Compositeinsulators possess certain advantages over ceramic types and their rate ofapplication has grown rapidly. For example, in China it is estimated thatcomposite insulators now account for more than a third of the insulatorpopulation on all transmission lines. In the case of new lines, their share ofall insulators installed is closer to 50% and when it comes to UHV they are byfar the dominant type.

This pastINMR article by Prof. GUAN Zhicheng of Tsinghua University, discussed thegrowing need to monitor their condition given the growing role they play notonly in China but on power networks worldwide.

The earliest silicone composite insulators have been in service now forover 40 years while in China their service history is about 30 years. Giventheir spiraling use and the many years in service, research into methodologiesfor in-service monitoring of defects or damage has taken on prime importance.There are a number of potential failures modes that represent possible hiddenthreats to the safe operation of composite insulators, including flashover,ageing and mechanical failure

Among the first category, lightning flashover is typically the most commonthreat to these insulators mainly because it depends on dry-arc distance withlittle relationship to insulator material. If the dry-arc distance of acomposite insulator is the same as that of a porcelain or glass string, it hasbasically the same lightning flashover withstand. While composite insulatorscannot increase lightning flashover voltage, they will certainly not lower aline’s lightning performance. Anyway, auto-reclosing in such cases is usuallysuccessful and therefore this potential problem has comparatively low impact onnetwork reliability.

Due to the superior hydrophobicity and hydrophobicity transfer property ofsilicone rubber housings, composite insulators offer excellent pollutionperformance. This translates into relatively few pollution flashovers, which isone of the principal reasons for their extensive application. On the otherhand, flashovers caused by bird excrement present a much bigger problem mainlybecause large birds release ‘streamers’ near the tops of insulators, i.e.essentially equivalent to a section of the conductor ‘floating in air’, therebygreatly distorting an insulator’s E-field distribution. Moreover, if the birdstreamer is close to the edge of the insulator, distortion in E-field is evengreater and more likely to cause flashover. Composite insulators are more proneto flashover due to bird streamers than are porcelain or glass strings sincethey have smaller shed diameters. Indeed, among flashover failures recorded asdue to undetermined cause, most are likely to have beentriggered by birds.

The second major category of threat to composite insulators – ageing –involves occurrence of surface phenomena such as cracking or crazing undernatural weathering. Ageing of silicone rubber is often accompanied by decreasein hydrophobicity and a corresponding reduction in pollution flashoverwithstand. However, ageing can also be due to other factors such as birdpecking that can damage silicone sheds or sustained strong winds that can tearsheds depending on diameter and design. Mechanical failure, the third maincategory of possible problem, basically involves core fracture of a compositeinsulator, causing a dropped conductor.

Basically, in-service monitoring of composite insulators involvesinspection to identify the type and extent of damage associated with thesepossible failure modes. In the case of lightning flashover, a lightninglocation system is typically available which can assist maintenance staff tolocate the affected tower. In China, the policy of power supply utilities is toidentify which composite insulator experienced lightning flashover and toreplace it so as to eliminate any risk of hidden resulting damage. On-lineleakage current monitoring is then usually used to evaluate their pollutionflashover withstand. In regard to flashovers caused by bird streamers, pass-byinspection is used to look for nests on towers or the presence of large birdsnear lines. Similarly, when it comes to ageing and damage to silicone sheds,visual inspection is the main technique – with or without assistance by opticaldevices.

Failure by fracture of the internal FRP core rod represents the mostserious potential failure mode of a composite insulator and is also the mostdifficult to monitor for. To make the situation even more complex, two uniquelydifferent types of fracture have now been identified. One is theclassical brittle fracture, which has beenthe focus of much research over the years and has been successfully simulatedin the laboratory. The other type has occurred several times in China in recentyears and results in a fracture section with a much different appearance. Forexample, a composite insulator with this failure mode was removed from a 500 kVAC line in Guizhou Province. One month prior to fracture, IR measurements haddetected a localized temperature increase on this insulator. When voltage wasapplied to the fractured insulator in the laboratory, discharge was detected atthe location having the increased temperature. This demonstrates that this newmode of fracture failure can be detected through pass-by field inspection usinga combination of IR thermography and UV camera. Indeed, several countries nowemploy helicopter inspection with both types of equipment.

Although there are presently a variety of in-service monitoring methodsfor composite insulators, few meet all the criteria of being simple, effectiveand convenient. Research to find improved in-service monitoring techniques musttherefore continue to ensure long-term reliable operation of the growingpopulation of composite insulators.