RTV Coatings Play Key Role at Chinese Substations
UTILITY PRACTICE & EXPERIENCE
RTV Coatings Play Key Role at Chinese Substations
After years of rapid urbanization and industrial development, many of China’s cities suffer from high pollution levels that also adversely impact performance of outdoor electrical insulation. In fact, reducing the risk of pollution flashover at substations located near major urban centers has become a key part of their design and maintenance strategies. INMR visits two substations from northwest to southeast China to report on application of RTV silicone coatings as one of the remedial measures increasingly used to combat pollution flashover.
Porcelain of GIS bushing at new Guandu 500 kV Substation near Zhengzhou shows evidence of contamination build-up and related arcing activity during wetting by light rain.
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Wubei Substation is located only some 30 kilometers from Urumqi, capital of China’s Xinjiang Province. Built in 2008, it operated for two years at 220 kV before being uprated to its full design capability of 750 kV in October 2010. Wubei’s pollution exposure is classified as Category IV (very heavy) due to the presence of a large local chemical industry and many coalfired power plants. A nearby waste incineration facility only adds to the problem as does the region’s topography and climate.
Porcelain tension insulators to gantry at Wubei when newly installed (middle) and one year later (bottom).
Photos: INMR ©
Li YaoZhong, Chief Engineer at the Electric Power Research Institute in Urumqi, points out that most of Xinjiang is barren and treeless. As a result, the province’s power network is typically exposed to high ESDD levels due to wind-blown dust. “While we are far from the ocean,” remarks Li, “the soil here is full of salt and other conductive contaminants. There are also large daily temperature fluctuations, especially during winter. If you combine this with frequent fog, it’s the classic recipe for flashover problems.”
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t Waste facility (smokestack in background) adds to pollution concerns at Wubei and resulted in all 750 kV breakers being equipped with silicone-housed bushings. q
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All porcelain support insulators on the 220 kV side at Wubei have RTV silicone coatings as well as antiicing booster sheds. q
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Li explains that this service environment has resulted in most overhead lines in Xinjiang being equipped only with silicone insulators and this policy also applies to the growing 750 kV transmission network that is a unique voltage level used only by the power grid in northwest China. For example, the first such line, built in 2006 between Manas Power Station and Wubei Substation, employs silicone insulators for all suspension applications. Explains Li. “they are not only less costly to buy but more important also require much less maintenance than porcelain strings which must be inspected annually for punctures.” There are also a growing number of 750 kV substations already in operation at this voltage – all, like Wubei, upgraded from their initial operating voltage of 220 kV. Li says that there are plans over the coming years to build additional 750 kV lines and that together these will result in a circle network covering much of China’s northwest.
Photos: INMR ©
Heavy pollution levels near Wubei have made it necessary to look for high performance insulation for most applications at the substation, from post insulators, to switches, to bushings and the preferred solution has been to coat all porcelain with RTV silicone. In most cases, booster sheds have then been placed on top of the coated porcelain to prevent ice bridging on relatively short insulators during the winter months.
Booster sheds offer anti-icing functionality and are placed on RTV-coated porcelain on the 220 kV side at Wubei.
Insulation at substations in Xinjiang is exposed to typically high ESDD levels. With large daily temperature fluctuations and frequent fog, it’s a classic recipe for flashover problems.
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According to engineers based at Wubei, the application of RTV silicone material was performed there late in 2010 and about two weeks were required to coat the entire station. Most RTV suppliers recommend cleaning and degreasing the porcelain prior to applying the coating evenly across the entire porcelain surface, ideally with a thickness of between 0.3 and 0.5 mm. However, some coated insulators at Wubei appear to have variable coverage and offer evidence that weather conditions during application as well as skill level of the maintenance staff involved can impact results. For example, if coating is carried out on a windy day, application thickness and extent of coverage can vary from insulator to insulator unless special care is taken. Indeed, to avoid the potential risk of variable quality of application, some porcelain insulator manufacturers in China and elsewhere now offer new pre-coated porcelain straight from the factory. In such a case, coating conditions are always controllable and this usually means more consistent application quality from one unit to the next. At almost the diagonally opposite side of China from Wubei is Luochong 220 kV Substation, near the center of Guangzhou in the southeast and one of the country’s largest urban centers. This substation belongs to the Guangdong Province Power Grid and is situated directly below a busy highway overpass, meaning constant high
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Evidence that RTV layer does not fully cover surface of porcelain, especially near flange. Note high levels of dust and contamination already deposited on coating.
levels of dust, which eventually settle onto insulators. Luochong was built in 2000 next to an oil-fired power plant that has since been taken down and replaced by numerous high-rise residential buildings. Liu Pingyuan, an Engineer with the Guangdong Power Grid, explains that the high local pollution affecting Luochong requires the station to be washed live at least once a year. However, this apparently is not sufficient and certain vital types of equipment, such as breakers and transformer bushings, have recently been coated with RTV silicone. Liu also notes that the use of such coatings by the Southern China Power Grid is still relatively new, dating back only to 2004/2005.
is whether those units with less than ideal coating coverage will perform as well as other insulators that have been properly coated. A paper on RTV coated line insulators published last year by researchers at the Shenzhen Campus of Tsinghua University reported that electrical and physical tests on field-aged samples taken down from 500 kV, 220 kV and 110 kV lines suggested that a service life of at least 6 years
could reasonably be expected. Of course, this applied to line insulators, which on average would probably have a much lower exposure to contamination than those at substations exposed to more or less constant heavy pollution. Other researchers from Tsinghua University took a different approach and looked at the contamination layer itself, studying whether this might affect flashover performance
One of the key questions ahead for maintenance staff at Wubei and Luochong will be what type of service life can be expected for the existing RTV coatings. Another issue
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RTV coatings applied in controlled factory environment ensure more consistent thickness and coverage.
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of the coating. Atomic absorption spectrophotography and ion chromatography were employed to study the composition while a scanning electron microscope analyzed the configuration of this contamination on coatings aged in service. 220 kV switchgear and transformer bushings on older transformers at Luochong are RTV coated.
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Findings suggest that while there were indeed significant changes in the polluted coated insulators studied compared to their original state, flashovers still did not occur during withstand tests, even at high voltage. In this regard, the researchers determined that the impact of different wetting behavior on the lower and upper insulator surfaces increased pollution flashover voltage more significantly than any reduction due to the presence of soluble salts
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Luochong lies directly below a busy highway overpass and extent of pollution accumulation is evident from glass discs at the end of one of the station’s insulator strings.
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Application of coating near flange of bushings in Luochong varies in thickness while some sheds show evidence of undesirable drip marks (see upper left).
on the coating surface. They therefore concluded that RTV coatings do indeed provide long-term flashover protection, even in areas with severe pollution problems.
As for the issue of whether less than ideal coating application (as clearly evident on some units at Wubei) might affect service performance, experts from the RTV silicone coating industry stress that it is always optimal to ensure coatings are applied uniformly and thoroughly to the entire porcelain surface. In those cases where the coating is either unevenly applied or does not reach the edge where the
porcelain meets the flange, coating performance will likely be reduced, although not necessarily critically. The main factor, it seems, is that a large portion of the coated surface retains its hydrophobicity, which will then play the dominant role in preventing pollution flashover.
Photos: INMR ©
In the case of Luochong, Liu reports that actual service experience seems to back this conclusion since, in spite of the obvious presence of contaminants on the coatings, there have been no flashovers recorded on coated insulators at the substation during the past 5 years.
One of the current transformers at Luochong has oil leaking onto the porcelain (above). RTV coating on porcelain switchgear housing (left).
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