Glass strings are still used in tension applications but have now been coated with RTV silicone material to improve performance.įortunately, the failed silicone insulator was in a double I-string configuration so there was no interruption of service. Engineers assess the line’s service environment as falling between Class II and IV in terms of the pollution scale used in China, depending on section of line and its exposure to winds sweeping in from the coast.īuilt just over a decade ago, the line was first insulated with glass cap & pin strings, but these were subsequently replaced by silicone long rods apparently due to problems of high noise.
The line’s pollution exposure comes mainly from the sea, which is only about two kilometers away at its nearest point. These circuits share the same towers along certain sections of line but employ different towers along other sections. Operated by the Shenzhen Power Bureau (part of the China Southern Power Grid) and running some 42 kilometers across mountainous terrain, the line is composed of two circuits, referred to as Jia and Yi. The 500 kV Lingshen Line is one of many that fan out across the countryside surrounding the Daya Bay Nuclear Power Plant in southeast China. In 2013, INMR visited the affected 500 kV line near Shenzhen to report on a rigorous inspection and replacement program that has since been put into place. Indeed, grading electric field near the live end is a critical requirement for composite insulators operating on high voltage lines and especially so in areas with elevated pollution levels combined with frequent wetting events. This allowed moisture and acids to penetrate into the core and erode the rod material. However, subsequent investigation revealed a much different explanation – namely severe erosion of the silicone rubber sheath due to intense corona discharges. It might be tempting to label this as another example of the same failure mode discussed above and linked with a manufacturing defect. Fortunately, since the tower involved was situated on a mountaintop, double strings had been specified as a safety measure and there was no dropped conductor. More recently, a small number of mechanical failures have also been reported in southern China, one of which took place in late November 2011 and involved an I-string insulator, also on a 500 kV line. The core was then progressively degraded by water-induced ageing processes until it was no longer strong enough to support the load. The finding there was that the failure, which did not show the characteristics typical of brittle fracture, resulted from internal moisture accumulation due to improper bonding of the sheath to the core rod during manufacture. The past issue of INMR featured the findings of a university research project into the mechanical failure of a 500 kV composite insulator in a relatively clean environment in central China.
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