With the dramatic growth in application of composite insulators, there have been periodic reports concerning mold growth and its possible impact on electrical performance. Users will therefore need to know what is the best course of action to follow should such situations arise on their networks. Here, it may prove helpful to know how other utilities have dealt with similar problems and with what results. This past INMR article provided an interesting case study by presenting the experience during the 1990s with serious mold contamination on silicone-housed bushings. The user in this case was a regional U.S. based utility located in the country’s humid south-east.
Lee County Electric Cooperative (LCEC) serves a customer base scattered over a 2500 square mile area in southwest Florida. In the past, the utility operated transmission lines at 230 kV (since sold to another operator) and 138 kV which feed the 25 kV distribution system. A majority of the service territory includes barrier islands and coastal areas which experience a subtropical climate, including ambient temperatures up to 30-38°C all year, high humidity, high levels of ultraviolet radiation and exposure to heavy salt contamination. Twice during the late 1980s, the network experienced tropical storms which were unusual in that they did not produce so much rain but rather brought salt-laden winds in from the Gulf of Mexico. These winds contained significant levels of contaminants which coated most electrical facilities resulting in tracking along insulators as well as bushings and causing several outages due to flashover. Two serious such outages involved failures of porcelain-housed bushings on dead tank circuit breakers. The insulation level and leakage distance requirements on the 230 kV system at the time were 900 kV BIL and 170 in. (4320 mm) respectively.
It was this combination of events which first aroused interest in applying non-ceramic bushings for high contamination areas. The timing seemed ideal since at the time the utility was also in the process of re-configuring the 230 kV bus at its 230/138 kV transmission substation. According to technical personnel, by the end of the 1980s there had already been a significant amount of information published on the performance characteristics of various organic materials, including EPDM and silicone, used in the manufacture of non-ceramic insulators. Unfortunately, from their perspective, there was still not enough field data on the long-term impact of high temperature, humidity, UV and salt contamination on these materials. Consequently, there were concerns as to how this material would behave and age in the environment of south Florida. As the re-design and re-configuration of the utility’s 230 kV bus progressed, a decision was made to utilize silicone bushings for six new 230 kV breakers which were to be installed. This decision was based on such factors as:
• LCEC had already been experiencing flashovers on porcelain bushings due to salt contamination, so something clearly had to be done.
• The silicone rubber bushings being offered provided an extra leakage distance (200 in or 5080 mm at 900 kV BIL rating compared to 170 inches or 4320 mm/900 kV BIL for porcelain) thereby offering better resistance to contamination flashover.
Six breakers were eventually purchased with a total of 36 silicone rubber bushings. The main issues to be resolved were what would be the life expectancy of these new insulators and would the utility see improved performance in its operaitng environment of high humidity and salt contamination.
Over the initial few years, there were two situations which presented cause for concern, the first of which ocurred during installation. While installing the bushings and mounting the flanges, the contractor gouged a piece of the insulator causing a tear of some 2 in (5 cm) by one in (2.5 cm) between the sheds. The damaged bushing was replaced with another, but this event forcefully illustrated that special care must be taken when installing or working around silicone bushings. The second situation was more unusual. During a routine inspection about 5 years after commissioning, the substation electrician noticed a greenish-black growth on the surface of the bushings. This growth seemed to be developing in the areas which were not directly exposed to sunlight, much like a mold or moss growth on the trunk of a tree. An obvious concern was what would be the impact of this growth on the bushings’ electrical integrity and what might be the proper method of removing it. The manufacturer had already advised LCEC that cleaning the bushings was not generally required. However, if they elected to do so, extreme care would have to be taken since use of detergents or alcohol could temporarily reduce hydrophobicity by removing the silicone oils. A decision was made not to immediately clean the affected bushings but rather to monitor the situation.
By the next year, however, it had become obvious that the mold seemed to be flourishing not only in the areas protected from sunlight but now also on the core of the housing as well as on the tops of the weathersheds. In addition, the areas not exposed to sunlight were now seeing extensive mold growth almost to the point of complete coverage. Moreover, the mold now appeared to be even growing on the surface of the breaker tank itself, even though no other devices or equipment in the substation were so affected. Visual inspection revealed that while the non-contaminated areas of the bushing exhibited very good hydrophobicity, those areas with mold coverage showed clear evidence of water tracking and ‘pooling’. Technicians quantified and evaluated the amount of mold growth on each bushing and also sent several samples to be analyzed at a nearby university laboratory.
According to the findings of this analysis, the silicone rubber of the insulation material had apparently been encapsulating and trapping air-borne pine pollen which was somehow also attaching itself to the breaker tank. The mold, which was the visible growth on the bushings, was in fact feeding on the pollen and this applied to the surface of the tank as well.
After performing a visual inspection to verify that there was no evidence of cracking, tears or chalking along the silicone insulators, the utility decided to conduct tests to evaluate the impact of the mold growth upon the electrical performance of the affected bushings. These tests were conducted only on that bushing which was most contaminated based on the percentage of surface area covered by the mold (circa 40 per cent). Leakage current measurements were taken under applied voltage with the objective of determining what, if any, was the reduction in electrical insulation as a result of the mold contamination to the silicone housings. The leakage currents at various voltages and at different locations along the bushing were examined. These tests also included comparison under wet and dry conditions. The test voltage was applied in steps of 20 kV up to a maximum of 200 kV DC across the bushing. Since the utility was somewhat limited by the test equipment available, voltage was first applied across the bushing at full leakage distance (200 in. or 5080 mm) and then reduced successively to 132 in. (3350 mm) and finally to 66 in. (1680 mm). This test procedure, it was felt, provided good information concerning how significantly the mold contamination was affecting insulation levels. Subsequently, the bushing was cleaned with high-pressure water and de-natured alcohol. After cleaning, it was sprayed with de-ionized water to assess the impact of cleaning on the bushing’s hydrophobicity. The bushing still exhibited water beading confirming the continued existence of the silicone oils. Finally, the cleaned bushing was re-tested under the same voltage variations and bushing placements as earlier.
Test results (Charts 1 and 2) revealed that the contaminated bushing demonstrated a dramatic increase in leakage current under wet conditions when applied at the reduced leakage of 66 in. (1680 mm). Since even at this reduced leakage, one would not normally expect the bushing to have a problem, this very rapid rise of leakage current was certainly a cause for concern.
After cleaning the bushing, the same test was repeated. However, this time when the bushing reached 200 kV at a leakage distance of 66 in. (1680 mm) a significantly lower leakage current was seen compared to that under mold contamination. This would support the belief that mold growing on the surface of these bushings does in fact reduce electrical insulation levels. The technical staff argued that while it might be a subject of debate as to the amount of degradation evident, from their perspective as a user cleaning the mold from the silicone rubber definitely provided improved electrical insulation levels.
According to engineering staff, although the voltage levels and field conditions did not fully replicate testing in a laboratory, enough information was nevertheless obtained at the time to confirm that the insulation capabilities of the silicone bushings improved once the mold contaminants had been removed. They then explained that this did not necessarily imply that the mold contamination on the bushing had resulted in an inadequate insulation level. Rather, it only suggested that a clean silicone bushing has better insulation than one covered by mold growth. Similarly, the utility determined that cleaning the silicone with de-natured alcohol and high-pressure water did not reduce hydrophobicity, even after years of service.