Polymeric insulators have been applied on overhead linesand at substations as far back as 1960. Several different polymers have beentried over the years, often with different results. For example,polytetrafluoroethylene (also known as Teflon) seemed promising at the startand was used in insulators made in Italy starting about 1965. But this materialwas soon abandoned as not suitable. Other polymers that have claimed superiorpollution performance versus porcelain and glass include ethylene propylene rubber(EPR), ethylene propylene diene monomer (EPDM), silicone rubber (SR) andvarious ‘alloys’ of these polymers. In this edited 2015 contribution to INMR,T&D expert Alberto Pigini discussed some of the factors to consider whenselecting the most suitable polymeric housing material for an outdoor insulatorapplication.

While each family of polymeric materialshas usually been referred to on the basis of its main bulk polymer, it isimportant to note that each insulator material is formulated using its ownunique ‘recipe’. Specific ingredients such as fillers, colorizers and otheradditives are added to the main bulk polymer to optimize it from the viewpointsof cost, production and performance. In fact, one issue that remains onlypartly resolved is establishing the best means to obtain a reliable‘fingerprint’ of each polymeric material. This is regarded as the best way toassure customers that the insulators shipped to them are exactly the same asthose for which type test certificates and field experience have been provided.

EPR, EPDM and SR (in their various proprietaryformulations) have been found to be the most suitable polymers with differentorders of merit from the standpoint of resistance to electrical, chemical,environmental and mechanical stresses. SR, for example, is a hydrophobicitytransfer material (HTM). This means that it not only exhibits intrinsichydrophobicity but also the unique ability to restore hydrophobicity at thesurface with low ‘recovery time’ should hydrophobicity be temporarily lost dueto service conditions such as heavy wetting. It is mainly because of thisadvantage that SR has prevailed over other polymers and indeed has becomethe de facto ‘standard’ for most HV applications inboth AC and DC – especially whenever enhanced pollution performance isdemanded. Field experience with this family of polymers has generally beenpositive for both line and substation applications, thereby reinforcing thestrong market preference.

At the same time, it is important to point out thatexperience suggests that SR insulation may not always meet user expectations inextremely harsh service environments (e.g. those having high soluble as well asnon-soluble deposits on insulators and with frequent wetting by fog). Undersuch challenging conditions, hydrophobicity recovery may not be fast enough,effectively nullifying the benefit. This behaviour was confirmed in recentyears by severe (perhaps overly severe) laboratory ageing tests where differentinsulator designs and materials were exposed for thousands of hours todifferent stress conditions including salt fog, rain, clean fog, drying periodsand UV (see Fig. 1). Fig. 2 shows examples of degradation experienced under DC.To get an indication of insulator condition after ageing, pollution withstandwas determined using the ‘quick flashover’ method at a salinity of 80 kg/m³. Fig. 3 shows a comparison of insulators in termsof unified specific creepage distance (USCD) required under DC. Under suchsimulated extreme conditions, it was found that the performance of EPDM and EPRinsulators was actually superior to that of SR. This was probably due to thefact that resistance to tracking and erosion by any polymeric material is moreimportant than hydrophobicity transfer in these types of extreme enviro

Finally, there are situations whereadopting composite insulators has been dictated not by superior pollutionperformance but rather by other considerations, such as improved safety. Infact, this has increasingly been the case for housings for AC substationapplications in relatively clean environments, where electrical design isdominated more by switching impulse performance than by pollution. While it ispossible that silicone insulators also offer the best solution in this case,technical and economic comparison of the ‘standard SR approach’ withalternative polymeric options not mentioned above should not be automaticallydiscarded. Too much standardization limits innovation.