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.