|Composite electrical insulator|
|Sunday, 10 December 2006|
High voltage insulators, as used for example in power transmission and distribution, typically comprise an elongate body of electrically insulating material and a fitting at each end for connection to a power conductor at high voltage and a point at another potential. The elongate body may comprise a ceramic material, for example porcelain or glass, or a composite material such as fiber glass generally with a coextensive sheathing of a polymeric material. Electrical insulators for high voltage use have traditionally been made of glass or porcelain as these materials are of low cost, high quality electrically and under normal conditions, have a long life in service. Ceramic or porcelain is a preferred insulating material in some applications because of its superior resistance to damage by electrical discharges, to weathering, and to chemical attack. Examples of porcelain insulator arrangements are potheads, which have an outer convoluted porcelain component filled, with oil or bitumen or other insulating material, whereby the conductor of a single core high voltage cable enters the porcelain chamber at one end and is terminated at a connector at the other end. At the cable inlet end, the metallic cable sheath is electrically connected to an earthed baseplate that is clamped to the porcelain, and the connector is secured to the other end of the porcelain body. While porcelain is a very suitable material due to its resistance to damage by electrical discharges, weathering and chemicals, it is relatively heavy and can shatter on impact. Thus, there have been cases of severe injury resulting from the shattering of porcelain insulators. In some places, they are damaged due to people deliberately throwing objects at the insulators or even shooting at them. To provide the necessary mechanical and electrical characteristics, porcelain insulators are typically quite heavy. Ceramic or porcelain insulators require specialized assembly fixtures or processes and are awkward and difficult to handle and ship. Moreover, such porcelain insulators are expensive and brittle and therefore porcelain insulators are subject to damage during shipment and insulation. The deficiencies of the porcelain insulators have led to the development of composite insulators which generally comprise a central core covered by an outer insulating coating. The central core is provided to perform the mechanical function of the insulator while the protective coating is provided to protect the central core from external agents and to prevent electrical discharge.
As an alternative to ceramics, composite materials were developed for use in insulators for transmission systems. Such composite insulators are also referred to as non-ceramic insulators (NCI) or polymer insulators, and usually employ insulator housings made of materials such as ethylene propylene rubber (EPR), polytetrofluoro ethylene (PTFE), silicone rubber, or other similar materials. Compared with the conventional insulators manufactured homogeneously from the material ceramic or glass, composite insulators consist of two components of different materials. A number of composite insulators made from lighter weight polymeric materials have been developed for use in such high voltage installations. Such composite insulators generally include a fiberglas rod having a number of weathersheds constructed of a highly insulating polymeric material attached to the rod along its length. Composite insulators including shield layers of a synthetic material are given preference mainly because the shield layer of a synthetic material, particularly silicone, is hydrophobic, i.e. the insulators employed mostly outdoors are highly water repellant which is conducive to repelling dirt and thus to low leakage current losses. And also, this is a lightweight structure which facilitates assembly. Composite insulators are generally produced by preparing the screens individually and then fitting the required number of them onto a shank coated with extrudate and vulcanising them with the coat, or by centrally placing a rod with a predetermined number of screens in a two-part mould and injecting all the screens at once. Composite insulators for high-tension use must conform to specific electrical requirements. The carrier rod must be electrically insulating in its axial direction and the insulating layer must be secured thereto in a manner that no electrical conduction can occur at the seam between the insulating cover and the interior carrier rod. The insulating cover performs several functions including providing resistance to weathering, UV, ozone, etcetera. The cover is also required to have good mechanical resistance to cold and good electrical tracking resistance. Desirably the insulating cover should be flexible, halogen-free and flame retardant.