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Interface in Composite Electrical Insulators

Most failures of composite insulators originate from within the various interface and could either be between different components which are macroscopic in nature and often visible to the unaided eye, or microscopic, i.e. within a particular component, such as internal to the rod or housing. Given the critical role played by interface in the long-term performance of composite insulators, it makes sense to study this subject in greater detail. At the same time, both users and manufacturers must know how effective existing standards are in screening out insulators which are likely to fail in service due to problems with design or manufacturing of their interface.

Electrical insulator is a very important component in the electric power systems such as sub-stations and distribution and transmission lines. In-depth study and practical knowledge on this subject is necessary for the electrical professionals in the electrical power field. In the early days, insulators were made of ceramic and glass materials. But in 1963, polymeric insulators were developed and its improvements in design and manufacturing in the recent years have made them attractive to utilities. It is consist of a fiberglass core rod covered by weathersheds of skirts of polymer such as silicone rubber, polytetrafluoroethylene, EPDM (ethylene propylene diene monomer) and equipped with metal end fittings. It is also called composite insulators, which means made of at least two insulating parts – a core and housing equipped with end fittings. Polymeric insulators have many advantages over the ceramic and glass insulators such as good performance in contaminated environment, light weight, easy handling, maintenance free, and considerably low cost etc. Because of these properties it is gaining popularity worldwide and replacing the conventional ceramic and glass insulators. The scope of this technical paper is to discuss about construction, types, designing, testing, and selection of polymeric high voltage insulators.
The basic construction of composite insulators for lines and apparatus essentially involves three main components: a fiberglass core, a polymeric housing and metallic hardware. Perhaps this is an oversimplified description since it masks the numerous variations which exist in formulation as well as in processing and production techniques among the many different suppliers. Service experience has shown clearly that the various interface between the components of such insulators are the principal locations where problems tend to originate. This has already been recognized and there are steps outlined in the standards (such as IEC 61109) to ensure certain minimum requirements are satisfied. Incidentally, this particular Standard has de facto become the universal one being used today by insulator users who specify it as the basic requirement in their specifications for composite insulators.

Insulator Formation
Polymer insulators have a core composed of a fiberglass rod covered by polymer weather sheds. Manufactures use various shed materials, designs, and construction methods. Basic polymer shed materials used are silicone rubber, EPM, EPDM, CE, and polytetrafluroethylene (PTFE or Teflon). To obtain desired electrical and mechanical properties these basic material are combined with various fillers, including aluminum trihydrate. The EP rubbers rely largely on the alumina trihydrate to avoid carbonaceous degradation. As the EP rubber surface is exposed to ultraviolet light and possible electrical arcing, the alumina trihydrate is gradually reduced to a white alumina powder on the surface. The alumina may affect the wet flashover level of the insulator. Although the silicone rubbers contain alumina trihydrate, they rely on hydrophobicity to prevent leakage currents and associated arcing. Most manufacturers use individual sheds that were slipped over the fiberglass rod. While the CE material was cast as a single housing on the rod, the manufacturer of the silicone rubber insulator use a continuous molding process to form one continuous housing on the fiberglass rod. Some manufacturer formed a sheath on the rod, slipped the sheds onto the sheath, and then vulcanized them into place. The metal end fittings are attached to the rod using various technologies, which include compression of the metal end fitting onto the rod (the most widely use method today), insertion of wedges into the fiberglass rod, cutting the rod end into a cone, or gluing the end fitting to the rod. As part of IEC 61109, some tests are to be performed on complete insulator assemblies while others are performed only on the various individual components. For example, there is a water immersion test conducted on the complete insulator and which is followed by a steep front impulse test as well as power frequency test. If there is a significant defect in the rod to housing interface, the insulator can be expected to fail internally in the steep front impulse test. Also, if there are major problems within the rod itself, the insulator could fail internally. Power frequency tests are performed to determine if there is any dramatic reduction in flashover voltage or if there are any punctures. Basically, this test evaluates the numerous microscopic interfaces in the housing and rod materials. It is quite likely that a diagnostic test such as power factor or tan delta would provide additional useful information. At the component level, there are tests for water absorption of the core material to ensure that there is no excessive intake of water through its constituents and the numerous interfaces within. All these are type tests and design tests. They are performed on a limited number of samples and they need to be repeated only should there be a major change in materials, design or manufacturing process. The criteria for acceptance or rejection are clearly stated in the Standard, whose intent, incidentally, is not to provide a ranking based on the data measured from the various tests and shows details of the hardware-rod-housing interface for two different composite insulators intended for the same application. This region is probably the most critical part of a composite insulator and the photo clearly illustrates the potentially large differences in how some suppliers design their products.
Both insulator types shown are presently being used by utilities, which means that they have both passed the relevant IEC 61109 tests. At the same time, however, it is clear that one has a more robust construction than the other, thereby providing a greater margin of safety (and comfort) for the user. Therefore, a more discriminating test than presently provided by 61109 would be useful so as to enable ranking important interfacial properties and assisting the process of insulator selection. A past research project evaluated the interfaces in the rod and housing materials used in composite insulators. Rods with variation in the type of glass (E glass versus ECR glass) and resin (epoxy, polyester and vinyl ester) were evaluated. Also included were silicone housings with variations in formulation. One of the goals of this project was to establish a ranking of the various interfaces in these insulators.
A voltage test was also performed in the manner described in IEC 61109. The current for all samples was less than 1 mA and therefore well within the specified limit and shows scanning electron microscope photos of one sample taken from each of the two main groups exhibiting different levels of water absorption. The samples had been cut and polished in an identical manner. These photos revealed clear differences in interface quality. The micrograph depicting the rod with lower moisture absorption was characterized by relatively smooth interfaces between the glass fibers and the resin. The rod with higher moisture absorption, by contrast, displayed obvious shortcomings in quality, such as rough fibers, cracks in the resin and voids in the fiber-resin interface.
Moisture absorption test results on silicone rubber samples of different formulations. Except for sample A, the rest of the samples all have low moisture absorption. Composite Insulators, samples, test, insulators, housing, interfaces, moisture, composite composite insulators, samples, test, insulators, housing, interfaces, moisture, composite Interfaces in Composite Insulators. The tests are fairly simple to perform and show significant differences in material properties which were not identified by the applicable Standard at the time. For applications on critical transmission lines, attention to such details in design and materials used in manufacturing composite insulators could make a significant difference in service experience.
Development of cost-effective diagnostic tools which can identify problematic interfaces in insulators both during production and later after being placed in service would probably contribute greatly to even wider application of composite insulators. While achieving this soon is perhaps still only a possibility, researchers must start by examining methods which can at least provide useful information in the laboratory. Candidates for consideration in this regard are methods employing ultrasonic devices, acoustics, vibration analysis, X-rays, MRI (magnetic resonance imaging) and possibly others.