In what must be seen as a rare meeting point of the fields of microbiology and HV engineering, a growing number of utility engineers are reporting the presence of biological growths on composite insulators. This phenomenon seems to be relatively independent of environment inasmuch as such growths have occurred on insulators installed on networks operating in temperate as well as tropical climates. Although these reports have typically gone no further than a simple record of this observation, in some cases there have been attempts to identify the specific source of this problem. For example, micro-organisms such as algae, fungi or lichen have been isolated from such insulators. Evaluation of the impact of these biological growths on insulation performance as well as of techniques to quickly identify affected insulators have also been carried out.
In spite of this work, however, relatively few reports have focused on the possible impact of such growths on the key properties of the silicone rubber material itself. Silicone rubber is known to be highly resistant to bio-degradation since the partly inorganic nature of the polydimethylsiloxane (PDMS) molecule makes it difficult for micro- organisms to use this polymer as a source of nutrients. Nevertheless, rubber polymers are clearly more sensitive to the potential for such degradation than are ceramic materials. Therefore, if inappropriate additives are chosen for production of the polymeric material, the resulting insulator can well become a target for biological colonization. Degradation of these additives or of the polymer matrix itself can then affect the bulk properties of the material under attack. Moreover, the mere presence of any micro-organisms on an insulator has the potential to change its surface properties.
The following past INMR article is based on research work performed in 2005 by PhD student Stina Wallström from the Royal Institute of Technology in Sweden under the supervision of Professor Sigbritt Karlsson. Areas of interest included identification of the micro-organisms causing these problems, examination of their impact on the chemical and mechanical properties of affected materials and evaluation of different silicone rubber formulations with respect to their ability to resist biological growths.
Micro-organisms colonizing bio-resistant substrates tend to form a film on the surface of the affected material. This biofilm consists of a mixture of different micro-organisms embedded in a highly-hydrated matrix of extracellular polymeric substances, mainly polysaccharides and proteins. Mixed populations of bacteria, fungi, protozoa and algae often co-exist in such a film. Generally, the composition of the biofilm affects its interaction with the support material. Therefore, it is important to know the specific composition of biofilms colonizing silicone rubber insulators in order to be able to best assess their impact on the properties of this housing material.
In this work, an analysis was performed of three different biofilms collected from silicone rubber insulators exposed to the outdoor environment in Sweden, Sri Lanka and Tanzania. Filamentous fungi and micro algae, in association with bacteria, were isolated from these samples. Even though the insulators were collected from three different continents, the compositions of the biofilms studied were found to be remarkably similar. Small unicellular green algae of the Chlorella family, living in symbiosis with bacteria, dominated these biofilms and traces of filamentous fungi were spread throughout the algae matrix. The various species identified are shown in Table 1. The striking similarity between the different biofilms indicates that the mechanism of bio-fouling of silicone rubber insulators may well be the same all over the world. This should then make it easier to develop a single bio-resistant housing material which can find application worldwide.
Damage to Housing Material
The structure and functionality of a polymeric material can be damaged by biofilms in a variety of ways. These undesirable effects can range from modifying surface properties due to the presence of accumulated biomass (via degradation of leaking components) to direct attack on the polymer matrix itself (see Figure 1).
How severe such biological attack can be depends on the composition of the polymer as well as on the nature of the biofilm. For example, it has been noted that not all polymeric insulators in the field are affected by biological growths, indicating that some formulations appear to offer effective protection against such attack.