It may surprise some but polymeric type insulators have been applied on high voltage lines and at substations since as far back as 1960. The best measure of their success in delivering real benefits versus traditional ceramic technologies is that they now account for a significant and still growing share of the insulator market. Several different polymeric housings have been tried over the years, with much different results. For example, polytetrafluoroethylene (or teflon) seemed promising at the start and was used to make insulators in Italy starting about 1965. But it was abandoned years later as not suitable. Other polymers that claimed superior performance to porcelain or glass included ethylene propylene rubber (EPR), ethylene propylene diene monomer (EPDM), silicone rubber (SR) as well as various ‘alloys’ of these polymers.
While each such family is usually referred to on the basis of its main bulk polymer it is important to note that each insulator material is formulated using its own ‘recipe’. Specific ingredients such as fillers, colorizers and other additives are added to the main bulk polymer in order to optimize it from the cost, production and performance points of view. Indeed, one issue that remains only partly resolved these days is establishing the best means to obtain a reliable ‘fingerprint’ of each material. This is regarded as the best way to assure any customer that the insulators shipped to them are exactly the same as for which type test certificates and field experience have been provided.
EPR, EPDM and SR (in their various proprietary formulations) have been found to be among the most suitable polymers with different orders of merit from the standpoint of resistance to electrical, chemical, environmental and mechanical stresses. SR, for example, is a hydrophobicity transfer material (HTM). This means that it not only exhibits intrinsic hydrophobicity but also the ability to restore hydrophobicity at the surface with low ‘recovery time’ should it be lost temporarily due to conditions such as heavy wetting. It’s mainly because of this advantage that SR has prevailed over other polymers and indeed has become the de facto ‘standard’ for most HV applications in both AC and DC – especially when enhanced pollution performance is demanded. Field experience has generally been positive for both line and substation applications, thereby reinforcing the strong market preference.
At the same time, it’s important to point out that some experience suggests that SR insulation may not always meet user expectations in extremely harsh environments (e.g. those having high soluble as well as non-soluble deposits on the insulators and with frequent wetting by fog). Under such conditions, hydrophobicity recovery may not be fast enough, effectively nullifying this benefit. This behaviour was confirmed by severe (perhaps too severe) laboratory ageing tests where different insulator designs and materials were exposed for 1000s of hours to different stress conditions including salt fog, rain, clean fog, drying periods and UV (see Fig 1). Comparison of different polymeric insulators after such AC and DC laboratory ageing are found in CIGRE Report 33-104 ‘Study of the performance of composite insulators in polluted conditions’ presented at the General Session back in 1994.
Fig. 2 shows examples of degradation experienced under DC. To get an indication of insulator condition after ageing, pollution withstand was determined using the ‘quick flashover’ method at a salinity (Sa) of 80 kg/m3. Comparison of insulators in terms of unified specific creepage distance (USCD) required under DC is provided in Fig.3. Under such simulated extreme conditions it was found that the performance of EPDM and EPR insulators was actually superior to that of SR. This is probably so because resistance to tracking and erosion by a polymeric material is more important than hydrophobicity transfer in these types of environments.
Finally, there are cases where adopting composite insulators has been dictated not by superior pollution performance but by other considerations, such as safety. In fact this is increasingly the case for housings for AC substation applications in relatively clean environments, where electrical design is dominated more by switching impulse performance than by pollution. While it’s possible that silicone insulators also offer the best solution in this case, technical and economic comparisons of the ‘SR standard approach’ with alternative polymeric material options (not necessarily among those mentioned above) should not be discarded a priori. Too much ‘standardization’ can limit innovation.