Electrical insulator is a very important component in the electric power systems such as sub-stations and distribution & transmission lines. 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 consists of a fiberglass core rod covered by weather-sheds or skirts of polymer such as silicone rubber, 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.
This write-up gives the salient features considered by GOLDSTONE in developing SILICONE RUBBER POLYMER INSULATORS. Two aspects are to be essentially considered in the development, one is the material suiting to the requirements and the other is design aspect. The following few lines will briefly describe the important aspects considered.
Insulator Construction
Silicone Rubber
Silicon rubber insulators give excellent performance in harsh polluted environment which is explained briefly in subsequent lines. The pollution problem is mainly due to the deposition of the pollutants on the insulator i.e. on the porcelain surface in the case of disc insulator strings. The pollutants, in nature, are hydrophilic, and hence chances of dry band arcing is more which may lead to a flashover and it is not a desired characteristic. Due to this reason periodical cleaning of the insulator is imperative. In case of silicone rubber insulator also, the pollutants will get deposited on the weather shed. But the hydrophobicity transfer characteristics of silicon rubber will play a major role in such circumstances. This property is controlled by proper selection of base Silicone rubber, dosage and particle size of ATH and degree of cross linking of Silicone rubber of the weather shed. The Silicone rubber compound used by Goldstone is having all the required properties to combat with the harsh environment.
The pollutants, which are hydrophilic in nature, are converted into hydrophobic nature due to the hydrophobicity recovery characteristics of silicon rubber which gives a good performance in harsh polluted environments in comparison to porcelain insulators which do not have such characteristic. The silicon rubber compound used by Goldstone has been tested for hydrophobicity after exposing the material to corona, UV radiation, arcing etc., and the results are very good. Hence, in view of the material characteristics, the silicon rubber compound used in manufacturing the insulators, will give good performance in the polluted environments as mentioned above.
In addition to corona, UV radiation, the prime importance of silicon rubber material is to possess good resistance to tracking and erosion. The material used by Goldstone has been tested for tracking and erosion as per IEC: 587.
FRP Rods
The FRP rods used as the core material in the manufacture of silicon rubber insulators is manufactured by pultrusion method using boron free ECR grade glass fibres. The FRP rods being Produced by Goldstone are having all the required mechanical and electrical characteristics required for the long term uninterrupted performance of High Voltage Composite Insulators. The FRP rods have been tested for Brittle Fracture Test. The rods met the requirements and passed in the test without even slight sign of brittle fracture.
Metal Fittings
Both the socket and ball fittings are of forged fittings in High Voltage Insulators, whose tensile strength, elongation, impact strength, melting temperature etc. are best suited for silicon rubber insulators for erecting in harsh polluted environments. Sharp corners have been avoided so as to avoid corona.
Dimensions
To suit the interchangeability, the overall length of the insulator has been kept within the range of overall length of tension and suspension insulator strings. The metal end fittings have been designed for 20mm designation in case of tension insulators and 16mm designation in case of suspension insulators for Extra High Voltage Insulators.
Creepage Distance
With reference to IEC: 60815, a specific creepage distance of 31mm/kV has been specified for very highly polluted condition. The description of polluted condition for Very High Level as per IEC: 60815 pertains to areas subjected to conductive dusts and to industrial smoke producing particularly thick conductive deposits, areas very close to the coast and exposed to sea-spray or to very strong and polluting winds from the sea and desert areas characterized by no rain for long periods, exposed to strong winds carrying sand and salt, and subjected to regular condensation. Hence our Polymer Insulators have been made to design for a specific creepage distance of 31mm/kV for High Voltage Insulators.
Shed Profile
Even though the profile parameters for the shed as well as whole insulator specified in IEC: 60815 are meant for porcelain insulators, the same have been followed for silicone rubber insulators. All the profile parameters given in IEC: 60815 have been obtained in the design of insulators.
Sheath Thickness
A normal sheath thickness of 3mm over the FRP rod is being adopted for silicone rubber insulators. But in our design, we have designed a sheath thickness of 5mm so as to get long time with standability of the sheath towards the pollution thus leading for a longer span of life of the silicon rubber insulator.
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