Failure Triggers Insulator Replacement & Inspection Program at Chinese Power Utility

Failure Triggers Insulator Replacement & Inspection Program

at Chinese Power Utility

UTILITY PRACTICE & EXPERIENCE

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INMR Issue 99.indd 58 15/02/13 1:09 PM

The last 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. The finding here 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 core was then progressively degraded by water-induced ageing processes until it was no longer strong enough to support the load.

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. 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.

It might be tempting to label this as another example of the same failure mode discussed above and linked with a manufacturing defect. However, subsequent investigation revealed a much different explanation – namely severe erosion of the silicone rubber sheath due to intense corona discharges. This allowed moisture and acids to penetrate into the core and erode the rod material. 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.

INMR visits the affected 500 kV line near Shenzhen to report on a rigorous inspection and replacement program that has since been put into place.

Failure Triggers Insulator Replacement & Inspection Program

at Chinese Power Utility

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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. 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. These circuits share the same towers along certain sections of line but employ different towers along other sections.

The line’s pollution exposure comes mainly from the sea, which is only about two kilometers away at its nearest point. Engineers assess the line’s service environment as falling

Examples of typical towers (right & bottom opposite page) supporting one and two circuits of 500 kV Lingshen Line.

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Replacement of failed I-string insulator on affected tower in June 2012 (top).

Daya Bay Nuclear Power Plant (left) supplies power to the region, including to nearby Hong Kong.

Built over a decade ago, the line was first insulated with glass cap & pin strings, but these were replaced by silicone long rods a few years later

apparently because of problems with high corona noise.

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.

Built just over a decade ago, the line was first insulated with glass cap & pin strings, but these were

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Two different RTV silicone coatings applied to glass strings on Lingshen line.

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subsequently replaced by silicone long rods apparently due to problems of high noise. Glass strings are still used in tension applications but have now been coated with RTV silicone material to improve performance.

Fortunately, the failed silicone insulator was in a double I-string configuration so there was no interruption of service. Engineers at the Shenzhen Power Bureau point to this as validation of the advisability of applying not only double-string configurations on important lines but also separate hanging points. As such, if one string breaks, as in this case, the other can still support the operating voltage and load.

Indeed, to increase the Lingshen Line’s reliability based on this experience, most single composite string towers with relatively large differences in altitude compared to the adjoining one or having longer than normal span lengths have subsequently been modified to double strings.

The insulator that fractured was removed and sent for analysis to determine the cause of failure. The main finding was that there was an unacceptably high level of partial discharge activity near the live end under high electric field intensity. This then led to severe erosion of the insulator’s silicone rubber housing.

Close-up of coated glass discs reveals pattern of pollution deposition, with apparent ‘dripping’ off the lower edges of coating (top photo) as well as evidence of discharge activity on disc closest to conductor (bottom photo).

Photos: INMR ©

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Given the line’s service

environment of pollution and

wetting, corona rings on the

500 kV insulators seem not to have been adequately

designed.


The implication was that, given the line’s service environment of pollution and wetting, the corona rings were not adequately designed for such conditions.

With this concern in mind, the Shenzhen Power Bureau instituted a program to inspect all the line’s insulators to determine if the problem was widespread or just isolated to a unit that may have been somehow defective or improperly installed.

A total of more than 600 strings were examined over the course of the following months as part of a systematic inspection program using a hand held infrared camera. This process revealed that some 85 strings were ‘running hot’, i.e. had abnormally high temperatures near their live ends versus ambient.

Any string whose maximum

temperature was found to

exceed ambient by at least 5°C would then be replaced, in

most cases by a double string.

Different views of failed insulator reveal evidence of severe erosion on shank as well as sheds. There is also evidence of corona activity on fitting at live end.

Fractured insulator removed for inspection.

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Maintenance inspector uses hand held IR camera to monitor temperature near end fitting of insulators on Lingshen line (bottom & inset).


In order to gather more data on the problem, ‘hot’ insulators were removed and separated into five categories, based on their elevated temperature gradient: 1-3°C; 3-5°C; 5-7°C; 7-10°C; and greater than 10°C. Each insulator was tagged with detailed information, including specific tower, phase sequence, location either to right or left side, etc. A total of 20 insulators, i.e. 4 units from each of these 5 categories were then sent for testing to study

the possible causes of overheating.

On the advice of experts at the Shenzhen Campus of Tsinghua University and from China Southern Power Grid, a maintenance program was then established to monitor temperature profiles of all the line’s insulators. According to these experts as well as the Technical Guide for infrared diagnostics of live high voltage equipment (DL/T 664-2008), any string whose maximum

temperature was found to exceed ambient by at least 5°C would then be replaced, in most but not all cases by a double string.

In the end, several dozen strings were replaced with insulators from a different supplier having the same basic shed geometry and creepage but featuring a different design of corona ring. While the original insulators were equipped with rings having a concave lower surface, rings

V-string with original insulator (top left) and replacement unit with different corona ring on right. Double string with new replacement insulators (top right). Hardware on double string with original insulators shows evidence of corona as carbon

marks on surface (bottom left). Single string with new replacement insulator (bottom right).

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Examples of recent infrared scans of single and double string insulators along Lingshen Line.

on replacement insulators have convex upper and lower surfaces.

According to experts at Tsinghua University in Shenzhen, this experience has confirmed the criticality of controlling corona on silicone rubber insulators resulting from partial discharge activity under high electric field. It has also offered insight into some of the practical remedial measures that can be taken should a situation such as this occur. Equally important, it highlights the need to systematically monitor the condition of composite insulators through established diagnostic procedures, such as infrared scans of their thermal profile, to detect incipient problems before they lead to failure.

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Failure Triggers Insulator Replacement & Inspection Program at Chinese Power Utility

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