely coating insulators at affected stations with a layer of RTV silicone material. Initial results proved satisfactory and, gradually, coatings came to become increasingly applied to all substation insulators where pollution flashovers occurred in the past or were felt likely to occur in the future.
INMR visited such a substation several years ago and reported on the background which led to the decision to employ coatings as well as on the subsequent experience.
The station yard at a large coal-fired power plant in South Africa provided a good example of use of RTV silicone coatings to counteract worsening pollution. The plant, built 50 years ago, was equipped with ten 200 MW generators at the time and had several cooling towers with two smoke stacks – one fitted with filters, the other not. The yard is situated parallel to the power plant and spreads out across about 1.4 km. When the station was built, an open-pit coal mine was located in the general vicinity but still a significant distance away.
During the first 15 years of operation, hardly any flashovers were experienced at the yard. However, as the growing coal pit approached the plant, up to 5 or 6 flashovers began to occur each year. In the classical manner, these took place at the end of the dry winter season when the first moisture began to appear. Because prevailing winds are parallel to the station, the end of the yard closer to the mine was more affected by these flashovers.
The first attempt to combat the problem involved applying silicone grease to insulators located at the more vulnerable portion of the station. While this proved beneficial, there were still concerns: the grease needed to be renewed regularly but it was not always apparent when this should be done. Therefore, to deal with the flashover problems affecting the other half of the yard, Eskom engineers decided to turn instead to coating the insulators with RTV silicone. Apparently, they had already contracted a local firm to apply such coatings at a number of other substations going back to 1991 and had been satisfied with the results. Therefore, the 400 kV bushings of several generator transformers in that portion of the yard were coated.
Other remedial work included using silicone insulators to replace strings of glass discs holding the overhead bus work and which had the relatively short specific creepage of only 19 mm/kV. Although the specific creepage distance of various other insulators in that portion of the yard varied from 18 mm/kV to 24 mm/kV, it was not felt that there was any need to increase this by utilizing shed extenders.
The performance of the RTV silicone-coated insulation proved satisfactory in dealing with the flashover problems. As a result, a decision was made to extend their application to that part of the station more affected by the coal mine. This was done section by section over a period of a few months and all insulators were coated, including bushings, circuit breakers, isolating switches and station posts. All silicone grease employed on insulators located in that portion of the yard nearer the mine was removed and replaced by the coating.
In preparation for the application of the coating, insulators were cleaned thoroughly. Also, since this was the more vulnerable portion of the yard, shed extenders were fitted to those which were felt to have insufficient creepage. This process brought all the insulators in this section to a uniform specific creepage of 24 mm/kV prior to coating.
At the time, a porcelain post insulator at the station had recently been taken down due to a broken shed. This insulator had been coated about three and a half years earlier and the coated surface still showed excellent hydrophobicity in spite of the severe local pollution. Moreover, that particular station post had also been previously fitted with shed extenders. When checking the adhesion of the extender to the porcelain, the silicone material of the extender actually tore before the bond to the surface failed.
The RTV-coated station yard at the 400 kV power plant operated for the next few years with only one recorded flashover. This occurred on a post and was probably caused by polluted water cascading directly onto the insulator from a structure located within the yard. The cause of that problem was then remedied.
When required by changing pollution conditions, as was the case here, the policy in South Africa has become to employ RTV silicone coatings for all insulators at existing power station yards and substations. Applying such coatings to insulators which have been in service for a number of years is generally more costly because these insulators need to be cleaned carefully prior to application of the coating. Therefore, new insulators destined for areas where pollution is likely to be severe are now routinely coated prior to being put into service.