The overhead line conductors ought to be supported on the poles or towers in such the simplest way that currents from conductors don’t flow to earth through supports i.e., line conductors should be properly insulated from supports. this is often achieved by securing line conductors to supports with the assistance of insulators. The insulators give necessary insulation between line conductors and supports and therefore forestall any discharge current from conductors to earth. In general, the insulators ought to have subsequent desirable properties.
The most unremarkably 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 a heat a mix of terra alba, felspar, and quartz. it’s stronger automatically than glass, gives less bother from discharge and is a smaller amount established by changes of temperature.
- High mechanical strength so as to face up to conductor load, wind load, etc.
- The high ohmic resistance of dielectric material so as to avoid discharge currents to earth.
- The high relative permittivity of dielectric material so as that material strength is high.
- The dielectric material ought to be non-porous, free from impurities and cracks otherwise the
permittivity is lowered.
- High magnitude relation of puncture strength to discharge.
Types of Insulators:
The self-made operation of Associate in Nursing overhead line depends to a substantial extent upon the correct choice of insulators. There square measure many kinds of insulators however the foremost unremarkably used square measure pin kind, suspension kind, strain nonconductor and shackle nonconductor.
Pin type insulators:
The half section of a pin kind nonconductor is because the name suggests, the pin kind 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 constant material because of the conductor.
Causes of nonconductor failure:
Insulators square measure needed to face up to each mechanical and electrical stresses. The latter kind is primarily thanks to line voltage and should cause the breakdown of the nonconductor. The electrical breakdown of the nonconductor will occur either by flash-over or puncture. in a very electric discharge, Associate in Nursing arc happens between the road conductor and nonconductor pin (i.e., earth) and therefore the discharge jumps across the air gaps, following shortest distance.
Suspension type insulators:
The cost of the pin kind dielectric will increase chop-chop because the operating voltage is exaggerated. Therefore, this kind of dielectric isn’t economical on the far side thirty-three potential unit. for prime voltages (>33 kV), it’s a usual observe to use suspension kind insulators comprises a variety of ceramic ware discs connected nonparallel by metal links within the kind of a string. The conductor is suspended at rock bottom 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 upon the operating voltage. as an example, if the operating voltage is sixty-six potential unit, then six discs during a series are going to be provided on the string.
Advantages of Suspension type Insulators:
- Suspension kind insulators square measure cheaper than pin kind insulators for voltages on the far side thirty-three potential unit.
- every unit or disc of suspension kind nonconductor is meant for low voltage, usually eleven potential units.
- Depending upon the operating voltage, the specified variety of discs are often connected serially.
- If anyone disc is broken, the entire string doesn’t become useless as a result of the broken disc are often replaced by the sound one.
- The suspension arrangement provides larger flexibility to the road. The association at the cross arm is such nonconductor string is liberal to swing in any direction and might take up the position wherever mechanical stresses square measures minimum.
- just in case of enhanced demand on the conductor, it’s found additional satisfactory to produce the larger demand by raising the road voltage than to produce another set of conductors.
- The additional insulation needed for the raised voltage is often simply obtained within the suspension arrangement by adding the specified variety of discs.
- The suspension kind insulators square measure usually used with steel towers. because the conductors run.
When there’s a dead finish of the road or there’s a corner or sharp curve, the road is subjected to larger 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 prime voltage transmission lines, strain nonconductor consists of AN assembly of suspension insulators. The discs of strain insulators are employed in the vertical plane. once the strain in lines is passing high, as at long stream spans, 2 or a lot of strings are employed in parallel.
In the period of time, the shackle insulators were used as strain insulators. however today, they’re ofttimes used for low voltage distribution lines. Such insulators are often used either in a very horizontal position or in a very vertical position. they will be directly fastened to the pole with a bolt or to the cross arm. shackle nonconductor 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 asynchronous through bimetallic 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 an electrical condenser C.
- this is often referred to as mutual capacitance or self-capacitance. If there have been mutual capacitance alone, then charging current would are an equivalent through all the discs and consequently, the voltage across every unit would are an equivalent i.e., V/3.
- However, in actual follow, the capacitance conjointly exists between the metal fitting of every disc and tower or earth. this is often referred to as shunt capacitance C1. thanks to shunt capacitance, charging current isn’t an equivalent through all the discs of the string.
- Therefore, the voltage across every disc is going to be completely different. Obviously, the disc nearest to the road conductor can have the utmost voltage.
Types of Insulators:
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…[ read more ]