A Study of Insulators
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Insulators are substances that do not conduct electricity and possess high resistivity, dielectric strength, mechanical strength, and the ability to withstand moisture and temperature. Common insulating materials include cotton, silk, rubber, mica, insulating oil, varnish, enamel, porcelain, and Bakelite, each with unique properties and applications. Types of insulators used in overhead lines include pin type, suspension type, strain type, and shackle type, which are designed for varying voltage levels and mechanical stresses.
A Study of Insulators
- 1. INSULATORS Insulators are substance which does not readily conduct electricity or Insulators are those substances which offer such a huge resistance. Di-electric strength is the term used for comparing insulation property of an insulator. Properties An ideal insulator has the following properties a) High Resistivity Insulators are those substances which offer such a huge resistance, that they allow practically no electricity. b) Di-electric strength All insulator conduct electricity if subjected to apply extreme voltage. The voltage at which the material lost his insulating property is known as di- electric strength or breakdown voltage. c) Mechanical strength The insulating material must be mechanically strong enough to carry tension and weight of conductor d) Ability to withstand moisture Their must not be any entrance on the surface of electrical insulator so that the moisture or gases can enter it. e) Withstand at high temperature There physical as well as electrical properties must be less affected by changing temperature. f) Low Permittivity
- 2. Commonly Used insulation Materials Cotton Cotton is used very largely for low voltage insulations. Cotton is much easier to handle and also serves mechanical support for other insulating materials Silk Silk is an excellent insulator. It is quite expensive and it is used in very small instruments. Paper Electrical insulation paper is paper type that is used for electrical insulations. The paper is made from manila fibres, wood pulp and cotton range. Rubber Rubber is a good insulator; it is used outside of wires that are used to conduct electricity. Rubber provides protection from the dangers of electrical charges being carried through the metal conductor. The mechanical properties of rubber can improved by addition of sulphur (vulcanizing). Mica Mica is a mineral consisting chiefly of a double silicate of aluminium and potassium. It is extremely stable when exposed to moisture and extreme temperature while maintaining superior electrical properties as an insulator.
- 3. Insulating Oil Mineral oil obtained from refining petroleum possesses good insulating properties. This oil has dielectric strength of 200KV/cm. It is used in oil as transformer, some types of high voltage capacitors, fluorescent lamps ballast, switch gears, etc. Varnish Varnish is an insulating material used to give protection against moisture and chemical corrosion environment Enamel Enamel is applied to the wires and strips, which are smaller in the cross-section. Thin enamel can give dielectric strength 600 KV/cm.
- 4. Porcelain Porcelain has very good electrical and mechanical properties as an insulator. It is used on overhead lines for bus-bar, switch-gear support and terminal bushing for transformer. It is made by chiefly of china clay, quartz and feldspar. Porcelain has a dielectric strength of 4-10 kV/mm. Bakelite It is also known as polyoxybenzylmethylenglycolanhydride. It is a thermosetting phenol formaldehyde resin, formed from a condensation reaction of phenol with formaldehyde. It is used in radio and telephone casing, for making switches, etc.
- 5. Insulators used in overhead line Pin type insulator Pin type insulator is used for the transmission of lower voltages (up to 11 kV). A pin type insulator is attached above the cross arm. The transmission line conductor passes through the groove on head of insulator Suspension type Insulators They are cost effective for higher voltage transmission. A suspension type insulator is suspended below the cross arm. It consists of number of disc connected in series with the help of a metal link. They are three types 1) Cemented cap suspension insulator 2) Interlinking/Hewlett suspension insulator 3) Link/Core suspension insulator
- 6. Strain type insulators They are horizontally placed suspension insulator. It is used to handle mechanical stress and take pressure off a conductor at the end. Shackle type insulator The shackle insulators are used on sharp curves, end poles and in section poles. A shackle insulator is used for low voltage distribution lines.