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The overhead line conductors ought to be supported on the poles or towers in such some way that currents from conductors don’t flow to earth through supports i.e., line conductors should be properly insulated from supports. this can be achieved by securing line conductors to supports with the assistance of insulators. The insulators give necessary insulation between line conductors and supports and therefore stop any escape current from conductors to earth. In general, the insulators ought to have subsequent fascinating properties.

  1. High mechanical strength so as to resist conductor load, wind load, etc.
  2. The high resistance of dielectric material so as to avoid escape currents to earth.
  3. The high relative permittivity of dielectric material so as that material strength is high.
  4. The dielectric material ought to be non-porous, free from impurities and cracks otherwise the permittivity is lowered.
  5. High quantitative relation of puncture strength to the arc.

The most ordinarily used material for insulators of overhead line is ceramic ware however glass, steatite, and special composition materials also are wont to a restricted extent. ceramic ware is created by firing at an extreme temperature a combination of china stone, feldspar, and quartz.

Types of Insulators:

The undefeated operation of the associated overhead line depends to a substantial extent upon the right choice of insulators. There are many kinds of insulators however the foremost unremarkably used ar pin sort, suspension type, strain nonconductor, and shackle nonconductor.

1. Pin sort insulators: The half section of a pin sort nonconductor is because the name suggests, the pin sort the nonconductor is secured to the cross-arm on the pole. there’s a groove on the higher finish of the nonconductor for housing the conductor. The conductor passes through this groove and is certain by the toughened wire of an equivalent material because of the conductor

2. Suspension sort insulators: the price of pin sort nonconductor will increase apace because the operating voltage is hyperbolic. Therefore, this kind of nonconductor isn’t economical on the far side thirty-three kV. for prime voltages (>33 kV), it’s a usual follow to use suspension sort insulators incorporates a variety of ceramic ware discs connected asynchronous by metal links within the sort of a string. The conductor is suspended at the all-time low finish of this string whereas the opposite finish of the string is secured to the cross-arm of the tower. every unit or disc is intended for low voltage, say 11 kV. the number of discs within the series would clearly rely on the operating voltage.

3. Strain insulators: once there’s a dead finish of the road or there’s a corner or sharp curve, the line is subjected to bigger tension. so as to alleviate the road of excessive tension, strain insulators are used. For low voltage lines (< eleven kV), shackle insulators are used as strain insulators. However, for high voltage transmission lines, strain nonconductor consists of Associate in Nursing assembly of suspension insulators as shown in Fig. 8.8. The discs of strain insulators are utilized in the vertical plane. once the stress in lines is passing high, as, at long stream spans, 2 or a lot of strings are utilized in parallel.

4. Shackle insulators: In the time period, the shackle insulators were used as strain insulators. But nowadays, they’re often used for low voltage distribution lines. Such insulators may be used either in a very horizontal position or in a very vertical position. they’ll be directly fastened to the pole with a bolt or to the cross arm.a shackle material fastened to the pole. The conductor within the groove is fastened with a soft binding wire.

Potential Distribution over Suspension Insulator String:

A string of suspension insulators consists of a variety of ceramic ware discs connected nonparallel through auriferous links.

  • shows a 3-disc string of suspension insulators. The ceramic ware portion of every disc is between 2 metal links. Therefore, every disc forms a condenser C as shown in. this can be referred to as mutual capacitance or self-capacitance.
  • If there have been mutual capacitance alone, then charging current would are constant through all the discs and consequently, the voltage across every unit would are constant i.e., V/3 as shown in
  • However, in actual apply, the capacitance conjointly exists between the metal fitting of every disc and tower or earth.
  • this can be referred to as shunt capacitance C1. because of shunt capacitance, charging current isn’t constant through all the discs of the string.

  • The voltage affected on a string of suspension insulators doesn’t distribute itself uniformly across the individual discs thanks to the presence of shunt capacitance.
  • The voltage affected on a string of suspension insulators doesn’t distribute itself uniformly across the individual discs thanks to the presence of shunt capacitance.
  • The disc nearest to the conductor has most voltage across it. As we tend to move towards the cross-arm, the voltage across every disc goes on decreasing.
  • The unit nearest to the conductor is underneath most electrical stress and is probably going to be punctured. Therefore, means that should be provided to equalize the potential across every unit.
  • This is absolutely mentioned in Art. If the voltage affected across the string were d.c., then the voltage across every unit would be the same. it’s as a result of material capacitances are ineffective for d.c.