MATERIAL CURRENTLY USED FOR INSULATOR DISCS & LIGHTNING ARRESTERS
The materialcomposition of a device plays a vital role in electrical system. Electrical Insulator is a very high resistive path through which practically no current can flow where as a Lightning Arrestor is used to divert the electrical flow. In transmission and distribution system, the overhead conductors are generally supported by supporting towers or poles. The towers and poles both are properly grounded. So there must be insulator between tower or pole body and current carrying conductors to prevent the flow of current from conductor to earth through the grounded supporting towers or poles.
A lightning arrester is normally an electrical insulator. However, under excessive voltage, it becomes a good electrical conductor, shunting the lightning surge safely to ground. Early arresters were simply metallic spark gaps. The spark gap was often a V-shaped "horn gap" or a pair of bent thick wire rods.Modern arresters use a series of sintered zinc-oxide disks (Metal-Oxide Varistors or MOV's) as high-voltage switching elements. In the OFF condition, they conduct little AC current, so no series spark gaps are necessary as with older silicon carbide arresters. Large MOV arresters used in electrical power systems consist of a series of zinc-oxide disk elements sealed within a ceramic or silicone rubber housing. Zinc-oxide MOV's are reliable, can handle large transient surge currents, and they recover quickly after the lightning transient ends.
Considering only their main body, disc insulators can be made of porcelain or glass or polymers.The best material used is the composite but the most used is the tempered glass for its better resistance against pollution and good rigidity.
The main active part of a High-Voltage (HV) surge arrester is its column of nonlinear resistors (varistors). Zinc Oxide (ZnO) varistors are mostly used in HV Surge Arresters. A Zinc Oxide varistor is a ceramic compound based on ZnO grains and intergranular layers formed by several dopant elements responsible for its nonlinearity. Silicon Carbide (SiC) based varistor was also used in the past but its nonlinearity is much lower than the one of ZnOvaristor. Due its poor nonlinearity, SiC arresters have gaps in series with the varistor column, while ZnO arresters are ordinarily gapless.
Porcelain Insulator: Porcelain in most commonly used material for overhead insulator in present days. The porcelain is aluminium silicate. The aluminium silicate is mixed with plastic kaolin, feldspar and quartz to obtain final hard and glazed porcelain insulator material. The surface of the insulator should be glazed enough so that water should not be traced on it. Porcelain also should be free from porosity since porosity is the main cause of deterioration of its dielectric property. It must also be free from any impurity and air bubble inside the material which may affect the insulator properties.
Glass Insulator: Now days glass insulator has become popular in transmission and distribution system. Annealed tough glass is used for insulating purpose. Glass insulator has numbers of advantages over conventional porcelain insulator
Polymer Insulator: In a polymer insulator has two parts, one is glass fiber reinforced epoxy resin rod shaped core and other is silicone rubber or EPDM (Ethylene Propylene Diene Monomer) made weather sheds. Rod shaped core is covered by weather sheds. Weather sheds protect the insulator core from outside environment. As it is made of two parts, core and weather sheds, polymer insulator is also called composite insulator. The rod shaped core is fixed with Hop dip galvanized cast steel made end fittings in both sides.
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